Abstract
This paper presents an investigation of the mechanism of the brittle–ductile cutting mode transition from the perspective of the mechanics. A mechanistic model is proposed to analyze the relationship between undeformed chip thickness, deformation, and stress levels in the elastic stage of the periodic chip formation process, regarding whether brittle or ductile mode deformation is to follow the elastic stage. It is revealed that, the distance of tool advancement required to induce the same level of compressive stress decreases with undeformed chip thickness, and thereby the tensile stress below and behind the tool decreases with undeformed chip thickness. As a result, the tensile stress becomes lower than the critical tensile stress for brittle fracture when the undeformed chip thickness becomes sufficiently small, enabling the brittle–ductile cutting mode transition. The finite element method is employed to verify the analysis of the mechanics on a typical brittle material 6H silicon carbide, and confirmed that the distance of the tool advancement required to induce the same level of compressive stress becomes smaller when the undeformed chip thickness decreases, and consequently smaller tensile stress is induced below and behind the tool. The critical undeformed chip thicknesses for brittle–ductile cutting mode transition are estimated according to the proposed mechanics, and are verified by plunge cutting experiments in a few crystal directions. This study should contribute to better understanding of the mechanism of brittle–ductile cutting mode transition and the ultra-precision machining of brittle materials.
Highlights
Brittle–ductile cutting mode transition is an important phenomenon in the ultra-precision machining of brittle materials [1,2,3,4]
The analysis of the mechanics in the cutting zone showed that, in order to reach a same level of maximum compressive stress, a larger undeformed chip thickness would require a larger distance of tool advancement and thereby a larger tensile stress would be induced below and behind the tool, which would cause brittle fracture when it exceeds the critical tensile stress for brittle fracture
Though previous studies have found that compressive stress would induce ductile chip formation under small undeformed chip thickness [16,17,18,19,20,21,22,23] and that tensile stress would cause brittle fracture under large undeformed chip thickness [22,34,35], the mechanics proposed in this study reveals an explicit relationship between the compressive and tensile stresses, and how their roles shift to determine the deformation mode when the undeformed chip thickness changes
Summary
Brittle–ductile cutting mode transition is an important phenomenon in the ultra-precision machining of brittle materials [1,2,3,4]. The core idea of this theory is that the energy of the brittle mode material removal is proportional to the second power of the machining scale while the energy of the plastic flow is proportional to the third power of the machining scale, which means that the ductile mode material removal would be energetically more favorable when the machining scale is small enough. This theory is widely adopted to guide the Micromachines 2018, 9, 49; doi:10.3390/mi9020049 www.mdpi.com/journal/micromachines
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