2018 年 36 巻 4 号 p. 253-263
Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out by using 980MPa 0.13%C high strength steel sheets of 1.0mm, 1.6mm and 2.0mm thickness. At the 5mm of nugget diameter full plug fracture were observed in cross tension resistance spot welded joint of 1.0mm, on the other hand partial plug fracture were recognized in cross tension resistance spot welded joint of 1.6mm and interface fracture were indicated in cross tension resistance spot welded joint of 2.0mm. These test results imply that it is necessary to consider the thickness effect in order to prevent deterioration of strength in resistance spot welded joint.
In case of interface fracture of cross tension resistance spot welded joint of 2.0mm, circumferential crack initiation due to separation of corona bond arose. The crack opening process without propagation (plateau crack length) was recognized until just before fracture and then the crack propagated to nugget immediately in brittle manner around CTS.
Fracture toughness (Mode I stress intensity factor and crack tip opening displacement) at CTS of nugget diameter of 3√t, 3.5√t, 4√t and 5√t in cross tension resistance spot welded joint of 2.0mm thickness were almost the same to the previous study of nugget diameter of 3√t in cross tension resistance spot welded joint of 980MPa 0.13%C 1.6mm thickness. Average fracture facet size of interface fracture and average packet size of nugget (fully martensite) were almost the same with meaningful scatter. These experimental results imply that fracture toughness were almost same independent of nugget diameter, sheet thickness and strength of steel sheet because the nugget of same C contents showed same hardness and same packet size with meaningful scatter as long as brittle chemical elements (P and S) were extremely changed.