Optical investigation of crack initiation and propagation during the single fibre pull-out test

The single fibre pull-out test is an important micromechanical test used to evaluate the fibre-matrix interface of polymer matrix composites primarily under shear stress. Models interpreting force-displacement curves from pull-out tests often rely on assumptions about the stress distribution along the loaded fibre and crack behaviour at the fibre-matrix interface. This study aims to answer two critical questions: does the crack initiation and propagation at the interface proceed as in elastic materials, or is the debonding instantaneous as in ductile materials? And if elastic, where does the crack initiate and how does it propagate? To address these questions, an in situ optical investigation of the pull-out test under polarised light was conducted to observe crack initiation and propagation at the fibre-matrix interface under load. It was found that the location of crack initiation and the subsequent crack propagation were dependent on the ratio of the embedding length to the fibre diameter ( L / D ) for a given fibre-matrix combination. For short embeddings ( L / D < 4), cracks initiated at the embedded fibre's end and grew toward the fibre's entrance into the droplet. Unstable crack growth occurred if the remaining stress on the interface was not reduced with further crack extension. For long embeddings ( L / D > 4), cracks first appeared at the entrance of the fibre into the matrix. As the crack propagated and the remaining contact area between fibre and matrix decreased, the stress along the fibre resembled the short embedding situation, leading to a second crack arising at the fibre's end. Further loading resulted in both cracks growing toward each other until the rising stress in the remaining contact zone exceeded that needed for stable crack growth, and the fibre was completely detached. These findings enhance the understanding of fibre-matrix interactions and provide valuable insights for improving the interpretation of pull-out test results.