Density functional prediction of the structural, elastic, electronic, and thermodynamic properties of the cubic and hexagonal (c, h)-Fe2Hf

Using density functional theory (DFT), the structural, elastic, electronic, and thermodynamic properties of Fe2Hf in the cubic and hexagonal solid phases with Fd-3m and P63/mmc are reported with generalized gradient approximations (GGA). To achieve energy convergence, we report the k-point mesh density and plane-wave energy cut-offs. The calculated equilibrium parameters are in good agreement with the available theoretical data. A complete elastic tensor and crystal anisotropies of the ultra-incompressible Fe2Hf are determined in the wide pressure range. Finally, by using the quasi-harmonic Debye Model, the isothermal and adiabatic bulk modulus and heat capacity of Fe2Hf are also successfully obtained in the present work. By the elastic stability criteria, it is predicted that Fd-3m and P63/mmc structures of Fe2Hf are stable in the pressure range studied, respectively.

Mechanical And Thermodynamic Stabilities, Half-Metallic Property of Co2CrAl Heuslerene: A DFT Study

In the present study, the physical properties of the ground state in bulk and Co2CrAl Heuslerene compound are investigated by density functional theory (DFT). The effects of exchange-correlation potential on the calculations have been also investigated by GGA, GGA+U, and GGA+U+mBJ approximately. Here, three graphene-like structures with the thickness of about 8 Bohr have been labeled as α, β, and ϒ phases. The results demonstrate the mechanical stability of bulk Co2CrAl since it passes the elastic stability test. Having proved the static stability of the bulk and three Heuslerene shapes of Co2CrAl, it is essential to study dynamic stability as well. The accessible region in the thermodynamic phase diagrams confirms the thermodynamic stability of bulk Co2CrAl and all 2D phases of the compound. According to our electronic calculations, the bulk phase of Co2CrAl is a half-metal whose values of magnetic moment and spin polarization is 3 μB and 100% at the Fermi level, respectively. Besides, α and ϒ phases show the metal behavior for both spin directions in all imposed approximations. Finally, β phase exhibits different magnetic properties for different approximations. From 3eV to 2eV, GGA and GGA+U reveal the magnetic anisotropic and isotropic nature. Besides, an extremely anisotropic nature is observed at the Fermi level by GGA+U+mbJ.

Comparative Analysis of Brake Disc Materials

A brake is a device that applies artificial frictional resistance to a revolving disc in order to stop the vehicle from moving, the frictional heat created at the disc pad interface can cause high temperature during the braking period, thermal elastic stability(TEI), early wear, brake fluid vaporization (BFV), and thermally stimulated vibrations can caused by frictional heat produced on the rotor surface (TEV), better thermal stability materials will decrease these causes, we investigate the thermal and structural characteristics in this research by finite element software, the solid brake disc is made up of various materials such as titanium alloy, structured steel and gray cast iron, further we analyze the brake disc using ANSYS 16.0 and CATIA V5 is used to design the model of brake disc, for this project the heat flux calculation have been made by considering various parameters of material as well as vehicle, finally a comparison made between grey cast iron, titanium alloy and structural steel materials. With respect to equivalent stress, temperature distribution, deformation values. This paper involves selecting a best suitable material to design a brake disc which leads to better safety to passengers.

