the role of failure analysis in establishing design rules for reliable plastic products

7/27/2019 THE ROLE OF FAILURE ANALYSIS IN ESTABLISHING DESIGN RULES FOR RELIABLE PLASTIC PRODUCTS

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PergamonMicroelectron. Reliah., Vol. 35, Nos 9-10, pp. 1275-1284, 1995

Elsevier Science LtdPrinted in Great Britain0026-2714/95 $9.50+.0(I

THE ROLE OF FAILURE AN AL YS IS IN ESTABLISHINGDESIGN RU LES FOR RELIABLE PLASTIC PRODUCTS

Jan L . Spo ormake r 1

AbstractPlas t i c products can fa i l i n a l a rge var ie ty o f modes because o f er rors i nth e des ign , p rocess ing and appl icat ion. T he des i gner mus t t ake i n t oaccoun t t hese e r ro r s i n o rde r t o p reven t t hese e r ro r s r a the r t han no t i cet h e m .W i t h o u t h a v in g k n o w l e d g e o f t h e c o m p l e x m e c h a n i c a l b e h a v i o u r o fp las t i cs and the s t ruc ture re la ted proper t i es , adequate des ign o f re l i ab lep roduc t s i s i mposs i b l e . T he des i gner mus t a l so be aware o f t he s pec i f i cfa i l u re modes , f a i l u re causes and f a i l u re mechan i sms and he can ach i eveth i s by fa i lu re analys is . Impor t an t i n f o rmat i on f r om ana l yses o f f a i l edp rodu c t s is t he hand li ng o f use rs in w ay s n o t f o reseen b y h i m .A c l as s i f ic a t i on o f f a i led p l as t ic p r oduc t s based on fa i l u re causes andfa i l u re mechanisms i s presented and shou ld be used in car ry ing outF a ilu r e M o des and E f f ec t s A na l y s is .

Fai lure analysis, re l iabi l i ty ,c o n c e n t r a t i o n s .

Keywordsdes ign, p las t i c products , fa t i gue, s t ress1. Introduction

De sign ing for re li ab i li ty o f p las t ic produ cts i s bec om ing increas ing ly imp -o r t an t i n Europe ma i n l y because o f t he concern abou t t he e n v i r o n m e n ta nd s t r i c t l i ab i l i t y doct r ine.Th is requ i res a c om p l e t e c on t r o l o f t he des ign p r oc es s , where the re l i ab i l -i t y and safe ty are des igned in to the product . In p las t i c products fa i l u recan, un l i ke in meta ls , be caused by a v a r ie t y o f f a il u r e m e c han i s m soccur r i ng a t am b i en t tem pera tu res .T h e v e r y u n w a n t e d f a i l u re mode "unexpected brittle failure" occurs eas i l yin p las t i c products due to fa i lu re causes l i ke s t r ess concen t ra t i ons ,

Facul ty of Indust r ia l Design Engineer ing,T echno l ogy , De l f t , T he Ne the r l ands1275

Del f t Un i ve rs i t y o f


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1276 J.L . Spoormakeri n a d e q u a t e p r o c e s s i n g , h i g h l y l o a d e d k n i t li n es a n d p o o r j o in i n gt e c h n i q u e s .A l t h o u g h v e r y m u c h i n f o r m a t i o n a b o u t th e fa i lu r e o f p la s t i c s is a v a i l a b l e[ 1 ] , o n l y l i tt l e o f t h i s i n f o r m a t i o n is k n o w n b y th e d e s i g n e r s . A w a y o fm a k i n g t h e m a w a r e o f th e p o s s i b l e f a i lu r e m o d e s is t o f a m i l i a r iz e t h e mw i t h t h e f a i l u r e s o f t h e i r an d o t h e r p l a s t i c p r o d u c t s . I t is f o r t h a t r e a s o ni m p o r t a n t t o v i s u a l i z e t h e i n f o r m a t i o n [ 2 ] a n d t o p r e s e n t i t a s c o n c r e t e a sp o s s i b l e .

