PDF - shotpeener.com

ERR-FIJ-1289
Structures and Materials
EFFECTS OF MACHINED SliKFACE Il9UGHNESS
ON THE FATIGUE LIFE OF D6ac STEEL
0. N , THOMPSON
RESEARCH &
ENGINEERIN6
EPARTM ENTS
61EhlEiRAL DYNAMldSL
Convair Aerospace Division
P, 0 , Box 748, Fort Worth, Texas 76 701
This wsrk was su3ported by the General Dynanics Research Program,
RDP Task Number: 414-24-511, 414-14-517.
NO? i CES
PAGE
ElGN HAT tON RELEASE
T h i s inforn@tlon i s furnished upon the condition that : t w i l l not be released
outside tka nx?&sorship o f G I DEP (Government-Industry Data Exchange Program)
w i thoot spcci f l c authosl t y o f the GIDEP Opn Ctr (Operations Center).
DISCLAIMER Of Ll ABl L l f Y FROM ACT OF TRANSHlnAL
When Government drawings, s p e c i f i c a t t o n s o r other data are used f o r any purpose
atner than i n cannection w i t h a d e f i n i t e l y r e l a t e d Government procurement operation,
the U p i t a d Stares Government thereby incurs no r e s p o n s i b i l i t y nor aoy o b l i g a t i o n
whatsoever; and the Fact that the Government may have formulated, furnished, or i n
any way supplied tho said drawings, s p e c i f i c a t i o n , or other data, i s not t o be
regarded b y i m p l i c a t i o n o r otherwise as i n any manner l i c e n s i n g the holder o r any
other person or corporation, o r conveying any r i g h t s or permission t o manufacture,
use, or s e l l any patented invention t h a t may i n any way be r e l a t e d thereto.
Dissemination o f sald information does not imply v e r i f i c a t i o n o r endorsement of the
information. The o r i g i n a t o r , i n submitting the material i s a c t i n g I n accordance
w i t h the requirements o f h i s contract, and n e i t h e r the o r i g i n a t o r nor the disseminat o r assumes any l i a b i l i t y t o p a r t i e s adopting any product, process o r p r a c t i c e
I t s presenting the success o r f a i l u r e
based upon the usage o f the information,
o f one (or several) p a r t number(s), model ( s ) , and l o t ( s ) under s p e c i f i c environment
and output requirements, does not imply t h a t other products not herein reported
on arc e i t h e r i n f e r i o r o r superior.
Any compliance by the r e p o r t o r i g i n a t o r w i t h reqtjests from r e c i p i e n t s for more
d e t a i l e d Information i n GIDEP reports originaced under Government contracts w i l l
be considered w i t h i n the scope o f present contractual o b l i g a t i o n s . Compliance
w i t h such requests w i l l be a t the d i s c r e t i o n o f the r e p o r t o r i g i n a t o r and w i l l be
performed w i thout cost or obl i g a t i o n t o the requester unless o t h e m l se negotiated
I n advance.
REPRODUCTION OF THlS REPORT
1
I1
Reproduction o r d u p l i c a t i o n of any p o r t l o o of t h i s r e p o r t I s expressly forbidden,
except by those organizations r e c e i v i n g i t d i r e c t l y from the GlDEP Opn Ctr o r
o r i g i n a t o r , f o r t h e i r i n t e r n a l use o r the use of t h e i r subcontractors. Reproduction
o r display o f a l l o r any p o r t i o n o f t h i s m a t e r i a l f o r any sales, advertising, o r
p u b l i c i t y purposes i s prohibited.
1,
E n t e r t h e e x a c t t i t l e a s I t appecrrs on t h e r e p o r t .
2.
E n t e r t h e program t o which t e a t c o s t s a r e c h a r g e d . F o r t e s t s n o t s o c h a r g e d ( e . g . ,
cuntpany-fr~nded) w r l t e CF o r M u l t i p l e , a s a p p l i c a b l e . Do n o t u s e weapon s y s t e m numbers.
Use g e n e r a l l y f a m i l i a r names.
3.
E n t t i r d a t e s t o t e s t c o m p l e t i o n and r e p o r t c o m p l e t i o n .
4,
E n t e r p a r t name and i d e n t i E i c a t i o n a s a s s i g n e d by o ~ ~ a n i z a t i o n t a g e n coyr i g i n a t i n g
the r e p o r t .
5.
E n t e r t h e o r i g i n a l r e p o r t i d e n t i f i c a t i o n number a s a s s i g n e d .
6.
E n t e r a d e s c r i p t i v e t e r m i d e n t i f y i n g m a t e r i a l t o be found i n t h e r e p o r t . T y p i c a l
examples i n c l u d e : Q u a l i f i c a t i o n T e s t , F a i l u r e A n a l y s i s , V e r i f i c a t i o n T e s t P r o c e d u r e ,
etc.
7.
Deldte t!ii;,-r ' , , u p e r s e d e s W o r "Supplements" by c r o s s i n g o u t t h e word n o t used.
If
ti-=-ward "S.2 ->!?rnentsn i s d e i e t e d , t h e r e p a r t b e i n g s u p e r s e d e d from t h e GIDEP f i l e i s
tc, be removeu" and d e s t r o y e d . F o r c r o s s - r e f e r e n c e p u r p o s e s , d e l e t e "Supersedes" and
rdent-iy the ;eport referenced.
8.
The manuf-actnrkr must be i d e n t i f i e d by name and by t h e H-4 Code d e s i g n a t i o n a s d e f i n e d
by tht: C a t a l o g i n g Handbook, Department of t h e Army Supply B u l l e t i n S . 8. 708-42.
Use
manufacturer-name a b b r e v i a t i o n from t h e approved GIDEP a b b r e v i a t i o n l i s t a s a p p l i c a b l e .
9.
E n t e r t h e M a n u f d c t u r e r P o s t Number i n t h i s box.
0 f o r the nuaeric zero.
10.
Use a
0
f o r t h e l e t t e r 0, and p l a i n
'%is box i s used t o d i f f e r e n t i a t e between g r o u p s o f p a r t s . Normally, a l l p a r t s grouped
a s a s i n g l e ITEM s h o u l d be i d e n t i c a l , b u t o c c a s i o n a l l y t h e l a r g e number of i t e m s p r o h i b i t s g r o u p i n g ( e . g , , s s i n g l e r e p o r t s p e c i f y i n g t e s t r e s u l t s on resistors o f d i f f e r e n t
r a t i n g s ) , Reasonable judgment must b e u s e d . I n most c a s e s , t h e p a r t t y p e s c a n be
separated.
11. . B r i e f t y s u m a r i z e m a t e r i a l d e t a i l e d fn t h e t e x t o f t h e r e p o r t .
12.
E n t e r i n a l p h a b e t i c a l o r d e r t h e s i n g l e symbol c o d i n g f o r e n v i r o n m e n t a l e x p o s u r e a s d e f i n e d
i n Codes f o r E s t a b l i h s i n g I n d e x Numbers, S e c t i o n 9 , P o l i c i e s and P r o c e d u r e s Manual.
13.
E n t e r "Key Words" t o b e used i n computer i n d e x i n g .
14.
E n t e r d a t e m a n u f a c t u r e r was n o t i f i e d . The p a r t m a n u f a c t u r e r muat b e n o t i f i e d by a
copy o f t h e r e p o r t . T h i s d o e s n o t imply t h a t t h e m a n u f a c t u r e r approved t h e t e s t r e e e l l p e r t i n e n t vendor
s u l t s , o n l y t h a t t e s t r e s u l t s were t r a n s m i t t e d t o him.
correspondence with t h e r e p o r t .
15.
