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Characterization of Regenerated Cellulose for Bio-based Epoxy Fibrous Composites

May 2013

Student Investigator: Issam Qamhia Advisor: Professor Rani El-Hajjar

Department of Civil Engineering & Mechanics; Materials Science & Engineering

University of Wisconsin-Milwaukee

UNIVERSITY OF WISCONSIN SYSTEM SOLID WASTE RESEARCH PROGRAM Student Project Report

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Introduction:

The objective of this research is to characterize regenerated and nano-cellulose fibers and their

composites with epoxy for mechanical properties, and to evaluate manufacturing techniques for these

composites. Cellulose is the most abundant natural material and is the main component of many solid

waste products of households, businesses and construction projects. Experimental and computational

fracture mechanics and strength approaches are used to suggest an optimum architecture of the

reinforcement to produce the desired mechanical properties. The resultant optimum design of the

reinforcement architecture is expected to give rise to a new method of producing high-strength, bio-based

and sustainable composites for different regenerated cellulose fibers and cellulose nano-fibers.

The results of this research are presented and published in the following conference and journal

publications:

1. Qamhia,  I.,  El-­‐Hajjar,  R.  F.,  “Processing  of  Nanocellulose  Scaffolds  for  Increased  Fiber  ContentThermosetting  Composites.”  1st  International  Conference  on  Natural  Fibers,  Guimarães,Portugal,  2013  June  9-­‐11.

2. Qamhia,  I.,  Shams,  S.  S.,  El-­‐Hajjar,  R.  F.,  “Analytical  Prediction  of  Elastic  Properties  In  TriaxiallyBraided  Regenerated  Cellulose  Composites.”  1st  International  Conference  on  Natural  Fibers,Guimarães,  Portugal,  2013  June  9-­‐11.

3. Qamhia,  I.,  El-­‐Hajjar,  R.  F.,  “Preparation  and  Thermomechanical  Characterization  ofNanocellulose  Scaffolding  /  Thermoset  Composites,”  Advancements  in  Fiber-­‐PolymerComposites:  Wood  Fiber,  Natural  Fibers  and  Nanocellulose  Conference;  2013  May  6-­‐7;Milwaukee,  WI,  USA

4. El-­‐Hajjar,  R.  F.,  Qamhia,  I.,  Shams,  S.  S.,  “Analytical  Characterization  Of  The  MechanicalProperties  In  Triaxially  Braided  Regenerated  Cellulose  Composites,”  Advancements  in  Fiber-­‐Polymer  Composites:  Wood  Fiber,  Natural  Fibers  and  Nanocellulose  Conference;  2013  May  6-­‐7;Milwaukee,  WI,  USA

5. Qamhia,  I.  I.,  El-­‐Hajjar,  R.  F.,  “Mechanical  and  Thermal  Properties  of  NanocelluloseScaffolding/Epoxy  Composites  Prepared  by  a  Vacuum  Assisted  Heated-­‐Press  Approach,”.  Postersession  presented  at:  Sustainable  Cities  and  Infrastructure.  10th  Annual  Sustainability  Summitand  Exposition;  2013  March  6-­‐7;  Milwaukee,  WI,  USA.

6. El-­‐Hajjar,  R.  F.,  and  Qamhia,  I.  I.,  “Modeling  and  Characterization  of  the  Moisture  DependentBilinear  Behavior  of  Regenerated  Cellulose  Composites,”  Journal  of  Wood  Science,  Accepted  forPublication.  2013

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Part I: Lyocell fibers and composites:

Objectives:

The goal of this research is to characterize regenerated cellulose fibers as a potential natural

reinforcement for composite materials. Understanding the characteristics of cellulose fibers and

composites means that potential applications can be deducted and the fibers can be produced from wood

waste products to help reduce waste in Wisconsin. The objectives of the research are:

− Perform initial characterization of the properties of Lyocell fibers strands.

