Mixed Mode Ⅰ+Ⅱ Fracture Toughness For Z-pin Enhanced Carbon Fiber/Resin Matrix Composite Adhesively Jointing Structures

Z-pin enhancement technology is to implant metal rod or carbon fiber rod in the thickness direction of laminate to enhance the fracture toughness of the composite layer and enhance the ability to resist delamination. Because of its low cost and good effect, it has been valued by experts. The delamination of composite structures is often mixed of mode I+II fracture in engineering practice. Therefore, it is of great importance to study the mixed mode I+II fracture toughness of Z-pin composites and to analyze bridging response of the Z-pins during mixed mode delamination.The mode I delamination toughness and the mode II delamination of the carbon fiber/resin matrix composite was analyzed by the DCB test and ENF test,respectively.Three point bending test was carried out on the reinforced structure of Z-pin reinforced carbon fiber/resin matrix composite. The effects of different Z-pin diameter and unit width of Z-pin implant on the fracture toughness of the composites were analyzed. Test results show that:(1)Increasing the Z-pin diameter or increasing the number of Z-pin in the unit width can improve the I+II mixed fracture toughness of the adhesive joint in the crack process of the adhesive joint. The delamination is dominated by the mode I fracture, but it also contains mode II fracture. Z-pin damage to pull out.(3)Energy consumption in the process of crack growth include the debonding of the bond interface and the elastic deformation of Z-pin as well as the friction slip.According to the experiment, the finite element model is established. Solid elements are used to establish Z-pin. Use cohesive element simulation of cracking process of the bond interface and in the Z-pin laminates and combined with introducing interface cohesion contact and friction contact simulation Z-pin degumming and friction during pull out process. The results show that:(1)The finite element simulation results are in good agreement with the experimental results.(2)Z-pin implantation can significantly improve the structure of the crack growth process under the load and the crack growth resistance.(3)Z-pin implantation can significantly enhance the load and crack growth resistance of the structure.(4)The longer the initial crack length is, the lower the stiffness of the end part of the bond head is, the smaller the load and the fracture toughness are.