The effect of neutron irradiation on the fatigue behaviour of AISI 316L - results of first in-pile tests

The effect of neutron irradiation on the fatigue properties of fusion reactor structural materials is mainly studied by means of post irradiation testing. This means that, for the case of low-temperature irradiations, the fatigue test is started on a material which is already fully hardened by the irradiation damage. The behaviour of a material under simultaneous fatigue and irradiation loading could be completely different. Therefore a system was developed which makes possible in-pile load-controlled tests. A first irradiation was performed during which the specimens were cycled in the BR2 reactor with a stress range of 580 MPa at 250deg C. Ruptures occurred after 10000, and 12500 cycles respectively for doses at rupture of 0.4 and 0.5 dpa. These fatigue lives are comparable with those obtained in out-of-pile tests. In order to explain this absence of an irradiation effect it is assumed that either moving dislocations prevent irradiation damage to build up or that crack initiation occurs before radiation can cause any hardening and that subsequent crack growth is almost unaffected by irradiation. (orig.)