Damage development in open-hole composite specimens in fatigue. Part 1: Experimental investigation

An extensive experimental program has been carried out to investigate and understand the sequence of damage development throughout the life of open-hole quasi-isotropic IM7/8552 carbon-fibre/epoxy laminates loaded in tension–tension fatigue. Samples were initially quasi-statically loaded to failure to determine an average strength, σUTS. Specimens were then cyclically loaded at 5 Hz at various amplitudes to 1 × 10+6 cycles or to failure (15% loss of stiffness), which ever occurred first. Interrupted fatigue tests were carried out with peak amplitude at 60% of the ultimate static load (60% severity) in order to determine the 3D sequence of damage events using X-ray Computed Tomography (CT). Matrix cracking at the surface ply and initiation of matrix cracks in the subsequent plies lead to delaminations that progress through the thickness, and ultimately to the propagation of delamination at the −45/0 interface all the way back towards the end tabs. The log (number of cycles to failure) decreased linearly as the maximum fatigue stress level increased.