Effect of Surface Roughness on Low-Cycle Fatigue Behavior of Type 304 Stainless Steel P. S. MAIYA

AND

D. E. BUSCH

The effects of s u r f a c e roughness on the l o w - c y c l e fatigue life of Type 304 s t a i n l e s s s t e e l at 593~ in a i r have been investigated. It is o b s e r v e d that, at a s t r a i n r a t e of 4 x 10 -z s "1 and a total s t r a i n r a n g e of 1 pct, the fatigue life (Nf cycles) d e c r e a s e s with an i n c r e a s e in s u r f a c e r o u g h n e s s . Information on c r a c k growth vs s t r a i n c y c l e s has been generated, as a function of s u r f a c e r o u g h n e s s , by the m e a s u r e m e n t of s t r i a t i o n s p a c i n g on f r a c t u r e d s u r f a c e s of s p e c i m e n s t e s t e d to f a i l u r e . Crack propagation follows the In a ~ : N (where a is the c r a c k length after N s t r a i n c y c l e s ) r e l a t i o n for longer s p e c i m e n fatigue l i v e s (Nf > 2700 c y c l e s ) and d e p a r t s f r o m In a c c N for s h o r t e r fatigue l i v e s . A quantitative e s t i m a t e is made of the number of c y c l e s No(R) to g e n e r a t e a c r a c k length equal to 0.1 mm (~ 1 grain diam). The initial s u r f a c e roughness significantly affects only the initiation component of s p e c i m e n life time. The effect of roughness on c r a c k initiation is d e s c r i b e d by No(R) = 1012R -~ where R is the s u r f a c e roughness ( r o o t - m e a n - s q u a r e value) in m i c r o n s .

F A T I G U E c r a c k s a r e initiated, in m o s t c a s e s , at the s u r f a c e s of s t r u c t u r a l components, and fatigue life is, t h e r e f o r e , s e n s i t i v e to s u r f a c e conditions. The i m p o r tance of tile r o l e of s u r f a c e s in fatigue c r a c k initiation has been d i s c u s s e d by s e v e r a l i n v e s t i g a t o r s . ~'2 I m p r o v e m e n t of s u r f a c e finish prolongs the fatigue life. %4 The magnitude of s u r f a c e - r o u g h n e s s effects on fatigue strength, however, depends on many v a r i a b l e s , such as fatigue t e s t i n g conditions (low-cycle or h i g h - c y c l e fatigue), method of producing s u r f a c e finish, t e m p e r a ture, and testing environment. Raymond and Coffin 5 have o b s e r v e d that, in a smooth s p e c i m e n subjected to axial push-pull cyclic loading, l o w - c y c l e fatigue f a i l u r e a p p e a r s to be g r e a t l y influenced by the changes in s u r f a c e topography, which include intensification in notch g e o m e t r y , s u r f a c e - r o u g h n e s s effects, and l o c a l ized s u r f a c e s l i p s t e p s . F o r example, they w e r e able to i m p r o v e the fatigue life significantly by i n t e r m i t t e n t machining to r e m o v e the changes in s u r f a c e topography that developed during fatigue testing. Because the s u r face is the c r a c k - i n i t i a t i o n site, with a few exceptions, it is important to quantitatively delineate the r o l e of s u r f a c e i r r e g u l a r i t i e s in the c r a c k - i n i t i a t i o n p r o c e s s . A g r e a t m a j o r i t y of the l o w - c y c l e fatigue t e s t s (axial push-pull c y c l i c loading) on s t r u c t u r a l m a t e r i a l s a r e being p e r f o r m e d on h o u r g l a s s s p e c i m e n s with s u r faces smoothed by final polishing or with a s u r f a c e finish in the longitudinal d i r e c t i o n . Inasmuch as r e a l s u r f a c e s a r e r a r e l y smooth, the d a t a obtained f r o m smooth s p e c i m e n s m a y not r e f l e c t the s u r f a c e conditions existing in actual s t r u c t u r a l components. E l e v a t e d - t e m p e r a t u r e l o w - c y c l e fatigue t e s t s a r e being conducted on Types 304 and 316 s t a i n l e s s s t e e l to o b tain design data for the l i q u i d - m e t a l f a s t - b r e e d e r r e actor applications. The p r e s e n t paper is concerned with the influence of s u r f a c e roughness on the lowcycle fatigue life of and c r a c k - i n i t i a t i o n and c r a c k propagation p r o c e s s e s in Type 304 s t a i n l e s s s t e e l at 593~ P. S. MAIYAand D. E. BUSCH are Metallurgistand Senior Technician, respectively,MaterialsScience Division,Argonne National Laboratory, Argonne, Ill. 60439. Manuscript submitted December9, 1974. METALLURGICALTRANSACTIONS A

