Numerical Investigation into Contact Fatigue of Small End of Engine Connecting Rod Based on Elastic-Plastic Damage Model

The connecting rod is one of the most important parts of the engine but suffers the risk of fatigue due to the alternating loads, especially at the small end where the stress is locally concentrated by its contact with the piston pin. In this study, a finite element model is established for the connecting rod in contact with the piston pin subject to combustion force that is experimentally obtained. The motion pattern of the connecting rod is equivalently modified to save the calculation cost, and the elastic-plastic behavior of its material is considered according to the nonlinear isotropic/kinematic hardening law. The damage-coupled elastic field can be conveniently obtained by the contact model developed in the finite element tool; nevertheless, the plastic stress/strain should be iteratively determined based on the plastic flow criterion. The damage evolution of the connecting rod is modeled by continuum damage mechanics, and it is induced by both the elastic and plastic components, which are determined by cyclic stress and accumulated plastic strain, respectively. The constitutive relations of the material for the damaged elements are updated at each analysis step by the user-defined material (UMAT) subroutine to consider the reduction of Young's moduli caused by damage and the effect of plastic deformation. It can be concluded that the damage accumulates gradually on the small end and significantly weakens the material's mechanical properties, leading to the decrease and redistribution of concentrated stress and plastic evolution, which in return accelerates the failure process of the connecting rod. The model considers the interactions of plastic zones and damaged areas and can potentially provide insights into the contact fatigue and damage evolution of engine connecting rods.