E n t e r s i g n a t u r e o f GIDEP R e p r e s e n t a t i v e and d a t e comp
C-XDEP.
16.
E n t e r name o f p a r t i c i p a n t a c t i v i t y . I f t h e t e s t was c o n d u c t e d u n d e r a s u b c o n t r a c t
from t h e p a r t i c i p a n t s u b m i t t i n g t h e r e p o r t , e n t e r t h e name o f t h e s u b c o n t r a c t o r .
17,
T h i s box t o b e c o m p l e t e d by t h e GIDEP A d m i n i s t r a t i o n O f f i c e .
18.
The i n f o r m a t i o n i n t h i s box must a g r e e e x a c t l y w i t h t h e n o m e n c l a t u r e e s t a b l i s h e d i n
S e c t i o n 9 , P o l i c i e s and P r o c e d u r e s Manual, a s d e f i n e d by t h e r e p o r t i n d e x number r e f l e c t e d i n box 19.
19.
E n t e r t h e r e p o r t i n d e x number a s e s t a b l i s h e d from "Codes f o r E s t a b l i s h i n g I n d e x ~ u m b e r s " ,
S e c t i o n 9. P o l i c i e s and P r o c e d u r e s Manual. When t h e g e n e r i c code number i s i d e n t i c a l
w i t h a p r & i o u s s u b m i t t a l , t h e l a s t two d
number.
was s u b m i t t e d t o
e
-t
I
t . OUTLINE, TABLE O C U N T E N T S ,
502,l3.?0.OO-EO-CR
570.55.OO. 00- EO- CR
AES m
--
T
The f a t i g u e l i f e o f 0,100 i n c h t h i c k % a c s t e e l , 220-240 KSI g t r e n g t h lwei
specimens c u t w i t h t h e end o f an end m i l 1 ;as determined f o r V ~ & a ? € i o n s* o f :
1.
( 6 0 GA,
125 tiA, z 2 5 0 AA and z 3 5 0 A.4 s u r f a c e f i n i s h h a v i n g p r o d u c t i o n
shop d e b u r r i n g and vacuum f u r n a c e h e a t t r e a t i n g t o p r e v e n t s c a l e f o r m a t i o n
2.
< 6 0 A4 and z 2 . 5 0 AA s u r f a c e f i n i s h , vacuum f u r n a c e h e a t t r e a t i n g w i t h
hand p o l i s h e d e d g e s ,
3. < 6 0 kA and z 2 5 G AA s u r f a c e f i n i s h w i t h p r o d u c t i o n shop d e b u r r i n g o f
e d g e s , p r o d u c t i o n h e a t t r e a t i n g f o l l o w e d by a s c a l e removal g r i t b l a s t h n g
p l u s shot peeninn.
+ ' ~ n ep r o d u c t i o n shop d e b a r r was t h e predominant c a u s e o f f a i l u r e o f t h e vacuum
h e a t t r e a t e d a s machined bpecimens. Hand p o l i s h i n g t h e e d g e s c a u s e d some improvement i n t h e f a t i g u e l i v e s of t h e s p e c i m e n s h e a t t r e a t e d i n a vacuum w i t h t h e 4 6 0 M
s u r f a c e f i n i s h specimens showing more improvement t h a n t h e &250 tiA s u r f a c e f i n i s h
specimens.
r h e specimens t h a t went t h r o u g h t h e normal p r o d u c t i o n shop d e h u r r i n g ,
" n e t t r e a t i n g , g r l c b l a s t i n g and s h o t p e e n i n g showed a v e r y l a r g e improvement i n
f a t i g u e l i f e w i t h no a p p r e c i a b l e d e m a r c a t i n between t h e f a t i g u e l i v e s of t h e
(60 a\ s u r f a c e f i n i s h specimens and t h e
250 *A s u r f a c e f i n i s h specimens;
;
%
,
L
DAY
14.
1
N. A .
MONTH
/
P
YEAR-
K E Y WORDS F O R I N D E X I N G
REPRODUCTION
PUBLICITY PURPOSES I S FRONIBITED
LIST OF FIGURES
No.
---
1
End M i l l s Used f o r Specimen P r e p a r a t i o n
2
F a t i g u e T e s t Specimen Design
3
As Machined and Heat Treated Specimens
4
( 6 0 AA S u r f a c e F i n f s h a t XSO M a g n i f i c a t i o n
5
"--I25 AA Surface F i n i s h a t X50 M a g n i f i c a t i o n
6
"-250 AA S u r f a c e F i n i s h a t X5O M a g n i f i c a t i o n
7
3350 A4 S u r f a c e F i n i s h a t X50 M a g n i f i c a t i o n
8
FatZgue L f f e of Specimens Cut w i t h End M i l l
Using Tungsten Carbide I n s e r t s Ground F l a t
9
F a t i g u e L i f e of Specimens Cut w i t h End M i l l
Using Round Tungsten Carbide I n s e r t s
10
F a t i g u e L i f e a f Specimens Cut w i t h End M i l l
Using Round ' h n g s t e n Carbide I n s e r t s
11
F a t i g u e L i f e .of Specfmens Cut w i t h High Speed
Cobalt S t e e l End M i l l
12
F a t i g u e L i f e of Specimens Cut w i t h End M i l l
Using Tungsten Carbide I n s e r t s Ground F l a t
13
F a t i g u e L i f e of Specimens Cut w i t h End M i l l
Using Round Tungsten Carbide I n s e r t s
14
F a t i g u e L i f e of Specimens Cut w i t h End M i 1 1
Using Round Tungsten Carbide Inserts
15
F a t i g u e L i f e of Specimens Cut w i t h High Speed
Cobalt S t e e l End M i l l
16
Specimens A f t e r G r i t B l a s t i n g and Shot Peening
iii
LIST OF TABLES
Results a £ Fatigue Test of Specimens Cut
at 27 Inches per Minute with End of End
Mill, Using Tungsten Carblde Inserts
Ground Flat, and Cycled at 40 KSI Mean Stress
Results 05 Fatigue Test of Specimens Cut at
27 Inches Per Minute with End of End Mill,
Using Round Tungsten Carbide Inserts and
Cycled at 40 KSI Mean Stress
Results of Fatigue Test of Specimens Cut at
54 Inches per Minute with End of End Mill,
Using Round Tungsten Carbide Inserts and
Cycled at 40 KSI Mean Stress
Results of Fatigue Test of Specimens Cut at
5.4 Inches per Minute, Using a High Speed
Cobalt Steel End Mill, and Cycled at 40 KSI
Mean Stress
Results of Fatigue Test 3f Specimens Cut at
27 Inches per Minute with End of End Mill,
Using Tungsten Carbide Inserts Ground Flat,
and Cycled at 80 KSI Mean Stress
Results of Fatigue Test of Specimens Cut at
27 Inches per Minute with End of E~idMill,
Using Bound Tungsten Carbide Inserts and
Cycled at 80 KSI Mean Stress
Results of Fatigue Test of Specimens Cut at
54 Inches per Minute with End of End Mill.,
Using Round Tungsten Carbide Inserts and
Cycled at 80 KSI Mean Stress
Results of Fatigue Test of Specimens Cut at
5.4 Inches per Minute, Using a High Speed
Cobalt Steel End Mill, and Cycled at 80 KSI
Mean Stregs
ACKNOWLEDGEMENTS
T h i s r e s e a r c h would not have been p o s s i b l e without t h e a s s i s t a n c e
o f many f e l l o w workers throughout t h e F o r t Worth D i v i s i o n and
my a p p r e c i a t i o n i s extended t o a l l .
