Similarity of Stress-Singularity Problems for Materials with Linear and Bilinear Behaviors

An analogy is established between the solutions of the problems of singularities of stresses in linear and bilinear elastic isotropic media. It is shown that the distributions of stresses and displacements in the vicinity of singular points on the boundary of the body (characterized by the singularities of stresses) are described, in both cases, by the same functional dependences on the space coordinates but with different characteristics of the material. We deduce expressions for the effective moduli of elasticity and Poisson's ratio of the bielastic medium including the parameter of hardening of the material. The solution of the problem of singularities of stresses in bilinear materials is obtained from the solution of the corresponding problem for the linear elastic medium by replacing the elastic constants with the corresponding effective values depending on the parameter of hardening of the material. The cases of wedge-shaped notches (for various boundary conditions imposed on their edges), two-component wedges, plane wedge-shaped cracks, and circular conic notches or rigid inclusions in the bielastic space are studied in detail.     

Influence of the Velocity of Turbulent Flow on Its Interaction with a Viscoelastic Coating

It has been shown that the maximum interaction between turbulent flow and a viscoelastic coating is observed when the coating thickness is equal to one quarter of the wavelength of the longitudinal deformation of the coating surface. The amplitudes of the longitudinal and lateral deformations are equal, which must be taken into account in analyzing the mechanism of operation of a compliant coating.     

Semi-solid rapid compression testing of spray-formed hypereutectic Al-Si alloys

The rheology of Al-30Si-5Cu and Al-30Si-5Cu-2Mg spray-formed alloys has been characterized after holding at various times and temperatures, using semisolid rapid compression to obtain load-displacement curves. Microstructures were examined after partial remelting and compression in an attempt to understand the flow behavior. The initial resistance to flow generally increased with holding time at temperature. This increase was attributed to the increased strength of a three-dimensional (3-D) solid silicon network present in these alloys in the semisolid state, with the initial resistance to flow smaller in the magnesium-containing alloys. With increased temperature, the initial resistance decreased due to increased liquid content and grain spheroidization. The viscosity vs shear rate response was studied using the approach of Laxmanan and Flemings. The materials shear thinned rapidly during the test conditions, in which the shear rate rapidly increased from approximately 2.5 to 92 s⁻¹. The results were fitted to a power-law model, and the viscosities showed a drop of approximately two to four orders of magnitude when extrapolated to a shear rate of 1000 s⁻¹ (a typical value in actual component manufacture). The rapid compression test, which is close to conditions present in component manufacture, was shown to have more in common with a shear rate jump experiment, rather than steady-state viscometry.     

Physicochemical principles of preparation of emulsion fuels

The basic physicochemical principles of preparation of emulsion fuel compositions based on heavy and extra-heavy crude cuts were analyzed with consideration of the nature and content of the phases constituting the emulsion, type of chemical additives — emulsifiers and stabilizers, and type of equipment for production of emulsion fuels. __________ Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 5, pp. 51–56, September–October, 2007.     

Vibration cavitation behaviour of selected Ni-Mn-Ga alloys

Vibration cavitation erosion tests were carried out on Ni-Mn-Ga alloys of three different crystal structures: (1) the cubic austenite, (2) the non-modulated tetragonal martensite (T) and (3) the five-layered martensite (5M). All Ni-Mn-Ga alloys exhibited cavitation behaviour characterized by a step-wise curve of mass loss versus test time. This behaviour is correlated to the microstructural nature of the alloys as well as the surface conditions of the pre-test samples. The type and concentration of the defects at the surfaces were critical to the cavitation resistance of the alloys. The best cavitation resistant alloy was of a cubic austenitic structure, followed with the alloy of a tetragonal T-martensite. The largest material loss was found in the alloy with a 5M martensite. All the studied Ni-Mn-Ga alloys had an excellent cavitation resistance compared to that of the reference stainless steel, and they even excelled some NiTi alloys found in literature. This may be due to the superelasticity of the cubic austenite and the twinning of the martensitic phases.     

Clay compaction modelling by homogenization theory

The modelling of hydromechanical behavior of clayey sediments by homogenization theory is described. The rheological model of clays buried in depth requires a three-dimensional approach and laboratory experiments to quantify the model parameters. An iterative process which can be coupled to any homogenization method is used to model the porous media behavior. Hydromechanical compaction results from oedometric experiments are simulated with the proposed model.     

Fractographic and numerical study of hydrogen–plasticity interactions near a crack tip

This paper offers a fractographic and numerical study of hydrogen–plasticity interactions in the vicinity of a crack tip in a high-strength pearlitic steel subjected to previous cyclic (fatigue) precracking and posterior hydrogen-assisted cracking (HAC) under rising (monotonic) loading conditions. Experiments demonstrate that heavier cyclic preloading improves the HAC behaviour of the steel. Fractographic analysis shows that the microdamage produced by hydrogen is detectable through a specific microscopic topography: tearing topography surface or TTS. A high resolution numerical modelling is performed to reveal the elastoplastic stress–strain field in the vicinity of the crack tip subjected to cyclic preloading and subsequent monotonic loading up to the fracture instant in the HAC tests, and the calculated plastic zone extent is compared with the hydrogen-assisted microdamage region (TTS). Results demonstrate that the TTS depth has no relation with the active plastic zone dimension, i.e., with the size of the only region in which there is dislocation movement, so hydrogen transport cannot be attributed to dislocation dragging, but rather to random-walk lattice diffusion. It is, however, stress-assisted diffusion in which the hydrostatic stress field plays a relevant role. The beneficial effect of crack-tip plastic straining on HAC behaviour might be produced by the delay of hydrogen entry caused by residual compressive stresses and by the enhanced trapping of hydrogen as a consequence of the increase of dislocation density after cyclic plastic straining.