− Evaluate the effect of moisture on the mechanical properties of Lyocell fibers and Lyocell fibers

composites.

− Examine the adhesion of thermosetting resins, especially epoxies, to regenerated cellulose fibers.

Epoxy is chosen since some of its types are bio-based, increasing the appeal for ‘green’ cellulose-

epoxy composites.

− Produce and evaluate the mechanical properties composites from regenerated cellulose fibers.

− Run a finite element analysis (FEA) model to correlate experimental data to it and use it to

predict the behavior of Lyocell/epoxy composites at conditions different from those set in

experiments.

Experimental procedure:

The tension behavior of the dry and wet Lyocell fibers (Tencel; Lenzing Fibers Inc., Axis, Alabama,

USA) and composites made from epoxy and different fiber volume fractions of Lyocell were studied.

Lyocell fibers used were in an uncrimped and unbleached state with staple fibers form. Experiments were

carried out at room temperature and a humidity level between 30-35%.

The first experiment was to characterize the mechanical properties of wet and dry Lyocell composites.

The ASTM ID: D3822 standard was used for determining the fiber properties. Tow strands of the same

weight and gauge lengths of 25.4 mm (1.0 in) to 254 mm (10 in) were attached to cardboards using a

small amount of epoxy (Figure 1.a). Samples were tested under dry conditions, wet conditions by soaking

in a water bath for a 120 ± 5 minutes period and wet conditions by extending the soaking period to 240 ±

5 minutes. Samples were tested under direct tension by applying load from an electro-mechanical loading

machine and using a load cell of maximum capacity of 2220 N (500 lb) as shown in figure 1.b.

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For the second experiment, composite specimens were prepared using a wet layup and resin infusion

method. The Lyocell Fibers were weighted to back-calculate the fiber volume fraction. The epoxy

chosen for this study (Super Sap 100 Epoxy; Entropy Resins Inc., Gardena, California, USA) contains

bio-renewable materials sourced as co-products or from waste streams of other industrial processes, such

as wood pulp and bio-fuels production. Samples with low fiber volume fractions (5-10%) were prepared

in a dumbbell shaped form according to the ASTM ID: D638 standard. For higher fiber volume fractions,

preparing panels using a resin-infusion and manual layup followed by degasing processes was introduced.

Panels produced using wet layup resulted in a maximum fiber volume fraction of 0.33 whereas the ones

with resin infusion resulted with a volume fraction of 0.35. Testing of composite coupons was performed

under direct tension according to the ASTM ID: D3039 standard. Strain properties were measured across

a 25.4 mm (1.0 inch) gage length in the middle of coupons by the use of an extensometer.

(a) (b) (c)

Figure I.1. (a) Lyocell tows attached to cardboards (ASTM D3822) (b) Testing of Lyocell Tows (c)

Testing of Lyocell/epoxy composite

Unit cell finite element model:

The constitutive response of the regenerated fibers was found to be dependent on the moisture condition

of the fibers. This suggests the need for a suitable analysis framework for these composites. Regenerated

Lyocell fibers possess a bilinear material behavior with elastic-plastic tendencies. Under moisture

exposure, the elastic response gradually dissipates and results in a largely plastic unrecoverable behavior.

For the purpose of this research, a multi-scale analysis approach for simulation of Lyocell/bio-based

epoxy composites that uses a representative unit cell is used. The behavior and mechanical properties of

the constituents is independently recognized. This modeling approach is implemented in a p-version finite

element analysis (p-FEA) approach. The primary advantage of this approach is the ability to check the

convergence of the solutions with increasing element order reducing the dependency on the mesh size.

StressCheck (Stress Check V9.0; ESRD, St. Louis, Missouri, USA) software was used to create and run

(b)

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the model. The typical unit cell model used for the hexagonal packing contained 178 elements consisting

of 72 hexahedral and 106 pentahedral elements. A hexagonal packing geometry was used to represent the

packing of the Lyocell fibers in the matrix. A bilinear elastic-plastic stress-strain relationship was used to

correlate the model to experimental data. The model was calibrated for one set of fiber volume fraction

(FVF) and was thereafter ran with different FVF for validation purposes of the modeling capability.

