The effect of clamping stress on the fatigue strength of bonded high-strength steel interfaces

It is well known that the fatigue strength of a welded joint does not increase proportionally with steel strength. For this reason, alternative joining techniques for high-strength steels are of great interest. Some studies have shown that hybrid joints, which combine mechanical fasteners and adhesives, have improved fatigue resistance as compared to welded joints in high-strength steel structures. For reliable design, an in-depth understanding of the clamped and bonded interface under cyclic loading is needed. In this study, the napkin ring specimen geometry has been chosen for a series of constant-amplitude fatigue tests. The test setup provides a bonded interface with well-defined normal and shear stress boundary conditions. Test conditions included four different static normal pre-stresses for the grit-blasted surface finish and one pre-stress level for the fine ground. Fatigue strengths of all the pre-stress/surface type conditions were obtained by a statistical method. For grit-blasted surfaces, the bonded specimens with high static normal pre-stresses behaved identically to the non-bonded specimens with the same pre-stress, i.e. the fatigue strength was not improved by the addition of adhesive even though the static strength had previously been shown to improve significantly for the same conditions.