The effect of Stone–Thrower–Wales defects on mechanical properties of graphene sheets – A molecular dynamics study

The mechanical properties of graphene may be strongly affected by defects. In our work, the effects of the orientation and tilting angles of Stone–Thrower–Wales (STW, 5-7-7-5) defects on the mechanical properties of graphene have been investigated based on molecular dynamics simulations. When there is one centred STW defect including STW-1 defect and STW-2 defect, our study reveals that the orientation with respect to the chirality governs the mechanical properties of graphene. For STW-1 defective graphene, the strength of the armchair direction is much lower than that of zigzag direction. While the circumstance for STW-2 defective graphene is opposite, the strength of the armchair direction is much higher than the strength of the zigzag direction. Furthermore, when there is more than one STW defect, the mechanical properties of graphene depends on the tilting angle of STW defects. The breaking strength of graphene decreases with the increasing tilting angle. Our present work could provide significant insights into the effect of STW defects on the mechanical properties of graphene.     

An efficient,simplified multiple-coupled circuit model of the induction motor aimed to simulate different types of stator faults

This paper proposes an original simplified model aimed to simulate, an easy way, inter turns short circuit fault, phase to phase fault and phase to ground fault. In this model, the stator is considered as six magnetically coupled windings and the rotor as three not magnetically coupled R–L circuits. The paper also presents the star- and delta-connected stator configurations of the simplified model. However, the proposed simplified model is suitable only for steady-state operation. The performance of the simplified model is first verified by a comparison between the simulated current of the multiple-coupled model and the simplified model. Then, since the stator faults have an impact on the symmetrical components of the stator current, this paper uses these components to validate the behavior of the simplified model by simulation and experimentally using a 1.1 kW motor. In addition, simulated results of the simplified model for a 110 kW motor are presented in order to generalize the use of the proposed model to larger motors.     

The influence of a thermoplastic toughening interlayer and hydrothermal conditioning on the Mode-II interlaminar fracture toughness of Carbon/Benzoxazine composites

Carbon fibre/Benzoxazine laminates with and without non-woven polyamide (PA) fibre veils at the interlaminar regions were manufactured using vacuum assisted resin transfer moulding (VARTM). The effect of the interlaminar thermoplastic veils on the Mode-II critical strain energy release rate (G_IIC), under both wet and dry conditions, was determined using two commercially available Benzoxazine resins: a toughened system and an untoughened system. In all samples the toughened system outperformed the untoughened system. The overall resistance to Mode-II crack growth was significantly improved by the inclusion of the interlaminar veils due to an increase in the thickness of the matrix-rich interlaminar region, plastic deformation of the PA fibres and a crack-pinning mechanism. Moisture caused an increase in matrix ductility, which improved the resistance to crack initiation; however, this was counteracted by a reduction in fibre/matrix interfacial strength causing a reduction in resistance to crack growth.     

Parametric study of texturing in convergent bearings

The effect of textured surfaces in hydrodynamic bearings is assessed using a mass-conserving numerical analysis that allows for arbitrary geometry and multiple regions of cavitation. The texture investigated consists of regularly spaced rectangular pockets through an (infinitely) long linearly convergent or parallel bearing. This arrangement leads to nine independent non-dimensional parameters including operating conditions. The effectiveness of texture at improving load support and reducing friction over a corresponding plain bearing is described in relation to these parameters and the important interaction between parameters is highlighted. The beneficial or detrimental effect of texture is explained in terms of the fundamental mechanisms of hydrodynamic pressure generation including inlet suction.     

Modeling and analysis of the material removal depth for stone polishing

In this paper, a model of predicting the material removal depth of the workpiece surface for the mould polishing by fixed abrasives is developed and an approach to achieve the material removal profile is presented. The effect of the grain size on material removal depth is considered. The distribution of the abrasive grain protrusion heights is taken to be Gaussian distribution. The relationship between the pressure and the depth of indentation is investigated by analyzing interaction of the abrasive grains and the workpiece. It is assumed that the pressure distribution is Hertzian at the contact between the tool and the workpiece surface. The theoretical model of linear removal intensity is presented by calculating the removal volumes of all abrasive grains participating in cutting. The depth of the material removal can be obtained by integrating the linear removal intensity along the polishing contact path formed by the polishing tool passing this position. The predicted results based on the current model are shown to be approximately consistent with the experimental results.     

Modeling and vibration characteristics analysis of a DMF rotor system

To analyze the vibration response of a rotor system with circumferential short spring dual mass flywheel (DMF) when the primary flywheel speed changes, the interactions (forces and torques) between the primary flywheel, spring seat, spring, and secondary flywheel are analyzed in detail, and the dynamic analysis model of the DMF rotor system is established considering the influence of clearance and friction between parts in the DMF in this study. The vibration response of the DMF is investigated by numerical method. By analyzing the bifurcation diagram, time history, phase trajectories, Poincaré section, and frequency domain of the relative angular displacement, the variation of vibration form of the system under different excitation frequencies are discussed. Besides, the effects of load, primary flywheel speed amplitude, and spring stiffness on system vibration are also analyzed. Finally, some of the results from the analytical study are verified through the DMF rotor system experiments.

