Material Failure

7/24/2019 Material Failure

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2D4: Integrity of Materials

& ComponentsProperties of Materials

Failure Modes and Prevention

Failure Prevention Strategies

Nondestru!tive "esting


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For!es on Materials

Tensile Force

Compressive Force

Shear Force


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#ending

Tension

Compression


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$%e!ts of oadingProvided t'at load is not e(!essive)material itself *ill return to its

original si+e *'en load is removed,material -e'aving in .elasti!/manner

0t in!reased stresses deformation-e!omes permanent material issaid to -e .plasti!/


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Main Modes of FailureDu!tile failure

#rittle failure

Metal fatigue

#u!1ling

Corrosionear

Creep


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Du!tile FailureMaterial moves into plasti! region and loses its originals'ape

"'ere is a redu!tion in !rossse!tional area *'i!'in!reases t'e stress ,stress is proportional to area3

Failure o!!urs at t'e pla!e of redu!ed !rossse!tionalarea

"emperature is a maor fa!tor

"'e 'ig'er t'e temperature) t'e more du!tilematerials -e!ome i5e5 t'ey !an elongate more -eforefailure


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#rittle Failure6!!ur very suddenly and *it'out *arning

6!!ur -e!ause t'e stru!ture of a materialdoes not slip) eit'er o*ing to t'e materialstru!ture or t'ere is insu7!ient time due to

t'e intensity of t'e load

Small !ra!1s spread t'roug' t'e material so8ui!1ly t'at massive failure is produ!ed

Speed of failure often results in some energy

in material -eing released as sound) giving-rittle failure a !'ara!teristi! .!ra!1/


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#rittle FailureNote t'at in ot'er failure modes) t'e a!tual failuremay -e -y -rittle mode) -ut t'is *ill only -e part oft'e se8uen!e of failure

Some fa!tors t'at promote -rittle failure:

o* temperatureImpa!t or Snat!' oading e5g5 lifting e8uipment

9esidual tensile stress e5g5 pretensioned -eams

In'erently -rittle material e5g5 glass and !erami!s

In appearan!e) a -rittle failure s'o*s no sign of any

deformation t'e parts !ould -e tted -a!1 toget'er5"'e surfa!e *ill -e -rig't and t'ere may -e !'evron

mar1ings a!ross t'e failed surfa!e/


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Metal FatigueMost !ommon type of failure *'ere !onditions produ!ingme!'ani!al vi-ration o!!ur

Flu!tuating stress !onditions produ!ed -y vi-ration ,e5g5air!raft *ings going up and do*n during ;ig't3 !an !auseformation of a !ra!1 t'at propagates t'roug' t'e material

Cra!1 redu!es t'e area of material resisting t'e stressesuntil t'e remaining material !an no longer resist t'estresses and fails

Failure is generally a -rittle failure as it is t'e less du!tile

materials t'at are used to resist !y!li! loading


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Metal FatigueInitial !ra!1 al*ays starts from t'e surfa!e and penetrates into

-ody of material

Surfa!e -lemis'es e5g5 ma!'ining mar1s and foreign -odyin!lusions are li1ely !andidates to set fatigue failure into a!tion) asare 'oles for -olts) rivets) inspe!tion 'at!'es et!5

"'e nal area of metal in pla!e *'en failure o!!urs is !learlyidentied -y its !olour t'e surfa!e of t'e !ra!1 *ill -e dull due tot'e e%e!ts of air and moisture -eing in !onta!t and setting up mild!orrosion) *'ile t'e ne*ly failed surfa!e *ill s'o* signs of ne*)!lean -rittle failure

Metal fatigue !ra!1s !an -e readily dete!ted -y ND"

Surfa!e defe!ts s'ould -e removed *'ere possi-le -y polis'ing


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#u!1ling'en a !ompressive for!e is put into a rod) -eam or -ar t'e for!eis resisted -y t'e material

0s for!e in!reases) t'e material distorts) preventing a straig't

transfer of stress t'roug' t'e material

Failure is due to tensile for!es on t'e e(treme !ausing du!tile

failure *'i!' propagates t'roug' t'e material and leads to!atastrop'i! !ollapse

Can -e -etter resisted -y a du!tile material t'an a -rittle materialdue to a-ility to resist tensile for!es introdu!ed -y t'e .-o*ing/on t'e outside surfa!e

Can -e limited -y installing intermediate supports to limitmovement from t'e straig't


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Corrosion0%e!ts only metals

9e8uires t'e presen!e of an ele!trolyte

,normally *ater3) potential di%eren!es-et*een metals to allo* ;o* of !urrent

C'emi!al !'ange in *'i!' -ody ofmetal loses atoms *'en repeatedmillions of times) loss of material !an-e o-served as !orrosion


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Corrosion9ate of !orrosion depends on:

Stress in material

Strengt' of ele!trolyte ,p< value3

$nvironment and e(posure ,'eat a!!elerates !orrosion3

9ea!tivity of metal

Metal impurities

Can -e o-served -y:

o!alised pitting in surfa!e of material6verall t'inning of material

Can -e dete!ted -y dis!oloration or ND" met'ods


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CorrosionCorrosion removes material) so t'ere isless !omponent to !arry t'e stress it *asdesigned to ta1e

