Rate-dependent behaviour of high-strength steel bolts

Bolts have been widely used in steel structures, and the resilience of the whole structural system under extreme loading conditions such as impact or blast, is closely related to the dynamic properties of the bolt materials. Bolted connections are possible to fail in modes different from the design expectations under high strain rate loading conditions due to the difference in strain rate sensitivity between bolts and the materials being connected. However, knowledge about their strain rate-dependent behaviour is still limited, and the effect of strain rates has been essentially disregarded in current standards and design codes. This paper presents tensile and compressive tests on grade 8.8 and grade 12.9 M12 high-strength steel bolts spanning quasi-static (2.5 × 10⁻⁴ s⁻¹) to high strain rates (10³ s⁻¹). The strain rate effect on the mechanical properties of the high-strength steel bolt material, in terms of the yield strength, tensile strength and the corresponding strain and elongation, was evaluated. A dynamic constitutive model was then developed based on the Cowper-Symonds model for predicting the stress-strain relationship of the two tested bolt materials at various strain rates and their dynamic strain energy absorption capacities were also compared. Both the deformation and energy absorption capacities of grade 12.9 bolts are weaker than those of grade 8.8 bolts, and the difference was found to increase with the increase in strain rate.