iñigo flores ituarte - bio-inspired metallic surfaces by means of mechanical processes

Iñigo Flores Ituarte ©

PFC 2009-2010

BIO-SURFACES BY MEANS OFMECHANICAL PROCESSES

©

2010

-INSPIRED METALLIC SURFACES BY MEANS OFMECHANICAL PROCESSES

AUTOR: IÑIGO FLORES ITUARTE TITULACIÓ: ENGINYERIA TÈCNICA MECÁNICA INDUSTRIAL TUTOR: ELENA BARTOLOMÉ DEPARTAMENT: MECÁNICA DATA: JULIO 2010

© Bachelor Final Thesis

1/92

INSPIRED METALLIC SURFACES BY MEANS OF MECHANICAL PROCESSES

MECÁNICA

Iñigo Flores Ituarte © Bachelor Final Thesis

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Bio-inspired metallic surfaces by means of mechanical processes Abstract

“Bio-inspired” design is the development of devices and applications founded on Nature

solutions. Bio-inspired functional surfaces have nowadays a great potential of implementation

in many areas, and intense research is being done to find out man-productions techniques that

allow industrial fabrication at competitive costs.

This work evaluates the feasibility of Bio-inspired metallic surfaces by means of mechanical

processes only. A review of the state-of-the-art of Bio-inspired solutions, manufacturing

techniques and materials available evidences that only “Sharkskin” riblet morphologies in the

micro-metric range are feasible by mechanical processes at present, whereas nano-metric

morphologies are difficult.

Finite element method (FEM) simulations have been used to evaluate the manufacturing of

riblet surfaces by rolling process and give guidelines for possible fabrication setups.

Results summarized in this memory show that drag reduction effect by mimic of Sharkskin

surface through riblet manufacturing is possible, and offers possibilities for future development

of this technology with a cost-effective process. This technology has a wide implementation

area in many industrial activities and it offers great environmental and technical benefits. The

benefits induced by the implementation of Sharkskin surfaces in airplanes would imply an

estimated of fuel consumption of around 1,5 %.

Keywords

Bio-inspired, functional surfaces engineering, Sharkskin, Lotus effect, Moth eye, Gecko feet’s,

Rolling process, micro-mechanical manufacturing processes, Finite element method, energy

save.


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Index

1. Introduction .......................................................................................................................... 6

2. Objectives ............................................................................................................................. 8

3. State - of - the - Art of Bio-inspired surface engineering ................................................... 10

3.1 Sharkskin and drag reduction effects .......................................................................... 10

3.1.1 Description .......................................................................................................... 10

3.1.2 Theoretical foundations ....................................................................................... 12

3.1.3 Manufacturing processes ..................................................................................... 15

3.1.4 Materials and applications ................................................................................... 18

3.2 Lotus effects, Self-cleaning and hydrophobic effects ................................................. 19

3.2.1 Description .......................................................................................................... 19




3.3 Motheye and biomimetic effects ................................................................................. 26

3.3.1 Description .......................................................................................................... 26




3.4 Gecko feet and dry adhesion effects ........................................................................... 32

3.4.1 Description .......................................................................................................... 32





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4. FEM simulations of riblet profile manufacturing ............................................................... 38

4.1 Simulation models ....................................................................................................... 38

4.2 Materials and Material models .................................................................................... 42

4.3 Simulation results ........................................................................................................ 44

4.3.1 “A” Model results ............................................................................................... 44

4.3.2 “B” Model results ................................................................................................ 48

4.3.3 Geometry parameters .......................................................................................... 52

4.4 Discussion ................................................................................................................... 54

5. Benefits of Sharkskin effect in aviation industry ............................................................... 57

6. Conclusions and future prospects ....................................................................................... 61

7. Project temporization ......................................................................................................... 65

8. Bibliography ....................................................................................................................... 67

9. Annexes .............................................................................................................................. 71


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1. Introduction

Biologically-inspired or “Bio-inspired” design is the development of engineered devices taking

ideas founded on Nature and evolution characteristics.

Nowadays, engineers and scientists are using many Bio-inspired solutions in several industrial

areas. Potentially, the design under this point of view offers new technological applications and

gives opportunities to improve the efficiency of actual devices.

Nature can provide models and templates to make and produce cleaner technologies, more

environment-friendly, technically more efficient and economically more profitable. [1]

All surfaces have a defined texture and structure. In productive processes these surfaces are

designed and manufactured to satisfy specific needs. Characteristics produced by means of

evolution and natural selection in animal and plant surfaces are for engineers and scientists a

great foundation in surface engineering. [2]

At the present, there are many Bio-inspired commercial solutions available in a few application

areas. For example, new generation swimsuits are inspired in Sharkskin effect and the reduction

of marks in swimming competitions is a fact, (see picture 1). Also, very popular dry adhesive

systems are inspired in fiber adhesion.

Some surfaces found in Nature, such as Sharkskin, Lotus effect, Moth eye and Gecko feet’s

have benefits for surface engineering. The effects produced by these bio-surfaces are drag

reduction, hydrophobic and self-cleaning, anti reflectivity and dry adhesion effects,

respectively. These surfaces offer significant advantages, such as a large energy save and

efficiency increase. [3]

The commercial development of these surfaces represents an important challenge. The

knowledge in the theoretical foundations and their implementation into mass productive

methods requires the close work of scientists and engineers for cost-effective development of

this technology. The next few years will reveal big steps in this novel area.


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Picture 1: Commercial swim suit with Sharkskin effect

Abbreviators Nomenclature

DR Drag reduction D Diameter MRF Micro riblet film L Longitude HSC Hydrophobic and self-cleaning h Height

WCA Water contact angle S Width

AR Anti reflective ReD Reynolds diameter

SEM Scanning Electron Microscopy CD Drag coefficient

FEM Finite Elements Method

GHG Green House Gases

Table 1: Abbreviators and nomenclature


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2. Objectives

This work was written and carried out in 2008-2009 during a 9 month internship at “VTT

Research Center of Finland”, where I worked as a research assistant profiting from an

“ERASMUS practicum” European grant.

VTT is a multi-technological contract research organization and it is part of the Ministry of

Industry of Finland. As an up to date European research center, this organization has a wide

array of technological branches, including knowledge areas such as bio-science, food industry,

electronics, energy, real estate and construction, machines and vehicles, service and logistics,

forest industry, process industry and environmental sciences.

Picture 2: VTT Research Center of Finland

This project was developed within the working cluster of “Manufacturing process simulation

and development”. This group is part of “Machines and vehicles department” in VTT. The main

working areas in this cluster are the modeling and simulation of: metal forming processes, steel

laminate and rolling process monitoring, crash and impact tests, welding and welding assembly

processes, heat treatment processes and the optimization of these industrial processes.

The purpose of this research project is to make an out-look of actual Bio-inspired surfaces

engineering capabilities, as a part of a prospect for a European project that VTT was working

on. Thus, this work would be used to have a description of Bio-inspired surface engineering.

This project makes an introduction to functional surface engineering steps and production

methods, available at present. Furthermore, the final objective is to research and evaluate the

possibility of manufacturing Bio-inspired metallic surfaces exclusively by means of mechanical

processes.


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In the first part of this memory the project will present the State-of-the-Art in Bio-inspired

surface engineering i.e. multifunctional surfaces inspired on biological structures. This section

analyzes and shows the theoretical foundations, morphology and shape characteristics, and

sketches the most common materials and productive methods in manufacturing of Bio-inspired

surfaces.

In order to study the possibility of mechanical manufacturing, metal forming FEM simulations,

presented in chapter 4, are used as predictive approach to evaluate the parameters for a reliable

implementation of these production methods into the common manufacturing processes in the

future.

Conclusion and future prospects of this work, in chapter 5, will serve to show future research

lines for VTT.


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3. State - of - the - Art of Bio-inspired surface e ngineering

The purpose of this section is to describe some of the most important Bio-inspired surfaces,

their geometrical characteristics, theoretical foundations and a review of the available materials

and manufacturing techniques.

3.1 Sharkskin and drag reduction effects

3.1.1 Description

Through natural evolution, sharks and several other aquatic animals have adapted their skin to

reduce friction with water. This method for saving energy was achieved by various mechanisms

of fluid control and boundary layer control.

The Sharkskin has tiny grooves called riblets aligned parallel in the direction of flow. Figure 1

shows a Scanning Electron Microscopy (SEM) photograph of real Sharkskin and an example of

a riblet surface imitating this Sharkskin surface.

Figure 1: a) Sharkskin photography, SEM [4] b) Example of riblets geometry [5].

The physical definition of friction is the combination of forces which tend to reduce the relative

motion of two surfaces in contact (Coulomb friction model) or a surface in contact with a fluid:

air on an aircraft (aerodynamic friction) or water in a pipe (hydrodynamic friction).

Even if intuition would say that this kind of rough texture increases friction, a riblet structure in

surfaces has the opposite effect, when the fluid conditions and riblet morphology are adequate.


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Micrometrical riblets and distances between tip and tip of riblet structures, or high-frequency of

riblet structures, Figure 2(a), produce the inhibition of transversal turbulences by a viscous

effect in surfaces in contact with parallel fluid in motion [6]. The transversal turbulences or

transverse instabilities keep away of the riblet structure and the flow becomes laminar between

the riblets. This effect produces friction reductions or Drag Reduction (DR).

Figure 2: Simplified behavior of a fluid into two different frequencies of riblets, a) High-frequency of riblets,

b) Low-frequency of riblet [6]

The theoretical demonstration that explains the DR due to the riblet structure is complex and

most of the knowledge on DR effect is based on direct experimental methods.

Experiments with riblet structure inside pipelines and tests with different Reynolds numbers

(ReD) show favorable values of DR [8]. Also, airplane flight tests and measurements in wind

tunnel [5], tests in pumps, exploratory experiments in compressor blades for aeronautic industry

or energy industry [9] [10] [11] are available. Moreover, there are direct numeric simulations

with different geometry of riblets [7] [12] [13].