Loading was applied in the form of a constant displacement on one face in the fibers direction. In and out-

of-plane displacements on the opposing face were restricted. The results of p-FEA modeling were

compared to those obtained by experiments in the fiber direction by calculating the average stress

resulting on the loaded surfaces.

Figure I.2. Unit cell FEA model for Lyocell/epoxy composites (micro-mechanical structure)

Significant results

• Dry and wet Lyocell fibers

The results show a bilinear elastic-plastic response in the mechanical behavior of the dry fibers. The dent

in the loading curves between the two regions can be attributed to a more preferable molecular orientation

of the fibers which is produced by stretching the fibers and stabilized by interconnections between

crystalline regions and by hydrogen bonding (Morton, 2008). The load versus crosshead displacement

results for the tow testing is shown in figure I.3. The behavior of the wet specimens shows a change in the

constitutive response. For surface dried fibers, an increase in weight by 1.8 times the original weight was

tracked for the two hours soaking and a 1.9 times the original weight for the four hours soaking. The loss

of modulus as the fibers are wetted is clearly shown by the loading curves. Some variability in the

mechanical behaviour is shown for 2 hours soaking but the results are more consistent for 4 hours soaking

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and show a drastic loss in modulus and a non-linear behavior of the fibers. A reduction in the failure

stress accompanied by a higher strains to failure is also observed for the wet fibers compared to the dried

ones. Additionally, the knee seen in the loading curves for the dry fibers is shifted toward the (0, 0) point

of the curves as the level of moisture increases in the fibers due to the removal of the hydrogen bonds

(Morton, 2008). The behavior of the wet fibers is plastic.

Figure I.3. Load-displacement curve for dry and wet Lyocell tows showing the effect of moisture on the

behavior of the fibers.

• Lyocell/epoxy composites

Panel preparation by wet layup followed by degasing was found to produce better samples than other

examined procedures. Pilling and fibrillation are inherent in the structure of Lyocell and adds difficulties

to the manufacturing process. Using resin infusion, thinner samples with higher FVF were produced.

However, good infusion of the resin with the fibers could not be achieved for the whole panel and some

areas were not properly wetted. Equal distribution of fibers among the panel and waviness prevention

were two major challenges of the wet layup method. By using tape to stretch and fix the fibers in position,

the effect of these challenges was reduced. Degasing the samples with a vacuum pump after wet layup

also insured minimal porosity.

• Finite element model

The p-FEA model proposed was calibrated for one set of experimental values using the specimens having

a fiber content of 33%. Error analysis is performed on the results by examining the global energy norm

versus the polynomial order and degrees of freedom. The predictive properties of the stress strain graphs

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are shown in figure I.4 superimposed with the FE calibration curve. The p-FEA model was successfully

used to capture the experimental stress-strain behavior of the composites and the FE model was calibrated

to average the differences seen in the plastic region for the samples. The results show that it is possible to

reproduce accurate results for different fiber volume fractions. Changes in the elastic and plastic modulus

of Lyocell-epoxy composites with different fiber contents is shown figure I.5. A linear change is seen

which can be attributed to the use of a bilinear stress-strain curve for Lyocell and a linear one for epoxy.

This linear mechanical behavior is expected to change with the wet composites where the non-linearity of

the Lyocell loading curve is expected to lead to nonlinearity in the stress-strain curve of the composites.

Some of the coupons with the high fiber volume fraction were tested for the effect of moisture on Lyocell

composites by soaking for two hours investigate how they compare to the model of dry Lyocell

composites. The result of these are shown in figure I.5.

Figure I.4. Experimental and FEA model results for stress-strain curves of Lyocell/epoxy composites for

different fiber volume fractions (19% and 33%).

