Effects of Ion Irradiation on Stage II Fatigue Crack Growth and Plasticity

Fatigue crack growth characteristics of ion irradiated compact tension (CT) specimens were evaluated in the Paris law region. Fatigue crack growth was monitored under tension-tension loading and the thermoelastic response was measured. Two stages of crack propagation were identified. In the first 300,000 cycles, crack growth rates of irradiated and unirradiated specimens were comparable and plastic zone area was found to be independent of crack length. Beyond 300,000 cycles, irradiated specimens showed a greater crack growth rate; additionally, the plastic zone area increased with crack length. The increase in crack growth rate was attributed to irradiation hardening. The plastic zone area was found to be dependent on the crack path, especially in the initial stages of crack propagation. Local peaks in the value of the area of the plastic zone were found to be associated with greater crack tortuosity and secondary cracks. As a result, decreases in plastic zone area were associated with greater crack growth rates. Heavy ion irradiation caused an increase in fatigue crack growth rate.Accelerated crack growth was attributed to irradiation hardening.Secondary cracks were found to correlate with identified peaks in plastic zone area.Drops in plastic zone area were associated with an increase in crack growth rate.