These experiments and simulations values show a significant DR for the different devices

producing an increment of the efficiency when riblet structures were applied, although it is

necessary to indicate that the efficiency of the riblet structure varies depending on the flow

conditions.


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3.1.2 Theoretical foundations

DR in applications like pipelines is obtained by passive or active techniques. Active method is

when DR is achieved by means of additive substances in the fluid. On the other hand, a passive

method only changes the contact surface morphology properties.

An active method changes the fluid properties or fluid temperature, in order to reduce fluid

viscosity and therefore decrease pressure drop. These techniques are at present more profitable

and common, although they require an energy input. [14] [15] [16]

A passive method, on the contrary, changes the fluid behavior and obtains DR only by changing

the contact surface characteristics. A surface with structure of riblet is an example of passive

DR. The theoretical DR can be increased up to 10 % in ideal cases, compared with a

hydrodynamic smooth surface.

The geometry of the riblet has influence on the fluid behavior and DR efficiency. Parameters

like the height and width of the riblets are decisive. The shape of the riblets plays also an

important role.

Figure 3 explains the relation between the different riblet shapes, and their efficiency level [12].

The empirical ratio between the riblet heights (h) to width (s) ratio is defined as:

s

hRiblet ratio = (3-1)

Experiments have shown that optimal DR is obtained for a riblet ratio of 0.5. Values between

0.2 < RibletRatio < 0.7 are acceptable, although efficiency decreases. [12]

Figure 3: Different shapes of riblet structure, and efficiency level. [17]


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The geometry of the riblet must be adapted to the local flow state for every particular

application. The choice for an optimal geometry involves many variables, and their impact is

not linear. In the prediction of an optimal geometry statistical techniques and experiments with

empirical solutions are mostly used.

One predictive solution is the empiric equation (3-2) that describes the maximum width Smax for

the geometry of riblet in a specific application [18]:

10/9

10/910/1

2/1

8/1

max 0225,0

37,0

∞

+=U

vxss , (3-2)

where s+ is a constant related with the flow state for the geometry characteristics of application

(e.g. for aircraft surface, high speed trains and turbine blades, s+=20 [18]), x is the position of

the boundary layer, v is the kinematic viscosity and U∞ is the free fluid speed.

This empiric equation has limitations and it is not possible to use it for every application,

however it is used for parallel flow with a plane surface, and Table 1 shows values for Smax for

most researched applications.

Application Smax

High speed trains 100µm

Aircraft skins 90µm

Turbine blades 70µm

Table 2: Estimation of Smax for different applications [18]

For applications like pipelines, there are experiments describing the behavior inside a wind

tunnel of two pipelines with the same geometrical characteristics (D=18 mm and L=200 mm)

but different surface morphologies. This experiment gives the values of drag coefficients (CD)

of the different pipes.[8]

One of the pipes has a hydrodynamic smooth surface and the other one includes a silicone

sawtooth Micro Riblet Structure (MRF) [19]. The geometry of the MRF was h = 180 µm and

s = 300 µm. Experimental conditions were such that the Reynolds number ranged between

2.5x103 < ReD < 3.8x104, changing U∞.


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Figure 4: Variation of DR with different ReD values. O: smooth pipe. ∆: MRF pipe. [8]

The example of Figure 4 shows the CD, for different Reynolds numbers. In the most favorable

combination, ReD =3.6x103 and U∞=3 m/s, the MRF pipe had a DR of 7.6 %, whereas for

ReD=3.6x104 and U∞=30 m/s, the friction increased a 4.2 %.

Theoretical expressions and tests demonstrate that DRs between 1 % < DR < 10 % are possible.

However, one must bare in mind that experiments have shown that a bad combination between

geometry of the riblet and adverse flow conditions produce an opposite effect, and friction could

even increase.

The key factor to increase the efficiency is to choose the best theoretical riblet geometry specific

for every individual application and find the equilibrium between the theoretical ideal geometry

and the real, manufacturable geometry inside an acceptable tolerance range.


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3.1.3 Manufacturing processes

• Micro - Rolling process

The rolling process techniques consist of replicating a riblet profile by means of plastic

deformation with a continuous roll machine.

Figure 5: Sketch of a rolling process, a) Detail of macroscopic rolling parameters, b) Thickness and detail

of microscopic parameters. Before and after the deformation [20]

Figure 5(a) shows the embossing of a DR profile in a work-piece by a continuous rolling

machine. One roll has a negative riblet profile and the other has a flat surface. The most

important parameters from the macroscopic point of view are: the width of the rolling area, the

linear speed of the rolling process, the hardness relationship of materials (tools and work-piece)

and the section reduction (thickness before and after the rolling). From the microscopic point of

view, Figure 5(b) shows the relevant geometrical parameters, negative riblet geometry in tool

and riblet geometry after the rolling in work-piece.

All these parameters have a direct influence on the final result of the process, and the shape of

the riblet must accomplish the shape requirements like the Smax for any application, and

RibletRatio to be efficient.

Figure 6: Sketch of tool manufacturing, a) Machining, b) Winding. [20]


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There are different mechanical techniques, like turning and winding, to manufacture the tool

and reproduce the negative riblet profile on it. In Figure 6(a), the riblet profile is produced by

direct machining with a conventional turning process. [18] [21]

In the winding technique, Figure 6(b), the base must be first machined with a turning technique,

to reduce the tendency of dislocation of the wire. Then it is possible to build up the roll with the

wire using the machined base like guide. [17] [20]

The most important parameters in tool manufacturing processes are the precision in axis of

turning and winding process machines, the material grain size and mechanical characteristics,

wire diameters, and last, the geometry of the machining tool and tip radio, which must be as

small as possible to produce a correct profile.

Experimental manufacturing and simulations reported in ref. [17] [18] [20] [21] show the

potential capacity of these techniques to reproduce riblet structures in small areas (w = 21, 30

mm of work-piece). Experiments show that riblets with approximately Smax = 400, 200, 100 µm

could be successfully manufactured accomplishing the RibletRatio for the fluid dynamics

requirements. Future experiments are focused to produce Smax = 75 µm to open the application

area.

• Micro - Grinding process

Micro-grinding processes consist of the machining of the DR profile by using a vitrified bonded

SiC grinding wheel with a tangential grinding machine.

Figure 7(a) sketches a grind process and shows the more general parameters in the grinding

process. The final shape of the work-piece after the machining in the experiment of ref. [22] can

be seen in Figure 7 (b) and (c).

Figure 7: a) Sketch of grinding process, b) SEM image of produced riblets, c) Micrograph section of

experimentally produced riblets. [22]


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The preparation of the tool with DR negative profile is made by means of a one diamond profile

roller and is sketched in Figure 8.

Figure 8: Sketch of the grinding wheel Sep up.

The set-up for the machining is very important. The final result is determined by the correct

combination of all the parameters, properties of diamond wheel tool, grain size, mechanical

characteristics and wear of the wheel. For the work-piece, mechanical and geometrical

parameters and also kinematics and dynamics in machining are very influent.

Experimental production and simulations show that it is possible to manufacture different

geometries of DR profiles with this technique and it is possible do it in big areas with a reduced

time, although there is a necessity to develop better abrasive wheels for cost effective

manufacturing. [22] [23] [24]


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3.1.4 Materials and applications

The application of riblet DR technique is very attractive for many industrial devices. Depending

on the application, a small or big surface area may be required, implying different

manufacturing techniques.

For small surface areas, materials like steel alloys, titanium alloys and nickel super alloys and

others are used to produce blades for jet engines and energy production turbines blades.

For instance, the work-piece material in experiments of ref. [18] [21] is Ti6Al4V. Different

stainless steels are used as experimental work-pieces, for example in ref. [22] the material is

X20Cr13 and in ref. [20] is X5CrNi 18-10. These materials are used to work at high

temperature environment with good mechanical requirements and commonly used in aero

engine blades manufacturing and energy generation turbines.

Furthermore, big surface areas like airplane skins, ship skins, high speed train skin etc.

aluminum and steel alloys are commonly used. Ref. [17] shows the experimental manufacturing

in Al99.5 used in airplane carriage manufacturing.

The development of DR profiles in turbine blades increases the efficiency and reduces stresses

increasing the admissible load cycles and effective life of the device. Also, the general friction

in high speed vehicles can be decreased with a consequent reduction in consumption.

Experimental grinding manufacturing and especially rolling manufacturing systems have a good

behavior in final embossed riblet shape, and they offer the possibility to include DR properties

in a new generation of turbines and high speed vehicles skins in the future.

Although the development of DR profile in pipes was tested with polymers films, the research

in this area and the development of new DR metallic pipes can provide more efficient fluid

transport systems implying large energy reduction. The negative aspect of manufacturing pipes

with an efficient riblet roughness is the poor cost-benefit ratio at present.


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3.2 Lotus effects, Self-cleaning and hydrophobic ef fects

3.2.1 Description

The lotus plant is a symbol of purity in many Asian religions. This symbolism has its origin on

the fact that the surfaces of lotus plant never retain dirt. The Lotus plant has hydrophobic

characteristics, Figure 9(a), and this effect allows a permanent self-cleaning action. This

Hydrophobic and self-cleaning (HSC) effect can be observed in other plants and animals in

Nature, and the advantages of HSC are obvious for many industrial applications.

The morphology of lotus flowers coatings has a double-structured surface, Figure 9(b). The

first presents a micro-scale roughness and the second one a nano-scale roughness. The first

micro roughness cells are covered by tiny nano-metrical almost cylindrical crystals, Figure 9(c).

This geometrical combination produces that the contact area is drastically reduced.

Figure 9: a) Photography illustrating the hydrophobic “lotus effect” [25], b) SEM image, surface of Lotus

Flower [26], c) Schematics of cells and wax crystals of lotus flower surface.

