Investigation of the statistical and geometrical size effect on the fatigue strength of EN-GJL-300 under consideration of the graphite morphology

Abstract

Grey cast iron (GJL) is one of the most important materials for numerous components in car, utility vehicle and ship engines, power generation units and machine beds in various branches of mechanical engineering. In order to optimize the design of grey cast iron components for applications subjected to cyclic loading, a precise knowledge of the fatigue mechanisms is necessary. In the framework of a research project, the statistical and geometrical size effect on the fatigue strength of EN-GJL-300 is investigated. The statistical size effect describes the relationship between the fatigue strength and a changing stressed volume or surface and can be expressed with the highly stressed volume (HSV) of the component. The geometrical size effect instead takes into account the influence of a stress gradient e.g. due to notches on fatigue strength. Knowledge of the statistical and geometrical size effect allows the transfer of the fatigue strength from small specimens with different notch factors to complex large-scale components. For the investigation, unnotched and notched specimens with different highly stressed volumes are removed from cast samples as well as from a gearbox housing to determine the cyclic material behavior by stress controlled axial fatigue tests. The investigations are complemented by metallographic analyses to describe the influence of the graphite morphology on fatigue strength. The data obtained in this study serve as a contribution to the development of a design and lifetime assessment concept enabling an estimation of the local lifetime of the component as a function of the highly stressed volume and the local microstructural properties.