Factors in deformation capacity loss of bolts under singular and consecutive impacts

Abstract

For structures subjected to extreme loading, such as a blast or impact, the plastic deformation of structural connections is vital to life safety. Despite its importance, a loss of deformation capacity in connections via fracture of bolts has been repeatedly noted experimentally and in situ. To further investigate the dynamic fracture of bolts, this work subjects A325 structural bolts in single shear to impulsive loads using a high-speed hydraulic actuator. This is the first study to report strength and deformation performance for structural bolts fractured via singular and consecutive impacts. Further, this work is the first investigation into the root cause of the deformation capacity loss that has historically coincided with shear bolt fracture. Previous split Hopkinson pressure bar experimentation suggests that rate-induced changes in strain hardening contribute to, but do not fully explain, the loss of deformation capacity in high-strength bolts. Further investigation with scanning electron microscopy suggests that loss of deformation capacity is also due to a rate-induced change in failure mechanism, adiabatic shear banding. These findings impact our understanding of connections that fail via dynamic bolt fracture – behavior changes on the macroscale are the result of underlying rate-dependent fracture mechanisms.