This surface micro-structure is not unique of lotus flowers; there are more combinations of

nano-micro-structures that produce HSC effect in Nature, in other plants or even insects. [25]

A combination between the chemistry of the surface, affecting directly the surface tensions, and

the double roughness geometry, produce these HSC bio-characteristics.

Nowadays, there are many developed and commercialized products with Lotus effect properties,

such as paintings, coverings, crystals, lenses etc. This HSC characteristic is produced commonly

in textile fibers, polymeric materials and crystals or ceramics.


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At the moment, the manufacturing techniques to produce commercial surfaces with HSC

characteristics are based on imitation of this double-structure with the best chemical

composition possible.

The most common techniques are the addition of composite materials, fibers and resins with

similar morphology characteristics to Lotus plant coatings by spraying and pulverization [27]

[28]. Also electrochemical treatments [29] and chemical routes or sol-gel routes [30] [31] [32]

[33] [34] [35] are researched to date.

Self-cleaning surfaces are in fact advantageous in whatever surface capable of getting dirty.

This properties HSC properties would be very convenient in metallic surfaces like aluminum

alloys [32] [36] [37], titanium alloys [38], magnesium alloys or steal alloys [34] [35]. The

research in this area can be profitable for many industrial areas and the development of new

technologies to optimize production is ongoing.


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A coating with HSC properties needs a close relationship between the chemistry and physical

properties of interaction elements and the surface morphology. This relationship determines the

final performance and HSC characteristics.

Figure 10: a) Sketch of surface tensions of the system: solid, liquid and gas. [39] b) Sketch of hysteresis

angle of a water drop.

Figure 10(a) shows the surface tensions and the interaction energies between elements of a

solid-liquid-gas system.

The final shape of the water drop or Water Contact Angle (α = WCA =θ ) is directly given by

the result of these three surface tensions.

The calculus of surface tensions implies the microscopically view and study of the internal

cohesion and repulsion of molecular forces of the different interaction elements states.

The traditional study of HSC properties is explained by Young´s equation based in Newton

second law:

∑ =i iT 0 θcos.,,, VLVSLS TTT =− , (3-3)

where TS,L is the surface tension between solid and liquid states, TS,V is the surface tension

between solid and vapor states and TL,V is the surface tension between the liquid and vapor

states.


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Nowadays, Young’s (3-3) equation is insufficient to explain the behavior of surfaces. The HSC

characteristics are affected by many parameters like the hysteresis angle, Figure 10(b),

roughness and porosity. At present, there are new models that explain better this effect and these

models include the above mentioned parameters. [26] [31] [40] [41]

One key factor for HSC property is the hysteresis angle condition, Figure 10(b). If the angle is

too much large the water drop cannot roll in the surface catching all the dirt, it only slips

producing a bad final result.

A water contact angle of WCA= 00 means a hydrophilic surface, whereas WCA = 0180 means

a completely hydrophobic surface. Surfaces with WCA between 0110 < WCA < 0140 are

considered hydrophobic and angles upper than WCA > 0140 with hysteresis angles less than

WCA < 05 have HSC characteristics. [42]

Figure 11 shows the evolution of polycarbonate polymer resin during the silica chemical

treatment [31]. The Figure 11(a) shows an uncoated PC surface and next Figures 11(b) and (c)

present the improvement of the WCA, self-cleaning and hydrophobic properties..

Figure 11: Sketched figure of chemical treatment, a) untreated surface, b) first step of treatment,

c) Treated surface. [31]



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• Micro - embossing process

One manufacturing technique to produce lotus flower surface profile and HSC morphology is

based on the plastic deformation of the work-piece.

Figure 12: Schematics of micro-embossing process: a) Description of elements, b) Embossing process

and plastic deformation, c) Final result.

The process to generate HSC morphology for functional surfaces needs the reproduction of a

negative coating in the tool with a nano-metric profile inside the micro-metric profile. The

manufacturing process by micro embossing is sketched in Figure 12.

The general parameters in micro embossing are the tool and work-piece mechanic properties. A

very important parameter is the relation of hardness in the tool and work-piece. Also, the

embossing temperature (hot or cold) and the forces in the embossing process can determine the

final result.

The techniques to manufacture nano-metrical coating in stamping or embossing tools require

alternative processes and non-exclusive mechanical processes, because the manufacturing

processes under the nanometric range by mechanical techniques is not possible nowadays.

For instance, one example of this is the machining of the negative tool profile by laser

machining. Figure 13 shows the effect of Nd: YAG laser treatment, which produces orthogonal

or hexagonal craters of 30, 40, and 50 µm of width and 20 µm of deep in the hard metallic tool.

Then the embossing of the work-piece is done by plastic deformation. [36]


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Figure 13: SEM image, a) Work tool, metallic hard surface, b) Work-piece, micro embossed 99.5 % pure

aluminum with 1mm thick. [36]

It is possible to use other techniques to reproduce HSC embossing tools, like electrochemical

routes, photolithographic techniques, electron beam writing processes and others. [36]


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Aluminum, steel, magnesium and titanium alloys are usually used for structural applications,

construction materials and many industrial applications.

The experimental manufacturing in aluminum allows, by means of stamping or embossing

process and then chemical treatments can produce WCA between 150o and 160o and also

successful values of hysteresis with acceptable HSC characteristics.

Magnesium or aluminum alloys are used for structures in many electronic devices, mobile

phones, laptops etc, where small surfaces are needed. The manufacturing of these structural

elements is made by stamping. Embossing tools with HSC morphology negative shape could be

included inside the manufacturing chain, and then make an efficient combination of the pre-

treated surface with chemical treatments to get the mentioned properties.

For large areas applications, the most used materials are steel and aluminum alloys. The

development of new generation rolling processes to emboss morphologic characteristics in

metallic materials can provide a first step to get HSC characteristic. Then, the combination with

chemical processes could give successful values of HSC properties and also a good cost-

benefits ratio.

HSC properties in metallic surfaces can provide a decrease in cleaning periods, maintenance

cost and also a reduction in corrosion reactions.

I should be pointed out that about a 5% of the gross domestic product (GDP) of an

industrialized country is spent on corrosion prevention and maintenance or replacement of

corrosion affected surfaces!


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3.3 Motheye and biomimetic effects

3.3.1 Description

Many insects, and specially moth species, have a nano-roughness structure in the surface of

their eyes and wings. With this structure the surface of insects presents a low refractive index

for light, and has therefore Anti-Reflective (AR) properties for camouflage and for escaping the

predator.

Figure 14(a) shows the Motheye effect, that is the low reflection of light in the moth surface

under light exposition. This kind of coating produces an AR layer on the surface.

Figure 14: a) Photograph illustrating the “Motheye” effect [43], b) SEM picture of Motheye [44], c) TEM

picture of two different species of moths [44].

The Motheye and surfaces based on the moth eye seem to be black. This effect is produced

because the surface has the property to absorb the light from any direction.

The Motheye surface is composed of an hexagonal array. Inside of this array, there are

nanoscopic roughness structures, Figure 14(b). The geometry and characteristics of this nano

structure varies between the different species, Figure 14(c). The common geometrical

dimensions for this nanoscopic roughness surface of different species varies between 100–300

nm for the width and 150–350 nm in height [44] [45].

Figure 15(a) shows a stochastic 3D representation of a wing surface, and Figure 15(b) plots

graphically the nanoscopic surface profile.

500nm

500nm

500nm

100nm


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Figure 15: a) Stochastic 3-D representation of cicada wings, b) graphic of profile dimensions. [45]

The investigation in this area is ongoing. Motheye technology is being used and is the object of

research in applications like solar cells for power generation industry [46], optic industry [47],

monitoring displays, optic fibers, detectors or many electronic devices [48].

Production techniques of surface with AR properties are generally based on chemical

techniques like sol-gel treatments [49] [50] or plasma treatments [51].


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The theoretical modeling of the reflection level of coatings can be explained by Fresnel

reflection equations and Snell’s laws. Parameters like wavelength of light, the angle of

incidence, refractive index of mediums, isotropy or anisotropy behavior of propagation of the

light through the medium and magnetism of materials and others, determine the average of final

reflection coefficient or the AR level.

A simplified model to calculate the AR level of Motheye coatings is based on Snell and Fresnel

equations. Figure 16 shows two isotropic media, air and water. Snell law states that the ratio of

the sinus of the angles of incidence vector P and refraction vector Q is a constant that depends

on the medium [52]. The mathematical expression is:

2

1

1

2

2

1

sin

sin

v

v

n

n ==θθ

or 2211 sin.sin. θθ nn = , (3-4)

where 1θ and 2θ are the incidence P and refraction Q angles respectively, 1n and 2n are the

refractive indices and 1v and 2v are the light velocities in the two different media, respectively.

Figure 16: Sketched figure of refraction (P, Q) and reflection (S) vectors. [52]

To evaluate the reflection coefficient of the coating, Figure 16 shows the sketch of reflected

vector S. Using trigonometric identities, and Snell law and Fresnel equations, it is possible to

evaluate mathematically the final Reflection coefficient (R).

2

2211

2211

)cos.cos.(

)cos.cos.(

+−=

θθθθ

nn

nnRQ and

2

1221

1221

)cos.cos.(

)cos.cos.(

+−=

θθθθ

nn

nnRP , (3-5)


PFC 2009-2010 29/92

where QR is the reflection coefficient of refraction Q vector and PR is the reflection coefficient

of incidence P vector.

The final reflection coefficient is calculated with the arithmetic average of this two reflection

coefficients:

2PQ RR

R+

= , (3-6)

where R is the final reflection coefficient of the surface. [53]

A manufactured surface with AR properties changes the refractive index 2n affecting strongly

the final reflection average (3-6) of the coating.

This basic 2D vector modeling formula gives first approach results of the AR level of the

coating. For more accurate results one must use rigorous diffraction theory, where the

propagation of the light is treated like a wave, and solve Maxwell equations under the proper

boundary conditions. [54]


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• Micro - embossing process

This manufacturing method is based on plastic deformation. The same technique already

sketched in Figure 12 is used to reproduce AR coating profile.

