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Experiences with Modeling Friction in Composite Bolted Joints

Finite element analyses of composite bolted joints are common in the literature. However, the important issue of friction is often given superficial treatment. Friction introduces added difficulties to an already complex contact problem in terms of numerical convergence, and there can be a temptation to accept any method that will give a convergent solution. However, friction can significantly alter the stress distribution in the laminate at the bolt-hole interface, and carries a major proportion of the load in torqued joints; hence is important to model correctly. In the present study, experiences with modeling friction in composite bolted joints using commercial code MSC.Marc are presented. Unlike previous studies, both physical friction parameters and nonphysical convergence parameters within the available models are examined in detail and the findings should be helpful to other researchers analyzing similar problems. Two available models within the code are examined for their ability to model load transfer by friction in torqued joints, and the stress distribution at the bolt-hole interface in a pinned joint. The torqued joints include a large clearance so that both static and kinetic friction effects occur as the joint begins to slide and clearance is taken up. Results from the torqued joint models are compared with the experimental results. The stress distribution at the bolt-hole interface of the pinned joint is compared with a solution from an analytical method. It has been found that only one of the two models available in the code is capable of producing satisfactory results, and even with that model significant modification to the default friction parameters was required. It has also been found that using friction coefficients measured under ideal (clean) conditions in the model of the torqued joints did not give very good agreement with the joint experiments, which involved routine handling of the specimens. Finally, the developed friction model is used in a case study of a multibolt joint with variable degrees of bolt torque and bolt-hole clearances, and it is shown that such models can provide useful information for the design of composite bolted joints.