Fatigue damage in a WC-Nickel cemented carbide composite
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THE application of cemented carbides as inserts in rotary rock bits stands apart from many other uses of this class of composites. Cemented carbide performance in this increasingly important role depends largely on deformation and fracture behavior at ambient temperatures. Service loads imposed during drilling are principally compressive and cyclic. Historically, the evaluation of rock-drilling carbides has focused on the control of the scale of deformation and fracture events as observed under drilling conditions. Through manipulation of microstructural parameters, it was sought to avoid large scale fracture while minimizing small scale fracture or wear. More recently, fracture mechanics concepts have been applied to the evaluation of cemented carbide mechanical behavior. Quantitative descriptions of crack stability in these "brittle" composites by linear elastic fracture mechanics (LEFM) parameters is becoming routine. At the same time, fracture origins and populations of nascent defects are being characterized. However, the nature of stable crack growth remains uncertain for cemented carbides. In particular, the role of fatigue in rock drilling applications is potentially significant and hitherto largely unexplored. Stable crack propagation under cyclic load has been observed in cemented carbides in compression and bend-type fatigue testingA 2 Crack propagation under the influence of cyclic loads has been observed in rockdrilling cemented carbides under laboratory conditions and also in association with surface spalls. 3 Early results indicate that crack growth rates in cemented carbides relate to the stress intensity amplitude by an exponent at least several times greater than those typical of metal
alloys. 4 However, it is not known how fatigue mechanisms influence stable crack growth in cemented carbides, nor has there been any study of fatigue damage accumulation in these materials. Fatigue response in a cemented carbide involves a number of factors pertaining to the composite nature of the system. One of these is the possibility of redistribution of the sizable differential thermal stresses typical in these materials. 5-u Also, the lack of a linear region in the stress-strain behavior in cemented carbides has been explained in terms of plasticity mechanisms which may also contribute to anomalous fatigue behavior. For example, plastic behavior in the WC-Co system has been linked with various dislocation and phase transformation mechanisms. 12-~8A primary consideration for prediction of fatigue behavior is the scale of a typical cermet microstructure; the development of dislocation structures observed in fatigue of pure metals and alloys may be significantly altered in cemented carbide composites as a result of the proximity of interfaces and the image forces arising from extreme property differences of the phases. In view of these considerations, the present work was designed as an initial study to characterize the physical and mechanical property changes which occur as a result of cyclic strain in a model
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