Experimental investigation on strain concentration in composite laminates with a circular hole

The effect of stitching reinforcement on strain concentration was investigated for the composite laminates containing a circular hole. The effects of stitching parameters, such as stitching needle distance, row spacing, edge distance, single stitching or double stitching, etc, were compared and analysed. The tensile strength and stiffness were measured for composite laminates containing a circular hole. The strain concentration and strain distribution regularity were investigated analytically and experimentally. The hole edge coefficients of strain concentration were measured for different stitching parameters, external load, the edge location of the hole, etc. The experimental results show a distinct efficiency of stitching reinforcement for composite laminates containing a circular hole and give the stitching design parameters on the hole edge. The experiments show that near the hole region in the loading direction, the strain concentration is not influenced under different loads; with the material bearing compressive deformation, near the hole region in the vertical line of loading direction, the strain concentration factors K_ε vary with load and can reach a very high value. Compared with isotropic plates, the K_ε of orthotropic laminates is variable at different edge locations of the hole.