Scatter of Fatigue Strength in Medium Carbon Steels

Abstract

A series of fatigue tests were conducted, from a statistical viewpoint to obtain foundamental data on the scattering characteristics of fatigue strength, on 3 kinds of commercial plain carbon steels of 0.45% C type and 2 kinds of 0.25% C type. The analysis of the results revealed that the scatter in fatigue strength of smooth specimens may generally be fitted to the normal distribution, and it is possible to obtain P-S-N curves by shifting laterally on S-N diagram the plot of median fatigue strengths estimated by Probit analysis from the data obtained at different numbers of cycles.The scatter in fatigue strength of smooth specimens made of the same steel subjected to a constant condition is affected by the difference in the sample rods from which the specimens were made. For the specimens subjected to different heat treatment conditions, the scatter tends to increase as the fatigue strength becomes higher. The variation coefficient of fatigue strength at 107 cycles is estimated to be approximately 2.5% for all the cases studied, except for the case of 0.45% C steel tempered at 350°C to achieve high brittleness, where it is 4.9%.For notched specimens of the same steel, on the other hand, the variation coefficients are 6.3% and 5.3% for the specimens tempered at 600°C and 350°C, respectively. From the fact that the notch causes a large increase in the variation coefficient for ductile materials but not so much for brittle materials, the process of crack propagation is thought to have great influence on the scattering phenomenon of fatigue strength.