Design and testing of an element test specimen for fatigue delamination crack growth initiating at a ply drop

In this work, we propose an element test specimen to explore the monitoring and predictability of fatigue delamination crack growth from a ply drop. The design procedure to create an element test specimen out of a full composite blade is presented. The focus of this work is on the design challenges of the specimens and the measurement of growth rate of the fatigue delamination crack. An analytical ply drop model is used to determine the design parameters of the element test specimen, i.e. the ply thickness, the thickness of the underlying layers, and the applied load levels. The present element test specimen is designed for cyclic tension–tension loading. During the testing, three damage evaluation methods, digital image correlation (DIC), acoustic emission (AE), and infrared (IR) thermography are applied to track the fatigue delamination crack growth. The growth rate of the fatigue delamination crack is estimated quantitatively from the crack tip displacement fields obtained by DIC and the localisation of events captured by AE sensors, respectively. Results show that after the crack has extended approx. 80 mm, the delamination crack tip propagates at an approx. constant growth rate consistent with steady state as the crack bridging zone is fully-developed. The crack extension values estimated by the three damage evaluation methods are in reasonably good agreement at high strains. However field blades in operation are loaded much lower than the test specimens. In field, AE might be possible for detecting subsurface delamination cracks, but IR thermography from the outside of a rotating blade is likely to be very limited.