Improving the open-hole tension characteristics with variable-axial composite laminates: Optimization, progressive damage modeling and experimental observations

This study consists of a numerical and experimental investigation on unnotched and open-hole tensile characteristics of fiber-steered variable-axial (also known as variable angle-tow and variable-stiffness) composite laminates. The fiber path was obtained from an optimization framework considering manufacturing characteristics of the Tailored Fiber Placement (TFP) process, in which both fiber angle and thickness are locally optimized; besides, unnotched coupons, open-hole samples with unidirectional fibers, fibers along principal stress directions and with optimized fiber path were manufactured and tested under longitudinal tensile loading. The tests were assisted by digital image correlation (DIC) to accurately capture both strain field and failure behavior. A progressive damage model was then developed to simulate the experimental observations. The notched strength-to-weight of the open-hole coupon with an optimized fiber pattern has a notched strength even higher than the unnotched sample. Optical measurements via DIC and numerical predictions evidenced no strain concentrations around the hole near the final failure for coupons with optimized fiber path, where the reinforcing mechanism alleviated the strain concentrations by redistributing the stress uniformly around the coupon.