8/6/2019 Influence of Porosity on Fatigue

1/4

J M E PE G ( 1997) 6 : 615 - 618 9 I n t e rna t i ona l

In f luen ce of Po rosity on Fat igueCu m ula tive Dam age o f S in tered I ronE.S. Palma

F a t i g u e r e p r e s e n t s a c o m m o n m e c h a n i c a l c o n d i t i o n d u r i n g s e r v i c e f o r m a t e r i a l s u s e d i n s t r u c t u r a l c o m -p o n e n t s . T h i s t y p e o f l o a d in g b e c o m e s p a r t i c u l a r ly c r i t i c a l w h e n a l t e r n a ti n g s t r e s s a m p l i t u d e m a y b e e x -p e c t e d t o v a r y , o r c h a n g e , i n s o m e w a y d u r i n g t h e s e r v i c e l if e . S u c h v a r i a t io n s a n d c h a n g e s i n lo a da m p l i t u d e m a k e t h e d i r e c t u s e o f s t a n d a r d S -N c u r v e s i n a p p l i c a b le , b e c a u s e t h e s e c u r v e s a r e d e v e l o p e da n d p r e s e n t e d f o r c o n s t a n t s t r e s s a m p l i t u d e o p e r a t i o n . I n t h e p r e s e n t p a p e r , r e s u l ts o f c u m u l a t i v e d a m -a g e u n d e r f a t ig u e o f p o r o u s m a t e r i a l s a r e r ep o r t e d . T h e s p e c i m e n s u s e d w e r e p r o d u c e d b y c o m p a c t i n ga n d s i n t e r i n g i r o n p o w d e r . T h e i r a s - s i n t e r e d p o r o s i t y c o n t e n t s w e r e P 0 = 4 . 1 a n d 1 2 . 4 % . U n i a x i a l f a t i g u et e s t s (t e n s i l e - c o m p r e s s i o n ) w e r e c a r r i e d o u t a t r o o m t e m p e r a t u r e o n a h y d r a u l i c t e s t in g m a c h i n e .

K e y w o r d s cumu lative damage, fatigue, porosity [

1 . I n t r o d u c t i o nT h e f a t i g u e s t r en g t h o f p o w d e r m e t a l lu r g y ( P / M ) m a t e r ia l s

i s m uc h l o w e r t ha n t ha t o f s i m i l a r po r e - f r e e m a t e r i al s , s o t he r ei s r e l uc t a nc e t o u s e po r ou s m a t e r i a l s f o r t he m or e r e s po ns i b l ec o m p o n e n t s . T h e p r e v a i l i n g o p i n i o n i s th a t th e m a r k e t d e m a n df o r P / M m a t er i al s w o u l d i n c r e a s e m a r k e d l y i f t h e p r o b l e m s o ff a t i gue i n the s e m a t e r i a l s c ou l d be s o l ve d .

A s a r e s u lt , m a n y r e s e a r c h e f f o r t s on f a t i gue in s i n t e r e d m a -t e r i a l s ha ve be e n unde r t a ke n , a nd s i gn i f i c a n t a dva nc e s ha veb e e n m a d e t o w a r d u n d e r s t a n d i n g t h e fa t i g u e b e h a v i o r o f t h e sem a t e r i a l s ( R e f 1 - 9 ) . I t i s w e l l - e s t a b l i s he d t ha t t he f a t igue e n du r -a nc e l i m i t ( S N ) o f po r ous m a t e r i a l s de c r e a s e s w i t h i nc r e a s i ngp o r o s i t y a n d d e c r e a s in g p o r e r o u n d n e s s .

W h i l e t he s e e ff o r t s a re o f g r e a t i m po r t a nc e , t he y c a nno t g i vea ba s i s f o r a c ons i s t e n t u s e o f P / M m a t e r i a l s w he r e h i gh l y r e l i -a b l e c o m p o n e n t s a r e re q u i r e d a n d f a t ig u e i s k n o w n t o b e a m a -j o r p r o b l e m . M o r e o v e r , t h e s t re s s a m p l i t u d e e x p e r i e n c e d b y ac om pone n t m a y o f t e n va r y du r i ng i t s s e r v i c e l i f e . I n t h i s s i t u -a t i on o f va r i a b l e - a m p l i t ude l oa d i ng , t he d i r e c t u s e o f S - Nc u r ve s i s no t pos s i b l e , be c a us e t he s e c u r ve s a r e de ve l op e d un -de r c ons t a n t s t r es s a m p l i t ude ope r a t i on . T h e pu r po s e o f t h iss t udy i s t o p r e s e n t t he e f f e c t o f a pp l y i n g s e ve r a l d i f f e r e n t s t r e s sa m pl i t ude s on t he f a t i gue s t r e ng t h o f si n t e r e d i r on w i t h t w o d i f -f e r e n t po r os i t i e s.1 .1 C u m u l a t i ve D a m a g e T h e o r ie s

