Open-hole flexural damage response of Z-fiber reinforced hybrid woven composites

Drilled holes and through-thickness reinforcement influence the mechanical behavior and damage propagation of load-bearing composite structures. This study examines the combined effects of hole diameter, inter-layer hybridization, and stitching on the flexural response and internal damage propagation of woven composites. Carbon fiber, glass fiber, and hybrid glass/carbon composites with identical layer counts were manufactured and reinforced through the thickness using Dyneema ® stitch yarn in a cross-stitch pattern. Circular holes of 4, 6, and 8 mm were introduced. Mechanical behavior was evaluated using three-point bending tests. Internal damage before and after flexural loading was characterized using ultrasonic C-scan imaging, followed by image-based damage quantification. Increasing hole diameter reduced flexural strength and flexural modulus due to increased stress concentration and reduced effective load-bearing area. Carbon fiber–based composites showed higher strength and modulus, whereas glass fiber–based and glass-dominated hybrids exhibited greater deformation capacity. Stitching decreased flexural modulus but increased strain capacity, promoting a more ductile-like flexural response, particularly in carbon-based hybrids. Ultrasonic C-scan results further indicated that stitching restricted damage propagation in specific composite configurations. Overall, hole geometry, fiber architecture, and stitching should be considered together when designing damage-tolerant hybrid composite structures.