Hygrothemal Degradation of Toughened Adhesive Joints: The Characterization and Prediction of Fracture Properties

The main objective of this work was to develop a framework to predict the fracture toughness degradation of highly toughened adhesive joints using fracture test data obtained by accelerated open-faced degradation method. First, the mixed-mode fracture resistance (R-curve) behavior of two rubber-toughened epoxy-aluminum adhesive systems was measured and could be fit in a bilinear R-curve model. Then, open-faced DCB (ODCB) specimens of the same adhesive systems were aged over a relatively wide range of temperature, relative humidity (RH) and time, dried and tested to characterize the irreversible evolution of the mixed-mode fracture R-curves. The R-curve bilinear model parameters of adhesive system 1 varied significantly with degradation while that of adhesive system 2 remained unchanged. The absorption and desorption of water in the adhesives cast wafers was measured gravimetrically. The absorption data were fitted to a new sequential dual Fickian (SDF) model while water desorption was modeled accurately using Fick’s law. A significant difference was observed between the amounts of retained water in the two adhesives after drying. An exposure index (EI) was defined as the integral of water concentration over time and calculated at all points in the ODCB and closed DCB joints. The fracture toughness of the closed joints was then predicted from these calculated EIs by making reference to fracture toughness data from the ODCB specimens degraded to various EI levels. To verify the predictions, fracture experiments and analyses were carried out for closed DCB joints. Good agreement was found between the predicted and experimentally measured fracture toughness values for the degraded closed DCB joints. Furthermore, the crack path and fracture surface characteristics were evaluated as a function of the degree of aging using optical profilometery. The unexpected crack path in the mixed-mode fracture of unaged open-faced DCB specimens was addressed. The results showed a strong relationship between fracture surface parameters and the critical strain energy release rate, Gcs, irrespective of the type of adhesive and exposure condition.Ph