material selection and fma

8/3/2019 Material Selection and FMA

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University of Cambridge

Materials Science & Metallurgy

Natural Sciences Tripos

Materials Science & Metallurgy

PART IIA and IIB

SELECTION OF MATERIALS

C2

Dr. E.R. Wallach

Easter Term 2009


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Materials Science Pt II Selection of Materials [C2]

- i -


The aims of the course, building on a basic knowledge of elementary mechanics and microstructures,are to:

summarise the basic steps in the design process;

show how materials, with a combination of appropriate properties, may be chosen for a givenapplication;

reacquaint students with the range and different combinations of properties that are availableusing the Cambridge Engineering Selector (CES) software, introduced in the Michaelmas Term;

indicate the synergy between shape and material properties to the design process and theresulting behaviour of a component;

consider what to do if things go wrong: failure analysis and what can be learnt.

The lectures are supplemented by practical studies, in examples classes, covering the examination ofclassical microstructures, specific household objects and actual objects that have failed in service.

The previous examples class in the Michaelmas Term provided an introduction to the CES software

(available on the computers in room 201) and the software can be used to underpin the conceptsintroduced in the course. The software, with its data available on a wide range of properties for manytypes of material and on fabrication methods, is useful for other courses as well as for Part III.

L ecture 1.

Classes of materials and types of properties.Types of design problems: original, developmental and variant.Steps in the design process: sequential and iterative progress.

L ecture 2.

Causes of failures in service.Specifications and standards: need and types (dimensional, quality, code of practice).Costs and cost effectiveness in design. Analysis of costs.

L ecture 3.

Materials data: required accuracy, sources.Combining materials properties for specific design problems (example of aircraft skin selection).Optimisation/ranking and expert systems. Use of weighting factors.Materials property charts without shape and their use in materials selection.

L ecture 4.

The effect of shape on materials selection. Shape factors (macro and microscopic).Performance indices which include shape, and materials property charts including shape.

L ecture 5.

Failure analysis: approaches to adopt when things go wrong.Reasons for failure.Analysis of failure for metals: types of failure and fracture surface examination.Introduction to examples of actual failures (to form the basis of independent study and an examplesclass to discuss the artefacts).

L ecture 6.

Analysis of failure for ceramics and polymers: types of failure and fracture surface examination.


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BIBLIOGRAPHY

D

esign

Ashby M.F., "Materials Selection in Mechanical Design", 3rd Edition, Elsevier, 2005. As43aElectronic book via Newton search on University Library website[www.lib.cam.ac.uk/electronicresources/ebooks.php enter Ashby in search field]

Ashby M.F. & Jones D.R.H., "Engineering Materials 2", 3rd Edition, AB97Butterworth-Heinemann, 2005.[www.lib.cam.ac.uk/electronicresources/ebooks.php enter Ashby in search field]

Charles J.A., Crane F.A.A. & Furness J.A.G., "Selection and Use of Engineering Materials", As403nd Edition, Butterworth-Heinemann, 1997.

Dieter G.E. "Engineering Design", McGraw-Hill, 1986. K114

Ashby M.F., Shercliff H., Cebon D., Materials: Engineering, Science, Processing and Design AB208Elsevier Science & Technology, 2007.[www.lib.cam.ac.uk/electronicresources/ebooks.php enter Ashby in search field]

Failure analysis

ASM International, "Fractography", Metals Handbook, 12, 9th Edition, 1987. R112

ASM International, "Failure analysis and prevention", R111Metals Handbook, 11, 9th Edition, 1986.

ASM International, "Handbook of case histories in failure analysis", Kw32Metals Handbook, 1, 1992.

Jones D.R., "Engineering 3: materials failure analysis", Pergamon, 1993. Kw31

Note: ASM Metals Handbooks are currently available electronically to members of the University via:

http://products.asminternational.org/hbk/index.jsp


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1. Background

1.1. Categories of materials

To date, dealt with a variety of engineering and electronic materials. These can be classified ina number of ways, e.g.

