Investigation of the translaminar fracture behavior of the fiber reinforced composites stitched perpendicular to their plane

In this study; while the carbon fiber plain woven and UD fabric layers stacked on top of each other were dry, they were stitched with polyester and carbon fiber thread in the direction perpendicular to the plane, and then they were turned into composite sheets by vacuum infusion method using epoxy matrix. It was aimed to determine the translaminar fracture toughness of laminated composites which applied stitch. Crack propagation and final damage which can occur in random directions need to be determined without using a preliminary estimate. With this purpose; the effects of crack angle, crack length, layer angle, layer stacking sequence, stitch density and stitch angle variation on the translaminar fracture toughness of carbon fiber reinforced composites were investigated experimentally and numerically. In the experimental study, the fracture tests were performed according to the ASTM E1922 standard, and the critical load, crack tip opening amounts and fracture toughness values were determined. In the numerical study, critical stress intensity factors were determined using the M-integral method and the displacement correlation method. Modelling and fracture toughness analyses were performed in the ANSYS finite element package program. In addition to homogeneous models, heterogeneous models showing the carbon fiber threads as a separate volume were used to define the crack region in the stitched specimens. Then, in order to find the crack propagation directions numerically, the model prepared in ANSYS was transferred to the LS-DYNA program and progressive damage analysis was performed. Crack propagation was numerically obtained for each specimen using the MAT_162 material card making calculations according to the Hashin damage criterion. Finally, the compatibility of crack propagation, crack opening displacement, and damage loads with experimental data is presented. In addition, the accuracy of the numerically calculated critical stress intensity factor values has been proved by the fracture criterion. Thus, it has been demonstrated that the translaminar fracture toughness of composites with random layer arrangement and crack angle can be accurately calculated without any preliminary estimation. Stitching the layered composites perpendicular to the plane increased the fracture toughness by 23.5-80.6% in plain woven fabrics and 1.41-9.38% in UD fabrics. Fracture toughness values increased with the increase of stitch density. This increase was highest in the longitudinal stitched specimen which the toughness increased by ~15.4% with the 100% increase in the stitch density. The highest fracture toughness was obtained with double-direction stitching having 1.25 mm stitch density. It was determined that the designed heterogeneous model gave more appropriate results by ~1-6% compared to the homogeneous model.