T h e b a s i c c o n c e p t o f c u m u l a t i v e d a m a g e t h e o r i e s is t h a t o p -e r a t i on a t a ny g i ve n c yc l i c s t r e s s a m p l i t ude g r e a t e r t ha n t he e n -du r a n c e l i m i t ( SN ) w i l l p r od uc e f a t i gue da m a ge . I n o t he r w o r ds ,t he m a t e r i al p r ope r t i e s a r e c ha ng e d a f t e r a c c um ul a t i on o f f a -t i g u e d a m a g e a n d a s p e c i m e n a c t u a ll y e x h i b it s a n e w a n d d i f -f e r e n t S - N c u r v e . T h is d a m a g e i s a fu n c t i o n o f th e n u m b e r o fc yc l e s o f ope r a t i on a t t ha t s t r e ss a m p l i t ude , a nd a l s o a f unc t i ono f t h e t o ta l n u m b e r o f c y c l e s t h a t w o u l d b e r e q u i re d t o p r o d u c ef a il u r e o f a n u n d a m a g e d s p e c i m e n a t t h a t st re s s a m p l i t u d e ( R e fE.S. P alm a, Department o f Mechanical Engineer ing, Federal Univer-s i ty of Minas Gerais--U FM G, A v. Ant6nio Carlos , 6627, 31260-90 lBe lo H or izonte - -MG , Braz il .

10 , 11 ). M a ny d i f f e r e n t c um u l a t i ve da m a ge t he o r i e s ha ve be e np r o p o s e d i n o r d e r to d e t e rm i n e f a t ig u e d a m a g e c a u s e d b y o p -e r a t i on a t a ny g i ve n s t r e s s l e ve l ( R e f 11 - 14 ) .1 . 1 . 1 L i n e a r D a m a g e A c c u m u l a t i o n R u l e( P a l m g r e n - M i n e r R u l e )

T h i s t h e o r y m a y b e d e s c r i b e d b y u s i n g t h e s c h e m a t i c S -Nc u r ve s h ow n i n F i g . 1 . I t ha s be e n p r opo s e d t ha t f a t igue f r a c t u r eund e r s t r e s s t ~a i i s a r e s u l t o f a l i ne a r a c c um ul a t i on o f pa r t i a l f a -t i gue da m a ge D i , w h i c h i s l ine a r l y p r o po r t i ona l t o t he r a ti o o f n it o Ni :

n iD i = -~- , n i < N i (Eq 1)

w h e r e n i is t h e n u m b e r o f c y c l e s o f o p e r a t i o n u n d e r t h is s t r e ssa m p l i t u d e a n d N i i s th e t o t a l n u m b e r o f c y c l e s t h a t w o u l d p r o -duce a f a i lure a t t ha t s t r e s s l eve l (Re f 11) .

I f the s t r e ss a m p l i t ude i s c ha nge d , a ne w pa r t i a l da m a ge i sc a l c u l a t e d f o r th i s ne w a m pl i t ude l e ve l , a nd t he a pp r op r i a t e N ii s ta k e n f r o m t h e S - N c u r v e . T h u s , o p e r a t i o n o v e r a s p e c t ru m o fd i f f e r e n t s t r e s s l e ve l s r e s u lt s i n a da m a ge f r a c t i on D i f o r e a c h o f

" 0. m

E 100 MPa. U nde r o a = 98 MPa, just on e speci-men ran out. On ly two speci mens with P0 = 12.4% ran out un-der o a = 40 or 44 MPa. All spe cimens failed before re aching 2x 106 cycles w hen subje ct to stress amplitude s Ga > 46 MPa .

4 . D i s c u s s i o nAs expected, the experimental results showed that fatigue

endurance limits decreased with increasing porosity. Theywere ver y similar to those found in the literature (Re f 2-9). It iswell known that the decrease of e ndurance limit with porosityis due to a reduction in true load-bearing cross-sectional area,

0Q .v" 0DQ.E