Experimental study on the creep performance of Grade 8.8 and 10.9 high-strength bolts at elevated temperatures

Abstract

High-strength bolts are widely used in beam-column connections which are typically subjected to significant pretension and experience high stress levels. Under fire conditions, high-strength bolts may exhibit noticeable creep deformation due to the combined effects of high stress and high temperature, which may result in significant deformation of the connection, thereby threatening the stability of the structure. This paper reports an experimental study on the creep behavior of Grade 8.8 and 10.9 high-strength bolts at elevated temperatures. A series of tensile tests are conducted on high-strength bolts at elevated temperatures, and varying stress ratios are set based on the yield strength measured form the tensile tests. Test results indicate that the temperature has significant influence on the mechanical properties of Grade 8.8 and 10.9 high-strength bolts. Through the entire temperature range, current specifications significantly overestimate the yield strength of high-strength bolts at high temperatures. At the same temperature, the development of creep strain over time is positively related to the stress ratio. An increase in stress ratio from 0.4 to 0.6 or 0.8 results in a maximum 7.8-time increase in creep strain. The temperature of 500℃ can be regarded as the critical temperature at which the creep effects of Grade 8.8 and Grade 10.9 high-strength bolts should be considered. The mechanical property prediction equations and a modified creep model for Grade 8.8 and 10.9 high-strength bolts were developed, which have high reliability in predicting the high-temperature mechanical properties and creep behavior of high-strength bolts.