Three-dimensional strain analysis of single-lap bolted joints in thick composites using fibre-optic gauges and the finite-element method

Bolted joints involving composite plates used to be almost entirely dedicated to aerospace applications. As the need for energy conservation has increased, the field of composite bolted joints has found new applications in ground armoured vehicles. Thick panels able to withstand large in-plane and impact loads are critical. The present investigation evaluates the interior strain field, through the thickness, of a composite plate connected to an aluminium panel with a single-lap bolted joint. The area of interest is the bearing plane region close to the hole because of the presence of stress concentrations that heavily modify the stress field. Experimental data for the bolted joint were recorded by fibre-optic strain gauges that were embedded in the bearing plane of the composite plate. Numerical analyses were performed using ANSYS as a pre-processor and LS-DYNA as a solver. The overall goal was to evaluate the magnitude of contact strains around the hole and through the thickness of the composite. These values were analysed and compared with the finite-element method results: the finite-element analysis correlated reasonably well with the experiments. An investigation of error causes was also carried out, in particular to evaluate the influence of incorrect gauge positioning and the effect of friction coefficients. General design considerations were finally provided, based on the complete three-dimensional finite-element analysis.