     

Fabrication of noncircular multicore microtubes by superplastic dieless drawing process

A superplastic dieless drawing process that requires no dies or tools is applied to the drawing of a Zn–22Al superplastic alloy for noncircular microtubes such as square, rectangular and noncircular multi core tubes having square inner and rectangular outer cross-sections. In this study, the effects of heating condition, such as heating length and the use or nonuse of cooling device, on deformation behavior are investigated. As a result, a square microtube with 0.58 mm side and a rectangular microtube of 0.75 mm × 1.3 mm were fabricated after 3-pass superplastic dieless drawing. In addition, the fundamental deformation behavior of noncircular tubes combined with square and rectangular tubes during the dieless drawing process has been clarified experimentally. The cross-sectional shape of the noncircular tubes after the superplastic dieless drawing process tends to be maintained on the basis of the similarity law in case of a wide heating length compared with a narrow heating length. Furthermore, a noncircular microtube, which has inner square tubes with a 335 μm side, and an outer rectangular tube of 533 μm × 923 μm were fabricated successfully after a 4-pass superplastic dieless drawing process. Consequently, it was found that the superplastic dieless drawing is effective for the fabrication of noncircular multicore microtubes.     

On the mechanism of squat formation on train rails – Part II: Growth

Longitudinal cross-sectioning of squats reveals characteristic features of internal crack front propagation. Leading crack planes propagate over longer lengths and greater depths as compared to more superficial trailing crack planes. A favourite depth of crack propagation occurs in the subsurface (2–3 mm), is related to the residual longitudinal stress profile, and may lead to an internal crack ‘terrace’. Especially during deeper crack propagation and branching oxidation processes are found to be metallurgical drivers of crack growth. Contact surface modification during squat growth can be distinguished between phases of transient local stress redistribution and of dynamic wheel–rail contact. If the hypothesized shearing wedge in the failure mechanism loses its load bearing capacity, this gives rise to a redistribution of normal stresses within the actual contact ellipse and the formation of a hardness envelope along the crack pattern. This may partially explain why maturing squats show decoloured and hardened surface areas bordering the surface-breaking cracks. A second effect occurs for contact patches not matching the failure ‘envelope’: due to the Poisson effect the surface overlying the crack planes settles slightly, experiences reduced contact, and corrosive products, ‘pumped’ from inside the cracks, may accumulate on the surface (as confirmed by SEM-EDX analysis). During progressive growth of the defect the harder and decoloured envelope as well as the original wedge is pressed into the deeper elastic material, accompanied by a gradual expansion of the contact band and a bilateral bridging of the defect. This may cause high-frequency impact, resulting into progressive internal crack growth affecting the global stress response and rail fracture.     

Internal battery temperature estimation using series battery resistance measurements during cold temperatures

A technique has been developed to estimate the internal battery temperature (T_bat) of secondary batteries by measuring the series battery resistance (R_B) at cold ambient temperatures. Tests were performed on both lead-acid and nickel metal hydride batteries at different cold temperatures to obtain useful plots of R_B versus T_bat. R_B was measured by using a pulse discharge circuit to apply a short-duration current pulse (I_B) directly to the battery. The test results indicated that R_B not only varies with temperature but also varies with the amplitude of I_B. The R_B versus T_bat plots were later utilized to predict T_bat from R_B during alternating current (AC) battery heating at cold ambient temperatures.     

Study on the influence of segmented fuel cell by grooving method and its application in oxygen starvation diagnosis

The oxygen starvation in fuel cells is an important reason for the deterioration of durability. The segmented fuel cell is a method to study the gas distribution inside the fuel cell. In order to study the influence of the grooving method on segmented fuel cell and its application in oxygen starvation diagnosis, a five-serpentine-channel three-dimensional two-phase simulation model is established by FLUENT. Through steady-state simulation, the effect of grooving method on fuel cell performance is studied. The overall performance (polarization curve) of the fuel cell drops slightly, but the current density distribution on the anode graphite plate changes greatly due to the grooves. The “current concentration” phenomenon is proposed based on the current density distribution. Through dynamic simulation, the oxygen starvation under current load mode and voltage load mode is simulated, and the “starvation coefficient” is defined as an oxygen starvation diagnostic index. In the current load mode, the “starvation coefficient” never exceed 15%, because when the oxygen starvation is severe, the simulation cannot converge or even cannot maintain, which corresponds to the voltage reversal in reality. However, in the voltage load mode, the “starvation coefficient” can reach up to 100%. The conclusions have important guiding significance for the judgment of the internal reaction uniformity of the segmented fuel cell by grooving method and provide a theoretical basis for judging whether a fuel cell is out of oxygen by segmented fuel cell.