Accuracy Study in Predicting Fatigue Life for a Welding Joint.

Abstract: This thesis describes a comparative study between two calculations methods for life estimates of a welding joint. The two studied methods are the Effective Notch and Linear Elastic Fracture Mechanics (LEFM). The effective notch method is today used at GKN and gives an equitable accuracy relative the work effort for the method. The use of LEFM is more accurate, but on the expense of calculation time and complexity. The aim with this thesis was to investigate if the LEFM method is feasible for the day-to-day work at GKN Driveline. In the effective notch method, inaccuracies and the stress concentration in the weld are collected in a fictional notch with a radius of 0.05 mm. The stress amplitude is evaluated in the notch and the relation between stress and fatigue cycles is collected in an S-N curve for the weld. In the LEFM method a small crack is introduced in the weld and a few number of fatigue cycles are performed to simulate crack growth. The real numbers of fatigue cycle necessary to fracture the weld are calculated by integrating Paris Law. The accuracy of the methods was measured with correlation between simulations and results from experiments. The accuracy for the effective notch was widely spread between different designs, indicating sensitivity to the notch configuration and location. The LEFM method gave better accuracy but at the cost of increased computational time. The LEFM method was not feasible to conduct on two of three investigated weld designs and to implement LEFM in GKN’s daily work requires that the method is developed to work on all design solution the company offers.