In this study, the nanogrinding process for single-crystal nickel was investigated using a molecular dynamics simulation. A series of simulations were conducted with different tool radii and grinding methods to explore the effects of chip morphology, friction forces, subsurface damage, and defect evolution on the nanogrinding process. The results demonstrate that the workpiece atoms at the back of the tool were affected by the forward stretching and upward elastic recovery when no chips were produced. Although the machining depth was the smallest, the normal force was the largest, and dislocation entanglement was formed. The small number of defect atoms indicates that the extent of subsurface damage was minimal. Moreover, when spherical chips were produced, a typical columnar defect was generated. The displacement vector of the chip atoms aligned with the machining direction and as the chips were removed by extrusion, the crystal structure of the chip atoms disintegrated, resulting in severe subsurface damage. By contrast, when strip chips were produced, the displacement vector of the chip atoms deviated from the substrate, dislocation blocks were formed at the initial stage of machining, and the rebound-to-depth ratio of the machined surface was the smallest.
Abstract-In this paper, the construction method of common measurement matrices is studied, adaptability of common measurement matrices for mechanical vibration signal is analyzed. Typical measurement matrices are selected from commonly used measurement matrices, Gaussian random measurement matrix and Bernoulli random measurement matrix are chosen from totally random measurement matrices, Circulant measurement matrix and Toeplitz measurement matrix are selected from deterministic matrices, partially random Fourier measurement matrix and Hadamard matrix are chosen from deterministic measurement matrices. The sensing performance of common measurement matrices for mechanical vibration signal is evaluated from the two perspective of reconstruction error and memory space. The simulation results show that two kinds of complete random matrices, Gaussian and Bernoulli matrices, can exactly reconstruct original vibration signal, but they occupy large memory space; deterministic matrices, Circulant and Toeplitz matrices, although need fewer memory space, obtained measurements which do not have information of global vibration signal lead to lower reconstruction results; partially random Fourier matrix is extremely coherent with sparse transforming base of vibration signal, so it has not exact result in the process of reconstructing original vibration signal, the requirement of Exponentiation of 2 seriously restrict its application.
Based on the log-linear virtual age process, an imperfect preventive maintenance policy for numerical control (NC) machine tools with random maintenance quality is proposed. The proposed model is a combination of the Kijima type virtual age model and the failure intensity adjustment model. Maintenance intervals of the proposed hybrid model are derived when the failure intensity increase factor and the restoration factor are both random variables with uniform distribution. The optimal maintenance policy in infinite time horizon is presented. A numerical example is given when the failures of NC machine tools are described by the log-linear process. Finally, a discussion is presented to show how the optimal results depend on the different cost parameters.
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