Abstract
Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out by using 980MPa high strength steel sheet of 1.6mm thickness. In proportion as nugget diameter increase from 3√t to 5√t (t; thickness) cross tension strength (CTS) increased, while fracture morphology simultaneously transferred from interface fracture to full plug fracture. In case of interface fracture, circumferential crack initiation due to separation of corona bond arose at early stage of loading. The crack opening process without propagation was recognized until just before fracture and then the crack propagated to nugget immediately in brittle manner around CTS. In full plug fracture, main ductile crack initiation from notch-like part at the end of sheet separation occurred with sub crack initiated at early stage. The ductile crack propagated toward HAZ and base material to form full plug fracture.
Mode I stress intensity factor was considered as a suitable fracture parameter for interface fracture because circumferential crack behaved pre-crack for brittle fracture in nugget region at final stage. Based on the FE analysis, mode I stress intensity factor was calculated 116MPa√m at CTS as fracture toughness for nugget. With respect to full plug fracture, ductile crack initiation behavior from notch-like part was expressed by concentration of equivalent plastic strain. On the assumption that ductile crack arose in critical value of equivalent plastic strain, the value was calculated 0.34 by FE analysis. Reasonable interpretation for interface fracture and full plug fracture in resistance spot welded joint was proposed due to first crack initiation by stress concentration, brittle fracture by using mode I stress intensity factor and ductile crack initiation by using equivalent plastic strain.