ALLOYSMETALS

MICROSTRUCTURE

ENGINEERING

CERAMICSBIO-COMPATIBLE

STRUCTURAL

COMPOSITESFUNCTIONAL

SILICA

GLASSESMETALLIC

PE, PS, PC

POLYMERSNANOTUBES

BUTYL RUBBER

ELASTOMERSSILICONES

Si BASED

ELECTRONICTHIN FILM

ALLOYSMETALS

MICROSTRUCTURE

ENGINEERING

CERAMICSBIO-COMPATIBLE

STRUCTURAL

COMPOSITESFUNCTIONAL

SILICA

GLASSESMETALLIC

PE, PS, PC

POLYMERSNANOTUBES

BUTYL RUBBER

ELASTOMERSSILICONES

Si BASED

ELECTRONICTHIN FILM

The evolution of individual, as well as classes of, material over time has been highly dependent on thetechnologies available during any period.

Now other factors are increasingly entering materials selection and development, namely energy andenvironmental considerations.

Relative importance of four major classes of engineering material as a function of time.


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erty

Tw

nking: ental and generally more subjective

figure or order of merit such as hardenability

ng on what is regarded as different )are needed for rational choice.

Fa

onality required

ent

ical and physical

outes and dimensional tolerances

sales

yalties, contracts

- social aspects, e.g. environmental concerns:

pollution e.g. CO2 emissions

le

- means of optimise and/or rank data

f steps

ich case, emphasis will need to be focussed

may be limited by a simple combination of material

can be

ht, maximise E0.5

/

duced in Pts IA andPt IB, and summarised in Section 2.2, page 4.

1.2. Types of prop

o major classes:

- fundamental: can be measured directly

- ra combination of several fundam

e.g. formability, machinability,

1.3. Selection criteria

The large number of available materials (over 50,000 dependimeans that selection criteria

ctors to consider include:

- requirement and functi

- design of compon

- lifetime planned

- properties of materials: mechanical, chem

- availability of materials: shape and purity

- fabrication r

- aesthetics

- lifetime anticipated and possible failure modes

- cost: development, materials, fabrication,

- legal issues: patents, ro

- health & safety issues

energy involved and

Need also for

- accurate data which is readily availab

2. Design

2.1. Design stagesFor a given design problem, there are typically a number oto consider, as shown on the left and on the following page.

A given component will be especially constrained by a few of the

bove factors, in whaon particular steps

For instance, if a particular failure mechanism is likely to dominate,he designtproperties

If the combination of properties can be identified, theysimultaneously optimised, see section 2.2. For example,

f failure is by buckling, then for minimum weigi

where Eis Youngs modulus andis density.

This is the approach of materials selection intro


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Design stages in going from concept to production and marketed product


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2.2. Optimisation of a combination of material properties, performance indices

To determine the least mass of a rod for different loading conditions.

F Let rod length and load F be fixed by design requirement.l

Mass m of rod of material with densityis given by

m = r2 l [A]l

Now consider possible failure mechanisms and hence appropriateequations which include material properties of interest in order toderive a performance index M, a ratio of material properties whichthen

F can be optimised to identify the most suitable materials.

2.2.1. Failure by plastic yielding

S y =

r

F

2

[B]

- radius r is the only free variable,

- all other terms are defined for a given application (S is a chosen safety factor e.g. 0.3)

- eliminate r between [A] and [B] to derive performance index for failure by plastic yielding

m =

y

S

lF

2.2.2. Failure by buckling

Fcrit =n

2

2

l

IE=

4

n 4

2

2 rE

l

[C]

where n depends on the end constraints

- eliminate r between [A] and [C] to derive performance index for failure by buckling

m =

2

n

2

E

F l

2.2.3. Failure by fast fracture

KIc

= a [D]

- no apparent free variable in above equation [D] where KIc

is fracture toughness and is ageometric factor depending on the crack location.

- in some situations, can assume crack length a is proportional to a dimension of body, e.g. r

- substitute for from equation [B] and eliminate r to derive performance index for fastfracture

m =( )

c3/4

IcK

where c is a constant = l

S

1/3

4/3

F

Note: equations [B], [C] and [D] represent constraints for different failure mechanisms.


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2.3. Design approaches

Three different design approaches can be defined.

