Experimental and Numerical Investigation of Failure Patterns of High-Strength Bolts in Conventional Tensile Test Methods

The tensile strength of high-strength bolts is determined by two test methods. In the first method, the tensile strength of the bolt is determined using a nut. The second method uses the device introduced in ISO instead of the nut. In the tested bolts, two failure patterns are observed,the bolt thread stripping and fracture in the threaded length of bolt. In this research, in addition to the experiment, the elastic-plastic numerical simulation of the bolt failure pattern was also performed in Finite element software. The results of the study show that the frequency of the thread stripping failure pattern is 4% in the bolt tension test with the ISO device and 90% in the bolt tension test with the nut. In addition, the tensile strength obtained from the bolt tension test with a nut is lower than the bolt tension test method with the ISO device. According to the frequency of the failure pattern in each method, the bolt tension test with the ISO device leads to the determination of the tensile strength of the bolt material, and the bolt tension test with the nut leads to the evaluation of the tensile strength of the bolt and nut assembly. Due to the compatibility of bolt and nut performance in steel structure, this research suggests performing tensile test of bolt and nut, in addition to other tests.