Effects of attached straight pipes on finite element limit analysis for pipe bends

The effect of the length of an attached straight pipe on the plastic limit load of a 90° pipe bend under combined pressure and bending is quantified, based on finite element (FE) limit analyses using elastic–perfectly plastic materials with the small geometry change option. Systematic FE limit analyses of pipe bends with various lengths of the attached pipe are performed. It is shown that the effect of the length of the attached straight pipe on plastic limit loads can be significant, and the limit loads tend to decrease with decrease of the length of the attached straight pipe. In the limiting case of no attachment, the limit loads are found to be close to existing analytical solutions.     

Thermo-mechanical modelling of a single-bead-on-plate weld using the finite element method

This work describes a three-dimensional thermo-mechanical finite element analysis of a single weld bead-on-plate of austenitic stainless steel performed as part of the NeT programme. The overall aim is to validate the use of finite element (FE) weld simulations to accurately predict residual stress states for use in the assessment of welded components. Here, the final residual stresses in the plate are predicted, which can later be verified through comparison with measured distributions. A one-way coupled analysis is carried out with the thermal and mechanical problems solved in sequence. For the thermal analysis, two approaches are adopted to simulate the welding process. In one case sections of the weld bead's elements are sequentially deposited, while in the other the whole bead is deposited simultaneously. A moving heat source is used to simulate the torch traversal over the weld bead in both cases. Predicted thermal profiles, residual stresses and equivalent plastic strains, due to the welding process are presented at certain locations in the plate.     

Transient conjugated heat transfer in thick walled pipes with convective boundary conditions

Transient conjugated heat transfer for laminar flow in the thermal entrance region of pipes is investigated by considering two dimensional wall and axial fluid conduction. The problem is handled for an initially isothermal, infinitely long, thick-walled and two-regional pipe for which the upstream region is insulated and solved numerically by a finite difference method for hydrodynamically developed flow with a step change in the ambient fluid temperature in the heated downstream region. A parametric study is done to analyse the effects of five defining parameters namely, wall thickness ratio, wall-to-fluid conductivity ratio, wall-to-fluid thermal diffusivity ratio, the Peclet number and the Biot number.     

Quasi-steady state temperature distribution in finite regions with periodically-varying boundary conditions

Reed and Mullineux applied in [3] a semi-numerical procedure for determining the quasisteady state solution of periodically varying phenomena. In the present study this problem is reduced to a system of matrix equations without using any approximation. The numerical results are compared with those of [3].     

Large scale multi-zone creep finite element modelling of a main steam line branch intersection

A number of papers detail the non-linear creep finite element analysis of branch pieces. Predominately these models have incorporated only a single material zone representing the parent material. Multi-zone models incorporating weld material and heat affected zones have primarily been two-dimensional analyses, in part due to the large number of elements required to adequately represent all of the zones. This paper describes a non-linear creep analysis of a main steam line branch intersection using creep properties to represent the parent metal, weld metal, and heat affected zone (HAZ), the stress redistribution over 100,000 h is examined. The results show that the redistribution leads to a complex stress state, particularly at the heat affected zone. Although, there is damage on the external surface of the branch piece as expected, the results indicate that the damage would be more widespread through extensive sections of the heat affected zone. This would appear to indicate that the time between damage indications on the surface using techniques such as replication and full thickness damage may be more limited then previously expected.     

The transient heat transfer analysis of solids with radiative boundary condition using finite element analysis

In this work, the system of equations to obtain the temperature distribution within a solid are derived using the finite element analysis. These nonlinear set of algebraic equations are solved using the Gauss-Seidel iteration technique. The results indicate that this method can be a valuable tool in the study of heat transfer process in the metallurgical or other related industries.     

Effect of convection on boundary layer structures in finite thermoelasticity

Abstract

Till now, all of the research on boundary layer structures in thermoelasticity has focused on conduction as the primary mode of heat transfer. In this article, we investigate the additional effect of convection on the deformation field boundary layer structures formed within a thin infinite slab made of a neo-Hookean material. We find that additionally introducing convection in finite thermos-elasticity shifts the boundary layer vertically, while retaining its shape.     