2.3.1. Original design

New principle or material involved

2.3.2. Developmental or adaptive design

Refine or improve an existing principle or component

Date Cleaner Dominant materials Exterior Fasteners Power Suck Weight Costparts W l/s kg

1900 Hand-powered Wood, canvas, leather 50 1 10 240

1950 Cylindrical Mild steel 11 28 300 10 6 96

1965 Spherical Mild steel 7 4 450 5.5 80

1985 Cylindrical ABS & PP 4 1 800 18 4 60

1997 Centrifugal PP & PC 1200 6.3 190

ABS = acrylonitrile butadiene styrene PP = polypropylene PC = polycarbononate

Costs recalculated and expressed in 1998 equivalent values.

[Adapted from Ashby M.F., "Materials Selection in Mechanical Design", 3rd Edition, Elsevier, 2005.]


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2.3.3. Variant design

Change of scale from model or pilot plant.

Design will proceed iteratively until model or pilot plant built and tested. Then need to scale up.

Consider variant design for three different failure modes.

a) plastic collapse for which dominant material property likely to be y (or, possibly, UTS)

neither affected as greater area when scaling up enables greater load to be carried

b) fast fracture

KIc

= y a

- critical crack length a can be related to a dimension of the component, e.g. r

- hence y will have to be reduced to avoid brittle failure

- let the scaling factor be when changing scale, and so assume aa which can arise as

crack detection can be more difficult in thicker sections (depends on detectiontechnique)

larger volume statistically more likely to contain a larger defect

- hence y will decrease by 1/

Cases (a) and (b) can be optimised by plotting stress against size versus scaled thickness t

applied

stress

scaled

thickness t

Ideal thickness or size

- most efficient use of material since optimises both properties

- safest design as find some plasticity can occur rather than catastrophic fast fracture

- flaws may be detected by leakage (e.g. pressure vessels) or

deformation (e.g. beam sagging)

c) corrosion

- want to make 1/6 scale model of traction engine with boiler of 10 mm thick mild steel plate

- require original model

to contain pressurised steel 6 mm 1 mm

for corrosion over lifetime 4 mm 4 mm

total thickness required 10 mm 5 mm

- in practice, to both minimise weight and improve aesthetics, may change boiler material

- use copper in model (too expensive for original) and, possibly, too low strength


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3. Lifetime of components

3.1. Factors to consider

Lifetime of object affected by:

design

properties of the materials used

dominant failure mechanism

defects

Planned lifetime involves knowledge of time-dependent properties particularly, for example

fatigue

creep

degradation

data obtained from accelerated tests

Unexpected failure can arise from:

design errors - under design leads to premature failure

over design overloads other component (plus unnecessary expense)

material or fabrication defects - standards / quality control

deterioration in service - misuse, change in expected use or loading.

3.2. Standards and specifications1

Invariably imposed on - materials: composition, heat treatment (temper) & extent of working

fabrication methods and tolerances on dimensions

inspection methods

Various types: dimensional or quantitative

quality: expectation from manufacturing process

specification of level of performance

code of practice: installation and/or measurement procedures

4. Cost

Components can be classified between extremes of

performance emphasis - space, military, medical

cost emphasis - domestic appliances, cars

Use terms such as

cost - price paid

value - extent to which performance criteria are satisfied for the cost

based on life expectancy

social expectations, e.g. increasing emphasis to conserve world resources

cost effectiveness - extent to which savings can be made by downgrading a property

- achieve by design changes

material selection

Examples of price per unit weight for different materials or complex products are shown in the

figure on the next page.

1Primo Levi, Periodic Table, Penguin Press,1975, Chapter on Chromium, page 152.


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Price per unit weight versus complexity of materials and products


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Breakdown of costs for fabricated components:

Total cost to consumer

Purchase price Cost of ownership

Costs of production Fixed costs Manufacturers (a) maintenanceprofit

(b) repairs(a) basic materials (a) factory overheads

- abundance - rent and rates (c) insurance- supply/demand - heating & light- quantity needed (d) amortisation- purity (b) administration- contract duration- exchange rates (c) sales & marketing

(b) manufacture costs (d) research & development- labour- equipment needed- equipment lifetime- quantity to make- energy demands

