Research on the relationship between the tightening torque and the preload of composite bolted structures

Bolted joints are widely used in assembly of composite structures that transmit large loads. The relationship between torque (T) and preload (F) is needed to be obtained when using the torque method to tighten bolts. In this paper, the T-F relationship in the process of bolt tightening composite laminates was analyzed through bolt tightening experiments. The variation behavior of friction coefficient in this process was also studied. On this basis, a progressive damage finite element model was established using the improved LaRC05 criterion to analyze the stress and damage of the composite laminate during tightening. The various stages in the bolt tightening were divided and the mechanical properties of each stage were analyzed. The results of the study showed that the thread friction coefficient mu(th) and the bearing friction coefficient mu(b) change with the preload during the tightening process. Its behavior can be described using an exponential function. Serious stress concentration and assembly damage can be formed in the direct extrusion area of the outer edge of the locknut when the composite laminate is bolted. The bolt tightening process of composite laminates can be divided into five stages. Research conclusions are valuable for understanding composite structures and formulating the bolt tightening process.