Investigation of fracture toughness of laminated stitched composites

The major objective of this research was to develop new test procedures to investigate the fracture toughness of laminated stitched composites. Graphite-epoxy laminated composites have very high stiffness-to-weight and strength-to-weight ratios that make them very attractive in structural applications. The properties of these materials depend strongly on fiber orientation and ratio of fiber to resin. The strength along the translaminar direction of this material is dominated by the matrix and is typically about 5% of the strength along the fiber direction. Hence, these materials are very easy to delaminate. Through-the-thickness stitching is one of the ideal ways to improve the translaminar properties and prevent crack propagation. In order to understand the stitch's effect, a series of test procedures was developed and was successfully used to test Mode I, Mode II and Mixed Mode fracture toughness of stitched laminated composites.; A secondary objective was to model stitched laminated composites to predict their properties. Finite element models were proposed to understand the effect of stitching on improving the fracture toughness for Mode I and Mode II of stitched laminated composites. From these models, “effective G” (effective strain energy release rate) and “parent G” (strain energy release rate of the parent laminate) can be isolated where stitches can be treated as “an extra structure.” The “parent G” is an intrinsic parent material property and almost never changes regardless of whether the laminated composites are stitched or not. However, the “effective G” is dramatically increased by the stitching. The author also found that changing the thickness of stitches has a more profound impact on the “effective G” than changing the density of stitches.