Fatigue Life Prediction of Welded Joints for Offshore Structures Under Variable Amplitude Loading

This paper describes the development of a fracture mechanics model for the prediction of fatigue life of welded joints subjected to variable amplitude loading. The prediction program is based on the integration of the Paris equation. A novel feature of this program are relationships which account for the development, growth and coalescence of multiple cracks that occur at the weld toe. These relationships or forcing functions were developed from extensive monitoring of the initiation and crack shape development of multiple cracks during experimental tests. Data is also presented on a series of variable amplitude tests performed on welded T-plate joints loaded in 3-point bending using the common load sequence (COLOS) at a stress ratio of R=−1. These results are presented in terms of effective and root mean square stress ranges. The experimental data are used to validate the life prediction results from the growth model. A discussion is also included on the proportion of the compressive stress range to be included as an effective stress range in the prediction model calculations.