Ref. [54] describes the fabrication process of nano-structured AR surfaces by hot embossing,

however, the production of the tool is not by mechanical processes. Manufacturing of nano-

metrical stamping or embossing tools requires alternative techniques. For instance, Figure 17

shows the manufacturing of the tool by lithography, etching and electrochemical techniques.

Figure 17: A sketched of nickel stamp manufacturing, a) E-beam exposure and development,

b) Cr reactive ion etch and resist ashing, c) Quartz reactive ion etching and Cr removal, d) Ag removal and

nickel electroforming, e) PC hot embossing, f) Replicated AR grating [54]

In ref. [54], the AR reflective negative coating was manufactured by Ni electroforming in a

stamper, while the hot embossing was in polycarbonate (PC).

This technique shows the experimental route to produce Ni stampers, combining lithographic

techniques with electro-chemical techniques. This process allows the production, through Ni

hardness tools, of PC surfaces with AR properties by plastic deformation.


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The most commonly used materials for AR properties are polymers, ceramics and crystals. AR

coatings could be applied in many different areas, although research is at present focused on the

development of better solar cells for energy production. Also in electronic devices like

monitoring displays, optics and detectors AR properties are advantageous.

Reproduction of AR coatings into metallic materials, like steels or copper, could have also

implementation areas. It will be interesting to evaluate AR coating behavior in applications

where most heat dissipation way is by radiation phenomena. An AR surface can offer optimized

surface characteristics to increase emissivity and the efficiency in micro waves or heat

exchangers.


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3.4 Gecko feet and dry adhesion effects

3.4.1 Description

Many creatures in Nature like flies, beetles, spiders and some reptile animals have the ability to

climb and walk in vertical smooth or roughness surfaces.

The adhesion technique in Nature for species like beetles or flies is by capillarity of secretions

in contact with the roughness surface, but spiders, and especially Gecko, have a dry adhesion

capacity. This adhesion force is produced largely by Van der Waals interaction forces between

the surface and the feet structure [55] [56].

Figure 18: a) Gecko Tokay feet, b) SEM and draft of lamellae and setae, c) SEM and draft of Setae,

d) SEM and draft of Spatulae. [57] [58]

Figure 18 shows the Gecko feet structure. It has a three level hierarchical system [57]: the first

big roughness surface, called lamellae, is covered approximately by hundred billion of foot hair,

called setae; at the tip of these setae fibers there are other nano-scale fiber structures, called

spatulae, which is the last contact surface [59].

The attachment area of one Gecko feet is approximately of 110 mm2 of lamellae structure. In

Figure 18(a), lamellae have between 1-2 mm of length and are cover by 1011 setae/m2 of hair

setae fibers.

The setae, Figure 18(c), have 30-130µm of length and 5-10µm of diameter and the top of the

setae is covered at the same time with the spatulae structure, Figure 18(d).


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The length of spatulae fiber is between 2-5 µm and the diameter is 0.1-0.2 µm. At the top of the

spatulae there are subfivers of 0.5 µm length and rectangular shape, 0.2-0.3 µm of width and

0.01 µm of thickness.

This triple fiber structure gives the property to generate a maximum contact area with the

surface, and capacity to penetrate into the nano/micro roughness of the contact surface. [57]

Mimic of Gecko structure is not useful for cost effective manufacturing. However, setae and

spatulae structure can be replaced by similar structures, Figure 19(a). [59]

There are actual products inspired in this Gecko feet effect. For example reusable adhesive

systems were developed inspired in fiber adhesion. Research institutes and resources are

nowadays developing new generation of dry adhesion inspired in this system.


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The geometrical parameters are not the unique responsible of the dry adhesion effect. The

roughness [59] or friction coefficients of the surfaces [56], mechanical properties of fibers,

dynamics of gecko [60] etc. must be included to predict and model the behavior of this dry

adhesive effect. [55] [57]

The adhesion between two bodies by theoretical van der Waals forces can produce big forces.

The theoretical contact strength thσ in one ideal contact between two surfaces can be calculated

with:

A

Tth ≈σ and 1221 TTTT −+= , (3-7)

where T is the adhesion tension, 1T and 2T , are surface tensions of the two bodies in contact

and 12T is the specific surface tension of the interface formed between them, and last A is the

characteristic length of surface interaction. [55]

Figure 19: a) Setae structure simplification, b) Graft of geometric parameter used for modeling

A more accurate model can be obtained based on ideal contact strength and geometrical

simplifications, Figure 19(b). Johnson-Kendall-Roberts (JRK) predicts the force necessary for

producing dry adhesion in contact area between a spherical solid and surface:

33

...4.

3

4aTE

R

aEF Π−= , (3-8)


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where F is the adhesion force, E is the Young modulus of the fiber, a is the contact area , R is

the radio of the sphere and T is the tension adhesion described in (3-7) produced by Van der

Walls interaction.

This theoretical model successfully describes experimental results is very accurate to predict the

adhesion behavior of manufactured fibers. [55] [57]


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The production of Gecko feet hair structure is not possible by mechanical processes. The

systematic mimic and reproduction of the different bio-structures is not possible. Gecko hair

structure needs nanometric elastic hairs to reproduce the effect of dry adhesion. Mechanical

methods like stamping, rolling or direct machining cannot reproduce the necessary elastic

characteristics of fibers.

Nowadays, experimental manufacturing and research areas are focused on alternative ways. For

example, one experimental manufacturing technique consists of synthesizing of carbon nano-

tubes aligned with the normal of the surface, Figure 20(a) [61]. Also, many techniques use

lithography to produce molds with nano cavities, and then use capillarity forces to reproduce

Gecko feet hair structure in polymers, Figure 20(b). [62]

Figure 20(a) shows an example of a small film of a few mm area with Gecko feet structure.

Experiments described in ref.[61] show that this dry adhesive film has approximately an

admissible strength of 2900N/m2 in normal direction to the surface.

Figure 20 : a) SEM of carbon nano tubes dry adhesion system and manufactured adhesive film [61] b)SEM

of two techniques of gecko hair production by lithographic and capillarity forces [62]


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This new generation of dry adhesion devices could have many applications areas, but the only

manufacturing materials at the moment are polymeric coatings.

In metallic surfaces this kind of adhesive surfaces could offer better quality in joints under

aggressive environmental conditions. In metallic structures, the first point of deterioration by

cracks or corrosion is in joints between bolts and, more exactly, between the contact surfaces of

the components of the joint. This dry adhesive surface would be ideal to get a better surface

contact in bolt joints and increase the life of the device.

The development of new generation dry adhesive films offers advantages compared to actual

systems. Reusability is one of the best points compared to other actual adhesion systems.


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4. FEM simulations of riblet profile manufacturing

From the literature overview presented in chapter 3, it can be concluded that Sharkskin riblet

profiles are the only structures that may be manufactured by exclusive mechanical processes at

present. Negative tool or riblet roll, can be machined by precision turning or cutting and the

process to generate metallic groove profile surfaces could be done by rolling process. All these

steps are described in chapter 3.1.3.

The purpose of this chapter is to analyze potential problems in “V” shape riblet rolling

manufacturing process of metallic surfaces, and evaluate microscopic and macroscopic

influential geometrical parameters to implement this technology in a real manufacturing

process.

Finite Element Method (FEM) simulations have been performed using the software DEFORM-

3D V5.1 to predict effective stresses, effective strains, loads and work temperature during the

rolling process for different materials. FEM programs are commonly used to determine stress

and strain distributions in complex-shaped structures.

The simulations of V riblet shapes have the objective to describe more in depth the parameters

in riblet rolling manufacturing. The principal objective is to predict the effective stress and

effective strain of the work-piece and tool, the macroscopic and microscopic geometric

parameters in tool and work-piece after the process, and one final set up for a reliable

development of the manufacturing process.

4.1 Simulation models

In order to obtain results in a reasonable computation time, the surface geometry and

manufacturing process had to be simplified. The simplification of geometry used in simulations

was possible considering symmetry conditions in parallel planes of one riblet structure and only

microscopic effects during the manufacturing process.

The rolling process was simplified by making the simulation of only one riblet structure, but

using symmetry tools of the software. Figure 21 sketches the geometrical model used in the

simulation. Moreover, it defines all the important boundary conditions and input parameters

required to describe and repeat the simulations:


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Figure 21: Overview of geometry simplification of the different models and model inputs, a) Riblet roll

b) Smooth roll c) Work-piece


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Two different models (“A” and “B”) with different geometrical parameters and boundary

conditions were considered, in order to evaluate and predict multi-physics behaviors produced

during the production process:

A) The “A” model considers a rigid material model for tools and a plastic material model for

the work-piece. Deformation, friction and heat transfer’s effects are included in this model. The

objective of the “A” model will be predicting the load in rolls to evaluate constructive

parameters of tools, study the stress generated during the manufacturing to evaluate the tool

material properties and the work temperature.

B) The “B” model considers an elastic-plastic material model for the “V” shape roll and a

smooth roll and a plastic material model for the work-piece. Only the deformation effect is

included. Simulations under “B” model will be focused on prediction of the effective stress,

effective strain and final embossed geometry of the work-piece. The elastic-plastic model

evaluates the effect of elastic recovering or spring back in the tool during the interaction

between solids.

The simplification of the “B” model does not include rotational dynamic motion of the

manufacturing tools. The rolls in this model are static because the software used in this work

has only the capability to generate rotational motion in rigid bodies.

The rotational motion and linear motion between tool and work-piece, respectively, and the

relative motion between contact points produce frictions during the real manufacturing. The

friction coefficient between bodies is equal to 0 in “B” model, for the compensation of the

absence of rotational motion of the tool.

The initial values for geometrical parameters are summarized in Figure 21, where d is the

diameter of rolls, So is the riblet width of rolls and work-piece, ho is the riblet height of roll, Lo is

the distance between rolls and to is the initial thickness of work-piece. The initial length of the

work-piece is also described.

