Journal of Applied Mechanics and Technical Physics最新文献

Abstract

A small-size elastic isotropic beam-strip loaded by variable body and surface forces is considered. On the front surfaces, surface shear stresses and inertia are taken into account within the framework of the Gurtin–Murdoch surface elasticity theory. Asymptotically correct equations describing the long-wavelength bending deformation of micro- and nano-beams taking into account shears and surface effects are derived by asymptotic integration of the two-dimensional elasticity equations over the strip thickness. The influence of surface stresses and inertia on the lower spectrum of natural vibrations of metal micro- and nano-beams is studied. It is shown that surface inertia has the same effect on the frequency spectrum of natural flexural vibrations as surface stresses.

The centrally symmetric problem of determining the velocity field in a continuous elastoplastic medium during a camouflet explosion has been solved assuming that the motion is non-oscillatory nature and that the medium in the plastic and elastic regions is incompressible. The solution was found using the camouflet equation — the relation for determining the pressure on the contact surface of the expanding explosion cavity. The solution can be used to estimate the dimensions of the expansion and plastic deformation regions and the impact of explosive disturbances on objects.

The molecular dynamics method is applied to calculate the stiffness constants of five structural configurations of graphyne. The latter is a carbon monolayer in which the atoms are arranged in a special way and characterized by (sp)- and (sp^2)-hybritization. It is revealed that the atom arrangement in a graphyne layer has a significant effect on the stiffness constants. It is determined that (gamma_2)-graphyne has the highest stiffness constant (c_{11}) (1091 GPa) and that (alpha)-graphyne has the smallest one (258 GPa). As shown in the study, (beta_3)- and (gamma_2)-graphyne are highly anisotropic structures.

The full Navier–Stokes equations are used to study the linear stability of plane Poiseuille flow in a channel with the lower wall corrugated along the flow, due to which the flow has two velocity components. A generalized eigenvalue problem is solved numerically. Three types of perturbations are considered: plane periodic (zero Floquet parameter), plane doubly periodic (finite values of the Floquet parameter), and spatial perturbations. Neutral curves are analyzed in a wide range of the corrugation parameter and Reynolds number. It is found that the critical Reynolds number above which time-growing perturbations appear depends in a complex way on the dimensionless amplitude and period of corrugation. It is shown that in the case of flow in a channel with corrugated wall, three-dimensional perturbations are usually more dangerous. The exception is the small amplitude of corrugation at which plane perturbations are more dangerous.

A high-velocity flow in an axisymmetric nozzle containing a central body and pylons is studied. The influence of the geometry of the main and additional pylons on the gas-dynamic and thrust characteristics at the nozzle exit in the flow regime with (n_{pr}= 2.25) ((n_{pr}) is the ratio of the pressure in the settling chamber to the ambient pressure) is determined. Azimuthal nonuniformity of the flow at the nozzle exit is detected. The maximum azimuthal nonuniformity is observed in the wake behind the pylons. It is shown that a three-dimensional transonic flow is formed in the nozzle duct with the pylons mounted in the minimum free cross section; local supersonic regions closed by weak shock waves are formed in this flow. It is found that the formation of such a shock wave structure is responsible for nozzle thrust reduction by 12%.

Three-dimensional numerical simulations of the air flow in the full human bronchial tree in situations of obstructive and chronic pulmonary diseases are performed. Based on the previously developed three-dimensional analytical model of the lower respiratory airways, the air distributions in the lungs (from the trachea to alveoli) in situations with lung injuries and bronchial asthma are calculated. Breathing modeling is based on a numerical technique of step-by-step computations, which allows one to avoid the loss of solution accuracy caused by the difference in the bronchus scales; moreover, the time needed to calculate the air flow in the lungs can be reduced by several times by using this technique.

Results on a nanoparticle impact onto a target calculated by the molecular dynamics method are presented. The first problem being solved is the nanoparticle impact onto a target under the conditions of cold gas-dynamic spraying. The second problem deals with nanoparticle extension, which adheres to the target due to the impact. It is shown that a chemical bond between the nanoparticle and target is formed during the impact. The bond in the case of the titanium nanoparticle impact onto an aluminum target is found to be stronger than that in the case of the aluminum nanoparticle impact onto a titanium target. The reason is that the titanium nanoparticle penetrates into the aluminum target to a greater depth.

Effects of various physical mechanisms at the stage of vapor bubble collapse and the subsequent formation of a shaped-charge jet in the process of laser-induced boiling near the end of a thin waveguide immersed in a cold liquid is studied numerically. Depending on the evaporation intensity, three process modes are identified and described.

A method is developed for predicting creep and long-term strength based on the behavior of a previously tested sample (leader sample, prototype) in the case of ductile fracture. It is assumed that a loaded material does not undergo instantaneous plastic deformation and the first stage of creep. The incompressibility hypothesis is fulfilled in this case. It is shown that, if a constant-stress creep curve and the time to fracture are known for a leader sample, then obtaining a diagram of rheological deformation and long-term strength of the material at other stress values requires knowing only the initial (at the initial time) minimum creep strain rate of the samples for these stress values. The relevance of the developed method is checked with experimental data in two types of tests. The first type is tension tests of 12Kh18N10T corrosion-resistant steel samples at a temperature of 850°C and titanium alloy samples at a temperature of 600°C and the second type is tension and torsion tests of D16T alloy samples at a temperature of 250°C. It is shown that the prediction results are independent of the choice of a leader sample from many samples tested at different stresses. The possibility of using the developed method in experimental studies of creep of materials until their fracture is discussed.