Evaluation of fatigue behavior of a casting aluminum knuckle considering influences of inhomogeneous microstructure

Abstract

Until now the fatigue behavior of casting aluminum components is usually evaluated in the safety analysis without considering the inhomogeneity of local properties. It can result in an underestimation of damage risk or an oversizing of the components. The purpose of the work is to quantify and to predict the influence of the inhomogeneity of microstructure and the corresponding local properties on the overall behavior of the selected casting aluminum knuckle under cyclic loading. Since no shrinkage pores are observed in the knuckle produced by the counter pressure casting process, influences of porosity on fatigue property are not taken into account in this study. Tensile specimens are extracted from different positions in the knuckle and tested under quasistatic and cyclic loading. The dependences of the flow stress, the fracture strain, and the S-N curve on position for specimen extraction are evaluated. Metallographic investigations are performed to reveal the relations between microstructure and the mentioned properties. A relationship between the S-N curves and the secondary dendrite arm spacing (SDAS) is derived from the experimental results. The corresponding parameters of the new fatigue damage model are determined and used to simulate the specimens and component tests. The distribution of SDAS in the component is determined by casting simulations and transferred to the FE-model for the fatigue analysis. The standard fatigue tests on knuckles are performed and the damage behavior is analyzed in view of the influence of microstructure. The simulations of the knuckle fatigue tests are performed with SDAS-dependent material parameters. The prediction of the fatigue behavior of the knuckles agrees with the experimental results well.