In both models, thickness reduction of the work-piece is from to = 2 mm, the initial thickness of

the work-piece, to Lo = 1 mm, the distance between rolls. This set of simulation proposed here,

makes the thickness reduction and the “V” riblet manufacturing into the same process.


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The kinematic boundary conditions of both models are also summarized in Figure 21, where Wz

is the angular speed and Vx is the linear speed of tools and work-piece, respectively. An

isothermal environment of 20oC is assumed during the process.

The manufacturing setup and materials used in this set of simulations have been selected by

using the literature overview already presented in the chapter 3.1. Basically, it has been

performed choosing the information presented in ref. [17] [18] [20] [21].

To perform the set of FEM simulations, a very fine mesh is required to describe correctly the

geometrical shape and produce a correct discretization of the model, in order to obtain accurate

results of the mechanical reactions, minimizing the numerical error of the calculus.

The criteria to create the mesh are different in the two models:

A) In the “A” model, only the work-piece material has been considered to be deformable. The

criterium to define the mesh is to consider a division among 3 of the smallest length of the

primitive geometry in work-piece. The value for maximum mesh length and remesh length

criterium is 0.100 mm. The number of elements in “A” model is about 25000 elements. The real

manufacturing time for this model is around 2.5 sec and the computation time required about 12

hours.

B) In “B” model all the solids are deformable. It considers a smaller discretization for more

accurate prediction. The general maximum mesh length is 0.100 mm with local mesh of 0.080

mm in contact spots between rolls and work-piece. The remesh criteria is of 0.050 mm in the

riblet roll and 0.080 mm for the smooth roll and work-piece. In “B” model the number of

elements is 70000, the real manufacturing time is 1 sec and the computation time is more than

30 hours.


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4.2 Materials and Material models

During the simulations, the work-piece and riblet manufacturing are assumed to be made in one

of these materials: Ti6Al4V super alloy, Al-1100 or AISI-1008 low carbon steel alloy. These

materials are commonly used in industry for production of many devices. The application areas

for these materials are described in chapter 3.1.4.

These materials are different from each other in composition and in mechanical properties. The

purpose of this selection is to compare the different behavior into the same process and get the

set up for a reliable manufacturing of different materials.

The tool material for manufacturing is assumed to be AISI-D2. This machinable cold work tool

steel is a high carbon multi alloy with hardness close to 60 HRC. It is used in many metal

forming processes like punching and cold rolling processes.

Tables 3 and 4 summarize the materials, material properties and material models assumed for

the simulations. The material flow stress data sets used in this work were taken from DEFORM-

3D V5.1 material data base.

Material Model Poison ratio Yield Strength Young modulus Ti6Al4V Plastic 0.31 860 MPa 115 GPa Al-1100 Plastic 0.3 75 MPa 68.9 GPa

AISI- 1008 Plastic 0.33 380 MPa 206.75 GPa

Table 3: Work-piece material properties and model used in simulations

Material Model Poison ratio Yield Strength Young modulus AISI-D2 Elastic-Plastic 0.3 1030 MPa 206.75 GPa

Table 4: Tool material properties and model used in simulations

Doing a pre-evaluation of the mechanical behavior under the rolling process of work-piece and

tool according with the high thickness reduction proposed in the model, DEFORM-3D software

has different methods of defining the flow stress curves of each material under these

manufacturing conditions.


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The material database and the material model used to perform the simulations takes into account

the flow stress of a material under uniaxial conditions. The flow stress curves of DEFORM-3D

material database are in dependency of the parameters described by this function:

= T,,

.

εεσσ , (4-1)

where σ is the effective stress, ε is the effective strain, .

ε is the effective strain rate and T is

the temperature.

Effective stress is evaluated by means of Von Mises stress criterium. Effective strain is

calculated through the extension of strain expression into three dimensions. Effective strain rate

is calculated measuring the rate of deformation or effective strain with respect to time, and the

temperature is calculated looking to the transformation of deformation energy into thermal

energy.

The material database of DEFORM-3D V5.1 and materials used to perform the simulations

have a wide array of flow stress curves as a function of the different values of strain rate and

temperature. The intermediate values are linearly interpolated between the different curves.


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4.3 Simulation results

Simulation models used to describe this manufacturing process and the results obtained

represent a powerful tool for prediction of multi-physics behaviors during the manufacturing.

These results are used to evaluate parameters for the implementation and predict the set up of

the future real manufacturing tests.

It should be noted, though, that the assumption of a macroscopic material model of the flow

stress curves to describe the behavior in micrometric range and the simplification and

idealization of the models are not entirely loyal with the real physical behavior during the

manufacturing process. Raw data of simulations can be found in the ANNEX of this memory,

whereas the most important results are summarized here.

4.3.1 “A” Model results

Results of the “A” model are the first approximation to a real manufacturing process. This set of

simulations was performed to describe tool properties and parameters of the manufacturing.

Figure 22: Ti-6Al-4V manufacturing, prediction of effective stress field, “A” model

Figure 22 shows the effective-stress in Ti-6Al-4V manufacturing. Values of effective-stress are

higher than those produced in the manufacturing of AISI-1008 and Al-1100. The maximum

effective stresses predicted for each work-piece manufacturing are 1260 MPa, 615 MPa and 125

MPa respectively.


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The highest stresses are located at the entry of the manufacturing tool, between P1 and P2 points

sketched in Figure 22. It is produced during the thickness reduction and it generates the highest

percentage of stress and strain reactions in tool and work-piece.

After these effective-stress results and looking material data base available in DEFORM-3D,

AISI-D2 was the selected material to evaluate the answer of the tool in, “B” model simulations

described later.

Figure 23: Plot of work temperatures at the hottest node during the manufacturing During the simulation, the “A” model assumes a cold manufacturing with an isothermal

environment of 20 oC. Figure 23 describes the temperature value located in the hottest node of

the work-piece, during the manufacturing process for three different work-piece materials.

The temperature of the work-piece saturates at 0.5 s, when energetic equilibrium of the media is

reached. The temperature values in the hottest node are 25 oC and 50 oC in the manufacturing of

Al-1100 and AISI-1008, respectively. Comparing the thermal behavior of the different

materials, Ti-6Al-4V manufacturing is one that saturates at the highest temperature, 124 oC.

In order to verify that FEM results were sound, the temperatures of work were also analytically

estimated considering the transformation of deformation energy into thermal energy, and taking

into account the empirically determined properties (density, heat capacity and flow stress curve)

of each material. Results were in reasonable agreement with FEM simulations.


PFC 2009-2010 46/92

The thickness reduction produces a high increase of stress along the flow stress curve of each

work-piece material. The stress accumulation is transformed in deformation energy and finally,

in dependency of specific heat and density of each work-piece material. Most of this energy is

transformed into thermal energy.

Figure 24: Above, Plot of temperature as a function of work-piece distance during the manufacturing of

the different materials, and bellow an example of AISI-1008 temperature longitudinal distribution

Figure 24 plots the temperature distribution along the work-piece material. The plot shows that

during the manufacturing the temperature of the work-piece decreases along the work-piece.

Heat transfer module in simulation input data includes effects of conduction in materials,

convection with the environment and radiation. These heat transfer effects are responsible of the

decrease of the maximum temperature produced through forming work done.


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The temperatures along the work-piece are 23 oC, 38 oC and 60 oC for Al-1100, AISI-1008 and

Ti-6Al-4V work-piece materials respectively.

Figure 25: Plot of Y load prediction during the manufacturing

Figure 25 plots the perpendicular load in the tool. After 0.8 seconds of manufacturing the curve

get saturated, this effect is due to the fact that from that time step the contact surface area

between tool and work-piece gets the maximum value. The value of load produced during the

manufacturing is in a range of 200 N and 2500 N.

The reaction force in a tool gives an idea of how the tool answers macroscopically, and how this

load could produce deflection in tool during the manufacturing process. The load used to

calculate tool macroscopic geometrical properties is after 0.8 sec of manufacturing. After the

transitory period the load becomes stationary, the stationary loads used are 200 N, 1160 N and

2500 N in Al-1100, AISI-1008 and Ti-6Al-4V manufacturing respectively.

This load is distributed along the longitudinal length “So” of the simulation geometry model.

During the evaluation of the macroscopic geometry and design for constructive parameters of

the tool, this perpendicular load is treated as a uniform load distribution along the tool with

values of 0.67 KN/m, 3.87 KN/m and 8.33 KN/m for Al-1100, AISI-1008 and Ti-6Al-4V

manufacturing respectively.


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4.3.2 “B” Model results

For an accurate prediction of the microscopic geometry in the embossed riblet profile of work-

piece and also mechanical analysis during the rolling process, it is important to include the

effect of deformation in the tool and work-piece. Predictions of final geometrical results and

mechanical behavior are more accurate after the equilibrium between tool deformation and

work-piece deformation.

The assumption of an elastic-plastic model for the tool and a plastic material model for the

work-piece are more precise with the real behavior. Thus “B” model gives more reliable values

and it evaluates better the mechanical effects during the riblet rolling process.

Figure 26 shows the obtained field of strain distribution in the cross section or thickness in

work-piece after rolling process.

Figure 26: Plot of strain distribution in the cross section of the different materials after the manufacturing,

and an example of Ti-6Al-4V work-piece manufacturing effective strain field

This effect of the deformation along the thickness is different in each material. The plot of

Figure 26 shows that the strain distribution in AISI-1008 and especially in Ti-6Al-4V is more

concentrated in the superficial layers.

On the other hand, the behavior of the Al-1100 work-piece under the deformation process is

more homogeneous along the cross section with a regular strain distribution.


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The plot of Figure 26 shows that the forming work done during the process generates the

highest plastic-strain values in superficial layer of the work-piece, mainly in the top of the riblet

structure.

