Characterizing the constitutive properties and developing a stress model for adhesive bond-line readout

Visible deformation on the exterior surface of adhesively-bonded automotive body panels in the vicinity of the adhesive application line is referred to as bond-line readout (BLRO). Differential shrinkage of the adhesive and substrate as an assembly cools from bonding temperature to room temperature is the primary factor responsible for BLRO. The gradual relief of bondline readout over time suggests that relaxation of residual stresses within the adhesive layer occurs. This work addresses determination of viscoelastic and thermal expansion characteristics of representative epoxy and polyurethane adhesives for input into numerical models to predict bondline readout. Besides the constitutive properties, usefulness of the bimaterial curvature technique was also investigated for measuring residual stresses in a coating layer, as well as the stress-free temperature. Knowing the constitutive properties and stress-free temperature, the residual stresses and deformations in a steel-epoxy bimaterial specimen were modeled using a simple recursive approach.