Fatigue properties and failure characterization of spot welded high strength steel sheet

Fatigue properties and failure characterization of high strength spot welded steels, such as DP600 GI, TRIP600-bare and HSLA340Y GI, have been conducted. Tensile shear and coach peel samples have been used in this investigation. HSLA340Y GI samples were used as the baseline material for comparison. Microhardness was measured to study the hardness change across the weld nugget. Under low load and high cycles situation all the materials show very similar fatigue strength. Crack initiation and propagation during the fatigue loading history has been experimentally determined and discussed. Microhardness tests show that DP600 GI samples have the highest hardness, about 420 HV in weld nugget and 250 HV in base metal, followed by TRIP600-bare, about 400 HV in weld nugget and 220 HV in base metal, and HSLA340Y GI, about 320 HV in weld nugget and 160 HV in base metal. It was also found that both DP600 GI and HSLA340Y GI display softening during high load and low cycles fatigue tests with HSLA showing more prominent softening behavior.