Special appreciation i s
extended t o those most c l o s e l y a s s o c i a t e d w i t h t h i s program:
to
1. M. Pemberton, g e n e r a l foreman of t h e machine shop who s o
g r a t u i t o u s l y allowed t h e experimental c u t t i n g and specimen prep a r a t i o n i n h i s shop; t o L. A . Johnson, manufacturing r e s e a r c h
e n g i n e e r , who conducted t h e manufacturing r e s e a r c h and development department p o r t i o n of t h i s c o o p e r a t i v e program; t o R. L, J o n e s ,
p r o j e c t engineering m e t a l l u r g i s t f o r s u p e r v i s i n g t h e f a t i g u e t e s t i n g ; t o Larry Watson, m e t a l l u r g i c a l l a b o r a t o r y mechanic f o r h i s
d i l i g e n c e i n keeping t h e t e s t machines loaded; and t o 2. E. Wolanski,
p r o j e c t engineering m e t a l l u r g i s t f o r h i s microscopic examination
of t h e s u r f a c e f i n i s h e s .
Pares manufactured for incorporation into military aircraft are
required to meet a prescribed surface finish in the "as machined"
condition regardless of subsequent processing.
Hand finishing
is required after most machining operations in order for the part
tn pass inspectton. This research was conducted to determine how
surface roughness af the "as machined" parts, without hand finishing, affected the fatigue life.
Only surface roughness due to machining with the end of an end
mill was investigated and only specimens of D6ac steel heat
treated to 220-240 KSI were tested.
(69,
125,
* 250 and
Groups of specimens having
350 a r i t h e t i r : average roughness
height rating, as measured by the method of ANSI B46.1, were made
using production shop deburring techniques and vacuum heat treati
No variation in fatigue life between groups was observed,
1
when tested in tension-tension fatigue, due to the predominance
of failures resulting from the deburr tool marks.
Additional specimens, as above, having 1 60 AA and
250 AA
surface finish but with hand polished edges were tested, Thcae
showed improved fatigue lives over the deburred edge specimen
fatigue lives with the
the most gain,
(
60 AA surface finish specimens showing
A final set of speciaens, sane having
4
60 AA as machined sur-
face roughness and some having e 250 AA as machined surface
r o u g h n e s s , were made using all production subsequent processes of
deburring, heat treating, grit blasting and shot peening.
The
Fatigue lives for these specimens showed a large improvement over
the fatigue Lives of specimens having as machined surface. No
appreciable demarcation between the fatigue lives of the specimens
having
< 60 AA as machined surface roughness and the specimens
having e 2 5 0 AA as machined surface roughness was found when the
specimens were grit blasted and shot peened prior to te8ting.
This research indicated that inspection and acceptance or rejection of parts in the as machined condition is meaningless and
the surface roughness at that stage has little or no effect on
the Earigue life of the finished part if subsequent processes such
as grit blasting and shot peening are applied. A new surface
finish criterion needs tc be established for the acceptance of
parts in their final, ready for use condition and a11 4s machined
surface finish inapectiors should be relaxed.
T h i s r e s e a r c h c o n s i s t e d of a c o o p e r a t i v e e f f o r t by t h e Research
and Engineering Department and the Manufacturing Research and
De"rs10prnent Department t o determine how machined s u r f a c e roughness r e l a t e s t o f a t i g u e l i f e of D6ac s t e e l h e a t t r e a t e d t o 220-
240 KSP.
Much hand f i n i s h i n g i s done throughout t h e a i r f r a m e
i n d u s t r y t~ reduce t h e machined s u r f a c e rgughness h e i g h t r a t i n g
t o some c o n t r a c t u a l l y s p e c i f i e d value.
A logical question e x i s t s ;
D Q ~ St h i s hand work and i t s added c o s t improve t h e f a t i g u e l i f e
and s t r u c t u r a l i n t e g r i t y of t h e a i r f r a m e o r i s i t wasted e f f o r t ?
S u r f a c e f i n t s h a s used i n t h i s r e p o r t p e r t a i n s t o t h e macroscopic
t o p o g r a p h y of machined s u r f a c e s with s p e c i a l emphasis on t h e sur-
faces of p a r t s manufactured f o r use i n a i r f r a m e c o n s t r u c t i o n .
The
v a r i a t i o n i n topography from a mean l i n e along any p r o f i l e on t h e
s u r f a c e of i n t e r e s t i s a measure of t h e s u r f a c e t e x t u r e .
t e x t u r e i n c l u d e s roughnesr, w a r i n e s s , l a y and f l a w s ,
Surface
Each of
t h e s e terms have been d e f i n e d f ~ rd e t a i l i n A N S I B46.1, "Surface
Text;ure" (Reference I) and w i l l b e used w i t h t h a t same meaning
herein.
T h c r e a r e several reasons f o r c o n t r o l l i n g t h e s u r f a c e t e x t u r e of
airplane part
B U C ~a s :
appearance
a d h e s i v e bonding o r sealing r e q u i r e m e n t s
r e d u c t i o n i n aerodynamic d r a g of exposed p a r t s
r e d u c t i o n i n f r i c t i o n between p a r t s d e s i g n e d t o
have r e l a t i v e motion
improvement i n f a t i g u e l i f e and r e l i a b i l i t y ,
I n t e r n a l s t r u c t u r e (such as wing s p a r s , wing s k i n s , bulkhead webs,
l o n g e r o n s , f i t t i n g s , e t c . ) t h a t a r e hidden from view, a r e n o t
bonded o r do n o t r e q u i r e s e a l i n g and have no r e l a t i v e motion o r
r u b b i n g w i t h r e s p e c t t o a d j a c e n t p a r t s r e q u i t e c a n t r o l of s u r f a c e
f i n i s h o n l y f o r improvement of f a t i g u e l i f e and r e l i a b i l i t y .
A
v e r y l a r g e p e r c e n t a g e o f t h e s u r f a c e s of p a r t s w i t h i n a n a i r c r a f t
f a l l i n t o t h i s category.
Modern h i g h performance a i r c r a f t d e s i g n s u t i l i z e i n t e g r a l s t i f f e n e r s
on s k i n s , bulkhead webs, e p a r s , e t c . t o s a v e w e i g h t and e l i m i n a t e
mechanical f a s t e n i n g of s t i f f e n e r s t o webs.
A stiffener/skin
combination c u t as a u n i t h a s a h i g h e r b u c k l i n g load t h a n a s i m i l a r
s t i f f e n e d s k i n i n which t h e s t i f f e n e r s are a t t a c h e d a t i n t e r v a l s
by mechanical f a s t e n e r s ,
T h i s i s due t o t h e r i g i d c o n t i n u o u s
a t t a c h m e n t o f t h e i n t e g r a l s t i f f e n e r t o t h e web and t h e i n c r e a s e d
end f i x i t y .
The c o s t of machining t h e i n z s g r a l l y s t i f f e n e d web,
t o g a i n the added e f f i c i e n c y , i s a n advantageous t r a d e - o f f w i t h
t h e lower e f f i c i e n c y and t h e c o s t of d r i l l i n g h o l e s i n s h e e t m e t a l
and a t t a c h i n g t h e s t i f f e n e r s w i t h mechanical f a s t e n e r s ,
As a
r e s u l t , rhousands of square f e e t of machined s u r f a c e s a r e
generated.
S p e c i f i c a t i n n s r e q u i r e t h e s e s u r f a c e s t o be machined
to some predetermined s u r f a c e f i n i s h r a t i n g ,
I n t e g r a l l y s t i f f e n e d p a r t s a r e predominantly machined from p l a t e
s t o c k by "pocket nilling" w i t h an end m i l l .
The web i s c u t w i t h
t h e end of t h e end m i l l whereas t h e u p s t a n d i n g s t i f f e n e r l e g i s
formed by c u t t i n g w i t h t h e s i d e of t h e end m i l l .
