A comparison of indentation crack resistance and fracture toughness of five WC-Co alloys
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A Comparison of Indentation Crack Resistance and Fracture Toughness of Five WC-Co Alloys
p//D3,~ = [2FE/KPD
]l/z
[2]
which r e l a t e s the i n d e n t e r load P to the c r a c k depth D by m e a n s of the effective f r a c t u r e s u r f a c e e n e r g y F, the Y o u n g ' s m o d u l u s E, and a d i m e n s i o n l e s s factor
/~D
:
(1 - p2)/rr 3 t a n 2 ~
[3]
where v is P o i s s o n ' s r a t i o and ,I, is the i n d e n t o r half angle c o r r e c t e d for f r i c t i o n . The effect of f r i c t i o n is here neglected. I n d e n t a t i o n f r a c t u r e was obtained with a V i c k e r s 136 deg d i a m o n d p y r a m i d i n d e n t o r on s p e c i m e n s polished on one s u r f a c e , a n n e a l e d in v a c u u m for 2 1/2 h at 1000~ to reduce s u r f a c e s t r e s s e s , and lightly r e polished. The s p e c i m e n s were cut f r o m s i n g l e - e d g e Table I. Alloy Characteristics Co, Vol Pet
d, #m
X, ~um
GIC, Jm -2
H V, GNm-2
5.1 10.1 14.8 23.6 30.6
1.06 0.90 1.16 0.96 1.04
0.128 0.237 0.276 0.395 0.741
130 184 220 280 497
16.7 15.7 13.6 11.5 10.4
Data obtained from Ref. 5. Nomenclature: d = mean linear intercept particle size of WC, h = true mean free path in binder, GIC = plane strain critical strain
energyreleaserate, H V = Vickershardness.
E D W A R D L. EXNER, JOSEPH R. PICKENS, AND JOSEPH GURLAND
VOL % i000
/
oL
/
The P a l m q v i s t indentation c r a c k i n g t e s t is s o m e t i m e s used for the c h a r a c t e r i z a t i o n of the t o u g h n e s s of c e m e n t e d c a r b i d e s , 1 The test p r o v i d e s a m e a s u r e of the i n d e n t a t i o n c r a c k r e s i s t a n c e of a b r i t t l e m a t e r i a l (from the length of c r a c k s induced at a h a r d n e s s i m p r e s s i o n and the applied load), n a m e l y ,
W = _P/L r
c o r r e l a t i o n between W and the c r i t i c a l s t r a i n e n e r g y r e l e a s e r a t e GIC obtained by a plane s t r a i n f r a c t u r e toughness t e s t , P e r r o t t 3 has applied e l a s t i c - p l a s t i c s t r e s s functions to i n d e n t a t i o n d e f o r m a t i o n and was able to r e l a t e W and GIC with good r e s u l t s for cem e n t e d c a r b i d e s u s i n g e m p i r i c a l e s t i m a t e s of the r e q u i r e d p l a s t i c and g e o m e t r i c a l c o n s t r a i n t f a c t o r s . The p r e s e n t work is c o n c e r n e d with an a p p l i c a t i o n of l i n e a r e l a s t i c f r a c t u r e m e c h a n i c s to i n d e n t a t i o n c r a c k i n g , as developed by Lawn and F u l l e r . 4 S t a r t i n g with the Griffith condition, they d e r i v e d an equation
z~ IOA x 148
/
80o
'
/ x /
o z3.6 v
30,6
60o
[1] L T (~.m)
where W is the s u r f a c e c r a c k r e s i s t a n c e , P is the load on a V i c k e r s d i a m o n d i n d e n t e r , and L T is the total length of the s u r f a c e c r a c k s e m a n a t i n g f r o m the c o r n e r s of the i n d e n t a t i o n .2 Although t h e r e is no s i m p l e ED
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