Preload relaxation behavior and its influence on the mechanical performance of bolted carbon fiber‐reinforced thermoplastic sheet molding compound joints

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Abstract
In this study, the relaxation of the initial tightening preload of bolted carbon fiber reinforced thermoplastic sheet molding compound (CFRTP‐SMC) joints was experimentally investigated under different temperatures and initial preload conditions. The influence of preload relaxation on the bolted structures was evaluated using static bearing tests, whereas the failure modes during the test were verified based on metallographic observations. The experimental results indicate that the speed of the preload relaxation can be accelerated by increasing the temperature and initial preload. The static bearing strength of the bolted SMC material decreased with a continuous increase in the preload loss. Furthermore, the existence of a lateral preload protected the material around the assembly hole from buckling, resulting in a higher static bearing strength. The shielding effect of the preload was weakened after the relaxation of the initial preload.Highlights: Elevated temperature and initial preload can expedite the relaxation process. Preload loss will result in a declined static bearing performance. Initial tightening preload can protect the bolted SMC material from buckling.

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