Experimental study of bonded, bolted, and hybrid braided CFRP joints with different stacking sequences and lapping patterns

Tensile tests of bonded, bolted, and hybrid braided CFRP joints were conducted using an MTS tester. Twenty-four groups of specimens were designed with four stacking sequences and two lapping patterns. The strain data around the bolt holes were recorded using a dynamic acquisition device. The macroscopic and microscopic failure modes obtained by SEM were analysed. The strength, stiffness, and bolt load distributions with different variables were investigated to determine the mechanical properties of the CFRP joints. The results show that the load–displacement curves for S1, S2, and S4 are approximately linear before fracture; the curve for S3 is nonlinear. Bolts 1 and 2 near the loading side have large strain values; bolts 3 and 4 have smaller strain values. The failure modes include adhesive debonding, fibre fracture, net-section failure, tearing failure, and bolt fracture. Hybrid joints exhibit a combination of failure modes with single joint failure. The SLJ exhibits better strength and stiffness than the SSBJ. The strength of bonded joints with high-performance adhesives can be greater than that of hybrid joints, and is related to the stacking sequence and lapping pattern. Bolt load distribution is a dynamic process. With addition of an adhesive layer, there is greater balance between the four bolts, which can effectively reduce fluctuation of the load distribution.