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Figure I.5. Experimental and FEA model results for the change in elastic and plastic moduli with fiber

volume fraction.

Part II: Cellulose nanofibers and composites:

Objectives:

The goal of this research is to lay the groundwork for using Cellulose NanoFibers (CNFs) Aerogels as

reinforcements in epoxy-based composites. This research is aimed to compliment ongoing efforts at

Forest Products Laboratory (FPL) to produce reinforcements from CNFs.

Specifically, the objectives are to:

− Perform initial investigations into the swelling and liquid flow into and through CNF scaffolds.

− Examine the adhesion of thermoset resins, especially epoxies, to CNFs.

− Investigate different fabrication techniques for CNF/epoxy composites to produce composites

with the highest possible CNF content and minimal porosity.

− Investigate the effect of the presence of cellulose on resin curing behavior.

Experimental procedure:

The material used for this study was a CNF scaffold provided by Forest Products Laboratory; Madison.

The experimental part of this research was to characterize the nanocellulose scaffolds, investigate the

wetting of the nanocellulose by different resin systems and to investigate different fabrication procedures

for the composites. Two resin systems were used to prepare composites, namely: Super Sap 100 Epoxy

(Super Sap 100 Epoxy ; Entropy Resins Inc., Gardena, California, USA) and Embed-It epoxy (Embed-

It™ Low Viscosity Epoxy; Polysciences, Inc, Warrington, Pennsylvania, USA). Composites were

prepared from the two resin systems by manual layup followed by degasing. The Samples with Super-Sap

epoxy showed very little wetting due to the high viscosity of the resin. Samples made with Embed-it

epoxy showed better wetting. Thus, Super Sap epoxy was eliminated and Embed-it epoxy was used due

to its low viscosity (65 cps). The fabrication techniques investigated were as follows:

1. Wet Layup followed by degasing

Samples in Embed-It epoxy were degased by a vacuum pump for 2 hours at 610 mm Hg (-24 in. Hg) over

a hot plate, and were cured for 16+ hours at 175 ºF. These samples showed good impregnation. Embed-It

resin-only samples were prepared as a control point. Samples prepared by this technique showed proper

wetting but resin content was very high, leading to low fiber content.

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Figure II.1 Sample preparation of CNF composites by wet

layup (left) and cured CNF/epoxy composites (right)

2. Resin infusion

The high viscosity resin only was used to fabricate composites by resin infusion. Samples produced were

highly pressed; yet a large void content was observed. Nanocellulose was not well impregnated by resin,

especially at midsections due to the high viscosity of the resin.

3. Wet Layup followed by degasing and hot pressing

This fabrication method was adopted to increase Nanocellulose content in the prepared composites.

Samples were prepared using Embed-It epoxy by manual layup followed by degasing for 30 minutes.

Samples were then inserted in a hot-press for 7 hours; then removed and left to complete curing overnight

at 175 ºF. Hot press was used to apply a pressure (100 lb load) as well as heat (160 ºF) to the samples and

led to 4-5 times higher FVF than the previous procedure. Fiber volume fractions up to 5-7% by volume

were obtained using this technique.

The mechanical testing part of this study included carrying out a three points bending test on the prepared

composites using the recommendations of ASTM D790-07standard. Sample dimensions fell within

acceptable limits recommended by the standard. Stresses and strains were calculated according to

standard formulas for three point bending tests:

!! =  3!"2!!!

       (1)

ɛ! =6!"!!

         (2)

Where σf and ɛf are the flexural stress and strain respectively, P is the applied load, L, b and d are the span

length, width and depth of the specimen respectively, and D is the maximum deflection at the center of

the beam. The bending test setup is shown below.

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Figure II.2 Sample preparation of CNF composites by hot pressing

(left) , an image of the hot press (center) and the three points bending

test setup (right).

