An Analytical Piecewise-Linear Joint Stiffness Model for Predicting Load Distribution in Multi-Bolt Metal–Composite Joints

Accurate prediction of load distribution in multi-bolt metal–composite joints relies heavily on high-fidelity modeling of single-bolt joint stiffness. Current models, however, inadequately capture the complex effects of bolt–hole clearance, including delayed load take-up and reduced bearing chord stiffness, as well as multi-interface friction interactions. To overcome these limitations, quasi-static tests were conducted on single-bolt, single-lap aluminum–CFRP joints with varying clearances. By integrating experimental findings with an analysis of the load-transfer mechanisms, we identified five distinct loading states and formulated corresponding analytical load-deformation equations along with explicit transition criteria, culminating in a novel piecewise-linear stiffness model. Enhancements over traditional tri-linear models encompass: (a) subdivision of the transition region into separate local and global slip phases, facilitating an accurate representation of asynchronous slip initiation at different frictional interfaces and (b) the implementation of a nonlinear correction approach to quantify the reduction in bearing chord stiffness induced by clearance. The proposed model exhibits excellent agreement with experimental observations across all clearance settings, affirming its predictive accuracy. Consequently, it offers a robust theoretical basis for improving load distribution prediction and facilitating design optimization in multi-bolt M-C joints. Furthermore, the methodology presented here shows considerable potential for adaptation to composite–composite joints.