Host: The Japan Society of Mechanical Engineers
Name : [in Japanese]
Date : November 02, 2019 - November 04, 2019
The threshold stress intensity factor range ΔKth for a mechanically long crack is constant for low hardness steel. However, the ΔKth value decreases depending on hardness for high hardness steel. Plastic strain gradient was proposed by previous study to contribute to decreasing the ΔKth value of high hardness steel, e.g. high-strength steel. To verify plastic strain gradient’s contribution to decrease of ΔKth, the plastic strain gradient ahead of the tip of a long notch which can be regarded as a mechanically long crack was investigated. Using high-strength steel and another material with lower hardness, which is manufactured by two kinds of thermal treatment on JIS-S45C steel, tensile experiments were carried out. Digital image correlation (DIC) analysis on the tensile test were performed, the strain distribution on the specimen surface was obtained. Comparing the gradient of the plastic strain from the mechanically long crack tip to the specimen edge, the plastic strain gradient being suppressed by hardness was found. Considering the reverse yielding and residual compressive stress during cyclic loading, the difference in plastic strain gradient can alter the plasticity induced crack closure (PICC) behavior. As the PICC would slow down the fatigue crack growth rate, the plastic strain gradient could be regarded as a reason for reducing ΔKth in high-strength steel of a mechanically long crack.