Implementation of boundary conditions and global mass conservation in pressure-based finite volume method on unstructured grids for fluid flow and heat transfer simulations

Proper boundary condition implementation can be critical for efficient, accurate numerical simulations when a pressure-based finite volume methodology is used to solve fluid flow and heat transfer problems, especially when an unstructured mesh is used for domain discretization. This paper systematically addresses the relationships between the flow boundary conditions for the discretized momentum equations and the corresponding conditions for the (Poisson type) pressure-linked equation in the context of a cell-centred finite volume formulation employing unstructured grids and a collocated variable arrangement. Special attention is paid to the treatment of the outflow boundary where flow conditions are either known to be fully developed (if a long enough channel is used) or unknown prior to solution (if a truncated channel is used). In the latter case, due to the singularity of the coefficient matrix of the pressure-linked equation, no pressure or pressure corrector solution can exist without explicitly enforcing global mass conservation (GMC) during each iteration. After evaluating published methods designed to ensure GMC, two new methods are proposed to correct for global mass imbalance. The validity of the overall methodology is demonstrated by solving the evolving flow between two parallel plates and the laminar flow over a backward-facing step on progressively truncated domains. In the latter case, our methodology is shown to handle situations where the outflow boundary passes through a recirculation zone.     

Finite element simulation of welding residual stresses in martensitic steel pipes

Abstract

Finite element (FE) simulations of the welding of two high grade steel pipes are described. The first is a P91 steel pipe welded with a similar P91 weld consumable, and the second is a P92 steel pipe welded with dissimilar nickel–chromium based weld consumables. Both welds are multipass circumferential butt welds, having 73 weld beads in the P91 pipe and 36 beads in the P92 pipe. Since the pipes and welds are symmetric around their axes, the FE simulations are axisymmetric, allowing high FE mesh refinement and residual stress prediction accuracy. The FE simulations of the welding of the P91 and P92 pipes comprise thermal and sequentially coupled structural analyses. The thermal analyses model the heat evolution produced by the welding arc, determining the temperature history throughout the FE models. Structural analyses use the computed temperature history as input data to predict the residual stress fields throughout the models. Post-weld heat treatment (PWHT) of both pipes has also been numerically simulated by assuming that the FE models obey the Norton creep law during the hold time period at 760°C. The residual stresses presented here have all been validated by corresponding experimental measurements. Before PWHT, it has been found that, at certain locations in the weld region and heat affected zone (HAZ) in the pipes, tensile hoop and axial residual stresses approach the tensile strength of the material, presenting a high risk of failure. It has also been found that PWHT substantially reduces the magnitude of residual stresses by varying degrees depending on the material.     

机车复轨器的有限元分析

复轨器是救援脱轨机车车辆的专用必备装置.在救援过程中,它能引导脱轨机车车辆的轮对滚动到钢轨顶部,复位到钢轨上,从而达到救援排障的目的,此间复轨器须承受较大的载荷.为了达到既满足强度和刚度的要求,又能减轻重量、节约材料等目的,对复轨器作较为精确的应力、位移等分析具有一定的实用价值.本文利用有限元分析软件ANSYS对某公司生产制造的人字型复轨器进行有限元计算分析,并且根据计算分析结果为以后此类复轨器的改进或重新设计提供重要的理论参考和工程实例.人字型复轨器由左"人"右"入"两个单件组成,其结构和组成如图1所示.     

基于显式有限元的机车碰撞模型研究

结合DF8B型机车,利用ANSYS/LS-DYNA软件分别建立了考虑转向架结构和考虑转向架质量的机车以10m/s的速度正面碰撞刚性墙的仿真模型,并对两种模型的仿真分析结果进行了比较。结果表明,考虑转向架实际结构的机车碰撞刚性墙有限元模型,能更准确地模拟机车的正面碰撞过程。     

Adaptive finite element analysis of axisymmetric freezing

The paper describes an adaptive finite element analysis of the transient axisymmetric freezing process. Adaptivity schemes are applied to both space and time tessellations. Error equidistribution adjusts the nodal positions and Taylor series analysis of time truncation error selects the time step. In implicit time integration, the location of the freezing front and the nodal temperatures at the next time is solved by full Newton all at once. The method appears to be accurate; in cases for which closed-form solutions are available, it agrees well with them. It also avoids the problem found in the Modified Isotherm Migration Method where the freezing front tends to retrogress when solid just forms on the outer surface cooled by convection.     