Other Experiments carried out to characterize the thermal behavior of the composites upon heating were

Differential scanning calorimetry (DSC) and Thermogravimetric analysis (TGA). DSC is a thermo-

analytical technique in which the difference in the amount of heat required to increase the temperature of

a sample and reference is measured as a function of temperature. Round samples of around 25mg in

weight were cored out from CNF/epoxy composites and pure Embed-IT epoxy resin samples for testing.

The main aim of the test was to see if cellulose is changing the glass transition temperature of the resin.

However, more important information about decomposition of the composites, and curing were obtained.

TGA is a type of testing performed on samples that determines changes in weight in relation to a

temperature program in a controlled atmosphere. Such analysis relies on a high degree of precision in

three measurements: weight, temperature, and temperature change. The main use for this technique was to

find the decomposition temperature of CNF/epoxy composites. Experiments were carried out in Argonne

gas to prevent oxidation.

Significant Results:

Transparency and density analysis reveal that the Super-Sap epoxy resin did not infuse well with the

aerogels for both resin infusion and wet layup techniques. Resin infusion produced better results but the

percentage of voids was still high. By the use of low viscosity epoxy resin (Embed-It), the curing time for

manual layup was relatively high (around 16-24 hours), but good results of infusion were obtained. The

use of the hot press enabled obtaining higher cellulose content in the composites (up to 7.5% by volume)

and ensured minimal porosity. Samples prepared by this technique showed higher elastic modulus in

addition to higher strength to failure when compared to the pure resin and to lower enforcement levels of

cellulose prepared by manual layup.

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The Embed-It resin was found to be sensitive to curing temperature and curing cycles. When the hot

pressing preparation method was used, the mechanical properties of the pure resin samples and the nano-

cellulose composites were improved in terms of modulus and failure stress. Results for samples prepared

by this technique shows that the nano-cellulose is not significantly improving the mechanical properties

(figure II.3). A 15% average increase in modulus is observed for reinforcement levels of 5-7.5%.

However, nano-cellulose samples are more brittle and fail at a lower stress. DSC results reveal the

importance of proper resin curing to the properties of the composites. No exothermic peaks are observed

for samples prepared by hot pressing indicating that the resin is well cured. Glass transition temperature

(Tg) for the pure resin was found to be around 265 ºC. When nanocellulose is added, data on Tg is

inconclusive. TGA results indicate that the decomposition temperature of cellulose/epoxy composites is

270 ºC; where a noticeable mass drop is observed (figure II.4).

Figure II.3 Flexural stress-strain behavior of CNF/epoxy and pure epoxy composites

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Figure II.4 Sample preparation of CNF composites by wet layup (left) and cured CNF/epoxy composites

(right)

Part III: BioMid fibers and composites:

Objectives:

The goal of this research is to characterize BioMid regenerated cellulose fibers composites for potential

use as naturally reinforced composites in industries. Understanding the mechanical behavior of these

composites means that potential applications can be deducted and the fibers can be produced from wood

waste products to help reduce waste in Wisconsin. The objectives of the research are:

– Examine different test methods (Mechanical, Acoustic Emission) for testing regenerated cellulose

fiber composites.

– Evaluate notched and un-notched test specimens for determining the mechanical behavior of the

Regenerated Cellulose Fibers/Bio-Epoxy specimens.

– Investigate the applicability of a proposed Modified Classical Lamination Plate Theory Model to

capture the mechanical properties relating them to the undulations in the bias yarns and fiber

content.

Experimental procedure:

A triaxially braided Regenerated cellulose BioMid fiber system with [-60, 0, 60] fiber orientation was

used for reinforcement in test specimens. A&P Technology, OH performed the braiding of the materials

used in this study. Fiber architecture is produced by having the bias yarns alternating two over and two

under the axial yarns. The epoxy used in this study is a Super Sap 100/1000 (Entropy Bio-Resins,

Gardena, California, USA). The resin also has a total calculated biomass of 50%. The actual fiber volume

fraction of the composite is determined by using the volume measurements of the matrix and fiber

constituents, resulting in an average fiber volume fraction of 0.60 with a sample standard deviation of

0.02. The spacing between the bias yarns was at 26 mm, and between the axial yarns the center-to-center

spacing was 5.2 mm. The axial bundle would fill a space of approximately 2.5 mm in width between the

bias yarns.

