STATISTICAL PROPERTY OF SMALL-CRACK GROWTH-RATE IN 0.21-PERCENT C STEEL SMOOTH SPECIMENS HAVING DIFFERENT FERRITE GRAIN SIZES

In order to investigate the scatter characteristics of growth rate for small cracks, rotating-bending fatigue tests of smooth specimens were carried out using 0.21% carbon steels with different ferrite grain si zes. Fifteen to eighteen specimens were fatigued at each stress amplit ude, and the growth rate of a crack which led to the fracture was meas ured for all the specimens. The growth data were analyzed statisticall y. The results show that the growth rate for each crack length can be expressed by a three- or two-parameter Weibull distribution. The CV, c oefficient of variance, for crack growth rate distribution was calcula ted, and using the CV the relations between the scatter in growth rate and the ferrite grain size were investigated.