Integrated digital imaging technique for correlation of damage evolution and stiffness degradation in non-crimp fabric composite materials under cyclic loading

A novel integrated digital imaging technique was developed to automatically characterize damage and simultaneously monitor stiffness degradation in non-crimp fabric glass fiber/reactive thermoplastic cross-ply laminates subject to tension–tension cyclic loading. Under a peak stress of 50 % UTS, the laminate exhibited four distinct stages of stiffness degradation, including a significant drop during the first cycle due to 90° fiber tow crack initiation, a gradual decrease due to crack multiplication and propagation up to saturation, a stable phase comprising localized delamination crack propagation, and a sudden drop prior to specimen failure. Under a peak stress of 75 % UTS, both 0° and 90° fiber tow cracks developed during the first stage followed by an accelerated growth rate and reduced 90° tow crack density at saturation. The integrated digital imaging technique proved to effectively correlate damage events with stiffness degradation, leading to a deeper understanding of the fatigue behavior of glass fiber/thermoplastic composites.