Fatigue strength of adhesively bonded deep drawn sheet specimens under multiaxial loading with variable amplitudes

Adhesive bonding is a frequently used joining technology in the manufacturing of vehicle bodies. In comparison to conventional spot-welded designs, adhesive bonding provides a higher stiffness due to a planar load transfer. Furthermore, there are advantages in damping behavior and chemical separation in multi-material designs. Up to now, there exists still quite some uncertainty in the fatigue assessment of adhesively bonded joints due to a lack of reliable assessment approaches. This is especially true for the case of more complex in-service loading conditions as variable amplitude loading, due to the sparse database of experimental fatigue results, which are needed for the development of new methods. For this reason, the results of such tests are presented in this paper on component-like specimens, so called bowl specimens, where even uniaxial loading results in multiaxial stress state due to the complexity of the geometry. These specimens consist of a deep drawn sheet that is bonded with a thermosetting structural adhesive to a planar sheet and are subjected under cyclic loading. They were loaded uniaxially and multiaxially, with (φ=90°) and without phase shift (φ=0°). As a complex shaped and stressed sheet metal specimen, the bowl specimen closely resembles components of e.g. vehicle bodies, which is why the conducted tests are presented for a better understanding of such vehicle structures. It is shown that a phase shift leads to an increase of the fatigue life. Furthermore, the cyclic stiffness degradation is discussed, which often serves as a failure relevant parameter in real structures.