The effect of helium on the fatigue properties of structural materials

Experimental and theoretical work on high temperature fatigue properties of metals containing helium is reported. The most important experimental results are: Above 5000C the reduction in the fatigue life to be attributed to helium increases with increasing temperature and decreasing frequency or strain rate, respectively. This reduction is generally accompanied by a transition from transgranular to intergranular failure. Details depend sensitively on the materials state. This embrittlement effect is attributed t the dynamic transformation of helium bubbles, breathing on grain boundaries under cycling stress, into unstably growing voids. Cavity stability under cycling stress (and analogously under pulsed irradiation) may be analysed in terms of attractor solutions of the cavity growth equation. The stability limit is defined by the appearance of a ''semi-stable'' limiting cycle. For given stress (strain) amplitude, the relaxed critical bubble size is significantly lower below a temperature dependent critical frequency than above it. Possible chemical and microstructural influences on the time available until the cavity stability limit is reached are discussed in reference to observed fatigue life times. (orig.)