EXPERIMENTAL

PROCEDURE

The s t a r t i n g m a t e r i a l is Type 304 s t a i n l e s s s t e e l (Heat 9T2796) in the form of h o u r g l a s s - s h a p e s p e c i mens (6.35 mm minimum diam). The c h e m i c a l c o m position of the alloy is shown in Table I. In the a s f a b r i c a t e d condition,* the s p e c i m e n s have a smooth *The specimenswerefabricatedat TascoTooland SpecimenCompany,Oak Ridge,Tennessee. s u r f a c e finish (an r m s value b e t t e r than 0.3 ~m) p a r a l l e l to the longitudinal d i r e c t i o n . Several h o u r g l a s s s p e c i m e n s w e r e solution annealed in evacuated quartz tubes for 30 min at 1092~ and aged for 1000 h at 593~ The aging t r e a t m e n t produces a r e l a t i v e l y m o r e s t a b l e m i c r o s t r u c t u r e in the s p e c i m e n s and has a s l i g h t b e n e ficial effect on the l o w - c y c l e fatigue s t r e n g t h . 6 F o l l o w ing the heat t r e a t m e n t , the s u r f a c e s of the s p e c i m e n s w e r e t r e a t e d to produce v a r i o u s d e g r e e s of s u r f a c e roughness. The s p e c i m e n was mounted on a lathe and turned at an a r b i t r a r i l y chosen speed of 600 r p m . A silicon c a r b i d e paper of a chosen g r i t was applied gently so that c i r c u m f e r e n t i a l grooves w e r e produced on the gage length of the s p e c i m e n . The p r o c e d u r e is capable of c o n s i s t e n t l y producing the s a m e d e g r e e of s u r f a c e roughness on s e v e r a l s p e c i m e n s . F o r longitudinal grooves, the s p e c i m e n was held s t a t i o n a r y while the grinding paper was applied along the longitudinal d i r e c t i o n . To obtain v e r y smooth s u r f a c e s , mechanical polishing or e l e c t r o p o l i s h i n g techniques

Table I. Chemical Analysis of Type 304 Stainless Steel (Heat 9T2796)

Element C P S Cu Si Mn Cr Ni Mo Fe

Analysis,WtPet 0.059 0.033 0.016 0.25 0.44 1.26 18.60 9.50 0.35 Ba/anee

VOLUME 6A, SEPTEMBER 1975-1761

w e r e u s e d . The e l e c t r o p o l i s h i n g of the gage s e c t i o n was done in a s o l u t i o n c o n s i s t i n g of 63 cc o r t h o p h o s p h o r i c a c i d and 15 c c s u l p h u r i c a c i d at a c u r r e n t d e n s i t y of ~ 4 5 0 to 500 m A p e r cm 2. The r e m a i n d e r of the s p e c i m e n was c o a t e d with m i c r o s h i e l d l a c q u e r to p r e v e n t e l e c t r o p o l i s h i n g of the m a t e r i a l . The s e v e r i t y of s u r f a c e r o u g h n e s s p r o d u c e d in v a r i o u s c a s e s is i l l u s t r a t e d b y the s e r i e s of s c a n n i n g e l e c t r o n m i c r o g r a p h s shown in F i g . 1. The depth of s u r f a c e r o u g h n e s s R for s p e c i m e n s g r o u n d on s i l i c o n c a r b i d e p a p e r was m e a s u r e d with a T a l y s u r f i n s t r u m e n t that t r a c e s the p r o f i l e of the s u r f a c e i r r e g u l a r i t i e s . E x a m p l e s of s u r f a c e p r o f i l e s r e c o r d e d by t h i s i n s t r u m e n t for the t e s t s p e c i m e n s a r e shown in F i g . 2. The s u r f a c e roughness is described by a root-mean-square (rms) v a l u e o r b y a m a x i m u m v a l u e for the depth of the s u r f a c e g r o o v e s . The v a l u e s for R a r e o b t a i n e d f r o m an a n a l y s i s of s u r f a c e p r o f i l e s such a s t h o s e d i s p l a y e d in F i g . 2. F o r the s m o o t h s p e c i m e n s ( m e c h a n i c a l l y o r e l e c t r o l y t i c a l l y p o l i s h e d ) , the s u r f a c e r o u g h n e s s was m e a s u r e d b y using i n t e r f e r o m e t r i c t e c h n i q u e s .

