Tensile low-cycle fatigue performance and life prediction of high-strength bolts

Abstract

Monotonic tensile tests and tensile low-cycle fatigue tests have been conducted to explore the low-cycle fatigue life of high-strength bolts, which is an important property to guarantee the connections do not occur low-cycle fatigue failure before their connected members under earthquakes. Large-deformation performance and failure modes of bolts were numerically simulated based on the damage constitutive relation which was investigated by the tensile tests of smooth and notched specimens. The results show that complete uniaxial constitutive relation of the bolt material, namely 20MnTiB steel, and the displacement from tensile load response of smooth specimens in the numerical simulation can be better described by the Swift flow stress model. The GTN damage model of the steel was calibrated by tensile tests and finite element models of notched specimens. With the decrease of the notch radius, the tensile capacity of the notched specimen increases while the ductility gets worse. The calibration parameters of GTN damage model can be applied to simulate the load-displacement response and fracture behavior of notched specimens accurately. In order to obtain a more targeted GTN model under different stress states, the quantitative relation between equivalent plastic strain during nucleation and stress triaxiality was proposed, which provided an important reference for establishing fatigue damage models of various bolted joints in the numerical simulation. Both the fatigue failure modes of the bolts under cyclic displacement with constant amplitude and the fatigue life based on the tensile capacity degradation were obtained from the fatigue tests. The fatigue brittle fracture and plastic elongation of the shank are the two fatigue failure modes of the bolt. Typical regional characteristics are presented in the bolt fatigue fracture, with beach-like fatigue bands in the expansion area. Under different failure modes, the capacity degradation can better determine the fatigue life of bolts. The methods for predicting fatigue life were proposed based on cyclic displacement amplitude, equivalent stress amplitude and local plastic strain. A good prediction result was proved by the phenomena that the predicted fatigue life based on the cyclic displacement amplitude and local plastic strain was basically within the scatter band of 1.5. Moreover, the fatigue design parameters of bolts given by the equivalent stress amplitude method provide a reference for establishing a unified fatigue design theory.