Vibration induced loosening of threaded fasteners: Mechanisms, modeling and design guidelines

Threaded fasteners find widespread use in assemblies due to their advantages, such as the ability to be disassembled for repairs. Fastener loosening is a common mode of failure in assemblies subjected to vibration. Such loosening is most severe in joints subjected to dynamic shear loads, where it has been shown to cause complete loss of fastener preload. This work presents a study of vibration induced loosening of fasteners subjected to dynamic shear loads.;The study begins with a fundamental analysis of fastener loosening based on basic physical principles. Fastener loosening is shown to be a result of slip at contact surfaces, and the requirements for loosening to occur are identified. Several factors that contribute to satisfying the requirement for loosening in a joint subjected to shear loads are discussed. Based on the analysis of these factors, it is shown that localized slip can occur at fastener contacts due to non-uniform load distributions. Four possible loosening processes based on either localized or complete slip at the head and threads contacts are identified. Experimental data in the form of hysteresis curves obtained from a transverse vibration test machine are used to illustrate the different loosening processes. In addition, a dimensionless loosening factor is introduced as a measure of loosening resistance of a fastener.;Next, loosening is simulated numerically using a non-linear three-dimensional finite element model of a joint with a threaded fastener. Numerical results illustrating the four different loosening processes are presented and compared to experimental data. The finite element model is shown to be capable of qualitatively and quantitatively modeling loosening. Results of a fractional factorial parameter study using the finite element model are presented. Design guidelines for joint parameter selection, such as fastener length, screw material and hole fit are provided based on these results.;Finally, the option of using placement of threaded fastener as means to minimize fastener loosening is explored. Using finite element analysis results and experimental data for simple assemblies, it is shown that optimum placement of a fastener in an assembly can be identified based on determination of shear load and slip between the clamped components.