Prote!tion:

Control environment

Sele!tion of !orrosion resistant material

Prote!t metal *it' !oatingSa!ri!ial anode of magnesium) aluminium or+in! *'i!' is dissolved instead of t'e!omponent


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earCan -e produ!ed -y:

S!u7ng

a!1 of lu-ri!ation -et*een moving parts5

Movement of parts !auses fri!tion

0-rasion6!!urs *'en small !omponents of foreign material *'i!' is 'ardert'an !omponent get -et*een moving parts e5g5 sand parti!les surfa!e t'en -e!omes s!ored

Pitting

Com-ination of a-oveParti!les produ!ed during s!u7ng -e!ome deta!'ed and 'ardened)-e!oming t'e e8uivalent of foreign parti!les and !ause a-rasion


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Creep'en a material is under stress near to itselasti! limit) it undergoes a pro!ess ofplasti! deformation 1no*n as !reep

$(tent to *'i!' !reep a!ts is dependant on:

"ime ,!reep is a slo* pro!ess3

"emperature ,!reep a!!elerated in 'ig'temperatures3

Not a true mode of failure as failure is ofteneit'er -rittle or du!tile


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Preventing Failure in

Design


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"esting and >uality

0ssuran!eC'e!1s at ea!' stage of a pro!ess

Management pro!ess to ensure t'e

!'e!1s are !arried out and re!ordedSystem is pro-a-ly -ased on t'e #SIS6?@@@ Series of do!uments

detailing 8uality assuran!e9e!ords are important in t'e event offailure


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Preventing Failure in AseAse *it'in manufa!turers statedparameters

Spe!i! maintenan!e e5g5lu-ri!ation

Nondestru!tive testing


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Failure Investigation

"e!'ni8uesColle!tion of samples

Fault "ree 0nalysis

0!!ident Investigation:

Bat'er samples of failed material

oo1 !losely at failed surfa!es

9e!ord information

a- analysis may -e ne!essary


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ND""est integrity of material *it'out destroying!omponents

"ypes of ND":

isual inspe!tion

Penetrant inspe!tion

Magneti! inspe!tion

9adiograp'y

Altrasoni! testing$ddy !urrent testing

$le!tri!al resistivity

"'ermal imaging

Summary of

NDT techniques
http://ndt%20summary.doc/http://ndt%20summary.doc/http://ndt%20summary.doc/


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isual Inspe!tionAse na1ed eye) mi!ros!ope ormagnifying glass

9e8uires good lig't sour!e

Surfa!e may re8uire !leaning

C'eap and easy -ut applies tosurfa!e defe!ts only


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Penetrant Inspe!tionAses dye penetrant to 'ig'lig'tdefe!ts for visual inspe!tion

6nly relates to surfa!e defe!ts

$asy to use o%site as dyes are inaerosol !ans


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Magneti! Inspe!tionComponent is magnetised and magneti!parti!les are applied

0ny defe!t *ill s'o* as it distorts t'emagneti! eld and t'e parti!les liedi%erently

6nly *or1s on materials *it' magneti!

propertiesDete!ts surfa!e !ra!1s and su-surfa!e!ra!1s !lose to t'e surfa!e


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9adiograp'yBamma or rays are passed t'roug'material and onto a strip of lm

9adiation triggers rea!tion in lm*'i!') *'en developed) s'o*s *'erematerial is sound and *'ere defe!ts

e(istBives permanent re!ord

9adiation 'a+ard present


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Altrasoni! "estingAses generator transmitting ultrasound*aves into material and dete!ting t'em*'en re;e!ted

$8uipment usually 'and'eld

9esults need to -e interpreted -y s1illedoperatives

Can dete!t defe!ts *it'in t'e material

Can -e used on metalli! and nonmetalli!o-e!ts


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$le!tri!al 9esistivity0 property of metalli! materials ist'at ele!tri!al resistan!e !an alter

*'en su-e!ted to for!esAses a small loop of *ire atta!'edto t'e o-e!t ,strain gauge3

Can -e used to monitor situationover long periods


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"'ermal ImagingCameras t'at !an dete!t 'eat and s'o*small variations onto !olour s!reen

Main use it to determine if a part isrunning 'ot I5e5 la!1ing lu-ri!ation orru--ing *'ere it isnEt supposed to

Cameras are small

6nly provides a snaps'ot

Comparison -et*een pi!tures is di7!ult-y eye) -ut !omputer !an -e used


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Previous $(am >uestions$(plain t'e purpose of nondestru!tive testing ,ND"3 onitems of plant ,2 mar1s3

6t'er t'an visual inspe!tion) outline t'e prin!iples)

-enets and limitations of t*o ND" te!'ni8ues ,mar1s3

0 serious a!!ident o!!urred *'en a t'readed steel rod int'e -ra1ing me!'anism of a 'oist failed5 0n investigation

revealed a fatigue failure of t'e rod5Des!ri-e t'e me!'anisms and !'ara!teristi!s of fatigue failure ,G3

6utline t'e fa!tors t'at may !ontri-ute to su!' a failure ,G3

Des!ri-e t'e measures t'at !ould 'ave -een ta1en *'i!' mig't'ave prevented t'e failure , mar1s3

Material Failure

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