Increase of Delayed Failure Strength by Partial Unloading Method

Abstract

The effect of three types of pre-stressing, i. e. partial unloading, one cycle loading, and perfect unloading, on the delayed failure strength was investigated using the notched specimen of JIS SNCM8 steel quenched and tempered at 200°C. Using the test stress, σ, the maximum stress, σ_max. (>σ), and the unloading stress, σ_a, the three types of pre-stressing are expressed as:
partial unloading: σ_max. →σ_max. -σ_a (=σ), one cycle loading: σ_max. →σ_max. -2σ_a→σ_max. -σ_a (=σ), and perfect unloading: σ_max. →O→σ_max. -σ_a (=σ). The results obtained are as follows:
1) All the pre-stressing methods can markedly increase the lower limit stress σ_scc if the unloading stress σ_a is chosen appropriately. Among the three pre-stressing methods, the partial unloading method is the most effective for increasing the delayed failure strength, and the lower limit stress obtained by this method is 4.5 times as high as that obtained by usual delayed failure test.
2) The minimum unloading stress necessary for increasing the lower limit stress σ_scc is smaller in the partial unloading method than the one cycle and perfect unloading methods.
3) The reason why the delayed failure strength is increased by each pre-stressing method can be explained by the decrease of surface stress at the notch root, which will suppress the corrosion reaction and prevent the invasion of hydrogen atoms into the material.