It produces values of plastic-strain between 0.8 and 0.95 at the top of the riblet structure and

values of plastic-strain in a range of 0.7 and 0.8 in the bottom of the section. These values are

dependent on the properties of each material.

The high strain values obtained in the superficial layer are interesting from the point of view of

production of work-piece material with a superficial high hardness. It may be useful for the

production of more durable riblet structures.

Figure 27: Comparison of maximum tool stress values during the manufacturing

Figure 27 shows a cross section of the contact area between tool and work-piece. It shows the

field of maximum effective stresses in the tool during manufacturing of the different materials.

The highest value of effective stress plotted in the legend is 1030 MPa. This value is an

estimated Yield strength of the tool material AISI-D2.


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Stress predictions of the Figure 27 evidence that the compressive effective stresses produced in

manufacturing of AISI-1008 and especially in Ti-6Al-4V work-pieces could be close to the

yield strength of the tool material.

In case of Ti-6Al-4V and AISI-1008 manufacturing, the effective stress produced could

overcome the reversible range of deformation in the tool. The prediction of effective stress

values in Al-1100 manufacturing are on the elastic range.

The process of rolling and the section reduction mostly produce mechanical compressive

reactions in Y axis. Reactions produced through the compressive stresses of section reduction

are responsible of the high values of effective-stress in tool during the manufacturing.

Although the tangential stresses produced by the friction between tool and work-piece relative

motion is not included in “B” model, the equivalent stress values in the “B” model are close

with equivalent predicted in “A” model, which means that the obtained results are accurate and

coherent with the real physical behaviour.

Figure 28: Comparison of maximum tool strain values during the manufacturing


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Figure 28 shows the field of effective plastic strain in the tool riblet structure for the same time

step and cross section of the contact area between tool and work-piece sketched in Figure 27.

Effective plastic-strain predictions in the tool evidences that during manufacturing values are

out of the reversible range of the deformation, that is, there is local plastic strain.

In the case of manufacturing of AISI-1008 there are low values of plastic-strain. On the other

hand, during the manufacturing of Ti-6Al-4V there is high local plastic-strain along the contact

points between roll and work-piece and especially on the top of the tool riblet structure.

Results of simulations in Figure 28 show that the set up of manufacturing done produces values

of plastic-strain bigger than zero (Plasticε > 0), in manufacturing of Ti-6Al-4V and AISI-1008.

In contrast, Figure 28 shows that the manufacturing of Al-1100 does not produce high strain

values and the manufacturing of this material is within the elastic range.

Although the prediction of effective plastic-strain values could be low, the high value of

effective-stress still indicates the possibility of local plastic deformation. Results show that

stresses produced during the manufacturing are higher than the tool yield strength and there is

plastic-strain. This set up of manufacturing predicts local irreversible plastic-strain in the tool

negative riblet in case of manufacturing of AISI-1008 and mostly TI-6Al-4V.


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4.3.3 Geometry parameters

Concerning the geometry characteristics of the tool and work-piece, predictions of

perpendicular load of the “A” model and calculated load distribution were used to describe the

macroscopic dimensional design of tool and the displacement of the tool. On the other hand

results of the “B” model gave the microscopic geometrical characteristics of the riblet geometry

after the manufacturing. Both geometrical aspects are used to evaluate parameters in the

development of a reliable manufacturing process.

Figure 29: Schematic view of the rolling process, “d” bending or deflection, “L” tool length and “w” uniform

load distribution.

Figure 29 shows the schematic view of the tool during the manufacturing process. The

deformation of the work-piece exerts a uniform load distribution against the rolling tools, this

load was already calculated during the “A” model analysis. The deformation of the tools

modifies the micro-riblet structure, using material resistance theory, the deflection of the tool as

a function of the tool diameter and length between supports has been calculated (see Figure 30).

The criterion used to design the diameter of the tool is to consider the maximum deflection in

the center of the tool.

This maximum deflection limit comes from the fact that high values of deflection can change

significantly the microscopic geometrical characteristics along the surface, affecting the final

shape of the embossed riblet structure.

The required diameter of the tool depends on the distance between supports and results show

reliable values for diameters in the range of 50 mm to 250 mm for different material

manufacturing and configuration set ups.


PFC 2009-2010

Figure 30: Deflection in tool as a

Table 5 summarizes the

process.

Table 5: Microscopic geometry

©

2010

Deflection in tool as a function of tool diameter, for tool length of 500mm, 300mm and 1

microscopic geometrical characteristics achieved

(mm)

(mm)

(mm) Ti6Al4V 0.141 0.300 0.998 AISI-1008 0.157 0.297 0.988 Al_1100 0.154 0.300 1.004

eometry of riblet after manufacturing


53/92

meter, for tool length of 500mm, 300mm and 150mm

achieved after the rolling

(mm)

ratio

1.068 0.469 1.066 0.527 1.081 0.512


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The RibletRatio was described in the chapter 3.1.2 through the equation (3-1). This empirical

expression describes the relation between the height (h) and width (S) of the riblet geometry, the

values for this expression must be close to 0.5.

The microscopic riblet geometry in the manufacturing tool and the final embossed riblet

geometry accomplish successfully the RibletRatio for fluid dynamics requirements.

4.4 Discussion

The set up used to simulate the riblet rolling manufacturing process considers a thickness

reduction of a half of the initial thickness, and also the embossing of V-shape riblet detail into

the same manufacturing process.

Considering the used software capabilities and evaluating results and predictions obtained

during the simulation of both models, the results are accurate enough to make an approach and a

first extrapolation to design the real production process.

The range of temperatures showed in Figure 23 and Figure 24 allow to conclude that the work

temperatures during the manufacturing do not change substantially the flow stress curves of

each material and it do not affect strongly the mechanical properties of the tool during the

manufacturing through the variation of temperature.

The perpendicular load results of Figure 25 have been used to evaluate the macroscopic

dimension of the manufacturing tool. Figure 29 shows the diameter of the tool as a function of

deflection, as a function of different lengths between supports. It shows the most adequate tool

geometry configuration for a set of manufacturing of the different materials.

Table 5 results allows to conclude that the manufacturing set up with the described geometrical

and boundary conditions has good results concerning to the final fluid-dynamics requirements

of the manufactured work-piece.

Effective-stress results in Figure 22 and Figure 27 show that the section reduction until the half

of the initial thickness is too high for manufacturing of laminate riblet V-shape structures in

AISI-1008 and especially in case of manufacturing in Ti-6Al-4V work-piece material.


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This high thickness reduction produces effective-stress higher than the yield strength of AISI-

D2 tool material.

Especially in case of manufacturing of Ti-6Al-4V and also AISI-1008, Figure 28 shows that the

tool suffers small local irreversible deformations or plastic-strain principally in the top of the

negative riblet structure. This high stress values and plastic-strain would produce a fast wear in

the tool, reducing drastically the life of the tool.

In case of Al-1100 manufacturing Figure 27 and Figure 28 show that the stress produced during

the manufacturing is close to the half of the yield strength of manufacturing tool AISI-D2.

These predictions do not show inconvenience to manufacture a high section reduction and also

the “V” shape manufacturing in the same manufacturing process.

Local plastic deformation and irregularities in the tool riblet structure will produce distortions in

the final embossed microscopic geometry of the surface. This fact might imply degradation of

the efficiency for hydrodynamic or aerodynamic requirements of the final application.

The uniformity of the riblet structure all around the produced surface area must be ensured in

the final manufactured product. Variation of stress values produced during the real

manufacturing, or the cyclic behaviour of stresses could produce vibrations affecting

significantly the final embossed riblet surface.

Moreover, stress produced in contact points between tool and work-piece during the thickness

reduction and the rotational movement of the tool creates a cyclic stress in tool. This cyclic

variation of stress produces a dangerous cumulative damage effect of fatigue in tool.

The most probable crack effect in tool during the manufacturing could be the fracture through

fatigue. Decreasing the stresses produced during the forming work up to values under the yield

stress of tool material would increase significantly the fatigue cycles of the manufacturing tool.

Considering and looking the high stress and strain values in the tool, produced during the

section reduction in AISI-1008 and mostly in Ti-6Al-4V manufacturing, it will be necessary to

make a different set up of manufacturing to generate less stress and strain in tools during the

manufacturing.


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It is necessary to do the manufacturing making the “V” riblet manufacturing without high

section reduction for these materials. A low section reduction during the manufacturing still can

accomplish the geometrical shape of the riblet structure according with the fluid-dynamics

requirements, and also, it is sure that it generates lower stress values in tool and work-piece

during the process.

In contrast, the low section reduction does not give surface hardness to the work-piece produced

through the forming work done. The high values of plastic-strain in superficial layer produced

by the high section reduction give higher surface hardness properties. This set up loses the

benefits offered by this property.

Furthermore, it will be necessary control the effect of spring-back or elastic recovering effect. If

the section reduction or the forming work done is too low, it cannot minimize this effect and the

final shape of the riblet structure would be inefficient.

To summarize FEM simulations have shown the multi-physical reactions and the most

important parameters in the manufacturing of metallic surfaces with riblet “V” structures. The

analysis has shown parameters and boundary conditions for manufacturing of laminate “V”

riblet shape metallic surfaces.

After the discussion, the conclusion is that it will be necessary to make a smaller section

reduction in order to decrease the stresses during the manufacturing and increase the life and

fatigue cycles of the tools, especially the manufacturing life of negative riblet tools.

On the contrary, manufacturing of AL-1100 laminate with riblet structures using the set up

proposed during the FEM simulations has offered guaranties of successful performance. The

proposed riblet geometry is in accordance with aircraft skin aerodynamics requirements.

These conclusions are based on results of FEM analysis described in chapter 4.3. It would be

necessary to make a new set of simulations with the proper geometrical and boundary

conditions for every new manufacturing set up in order to evaluate and predict more accurately

all the mechanical reactions.


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5. Benefits of Sharkskin effect in aviation industr y

Over the last decades, and particularly since the 70s, due to the oil crisis, all the industrialized

countries have been trying to decrease, control and optimize the amount of fuel consumption in

combustion vehicles and devices.