Buckling between soft walls: sequential stabilization through contact

Motivated by applications of soft-contact problems such as guidewires used in medical and engineering applications, we consider a compressed rod deforming between two parallel elastic walls. Free elastica buckling modes other than the first are known to be unstable. We find the soft constraining walls to have the effect of sequentially stabilizing higher modes in multiple contacts by a series of bifurcations, in each of which the degree of instability (the index) is decreased by one. Further symmetry-breaking bifurcations in the stabilization process generate solutions with different contact patterns that allow for a classification in terms of binary symbol sequences. In the hard-contact limit, all these bifurcations collapse into highly degenerate ‘contact bifurcations’. For any given wall separation at most a finite number of modes can be stabilized and eventually, under large enough compression, the rod jumps into the inverted straight state. We chart the sequence of events, under increasing compression, leading from the initial straight state in compression to the final straight state in tension, in effect the process of pushing a rod through a cavity. Our results also give new insight into universal features of symmetry-breaking in higher mode elastic deformations. We present this study also as a showcase for a practical approach to stability analysis based on numerical bifurcation theory and without the intimidating mathematical technicalities often accompanying stability analysis in the literature. The method delivers the stability index and can be straightforwardly applied to other elastic stability problems.

Laboratory Study on the Stability of Large-Size Graded Crushed Stone under Cyclic Rotating Axial Compression

In this paper, the stability of large-size graded crushed stone used for road base or cushioning under repeated load is investigated. Using an in-house developed device, large-size crushed stone mix was compacted and molded by the vibration and rotary compaction method. Cyclic rotating axial compression was applied, and the shakedown theory was used to study the cumulative deformation of the large-size crushed stone specimens. The effects of gradation parameters on the cumulative strain and stability behavior were analyzed, and the critical stability and failure loads were determined according to the shakedown theory. The test results indicate that there are three obvious instability behavior stages of large-size graded crushed stone under cyclic rotating axial compression: elastic stability, plastic creep, and incremental plastic failure. Large-size graded crushed stone has a higher critical stability load stiffness than conventional-size graded crushed stone. The critical shakedown load of the specimen is mainly affected by the skeleton structure performance, and the critical failure load by the properties of the crushed stone material. Increasing the content and compactness of large-size crushed stone in the specimen can improve the stiffness and stability performance, and to achieve improvements, the content of large-size crushed stone should be controlled between 22% and 26%. The critical shakedown load increases with the increase in the California bearing ratio (CBR) value, while, on the other hand, the CBR value has little relationship with the critical failure load.

A Sliding-rod Variable-strain Model for Concentric Tube Robots

<div>In this work, the Piecewise Variable-strain (PVS) approach is applied to the case of Concentric Tube Robots (CTRs) and extended to include the tubes’ sliding motion. In particular, the currently accepted continuous Cosserat rod model is discretized onto a finite set of strain basis functions. At the same time, the insertion and rotation motions of the tubes are included as generalized coordinates instead of boundary kinematic conditions. Doing so, we obtain a minimum set of closed-form algebraic equations that can be solved not only for the shape variables but also for the actuation forces and torques for the first time. This new approach opens the way to torque-controlled CTRs, which is poised to enhance elastic stability and improve interaction forces’ control at the end-effector. </div>

Investigations on Cmc21-Si2P2X structures and physical properties by first-principles calculations

The new structures, Cmc21-Si2P2X (X=S, Se, Te, and Po), are predicted, and their mechanical, electronic and optical properties are investigated with the density functional theory, by first principles calculations. The elastic constants of the four compounds are calculated by the stress-strain method. The calculations of the elastic stability criteria and phonon dispersion spectra imply that they are mechanically and dynamically stable at zero pressure. The mechanical parameters, such as shear moduli G, bulk moduli B, Young's moduli E and Poisson's ratios v are evaluated by the Voigt-Reuss-Hill approach. The Cmc21-Si2P2X has the largest hardness due to the largest Young's modulus in the four compounds, and it is a covalent crystal. The anisotropies of their mechanical properties are also analyzed. The band structures and densities of states, which are calculated by using HSE06, show that Cmc21-Si2P2X compounds are indirect bandgap semiconductors, and the values of the band gaps decrease with increasing atomic number from S, Se, Te, to Po. In addition, the longitudinal sound velocity and transverse sound velocity for Cmc21-Si2P2X have been investigated. The dielectric constant, electron energy loss, refractive index, reflectivity, absorption and conductivity are analyzed to gain the optical properties of Si2P2X.