Theses and Dissertations

ORCID

https://orcid.org/0009-0006-3591-5253

Advisor

Hongjoo, Rhee

Committee Member

Whittington, Wil

Committee Member

Paudel, Yubraj

Date of Degree

5-15-2026

Original embargo terms

Embargo 6 months

Document Type

Dissertation - Open Access

Major

Engineering (Mechanical Engineering)

Degree Name

Doctor of Philosophy (Ph.D.)

College

James Worth Bagley College of Engineering

Department

Michael W. Hall School of Mechanical Engineering

Abstract

Understanding and improving the performance and durability of tool steels is critical for industrial applications, including drilling, forming, and high-speed cutting. This study presents a comprehensive experimental and numerical investigation aimed at optimizing tool steel compositions and microstructures to enhance driver bit performance under both quasi-static and high-strain-rate torsional loading. The study begins with a detailed literature review on tool steels, emphasizing the roles of alloying elements, heat treatment, and phase transformations in controlling microstructure and mechanical behavior. Experimental investigations were conducted on three industrial tool steel compositions (SVCM+, X8, and S2M), with a focus on the effects of heat treatment. Mechanical behavior was characterized using indentation and torsional testing, while manual testing evaluated real-world performance through screw drilling cycles. X-ray computed tomography (XRCT) scans were subsequently used to analyze failure mechanisms and tip wear, providing a non-destructive approach for visualizing internal deformation and fracture patterns. Finite element simulations were conducted in Abaqus to complement the experimental work. Quasi-static torsion simulations validated the model against experimental torque curves, followed by high strain rate simulations to investigate strain-rate-dependent deformation and failure mechanisms. Plastic equivalent strain (PEEQ) served as a key metric for assessing localized deformation at the tip and shock zone regions. A performance index (PI), derived from experimental results, was incorporated into the simulation framework to correlate material properties with PEEQ, enabling performance prediction across both domains and linking ultimate tensile strength (UTS), UTS ratio, and PEEQ to durability. The results indicate that optimizing hardness gradients by strengthening the exterior while introducing a softer core enhances energy redistribution and delays failure, whereas surface hardening alone promotes brittle fracture. High strain rates lead to increased flow stress and reduced plastic deformation, resulting in a stiffer, less ductile response with a higher tendency for localized deformation and brittle failure. Overall, this work integrates metallurgical design, experimental testing, and advanced finite element modeling to provide new insights into the wear and failure behavior of tool steels. These findings provide a robust framework for developing next-generation driver bit materials by combining optimized microstructures with predictive performance modeling to extend service life under demanding operational conditions.

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Available for download on Thursday, December 10, 2026

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