T a b l e II s u m m a r i z e s the v a r i o u s v a l u e s * of R o b *For the purpose of relatingR to fatigue behavior, the rms value was used throughout. It is important to note that the value of R does not take into account the shape of the surface grooves, microstructural changes, and other effects associated with the procedure adopted for producing different degrees of surface roughness,

t a i n e d on the fatigue s p e c i m e n s b y using the d i f f e r e n t surface preparation procedures.

Table I I. Surface Roughness Obtained by Various Methods

Surface Preparation Ground on silicon carbide paper 600 grit 240 grit 50 grit Polished Mechanically Electrolytically

Surface Roughness R, pm (rms)

0.045 0.48 2.9

0.18 1.8 8.9

~0.0075 ~0.0075

<0.027 <0.027

(a)

(b)

(c)

(d)

J IO0

Maximum Depth of Surface Grooves, ,urn

.m !

Fig. 1 - - S c a n n i n g e l e c t r o n m i e r o g r a p h s of s p e c i m e n s with d i f f e r e n t s u r f a c e r o u g h n e s s . (a) M e c h a n i c a l l y p o l i s h e d . (b) g r o u n d on

600-grit paper, (c) ground on 240-grit paper, and (d) ground on 50-grit paper. 1762-VOLUME 6A, SEPTEMBER 1975

METALLURGICAL TRANSACTIONS A

The fatigue t e s t s w e r e p e r f o r m e d in a i r in s e r v o c o n t r o l l e d , h y d r a u l i c a l l y actuated fatigue m a c h i n e s in axial s t r a i n c o n t r o l at a s t r a i n r a t e of 4 x 10 -3 s -~ and a total s t r a i n r a n g e of 1 pct. Thus, the f r e q u e n c y of loading, t e m p e r a t u r e , and e n v i r o n m e n t w e r e the s a m e for all s p e c i m e n s . The e x p e r i m e n t a l p r o c e d u r e for fatigue t e s t i n g is s i m i l a r to that d e s c r i b e d by Slot,

e t a l . ~ All t e s t s w e r e p e r f o r m e d to f a i l u r e (complete

s e p a r a t i o n ) . The f r a c t u r e d s u r f a c e s w e r e e x a m i n e d by s c a n n i n g e l e c t r o n m i c r o s c o p y . RESULTS AND DISCUSSION It is observed that fatigue life depends on the s u r face roughness and decreases with an increase in s u r face roughness for the circumferential grooves. The surface fatigue factor (where is the number of cycles to failure for a specimen with roughness R, and is the fatigue life of a specimen with a smooth surface) as a function of R is shown in Fig. 3. For the extreme surface roughness R = 2.9 ~m, the fatigue life is reduced by a factor of two. This is significant considering the fact that in all cases the two tests run under the same conditions yield the same results to within :~7 to 15 pcto Scatter of this magnitude is typical when a much greater number of specimens are tested under the same conditions. It is also clear that longitudinal grooves have a much less detrimental effect on fatigue life (Fig. 3) and therefore will not be discussed further. The f r a c t u r e d s u r f a c e s have b e e n e x a m i n e d by s c a n ning e l e c t r o n m i c r o s c o p y , and all s p e c i m e n s showed fatigue s t r i a t i o n s ; e x a m p l e s a r e shown in Fig. 4. F r o m s t r i a t i o n s p a c i n g m e a s u r e m e n t s and a s s u m i n g that each s t r i a t i o n is c a u s e d by one s t r a i n cycle, it is p o s s i b l e to obtain i n f o r m a t i o n on c r a c k length v s s t r a i n c y c l e s , s's The p r o c e d u r e for o b t a i n i n g c r a c k - g r o w t h m e a s u r e m e n t s c o n s i s t s of taking a s e r i e s of s c a n n i n g e l e c t r o n m i c r o g r a p h s (typically 25 to 30 for each s p e c i m e n ) f r o m the c r a c k - i n i t i a t i o n site to the final t e n s i l e r u p t u r e r e g i o n . The s c a n n i n g e l e c t r o n m i c r o g r a p h s a r e r e c o r d e d in such a way that o v e r l a p p i n g m i c r o f r a c t o g r a p h i c f e a t u r e s a r e d i s c e r n i b l e in s u c c e s s i v e p h o t o m i c r o g r a p h s so that the e n t i r e f r a c t u r e s u r f a c e

