Mismatch Stress Effects on Microstructure-Flaw Size Dependence of KIC and Strength of Metal Bonded Carbides
This paper addresses two related effects of the mismatch stresses due to differences in thermal expansion between metal and carbide phases. First, a mathematical model, developed for the microstructural dependence of fracture energy, due to mismatch strains that exist between noncubic grains in a single phase body, is extended to such mismatch strains in two phase bodies, such as Co bonded WC. The combination of mismatch strains and the applied stress in the vicinity of a crack tip are treated as the source of microcracking in, or between, WC grains and local plastic deformation in the Co phase. Resultant calculations show good comparison with literature data on fracture energy, as well as strength data for flaws sufficiently larger than the microstructure.
The second effect the paper addresses are deviations from the strength behavior predicted by K IC measurements or calculations at smaller crack sizes when cracks no longer encompass a complete range of grain misorientations and hence mismatch stresses. Some cracks will then be associated with grain combinations having net tensile components, which will add to the applied stress to aid failure. Recent literature data is examined, showing support for the occurrence of this mechanism.
KeywordsFracture Energy Main Crack Transverse Rupture Strength Spontaneous Fracture Mismatch Stress
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