Band 24, Heft 5-6

Although, the evaluation of the nanohardness of amorphous silicon (a-Si) has been the subject of a few experimental works but, to date, it has not been addressed theoretically yet. In this work, first principles Kohn-Sham density functional theory (DFT)-based molecular dynamics (MD) in combination with Mohr-Coulomb criterion is employed to calculate the ideal shear strength of the damped MD annealed a-Si sample containing dangling and floating bonds which are pertinent to the threefold- and fivefold-coordinated defects, respectively, as well as distorted tetrahedral bonds. The stress state beneath the nanoindenter is triaxial, and is accounted for properly. The calculated values of nanohardness are in reasonable agreement with those values measured experimentally. Consideration of the electronic charge distribution under the state of triaxial tension test reveals that the yield phenomenon in a-Si is accompanied by the transformation of a threefold-coordinated Si atom to a fourfold-coordinated.

This work presents a variational material model for transformation-induced plasticity in steels. We will use the principle of the minimum of the dissipation potential to develop a coupled material model for plastic deformations and phase transformations that simultaneously accounts for the hardening effects that play an important role. We will use a polycrystalline approach and introduce a combined Voigt/Reuß bound and a coupled ansatz for the dissipation functional to model the simultaneous effects of plastic deformations and phase transformations. Finally, we will present the first numerical results for a tension/compression test.

The occurrence of alternating damage zones surrounding underground openings (commonly known as zonal disintegration) is treated as a “far from thermodynamic equilibrium” dynamical process or a nonlinear continuous phase transition phenomenon. The approach of internal variable gradient theory with diffusive transport, which may be viewed as a subclass of Landau’s phase transition theory, is adopted. The order parameter is identified with an irreversible strain quantity, the gradient of which enters into the expression for the free energy of the rock system. The gradient term stabilizes the material behavior in the post-softening regime, where zonal disintegration occurs. The results of a simplified linearized analysis are confirmed by the numerical solution of the nonlinear problem.

Various failure criteria for rocks are discussed with emphasis on intrinsic temporal and structural features. It is shown that such internal-time/structure-based fracture criteria are different expressions of the governing structural changes under external action. Rock mass has a complex internal structural hierarchy, and failure is intimately related to the development of cracks, their growth and coalescence. In order to describe properly deformation and fracture in rocks it is necessary to consider the internal structural hierarchy of the rock mass and the finiteness of crack propagation. Critical issues including the essence of incubation time, the nature of limit failure strain, and the type of failure criteria for different strain rate sensitivity regimes, are discussed.

In the present paper, the equation of motion for an incompressible transversely isotropic fibre-reinforced elastic solid is derived in terms of a scalar function. The general solution of the equation of motion is obtained, which satisfies the required radiation condition. The appropriate impedance boundary conditions are also satisfied by the solution to obtain the required explicit secular equation for the Rayleigh wave speed. The numerical values of non-dimensional speed of a Rayleigh wave are obtained with the application of Iteration method. The dependence of the non-dimensional wave speed on non-dimensional material parameter and impedance parameters is shown graphically.

Foam core sandwich composites were subjected to (i) creep to failure, (ii) cyclic creep-relaxation and (iii) fully reversible cyclic creep loading in seawater in order to mimic an actual ship hull’s service lifetime scenario. The results indicate a strong dependence of lifetime on the mode of loading. A significant reduction in the overall life was observed under cyclic creep as compared with the conventional creep to failure. Creep relaxation (R=1) tests were performed at loading-relaxation periods of 24/24, 24/12, 24/6, 12/12 and 6/6 h, while the fully reversible (R=-1) creep tests were conducted at loading-reversed loading times of 36/36, 24/24, 12/12, 6/6, and 3/3 h. The results suggest that creep-relaxation lifetime characteristics depend predominantly on the relaxation time as opposed to loading times, i.e. longer relaxation periods lead to shorter life. Whereas, fully reversible creep appears to be dependent upon the number of reversals whereby, life is observed to reduce as the number of reversals increase. These significant observations are explained in terms of various possible paths to interface cell wall collapse. Modes of failure were predominantly indentation and core compression in the vicinity of the loading site.

AA 2519-T87 is an aluminium alloy that principally contains Cu as an alloying element and is a new grade of Al-Cu alloy system. This material is a potential candidate for light combat military vehicles. Fusion welding of this alloy leads to hot cracking, porosity and alloy segregation in the weld metal region. Friction stir welding (FSW) is a solid state joining process which can overcome the above mentioned problems. However, the FSW of age hardenable aluminium alloys results in poor tensile properties in the as-welded condition (AW). Hence, post weld heat treatment (PWHT) is used to enhance deteriorated tensile properties of FSW joints. In this work, the effect of PWHT, namely artificial ageing (AA) and solution treatment (ST) followed by ageing (STA) on the microstructure, tensile properties and microhardness were systematically investigated. The microstructural features of the weld joints were characterised using an optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The tensile strength and microhardness of the joints were correlated with the grain size, precipitate size, shape and its distribution. From the investigation, it was found that STA treatment is beneficial in enhancing the tensile strength of the FSW joints of AA2519-T87 alloy and this is mainly due to the presence of fine and densely distributed precipitates in the stir zone.

This study hinges on the feasibility of strengthening Al and Al-Mg wires by adding Al nanocomposite pellets containing MgB 2 nanoparticles into the melt upon fabrication. These MgB 2 nanoparticles were obtained by fragmentation using a high-energy ball mill, and were, afterward, mechanically alloyed with pure aluminum. The resulting MgB 2 /Al nanocomposite pellets were sintered at 260°C to be subsequently added into molten aluminum and an Al-Mg alloy melt. Cold rolling intercalated with stepwise annealing allowed the fabrication of 1 mm diameter wires with a final area reduction of 96%. Mechanical and physical properties of the treated wire specimens were compared to those of similarly processed pure aluminum wire. The ultimate tensile strength of the treated wires increased approximately double fold with respect to untreated wires at the expense of some loss in electrical conductivity.