Preparation And Study On The Properties Of The Stitched Composite Laminates

Stitched composites have been widely used in many areas due to their excellent interlaminar property, high impact resistance and unique interlaminar fracture toughness. In this dissertation, stitched composite laminates were prepared by variable angle tow (VAT) placement technology and vacuum assisted resin infusion molding (VARIM) process. The effects of the stitch density (0*0,8*8,5*5,3*3) on the plane mechanical properties, low velocity impact response, impact damage and interlaminar fracture toughness were investigated in details. The VAT placement technology provides a new way for the manufacturing of stitched composites. Also the research here is helpful for promoting the practical application of stitched composites.This dissertation addresses the. following work:1. The effects of the stitch density on tensile, flexural properties and laminar shear strength were investigated. The results show that the small stitch density has little effect on the tensile and flexural properties, while the high stitch density leads to a reduction of the tensile and flexural properties with a range of10%. The tensile and flexural properties of the stitched composite laminates were influenced by the combination of fiber compaction effect, fiber straight degree and resin-rich area. The interlaminar shear strength of stitched samples increased up to7.3%, compared to the unstitched sample, which is caused by the weak resin-rich area around the stitch fibers.2. The effects of the stitch density and impact energy on the low velocity impact response and damage of the stitched composite laminates were investigated by small drop hammer impact test and ultrasonic C-scan. It is found that the change history of contact load, energy absorption and deformation are similar in the low velocity impact test, and stitching has little effect on the maximum contact force and contact time, with a fluctuation of5%. With the increasing of the impact energy and the stitch density, the absorbed energy and the energy absorption rate gradually increase, while the maximum deformation displacement and the residual deformation displacement decrease with a maximum reduction of20%. The C-scan projections show that the damaged areas are circular or elliptical. The impact damage area and the residual deformation displacement reduce with the increasing of the stitch density, while the compression strength after impact (CAI) increases. The main reason is that stitching can effectively prevent the generation of delamination and crack propagation, which results in the improvement of damage resistance and damage tolerance.3. The effects of the stitch density on the interlaminar fracture toughness (Mode I energy release rate, G1C) were investigated by the double cantilever beam test. The test results indicate that stitching can significantly improve the interlaminar fracture toughness (G1C) of the composite laminates. The G1C of the3*3stitched sample is4.2times of that of the unstitched sample. The test results show that the interlaminar fracture mechanism of the unstitched composite laminates is the damage of the fiber/matrix interface, while the fracture modes of the stitched composite laminates is the damage of the fiber/matrix interface together with pullout and fracture of the stitch fiber.