Abstract
The effects of two types of pre-stressing, i.e. partial unloading (0 →K max → K) and perfect unloading (0 → K max → 0 → K) on the delayed failure strength were investigated using pre-cracked specimens of JIS SNCM8 steel quenched and tempered, where K max is the maximum stress intensity factor at pre-stressing, and K is the stress intensity factor under which delayed failure test is carried out. Both pre-stress methods can markedly increase the delayed failure strength or the lower limit stress intensity factor K ISCC . The partial unloading method is superior to the perfect unloading method in each tempering condition (200 or 400°C) and in each environment (distilled water or 3% NaCl water). The reason why K ISCC is increased by each pre-stressing can be explained by the decrease of surface stress at crack tip, which will suppress the corrosion reaction and prevent the invasion of hydrogen atoms into the material.
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