Hygrothermal Degradation And Recovery Of Properties Of An Automotive Structural Adhesive: Phenomenon, Mechanism, Prediction

Automobile weight reduction boosts the application of new materials in car body structures. Secure connection between body components made of new materials is obtainable with the adoption of adhesive bonding technology, which features a slew of unique advantages over traditional joining methods. However, the hot humid automotive service environment poses a threat to the mechanical properties of adhesive joints. In this study, we aim to achieve a better understanding of automobile structural adhesive hygrothermal degradation(HTD) by means of experimental measurements, molecular dynamics(MD) simulation and finite element(FE) simulation.In the experimental study section, the phenomenon of adhesive HTD was explored, with great emphasis placed on total strain energy and bulk–interface decoupling. Bulk adhesive, including uniaxial tension(UT) and notched tension(NT) specimens, was tested to examine the energy absorption(EA) and HTD behaviours, with the impact of specimen flaw carefully considered. EA was later corrected with double cantilever beam(DCB) test and FE simulation. A series of adhesive joint tests, incorporating DCB and cross tension(CT) specimens, were finely tuned to generated prescribed proportion of interfacial failure, which revealed the reduction and recoverability of interfacial strength owing to HTD.In the MD simulation section, the mechanisms of adhesive HTD were investigated using quantitative method from a micro-scale perspective. Firstly, the molecular model of the adhesive was constructed, based on simplification of an industry level epoxy adhesive. Secondly, HTD and dehydration were reproduced in the model by means of water inclusion and bond scission. Thirdly, the mechanical properties of the molecular system were predicted. Existing studies, all based on qualitative speculations, have attributed the recoverable part of HTD to plasticisation and the irrecoverable part to hydrolysis. The above statement was quantitatively examined at micro-scale. Findings suggest that both plasticisation and hydrolysis are recoverable.In the FE simulation section, this study compared and established connection between two of the most prevalent simulation techniques, i.e. representing the adhesive with solid element(SE), and with cohesive interface(CI). Based on adhesive bulk and DCB joint test data, the parameters of SE and CI were calibrated, which were then validated using CT test results. Good correlation was achieved in both cases, indicating the intrinsic relation between the two methods. Therefore, an integrated simulation method, incorporating both SE and CI, was proposed to obtain both shorter data acquisition time and lower computation cost.