Short-term preload relaxation in composite bolted joints monitored with reusable optical sensors

Abstract

Bolted fastening is one of the oldest joining techniques, and it is widely used in vast number of applications. Although such fasteners provide many advantages, bolted joint assemblies and in-service behaviors demonstrate complex phenomena and stress distributions. Among these factors, bolt tension affects joint reliability and residual lifetime most significantly. One of the major concerns in bolted joints with composite panels is the effect of creep in the throughthe- thickness direction of the material, which leads to preload reduction and premature failure of the loosened bolted joint. Thus, it is essential to monitor the preload after joint assembly. In this work, a previously developed bolt tension monitor that is instrumented with a reusable optical sensor was used to evaluate the short-term relaxation of preload in similar and dissimilar composite bolted joints. A single-parameter model was used to fit experimental data and obtain a relaxation parameter, which was assumed to be an extensive property. Preload reductions in lap joints with steel and glass fiber reinforced plastic (GFRP), and GFRP with GFRP were measured for ca. 9 min after the application of two levels of initial preloads. It was found that, depending on the joint configuration, the preload reduction varied considerably; and an increase in initial preload tends to reduce the preload relaxation. Overall, this technique provides robust and cost-efficient health monitoring in composite bolted joints.