Fatigue Analysis of a Welded Lap Joint Under Bending Load

Welded lap joints subject to cyclic loading are used in industrial vehicles and equipment. Cyclic bending loads can result in fatigue failure of these joints. It is desirable to predict the expected life of the joint as a function of a load-stress parameter. To predict the life of a typical welded structure, additional types of weld joints and loads must also be considered. The lap joint work covered here provides a building block for the structure life prediction. For the work reported here, lap joints were formed by an industrial manufacturing wire feed weld process by certified welders. Strain gages were applied to one of the joint members. A fixture was designed and built to apply a pure bending load to the welded members at the joints, and this was installed in a fatigue test machine. A number of fully reversed tests were run to failure at various loads and some were run to effective infinite life. A resulting set of stress life curves was generated based on mean life and mean life less one and two standard deviations as a function of the “hot spot” stress. This hot spot is the region near the weld toe where the stress distribution starts to deviate from linearity due to the geometric stress riser of the weld fillet and joint geometry. Measurements show that this stress can be approximated by the calculated beam bending stress for the specimens and loads used in the tests. The results of this work provide guidelines for design of welded lap joints for a desired cycle life and for predicting the expected cycle life of an existing joint including uncertainties in life due to weld and test variables. The stress needed to predict the life of a joint is obtained by strain measurements on the actual structure. For design, the stress may be approximated by calculated stresses.