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research-article

Stress-State, Temperature, and Strain Rate Dependence of Vintage ASTM A7 Steel

[+] Author and Article Information
Shane Andrew Brauer

Mechanical Engineering, Mississippi State University, Mississippi State, MS 39762
sbrauer@cavs.msstate.edu

Wilburn Whittington

Center for Advanced Vehicular Systems, Center for Advanced Vehicular Systems, Mississippi State University, Starkville, MS 39759
wrw51@cavs.msstate.edu

Hongjoo Rhee

Center for Advanced Vehicular Systems, Center for Advanced Vehicular Systems, Mississippi State University, Starkville, MS 39759
hrhee@cavs.msstate.edu

Paul Allison

Department of Mechanical Engineering, University of Alabama, Tuscaloosa, AL 35406
pallison@eng.ua.edu

Doyl Dickel

Center for Advanced Vehicular Systems, Center for Advanced Vehicular Systems, Mississippi State University, Starkville, MS 39759
doyl@cavs.msstate.edu

Charles Kennan Crane

Geotechnical and Structures Laboratory, U.S. Army Engineer Research and Development Center, Vicksburg, MS 39180
charles.k.crane@usace.army.mil

Mark F. Horstemeyer

Mechanical Engineering, Mississippi State University, Mississippi State, MS 39762
mfhorst@cavs.msstate.edu

1Corresponding author.

ASME doi:10.1115/1.4041388 History: Received June 03, 2018; Revised July 20, 2018

Abstract

The structure-property relationships of a vintage ASTM A7 steel is quantified in terms of stress state, temperature, and strain rate dependence. The microstructural stereology revealed primary phases to be 15.8% ± 2.6% pearlitic and 84.2% ± 2.6 ferritic with grain sizes of 13.3 µm ± 3.1 µm and 36.5 µm ± 7.0 µm, respectively. Manganese particle volume fractions represented 0.38-1.53% of the bulk material. Mechanical testing revealed a stress state dependence that showed a maximum strength increase of 85% from torsion to tension and a strain rate dependence that showed a maximum strength increase of 38% from 10-1 to 103 s-1at 20% strain. In tension, a negative strain rate sensitivity was observed in the quasi-static rate regime yet was positive when traversing from the quasi-static rates to high strain rates. Also, the A7 steel exhibited a significant ductility reduction as the temperature increased from ambient to 573 K (300oC), which is uncommon for metals. The literature argues that dynamic strain aging can induce the negative strain rate sensitivity and ductility reduction upon a temperature increase. Finally, a tension/compression stress asymmetry arises in this A7 steel, which can play a significant role since bending is prevalent in this ubiquitous structural material. Torsional softening was also observed for this A7 steel.

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