5. Materials data and sources

5.1. Accuracy of data

structure insensitive density

modulus

thermal expansion

specific heat

y

engineering polymers (E/50)

thermal conductivity

electrical conductivity

hardness of ceramics

y

and UTS

for metals

structure sensitive KIc for all materials

10% error

50% error

composition

depends on processingheat-treatment

5.2. Data sources

Books e.g. ASM Metals Handbook

Manufacturers data sheets

On-line manufacturers data and collated data

Databases: need to know and select relevant properties plus how to combine/weight propertiesfor given projected use

do not necessarily show data reliability, e.g. structure insensitive versus those that

are highly dependent on, say, composition and/or processing history


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6. Materials selection approaches

6.1. Expert systems: data optimisation/ranking

Expert systems are based on computerised selection with ranking/ weighting factors.

Utilise stored databases which are interrogated by programs which poses questions to a user toobtain answers and hence information to define the problem and appropriate weighting factors.

Generally will combine several sets of properties, suitably weighted, in order to provide anoverall optimised index to advise the user.

Can be fast and efficient for simple problems.

Dangers:

- limited by skill of programmer to have foreseen all possible situations;

- may be limited by data in database although on-line systems can minimise this;

- reliant on weighting factors, the basis of which may not be seen by the user;

- may always attempt to provide an answer even when inappropriate to do so.

6.1.1. Example of choosing a casting mould material

a. Go / no-go approach, based on simple acceptability

a = acceptable, U = underprovision, O = overprovision, E = excessive__________________________________________________________________________________

Material Heat Rigidity Resistance to Mouldability Cost Decisionresistance stress cracking

__________________________________________________________________________________M1 a a a a E RejectM2 a a a a aM3 O a U U a RejectM4 U U a a a RejectM5 a O U a aM6 a a O a a

__________________________________________________________________________________

b. Degree of merit, based on numerical rating of 1 (worst) to 5 (best)__________________________________________________________________________________

Material Heat Rigidity Resistance to Mouldability Overall ratingresistance stress cracking (maximum =20)

__________________________________________________________________________________M1 4 3 3 3 13 = 0.65M2 2 3 4 3 12 = 0.6M3 5 4 1 1 11 = 0.55M4 1 1 4 3 9 = 0.45M5 4 5 1 3 13 = 0.65M6 3 2 5 5 15 = 0.75

__________________________________________________________________________________

3. Weighting factors__________________________________________________________________________________

Material Heat Rigidity Resistance to Mouldability Overall ratingresistance stress cracking (maximum =75)

x 5 x 5 x 2 x 3__________________________________________________________________________________

M1 20 15 6 9 50 = 0.67M2 10 15 8 9 42 = 0.56M3 25 20 2 3 50 = 0.67M4 5 5 8 9 27 = 0.36M5 20 25 2 9 55 = 0.73

M6 15 10 10 15 50 = 0.67__________________________________________________________________________________


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Limitations in above example are:

- real data are not shown so extent to which different properties are markedly different is not seen

- weighting factors are somewhat arbitrary

- very subjective approach.

6.1.2. Example of selecting a metallic alloy for civilian aircraft wing material

a. Data for possible materials.

b. Potential problem is units are dissimilar so normalise with respect to highest value for eachproperty and then average to obtain overall rating.

c. Can also include weighting factors although somewhat subjective choice of values.

+

+

+

+

+ ++

Note that temperature limit introduced. Even if highest speed is Mach 2, overall skin temperature willbe < 200C. Hence stainless steel and titanium both are clearly over design but for different reasons

(density is problem for steel while cost is for titanium).Aluminium alloy 2 is to be avoided due to poor fracture toughness.


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6.2. Materials selection charts

6.2.1. Approach

Use simple combinations of material properties to optimise design for a given problem

Approach as in section 2.2, page 4:

- set up equations combining material properties and geometry of problem;

- identify free variable not specified by constraints of problem and eliminate from equations;

- resulting equation includes a performance index, a ratio of material properties for optimisation.