2. The mechanical properties of plasticsW i t h o u t u n d e r s t a n d i n g t h e s t r u c t u r e r e la t e d p r o p e r ti e s o f p l a s t ic s i t i sv e r y d i f f i c u l t t o u n d e r s t a n d t h e f a i l u r e b e h a v i o u r o f p l a s t i c p r o d u c t s . T h em e c h a n i c a l p r o p e r t i e s o f p l a s t i c s s t r o n g l y d e p e n d u p o n t h e s t r u c t u r e a n dm o r e o v e r u p o n t e m p e r a t u r e a nd l o a d i n g r a t e o r f r e q u e n c y . In order toi l lu s t r a t e t h e s e d e p e n d e n c i e s i n f i g u r e 1 s t r e s s - s tr a i n c u r v e s o f P M M A f o rd i f f e r e n t t e m p e r a t u r e s a r e d e p i c t e d . T h e t r a n s i t i o n f ro m b r i t tl e t o d u c t i leb e h a v i o u r a t a b o u t 4 5 " C i s d u e t o t h e p o s s i b i li ty o f r o t a t i o n o f th e s i d eg r o u p a b o v e t h i s t e m p e r a t u r e ~ 8 -r e la x a tio n ).T h e m e c h a n i c a l p r o p e r ti e s a r e m o r e o v e r a ls o d e p e n d e n t o n t h e m o l e c u l a rm a s s , t h e o r i e n t a t i o n , t h e t r a n s i t io n s , t h e d e g r e e o f c r y s t a l l is a t i o n , t h ew a t e r c o n t e n t e t c . [ 1 ] , [ 2 ] a n d [ 3 ] .

3 . Des ign review and failure mode and effects analysisDesign Review and Checklists.D e s i g n R e v i e w i s a f o r m a l , d o c u m e n t e d a n d s y s t e m a t i c s t u d y o f a d e s i g nb y s p e c i a l i s t s n o t d i r e c t l y a s s o c i a t e d w i t h i t s d e v e l o p m e n t . A l t h o u g h t h em e t h o d is k n o w n f o r m a n y ye a r s , t h e i m p l e m e n t a t i o n o f t h is m e t h o d isn o t y e t g e n e r a l in m a n y c o m p a n i e s . E s p e c i a ll y f o r p l a s t i c p r o d u c t s d e s i g nr e v i e w i s v e r y u s e f u l b e c a u s e s p e c i a li s ts i n m o u l d d e s i g n an d p r o c e s s i n go f p l a s t i c s c a n b e i n v o l v e d i n t h e d e s i g n p r o c e s s a t a s t a g e a t w h i c hd e s i g n e r r o r s c a n e a s i l y b e c o r r e c t e d . A f t e r c o m p l e t i o n o f a m o u l d t h e


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F i g u r e 1 : S t r e s s - s t ra i n c u r v e s o f P M M A f o r d i f f e r e n t t e m p e r a t u r e s

C h e c k l is t s a r e v e r y h e l p f u l i n c a r r y i n g o u t d e s ig n r e v i e w s a n d F M E A ' s . Av e r y i l lu s t r a t i v e c h e c k l i s t h a s b e e n p u b l is h e d in M a c h i n e D e s i g n [ 4 ] a n dm a n y p l a s t i c m a n u f a c t u r e r s a l s o p r o v i d e c h e c k l is t s . A c h e c k l i s t w i t h t h em a i n f a i lu r e c a u s e s a n d f a i l u r e m e c h a n i s m s f o r p l a s t ic p r o d u c t s is g i v e n i nT a b l e 1 . A s c a n b e e n s e e n in t h i s t a b l e s tr e s s c o n c e n t r a t io n s h a v e b e e npu t on t op o f t he l i s t and i n [ 4 ] and [5 ] i t has a l so been s ta ted t ha t sha rpc o r n e r s a r e t h e s i n g l e g r e a t e s t f a i l u r e c a u s e o f p l a s t i c p a r t s . D e s i g n e r ss h o u l d b e c o n f ro n t e d w i t h t h e s e fa i lu r e c a u s e s a nd m e c h a n i s m s f r o mp r a c t i c a l e x a m p l e s a n d t h e r e f o r e a c o l l e c t i o n o f p l a s t i c p r o d u c t s f a i l e d i nse rv i ce i s bu i l d up i n ou r l abo ra to ry .


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1278 J . L . SpoormakerTh e Failure M ode and Effects Analysis.T h e F a ilu r e M o d e a n d E f fe c t s A n a l y s i s is a s y s t e m a t i c p r o c e d u r e in w h i c hf o r e a c h p o s s i b l e fa i l u re m o d e o f a c o m p o n e n t o f a p r o d u c t t h e f a i l u r ec a u s e s , t h e f a i l u r e m e c h a n i s m s a n d i t s e f f e c t s o n t h e r e l i a b i li t y o f t h ep r o d u c t a r e l i s t e d .

