Laboratory of Materials, IOMP, UniversitO de S#tif 19000, Algeria. G. ORANGE, G. FANTOZZI. GEMPPM Laboratory, UA 341, INSA, Villeurbanne 69621, France.
J O U R N A L OF M A T E R I A L S SCIENCE L E T T E R S 13 (1994) 1691-1693
Temperature effect on the fracture toughness of W C - 2 5 % Co cermet N. BOUAOUADJA, M. HAMIDOUCHE, H. OSMANI Laboratory of Materials, IOMP, UniversitO de S#tif 19000, Algeria G. ORANGE, G. FANTOZZI GEMPPM Laboratory, UA 341, INSA, Villeurbanne 69621, France
The fracture mechanics approach by using the Kic parameter to predict unstable crack extension has proved to be successful for the limited case of linear elastic plane strain fracture [1]. Structures may have stresses in some regions that exceed the elastic limit. This has created the need for a fracture criterion that also includes elastic-plastic to fully plastic behaviour.
In a previous work [2], the concepts of linear elastic fracture mechanics (LEFM) were successfully applied to measure the fracture toughness in plane strains of WC-25% Co cermet as a function of temperature. The variation of the critical stress intensity factor versus temperature (Kid = f ( T ) ) has been established. However for this grade, valid measurements were restricted to low temperatures (< 600 °C). At higher temperatures, extensive plasticity occurred at the crack tip, therefore invalidating the use of LEFM. In these conditions, other methods taking into account this plasticity at the crack tip, such as the J integral, can be used. In this letter, the method of Garwood et al. [3] is used to determine the J - A a variation for WC-25% Co cermet between 600 and 1000 °C. This method has been used successfully in other works [4, 5]. The experimental procedure for determining the Jn energies consists of subjecting single edge notched beam samples to successive load-unload cycles,
7 :
i ..... i
leading to an increasing crack length. It is normal to consider that the sample compliances C start to vary from the critical load Pc (crack initiation point), which allows the determination of Jic value. The crack extension Aa measurement was determined from compliances variation. The initial energy J0 or Jic is given by Rice's formulae: Jo = 2 U o / [ B ( W - a0)}
(1)
where U0 is the initial energy, B is the width of the samples, W is the depth of the samples and a0 is the initial crack length. This method was applied at different temperatures. An example of a load P against displacement 6 cycled curve, established at 900 °C for WC-25% Co, is presented in Fig. 1. From Fig. 1, it can be observed that the residual deformation is more important, and that the hysteresis effect increases as long as the deflection increases. These may be attributed to the plastic deformation at high temperature and to irreversible phenomena such as cobalt ligaments (Fig. 2a) and the "unsticking" of individual or agglomerate WC grains (Fig. 2b). The determination of the J - A a curve using the method of Garwood et al. requires accurate determination of the initiation energy J0, because it
i .... ~ --J =..... i
68o
510
'~i
iJ
0
I x mfrs..
16
. . . . . . -4-
32
48
-I
-'
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c (gm)
80
96
12
128
Figure 1 Load-unload cyclingcurveobtainedat 900 °C on WC-25% Co.
0261-8028 © 1994 Chapman & Hall
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Figure 2 Micrographs obtained at 1000 °C showing (a) cobalt ligaments and (b) "unsticking" of individual and agglomerate WC grains.
is an iteration method. Consequently, all of the superior order terms depend on the value of J0. Therefore, J0 determination depends on that of the critical load Pc, which (as in some works) has been taken to be equal or slightly inferior to the maximum load Pmax" For this purpose, the l I P = f ( C ) variation has been established to resolve this problem. In fact, this variation permits us to find Pc accurately. An example of this curve established at 1000 °C is plotted in Fig. 3. The discontinuity point I corresponds to the begining of compliances variation, independently of the step taken between the load-unload cycles. The hysteresis effect increases especially when the unloading level is significant. This phenomenon increases with material plasticity. Also, the observed loops (for the same stress level) are more significant for higher temperatures. This is directly due to the material microstructure and the stress field located in the crack tip. According to Sakai et al. [6], the hysteresis loop surface represents the energy dissipated by the plastic deformation created under the effect of load-unload cycles. In other words, it is worth mentioning that the hysteresis loops are completely alleviated during stress relaxation tests [2]. The temperature effect on the WC-25% Co fracture toughness behaviour can be shown from J - A a evolution. An example of this variation, established at different temperatures, is plotted in
Fig. 4. It can be seen that the J0 mean values (corresponding to Aa = 0) increase with increasing temperature. Similarly, crack extension increases with temperature evolution. 12000 /
.-~ .......
-~. . . . . .
(d)
10000' ...............
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~ W
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4000.
20001
O0
~(a) 0'.2 0'.4 0'.6 0:8 1:0 1'.2 114 1:6 1'.8 2'.0 A a(mrn)
Figure 4 J = f(Aa) curves determined at (a) 700, (b) 800, (c) 900 and (d) 1000 °C.
/
/ Notch / sawcut
24 ¢o 21
Notch / tip
~18
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8
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. . . . i
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i
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i
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i
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i
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i
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Figure 3 Variation of lIP = f(C) established at 1000 °C. At point I, P = 328.6 N and C = 0.044 ~m N -I.
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Figure 5 Microscopic observation on the crack surface showing the presence of microcracks in the notch tip.
From Fig. 4, it can be observed that the curve level increases with increasing temperature. As a general behaviour, the variations tend to manifest an energy increasing for the lowest crack extension values until Aac. From beyond this critical value, J experimental points levelled off. A significant J0 scattering value has been noted in the case of the studied grade (up to about 23%). This scattering can be explained from the crack surfaces by microscopic observations (Fig. 5). The micrographs show the presence of microcracks on the notch tip. The notch was produced by electrical discharge machining (EDM), and it is important to remember that during EDM tests, the machining conditions were very refined in order to avoid microcrack initiation. The microcracks are probably generated at the start of the specimens' loading. Under the applied loads, the microcracks do propagate, with the creation of small tongues along the
crack. The latter are expressed in terms of energy, which is higher than that necessary to initiate and propagate one principal crack.
References 1. 2. 3. 4. 5. 6.
w . F. B R O W N and J. E. S R A W L E Y , A S T M STP 410 (1966). N. B O U A O U A D J A , Doctoral thesis, INSA, Lyon (1986). S . J . G A R W O O D , J. N. R O B I N S O N and C. E. T U R N E R , Int. J. Fract. 11 (1975) 526. K. K R O M P and R. F. P A B S T , J. Amer. Ceram. Soc. 66 (1983) 106. M. R ' M I L I , D. R O U B Y and G. F A N T O Z Z I Compos. Sci. Technol. 37 (1990) 207. M. S A K A I , K. U R A S H I M A and M, I N A G A K I , J. Amer. Cemm. Soc. 26 (1983) 868.
Received 25 February and accepted 4 July 1994
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