Effect of bolt preload uncertainty on fatigue reliability of single-lap, countersunk composite bolted joints considering forced assembly interaction

Bolt preload plays a crucial part in ensuring the security of composite bolted joints. The actual bolt preload level displays noticeable fluctuation during aircraft assembly, impacting the fatigue reliability of joints. Assembly gaps commonly arise at the mating surface of composite airframes, and forced assembly is a prevalent method for closing the gaps before applying bolt preload, which leads to the bolt head fatigue cracking, weakening the fatigue performance of joints. In this study, the effect of bolt preload uncertainty on fatigue reliability of single-lap, countersunk composite bolted joints considering forced assembly interaction was systematically evaluated. The combination of progressive fatigue damage model (PFDE) and extended finite element method (XFEM) was developed to characterize the hole bearing damage and the bolt head fatigue cracking respectively. The findings show that the joints with forced assembly present a shorter fatigue life with increasing bolt preload under the fatigue load level of 70% $F_x^{\mathrm{bro}}(2\%)$ due to bolt head fatigue cracking. Higher bolt preload makes the bolt head fatigue cracking occur earlier, thus stiffness degrades and fatigue failure more rapidly. The bolt head crack size shows a growing trend with increased bolt preload, leading to a lower residual strength of joints. With a Gaussian distribution of actual bolt preload, the composite bolted joints after forced assembly presents a right-skewed distribution fatigue life under the fatigue load level of 70% $F_x^{\mathrm{bro}}(2\%)$, since the bolt head fatigue cracking would be triggered at a bolt preload lower than the mean value.