The effective mechanical and acoustic properties of two-dimensional pentamode metamaterials (PMs) with different structural parameters are investigated in this paper. It is found that with varying structural parameters, the effective bulk modulus and density remain constant as the same as those of water, while the figure of merit, i.e., the ratio of the bulk modulus to the shear modulus (B/G) gradually increases due to the decrease of the shear modulus. However, full wave simulations reveal that with the increase of B/G, the acoustic scattering becomes more and more intense, which indicates that the acoustic properties of pentamode metamaterials gradually deviate from those of water. These anomalous acoustic behaviors are proposed to arise from the existence of the bending modes in pentamode microstructures. Our results show that for pentamode metamaterials, the mechanical properties cannot be simply translated to their acoustic properties, and the structural parameters affect the mechanical and acoustic properties in much different ways.

Pentamode materials approximate tailorable artificial liquids. Recently, microscopic versions of these intricate structures have been fabricated, and the static mechanical experiments reveal that the ratio of bulk modulus to shear modulus as large as 1000 can be obtained. However, no direct acoustic experimental characterizations have been reported yet. In this paper, a water-like two-dimensional pentamode material sample is designed and fabricated with a single metallic material, which is a hollow metallic foam-like structure at centimeter scale. Acoustic simulation and experimental testing results indicate that the designed pentamode material mimics water in acoustic properties over a wide frequency range, i.e., it exhibits transparency when surrounded by water. This work contributes to the development of microstructural design of materials with specific modulus and density distribution, thus paving the way for the physical realization of special acoustic devices such as metamaterial lenses and vibration isolation.

In micro scale cutting, tooling geometry plays a significant role in determining machining quality and tool life, and the knowledge of tooling geometrical effects on process performance potentially benefits engineers on improving tool designs and selecting optimum cutting conditions. This research aims to comprehensively investigate tooling geometrical effects on the process performance in micro milling using a finite element method supported with well-designed cutting trials. In the study, a benchmark three-dimensional tooling model, incorporating rake angle, relief angle, helix angle, diameter and cutting edge radius is initially developed for simulating the micro milling process under large deformations. The simulation is then experimentally validated and the predicted micro chip formation and cutting forces are in reasonable agreement with measured results in cutting trials. Furthermore, finite element-based simulations are performed under different helix angles, rake angles and cutting edge radius, and comparisons of cutting forces, tool stresses, tool temperatures, chip formation and temperatures are presented and discussed. It is found that the cutting edge radius is the most influential factor on the tool’s process performance, followed by helix angle, and rake angle has less effect.

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In micro milling, unpredictable tool life and premature tool failures are the major constraints for its industrial applications, and prolongation of the tool life so as to enhance the tooling performance presents great challenges. Appropriate coating techniques potentially offer a feasible and promising solution. In the research presented, diamond-like carbon (DLC) films are deposited on a Ø500µm diameter tungsten carbide (WC) micro end mill by the plasma enhanced chemical vapor deposition (PE-CVD) process. Coating characterization has been undertaken and the DLC coatings are found in good coverage on the tool except for a slight delaminating on the edge corners. Besides, the surface of the amorphous coatings is much smoother than that of WC. In addition, comprehensive cutting performance of the DLC coated tool in dry slot milling of Al 6061-T6 has been compared with those of uncoated tools in both dry and wet conditions. It is observed that the use of DLC coatings can reduce the cutting forces, lessen the tool wear, improve the surface roughness and minimize the microburr formation as compared to the corresponding performance of an uncoated tool in dry cutting. However, the performance improvement is still unreachable to those resulting from the cutting fluid influence.