Table 1: L i s t o f t h e m a j o r F a i lu r e C a u s e s a n d M e c h a n i s m s f o r p l a s t i cp r o d u c t s .

FAILURE CAUSEs t r es s c onc e n t r a t ionsl o w m a s s a n d / o r m o u l dt em pe r a t u r eh i gh l y s t r es s ed w e l d li nesf au l t y r ibb i ngt oo h i gh s t i f f nes si nc o r r ec t j o i n i ng

F A IL U R E M EC H A N ISMc reeps t ress re laxa t i onw e a rfa t i gueage ingUV-degrada t i onenv i r onm en t a l s t r es sc rack ing

F r o m t h i s l i s t t h e c r i ti c a l i t e m s m u s t b e d e r iv e d a n d f u r t h e r a n a l y s e s a r ef o c u s e d o n th e s e c r i t ic a l i t e m s . I n o r d e r t o s t i m u l a t e t h e d i s c u s s i o nb e t w e e n d e s i g n , p r o d u c t i o n a n d s e r v i c e a s e a r l y a s p o s s i b l e in t h e d e s i g ns t a g e a d i s t i n c t i o n b e t w e e n f a i l u re c a u s e s in d e s i g n , p r o d u c t i o n a n d u s es h o u l d b e m a d e . F o r t h e f a i lu r e m o d e f r a c t u r e t h i s i s i ll u s t r a t e d i n t a b l e 2 .

Table 2: P a r t o f a F M E A

FAILURE CAUSE S DUE TO:DESIGNs h a r p c o r n e r so n t h e d r a w i n g

PRODUCTIONno rad ius i nco rpo ra tedi n t h e m o u l d

USEi m p a c t l o a d i n g a tl o w t e m p e r a t u r e s


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Fai lure analysis 1279A n i m p o r t a n t a s p e c t o f th e F M E A is t h e d e s ig n i m p r o v e a n d e x a m p l e s o ff a il ed p rod uc t s shou l d a lso inc lude des i gn i mp rove m en ts as sh ow n inf i gu re 2 .

4. Failure analysisStress Concentrat ions.T he mos t occu r r i ng f a i l u re causes i n p l as t i c p roduc t s a re t he ab rup t geo-met r i ca l changes i n t hese p roduc t s . T hese changes resu l t i n s t r essconcen t ra t i ons and because o f t hese t he s t r a i ns and s t r esses can l oca l l yreach very h igh va lues. The very loca l i zed ra i se o f the s t ra in and s t resso f t en resu l t s i n c rack f o rmat i on and /o r f r ac tu re . T h i s happens espec i a l l yin br i t t le p las t i cs , but p las t i cs ha ve a br i t t le b eha v iour a t l owtempera tu res and h i gh s t r a i n r a tes .Sharp geomet r i ca l changes o f t en a r i se f r om the mach i n i ng o f t he mou l dand an example o f th i s i s dep ic ted in F ig . 2 , where a deta i l o f a c lampingdev ice o f p las t i c i s g i ven. The loca l i ncrease o f the s t ress in the case o f al i near e las t i c behav iour o f the p las t i c i s expressed in the stressconcentration factor (SCF) deno ted by K , being th e ra t io o f the h ighe stoccur r ing loca l s t ress d i v ided by the nomina l s t ress . For smal l f i l l e t rad i it he exp ress i on f o r t he s t r ess concen t ra t i on f ac to r i s :

K , : c (1 }

w h e r e C is a c o n s t a n t t a k i n g a c c o u n t o f t h e s p e c i fi c g e o m e t r y a n d p i sthe f i l l e t rad ius .

Insufficient flexibil ity fixing devices.Espec ia l l y f i x ing dev ices l i ke snap f i t assembl ies are sub jec t to prescr ibedde f l ec t i on and t he f l ex i b i l i t y o f t he snap f i t f i nge r shou l d be h i gh enough .


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1280 J.L. Spoormaker

Failed product Failure cause

Stress concentration

Improvement

Rounded comersFigure 2 : C lamping dev ice w i th sharp geo m etr ica l changes

Fai led product Failure causeWT

Y / / / / A

Improvement

F i g u r e 3 : F ix ing dev i ce fo r CD ' s w i th f l ex ing f i ngers w i th a h ighs t i f f ness .

