Failure mechanism of resistance spot-welded DP600 steel under high cycle fatigue

Abstract

Dual phase (DP600) steel is one of the most predominately used advanced high strength steel in the vehicle manufacturing. Resistance spot welding is the most frequently used joining methods in automotive industry. The vehicle during its lifetime in service experiences fatigue loads because of rough roads, load excursions, harshness, and vibrations. The fatigue failure of the spot-welds mostly occurs at lesser than that of the static joint strength. This kind of failure is of serious concern as it will directly affect the overall crashworthiness of the vehicle. DP600 steel of thickness 1.4 mm were resistance spot-welded at a weld current of 8.5 kA, weld time of 225 ms, and electrode force of 3.8 kN in tensile-shear configuration. The microstructure and failure behaviour were examined by optical microscopy. The microhardness distribution in the welds was determined by microhardness tester. The tensile-shear tests and fatigue tests were carried on universal testing machine. The objective of present work is to investigate failure mechanisms of DP600 steel by high cycle fatigue tests. The major conclusions from the present work: The load carrying ability is 22 kN and energy absorbed is 36 J for the joint under tensile-shear loading. The fatigue crack initiation primarily occurs from heat affected zone (HAZ) or from the region close to fusion zone. In the next stage fatigue crack would propagate in through thickness and width direction to complete the final failure. The failure mechanism of spot-welds under fatigue loading is primarily governed by stress concentration at the interface of two conjugated sheets, stress intensity factor, and strength of the base metal.