A Predictive Approach To The In-plane Mechanical Properties Of Stitched Composite Laminates

As a new type of composite, stitched composite has an encouraging prospect of applications in aeronautic and aerospace industries because of the excellent through-thickness properties and lower cost of manufacture. During the process of stitching, however, there are some in-plane local damages, which have effects on the in-plane mechanical performances unintentionally. In this thesis the Fiber Distortion Model (FDM) is presented to analyze the effect of stitching and geometry parameters on the equivalent modulus and tensile strength of the stitched composite laminates. The fiber distortion model is set up based on the microstructure of fibers near the stitch holes. The distortion of in-plane fibers is considered to be the main reason for the effect of stitching on the in-plane mechanical properties, and the fiber distortion region, the resin-rich pocket and the through-thickness reinforcement section are taken into account. The fiber misalignment angle and the inhomogeneous fiber volume fraction caused by stitching have been determined within the lamina. The elastic constants of a lamina are calculated using a micromechanical model, then the constitutive equations of the stitched composite laminates can be composed. It is clearly that only two geometrical parameters i.e. maximal misalignment angle () and distortion width (S) need to be determined experimentally.The engineering constants around the stitch hole in the composite laminate are analyzed using the FDM. It is found that the elastic constants have distinct region character and the traces of the resin-rich pocket in different lamina are clear. The longitudinal Young's modulus and the in-plane shear modulus decrease with the increase of the maximum misalignment and the distortion width, but the transverse Young's modulus and main Poisson's ratio have no obvious change. The in-plane equivalent elastic modulus and tensile strength of stitched composite laminates are studied, and it's found that the in-plane equivalent elastic modulus decreases with the increase of stitch density, stitch thread diameter, maximal misalignment angle and the distortion width, and the range is about 5%. The tensile strength of stitched composite laminate increases with the increase of stitch step and decreases with the stitch space. Theincrease of the stitch thread diameter, maximal misalignment and the distortion width will reduce the tensile strength of stitched composite laminates in the range of 10-20%.