Nf(R)/Nf(S)

5o.8 ,.

I

(a)

254H.m

5.08p.m

.

.

.

.

,

.

0.508/~m

L

(c)

Fig, 2--Surface roughness profile of Type 304 stainless steel. (a) R = 2,9#m, (b) R = 0.48 #m, and (c) R = 0.045 ~m.

. . . . . .

"1

Nf(R)

Nf(S)

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Fig. 3--Fatigue life of Type 304 stainless steel as a function of surface roughness at 593~

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STRAIN

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RATE =4.02 xlO-3sec -t

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TOTAL STRAIN RANGE =1% o CIRCUMFERENTIAL GROOVES

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SURFACE ROUGHNESS, Fm (RMS VALUE) METALLURGICALTRANSACTIONS A

VOLUME 6A, SEPTEMBER t975-1763

(a)

, IOOp, m , |

!

(b)

(c)

t 40/

m

Fig. 4--Scanning electron micrographs of the fracture surfaces of specimens. (a) R < 0.027 #m, (b) R = 2.9 ~m, and (c) R = 0.045 ~m. in the d i r e c t i o n of c r a c k p r o p a g a t i o n can be displayed. F a t i g u e s t r i a t i o n s have fine s p a c i n g only in the v i c i n ity of c r a c k i n i t i a t i o n . However, within a few tenths of a m i l l i m e t e r f r o m the c r a c k - i n i t i a t i o n site, the s t r i a t i o n s b e c o m e v i s i b l e , t y p i c a l l y at a m a g n i f i c a t i o n of 2000 t i m e s . When o b s e r v e d at the a p p r o p r i a t e m a g nification, they a r e v i s i b l e and well defined until the b e g i n n i n g of f i n a l t e n s i l e f a i l u r e . The m e a s u r e m e n t s 1764-VOLUME 6A, SEPTEMBER 1975