S t i f f fl ex ing m em bers requ i re m oreover sma l l de f l ec t ions to o b ta in therequi red react ion force and are hence very sens i t i ve to to lerances. A very


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F a i lu r e ana ly s i s 1281i l l u s t r a t i v e e x a m p l e o f f a i l u r e d u e t o i n s u f f i c i e n t f l e x i b i l i t y a n d u n r e a l i s e du s e is th e f i x i n g d e v i s e f o r C D ' s . In f ig u r e 3 a n e x a m p l e o f a w a y o f f i x i n ga C D i n a b o x , t h e d e s i g n e r h a d n o t i m a g i n e d a n d h e n c e t h e f l e x i n gf i n g e r s o f t e n r u p t u r e d .B y a p p l y i n g t h e e x p r e s s i o n f o r t h e h i g h e s t o c c u r r i n g s t r a i n i n a b e n d i n ge l e m e n t :ema = 3 " 1 . _ . / " w ( 2 )12 W

w h e c e / i s t h e s e c o n d m o m e n t o f a r ea , W i s t h e s e c t i o n a l m o d u l u s , / i st h e l e n g t h o f t h e f l e x i n g e l e m e n t a n d w is t h e p r e s c r ib e d d e f l e c t i o n , t h em a x i m u m s t r a i n c a n e a s i l y b e p r e d i c t e d .

Fat ique crack ini t iat ion due to f lashing.L i k e m e t a l s p l a s t i c s s u f f e r f a i l u r e u n d e r c y c l i c l o a d i n g a t s t r e s s e s e v e nb e l o w t h e y i e l d s t r e s s a n d t h i s i s r e f e r r e d a s f a t i g u e . T h e f a t i g u em e c h a n i s m i n p l a s t ic s i s n o t w e l l u n d e r s t o o d , b u t i t is l ik e l y t h a t w h e n ap o l y m e r i c m a t e r i a l i s s t r e s s e d t h e m o l e c u l a r c h a i n s w i l l b e s t r e t c h e d a n dc h a i n s l i p o c c u rs , w h i c h m i g h t le a d t o c r a ze s . W h e n t h e m a t e r i a l iss u b s e q u e n t l y c o m p r e s s e d t h e b u n d l e o f m o l e c u l a r c h a i n s i n t h e c r a z em i g h t k i n k an d th e m i c r o c r a c k in g o c c u rs .T h e d e p i c t e d p r o d u c t in F ig . 4 is a b e n d o f a v a c u u m c l e a n e r a n d i tsc r i t i c a l c r o s s s e c t i o n . F a t i g u e c r a c k i n i t i a t i o n o c c u r s s o m e t i m e s a tu n e x p e c t e d l o c a t i o n s a s in s h o w n i n F ig . 5.T h e b e n d is i n j e c t io n m o u l d e d a n d s o m e t i m e s f la s h i n g o c c u r r e d d u e tom i s r n a t i n g a n d / o r i n s u f f i c i e n t p r e s s u r e e x e r t e d o n t h e t w o c o r e s .

F l a s h in g c a u s e s n o t c h e f f e c t s a n d t h i s f a c i l i ta t e s c r a c k i n i t ia t i o nc o n s i d e r a b l y . T h e s t r ia t i o n s f r o m t h e c r a c k i n i t ia t i o n p o i n t a r e c l e a r l y v is -i b l e b y t h e n a k e d e y e a n d a r e s k e t c h e d i n F i g . 6 . T h e f a t i g u e s t r e n g t h o f


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1282 J.L. Spoormakert h e b e n d s w i t h f la s h e s w a s c o n s id e r a b ly l o w e r t h a n th e b e n d s w i t h o u tf l a s h e s .

c r o s s s e c t i o n A - A

F i g u re 4 : B e n d o f a v a c u u m c le a n e r w i t h t h e c r i t ic a l c r o s s s e c t i o n

