Date of Completion

8-13-2013

Embargo Period

8-13-2013

Keywords

Shear Bands, High Strain Rate, HSLA, Steel, Hardness, Microstructure

Major Advisor

Bryan D. Huey

Associate Advisor

Rainer Hebert

Associate Advisor

Harris Marcus

Associate Advisor

Puxian Gao

Associate Advisor

Avinash Dongare

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

The Department of Homeland Security (DHS) funded National Transportation Security Center of Excellence (NTSCOE) at the University of Connecticut focuses on developing and characterizing new materials systems and testing methods for the protection of the country’s infrastructure. This study has provided insight into the mechanical behavior of a high strength low alloy (HSLA) steel, ASTM A913 grade 65, under shock loading conditions and elevated temperatures. Shock loading conditions incorporate deformation rates greater than 10s-1. Studies have shown that the high rates of deformation, as experienced in an explosion, followed by elevated temperatures such as during a fire, can severely weaken steel structures to the point of failure. High strain rates have the propensity to create localized shear in metals, most catastrophically in the form of adiabatic shear bands (ASBs). ASBs are detrimental to a material’s performance as they are more brittle that the surrounding matrix, and thus are likely sites for crack propagation. A drop-weight test fixture and specialized specimen geometry was designed to deliver impact loads sufficient to generate ASBs within the A913 material as well as for A992, an industry standard. A microstructural investigation of the resulting shear bands and surrounding matrix was carried out by light microscopy, indicating that A913 is more susceptible to ASB formation due to the finer initial grain structure and additional microstructural constituents of this advanced thermally processed alloy. The hardness was mapped across such ASBs using Instrumented Indentation Testing (IIT), both in the as formed state and upon annealing at 600°C for 1 hour. Post annealing, the hardness of the shear bands returns to the matrix values as the grain structure heals while the parent microstructure remains unaffected, decreasing the likelihood of brittle failure but maintaining structural integrity. Finally, a 3-point test fixture was designed and loaded in a furnace heated according to modified ASTM E119 standards for fire tests of building materials. The A913 out-performed A992 in these elevated temperature tests with lower rates of deformation under equal static loads.

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