In order to evaluate the effect of thermal history on environmental stress cracking (ESC), three different thermal conditions were employed for preparation of the samples. These samples were fractured under methanol environment by using two loading conditions; one was dynamic and the other was static. The relation between the stress intensity factor range ΔK and the cyclic crack growth rate aN was investigated under dynamic loading conditions. Similarly, under static loading conditions, the relation between the stress intensity factor K and the crack growth rate a was investigated. The log aN vs. log ΔK curve was markedly influenced by the thermal history of the samples. The same trend was observed under static loading conditions. From these investigations, it is found that the high ESC resistance of the quenched sample can be well represented by using the fracture mechanics concepts. Furthermore, the mechanism for the high ESC resistance was discussed by using the Williams theory. It is proposed that the high ESC resistance of the quenched sample is caused by the marked relaxation of stress in the failure zone.
