Viscoelastic adhesive modeling of ductile adhesive-composite joints during cyclic loading

Flexible adhesives have recently gained increased attention for their use in the joints of fiber-polymer composite structures because of their pseudo-ductile response and energy dissipation capacity. However, their structural application requires further study as their viscoelastic mechanical behavior is highly nonlinear and sensitive to the displacement rate in particular. In this study, a phenomenological model was developed, consisting of two parallel acting conventional-linear and extended-nonlinear Maxwell units, to model the monotonic and cyclic loading of an adhesive joint under various displacement rates; the viscoelastic parameters were calibrated by experimental results. Power-law relationships between 1) the monotonic viscoelastic parameters and the applied displacement rates, 2) the cyclic viscoelastic parameters and the maximum cycle displacements, and 3) the constants of the cyclic viscoelastic parameters and the applied displacement rates, well captured both the pre- and post-yield monotonic and cyclic responses, such as stretching of molecular chains (strain hardening), and formation and accumulation of damage (softening). The model was also able to predict the cyclic envelope curves and cyclic energy dissipation well.