Fiber–matrix interface debonding and transverse cracking in macro fiber composites

Fiber–matrix interface debonding is a precursor to transverse matrix cracking at the mesoscopic scale in fiber composites. The mechanisms controlling fiber–matrix interface debonding and subsequent transverse crack formation have been explored primarily by computational methods with limited experimental verification. This study aims to establish an experimental approach for characterizing fiber–matrix interface debonding in model macro fiber specimens that replicate realistic microstructures. The primary goal is to measure strain fields at individual fiber–matrix interfaces using optical digital image correlation (DIC) and link these measurements to the initiation and propagation of transverse cracks. Macro fiber composite specimens are fabricated by embedding dozens of randomly distributed glass macro fibers (1 mm dia.) in an epoxy matrix. These specimens are then subjected to controlled transverse loading, and their local strain fields are monitored and quantified with high-magnification optical DIC. The experimentally obtained kinematic fields are first used to connect global and local deformation responses and to investigate the mechanisms governing matrix cracking between the fibers. The experimental data are then used to set up and validate a modeling framework created based on cohesive zone and phase field formulations to investigate fiber–matrix interface debond initiation and matrix cracking, respectively. The experimental protocols described here provide a practical approach for characterizing deformation and failure at the fiber–matrix interface and tracking their evolution into larger transverse cracks. Complementary simulation studies highlight the significance of boundary conditions and the uncertainty in the fiber–matrix interface fracture properties in realistic and reliable predictions of debonding kinetics and matrix crack formation. The presented approach is transferable to smaller length scales to enable the quantitative assessment of the effects of geometric and morphological factors, such as inter-fiber distance and angle, on transverse crack formation in fiber composites.