Finite fatigue life behavior and fatigue crack-growth resistance of a fine-grained WC-Co cemented carbide

Abstract

In this work, fatigue life and fatigue crack growth (FCG) testing, using pristine and pre-cracked samples respectively, are combined for studying the mechanical behavior of a fine-grained WC-Co cemented carbide. The main objective is to correlate fatigue lives measured with estimated and experimentally determined FCG data for natural and long through-thickness cracks respectively. It is done on the basis that fatigue failure, within the finite fatigue life regime, is controlled by the subcritical propagation of pre-existing flaws. In doing so, fatigue strength for the finite life defined as run-out (200,000 cycles) is first evaluated using two protocols based on the stair-case methodology. Then, strength data experimentally measured under monotonic and cyclic loading are correlated to each other for estimating FCG data for intrinsic defects. As a result, the dependence of FCG rates with the maximum applied stress intensity factor estimated for natural small flaws is found to follow trends similar to those experimentally determined for artificial long cracks. However, FCG rates and effective threshold for crack extension for the former are determined to be significantly lower than for the latter. The similitude found by both crack types regarding fatigue and fracture micromechanisms, as discerned from scanning electron microscopy inspection of stable and unstable crack extension phenomena, points out that distinct FCG behavior exhibited by small and long cracks is a consequence of extrinsic issues, such as length-scale of crack size or environmental aspects linked to location of the flaw either in the bulk or at the surface, rather than physically-based intrinsic ones.