Hydro-thermal fatigue of polymer interfaces Original Research Article

We have developed an experimental procedure for measuring the crack growth along polymer interfaces under hydro-thermal (HT) fatigue conditions [relative humidity (RH) is held constant while the temperature is cycled]. The technique has been used to test an interface between an anhydride-cured epoxy and a polyimide (PMDA/ODA). Our sample is a trilayered cantilever beam with the epoxy bonded over a polyimide coated metallic beam. The displacement of the metallic beam at the lowest temperature in the thermal cycle is measured and is converted into the HT crack growth along the interface. The experimentally determined HT crack growth per unit temperature cycle (da/dN) is correlated to the maximum difference in the thermal strain energy release rate at the crack tip during each temperature cycle (ΔG). While the Paris law relationship (where da/dN increases as a power of ΔG) holds for thermal (T) fatigue, it does not hold true for HT da/dN measured under high RH. We have modeled the HT da/dN as a summation of two components: (a) the da/dN due to T-fatigue and (b) the da/dN due to the stress-assisted water attack (SAWA) along the interface for a given T-cycle. While an empirical power law relationship describes the T-fatigue, the SAWA is described by a thermally activated kinetics model. Our model predicts the experimental observations and measurements well and provides an insight into the mechanisms behind the failure of the polyimide/epoxy interface during HT fatigue.