The Kyoto Protocol signed and ratified by most of the industrialized countries in the world in

1997, established new parameters of emissions of greenhouse gases (GHG). The objective of

the Kyoto Protocol was to establish lower GHG concentrations in the atmosphere in order to

stop interferences with the climate and slow down the climate change.

Aviation and aircraft industry have an important impact on the amount of GHG emissions

produced by each country every year. International institutions, governments and also, civil

aircraft manufacturers are making big efforts to reduce these emissions in commercial aircraft

flights.

Since the beginning of the Aviation Industry, engineers and designers have been trying to

design aircraft that reduces the fuel burn.

The traditional technique to reduce fuel consumption is to minimize the weight of the aircraft.

Nowadays, most of the primary material of the aircraft fuselage is build up with light aluminum

alloys and composites are the secondary material. Using these lightweight materials the overall

weight of the airplanes has been reduced significantly.

Another great improvement was to include electronic control systems. Fully electronically

controlled aircraft is nowadays a reality, the automatic control of the most important parameter

during the flight permits to reduce, control and optimize the fuel consumption in every phase of

the flight.

Moreover, designing of more efficient engines is an ongoing project for many aircraft

manufactures. The petroleum based combustibles would combine biologically synthesized

combustibles. These engines will be more environmental friendly, increasing the efficiency and

reducing the fuel consumption levels in commercial flights.


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Efficient aerodynamics in aircraft design is another key factor to optimize the aircraft

performance during the flight. Into the involved aerodynamic variables, the drag effect with the

aircraft skin is one of the most important variables that has to be controlled in order to decrease

the fuel consumption.

Sharkskin effect has direct benefits to DR effect in aircrafts. The physical effect of micro-riblets

structures in the direction of airflow has impact on the reduction of turbulent flow areas in

contact with the aircraft skin, and this effect produces DR.

This work has shown the State-of-the-Arts, as well as the most important ongoing research

projects to get economical manufacturing methods in order to implement this technology into

the common aircraft manufacturing processes. In addition, the chapter 4 of this work has used

FEM simulations tools to describe manufacturing parameters to produce laminate AL-1100 with

micro-riblets structures.

To analyze the real benefits of sharkskin surfaces for drag reduction in airplanes, empirical test

into wind tunnel conditions and direct flight test have been performed, and have shown that

drag effect reduction of 10 % could be achieve in ideal cases [5] [13] [20].

In the same way, estimations and experimental flight tests have shown that the implementation

of micro-riblets structures in aircraft aluminum skin permits to decrease the fuel consumption to

1.5% [18] [21].

Approximately 2-5% of the total amount of human produced GHG is induced by the aviation

industry. This percentage is relatively small compared to that owed to energy generation by

power plants. In spite of this, the application of sharkskin effect would have direct influence in

the reduction of GHH emissions due to airplanes. [63]

The main topic of this work was not to evaluate in depth the induced economical and technical

benefits of sharkskin effect. In this section of the work, I present a very rough estimate of

economical and environmental benefits that may follow from the implementation of Sharkskin

aluminum surfaces in airplane industry. Although many assumptions had to be made to simplify

the calculations, results give an idea of the achievable reduction of GHG emissions and

potential economical benefits.


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Most of the actual commercial airplanes are using Kerosene or Jet Fuel as a combustible. To

make an estimation of the impact of the sharkskin effect, I have assumed that the CO2 emission

rate in commercial airplanes is 2,44 kg CO2 /liter of fuel burn and the average fuel consumption

rate in commercial aircraft is 3025 liter/hour. [64]

Amount Units Average fuel consumption 3025 Liter / hour

Reduction of fuel consumption 1,5% 2980 Liter / hour

Average CO2 emissions 7381 Kg CO2 / Hour

Reduced CO2 emissions 1,5% 7271 Kg CO2 / Hour

Table 6: Commercial airplane fuel consumption and CO2 emissions parameters, estimation of benefits of sharkskin effect.

In order to summarize the results of Sharkskin effect table 6 show makes an overview of the

obtained benefits induced by this effect in commercial aviation. I have assumed that the CO2

emission is linear with the fuel consumption.

To estimate the economical impact of sharkskin, we could analyze the fuel burn reduction in

aircraft skin with sharkskin effect. To make and approach of the economical benefits, a

commercial flight which goes from Barcelona to Helsinki takes approximately 5 hours. The

average amount of fuel burn would be 15125 liters and the CO2 emission is 36Tn of CO2.

If we assume that the average cost of the commercial airplane fuel for the regular companies is

0,36 € / liter, the cost of this flight in fuel consumption would be approximately 5400 €. If the

reduction of fuel burn is 1,5 % the amount of money saved during this flight would be 81

€/flight. Nowadays there are companies that offer this flight 3 times per week during all the

year. Only considering this shuttle service, the save of money in one year for this company

would be approximately 25300 €/year.

Just considering this flight sketch and extrapolating this reduction in cost to all the global

aviation activity, it is easy to figure out the great economical benefit produced by the sharkskin

effect in airplane skin. Furthermore, the decrease of GHG emissions induced by the reduction of

fuel consumption also would be significant and this impact must have into consideration.


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6. Conclusions and future prospects

The main purpose of this work was to evaluate the possibility of making Bio-inspired metallic

surfaces by mechanical processes, which can be implemented in mass-production.

The State of the Arts presented in chapter 3 allows to conclude that surface characteristics in the

micro-metric range can be manufactured by mechanical processes, while manufacturing of

nano-metric structures would require alternative techniques.

- Sharkskin characteristics and riblet production in the micrometric range could be

manufactured exclusively by means of mechanic techniques. Riblet structures could be

implemented in the future for many technological applications, and the results of this work and

references show that it will be possible to develop an efficient technique to produce metallic

surfaces with DR properties. Sharkskin effect has direct benefits in aviation industry, the

research and implementation of this technology is an ongoing project.

- On the other hand, the HSC metallic surfaces based exclusively on mechanics

manufacturing techniques are not possible. The mimicking of nano-metric geometry

characteristics could be possible but the need of chemical treatment after the embossing to get

accurate HSC properties decreases the benefits offered by this technique. To make this

technique competitive the process could be optimized by developing a sustainable process

combining mechanical and chemical processes for a final good performance.

- Although the nano-metric ranges of AR geometry properties and the actual production

techniques require alternative techniques for manufacturing, mechanic embossing processes

could be viable as mass production system. It is also important to notice that the application area

of AR coatings is not focused on metallic materials and research efforts are located in other

directions.

- The manufacturing of Gecko feet dry adhesive systems by mechanics processes (like

deforming or machining) is not possible nowadays. The required geometric characteristics and

the necessity of elastic fibers to mimic this effect make mechanical production impossible.

Manufacturing materials for mimicking Gecko feet characteristics are polymers and all the

research efforts for manufacturing are focused on alternative methods.


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A possible solution for economic and mass production of Bio-inspired large surface areas is the

development of manufacturing techniques based on continuous rolling processes. This

technique gives options to produce a wide array of sizes and geometries in a reduced time in a

continuous manufacturing chain.

In contrast, for applications where small surface areas are needed, techniques like embossing or

stamping could be competitive.

Bio-inspired functional surfaces offer great opportunities for many industrial areas. Technical

and environmental benefits of Bio-inspired properties in actual devices are clear. However, the

development and implementation of these technologies into common productive methods is still

very difficult.

The difficulties for the implementation of these technologies into actual productive processes

and produce Bio-inspired functional surfaces are the high price of manufacturing technologies

and measurement equipments. Moreover, verification of the dimensional specifications of the

surfaces is still a challenge because the measurement instruments must allow resolutions in

nanometric range.

At present, researches are making progress in order to solve and overcome all the mentioned

difficulties. The literature overview shows that the investigation is ongoing.

After the literature overview, the conclusion is that Sharkskin riblet structures are the only Bio-

inspired kind of surfaces that can be implemented by role mechanical manufacturing at present.

Looking the references available and after the FEM analysis of Sharkskin riblet rolling

manufacturing in chapter 4, the conclusion is that this technology has possibilities to be

implemented in the future.

The multi-physics FEM analysis has allowed us to analyze the mechanical effects and most

important parameters in riblet rolling manufacturing. We have proposed a set up for production

of laminate metallic surfaces with micrometrical “V” shape riblet structures.

The implementation of Sharkskin surfaces in aircraft to achieve a drag reduction effect may

imply a significant decrease of fuel consumption, followed by a saving of CO2 emissions.


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For the future implementation of riblet manufacturing into productive chains, and in order to

open the application area of the riblet structures and get a cost-effective production system, it

would be necessary to allocate more efforts into:

- Basic research focused into the description of optimum riblet shape and geometry

characteristics for a set of possible applications with fluid dynamics requirements.

- Engineering research focused on the manufacturing of hard-steel riblet rolling tools,

with the optimum negative riblet shape and geometry for every possible application.

- The manufacturing simulation showed in this paper was focused in the production of

laminate shape work-pieces. It will be necessary make engineering research describing

production methods of work-pieces with more complex 3D geometries.

- Last, it would be necessary to make experimental manufacturing of real industrial

components with riblet structures and direct tests describing fluid dynamic behavior of it.


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7. Project temporization

As noticed in the chapter 2, this project has been carried at VTT Research Center of Finland.

The internship duration was 9 months and the working period started in 1 of September 2008

and finished in 29 of May 2009.

During the internship period, I was taking part at the same time in many other activities. I have

been working and using many simulations tool like: AnSYS, Abaqus, Python, hyper-mesh, pro-

engineer and so on, to help my colleagues in many others research activities.

Table 6, summarizes the project timing and temporization. The start of the work required the

collecting of the information and bibliography related to the topic of the work. The State-of-the-

Arts was written just before the evaluation of the bibliography and right away, simulation tools

were introduced to complement the work.