6.2.2. Summary of minimisation of weight for different forms of loading____________________________________________________________________

minimise weight for a given- - - - - - - - - - - - - - - - - - -------- - - - - - - - -

mode of loading stiffness ductile brittlestrength strength

____________________________________________________________________

tie (slender column)

E

y

Ic

K

bending of rod/tube

2/1E

2/3

y

3/2

IcK

buckling of rod/column

2/1E- -

bending of plate

3/1E

1/2

y

2/1

IcK

____________________________________________________________________

For minimum cost, replace withCR

where CR

is relative cost per unit weight of material =steelmildkgpercost

materialkgpercost

____________________________________________________________________

Above shows that same key materials properties are relevant to similar modes of loadingalthough material property exponents alter

Example: consider stiffness where

n1/E= k where k is a constant

hence log (E) = n log () + k'

Graphs of log(E) versus log() will give straight lines of gradients 1, 2 or 3corresponding to the modes of loading above.


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6.2.3. Use of materials selection charts

Charts:

- allow visual comparisons of properties and combinations of properties;

- materials on the same line of the appropriate gradient for a given loading have the same normalisedproperty, i.e. are equivalent in terms of the ratio of the two properties;

- allows fast evaluation of suitable materials, including relative merits from position on graph;- on-line version allows interactive manipulation of diagrams to see details;

- on-line version enables more than one combination of properties per graph;

- on-line version shows progressive elimination of materials as additional constraints used.

Problems remain that:

- need independent knowledge to assess dominant properties for a given design and application,similarly to estimate weighting factors;

- structure sensitive data still not catered for (though can use lowest values)

- still have to assume likely failure mechanism from simple tests or assessments, yet it is not alwaysreliable to extrapolate from short to long term as different mechanisms may be involved.

Schematic Youngs modulus versus density chartsshowing (a) primary constraints and (b) lines of different gradient


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6.3. Multiple constraints

For aircraft wing material (section 6.1.2., page 10), weight must be minimised but simultaneously needgood stiffness, strength, toughness.

C

an be difficult to handle if more constraints than free variables - sometimes called overconstrained.

Approaches:

a) Sequential performance indices (iterative approach as used in CES)- exercise judgement by identifying most two important constraints (e.g. mass and yield strength);

- using these two constraints (and ignoring others), eliminate the free variable;

- hence derive a performance index and identify subset of possible materials

- use remaining constraints repeatedly to eliminate free variable(s)

- derive further indices and hence refine existing subset of materials (may have to enlarge originalsubset of materials in light of other indices)

b) Use of coupling equations

As an example, consider a rod length lwhich while having a low weight must be both

strong (to support a load F) and

stiff (not extending its original length lmore than u),.

As previously, m =A l andstrength = F / A then eliminating free variableAl

m = F ( / ) [X]l

Similarly for the elastic stiffness constraint, using E = / = (F/A) / (u/l)

m = (F / u)2

(/ E) [Y]l

Since these two expressions for mass are for same rod, then [X] & [Y] can be equated to give:

=

uE l or

uE l

// =

Best material is that which maximises (E /) and also ( / ) coupled in the way shown by equation.

Plot lines of the specification (l/u) on appropriate selection charts as coupling lines

Best material will be: on the coupling line and as high as possible (to optimise both E/and /)

In the search box, i.e. the area defining optimal values for both above ratios.

Index 2,

E/

Index 1, /

coupling line:gradient = ul /


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7. Effect of shape on materials selection

7.1. Shape factors

Occasionally, section shape is not a factor in designing a component anddesign optimised solely by material choice.

More generally, design combines section shape with material choice and so

shape factors are required.

Need to consider macroscopic shape factors: overall bulk shape of a section

microscopic shape factors: structural anisotropy within a bulk section

7.2. Macroscopic shape factors

Macroscopic factor: is dimensionless quantity e.g. elastic bending of beammodefailureLOADINGOFTYPE

eB

twisting of beam to failure fT

- measures the structural efficiency of a section shape relative to a solid round bar of the samecross-sectional area, under equivalent loading;

- equals 1 for solid bar of circular cross-section and increases to 10 for I-beam section, hencerecognises the mass distribution around a central axis for different geometries;

- depends on shape solely and, as it is a ratio, is independent of size or scale.

7.2.1. Determination of macroscopic shape factor

For elastic bending, the stiffness or bending resistance is determined both by the material propertiesand also by the second moment of area, I, about the axis of bending, where Ihas dimensions of length

4.

