Date of Award

12-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Committee Chair/Advisor

Hongseok Choi

Committee Member

Laine Mears

Committee Member

Xin Zhao

Committee Member

Gang Li

Abstract

As global warming, environmental pollution, and energy shortage intensify, a low-carbon and environmentally-friendly economy has become every country’s aim in this century. However, fossil fuel is still the most widely used energy in human activities, especially in automobiles and aerospace. Therefore, limiting the consumption of fossil fuels is of great importance to alleviate the crisis and pursue carbon-neutral. In the recent few decades, there has been an increasing interest in applying high-strength and lightweight materials.

With the superior specific strength weight ratio, aluminum alloys, titanium alloys, and Carbon Fiber Reinforced Polymer (CFRP) has been extensively used to lower the vehicle’s weight. However, the soft nature of the aluminum, low thermal conductivity of the titanium gives significant problems in machining. Moreover, the heterogeneous property of the CFRP made it even more challenging in its drilling process. Combining the numerical modeling with experimental methods, this study was intended to explore and gain a better understanding of the machining process of these three widely used but difficult-to-machine materials.

Firstly, Finite Element (FE) models of orthogonal cutting of A2024 and Ti6Al4V were developed using the commercial package Abaqus/Explicit. The effects of different numerical formulations on the computational prediction of the segmented chip formation were systematically studied in terms of chip morphologies, cutting force, strain, and temperature distributions. The results demonstrated that the pure Lagrangian formulation model delivers the closest chip morphology to the experiment. In contrast, both the ALE and CEL formulations have adverse effects on chip formation and maintenance with suppression on predicting the adiabatic shear fracture, which has been considered one of the essential factors of the segmented chip formation. Among the three numerical formulations, the Lagrangian formulation showed the best computational efficiency.

Secondly, high-speed orthogonal cutting tests were additionally performed for A2024-T351 aluminum to inspect the chip segmentation further. The results provide validation to the numerical modeling results and provide evidence of the numerical findings. It confirmed that the Adiabatic Shear Band (ASB) initiates from the chip root and the free chip surface. The ASB is still the root cause and precursor of cracks in chip segmentation.

Thirdly, the drilling experiments were performed for CFRP with Lightning Strike Protection (LSP) System using both twist and dagger drills. Its machinability was characterized in terms of thrust force, hole quality, and tool wear. The results revealed that LSP has effects on the thrust force and introduces additional burr and delamination. Moreover, the dagger drill showed minor wear damage, while the twist drill suffered significant wear due to their different geometry, precisely the point angle.

Finally, the numerical models of drilling of CFRP with LSP were developed both mesoscopically and microscopically. The modeling results helped to reveal the mechanisms of the LSP-introduced burr and delamination in the drilling process. The ductility of the LSP was confirmed to be the main reason for the notable burr and delamination formation.

Available for download on Wednesday, November 30, 2022

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