The " t o o l mark"
p a t t e r n g e n e r a t e d by t h e end o f t h e end m i l l i s very d i f f e r e n t
from t h e p a t t e r n g e n e r a t e d by t h e s i d e of t h e end m i l l .
p a t t e r n should be c o n s i d e r e d a s a s e p a r a t e problem.
Each
Only t h e
r e l a t i o n between f a t i g u e l i f e and s u r f a c e roughness g e n e r a t e d by
t h e e n d of t h e end m i l l was c o n s i d e r e d i n t h i s r e s e a r c h .
There i s a g e n e r a l b e l i e f t h a t a p a r t w i t h a smooth s u r f a c e s h o u l d
have a l o n g e r f a t i g u e l i f e t h a n a s i m i l a r p a r t w i t h a rough s u r f a c e
when exposed t o t h e same c y c l i c l o a d i n g c o n d i t i o n s .
As a result,
s u r f a c e roughness l i m i t s a r e imposed on " f a t i g u e c r i t i c a l " p a r t s .
V i r t u a l l y a l l o f t h e p r i m a r y s t r u c t u r e of a h i g h performance a i r plane is classed a s "fatigue c r i t i c a l . "
F a t i g u e c r i t i c a l as u s e d
h e r e i n i n d i c a t e s t h a t a p a r t , when i n s e r v i c e , w i l l be s u b j e c t e d
t o a c y c l i c s t r e s s spectrum c a u s i n g f a t i g u e f a i l u r e t o b e t h e
primary concern i n d e t e r m i n i n g t h e margin of s a f e t y a l l o w e d for
the part.
The ideal design is one that: will provide the required
fatigue l i f ~wF;h
the least weight and lowest manufacturing cost.
The question a r i s e s as "how does surface roughness relate to
fatigue life and what criterion should be used to set the surface
finish requirements for fatigue critical parts."
The procurement
specification for the F-111 airplane required that all fatigue
critical machined surfaces be finished to a 63 (or less) microinch
arithmetical average roughness height rating per MIL-STANDABD 10A.*
The 1970 edition of the Air Force Systems Command Design Handbook
(AFSC DH 2 - 1 ) Chapter 2 , Section 2A, Design Note 2A3, Paragraph 4
t
Surface Roughness states verbatem:
"In predicting useful life, consider surface roughness of
individual components that are subject to repeated stresses,
the tension component of which is 50% of the material
specification minimum yield strength or higher, using
applicable'stress concentration factors, Those components
considered to be critical in fatigue must have a surface
roughness not to exceed 63 AA (arithmetical average) as defined by ANSI ~ 4 6 . 1 * or must be shot peened, with a surface
roughness prior to peening not to exceed 1 2 5 AA.
*See Appendix A
Surface
should be f r e e of d e f e c t s such as gouges, t o o l marks,
c s a t c h e s , or s i m i l a r s u r f a c e imperfecrions."
An examination of t h e i n t e n t of AXTI ~ $ 4 6 ~i s1 i n o r d e r b e f o r e t h i s
s p e c i f i c a t i o n , f o r measuring s u r f a c e roughness, i s made p a r t of
a s u r f a c e f i n i s h requirement.
a c c o r d i n g to kW
The measure of roughness made
. I i s t h e a r i t h m e t i c a l average d e r i v a t i o n
from t h e c e n t e r l i n e through t h e s u r f a c e p r o f i l e .
The c e n t e r l i n e
i s t h e l i n e f o r which t h e sum of t h e enclosed a r e a s between t h e
l i n e and t h e s u r f a c e p r o f i l e i s z e r o surmned a l o n g t h e l e n g t h of
p r o f i l e being measured.
The a r i t h m e t i c a l average roughness h e i g h t
r a t i n g Y i s t h e n given a n a l y t i c a l l y by
where
y
=
1=
x
=
,
t h e o r d i n a t e between t h e c e n t e r l i n e and t h e
profile
l e n g t h of p r o f i l e being measured
v a r i a b l e d i s t a n c e from t h e s t a r t i n g p o i n t on
as illustrated
t h e l i n e t o t h e end p o i n t
f,
be low
Electronic, stylus activated, direct reading measuring instruments
are available and are usually used ta perform the integration as
t h e stylus traverses the path to be evaluated.
The arithmetic average roughness height rating supplies no information as to the notch acuity at the bottom of the profile or the
depth of an individual notch with relation to adjacent peaks.
For a symmetrical periodic wave form profile such as
the mean line is located at h/2 and the arithmetic mean roughness
height rating can be shown to be h/4 for both cases even though
the notch effect is vastly different.
The writers of AN~IB46.lprepared the specification for use on
surfaces having relative motion as a means of controlling friction
and wear and had no intention for the specification to be used as
a measure of resistance to failure by fatigue. To quote verbatfrom ANSI £345.1
Para. D-4 ' Q a r k i n g s u r f a c e s such a s bearings, p i s t o n s , and gears
are t y p i c a l of surfaces f o r w h k h optimum performance
may r e q u i r e c o n t r o l of t h e s u r f a c e c h a r a c t e r i s t i c s i n
accordance w i t h t h e procedure o u t l i n e d i n t h e foregoing
standard.
Nonworking s u r f a c e s such a s t h e w a l l s of
transmission c a s e s , crank c a s e s , o r d i f f e r e n t i a l housings
seldom r e q u i r e any s u r f a c e c o n t r o l such a s t h a t with
which t h i s standard i s concerned, t h e only exceptions
i n t h e s e i n s t a n c e s being r e s t r i c t i o n s t h a t may be
necessary f o r process c o n t r o l and f i n i s h required f o r
sake of appearance."
I t should be s t a t e d t h a t a r i t h m e t i c average roughness h e i g h t r a t i n g
p e r A N S I B46.1 cannot be used a s a c r i t e r i o n f o r measuring t h e
resistance sf a part to fatigue failure.
To f u r t h e r i l l u s t r a t e ,
consider a s u r f a c e generated by t h e s i d e of a n end m i l l .
If alumi-
num a l l o y i s being c u t , it i s economical t o i n c r e a s e t h e machining
feed r a t e t o t h e e x t e n t t h a t a f l u t e d s u r f a c e w i l l r e s u l t a s shown
exagerated below.
The radius of t h e notch is large and i s , in effect, the radius
of the cutter. Theoretical and photoelastic stress analysis show
that the large radius causes a low stress concentration and that
the peaks have little or no influence on the stress at the bottom
of the notch. That is, the tops of the peaks can be removed thereby
changing the roughness height rating without altering the stress
at the bottom of the notch.
Now,
consider a surface finish pattern to be an exact inversion of
the previous pattern.
The arithmetic average roughness height rating is exactly the same
as for the previous case. However, in this case the notch has a
sharp root radius resulting in a high stress concentration factor.
Fatigue cracks originate at points of high stress. The high stress
concentration caused by t h e sharp notch will l m e r the fatigue life
much below the fatigue life for the rounded notch in the first
illustration. Thus, a wide range of surface profiles can have a
wide range of stress concentrations and resulting fatigue lives
even though the arithmetic average roughness height ratings are
identical.
8
l7
IZ t h e above observations and conelusions are accepted as valid,
q i ~ e s t i a n sarise such a$:
I.
How does surfsee roughness affect fatigue life?
2.
Is the hand finishing to achieve a smooth finish
required per specificaticn, as measured by ANSI
046.1 procedures, a benefit to fatigue life or an
unnecessary expenditure?
The only way to answer these questions was to resort to controlled
laboratory tests wherein the surface roughness height rating was
varied in steps and the fatigue life determined for each step. The
only meaningful evaluation of fatigue life was to generate complete
stress-cycles to failure curves ( S - N curves), varying from the
t.2 - L A - + .
