Abstract
An attempt was made to clear up the reason why the relationship between fracture stress and the size of fracture-initiating defect of the cemented carbides (WC-Co alloys and TiC-Ni base alloys) changes at a certain defect-size.
The fracture strain of the cemented carbides depends upon the defect-size similarly to the fracture stress. Irrespective of the defect-size, the fracture crack is always propagated from a pre-existing defect and the critical condition for fracture obeys the Griffith-Irwin criterion. The toughness of the cemented carbides is reduced due to the plastic flow of the binder phase and the fracture toughness KIC is lowered gradually with increase in the applied stress beyond the yield strength. Therefore when the cemented carbides undergo a fracture under a high applied stress, a small variation of the fracture stress with the change of defect-size is obscured by scattering of the experimental data. Consequently, the fracture stress is apparently observed independent of the defect-size and this can be designated as the limiting strength.
