Analysis of multi-bolt composite joints with a user-defined finite element for the evaluation of load distribution and secondary bending

The present paper aims to demonstrate the capabilities of the Composite Bolted Joint Element (CBJE) methodology in the framework of single-lap multi-bolt joints analysis. This FE modeling approach exploits the CBJE, a user-defined finite element. It is founded on the complete elastic characterization of the bolted region through analytical models. The elastic behavior of a circular portion of the laminates enclosing the bolt is determined in the context of thick composite plate mechanics; a set of radial beam elements is made structurally equivalent through the custom definition of their stiffness matrix. The interaction between the bolt and the plates is modeled as a beam on elastic foundation. The stiffness contributions of the bolt and the specific properties of the joint, such as preload, bolt-hole clearance, contact stiffness, and friction effects are simulated utilizing an assembly of beam and spring elements. In this way, it is created a FE model from the theoretical solution of the bolted region featuring high fidelity and computational efficiency, as it requires a low amount of degrees of freedom. Experimental data are used to validate the FE model with CBJE of the single-lap three-bolt composite joint. Specimens with varying lay-up, radial clearances, bolts preload and bolt number are also analyzed to evaluate the bolt load distribution and out-of-plane behavior. The CBJE results are successfully compared with those of reference solid models.