of s t r i a t i o n s p a c i n g s a r e m a d e on the p h o t o m i c r o graphs. Plots of In a vs s t r a i n cycles N (where a is the c r a c k length after N cycles), as i n f l u e n c e d by R, a r e shown in Fig. 5. The s t r i a t i o n s w e r e v i s i b l e throughout the e n t i r e c r a c k - g r o w t h a r e a e x a m i n e d , and a l l c y c l e s shown in Fig. 5 a r e taken into a c c o u n t by counting the s t r i a t i o n s . The t r e n d of t h e s e r e s u l t s s u g g e s t s that the n u m b e r of c y c l e s to i n i t i a t e a c r a c k d e c r e a s e s m a r k e d l y with an i n c r e a s e s in R. Some u n d e r s t a n d i n g of the effects of R on fatigue life can be achieved by c o n s i d e r i n g c r a c k - i n i t i a t i o n and c r a c k p r o p a g a t i o n p r o c e s s e s s e p a r a t e l y . The l o w - t e m p e r a t u r e , l o w - c y c l e fatigue, b a s e d on c o n s i d e r a t i o n s of c r a c k i n i t i a t i o n and c r a c k propagation, has b e e n d i s c u s s e d by Manson and by Manson and H i r s c h b e r g , 9'1~ A useful approach has b e e n s u g g e s t e d to e s t i m a t e the n u m b e r of cycles r e q u i r e d to i n i t i a t e a c r a c k of a c e r t a i n s i z e ( e n g i n e e r i n g s i z e c r a c k , e . g . , 0.076 m m length). C o n s i d e r a b l e d i f f i c u l t i e s s t i l l p r e v a i l , howe v e r , and no g e n e r a l a g r e e m e n t e x i s t s for d e f i n i n g what c o n s t i t u t e s a fatigue c r a c k and the n u m b e r of cycles r e q u i r e d to i n i t i a t e such a c r a c k . Recently, Solomon 11 has shown that, in a notched l o w - c y c l e fatigue s p e c i m e n , In a (a is the s u m of notch depth and c r a c k length) is p r o p o r t i o n a l to N at c o n s t a n t t e m p e r a t u r e and f r e q u e n c y of loading. The p r e s e n t r e s u l t s i n d i c a t e that a s i m i l a r r e l a t i o n ship is v a l i d for smooth s p e c i m e n s , but a c o n s i s t e n t deviation from the c r a c k - g r o w t h law a p p e a r s to occur after the c r a c k has p r o g r e s s e d to a length e x c e e d i n g 1 to 2 m m in s p e c i m e n s with R = 0.48 and 2.9 tzm (Fig. 5). Although all c r a c k - p r o p a g a t i o n c h a r a c t e r i s t i c s a r e difficult to u n d e r s t a n d f r o m a m e c h a n i s t i c viewpoint, the In a v s N plots shown in Fig. 5 offer an a p p r o x i m a t e method of d e t e r m i n i n g the n u m b e r of cycles to initiate a c r a c k of length ao by e x t r a p o l a t i o n . I n a s m u c h as the g r a i n d i a m e t e r of the s p e c i m e n s is of the o r d e r of 0.1 m m , it is r e a s o n a b l e to a s s u m e a o = 0.1 m m . This a s s u m p t i o n is c o n s i s t e n t with the o b s e r v a t i o n made by s c a n n i n g e l e c t r o n m i c r o s c o p y : no fatigue s t r i a t i o n s w e r e v i s i b l e in the c r a c k - i n i t i a t i o n zone ~ 0.1 to 0.2 m m f r o m the c r a c k - n u c l e a t i o n site. Thus, one can d e t e r m i n e No, which is the n u m b e r of c y c l e s to i n i t i a t e a c r a c k length ao = 0.1 m m . The a c c u r a c y in N o is, of c o u r s e , affected by the r e p r o d u c i b i l i t y of the fatigue l i v e s and the v a l i d i t y of the c r a c k growth r e l a t i o n , and for the p r e s e n t work it is e s t i m a t e d to be within 15 pct. F o r the c a s e of smooth s p e c i m e n s p r e p a r e d by either m e c h a n i c a l or e l e c t r o p o l i s h i n g t e c h n i q u e s , the fatigue l i v e s and c r a c k growth c u r v e s a r e the s a m e and have a p p r o x i m a t e l y identical crack-initiation lives. The d e c r e a s e in i n i t i a t i o n t i m e with an i n c r e a s e in s u r f a c e r o u g h n e s s R, shown in Fig. 6, may be e m p i r i c a l l y d e s c r i b e d by

No(R) = 1012R -~

[1]

w h e r e R is in m i c r o n s . The effect of R on the total fatigue life is

Y f ( R ) = 1012R -~

+ Np(S),

[2]

w h e r e Np(S) is the n u m b e r of c y c l e s for c r a c k p r o p a gation in a smooth s p e c i m e n . Eq. [2] thus p r e d i c t s the fatigue life of a s p e c i m e n with a r o u g h n e s s R f r o m the smooth s p e c i m e n data. It should a l s o be pointed out METALLURGICALTRANSACTIONSA

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SURFACE ROUGHNESS (rms vQlue) o 2.9 u.m 0 0.48~m Nf=2224

O 0.045 y,m --

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Nf : 2 6 5 4

Nf =4011~

Nf=2

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o <0.027t~m ..(;y (ELECTROPOLISHED, SMOOTH 7 "

SURFACE)

~5

V
p

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Fig. 5 - - E f f e c t of s u r f a c e r o u g h n e s s on p l o t s of In a vsN.