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Figure III.1 Braided BioMid sample showing yarns orientation

For the early trials, a wet layup technique was used to prepare the composites, but the fibers weren’t fully

infused, especially the axial yarns. The early trials of resin infusion produced medium levels of porosity, a

modification of the technique was implemented; in which silicon connectors were inserted inside the

vacuum bag and the pipes were attached to them and insulated with tape from outside the bag. With this

modification, the levels of porosity reduced drastically. The panels were infused under room temperature

conditions. BioMid fibers were stored at room temperature and a humidity level of 30%. Resin infusion

was carried out at room temperature and a humidity level of 30%. Panels were cut into 305 mm long by

38 mm wide coupons in transverse and longitudinal directions. Some panels were also cut into 152 mm

by 76mm wide samples. For these samples, a 19mm notch was cut on both sides of the sample towards

the middle. It is difficult to obtain reliable strength properties due to failure mode inconsistencies.

Notched specimens are more suited to strength measurements because all bias fiber tows are gripped.

Notched samples are also easy to fabricate and are expected to generate higher measured loads.

The elastic properties were obtained by testing the specimens in tension according to ASTM ID: D3039.

Standard for tensile properties of polymer reinforced fiber composites. The experiments were performed

in a displacement-controlled mode at a displacement rate of 1.3 mm/min.

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(a)

(b)

Figure III.2 (a) preparation of BioMid/epoxy composites by resin infusion (b) tensile testing of notched

and un-notched samples.

For one longitudinal coupon and one transverse coupon, simultaneous mechanical testing, and acoustic

emission (AE) tests were carried out. Acoustic emission was used to investigate damage initiation and

damage propagation through the energy bursts levels of events and the timing of these events and relating

these to changes in the loading curve obtained by mechanical testing. A Physical Acoustics WS-alpha

transducer was used for measuring the AE signals. The sensor has an operating frequency of 100-1000

kHz with a 63.74dB peak sensitivity occurring at 523.23 kHz. In order to improve the quality of the

captured signals; a pre-amplifier coupled with a bandpass filter adjusted to pass frequencies in the range

of 10-1200 kHz was used. A Mistras 2001 data acquisition system was used for recording the data,

including waveforms.

Analytical model:

An analytical model was used to correlate experimental results for mechanical properties and provide a

solution to determine the sensitivity of the braided materials to braiding properties such as crimp angle

and bias yarns angles as well as the composites properties such as fiber content. A discrete 3-layer

analytical model was evaluated for this purpose. The analytical model uses a representative volume

element (RVE) or a representative unit cell to predict the macroscopic properties of triaxial composites

from constituent microscopic material properties. The model presents yarns architecture as a total

equivalent stiffness matrix of the triaxially braided composite in a unit cell. El-Hajjar et al. (2013)

examined various analytical methods and found consistent results with experiments on glass fibers using

this approach. The global stiffness of the composites is given as (Shokrieh and Mazloomi, 2010):

0 0[ ] [ ] [ ] [ ]RUCglobal global globalC t C t C t Cθ θ θ θ+ + − −= + + (3)

where t θ+ , t θ− and 0t are the thickness of each layer to the thickness of the laminate. In this research we

use the actual thickness values obtained from microstructural examinations of the cross sections of the

prepared composites as inputs for the model.

