Procedia IUTAM最新文献

The influence of the laser-beam radial distribution of the energy flux density is theoretically studied for the Gaussian distribution (mode TEM₀₀), and doughnut distribution of TEM_01* mode for the values of the Peclet number from 0 to 3. The model of linear thermal conduction in the target indicates that profile TEM₀₀ is the best for thermo-activated treatment processes that can be accomplished in a wide temperature range and profile TEM_01* can be advantageous for a narrow range of the permissible processing temperature. If the phase transitions of melting/solidification and evaporation are included into the model, the estimate of the width of the laser-treated band is reduced but the tendencies predicted by the linear model are not changed.

In rigid-body mechanics, models that capture collisional contact as an instantaneous exchange of momentum may exhibit dynamics that include infinite sequences of impacts accumulating in finite time to a state of persistent contact, often referred to as chatter. In this paper, we review theoretical tools for the analysis of transient and steady-state behavior in the vicinity of critical periodic orbits for which chatter terminates at a point corresponding to the imminent release from persistent contact, and illustrate the application of this theory to a simplified model of a mechanical pressure relief valve. A general theory for single-degree-of-freedom impact oscillators, previously described in an unpublished manuscript by Nordmark and Kisitu¹, is shown to yield both qualitative and quantitative agreement with model simulation results. The predicted bifurcation structure shows that the border orbit unfolds supercritically into a universal cascade of local attractors with nontrivial scaling relationships.

Experiments designed by students at any level is a fun way to learn physics. Laboratory activities can help students acquire, integrate and construct knowledge in a friendly way. For the last 15 years, we have promoted experimental work as projects in theoretical courses. The enthusiasm students have shown, even though they have to spend many extra hours, contrasts their dislike for traditional laboratory courses where they follow recipes. In this paper, we present the original project and its influence in other courses and in new curricula. We will not address the epistemological aspects [1] and give only a slight overview of pedagogical advantages of this approach.

The structural and mechanical properties of ZnO nanowires (NWs) have been systematically investigated by using molecular dynamic simulations based on the empirical Buckingham potential. Under tensile loading in <0001> direction, ZnO NWs undergo four-stage deformation: elastic stretching of initial Wurtzite structure, Wurtzite to body-centered tetragonal (BCT) phase transformation, stretching of the resulting BCT structure and eventually brittle fracture. The entire deformation process is significantly size dependent. As the NW size decreases, the Young's modulus dramatically increases. The critical stress for both phase transformation and fracture decreases while the critical strain increases with increasing the NW size; both converge to constant values when the size is sufficiently large. The strain energy density for the initiation of phase transformation appears independent of the size, which implies that the size-dependent phase transformation is dominated by the size effect of the Young's modulus.

The present paper provides a systematic treatment of modern differential-geometrical methods for modeling of incompatible finite deformations in solids and thin walled structures. The incompatibility of deformations may be caused by a variety of physical phenomena; among them are: growth, non-uniform thermal fields, shrinkage, etc. Incompatible deformations results in residual stresses and distortion of geometrical shape. These factors are associated with critical parameters in modern high-precision technologies, particularly, in additive manufacturing, and considered to be essential constituents in corresponding mathematical models. The methods in question are based on the representation of a body and physical space in terms of differentiable manifolds, namely material manifold and physical manifold. These manifolds are equipped with specific metrics and connections, non-Euclidian in general.

This paper investigates regenerative and frictional grinding chatters affected by mass eccentricity in the workpiece. Time delays and velocity-soften friction coefficient are employed to represent regenerative and Stribeck effects in normal and tangential grinding forces. Eigenvalue calculation and continuation scheme are used to find stability boundaries for both regenerative and frictional instabilities, illustrating that a deep grinding enhances the regenerative stability but impairs the frictional one. Near each kind of boundaries, numerical simulations and bifurcation analyses are adopted to present various chatter motions in the grinding, either with or without mass eccentricity. It is found that the frictional chatter is prone to be quenched by the external excitation due to the mass eccentricity. On the contrary, the regenerative instability still persists, but is perturbed to be quasi-periodic.

This paper compares two numerical methods applied to compute the starting vortex flow past a flat plate. The plate is inclined relative to a constant background flow at angle α, with α = 90°, 60°, 30°. The numerical methods considered are (1) direct numerical simulation of the viscous flow (DNS), and (2) an inviscid vortex sheet model. The viscous DNS solves the Navier- Stokes equations by an operator splitting finite-difference method, for Reynolds numbers Re = 250, 500, 1000, 2000. The inviscid flow is computed by a regularized vortex sheet method, with the unsteady Kutta condition imposed at the edges of the plate, for regularization parameters δ = 0.2, 0.1, 0.05. We present viscous vorticity contours, and compare streaklines and shed circulation obtained with both methods. Good agreement is found in the large-scale features of the separated spiral streaklines and the shed circulation as Re increases and δ decreases. For small inclination angle α, secondary separation on the downwind side of the plate introduces small-scale features in the viscous flow that are absent in the inviscid model. The vortex sheet model is much less costly than the viscous DNS, but it is limited by the omission of the boundary layers present in the viscous flow.

A system of optical monitoring and diagnostics of selective laser melting process with alternative beam power density distributions is developed. The experimental work with input parameters variation showed the correlation between obtained power density distribution and geometrical parameters of single tracks. Technological gaps of stable track formation for different distributions have been detected, and high-speed process photography have been realized.

During machining, the use of variable helix tools can potentially improve the system's stability to regenerative chatter. However, this configuration of tool has a distributed time delay, which makes the stability analysis more complex. The analysis is further exacerbated by the time-periodic coefficients that occur during milling. The present contribution demonstrates how the Fourier transform and harmonic transfer function approach can be used to analyse the system stability. This provides new insight into the stability of these tools, based on a mathematically elegant approach that makes extensive use of the shift theorem.

The present paper aimed at experimental study for the distortion of geometrical shape of thin walled structures manufactured by stereolithography technologies. It is proposed that total distortion occurring due to layer-by-layer photopolymerisation can be estimate using experimental data about distortion caused by curing single intermediate or final layer. The experiments used a sample in the form of a hollow cube. The distortion of the cube is recorded using time-average holographic interferometry.