Evaluation of the fracture toughness of butt-welded joints using the boundary effect model

Yielding loads (Py) from small three-point-bending (3-p-b) specimens with notch tips in the heat-affected zone, fusion zone, weld metal, and base metal of Q235 (common carbon steel in China) were used to estimate the corresponding fracture toughnesses. To model the elastic and plastic fracture around the Q235 welded joint, the boundary effect model initially developed for quasi-brittle fracture of heterogeneous solids with a large crack-tip fracture process zone was adopted in this study, bypassing the stringent ASTM standard requirements on the specimen size, initial crack length, and un-notched ligament. It was found that the weld metal had the highest fracture toughness value around 130 MPa√m, followed by the heat-affected zone of 109 MPa√m, the base metal of 83 MPa√m, and the fusion zone of 77 MPa√m. Microstructures and notch-tip plastic zones in each section of the welded joint were measured and used to explain the fracture toughness measurements.