Investigating the mode II delamination of composite laminates using in-situ X-ray computed tomography

Delamination is among the most prevalent life-limiting failure modes in laminated composites. Mode II delaminations, usually triggered by off-axis cracks in tensile loading or through interlaminar shear stresses in flexural or impact loading, are more challenging to hinder than mode I delaminations. We, therefore, conducted a novel experiment using in-situ X-ray computed tomography to capture the mode II delamination initiation and propagation at the meso- and microscale for a thin-ply carbon/epoxy composite laminate. Such observations of delaminations at the microscale have only been performed in mode I, and this work is the first to deal with mode II. We employ the four-point end-notched flexure configuration that offers more stable crack growth and less X-ray beam interactions with the fixture than the standardised three-point version. This work demonstrates the benefits of in-situ X-ray computed tomography in determining the microscale damage mechanisms involved in mode II fracture toughness testing. The study can be extended to determine the influence of interface ply orientations and quantify mode II fracture toughness. The obtained information will hopefully contribute to answering the critical question of how the local microstructure affects the mode II delamination initiation and growth.