Shape factor =e

B areasectional-crosssameofbeamsolidcircularofstiffness

beamshapedofstiffness

oB

B

oI

I

S

S==

hence = 1 for a solid circular cross-section bareB

Consider a rectangular bar and a circular bar

12

3b hI = oI

A

rI ===

44

24

Hence =e

B 4

2A

I

Note that there will be different expressions for different loading conditions or failure modes.


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7.3. Microscopic shape factor eB

Various structural materials have in-built shape factors, many of which mimic structures found innature, e.g.

honeycombe wood

fibre composite palm wood

concentric cylindrical plant stems

layered structures cuttlefish shell

Effects often shown by anisotroic properties.

Can treat microscopic shape in similar fashion to macroscopic shape using eB

e

B

e

Be

B

e

B

microscopic macroscopic overall structure (multiply shape factors)

7 .3 Summary: use of shape

May optimise a design by choosing sections with higher values of second moment of area, which canthen help to reduce weight.

However, need to be aware that might introduce a different failure mode, e.g. thin wall tubes may failby buckling when under compressive load rather than by plastic yielding.

8. Selection based on fabrication methods and environmental factors

Materials can be selected according to their properties, as has been shown in many of the examplesused above.

However, the design stages (see section 2.1, page 2) need to consider many other criteria, as aresummarised in section 1.3 (page 2) including fabrication methods (shaping, surface treatment and

joining) and environmental factors such as embodied energy and CO2 emissions.

The CES software allows selection using such criteria in combination with material properties orseparately.

8.1. Fabrication methods

A summary of bulk fabrication methods is given on the next page, followed by two CES charts showingpossible fabrication methods, the second as a function of the mass of the possible components.

8.2. Environmental factors

Representative CES charts, showing the embodied energy and CO2 emissions associated with themaking annually of different materials, are provided on page 20.


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MATERIAL PREPARATION

Extraction

Alloying

Refining

Reclamation

SHAPING

CASTING

------> Casting: polycrystaline

single crystal

directional solidification

FORMING

Bulk: rolling

extrusion

drawing

forging

Consolidation: powder routes

liquidvapour solid

C.V.D. or P.V.D.

FINISHING

MACHINING

Milling Turning

Drilling Grinding

Spark erosion

Ultrasonic drilling

Laser machining

COATING

Electroplating

Electroless

Plasma spray

Ion coating

Laser coating

JOINING

Mechanical

Adhesive

Brazing & soldering

Fusion welding

Solid-state welding

N.D.T.

PRODUCT

Summary of bulk fabrication methods


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Embodied energy per mass for different materials

[Ashby M.J., Materials and the environment, Elsevier, Chapter 6, p 117, 2009]

Annual carbon dioxide emissions to atmosphere from material production

[Ashby M.J., Materials and the environment, Elsevier, Chapter 6, p 119, 2009]


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Possible material fabrication routes for different classes of material

[Ashby M.F et al, Materials: Engineering, Science, Processing and Design, Elsevier, p 414, 2007]

Possible material fabrication routes for different classes of material

[Ashby M.F et al, Materials: Engineering, Science, Processing and Design, Elsevier, p 414, 2007]


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9. Failure analysis

9.1. General approach

Aimfind the primary cause of failure

initiate corrective action to prevent repetition.

Method

field assessment: obtain samples & controls

record background data (plus service history)

preliminary examination of failed part

reconstruction of events

initial assessment: non-destructive evaluation

macroscopic examination (fracture surfaces, cracks)

microscopic examination (including microhardness)

collection of information on (history of) suspect components

detailed assessment: mechanical testing

chemical analysis (bulk, local, surface, corrosion/wear products)

test under simulated service conditions

diagnosis: ensure data are self consistent

report: analysis of all data

suggestions for the future

action: implementation of report (no action, modifications, withdrawal)

Reasons for failure

design deficiency

material's problem

overload (abuse)

failure to observe specification.

History of component

design criteria: specifications - codes of practice

safety factor

materials selection: specifications

substitution

manufacturing practice: codes of practice

records

service history: loads

displacements

temperature

environmentstatistical data.


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10. Analysis of failures metals

10.1. Types of failure

Ductile

Unusual as earlier plastic deformation generally detected and classed as failure.

