EFFECT OF INTERNAL CREEP MICROCRACKS ON FATIGUE MACROCRACK PROPAGATION AT HIGH TEMPERATURES

Abstract

The cycle-dependent pure fatigue crack propagation and the time-dependent creep-fatigue one, in general, were scarcely accompanied with microcracks and voids or cavities in the vicinity of the tip of propagating cracks inside the material when subjected to relatively high strain rate cycles at intermediate temperatures. In very low strain rate and/or high temperature conditions, however, diffusion-controlled creep cavities and microcracks tend to be generated along grain boundaries at the mid-thickness. These cavities and microcracks affect the macrocrack propagation in high temperature fatigue in two ways; one is the decrease in the crack propagation resistance and the other is the decrease in the crack driving force, i.e., the J-integral. Consequently, the creep-fatigue crack propagation is accelerated by several times and the succeeding fatigue crack propagation in the pre-creep damaged material by ten times at most for a given J-integral range, ΔJ.