A MULTIPLE CRACK MODEL FOR FATIGUE IN WELDED-JOINTS

Several linear elastic fracture mechanics (LEFM) models have been developed to simulate the fatigue performance of welded joints, particularly for offshore structures. Such models are used to estimate residual life when cracks are discovered in service. These models are also used to interpret available experimental results and may eventually be used in design. A key element to the success of such models is the realistic modelling of fatigue crack shape development. In all fatigue testing of welded T-plate, pipe-plate and tubular joints in the Canadian Offshore Research Programme, care was taken to monitor crack shape development using beach marks and potential-drop techniques. Crack shape development was significantly influenced by specimen thickness, stress distribution and environment. A fatigue model is proposed which explicitly models the growth and coalescence of multiple semi-elliptic fatigue cracks. Such a model has the potential of modelling geometry and environmental effects through their influence on crack shape development. The basics of this model, including coalescence and the calculation of stress intensity factors, are discussed. Verification studies involving both T-plate and pipe-plate specimen geometries are presented.