Numerical and experimental evaluation of stress relaxation in hybrid composite-metal bolted joints

In the present paper, the preload reduction over the time attributed to the relaxation phenomenon which involves hybrid (composite-metal) bolted joints has been experimentally and numerically investigated. Unlike conventional materials (steel, aluminium, etc.), composite ones, due to the presence of the polymeric matrix with viscoelastic properties, feel more considerably the relaxation phenomenon, which begins at room temperature (around 20°C). As a result, the loss in the mechanical fastening is a very critical aspect to consider when composite components have to be bolted to other; further investigations are still mandatory to increase the effectiveness of such jointing techniques. The test article consists of two bolted plates made of mild steel and a composite laminate made by E720 770gms 3×1 T E-Glass roving 1200 Tex 35% RW, respectively. Under this purpose, the preload reduction in the bolt has been experimentally emphasized by developing a novel strain-gauged bolt. Moreover, the relaxation phenomenon has been numerically simulated by developing a complex Finite Element (FE) model.