On the transition from arc debonding to tunneling at fiber–matrix interfaces

The mechanical performance of fiber-reinforced composites critically depends on the integrity and properties of the fiber–matrix interfaces. This study revisits the mechanics of interface fracture with the goal of developing a framework suitable for engineering prediction of fiber–matrix debonding in composite microstructures. Classical interface fracture mechanics are applied to circular fiber inclusions to characterize the conditions governing circumferential fiber–matrix debonding. The analysis is extended to describe the transition from arc debonding to unstable tunneling along the fiber length – a fracture mode associated with decreased mechanical properties and often observed in transversely loaded unidirectional composites. Finite element simulations of circumferential debonding incorporating contact effects overcoming limitations of classical analytical solutions. These are integrated with closed-form results into a framework for analyzing circumferential and longitudinal debonding development, with a specific focus on the onset of tunneling. The analysis is applied systematically over a range of bimaterial systems and interface fracture properties to construct maps in terms of critical angles and applied stress associated with unstable tunneling. The resulting framework accurately predicts the transition from arc debonding to unstable tunneling and explains experimentally observed debond morphologies. The findings highlight how interaction of elastic mismatch and mixed mode fracture toughness govern debond growth, offering new opportunities for analyzing composite fiber interfaces to elucidate failure behavior.