Abstract
The statistical fatigue properties were investigated for a commercial 0.45%C type carbon steel, most commonly used in mechanical constructions, in order to obtain quantitatively the scatter in fatigue lives and strengths. The fatigue tests were carried out in rotating bending for both smooth and notched specimens which had been subjected to different heat treatments so as to study especially the influence of the resilience of the material on the scatter in fatigue.
Since the scatter in fatigue strength was expected to follow a normal distribution in the same manner as those in tensile strength and Vickers hardness, it was possible to obtain the probability fatigue curves (P-S-N curves) by the method proposed formerly3), which consists of shifting the median fatigue strength curve determined by Probit analysis.
The variation coefficients of fatigue strength at 107 cycles were 2.1, 2.0 and 4.9% for the normalized, the 600°C tempered and the 350°C tempered smooth specimens, respectively, and 6.4 and 5.4% for the 600°C and the 350°C tempered notched specimens. From the variations in other mechanical properties and the results of microscopic observations of fatigue failure, it is considered that the variation of fatigue strength is particularly great for the 600°C tempered notched specimens, due principally to the variation introduced in the process of fatigue crack propagation.
