failure of metals

8/11/2019 Failure of Metals

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Chapter 6 : Failure of Metals

1) Cite the three usual causes of failure.

2) (a) Cite the two modes of fracture and the dierencesbetween them.(b) Note which tpe of fracture is preferred! and "i#e tworeasons wh.

$) %escribe the mechanism of crac& propa"ation for bothductile and brittle modes of fracture.

') %escribe the two dierent tpes of fracture surfaces forductile metals! and! for each! cite the "eneral mechanicalcharacteristics of the material.

) rie* describe the mechanism of crac& formation and"rowth in moderatel ductile materials.

6) rie* describe the macroscopic fracture pro+le for amaterial that has failed in a brittle manner.

,) Name and brie* describe the two crac& propa"ationpaths for polcrstalline brittle materials.

-) %e+ne fatigueand specif the conditions under which itoccurs.

) Name and describe the three dierent stress/#ersus/timeccle modes that lead to fati"ue failure.

10) i#en a sinusoidal stress/#ersus/time cur#e! be able todetermine the stress amplitude and mean stress.

11) %escribe the two dierentl tpes of fati"ue surfacefeatures! and cite the conditions under which the occur.

12) Cite +#e measures that ma be ta&en to impro#e thefati"ue resistance of a metal.

1$) %e+ne creep and specif the conditions under which itoccurs.


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1') Ma&e a schematic s&etch of a tpical creep cur#e! andthen note on this cur#e the three dierent creep sta"es.

1) i#en a creep plot for some material! determine (a) thestead/state creep rate! and (b) the rupture lifetime.

Fracture Fracture is a form of failure where the material separates in pieces

due to stress, at temperatures below the melting point

The usual causes of fracture are improper materials selection, wrong

processing technique, inadequate design of the component andmisuse.

The fracture is termed ductile or brittle depending on the ability of a

material to experience plastic deformation

Any fracture process involves two steps

1) crac formation!) crac propagation

Ductile Fracture

"uctile fracture is always preferred because #

1) the presence of plastic deformation gives warning that fracture isimminent $due to happen soon), allowing preventive measures tobe taen. Alas, brittle fracture occurs suddenly andcatastrophically without any warning as a consequence of the

spontaneous and rapid crack propagation


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!) more strain energy is required to induce ductile fracture becauseductile materials are generally tougher

%acroscopic fracture profile of #

$a) &ighly ductile fracture in which the specimen necs down to a point$b) %oderately ductile fracture after some necing$c) 'rittle fracture without any plastic deformation

The most common type of tensile fracture profile for ductile metals is

shown in Figure $b) in which fracture is preceded by only a moderateamount of necing.

The ductile fracture process occurs in several stages.


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First, afternecing begins $Figure a), small cavities, or microvoids,

form in the interior of the cross section $Figure b)

(ext, as deformation continues, these microvoids enlarge, cometogether, and coalesce to form an elliptical crac, which has itslong axis perpendicular to the stress direction $Figure c).

The crac continues to grow in a direction parallel to its maor axis

by this microvoid coalescence process $Figure d).

Finally, fracture ensues by the rapid propagation of a crac around

the outer perimeter of the nec by shear deformation at an angle of

about *+ with the tensile axis - this is the angle at which the shearstress is a maximum $Figure e).

"uctile fracture is also termed cup-and-conebecause one of the

mating surfaces is in the form of a cup, the other lie a cone.


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The fractured specimen of ductile material shows that the central

interior region of the surface has an irregular and fibrousappearance.

Fractographic examination of the fibrous central region byscanning electron microscope shows that it consists of manyspherical dimples/. At *+ shear lip of the cup-and-cone fracture,elongated or 0-shaped dimples are formed.

0up-and-cone fracture in aluminum

scanning electron fractograph showing$a) spherical dimples$b) elongated or 0-shaped dimples

Brittle Fracture n brittle fracture, there is no appreciable deformation occured and

crac propagation is very fast.


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'rittle fracture in mild steel

n most brittle materials, crac propagation $by bond breaing) isalong specific crystallographic planes $cleavageplanes). This type of

fracture is transgranular $through grains) producing grainy texture $orfaceted texture) when cleavage direction changes from grain to grain.n some materials, fracture is intergranular

a) 2chematic cross-section profile showing crac propagation through the interiorgrains for transgranular fracture

b) 2canning electron fractograph of ductile cast iron showing a transgranular fracturesurface


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a) 2chematic cross-section profile showing crac propagation along grainboundaries for intergranular fracture

b) 2canning electron fractograph showing an intergranular fracture surface

Ductile Brittle

Plasticdeformation

extensive little

crack propagation slow, stable, needs stress Fast, unstable

type of materials most metals $not too cold) ceramics, ice, cold metals

warning permanent elongation none

strain energy higher lower

fractured surface rough smoother

necking yes no

Variation of properties with temperatures


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The yield strength and tensile strength vary with prior thermal and

mechanical treatment, impurity levels, etc. This variability is related tothe bahaviour of dislocations in the material. 'ut elastic modulus isrelatively insensitive to these effects

The yield strength, tensile strength and modulus of elasticity

decrease with increasing temperature, ductility increases withtemperature.