IlL;E411GZ3L
- ~ n l -
PTCSK
JLLGBJ
,...rrrw&rrrl
GApGbCCU
4-
ALI
.rA.rrrlan
J G L V A C G
CLU
+ha
b L L G
aorl..rs-na
F L L U U L W L L b F
14-4
t
& L L I I & b
stress, and comparing the resulting curves,
The hilitial plan was only to evaluate "as machined'' surface roughness since the specifications require the finish to be measured in
the "as machined" condition.
Special care was to be exercised to
prevent damage to the surfaces during heat treatment, This fnformation would answer the initial question of how machined surface
roughness affects fatigue life but the answer would have limited
use.
P a r t s t h a t are w e l d e d a r e g r i r b l a s t e d a f t e r welding to remove
s c a l e p r i o r to I n s p e e t i o n of t h e w e l d .
In actual practice a l l
s t e e l p a r t s r e c e i v i n g a produetion heat treatment r e c e i v e some
degree of o x i d a t i o n a s t h e p a r t i s moved from t h e f u r n a c e thrcugh
t h e a i r t o t h e quench bath.
T h i s s c a l e i s subsequently removed by
g r i t blasting and t h e n t h e p a r t s a r e s h o t peened.
During each g r i t
b l a s t o p e r a t i o n t h e g r i t erodes t h e s u r f a c e t o t h e e x t e n t t h a t
m e t a l i s removed.
Each g r i t b l a s t o p e r a t i o n removes =ore m e t a l ,
A f t e r s e v e r a l such g r i t b l a s t o p e r a t i o n s t h e r e is l i t t l e resemblance
of t h e f i n a l s u r f a c e t o the "as machined" s u r f a c e .
For t h i s
r e a s o n , a r e a l i s t i c e v a l u a t i o n of f a t i g u e l i f e should b e made on
p a r t s having t h e i r f i n a l s u r f a c e p r e p a r a t i o n r a t h e r than on p a r t s
i n t h e ''as machined" c o n d i t i o n .
Likewise, s u r f a c e finish acceptance
c r i t e r i a should be based on "as used" s u r f a c e f i n i s h r a t h e r than
"as machined" s u r f a c e f i n i s h .
To e n l a r g e t h e scope and provide u s a b l e d e s i g n d a t a , t h i s r e s e a r c h
was extended t o compare t h e f a t i g u e l i f e of t h e "as machined" and
h e a t t r e a t e d specimens w i t h t h e f a t i g u e l i f e o f s i m i l a r l y prepared
specimens w i t h a subsequent g r i t b l a s t p l u s s h o t peen t r e a t m e n t .
DXSCFBSO
IW AND RESUI,TS
The i n i t i a l purpose of th3.s r e s e a r c h was t o determine how s u r f a c e
roughness, of a s u r f a c e c u t with t h e end of an end m i l l , r e l a t e d
t o fatigue l i f e ,
Only D6ac s t k e l machined i n t h e normalized and
tempered condition and subsequently h e a t t r e a t e d t o 220-240 KSI
s t r e n g t h range was t e s t e d .
The r e s u l t s a r e t h e r e f o r e l i m i t e d t o
this one w t e r i a l and t y p e of c u t and should n o t be construed t o
apply t o o t h e r m a t e r i a l s and t y p e s of c u t s without f u r t h e r t e s t i n g
for verification.
The conventional c u t t i n g t o o l used f o r pocket m i l l i n g l o w a l l o y
s t e e l i n t h e normalized and tempered c o n d i t i o n i s s c o b a l t a l l o y
h i g h speed s t e e l end m i l l ,
T h i s c u t t e r s h m i n F i g u r e 1 was used
as a Lase l i n e c u t t e r f o r g e n e r a t i n g t e s t specimen s u r f a c e s ,
Also
shown in F i g u r e 1 i s an in-house designed end m i l l used t o g e n e r a t e
t h e smoother t e a t specimen s u r f a c e s .
The in-house designed end
m i l l was developed by Manufacturing Research and Development Department personnel f o r making f i n i s h i n g c u t s ,
The in-house designed
end m i l l u t i l i z e s t u n g s t e n c a r b i d e i n s e r t s t o form t h e c u t t i n g edges
of a two f l u t e d end mill,
I t cannot be used f o r plunge m i l l i n g o r
s i d e m l l l i n g a s is done w i t h the conventional end m i l l .
Three e i g h t h s i n c h thick p l a t e s t o c k of n o r m a l i z ~ dand tempered
Dbas s t e e l was f a c e m i l l e d t o remove c k e as r o l l e d s u r f a c e and
mikL scale.
The p l a t e s t o c k was t h e n c u t i n t o i n d i v i d u a l f a t i g u e
s p e e f n c n s having a c o n f i g u r a t i o n as s h m i n Figure 2 .
The i n -
w e r e t h e n helc! i n a v i c e b o l t e d t o t h e t a b l e
d i ~ i d specimens
~ l
o f a C i n c i n n a t i Model 550-20 m i l l i n g m a c h i ~ eand were machined on
one face o n l y t o produce t h e v a r l o ~ ss u r f a c e f i n i s h e s d e s i r e d for
testing.
The o p p o s i t e s i d e s of t h e spectmens were s u b s e q u e n t l y
ground t o a u n i f o m . s u r f a c e f i ~ i s hf o r a l l s p e c h e n s r e d u c i n g t h e
-
specimen t h i c k n e s s t o 0.100 +.001 i n c h .
The c o r n e r s were d e b u t r e d
us i n g p r o d u c t i o n machine shop p r o c e s s i n g .
Four d e g r e e s e-. roughness were g e n e r a t e d f o r t e s t p u r p o s e s .
roughest s u r f & - L (*350
The
AA) was produced by t h e c o n v e n t i o n a l c o b a l t
s t e p 1 end m i l l ( , O 6 8 i n c h deep c u t a t 5.4 i n c h e s p e r minute f e e d
r a t e ) as t h e b e s t s u r f a c e o b t a i n a b l e with t h a t t y p e o f end m % l l .
Three smoother f i n i s h e s were produced f o r c o r n p a r i s m u s i n g t h e MR&D
in-house d e s i g n e d e n d m i l 1 w i t h o n l y one t u n g s t e n c a r b i d e i n e e r t
1
installed.
rhe smaoehest s u r f a c e ( < 6 0 AA) webs cut a t 27 i n s h e s
I
p e r m i n u t e feed r a t e , .O60 i n c h deep c u t u s i n g a t u n g s t e n c a r b i d e
i
i
i
i n s e r t grcund t o a f l a t c u t t i n g edge.
The n e x t smoothest surface
(%I25 AA) r e s u l t e d from c u t t i n g a t a feed r a t e o f 27 i n c h a s per
Ii:
I
m i n u t e , ,060 i n c h deep c u t u s i n g a t u n g s t e n c a r b i d e i n s e r t i n i t s
r o u n d "as r e c e i v e d " condition.
To g e t a rough c u t f o r f u r t h e r
comparison ( Z 2 . 5 0 PA), t h e feed r a t e was i n c r e a s e d t o 54 i n c h e s
p e r minute u s i n g t h e same t y p e c u t t e r and d e p t h of c u t a s used t o
o b t a i n t h e I25
finish,
A l l specimen machining was done under s u r v e i l l a n c e of t h e Manu-
f a c t u r i n g Research a n d Cevelopmcnt Department,
A complete
d e s c r i p t i o n of t h e specimen otackining p r o c e s s i s g i v e n i n R e f e r ence 2 .
