A detailed finite element investigation of composite bolted joints with countersunk fasteners

This paper presents a very detailed FE model of a single lap composite bolted joint, with countersunk fasteners, under static tensile load. The stress states of the CFRP plates and titanium bolts are discussed, the evolution of the contact between the bolts and the holes is analysed, and the numerical results are compared to experimental data. Parametric studies have been performed to study the influence of bolt clamping force, coefficient of friction and bolt-hole clearance on the joint behaviour. It has been found that the model is able to identify correctly the critical locations in the joint (head-shank transition and first thread in bolts and edges of the holes at the faying surface for the plates) and reproduce with accuracy the experimental load-displacement test curves (including an unloading-reloading loop) up to the point where bearing damage occurs. A correlation between the joint stiffness and the contact status between bolts and holes has been found. Five stages have been identified in the joint behaviour: (i) No-Slip, (ii) Slip, (iii) Full Contact, (iv) Damage and (v) Final Failure. The results show that the joint stiffness is higher in the No-Slip stage than in the Full Contact stage, and that this is independent of coefficient of friction and bolt clamping force. The clearance controls the length of the Slip stage and modifies the joint stiffness in the Full Contact stage.