Fracture Toughness of Stitched CFRP Composites Using Optical Fiber Strains

Abstract

The use of lightweight polymer matrix composite materials in primary load-bearing applications has increased in the aerospace industry due to their high strength and stiffness. However, delamination caused by low interlaminar fracture resistance can cause stiffness degradation and affect the integrity of the structural part. Throughthickness stitching is used to improve the delamination resistance by providing translaminar reinforcement. In order to characterize fracture behavior in composite laminates, fracture energy is determined by performing double cantilever (DCB) tests. Stitched DCB specimens with a quasi-isotropic layup configuration were fabricated using vacuum-assisted resin transfer molding. Dry preforms were stitched using a robotic single-sided stitching technique. To obtain the internal strain data and internal crack growth, an optical fiber was embedded at the midplane of the specimens. The apparent fracture energy was estimated using the modified compliance calibration method. The strain data obtained from the optical fibers enabled real-time crack monitoring of the DCB specimens.