Derivation of a fatigue damage law for an adhesive from in-situ bending tests

Abstract

The adhesives used for the attachment of electronic components to space boards should withstand harsh vibrations of the space launch, which requires their characterization in fatigue. The present study investigates Ablestik 8-2 epoxy adhesive within this context. Novel adhesive test assemblies were devised, which consist of a rigid ceramic component bonded to a resonant flexible epoxy-fibreglass (E-glass) support. Cantilever and square E-glass supports produced uniaxial and biaxial bending, respectively. The in-situ fatigue tests, conducted on batches of uniaxial/biaxial bending adhesive assemblies, led to distinct data sets of maximum support deflection versus the number of cycles to crack initiation/total failure. The derivation of an intrinsic fatigue damage law of the adhesive relied on the substitution of deflection by the maximum principal strain of each adhesive point while keeping the Basquin’s form. In so doing, the adhesive strain was computed from finite element models of test assemblies built and simulated under Abaqus. The adhesive layer was meshed by cohesive elements created through a Fortran user-element subroutine coupled to Abaqus. The subroutine incorporated an already validated static damage together with the strain-based fatigue damage law sought. The retained Basquin’s fatigue damage law has undergone calibration against uniaxial bending test data and validation by biaxial bending test data.