Avoiding crack nucleation and propagation during upset bulging of tubes

As an alternative to joining by fusion welding, joining by upset bulging (JUB) can be applied in cases where tubes are joined to plates, sheet metal, or other tubes and profiles. If a tube is joined to a pierced flat plate, joining is accomplished by creating bulges in the tube by axial compression, which enclose and securely lock the plate. The JUB process has a large potential for reducing cycle times, realizing joints between different materials and for increasing the dimensional accuracy of the joints compared to fusion welding. The joints produced by this forming technique are free from the negative impact of heat affected zones on the material properties. However, the material undergoes large plastic deformation during the JUB process. When the bulges are fully compressed, local damage and failure can be observed which reduce the service life properties of the joints. This paper presents an experimental and numerical study of the damage evolution and crack initiation in mechanical joining by upset bulging. Experiments were carried out to analyse the occurrence of failure. The results are supported by FE analyses. Nucleation of cracks strongly depends on the final bulge height and consequently on the degree of deformation on the inner side of bulges. Tensile tests show that cracks in the bulge reduce the strength of the joint to half of the undamaged material. A new bulge design with a modified shape is presented, which reduces the damaging effect of the upset bulging process and thus improves the strength of the joints.