The mechanics of bondline thickness in balanced sandwich structures

Assuming negligible in-plane stress, non-varying shear stress, and linearly varying transverse stress along the thickness of the adhesive, the closed-form solutions for the interfacial shear and the interfacial peeling stresses in balanced bonded sandwich structures was derived. The nil-shear-stress condition at the free edge of the adhesive was enforced through the use of a decay function. The solutions for the shear and the peel stresses along the interfaces right up to the free edge were validated with the finite element analysis. The solutions were used to investigate (i) the observed increase susceptibility of single lap joints with increase bondline thickness and (ii) the nil report of such susceptibility for electronic assemblies experiencing differential thermal strain. The first investigation was compromised by the inability of the solutions to model the singular stress/strain field at the free-edge of the interfaces. The second investigation revealed a far higher rate of reduction in the magnitudes of the interfacial shear and peeling stresses in structures (electronic assemblies) that experience differential thermal strain than in structures (single lap joints) that experience bending strain.