A f t e r t h i s f i r s t group of specimens were machined, t h e y were h e a t
t r e a t e d t o 220-240 KSI s t r e n g t h l e v e l i n a vacuum t o p r e c l u d e s u r -
face damage due t o o x i d a t i o n and s c a l i n g .
As a f i n a l o p e r a t i o n p r i o r
t o t e s t i n g , the machined finish Was r e m ~ v e din t h e g r i p area on
each end by s a n d i n g and a f l a s h copper p l a t i n g was a p p l i e d i n t h a t
r e g i o n t o r e d u c e f r e t t i n g c o r r o s i o n and premature f a i l u r e i n t h e
A photograph of each of t h e completed specimens is shown i n F i g u r e
3.
Photographs of t y p i c a l s u r f a c e s t a k e n e,"i54)
magnification
a r e shown i n Figures 4 through 7 ,
Specimens of e a c h o f t h e f o u r surface f i n i s h t y p e s were t e s t e d i n
t e n s i o n - t e n s i o n f a t i g u e t o g e n e r a t e s t r e s s - l i f e c y c l e (S-N) curves
FIGURE 4
(
60 AA SURFACE FINISH AT X50 MAGNIFICATION
FIGURE 5 z 1 2 5 AA SURFACE FINISH AT X50 MAGNIFICATION
17
FIGURE 6 =250
AA SURFACE FINTSIi AT X50 MAGNIFICATION
FIGURE 7 -350
AA SURFACE FINISH AT X50 MAGNIFICATION
18
a t mean s t r e s s l e v e l s oE 40 K S I and 80 KSI.
The t e s t r e s u l t s f o r
each t y p e of f i n i s h a r e shown in Tables I through V I f T and p l o t s
of t h e r e s u l t i n g t e s t p a i n t s a r e shown i n F i g u r e s 8 through 15.
The l i n e s shown on t h e c h a r t s a r e f o r r e f e r e n c e o n l y .
The l i n e s
on F i g u r e s 8 through 11 have i d e n t i c a l c o o r d i n a t e s , and t h e l i n e s
on F i g u r e s 12 through 15 have i d e n t i c a l c o o r d i n a t e s .
The l i n e s
were drawn a s a means of r e l a t i n g t h e t e s t p o i n t p o p u l a t i o n on
one c h a r t w i t h t h e t e s t p o i n t p o p u l a t i o n f o r any o t h e r f i n i s h on
a n o t h e r c h a r t and a r e n o t intended t o be used a s S-N c u r v e s .
A f t e r a c o n s i d e r a b l e amount of t e s t i n g was accomplished f o r each
type of f i n i s h and mean s t r e s s l e v e l it became a p p a r e n t t h a t t h e
S-N t e s t d a t a f e l l I n t h e same s c a t t e r band r e g a r d l e s s of s u r f a c e
roughness.
This r e s u l t was not expected s i n c e t h e s u r f a c e f i n i s h e s
v a r i e d from a smooth f i n i s h t o a rough f i n i s h w i t h g r o s s s u r f a c e
t e a r i n g a s shown i n t h e X50 m a g n i f i c a t i o n of t h e m 3 5 0 AA s u r faces (Fig. 7 ) .
Ac examination of t h e f a i l u r e o r i g i n s showed t h a t
t h e production d e b u r r o p e r a t i o n l e f t s c r a t c h e s on t h e c o r n e r s of
t h e specimens which served a s p o i n t s of s t r e s s c o n c e n t r a t i o n a s
s e v e r e o r more s e v e r e t h a n t h e machined s u r f a c e t o o l marks.
S i n c e a l l specimens were deburred u s i n g t h e same technique, t h e
d e b u r r i n g was t h e predominant f a c t o r c o n t r o l l i n g t h e f a t i g u e l i f e
of a l l specimens except f o r t h e specimens w i t h s u r f a c e s machined
.
4
I
.
.
----*-
I
.
4 . -
I".
4;'
-..,.- - - - --...*--; - .-. ,- . ,
? J.,.
--+
L . - - l... .-+ .--. - .., . - - - - --
-,
;t--f
.....
;
.
*-it
--.-A
4
TABLE I V
RESULTS OF FATIGUE TEST .3F SI?ECIXENS CUT AT 5.4 INCHES PER MINUTE,
USING A HIGH SPEED COBALT !STEEL Ehl) MILL, AND CYCLED AT 40 K S I
MEAN STRESS
AS RECEIVED (a)
MAX
STRESS ( K S I )
KILOCYCLES
TO FAI1VP.E
FAILURE
ORIGIN*
1
1
1
1
1
1
1
1
2
1
1
3
NF
1
1
1
1
NF
1
2
1
1
1
3
1
NF
NF
NP
2
NF
Machined to 23 3 5 0 M fintsh, deburred and heat treated in vacuum.
(1) machined surface, (2) ground surface, (3) corner.
NF indicater test war stopped with no failure.
*(a)Failure origin on:
TABLE V
RESULTS OF FATIGUE TEST OF SPECIMENS CUT AT 27 INCHES PER MIXmE
WITH END OF END MILL, UIjlNG TUNGSTEN CARBIDE INSERTS GROUND FLAT,
Am CYCLED AT 8 0 KSI h-EN STRESS
MAX
STRESS (KSI)
AS RECEIVED (a)
KILOCYCLES
TO FAILURE
FAILURE
ORIGIN*
POLISHED CORNERS (b;
--MKX
KILOCYCLES
STRESS (KSX)
TO FAILURE
FAILURE
ORIGIN*
Machined t o < b0M f i n i s h , deburred and heat treated i n vacuum.
Same a s (a) except corners polished t o remove deburr tool marks.
( c ) Same a s (a) except f o r production heat treatment plus g r i t b l a s t cleaning and shot peening.
Failure origin on: (1) machined surface, (2) ground surface, (3) corner, (4j undefinable.
NF indicates t e s t was stopped with no f a i l u r e .
(a)
(b)
*
6
rrr
SHOT^^^^^'
GRIT BLASTED kNI)
MAX
KILOCYC'LES
STRESS (KSZ)
TO FATLURE
FhILiXE
ORIGIN*
TABLE VI
RESlJLTS OF FATIGUE TEST OF SPECImNS CUT AT 2 7 INCHES PER MINLKE
WITH END OF EM, MILL, USING ROUND TUXSTEN CARBIDE INSERTS AND
CYCLED AT 80 KSI MEAN STRESS
AS RECEIVED(~)
KIWYCLES
STRESS (KSI)
TO FAILURE
MAX
FAILURE
ORIGIN*
(a) mchined to =Vu125M finish, deburred and heat treated in vacuum.
Failure origin on: (I) machined surface, (2) ground surface, (3) corner, (4)undefinable.
RV indicates test
WAS
stopped with no failure.
TABLE '$11
RESULTS OF FATIGUE TEST OF S P E C I E N S CUT AT 54 INCHES PER M I N m E
WITH END OF END MILL, USING ROUND TLMCSTEN CARBIDE INSERTS AND
CYCLED AT 80 KSI E 4 N STRESS
AS RECEIVED(*)
HAX
TRESS (KSI)
KIWYCLES
TO FAILURE
FAILURE
ORIGIN*
160
160
150
160
140
140
140
140
140
130
130
130
130
130
120
120
120
120
120
120
120
115
115
115
115
115
115
115
28
47
22
21
41
275
22
28
49
226
58
33
61
78
349
215
146
198
206
1,363
1,954
6,613
1,813
1,828
1,207
12,560
2,717
463
1
1
3
3
3
3
3
3
3
NF
1
3
3
2
POLISHED CORNERS(^)
KIIKYCLES
FAIUfRE
STRESS ( K S I )
TO FAILURE
ORIGIN*
MAX
160
160
160
140
140
140
130
130
130
120
120
120
26
26
30
44
83
98
162
106
161
10,057
10,000
265
-G RMAX
I T BLASTED AND
s ~ E E EF
~ ~AE I ~ L( ~~ ~
KILOCYCLES
STRESS ( K S I )
1
260
1
1
1
1
1
1
2
1
200
NF
NF
2
TO FAILVnE
180
180
180
170
170
170
lbi)
160
160
160
160
hT
NF
1
3
1
L
3
1
NF
NP
NF
NF
NF
3
3
( a ) Machined t o 5 2 5 0 * h f i n i s h , deburred and h~eatt r e a t e d i n vacum.