Ideally would neck to a point but triaxial stress state occurs in sample centre leadingto failure initiation, often at inclusions.

Final failure at edges gives characteristic shear lips.

Brittle

Transgranular as insufficient slip systems available due to:

crystal type - e.g. hcp compared to fcc strain rate

temperature below ductile brittle transition TDBTT stress concentration

Intergranular a. 70:30 brass b. H in steel

Due to segregation to grain boundary (gb), e.g. H, P, Sn, Sb, S in steels.

May also arise due to second phases forming at gb, or precipitate distribution.

Embrittlement (gas, liquid metal): characteristic intergranular as above, e.g. H in steel (b above)

Stress corrosion require susceptible alloy, e.g.

stainless steels (as shown)

Al-Zn-Mg alloys (7xxxseries),

brasses in ammonia


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Corrosion

Surface oxide debris evident

Pitting and crevice corrosion is especially common

Fatigue

Al 7178 varying loads (beach marks)

See characteristic striations associated with stage 2

Stage 3 can be ductile or brittle failure

Creep

Voiding and gb sliding may be observed in stage 3

No coarse microstructural changes in stages 1 & 2 (could observe in tem)

High-temperature degradation. Signs of oxidation, gb local melting, grain growth

Wear erosion solid particles carried in a fluid

adhesion transfer of one solid (softer) to another

abrasion cutting of softer solid by harder so material removed


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10.2. Fracture surface examination non-destructive

presence of colour or texture changes

temper colours

oxidation

corrosion products

presence of distinctive features on surface

shear lips

beach marks

chevron marks

river lines

gross plasticity

large voids or inclusions

secondary cracks

direction of fracture propagation

fracture initiation site

nearby stress raisers

mode of loading


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Beach marks: distinct regions on a fracture surface which indicate a change in the fracturemechanism or rate of crack propagation

.

A typical example is illustrated above for fatigueshowing the three areas corresponding to

crack initiation,

crack propagation and

fast failure

Chevron marks: arise when a crack in a steel initiates along the centre line of a plate (so may beassociated with inclusions at which cracks initiate) and then runs to the surface of the plate.

The marks point in the direction away from that of crack propagation and so can be used totrace the direction of crack growth and ultimately the source of origin of fracture.

River lines occur especially in relatively inclusion-free steels.

Brittle failure (cleavage) results in roughly flat surfaces which are normal to the applied(tensile) stress and which often are on a particular set of planes.

When the crack crosses a grain boundary, especially tilt, many small parallel cracks can formwith steps between them.

These then continue to grow and run into each other to form a single crack with a larger step.

The characteristic pattern is the river pattern or river lines. Hence can show the direction ofcrack propagation and is a clear sign (with the characteristic failure surface) of brittle failure.


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10.3. Fracture stages

local degradation: wear

fretting corrosion

pitting

oxidation

crack initiation: cracks < 100 m hence difficult to see optically

cracks either grow to be catastrophic or may be stable

cracks often initiated externally at stress concentrators

internally at inclusions as shown below in steel

slow crack growth: ductile tear

fatigue

creep

stress corrosion cracking

spend high proportion of life time growing crack hence can

[can define suitable quality control or inspection period

can evaluate risk of leaving other parts in service]

fast crack growth: onset of instability at critical crack length and leads to

brittle failure

ductile tearing

plastic collapse


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11. Analysis of failures ceramics and glasses


brittle (generally) from

design deficiency

defects introduced during fabrication or machining

service damage - impact especially

thermal shock

oxidation & corrosion related

ductile at high temperature for

glasses (viscous flow and so strain rate dependent)

crystalline ceramics where slip systems thermally activated

11.2. Fracture appearance

Frequency of cracks is measure of: energy introduced and residual stresses in the body

Little branching in thermal shock see below for crack in glass

Crack direction reflects type of loading and magnitude of stresses applied

Cr

Direction of crack/s also affected by near stress raisers such as

machining marks

change in section thickness

hole or internal porosity

internal defect, impurity


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Fracture generally radiates out from a single point or small area

Branching occurs when:vcrack > vs where vcrack is crack velocity

vs is speed of sound in the ceramic or glass

multiplebifurcation

A B

fracture origin

Crack patterns also can be determined from fragments and/or "witness mark" - smallarea with debris or burnished appearance