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2400 2800 STRAIN CYCLES, N

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Fig. 6 - - C r a c k - i n i t i a t i o n life of Type 304 s t a i n l e s s s t e e l a s a function of s u r f a c e roughness.

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O.I

SURFACE

that the s h a p e of the g r o o v e s (e.g., r o o t r a d i u s / d e p t h r a t i o ) , s u r f a c e s t r e s s e s , m i c r o c r a c k i n g , and depth of d a m a g e i n t r o d u c e d by g r i n d i n g with s i l i c o n c a r b i d e p a p e r s of d i f f e r e n t g r i t do not e n t e r into the r o u g h n e s s p a r a m e t e r R. Although s o m e of the f a c t o r s such a s s u r f a c e s t r e s s e s m a y not be i m p o r t a n t in l o w - c y c l e fatigue, o t h e r f a c t o r s such a s m i c r o c r a c k i n g and l i n k up of m i c r o c r a c k s m a y w e l l e x p l a i n the e n h a n c e d c r a c k - g r o w t h b e h a v i o r (a s y s t e m a t i c d e v i a t i o n in the c r a c k - g r o w t h b e h a v i o r shown in Fig. 5) o b s e r v e d in s p e c i m e n s containing c i r c u m f e r e n t i a l g r o o v e s (R > 0.5 tzm).

It is a l s o i m p o r t a n t to note that s u r f a c e r o u g h e n i n g in a s m o o t h s p e c i m e n r e s u l t s f r o m s t r a i n cycling, which e v e n t u a l l y l e a d s to the i n i t i a t i o n of a c r a c k . 5 This p l a s t i c i n s t a b i l i t y has b e e n m o d e l e d by L a i r d and K r a u s e 12 using s i m p l e g e o m e t r i c a l a s s u m p t i o n s to c a l c u l a t e N o. Q u a l i t a t i v e l y , if s u r f a c e r o u g h e n i n g is a p r e c u r s o r to c r a c k i n i t i a t i o n in a s m o o t h s p e c i m e n , METALLURGICAL TRANSACTIONSA

I

1.0

I

I

I t I ii

IOD

ROUGHNESS, /.Lm (RMS VALUE)

c r a c k i n i t i a t i o n t i m e can be c o n s i d e r a b l y r e d u c e d by the i n t r o d u c t i o n of the i n i t i a l s u r f a c e r o u g h n e s s . It is o b s e r v e d that, in a s m o o t h s p e c i m e n at 593~ and at a t o t a l s t r a i n r a n g e of 1 pct, c r a c k n u c l e a t i o n and Stage I growth ( c r a c k i n i t i a t i o n ) o c c u p i e s 70 p c t of the total fatigue life. The f r a c t i o n of c r a c k - i n i t i a t i o n l i f e o b s e r v e d in our w o r k for Nf = 4018 c y c l e s is not i n c o n s i s t e n t with the c r a c k - i n i t i a t i o n d a t a r e p o r t e d by L a i r d and Smith 13 for A1 and Hi. F u r t h e r m o r e , Fig. 5 s u g g e s t s t h a t s u r f a c e r o u g h n e s s m a y have an effect on c r a c k p r o p a g a t i o n r a t e in the e a r l y s t a g e of growth. However, the d i f f e r e n c e s o b s e r v e d a r e s m a l l and a r e not c o n s i d e r e d s i g n i f i c a n t . F u t u r e e f f o r t should be d i r e c t e d t o w a r d d i s t i n g u i s h i n g b e t w e e n the g e o m e t r i c a l effects p e r t a i n i n g to s u r f a c e t o p o g r a p h y on c r a c k - i n i t i a t i o n p r o c e s s and the effects r e s u l t i n g f r o m the m a n n e r in which the s u r f a c e r o u g h n e s s is p r o d u c e d . Only the s u r f a c e - r o u g h n e s s e f f e c t s at one s t r a i n r a n g e have b e e n included in the p r e s e n t work. E x a m i n a t i o n of s u r VOLUME 6A, SEPTEMBER 1975-1765