Mechanisms and computer modelling of transition element gettering in silicon

This paper starts out by summarising the modelling and computer simulation of phosphorus diffusion gettering (PDG) of Au. The mobilisation of precipitated impurity atoms is discussed in the light of the silicon interstitial supersaturation provided by the phosphorus diffusion (PD). We then extend the gettering model to Co using bulk solubility data of highly P-doped Si, and find satisfactory agreement with experimental profiles of the total Co-concentration. Yet the pointed disagreement between the CoP/Co_s-ratio obtained through simulation and Mößbauer data leads to the conclusion that, in the case of phosphorus silicate glass (PSG) growth, segregation alone cannot unambigiously account for the observed gettering efficiency. Instead, it is proposed that PD induced silicide formation provides a more suitable explanation of the high efficiency of PDG accompanied with PSG growth.     

Transient elastodynamic boundary element formulations based on the time-stepping scheme

A new method is presented in this paper to analyze transient elastodynamic problems with damping by the boundary element method formulated in terms of the fundamental solution for steady elastodynamics after approximating acceleration and velocity components by finite difference or implicit time integration schemes such as the Newmark method and the Wilson θ method. The effects of the approximation scheme and time step on accuracy are examined through numerical examples for one-dimensional transient response.     

Cyclic thermo-mechanical material modelling and testing of 316 stainless steel

A programme of cyclic mechanical testing of a 316 stainless steel, at temperatures of up to 600 °C under isothermal conditions, for the identification of material constitutive constants, has been carried out using a thermo-mechanical fatigue test machine (with induction coil heating). The constitutive model adopted is a modified Chaboche unified viscoplasticity model, which can deal with both cyclic effects, such as combined isotropic and kinematic hardening, and rate-dependent effects, associated with viscoplasticity. The characterisation of 316 stainless steel is presented and compared with results from tests consisting of cyclic isothermal, as well as in-phase and out-of-phase thermo-mechanical fatigue conditions, using interpolation between the isothermal material constants to predict the material behaviour under anisothermal conditions.     

Non-linear finite element analysis and high temperature defect assessment

In recent years a methodology has been developed for predicting the complex phenomenon of creep-fatigue crack growth. This is of considerable importance in a wide variety of modern components and structures, for example gas turbines, high temperature pressure vessels and pipework. Essentially the technique involves calculating rates of crack growth for the limiting cases of rapid cycling and steady loading, and then adding the rates to give combined effects. As well as creep data and the well known Paris Law for fatigue crack growth, the method requires linear and non-linear fracture mechanics quantities K and C* to be calculated. K is the well known elastic crack tip stress intensity factor. C* is the creep analogy of the J-integral, defined by:

The paper describes the methodology for determining creep-fatigue crack growth and the application of ABAQUS to determine K and C* for a test specimen and a turbine blade. The use of approximate techniques is also evaluated.     

Effects of geometric design on thermal performance of star-groove micro-heat pipes

This work is aimed to study the effects of geometric design on the thermal performance of star-groove micro-heat pipes. A mathematical one-dimensional, steady-state model is developed from the first principles where the continuity, momentum, and energy equations of the liquid and vapor phases, together with the Young–Laplace equation, are solved numerically to yield the heat and fluid flow characteristics of the micro-heat pipe. The heat transport capacity and the corresponding optimal charge level of the working fluid are evaluated for different geometric designs and operating conditions. The unique geometrical design of the star-groove micro-heat pipes provides a better insight into the effects of various geometrical parameters, such as the cross-sectional shape, the acuteness of the corner apex angle, the number of corners, cross-sectional area and total length. Comparisons of the performance between the star-groove and regular polygonal micro-heat pipes are performed and the factors contributing to the enhancement of heat transport due to the variations of geometrical parameters are identified and discussed.