Mechanical performance and failure behavior of screw-bonded joints of aluminum sheets and cross-laminated birch veneer plates

Integrating wood-based lightweight components into advanced multi-material car bodies requires capable and reliable technologies for joining or bonding of dissimilar materials. Therefore, the present experimental study investigates hybrid lap joints of 1.5 mm-thick sheets of EN AW-6016-T4 aluminum alloy and 9 mm- or 13 mm-thick plates of cross-laminated 1 mm-thick birch veneers. Two self-cutting screws and single-component polyurethane-based adhesive were used for each joint. The mechanical performance and the fracture behavior were tested under both quasi-static and cyclic shear-tensile loadings. At each load condition fracture of the joint occurred, but neither fracture of the aluminum alloy sheet nor fracture of the veneer plate were observed. Debonding of the aluminum alloy sheet from the birch veneer plate was identified as critical failure mechanism. The adhesive provided the main contribution to both the static strength and the fatigue performance of the joints. However, the energy absorption of the joint was mainly determined by the pull-out resistance of the screws, which was dependent on the thickness of the veneer plate. To optimize the strength-to-weight ratio of the joint and to exploit the load-bearing potential of the materials, the focus should be placed on the improvement of the bonding conditions or on the adaptation of the sheet/plate thicknesses.