Development and implementation of a simultaneous fatigue crack growth test setup for polymeric hybrid laminates

Delamination is a common and highly relevant in-service failure mode of laminated structures. Cyclic mechanical loads superimposed with environmental factors, such as hot-humid air, are most critical. To evaluate the cyclic debonding resistance of polymeric hybrid laminates, fracture mechanics approaches based on double cantilever beam (DCB) specimens are well established. However, conventional fatigue test setups allow for measurement of just one specimen which is rather time consuming. The main objective of this paper was to develop and implement a novel fatigue test setup for simultaneous characterization of multiple DCB specimens. To validate the simultaneous fatigue testing system, steel/epoxy laminates of different number of steel plies and epoxy adhesive layers with or without filler were characterized using the novel fatigue test setup and a conventional electrodynamic test machine. Fatigue crack growth data was fitted by a form of the Hartman-Schijve crack growth equation. To derive a statistically significant threshold value a fitting procedure was employed. A good agreement of the crack growth kinetics data at ambient conditions was obtained confirming the high quality and reliability of the novel simultaneous fatigue test setup. Moreover, reliable data were gathered at the beginning of the displacement controlled experiments in the instable crack propagation regime, as the displacement amplitude was reached from the first cycle onward. The novel test methodology is of special relevance for development and screening of adhesive formulations.