Estimation of Fatigue Life for Welded Gears Using LEFM

Abstract: The aim of this thesis is to estimate the high cycle fatigue life of welds using finite element methods and liner elastic fracture mechanics (LEFM). Develop a new method of calculating the fatigue life that can replace the old method in use today at GKN Driveline. This is a continuation of an earlier master thesis1. When gears have been subjected to high cycle fatigue testes, cracks have been observed propagating though the welds and resulted in component failure. The current method of estimating the welds fatigue life has a tendency to overestimate the life and thus a new method with better accuracy is sought after.The crack propagation is simulated by starting with an initial crack in the weld which is then incrementally increased in a pre-determined direction. The energy release rate for each node at the crack tip is calculated using virtual crack closure technique (VCCT). Due to multiaxial loads on the weld, an effective stress intensity factor is used together with a Paris Law type equation to estimate the propagation speed. The total amount of cycles it takes the crack to propagate between each of the increments is then estimated.The results show a good correlation to experimental data and takes approximately 40% longer time compared to the old method. The new method has a high dependency on good material parameters and thus important that these are chosen accurately, which can be especially hard for welds. The results can also be linearized which means that the life can be estimated for different load levels from one single simulation.