Analysys and optimization of an end double-lap bonded joint for GFRP composite sandwich panels

Although the best configuration of the joints used to connect a common GFRP sandwich panel with the main structures is the adhesively bonded one, that permit high static and fatigue performances as well as to avoid stress concentrations, mechanical or hybrid joints are still widely used in the industrial field. After a preliminary theoretical design, the optimal configuration of an end adhesively bonded double-lap joint constituted by a simple insert as internal adherent and the same face sheets as external adherent, has been researched. In detail, several experimental tests and successive numerical simulations under tensile and bending loading, performed by varying the main influence parameters as the overlap length and the material of the internal insert (aluminum, steel, CFRP), have been carried out. In brief, the experimental and numerical analyses have shown that, due to the limited effects of the stiffness unbalancing of the joints, as well as to the appreciable peel stress values associated with tensile and especially bending loading, the optimal joint configuration is obtained in practice by using an insert made by CFRP with an overlap length equal to about a double the theoretical overlap value. Also, due to the different damage tolerance of the epoxy adhesive to the shear and peel stresses, the accurate strength prediction of such joints has to be performed by assess the failure processes that occur at the two attach points of the double lap joint. In detail, the delamination growth that can occurs at the attach point of the face sheets, can be predicted by using a delamination criterion along with a point stress approach, whereas the unstable delamination growth that can occur at the opposite point of attach of the insert, can be evaluated by using a simple delamination criterion with the classical approach of the maximum stress.