In-situ tensile tests of welding joints of bainite high strength steels with notch specimens were studied.Fracture processes of welding joints of this high strength steel were observed by in-situ tensile tests,and fracture mechanism was also analyzed.By
these studies, the weak link of welding joints was determined.It is found that:plastic deformation is produced at notch tips with applied load increasing.For slight notch specimens, plastic deformation appears at the notch root, a short plastic crack is initiated, propagated, blunted and repeated process are done, finally, the specimens are fractured along notch root. For the specimens with a large arc, at the action of shear stress, plastic deformation appears firstly, and necking are produced, then microcracks are initiated and
propagated. The processes are repeated until the specimen fracture. Initiated stress and fracture stress are almost the same for large arc specimens and smooth specimens, so sudden fracture are produced. In the smooth tensile specimens, fracture is produced at weld metals zone, so weld metals are the weak link of welding joints.