After all, I wrote a final report, which was evaluated and corrected by my cluster colleagues in

VTT. My supervisor at the university in Barcelona has strongly participate in all the steps which

were done during the internship, and until the project was presented.

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Week Bibliography State-of-the-arts Presentations Simulations

Redaction PFC VTT

PFC Revision

35.2008 36.2008 37.2008 38.2008 39.2008 40.2008 41.2008 42.2008 43.2008 44.2008 45.2008 46.2008 47.2008 48.2008 49.2008 50.2008 51.2008 52.2008 1.2009 2.2009 3.2009 4.2009 5.2009 6.2009 7.2009 8.2009 9.2009 10.2009 11.2009 12.2009 13.2009 14.2009 15.2009 16.2009 17.2009 18.2009 19.2009 20.2009 21.2009 20.2010 21.2010 22.2010

Table 7: Project timing and temporization


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of the 2nd ICNFT, 185-194

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Drag Reduction Effects on Curved Objets.

Department of Mechanical Engineering, Pohang University of Science and Technology, Korea

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Production Engineering Research and Development 1:233-237

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microprofiles. Production Engineering Research and Development

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Eng 13(1): 31- 34 WGP


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[24] Hesselbach J, Hoffmeister H-W, Hlavac M (2005) Micro grinding- efficient technique for microstructuring of hardened steels.

Prod Eng 12(1):1-4

[25] Kerstin Koch, Hans-Jurgen Ensikat (2008) The hydrophobic coatings of plant surfaces: Epicuticular wax crystals and their

morphologies, crystallinity and molecular self-assembly.

Institute for Biodiversity of Plants Bonn Germany, Micron 39:759-772

[26] Jun Li, Zhuqing Zhang, Jianwen Xu, CP Wong (2005) Smart Self- Cleaning, Lotus Effect Surfaces.

[27]Chien te Hsieh, Fang Lin Wu, Shu Ying Yang (2008) Superhydrophobicity from composite nano/microstructures: Carbon

fabrics coated with silica nanoparticles.

Surface & Coatings Technology 202: 6103-6108

[28]Ailan Qu, Xiufang Wen, Pihui Pi, Jiang Cheng, Zhuoru Yang (2007) Preparation of hybrid film with hydrophobic surfaces

based on irregularly structure by emulsion polymerization.

Applied Surface Science 253: 9430-9434

[29] Minglin Ma, Randal M. Hill (2006) Superhydrophobic surfaces.

Current opinion in colloid & interface science11:193-202

[30]Weici Wu, Miao Chen, Shan Liang, Xiaolong Wang, Jianmin Chen, Feng Zhou (2008) Superhydrophobic surface Cu-Zn alloy

by step O2 concentration dependent etching.

Journal of Colloid Interface Science 326: 478-482

[31] H. Dodiuk, P.F. Rios, A. Dotan and S. Kenig (2007) Hydrophobic and self-cleaning coatings.

Polymers for advanced technologie 18:746-750

[32] M. Thieme, H. Worch (2006) Ultrahydrophobic aluminium surfaces: properties and EIS measurements of different oxidic and

thin-film coated states. J. Solid State Electrochem 10:737-745

[33]Rosa Taurino, Elena Fabbri, Massimo Mesori, Francesco Pilati, Doris Pospiech, Alla Synytska (2008) Facile preparation of

superhydrophobic coatings by sol-gel processes.

Journal of Colloid and interface Science 325:149-159

[34]Anne Pahkala, Juha Mannila, Marke Kallio, Juha Nikkola, Amar Mahiout,Jarmo Siivinen, Anne-Christine Ritschkoff, Riitta

Mahlberg, Olli Posti,Mia Löija, Soili Takala (2006) Anti-fouling and scratch resistant hybrid sol-gel coatings

Functional surfaces and coatings applications VTT SYMPOSIUM 244:177-187

[35] Juha Nikkola, Marke Kallio, Juha Mannila, Anne Pahkala, Mika Kolari, Riitta Mahlberg, Olli Posti , Amar Mahiout (2006)

The effects of chemical parameters and topography on the properties of the hybrid sol-gel coatings

Functional surfaces and coatings applications VTT SYMPOSIUM 244:154-165

[36] M. Thieme, Ralf Frenzel, Sylvia Schmidt, Frank Simon, Anja Hennig, Hartmut Worch, Klaus Lunkwitz, Dieter Scharnweber

(2001) Generation of Ultrahydrophobic Properties of Aluminium, A first step to Self- cleaning Transparently coated metal surfaces.

Advanced engineering materials 9:691-695

[37] D. Triantafyllidis, L. Li, F.H. Stott (2005) The effects of laser induced modification of surfaces roughness of Al2O3 based

ceramics on fluid contact angle. Materials Science and Engineering 390:271-277

[38] F. Burmeister, C. Kohn, R. Kuebler, G. Kleer, B. Bläsi, A. Gombert (2005) Applications for TiAlN and TiO2 coatings with

nanoscale surface topographies. Surface & Coatings Technology 200:1555-1559

[39] http://lotus-shower.isunet.edu/the_lotus_effect.htm

[40] Michael Nosonovsky, Bharat Bhushan (2009) Superhydrophobic surfaces and emerging applications. College of Engineering &

Applied Science University of Wisconsin, Milwaukee

[41] Christian Dorrer and Jürgen Rühe (2007) Condensation and Wetting Transitions on Microstructured Ultrahydrophobic

Surfaces. Langmuir23:3820-3824

[42] Edwin Nun, Markus Oles, Bernhard Schleich (2002) Lotus- Effect® Surfaces. Macromol.Symp 187:677-682

[43] http://sciencedaily.com/releases/2007/11/071126115318.htm

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atomic force microscopy. School of science, Griffith University Brisbane Australia, Applied Surface Science 235:139-144

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[47] Alexander Disch, Jörg Mick, Benedikt Bläsi, Claas Müller (2007) Nanostructures on microstructured surfaces

[48] H. Kasugai, K. Nagamatsu, Y. Miyake, A.Honshio, T, Kawashima, K. Lida, M. Iwaya, S. Kamiyama, H. Amano, I. Akasaki,

H. Kinoshita, H. Shiomi (2006) Ligth extraction process in moth-eye structure Phys. stat .sol 6:2165.2168

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9. Annexes


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A0.Units system

Unit s Stress effective MPa Strain effective Dimension less Load N Deformation energy MJ/m3 Temperature oC Length mm Time Second

A1.Results from Deform 3D V5.1

“A” model results ........................................................................................................................ 74

Work piece AISI-1008 ............................................................................................................ 74

Stress effective during the rolling process: ........................................................................ 74

Work temperature as a function of longitudinal distance during the rolling process: ........ 74

Temperature as a function of time during the rolling process in the hottest node: ............ 75

Load as a function of time during the rolling process: ....................................................... 75

Work piece Ti-6Al-4V ............................................................................................................ 76



Maximum work temperature as a function of time during the rolling process: ................. 77


Work piece Al-1100 ............................................................................................................... 78



Maximum work temperature as a function of time during the rolling process: ................. 79


Comparison of work temperature as a function of time: ........................................................ 80

Comparison of temperature as a function of longitudinal distance: ....................................... 80

Comparison of perpendicular load in tool as a function of time: ........................................... 81


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“B” model results ........................................................................................................................ 82

Work piece AISI-1008 ............................................................................................................ 82

Geometry after rolling process: .......................................................................................... 82

Strain as a function of the riblet thickness: ........................................................................ 82

Strain and deformation energy as a function of the riblet thickness: ................................. 83

Work piece Ti-6Al-4V ............................................................................................................ 84




Work piece Al-1100 ............................................................................................................... 86




Comparison of Strain (Thickness) and Deformation Energy (Thickness) functions: ............. 88

Comparison of maximum stress & strain values of the tool during the rolling process: ........ 89

A2. Microscopic and macroscopic geometry parameters

Microscopic geometry, geometry of “V” riblet after the rolling process: .............................. 90

Macroscopic geometry, diameter of tool as a function of deflection: .................................... 91


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A1. Results from Deform 3D V5.1 “A” model results

Work piece AISI-1008

Stress effective during the rolling process:

Work temperature as a function of longitudinal distance during the rolling process:


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Temperature as a function of time during the rolling process in the hottest node:

Load as a function of time during the rolling process:


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Work piece Ti-6Al-4V




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Maximum work temperature as a function of time during the rolling process:



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Work piece Al-1100




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Maximum work temperature as a function of time during the rolling process:



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Comparison of work temperature as a function of time:

Comparison of temperature as a function of longitudinal distance:

©

2010

Comparison of work temperature as a function of time:

Comparison of temperature as a function of longitudinal distance:


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Comparison of perpendicular load in tool as a function of time:

©

2010

Comparison of perpendicular load in tool as a function of time:


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“B” model results

Work piece AISI-1008

Geometry after rolling process:

Strain as a function of the riblet thickness:


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Strain and deformation energy as a function of the riblet thickness:


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Work piece Ti-6Al-4V




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Work piece Al-1100




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Comparison of Strain (Thickness) and Deformation Energy (Thickness) functions:


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Comparison of maximum stress & strain values of the tool during the rolling process:


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A2. Microscopic and macroscopic geometry parameters

Microscopic geometry, geometry of “V” riblet after the rolling process:

Figure 1: Draft of characteristics lengths

Where: hF is the final height of riblet, SF is the final width of riblet, LF is the final length under

riblet, ft is the final nominal thickness and h/s is the ratio of riblet efficiency.

h/S ratio Ti6Al4V 0,141 0,300 0,998 1,068 0,469 Al_1100 0,154 0,300 1,004 1,081 0,512 AISI-1008 0,157 0,297 0,988 1,066 0,527

Table 2: Riblet geometrical parameters after rolling


PFC 2009-2010

Macroscopic geometry, diameter of tool as a function of deflection:

©

2010

Macroscopic geometry, diameter of tool as a function of deflection:


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