11.3. Fracture initiation site

Fracture site appearance changes as vcrack increases, leads to mirror, mist and heckle.

mirror: crack accelerates from initiation site (often surface defect, inclusion or stress raiser)

crack initially proceeds on one plane

estimate size of mirror (indicated by diameter AB in above sketch

(i) = Kcr 0.5

where = 3.5 0.3

equation is reasonably material independent but is more complex if defectspresent, then have to introduce stress intensity factors;

note similarity to fracture toughness KIc = f(a)0.5

(ii) fr0.5 = constant - value of the constant is material dependent: glass 2.3Al2O3 9Si3N4 14

Hence can estimate critical defect size cby;

ln f

lnr

- running controlled tests and plot f against r

- later can estimate f for any observed rvalue

- use Griffith to estimate defect size c

f = A (E /c)0.5

or can use Evans & Tappin

f = Z/Y (2E /c)0.5

- Y is dimensionless term (1.7 2.0), depends on flaw depth & test geometry

- Z is dimensionless term (1 2), depends on flaw configuration.


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mist: as crack velocity increases, may intersect inclusion or shift in direction of principal stress

slight deviation from original plane

small radial ridges (although generally not seen on crystalline ceramics)

hackle: - larger ridges than mist and transforms to crack branching- if abrupt change in stress field, points in new direction of crack movement,

100 m

Fracture initiation in silicon nitride Fracture in glass[http://cems.alfred.edu/ces252/Fracto.html]

11.4. Wallner linesSimultaneous propagation of crack front & elastic shock wave

||- as each wave overtakes the primary fracture crack

principal stress momentarily deviated/disturbed

- curvature

approximate shape of crack front (assuming wave intersects with entire fracture front)

direction of crack propagation and indication of stress distribution (distance of eachpart of line from crack origin).


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12. Analysis of failures polymers

S

ome phenomena apparently similar to metals although mechanisms quite different, e.g.

ductile-brittle transition

tough

ness & fracture mechanics

fatigue

stress corrosion.

F

racture behaviour influenced by:

type of bonding and extent of cross-linking

chain pack

ing (amorphous versus crystalline polymers)

extent of crystallisation and average crystallite size.

12.1. Deformation catagories

(i) dilational: crazes, voids, microcrackscrazing: - principally in amorphous polymers, brittle in tension

e.g. polystyrene (PS), polymethyl methacrylate (PMMA)

Crazing ahead of crack in PS Fibrils in crazes in polystyrenelow molecular with 1% of higherweight molecular weight

- limited extent in semi-crystalline

e.g. polycarbonate in tensile-fatigue loading c.f. tension failure by shearbanding.

- slit-like microcracks spanned by oriented fibrils as illustrated above

o width 1-2 m and up to several mm in length

o grow normal to applied stress

o fibrils strength & density depend on molecular weighthigher molecular weight have fewer, longer stable crazes

- precedes crack, c.f. plastic zone ahead of crack in metals

- abs

orbs energy so improves toughness

- whitening effect (especially if associated voiding) due to scattering of light

(ii) non-dilational: shear bands.

most polymers in compression exhibit this behaviour

microscopic localised deformation along shear planes

at 45 to applied compressive load

strain magnitude ~ 2-3 locally


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ductile due to: mechanical overload

effects of liquids

particlulate fillers

brittle failure in ductile material (often) associated with

choice of polymer

design - stress concentration

mould design

poor joining

processing - inhomogeneous melt

degraded melt

surface defects

non-uniform dispersion of additives

non-uniform cooling - coarse spherulites

high chain orientation

imperfect internal welds

service factors -prolonged loading

fatigue

thermal degradation

photochemical degradation

corrosion

stress corrosion


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PAST TRIPOS QUESTIONS

In addition to the following specific questions, see also questions in the essay parts of the papers asthese often have a materials selection bias.

1998 1j, 9, 28

1999 1b, 8

2000 1b, 3, 25

2001 10i, 11, 36

2002 10j, 17

2003 10d, 14

2004 1j, 7

2005 10b, 25

2006 1e, 18

2007 5, 10j

2008 10b, 38

material selection and fma

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