Measuring the influence of laser welding on fatigue crack propagation in high strength steel

Laser welded products often have to be designed and optimized for fatigue load. The most common method to characterize the fatigue strength is periodic loading of a weld for different peak loads to identify an S-N-curve. To obtain information on how metallurgy influences fatigue cracking behaviour, three-point bending tests with a laser weld normal to the cracking direction were performed. The crack length was derived from measurement of the compliance of the specimen during fatigue loading in three point bending and the growth rate was obtained through differentiation. The weld surfaces were machined off to eliminate stress raisers. 8 mm thick high strength steel was laser welded, as bead-on-plate. When crossing the weld, a difference of the crack propagation rate was found when compared with the base material. Changes in the propagation rate are either caused by varying microstructure or by residual stress. Different steels and weld parameters were investigated.