School of Civil and Environmental Engineering 

Ph.D. Thesis Defense Announcement 

Fracture and Residual Capacity of Steel Bolted Connections Subjected to Singular and Consecutive Impulsive Loads

By Maria Warren

Advisor: 

Dr. Lauren Stewart

Committee Members:  Dr. Antonia Antoniou (ME), Dr. Nan Gao (CEE), Dr. Laurence Jacobs (CEE), Dr. Ryan Sherman
(CEE)

Date and Time:  Tuesday April 5, 2022, at 1:30pm

Location:SEB 122 / Online at bit.ly/mariawarren22

Complete announcement, with abstract, is attached. 

     Civil structures may be subjected to high-magnitude, short-duration loads in scenarios such as weapons attacks or accidental explosions. Both during and after an impulse, the integrity of a structure is dependent on the sufficient performance of its connections. During the impulsive event, the plastic deformation of connections is relied on to dissipate energy. After the event, connections must maintain the capacity to re-route service loads around local member failures to minimize the likelihood of global collapse. Thus, an understanding of both the dynamic failure and residual behavior of connections is necessary for structural safety. 
      Limited experimentation on the residual capacity of steel structures, especially steel connections subjected to impulsive events, impedes the accuracy of numerical simulations used to predict post-blast behavior. To quantify the behavior of structural bolts, double and triple impulses are applied to A307 and A325 bolts in single shear using a high-speed hydraulic actuator. To decouple the effects of repetitive loading from the effects of dynamic loading, quasi-static single-cycle unloading experiments are performed. Instead of focusing solely on residual strength, as is typical in residual capacity studies, this work also studies the residual deformation capacity and strain energy absorption. These three metrics suggest differing levels of damage and rates of degradation, highlighting the importance of evaluating residual capacity based on a group of metrics that describe the essential behavior of a structural component. Although high-energy impacts demonstrated improvement for some residual properties, especially energy absorption, repeated low-energy impacts run the risk of diminished total deformation capacity and energy absorption response. This work also related the residual properties and subsequent loading characteristics to permanent deformation in the bolt, as a first step to using forensic field measurements to infer the remaining capacity of connections. 
     Although a loss of ductility in connections via bolt failure has been noted experimentally and in situ for decades, this work is the first investigation into its root cause. Experimental results show that the ductilityvariation of A307 bolts, both among dynamic tests and compared to quasi-static tests, is attributable to strain hardening and mode mixity of the fracture. However, the cause of ductility loss in high-strength A325 bolts is multi-faceted. Split Hopkinson Pressure Bar (SHPB) experimentation suggests that rate-induced changes in strain hardening contribute to a loss of ductility in high-strength bolts, but do not fully explain the marked degradation observed historically. Further investigation with scanning electron microscopy suggests that the ductility loss observed in high-strength A325 bolts is also due to a change in fracture mechanism. 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.