Study On Hole Expansion Strengthing Technology Of CFRP Composite Bolted Jionts

In advanced engineering fields, such as aerospace engineering, CFRP(Carbon Fiber Reinforced Polymer/Plastic) composite materials have wide applications because of their high strength-to-weight and stiffness-to-weight ratios. In aircraft structures, bolted joints are still the dominant fastening mechanisms used in jointing of CFRP composite materials structures. Fasten holes are the stress concentration and weak link in CFRP composite material structures. Therefore, applying the reasonable and effective strength manufacturing technology in the weakness structures is one of the important technical measures in enhancing the bearing strength of aircrafts. In order to improve the fatigue life of CFRP composite bolted joints, the hole expansion for mechanical connection structure of CFRP composite materials was conducted in this study.The main contents are summarized as follows:(1) Expansion process and experimental apparatus for hole expansion of CFRP was designed,the seamless sleeves which is suitable for hole expansion of CFRP were developed from TC4 titanium.The test pieces for hole expansion of CFRP were prepared, hole expansion process of CFRP was carried out. The changes of extrusion force, extrusion quality and damage of hole wall were studied,and the expansion quality was evaluated.(2) The finite element analysis model of cold expansion for CFRP composit materials structures which contain end holes and composite joints in static tensile load condition are established. VUMAT user subroutine is applied to models for realize damage evolution which based on Hashin failure criteria and partial degradation model of material property. Through simulation results, the stress distribution, elements damage and failure, deformation of hole edge were studied. The influence of cold expansion to the load carrying capability of CFRP composite materials bolted joints was discussed.(3) Tensile and fatigue tests were carried out for CFRP composite materials bolted joints in different cold expansion values. Connection strength and fatigue life of CFRP composite materials bolted joints in different cold expansion values are achieved. The influence of cold expansion to the carrying capacity, failure mode, fatigue deformation of CFRP composite materials bolted joints is discussed. The hole expansion effect to the fatigue gain was verified, and proper expansion parameters was achieved.