A novel stress-guided method to simultaneously improve fatigue resistance and anti-loosening of threaded fastener

Threaded fasteners are prone to fatigue and loosening failure under alternating external forces. In ordinary fasteners, stress is concentrated at the first engaged thread, and fractures occur here frequently. At present, the main approach to improve fatigue resistance is to enhance material strength by surface treatments or developing new materials, which increases the costs yet still cannot completely prevent fractures. Conventionally used frictional anti-loosening methods, although being effective, tend to increase the bolt stress with a negative impact on fatigue resistance. Thus, achieving simultaneous optimization of fatigue and anti-loosening resistance is challenging. Our study is the first to analyze two stresses that affect fatigue resistance and anti- loosening ability. The stresses distribution defects causing failures of ordinary fasteners were identified. Based on these findings, we propose an ideal distribution pattern of both stresses. Using pitch difference fastener, we experimentally and analytically proved that this stress patterns indeed yield superior performance. It was also revealed that manipulating only one geometric parameter (pitch difference) did not allow achieving simultaneous fatigue and anti- loosening improvement because these two respective stresses interfered with each other. Next, by adjusting two geometric features of a fastener (pitch difference and interference volume) joint enhancement of fatigue and loosening resistance was achieved. Finally, stress-guided optimization method was proposed to design different types of fasteners with desired anti-loosening and fatigue performance for a variety of engineering applications.