(b) Same a s (a) except corners polirhed t o remove deburr t o o l marks.
(c) Same a s (a) except f o r production heat treatment plus pit b l a s t cleaning and shot peening.
* F a i l u r e o r i g i n on: (1) machined surface, (2) ground surface, (3) corner. (4) undefinable.
NF i n d i c a t e s t e s t war stopped with no f r i l u r e .
47
23
23
125
62
ti2
2.435
49
2,111
9,550
8,974
50
60
ORIGIN*
4
4
4
4
4
4
4
4
NF
4
NF
4
4
TABLE V I I I
RESULTS OF FATIGUE TEST OF SPFCIMENS C'JT AT 5.4 INCIIES PER MIKUTE,
U S I N G A HIGH SPEED COBALT S E E L EWD MILL, AND CYCLED AT 80 KSI ME4N
STRESS
AS RECEIVED_((")
KILOCYCLES
FAIEL~E
STRESS (KSI)
TO FAILURE
ORIGIN*
MAX
160
160
160
160
160
140
140
140
140
140
130
130
130
130
130
120
120
120
120
120
120
115
115
115
115
115
115
110
110
110
a)
16
22
18
11
16
57
65
66
48
40
64
61
204
93
286
227
69
114
110
278
142
123
10,000
361
188
7,916
325
2,525
10,000
199
3
1
1
1
1
1
1
1
1
1
1
1
3
1
NF
1
1
3
1
1
NF
NF
HF
3
3
1
Z
NF
NF
1
~ a c h f n e dt o 2 5 3 5 0 M f i n i s h , deburred and heat t r e a t e d i n vacuum.
F a i l u r e o r i g i n on: (1) machined surface, (2) ground surface, (3) comer.
i n d i c a t e s t a r t war rtopped with no f a i l u r e .
with the cobalt steel end mill (z350 AA),
The l o c a t i o n of t h e
o r i g i n of f a i l u r e was identified and i n d i c a t e d on the c h a r t s of
S-N d a t a , Figures 8 through 15, by the open symbol t e s t p o i n t s .
Note t h e predomicance of c o r n e r f a i l u r e s .
To remove t h e i n f l u e n c e of t h e deburr o p e r a t i o n , s o t h a t t h e
machined s u r f a c e e f f e c t s could be evaluated, t h e remainder of t h e
4 6 0 AA specimens and t h e "-- 250 AA specimens were hand polished
on the corners u n t i l a l l evidence of t h e deburr s c r a t c h e s were
rernaved.
The c y c l i c t e s t i n g was continued a s b e f o r e .
These t e s t
p o i n t s a r e shown a s s o l i d symbols on t h e Figures 8 , 10, 12 and 14.
A n o t i c a b l e s h i f t i n t h e s c a t t e r p a t t e r n can be observed f o r t h e
smooth ( 4 6 0 AA) f i n i s h specimens whereas t h e rougher ( 2 2 5 0 AA)
----E.. - - fiiiisf; specimens ohwed
YUL LaGG
a 1~_ssef:
benefitr
Polishing the
corners showed a d e f i n i t e s h i f t i n o r i g i n of f a i l u r e from c o r n e r
t o f l a t s u r f a c e as i n d i c a t e d on Figures 8 , 10, 12 and 14.
L
A very l i m i t e d e l e c t r o n and o p t i c a l microscope examination of a l t
f o u r types of s u r f a c e s was made.
The s u r f a c e s c u t w i t h t h e end
m i l l using t u n g s t e n c a r b i d e i n s e r t s appeared t o be f r e e of s u r f a c e
t e a r i n g t o t h e naked eye, whereas, t h e s u r f a c e s c u t u s i n g t h e c o b a l t
s t e e l end m i l l had obvious s u r f a c e t e a r i n g .
amination showed m i c r o t e a r i n g i n a l l cases.
The microscopic ex-
The cause of f h e rearing action was pointed out by E,G. Coker
Fn 1925 in h i s r e p s t to the Institute of Mechanical Engineers
(Landon) on "The Action of Cutting Taofs" (Reference 3).
showed by
Coker
analysis and confirmed his analysis by the use of
photoelasticity that the chip is t o m from the base metal by
shearing and tensile forces. The tensile component may be small,
depending on the rake angle of the cutting tool, but it is sufficiene to form a crack that runs ahead of the tool tip.
Such
action is sufficient to form microsurface fissures even though
the surface appears to be smooth.
During a production heat treatment followed by grit blasting to
remove heat treat scale the top few thousands of an inch of
surface metal are eroded from the surface thus removing some or
all of the fissured metal.
The "as machined" surface and the grit
blasted plus shot peened surfaces should have different fatigue
characteristics. To determine the change in fatigue life due to
grit blasting plus shot peening, a sesond set of specimens were
made using the same machining and deburring procedures as before
for specimens having; ( 6 0 AA surface finish and
%
250 AA surface
finish. These specimens were heat treated to 220-240 KSI in the
production heat treating facility, They were descaled by grit
blasting and then shot peened to an Almen A inteneity of
approximately 8. Photographs of these specimens are shown in
F iwrz 16.
These grit blasted and shot peened speciuens were tested in
tension-tension fatigue using the same test machines and procedures as was used for the initial set of specimens. The test
points generated were added ro Tables I, 111, V and VII and
Figures 8, 10, 12 and 14. Again, the lines t h m the added test
points on Figures 8 and 10 have identical coordinates and the
lines through the added test points on Figures 12 and 14 have
identical coordinates.
It is easily seen by comparing the added test points that the
grit blasting plus shot peening raises the endurance limit stress
from approxfmately 80 KSI to approximately 120 KSI for specimens
tested with a mean stress of 40 KSI.
The specimens tested with
a mean stress of 80 KSP showed an approximate improvement in
endurance limit stress from 110 KSI to 160 KSI.
The rougher as
machilned plus grit blasted and shot peened surfaces shawed somewhat
more scatter in fatigue life for any given maximum stress level
but the mean values are nearly the same for the two conditions.
How much a f the improvement in fatigue life is due t o shot peening
and how much is due to removing the microtears is not known.
1'
The
g r i t erodes away t h e s u r f a c e damage done during machining but
l e a v e s i t s awn p a t t e r n o f s u r f a c e roughness.
The s h o t peening
i n t u r n modifies t h e g r i t b l a s t e d s u r f a c e roughness i n t o a new
p a t t e r n o f roughness.
f o r f a t i g u e damage.
Shot peening i s t o u t e d t o be a c u r e a 1 1
Most of t h e d a t a showing f a t i g u e l i f e i m -
provements due t o s h o t peening was generated on low t o medium
s t r e n g t h steels w i t h l i t t l e o r no i n f o r m a t i o n given on t h e
o r i g i n a l s u r f a c e f i n i s h o r how i t was changed due t o s h o t peening.
The theory of how s h o t peen!.ng produces r e s i d u a l compressive
s t r e s s e s on t h e s u r f a c e which i n t u r n improves f a t i g u e l i f e
appears t o be v a l i d .
