An investigation of relationships between statistical dispersion of crankshaft’s fatigue properties and initial damage

Abstract

Premature failure due to fatigue would occasionally happen even in a well-designed crankshaft. The microstructural nonuniformity (including micro-defects) plays an essential role in the premature failure due to fatigue, which can lead to the scatter of strength and fatigue life properties. In this paper, the statistical distribution of initial damage corresponding to microstructural nonuniformity has been introduced to uniformly characterize scattering behaviors of tensile strength, fatigue life and impact energy in sampling tests of crankshafts. The goal of this study is to investigate quantitative relationships between the dispersion of initial damage and scattering behaviors of the above sampling tests, so that dispersion properties of a crankshaft can be determined only by relatively simple experiments. By impact sampling tests and several single-throw crankshaft fatigue tests, probability density functions of initial damage have been determined for two crankshafts provided by different manufacturers. It is found that the fatigue reliability of a crankshaft depends on its actual dispersion properties. For a crankshaft with large dispersion of initial damage, the safety factor in the fatigue strength design should be sufficiently large to prevent its premature failure due to fatigue, rather than the one directly recommended in the design manual.