Variable-amplitude fatigue behavior of M30 high-strength bolts in end-plate connection joints

High-strength bolts are widely used in connection joints for steel structures. In this paper, the variable-amplitude fatigue behavior of M30 high-strength bolts in end-plate joints is studied. Variable-amplitude fatigue tests were carried out on 19 M30 high-strength bolts using an Amsler pulse fatigue tester. Four cyclic stress amplitude loading modes, including Low to High (L-H), High to Low (H-L), Low to High to Low (L-H-L), and High to Low to High (H-L-H), were applied to the test specimens. In addition, the variable-amplitude fatigue life of M30 highstrength bolts was studied using two forms of cumulative damage models. The stress - fatigue life (S-N) curve was plotted using the equivalent stress amplitude. The obtained S-N curve was compared with the existing bolt fatigue S-N curve and the current specifications. Finally, fatigue fracture was analyzed from macro and micro perspectives. It was found that for most specimens, fatigue fracture occurs at the first joint thread between the bolt and nut, while for a few bolts, fatigue fracture occurs at the joint between the bolt head and bolt. With a fatigue life of 2 million cycles as the threshold, the fatigue limit is 107.5 MPa, which is 2.67 times higher than that specified by ANSI/AISC 360 - 16. Additionally, the fatigue properties of high-strength bolts in end-plate connections are better than those in bolted sphere joints at higher stress levels. At low stress levels, the contact pressure formed by the extrusion of the end-plate reduces the fatigue life of the bolts. Furthermore, unlike the constant-amplitude fatigue fracture, the variable-amplitude fatigue fracture has an obvious load change boundary line.