Numerical investigation of plastic flow and residual stresses generated in hydroformed tubes

During tube hydroforming process, the friction conditions between the tube and the die have a great importance on the material plastic flow and the distribution of residual stresses of the final component. Indeed, a three-dimensional finite element model of a tube hydroforming process in the case of square section die has been performed, using dynamic and static approaches, to study the effect of the friction conditions on both plastic flow and residual stresses induced by the process. First, a comparative study between numerical and experimental results has been carried out to validate the finite element model. After that, various coefficients of friction were considered to study their effect on the thinning phenomenon and the residual stresses distribution. Different points have been retained from this study. The thinning is located in the transition zone cited between the straight wall and the corner zones of hydroformed tube due to the die–tube contact conditions changes during the process. In addition, it is clear that both die–tube friction conditions and the tube bending effects, which occurs respectively in the tube straight wall and corner zones, are the principal causes of the obtained residual stresses distribution along the tube cross-section.

Spatially averaged haemodynamic models for different parts of cardiovascular system

This paper revisits the usage of spatially averaged haemodynamic models such as non-stationary 1D/0D in space and stationary 0D in space models. Conditions of equivalence between different 1D model formulations are considered. The impact of circular and elliptic shapes of the tube cross-section on the friction term and the tube law is analyzed. Finally, the relationship between 0D lumped and 1D models is revealed.

An Analysis of a Reheater Failure and a Proposal to Upgrade the Device Design

The aim of this paper is to present an example of damage to the reheater tubes and conduct the material and numerical analyses to establish the cause of the device failure. Cracks were observed on the first, second, and third tube row. Close to the damaged area, a ferritic structure could be observed with highly degraded bainite areas, characterized by coagulation and coalescence of precipitates. The cause of the damage was analysed using the finite element method (FEM). The big yield of the tube cross-section confirmed that the tube may get damaged during subsequent cycles of the boiler operation, which was also proved by the microstructure testing results. For the reheater under analysis, the tubes have to be lengthened to achieve a reduction in stresses, arising due to thermal loads to values lower than allowable stresses according to Standard EN 13480-3. The modelling results confirmed the correct operation for the upgraded system.

Tracing sub-surface swept profiles of tapered toroidal end mills between level cuts

Development of closed-form solutions and algorithms for constructing sub-surface swept profiles (SWP) of toroidal and conical bodies is presented in this paper. While the problem of identifying the entire SWP of such surfaces has been extensively investigated in extant studies, construction of subsurface SWPs has rarely been addressed despite the subject being of great significance to machining process employing nonstandard-shaped NC tools. Torus shapes considered in extant literature are restricted to the fourth quadrant of a tube cross section. In industrial applications, however, profile cutters contain different regions of a toroidal surface. To identify SWP elements in the proposed study, a single analytical expression in one variable has been deduced using two moving frames. The basic idea behind such a formulation is to employ the one-to-many strategy, which greatly reduces the computational cost and effort. Algorithms to identify feasible domains of SWP parameters at each level cut, where toroidal and conical surfaces meet, have also been proposed in this study. This is important, since cutting a tool surfaces along the rotation axis divides SWP-parameter domains into non overlapping sets of intervals that must be addressed for each tool posture. In addition, this study demonstrates that for certain tool postures, while C1 continuity between sub-surfaces is satisfied, the SWP connectivity is lost at some points. To locate these so called singular-characteristic points, some precomputation steps have been performed. Lastly, several factors affecting the smoothness of SWPs have been identified and discussed. Highlights Closed form solutions have been derived for constructing the sub-swept profiles of toroidal tools. Three algorithms have been presented to identify the feasible domains of swept profile parameters. In order to locate the singular-characteristic points some precomputation steps have been carried out. Finally, several factors, affecting the smoothness of the swept profiles, have been identified.

Problem of conjugate heat transfer between main gas pipeline and frozen ground

Mathematical model of non-isothermal gas flow within the framework of tube hydraulics including change of tube cross-section due to hydrate formation and the dependence of coefficient of heat transfer between gas and hydrate layer on varying flow area is proposed. The corresponding conjugate problem of heat exchange between imperfect gas in the pipeline and the environment is reduced to the solution of differential equations describing non-isothermal flow of gas in pipes and heat transfer equations in ground with the corresponding conjugation conditions. In the quasi-stationary mathematical model of hydrate formation (dissociation), the dependence of gas-hydrate transition temperature on gas pressure is taken into account. Some decisions taken in the design of the first section of the main gas pipeline «Power of Siberia» have been analyzed. It has been shown that if gas is not sufficiently dried, outlet pressure may drop below the technological limit in about 6-7 hours. At the same time, for completely dry gas ,it is possible to reduce the cost of thermal insulation of the pipeline at least two fold.

Calculation of Resonant Radiation Power of a Direct Sodium Lamp With Low Pressure

They consider the calculation technique for the resonance radiation power of a direct low-pressure sodium lamp with a sickle-shaped discharge tube cross-section. The calculation of the sodium discharge radiation power is difficult in such a tube, as compared with the calculation in a cylindrical shape tube. The dependence of the relative output of sodium discharge resonant radiation occurring in equivalent discharge tubes in the form of a parallelepiped and a cylinder is based on the involvement of the theory by M. Cayless. The obtained results were used to determine the power of DNaO-85M lamp resonant radiation. They presented the scheme of the device to study the influence of the tube geometry on sodium discharge power. It is shown that the power of the resonance radiation generated by an equivalent discharge will be the greater, the larger the cross section of the discharge tube. The discrepancy between the calculated and the experimental data for resonant radiation power determination in a crescent-shaped tube is less than 4%.

Effects of Notch Depth and Direction on Stability of Local Sharp-Notched Circular Tubes Subjected to Cyclic Bending

Cyclic bending of tubes leads to progressive ovalization of the tube cross-section, and persistent cycling causes catastrophic buckling of the tube. This paper presents the response and stability of SUS304 stainless steel tubes with local sharp-notched depths of 0.2, 0.4, 0.6, 0.8, and 1.0[Formula: see text]mm and notch directions of 0[Formula: see text], 30[Formula: see text], 60[Formula: see text], and 90[Formula: see text] under cyclic bending. The experimental results reveal that the moment–curvature relationship first exhibits cyclic hardening and then a steady loop after a few cycles. Because the notches are small and localized, notch depth and direction show minimal influence on the moment–curvature relationship. In contrast, the ovalization–curvature relationship demonstrates an increasing and ratcheting pattern along with the bending cycle, whereas notch depth and direction show a strong influence on this relationship. Finite-element analysis via ANSYS is used to simulate the moment–curvature and ovalization–curvature relationships, and an empirical model is proposed to simulate the relationship between the controlled curvature and number of cycles required to ignite buckling. The experimental and analytical data agree well with each other.