Fatigue performance of weldbonded high strength sheet steels tested in Arctic, room temperature and tropical environments

Weldbonding was performed on a galvannealed rephosphorised sheet steel of 0.8 mm thickness. A one-component structural epoxy resin for hot curing was used. Fatigue testing was performed on H-beams in peel mode. The specimens were preconditioned in high humidity at room temperature before fatigue testing at -20°C and at +50°C / 90% relative humidity. Reference test series were performed without preconditioning and with testing at ambient laboratory conditions. Two different degrees of filling of the adhesive was investigated. Tests were performed at constant amplitude with R=0.1. Different fatigue life criteria were evaluated based on stiffness losses mainly in the bond and finally in the spot welds. Fracture surfaces after the failures were classified. A model was proposed for prediction of fatigue life. It was based on fracture mechanics and the finite element method. It describes how a cohesive crack propagates mid through the epoxy resin. The model illustrates that the spot welds deteriorate the bond quality so that life is reduced compared to a bond without spot welds. Significant differences were observed between fatigue strengths of weldbonded peel specimens at the different temperatures and environments. Weldbonding produced significantly higher fatigue strength than spot welding.