Significant Results:

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The stress versus strain curves for longitudinal and transverse specimens loaded in tension shows that are

shown in figures III.3 and III.4. Two distinct regions can be identified for the stress-strain curve in which

the first is linear elastic and the second is the region with a lower stiffness. The loading properties are

similar to other regenerated cellulose composites, eg. Lyocell/epoxy composites (El-Hajjar and Qamhia

2013). The loading behavior and modeling of Lyocell composites was discussed in part I. The results of

cumulative acoustic emissions (figure III.5) show that the initiation of damage for transverse specimens

starts at a much lower stress level compared to the early emissions observed in the longitudinal

specimens. The initial emissions are most likely attributed to the matrix cracking. Higher energy

emissions before failure are observed for the longitudinal specimens where the final cumulative energy at

failure is 35% higher. The reason for this is thought to be associated with the fracture of the axial fibers

oriented in the direction of loading. The incidence of high acoustic emissions can be used as an indication

of the point of detrimental irreversible damage. For the fracture study, the analysis of the data is still in

progress and will be assessed in future work.

Figure III.3 Stress versus strain plots of BioMid/epoxy quasi-isotropic composites (longitudinal)

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Figure III.4 Stress versus strain plots of BioMid/epoxy quasi-isotropic composites (transverse)

Figure III.5 Acoustic emission cumulative energy plots of Biomid/epoxy quasi-isotropic composites

The results of the discrete three-layers model show a good corellation to the experimental data (figures

III.3 and III.4). The variation of longitudinal and transverse elastic modulus with respect to the fiber

volume fraction is shown in Figure III.6(a). The elastic moduli in both in-plane directions are very

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sensitive to the change in reinforcement content. The effects of changing the crimp angle (or the aspect

ratio) of the bias yarns in the braided fiber architecture on the elastic modulus of the composites are also

investigated. Figure III.6(b) indicates a high sensitivity of the transverse modulus to the change in crimp

angle. On the other hand, there is no significant variation in the predicted longitudinal stiffness as the

behavior in the axial direction is mainly dominated by the longitudinal fibers that have no undulations.

Figure III.6 Prediction of the elastic modulus versus fiber volume content Aspect ratio

Conclusion

The objective of this research was to characterize regenerated cellulose fibers and their composites with

epoxy for mechanical properties, and to evaluate manufacturing techniques for these composites.

Cellulose is the most abundant natural material and is the main component of many solid waste products

of households, businesses and construction projects. Experimental and computational fracture mechanics

and strength approaches will be used to suggest an optimum architecture of the reinforcement to produce

the desired mechanical properties.

The study performed an initial investigation into the swelling and liquid flow into and through

regenerated cellulose fibers, and examined the adhesion of thermoset resins, especially epoxies. Three

different cellulose based materials (Lyocell, CNF and BioMid) where characterized for their mechanical

properties. The results reveal a good potential for the use of regenerated cellulose fibers as reinforcement

in composite materials to replace other man-made fibers. The main issue is to recognize the moisture

dependent behavior of regenerated cellulose fibers. The potential of the use of CNF scaffolds needs to be

further assessed to understand its behavior. Part of my future work will include making and testing

composites from CNFs with uniformly oriented nanofibers which are expected to give higher mechanical

properties than the randomly oriented fibers scaffolds used in this study.

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References:

– El-Hajjar, R.F. Shams, S.S. and Kehrl, D.J. Closed form solutions for predicting the elastic

behavior of quasi-isotropic triaxially braided composites. Composite Structures, 2013. In press.

– El-Hajjar, R.F. and Qamhia, I. I., “Modeling and characterization of the moisture-dependent

bilinear behavior of regenerated cellulose composites,” Journal of Wood Science, 2013. In press.

– Morton, W.H., J, 2008. Physical properties of textile fibres, 4 ed. Woodhead Publishing.

Mottershead, B., Eichhorn, S.J., 2007. Deformation micromechanics of model regenerated

cellulose

– Shokrieh M.M., Mazloomi M.S. An analytical method for calculating stiffness of two-

dimensional tri-axial braided composites. Composite Structures. 2010;92:2901-5.