5. Documentation of fai led productsW i t h i n o u r l a b o r a t o r y f a i l e d p l a s t i c p r o d u c t s a r e b e i n g a n a l y z e d i n o r d e r t od e t e r m i n e t h e c a u s e o f t h e f a i l u r e a n d t h e f a i l u r e m e c h a n i s m s . T h e a i m o ft h e d o c u m e n t a t i o n o f a ile d p r o d u c ts is t o c o n f r o n t d e s ig n e r s w i t h a llp o s s i b l e f a i l u r e m o d e s a n d c a u s e s a n d t o c o n v i n c e t h e m o f a c c o u n t i n gf o r t h e s e f a i l u r e s i n t h e d e s i g n s t a g e . T h e r e s u l t s o f t h e d o c u m e n t a t i o nare a lso used in F a i l u r e M o d e a n d E f f e c t A n a l y s e s f o r s tu d e n t s . A g o o dg u i d e l i n e f o r t h e f a i l u r e a n a l y s i s a n d s p e c i m e n p r e p a r a t i o n i s f o u n d i n t h eb o o k B O h m e [ 5 ] in w h i c h a ls o v e r y m a n y e x a m p l e s h a v e b e en c o m p i le d . I ti s r a t h e r d i f f i c u l t t o o b t a i n f a i l e d p r o d u c t s a n d t h e i m p r o v e d p r o d u c t sf r o m i n d u s t r y , b u t if a g o o d c o o p e r a t io n w i t h a n in d u s t r y e x i s ts o n e m i g h t


9/10

1283

Figure 5:

f l a s h

c r ac k i n i t i a t i on

Cause o f f lash ing due to mismatch ing o f cores

crack initiation ..~

Failure ana lysis

Figure 6 : Crack in i t ia t ion a t the outer f ib re and a t the f lash ingsucceed in obta in ing these products, We are convinced tha t showingfa i led and improved products prevents the un ivers i ty f rom b laming theindust ry , who is ext remely sensi t ive fo r admi t t ing fa i lu res.

MR35:9/10.F


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1284 J , L . Spoorm aker6 . C o n c l u d in g r e m a r k s

Designing of re l iable plast ic products is hard ly poss ib le w i thout hav i ng ap ro found unders tand i ng o f t he st ruc tu re re lated proper ties o f p las t ics .F undamen ta l r es ea r c h on severa l fa i l u re mechanisms [7 ] and [8 ] i sessen t ia l in obta in ing a pro foun d unde rs tand ing ab ou t the fa i l u rebehav i ou r o f p l as t i c p roduc t s .T h e fa i lures re lated to the mould const ruc t ion and the process ing ofp las t i cs are o f t en ve ry i mpor t an t and can never be neg l ec ted i n des i gn i ngfor re l i ab i l i t y o f p las t i c products .Ana lys i s and documenta t ion of fa i l ed p las t i c products i s essent ia l fo rp reven t i ng des i gners f r o m repea ti ng t he same e r ro r s a ll t he t i me .Tes ting o f s pec i men s and p r oduc ts shou l d be ca r r i ed ou t ex t reme l yca re fu l l y because o f t he comp l ex mate r i a l behav i ou r o f p l as t i cs .

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R e f e r e n c e sKun s ts to f f -W erks to f fe im GesprAch (Aufbau und E igenschaften),B A S F A k t i e n g e s e l ls c h a f t , L u d w i g s h a f e n , G e r m a n y .Young , R . J . , Introdu ct ion to Polym ers, Chapman and Ha l l , London1 9 8 7 .McCrum N .G . , C .P . Buck l ey and C .B . Buckna l l , Pr i nc i p l es o fpo lyme r Eng ineering , O xfo rd Un i ve rs i t y p ress , O x fo rd 1988 .Structura l m ateria ls , ed. G. Weidmann, P . Lewis and N. Reid . OpenU n i v e r s i t y , B u t t e r w o r t h , L o n d o n 1 9 9 0 .BShme E . , Fai lure An alysis using M icrosco pic Techn iques, D u P o n t ,G e n e v a 1 9 8 7Bros tow , W. and R .D . Corne l i ssen , M echanica l F ai lure o f P las tics ,Car l Han ser Ver lag, MOnchen 19 85Schoolenberg, G.E . , A s tudy of the U l t ra-Vio let Degradat ion ofPo lypropy lene Po lymer . Doc to ra l T hes i s , De l f t Un i ve rs i t y o fT e c h n o l o g y , D e l f t 1 9 8 8 .R e m m e r s w a a l , J . A . M . , F a t i gue o f am or phous poly mer s, Doc to ra lT hes i s , De l f t Un i ve rs i t y o f T echn o l ogy , De l f t 19 90 .

the role of failure analysis in establishing design rules for reliable plastic products

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