Fatigue and Creep

Fatigue

Fatigue is a form of failure that occurs in structures subected todynamic and fluctuating stresses.

3nder these circumstances it is possible for failure to occur at a

stress level lower than the tensile or yield strength for a static loadand after a lengthy period of repeated stress or strain cycling.

Fatigue failure can happen in bridges, airplanes, machine

components, etc.

t is the most usual $456) of metallic failures $happens also in

ceramics and polymers)

The failure is brittle-lie even in ductile metals, with little plastic

deformation

t occurs in stages involving the initiation and propagation of cracs

Cyclic Stresses These are characteri7ed by maximum, minimum andmean stress,

the stress amplitude, and the stress ratio. The three types of stress

cycle are#


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$a) reversed stress cycle $the stress alternates from a maximumtensile steress $8) to a maximum compressive stress $-))

$b) repeated stress cycle $maximum stresses are asymmetricalrelative to 7ero-stress level)

$c) random stress cycle.

%ean stress,

2

minmax

+=

m

9ange stress,

minmax =

r

2tress amplitude,

22

minmax

==

r

a

2tress ratio,

max

min

=R

The S-N Cure

The curve is obtained by using fatigue testing apparatus


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The specimen is subected to stress cycle at large maximum stress

amplitude then the number of cycles to failure is counted.

The procedure is repeated at decreasing maximum stress amplitude.

"ata are plotted as stress 2 $normally taen as stress amplitude)

versus the logarithm of the number ( of cycles to failure

The higher the magnitude of the stress, the smaller the number of

cycles the material is capable of sustaining before failure

Fatigue limit !endurance limit": a limiting stress level below whichfatigue failure will not occur. n this case, the SN curve becomeshori7ontal at large N

Fatigue strength : the stress level at which failure will occur for

some specified number of cycles

Fatigue life Nf: the number of cycles to cause failure at a specified

stress level


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The scatter in results $fatigue data) is a consequence of the fatigue

sensitivity to a number of test and material parameters that areimpossible to control precisely. The parameters include specimen fabrication and surface preparation

metallurgical variables specimen alignment in the apparatus

mean stress

test frequency

Crack #nitiation and Propagation

Fatigue failure is characteri7ed by three steps#

. crac initiation $a small crac forms at some point of high stressconcentration)

. crac propagation $this crac advances incrementally with eachstress cycle)

. final failure $occurs very rapidly once the advancing crac hasreached a critical si7e)

0rac nucleation sites include surface scratches, sharp fillets,

eyways, threads, dents etc.

The region of a fracture surface that formed during the crac

propagation step may be characteri7ed by two maring, beachmarsand striations.

'oth features indicate the position of the crac tip at some point in

time and appear as concentric ridges that expand away from thecrac initiation site in a circular or semicircular pattern.


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'eachmars are found for components that experienced interruptionsduring the crac propagation stage

2triations

;ith regard to si7e, beachmars are normally macroscopic

dimensions and may be observed with the naed eye< fatiguestriations are of microscopic si7e and it is necessary to observe themusing electron microscopy.

;ith regard to origin, beachmars result from interruptions in the

stress cycles< each fatigue striation is corresponds to the advance ofa fatigue crac during a single load cycle.


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Factors That $ffect Fatigue %ife %ean stress $lower fatigue life with increasing mean stress m). 2urface effects

Design factors$any notches or geometrical discontinuity act as a

stress raiser and fatigue crac initiation sites - scratches, sharptransitions and edges)

Surface treatments$surface marings can limit the fatigue life)

Four measures that may be taen to increase the fatigue resistance

of a metal alloy are#1) =olish the surface to remove stress amplification sites!) 9educe the number of internal defects $pores etc.) by means of

altering processing and fabrication techniques>) %odify the design to eliminate notches and sudden contourchanges

*) &arden the outer surface of the structure by case hardening$carburi7ing, nitriding) or shot peening

Creep

0reep is the time-varying plastic deformation of a material stressed

at high temperatures $greater than about 5.*Tm). ?xamples# turbineblades, steam generators. @eys are the time dependence of thestrain and the high temperature

&enerali'ed Creep Behaior A typical creep curves $strain vs. time) exhibit three distinct

regions.


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rimaryor transientcreep# the slope of the curve diminishes with

time

Secondary creep$steady!state creep) # the rate is constant and the

plot becomes linear

Tertiary creep# there is an acceleration of the rate and ultimate

fracture $rupture)

Stress and Temperature (ffects

'oth temperature and the level of the applied stress influence the creepcharacteristics.

0reep becomes more pronounced at higher temperatures. There isessentially no creep at temperatures below *56 of the melting point.

0reep increases at higher applied stresses.

;ith increasing stress or temperature, the following will be noted#1) the instantaneous strain at the time of stress application increases


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!) the steady-state creep rate is increased>) the rupture lifetime is diminished

"ependence of creep strain rate on stress

s=K1Bn

$lloys for )igh-Temperature *seThese are needed for turbines in et engines, hypersonic airplanes,nuclear reactors, etc. The important factors are a high meltingtemperature, a high elastic modulus and large grain si7e

2ome creep resistant materials are stainless steels, refractory metalalloys $containing elements of high melting point, lie (b, %o, ;, Ta),and superalloys $based on 0o, (i, Fe.)

failure of metals

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