Mechanical properties and stress-strain models of large diameter high-strength bolts

High-strength (HS) bolts (such as grades 8.8, 10.9, 12.9) are extensively used in various types of steel structures. In certain high-altitude or coastal regions, the extremely complex and harsh climatic condition is a great challenge to the long-term serviceability and durability of HS bolts. Based on a steel television tower under construction, grade 8.8 heavy hex large diameter HS bolts were used. In order to obtain their mechanical properties and stress-strain relationships, the extensively used M36 and M48 bolts were selected to be subject to standard tensile tests. The impact of different sampling positions and surface treatment methods (whether or not hot-dip galvanizing) on the mechanical properties was taken into account. The typical mechanical performance, ductility index, bolt pre-tension and tightening torque of the large diameter HS bolts were obtained. The test results indicate that the mechanical properties of HS bolts were slightly influenced by hot-dip galvanizing and sampling positions. Based on the measured engineering stress-strain curves, the true stress-strain curves throughout the entire deformation history were obtained. Constitutive models based on an improved Ramberg-Osgood model for engineering stress-strain relations were proposed and compared with the existing constitutive models. It is recommended to use the high accurate models to simulate the stress-strain relationship in the design and modelling of HS bolts with heavy hex and large diameter.