Experimental evaluation of fatigue life and fatigue crack growth in a tension bolt–nut threaded connection

Threaded components are structural elements that play an important role in the integrity of an efficient engineering design. The assessment of their fatigue and fracture behaviors under real load conditions becomes many times extremely complex. This is because of the experimental difficulties involved in the laboratory simulation of the “nut–bolt” interaction. As a consequence, crack initiation and crack growth data are scant. This paper proposes an experimental method that overcomes these difficulties allowing an analysis of the behavior of the coupling under a state of tensile cyclic loading. The method enables to study the stages of initiation and crack growth. In the first, the crack nucleation period over different test conditions can be determined. In the second, the crack size as a function of the thread stiffness is estimated. Moreover, knowing the Paris’s constants C and m for the tested material, the dimensionless stress intensity factor Y(a/d) acting in the assembly was also estimated. Tests were conducted on M12 × 1.75 threaded rods, with a load ratio R = 0 and the obtained results demonstrated the suitability of the method.