f a c e - r o u g h n e s s e f f e c t s at o t h e r s t r a i n r a n g e s is e x p e c t e d to y i e l d i n f o r m a t i o n on the i m p o r t a n c e of s u r f a c e - r o u g h n e s s at l o w e r s t r a i n r a n g e s . Also, an i n v e s t i g a t i o n will be m a d e on the effects of t e m p e r a t u r e and f r e q u e n c y of c y c l i n g on the c r a c k - i n i t i a t i o n p r o c e s s and the p o s s i b l e e f f e c t s of s u r f a c e r o u g h n e s s on early crack-growth rates. SUMMARY A s e r i e s of l o w - c y c l e fatigue t e s t s has b e e n p e r f o r m e d on Type 304 s t a i n l e s s s t e e l at 593~ to e v a l u ate the i m p o r t a n c e of s u r f a c e r o u g h n e s s . The fatigue l i f e d e c r e a s e s with an i n c r e a s e in s u r f a c e r o u g h n e s s (i.e., with an i n c r e a s e in the depth of c i r c u m f e r e n t i a l g r o o v e s ) . Using the c r a c k - g r o w t h d a t a o b t a i n e d f r o m f a t i g u e - s t r i a t i o n - s p a c i n g m e a s u r e m e n t s , an a t t e m p t has been m a d e to d e f i n e the e f f e c t s of s u r f a c e r o u g h n e s s on t o t a l fatigue l i f e in t e r m s of the c r a c k - i n i t i a tion ( c r a c k n u c l e a t i o n p l u s e a r l y growth a c r o s s one g r a i n ) and c r a c k - p r o p a g a t i o n p r o c e s s e s . The d e c r e a s e in the n u m b e r of c y c l e s r e q u i r e d to i n i t i a t e a c r a c k length of ~ 1 g r a i n d i a m , r e s u l t i n g f r o m s u r f a c e r o u g h n e s s , and the r e l a t i o n s h i p b e t w e e n s u r f a c e r o u g h n e s s and c r a c k i n i t i a t i o n t i m e h a v e b e e n d i s c u s s e d q u a n t i t a tively.

1 7 6 6 - V O L U M E 6A, SEPTEMBER 1975

ACKNOWLEDGMENTS The a u t h o r s a r e g r a t e f u l to A. P. L. T u r n e r for f r u i t f u l d i s c u s s i o n s , and W. F. Burke for his help in p e r f o r m i n g the fatigue t e s t s . The continuing s u p p o r t and e n c o u r a g e m e n t of R. W. W e e k s is a p p r e c i a t e d . W o r k s u p p o r t e d by the U. S. E n e r g y R e s e a r c h and D e velopment Administration. REFERENCES 1. J. C. Grosskreutz and D. K. Benson: Surfaces and lnterfaces II, ]. J. Burke, N. L. Reed, and V. Weiss, eds., p. 61, Syracuse University Press, Syracuse, New York, 1966. 2. A. S. Argon: Cot~osion Fatigzte, O. F. Devereaux, A. J. McEvily, and R. W. Staehle, eds., p. 176, NACE, 1972. 3. W. N. Thomas: Engineering, 1923, voi. 116, p. 449. 4. P.G. Fluck:Proc. ASTM, 1951,vol. 51,p. 584. 5. M. H. Raymond and L, F. Coffin: Trans. ASME, Basic Engineering, Series D, 1963, vol. 85, p. 548. 6. C. Y. Cheng and D. R. Diercks: Met. Trans., I973, vol. 4, p. 615. 7. T. Slot, R. H. Stentz, and J. T. Bering: STP 465, p. 100, ASTM, Philadelphia, Pa., 1969. 8. C. Laird: STP 415, p. 13i, ASTM, Philadelphia, Pa., 1967. 9. S. S. Manson: Exp. Mech., 1965, vol. 5, p. 193. 10. S. S. Manso'n and M. H. Hirschberg: NASA TN-D-3146, Lewis Research Center, Cleveland, Ohio, 1967. 11. H. D. Solomon: Met. Trans., 1973, vol. 4, p. 341. 12. C. Laird and A. R. Krause: Int. J. Fract. Mech., 1968, vol. 4, p. 219. 13. C. Laird and G. C. Smith: Phil. Mag., 1963, vol. 8, p. 1945.

METALLURGICAL TRANSACTIONS A