However, t h e r e i s some evidence t h a t b l a s t -
i n g u l t r a high s t r e n g t h s t e e l hard enough t o c o l d work t h e s u r f a c e
does a s much damage to p o l i s h e d s u r f a c e s as it does t o improve t h e
s u r f a c e s t r e s s c o n d i t i o n w i t h no n e t g a i n i n f a t i g u e l i f e (References 4 and 5 ) .
A stlmmary of t h e r e s u l t s i s a s f o l l w s :
1,
The d e b u r r process t o o l marks a r e a s d e t r i m e n t a l t o f a t i g u e
l i f e a s t h e roughest s u r f a c e f i n i s h t e s t e d , causing a l l
s p e c h e n f a t i g u e l i v e s t o f a l l i n t h e same s c a t t e r band,
r e g a r d l e s s of machined s u r f a c e f i n i s h .
2.
Removing the deburr t o o l marks caused a s h i f t from p r e dominantly c o m e r o r i g i n f a i l u r e s t o f l a t surface o r i g i n
f a i l u r e s w i t h t h e g r e a t e s t gain i n f a t i g u e l i f e ~ h w n
y t h e s p e e l m e ~ sh a v i n
smoothest ( 4 6 0
u) surface
fLrrlsh.
3,
G r i t : b2a tint3 f o l l ~ e dby S'flot: peening ~IludifLedthe
eEZects of t
the f a t i g u e l i f e considerably.
The r a t i o of a l t e r n a t i n g
s t r e s s t o m a n s t r e s s a t the endurance l i m i t was r a i s e d
proximete r a t i o of 2:1 f o r the a s machined
specimens to an approximate r a t i o of 3 : l for the g r i t
blasted plus shot peened specimens when cycled about a
mean stress of 40 KSI.
o f 80
When cycled about a mean s t r e s s
KSI the comparable r a t i o s were raised from approxi-
msrely 1 . 4 : l up t o 2 : l .
CONCLUSIONS
There are several conclusions that can be drawn frcm this
research as follows:
1. The conventional deburring process applied to
parts with corners is a prime contributor to
the determination of fatigue life of parts
used in the as machined condition; however,
there are no specifications or requirements
for the inspection and acceptance of the
deburred corners.
Surface finish can vary over a wide range,
when measured by the method of ANSI B66.1,
without a corresponding variation in fatigue
life. Therefore, ANSI £346.1 is not an appropriate specification for use where fatigue life
of parts used in the as machined condition is
to be controlled,
Processes applied to parts after final wchining
can alter the fatigue life by a huge amount, In
this research, the subsequent grit blasting plus
shot peening increased the allowable maximrrm stress
a c o n s i d e r a b l e amount f o r any specified fatigue
life.
C o n c e i v a b l y , o t h e r processes might Lmer
the allowable stress,
The surface finish re-
quirements and acceptance specification should
be a p p l i e d t o t h e final f i n i s h and not t o the as
machined surface f i n i s h .
This research was limited to one type of surface finish and
one type of steel at only one strength level. The results
are definite enough to show that, for the material and strength
level tested, t h e "as used" final surface finish controls
fstigue life, whereas, the intermediate "as machined" surface
finish has little or no influence on the fatigue life of
specimens receiving subsequent processing.
It is recommended
that other types of machined surface fbnishes, materials and
strength levels be evaluated to determine if the conclusions
drawn from this limited research can he generalized for all
machined surfaces.
Specifying a low surface finish average roughness height rating
per ANSI B46.1 does not assure an improvement in fatigue life.
Xt is recommended that an entirely new specification be developed,
based on test results, that defines the acceptable surface top-
ography far each final finishing process in terns of a true
surface comparison,
The conventional deburring process applied to corners of parts
used in the as machined condition can have a very deleterious
a f f e c t on t h e f a t i g u e l i f e of t h e p a r t .
I t i s recamended t h a t
improved debursing processes be developed t h a t remove t h e b u r r s
and rounds t h e c o r n e r s without ldaving s c r a t c h e s a l o n g t h e edge.
Tmbling t h e p a r t s i n an a b r a s i v e p a r t i c l e b a t h i s one such
process t h a t might be used.
A c o n s i d e r a b l e gain i n f ~ t i g u el i f e r e s u l t e d from g r i t b l a s t i n g
p l u s s h o t peening a f t e r h e a t t r e a t i n g t h e specimens,
How much
of t h e improvement was due t o t h e change i n t h e s u r f a c e due t o
g r i t b l a s t i n g and how much was due t o s h o t peening was not
determined.
I t i s recommended t h a t t h e s e e f f e c t s be i s o l a t e d
and evaluated s e p a r a t e l y .
I n r e f e r e n c e s 4 and 5 i t was pointed o u t t h a t s h o t peening
polished u l t r a high s t r e n g t h s t e e l s d i d n o t improve f a t i g u e
l i f e but p o l i s h i n g a f t e r s h o t peening t o remove s u r f a c e damage
due t o t h e s h o t peening d i d show an improvement i n f a t i g u e l i f e .
I t i s recommended t h a t t h i s a f f e c t be i n v e s t i g a t e d f u r t h e r a s a
means of improving f a t i g u e l i f e where no o t h e r approach i s
available.
REFERENCES
1.
AWSI R 4 6 . 1 - 1 9 6 2 , "Surface Texture," American National
Standards Inst ttute.
2.
Johnson, L. A., "~urfaceIntegrity of Metal Finishes,"
General Dynamics Corp. Report FMR 70-1915, 9 November 1971.
3.
Coker, E. G., Cutting Tools Research Committee Report on
he Action of Cutting Tools ,'I Proceedings of Institute of
Mechanical Engineers, London, 1925.
4.
Coombs, A.G.H., Sherratt, F. and Pope, J. A , , "An Analysis
of the 2fEects of Shot Peening Upon the Fatigue Strength of
Hardened and Tempered Spring steel," International Conference
on Fatigue of Metals, The Institute of Mechanical Engineers
and the American Society of Mechanical Engineers, 1956,
published by the Institution, London.
5.
Sherratt, F., "The Influence of Shot-Peening and Similar
Surface Treatments on the Fatigue Properties of Metals,"
Nottingham University, Office of Naval Research, London,
England, Memorandum Report, S&T Memo 2/66, February 1966.
APPENDIX A
There is only one specification for measuring surface roughness
accepted by the military services, That specification originated as a result of a cooperative effort between the Society
of Autoaotive Engineers and the American Society of Mechanical
Engineers. The draft was submitted to the American Standards
Association and designated as an American Standard in 1947.
The military services adopted the standard for military use and
issued it as Mil-Standard 10, "Surface Roughness, Waviness and
Lay" dated 2 August 1949.
The Ansrican Standard was revised
and reissued on January 11, 1955 as ASA B46.1-1955.
The military
services accepted the revisions and reissued Mil-Standard 10 as
Mil-Standard LOA dated 13 October 1955.
American Standard ASA B46.1-1955 was again revised and reissued
as ASA 346.1-1962. Following this revision three change notices
were issued against Mil-Standard 10A. The first change notice
instructed all users of Mil-Standard 10A to use ASA B46.1-1962 in
its place, The second change notice rescinded the first change
notice, The third change notice dated 3 January 1966 cancelled
Mil-Standard 10A for all future use and replace it with ASA B46.l1962.
Since that time the American Standards Association has been
renamed the American National Standards Institute and ASA
B46.1-1962 is now designated as ANSI B46.1-1962,
An extensive effort was expended by the preparing committee from
the beginning of the specification coordination activity in 1932
to the issuance of the ANSI B46.1-1962 as it now stands. This one
and only specification has served well in the purpose for which
it was intended and those men who created the specification deserve
a commendation.