This work aimed to design an undermatched lap joint that has an equal load-carrying capacity (ELCC) with a traditional equalmatched joint under out-of-plane bending. A weld strength calculation method was proposed based on the similarity of a lap joint and a T joint, as shown using linear elastic finite element (FE) analysis, and then applied in the analysis of a lap joint and the design of an ELCC lap joint. A single lap joint of HQ785 steel was chosen for experimental verification. The bending force limit of the ELCC joint was 93.35% of the theoretical prediction and 96.90% of the traditional equalmatched joint. The results show that the weld strength calculation method and the ELCC design method are reasonable and feasible.

In order to reduce the weld size of weld joints designed based on equal load carrying capacity (ELCC) further, the shape of under-matching butt joints is designed with the parameter of plastic limit loads. The realization condition of ELCC is that the plastic limit loads of the weld equals to that of the base metal. Through analysing the tensile/bending load carrying capacity of weld zone, the weld shape was determined, and the method of tensile/bending ELCC design for butt joints is given. The ELCC joints are designed and their elastoplastic finite element analyses are carried out to verify their load carrying capacities. The tensile ELCC joint fractures at the base metal in the mode of overall yield fracture and its load carrying capacity reaches the tensile strength of the base metal. The load carrying capacity, deformability and stiffness of the bending ELCC joint are better than that of the base metal. The results indicate that the joints with acceptable size meet the requirements of ELCC, which is of great significance to the application of ELCC joints.