Failure characterization of spot welds under combined axial–shear loading conditions

This paper proposes an accurate failure criterion of spot welds under combined axial and shear loading condition. Test fixtures and a specimen were newly designed with the aid of information from finite element analysis results in order to obtain the failure load of a spot weld under the combined load with the constant ratio of the shear load to the axial load. The testing apparatus designed involves a pin-joint between a loading frame and a testing fixture for improved constraint conditions and a cross-type specimens spot welded using guide plates in order to assure the constant ratio of the shear load to the axial load preventing a spot weld from rotating during the test. Using the designed test fixture and specimens, failure tests of spot welds were conducted with the variation of seven different loading angles in order to obtain failure loads and identify failure modes at each angle. Failure loads of spot welds were investigated with experiments for three different materials of a mild steel, a high strength steel and an advanced high strength steel (AHSS). The failure loads obtained from experiments at various loading angles are utilized to propose a failure criterion for description of failure behavior of a spot weld. The failure criterion of a spot weld as a function of the axial load and the shear load is expressed as a different function from an elliptic function, which was proposed in previous researches: the different function is called a β-norm function in this paper. It was found that the failure criterion proposed provides a fairly accurate description of the failure load obtained from experiments under combined axial and shear loading conditions.

► This paper proposes a new failure criterion for failure load of spot welds under combined loading.
► The proposed apparatus involves a pin-joint in pull bars to prevent unfavorable rotation of nuggets.
► The specimen is attached to two guide plates to prevent unfavorable rotation of nuggets.
► Failure tests of a spot weld were conducted with seven different loading angles.
► The failure criterion proposed takes a shape of a β-norm failure contour for better description.