Environment assisted cyclic fatigue crack growth testing of polymer/inorganic laminates under strain energy release rate control

Abstract

We focus on the development and implementation of an environmental (hot-dry or hot-humid air) testing methodology for adhesively bonded laminates under maximum strain energy release rate (SERR) control. The technique overcomes disadvantages of displacement- and force-controlled fatigue crack growth testing. Fracture mechanics based double cantilever beam (DCB) specimens comprising steel/epoxy- and glass/polyolefin-laminates were investigated using an electrodynamic test system with automated software control. Crack length was evaluated in-situ with a compliance calibration approach. User-defined SERR values were set with a control channel. The SERR was changed every 200,000 cycles or after exceeding crack length milestones. For steel/epoxy-laminates with an adhesive thickness of approximately 5 µm a good agreement of crack growth rates was obtained by displacement and SERR control. Moreover, the negative effect of humidity (50% vs. 10% rh) on the fatigue delamination resistance was confirmed at 60 °C. In contrast, significant deviations of crack growth rates under displacement and SERR control were deduced for glass/polyolefin-laminates with an adhesive thickness of 250 µm under hot-dry (60 °C, 1%rh) conditions. The deviations were attributed to differences in the damage zone size for the soft polymeric adhesive. Under hot-humid (60 °C, 80%rh) conditions a reasonable agreement of displacement and SERR controlled test data was observed and attributed to a less pronounced damage zone and a failure of the glass/polyolefin-laminates in an interfacial manner.