Constant amplitude fatigue test research on M20 high-strength bolts in grid structure with bolt–sphere joints

The grid structure with bolt–sphere joints is one type of large-span spatial structures. Due to small induced force, light weight, large stiffness, and good seismic performance, such structural system has been widely adopted in industrial plants with suspending cranes. The damage of the grid structure with bolt–sphere joints tends to occur at high-strength bolt joints, with fatigue fracture as the primary failure mode. The alternating and reciprocating action of the suspending crane will cause fatigue problems to the structural components of the grid structure, including rod, cone, sealing plate, bolted sphere, and high-strength bolt. The threads in high-strength bolt result in a significant stress concentration leading to bolt fatigue which is the key issue of fatigue failure. In this study, systematic constant amplitude fatigue tests were conducted for 18 M20 high-strength bolt made of 40Cr material using the developed efficient and smooth loading equipment combined with the Amsler fatigue testing machine, in which the tension condition of the high-strength bolt in the grid structure was simulated, simulation of high-strength bolt in tension works of grid structure. In addition, the signal-to-noise curve and the calculation method of constant amplitude fatigue for the high-strength bolt are proposed in grid structure with bolt–sphere joints.