The Strength of Bolted and Bonded Single-Lapped Composite Joints in Tension

Abstract

This work is primarily concerned with a failure analysis of single-lapped bolted and bonded mixed-composite joints, representing practical situations, both in terms of experimental data and theoretical appreciation. Two kinds of stacking sequences in adherends are tested: quasi-isotropic and cross-ply. The adhesives include thermal-setting and thermal-plastic types. The bolts used to clamp the bonded joints are made of 304 stainless steel, whilst the rivets are of NAS 1919 C05 according to National Aerospace Standards. The important invented techniques in making riveted and bolted joints and measuring their respective clamping pressure are disclosed, and the main contribution of systematic experimental data in contrast with numerical prediction is highlighted as follows. The bolting arrangements can be divided into one (1 x 1), three (1 x 3, 3 x 1), six (2 x 3) and nine (3 x 3) configurations. The empirical results indicate that the joint of six (2 x 3) bolts possess the highest strength, nine (3 x 3) bolts the second strongest, followed by (1 x 3), (3 x 1); while the one bolt geometry is the weakest. The mixed joints with more than three bolts will fail by adhesive delamination and tension failure in adherends transversely. However, the failure mechanism for the one bolt system is different, in that delamination occurs first, then bolt bending and shearing, and finally a tension-type failure. A typical tensile stress-strain diagram is linear before delamination. We also adopt the C-scan non-destructive testing (NDT) technique to examine damage initiation sites and to compare this information with the numerical prediction obtained using the finite element method. The comparison is satisfactory. Finally, the joint strength increases with increasing overlap length, until the strength approaches asymptotically a constant value for longer overlap lengths.