Post-curing process and visco-elasto-plastic behavior of two structural adhesives

The aim of this paper is to reveal original visco-elasto-plastic phenomena for two commercial epoxy adhesives (D609 and E20HP) subjected to uniaxial tension and compression. First, a post-curing heat treatment is proposed by means of thermal analyses in order to ensure stable mechanical properties. Bulk adhesive specimens are prepared to analyze the mechanical response of both materials. Monotonic tensile and compressive tests are carried out at different strain rates. Both adhesives exhibit first a linear elastic behavior but once a yield stress is reached, a visco-elasto-plastic behavior appears. Creep tensile tests are also carried out and confirm that strain rate phenomena take place and that non-negligible negative volumetric inelastic strains appear. Cyclic tests are also performed and reveal ratcheting effects. The applicability of the results to thin bondlines is discussed. The experimental observations must be taken into account in any model which aims at predicting accurately the behavior of the adhesives considered in this paper.