Effect of Hardness HV on Plastic Strain Gradient Around Sharp Stress Concentration Source of S45C Steel with a Mechanical Long Crack

<p>The threshold stress intensity factor range Δ<i>K</i>_th for a mechanically long crack is constant for low hardness steel. However, the Δ<i>K</i>_th value decreases depending on hardness for high hardness steel. Plastic strain gradient was proposed by previous study to contribute to decreasing the Δ<i>K</i>_th value of high hardness steel, e.g. high-strength steel. To verify plastic strain gradient's contribution to decrease of Δ<i>K</i>_th, 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 Δ<i>K</i>_th in high-strength steel of a mechanically long crack.</p>