Parametric study on the fatigue life of railways under rolling contact fatigue by three-dimensional numerical analysis

The current paper presents the results of parametric analyses on the stress intensity factor (SIF) of railways with inclined cracks under rolling contact fatigue (RCF). A 3D finite element (FE) model was proposed to demonstrate the shear mechanism in RCF. The feasibility of the suggested numerical model was verified through the SIF (K) obtained from advanced 3D FE analysis compared with existing 2D FE results. Based on the series of FE analyses, the sensitivity analysis on the cracked depth, surface/crack friction coefficients, and inclined angle, which mainly affected SIF history at the cracked tip, was examined. SIF distributions for various locations of the wheel along the cracked tip were also presented.     

A study on process parameters for cold U-bending of SUS304L heat transfer tube using rotary draw bending

Steam generator is one of enormous heat exchangers that use heat energy derived from a reactor of nuclear power plant for generating steam. The steam obtained is drained into a turbine, and plays an important role for power generation. The heat transfer tubes of each steam generator used in a pressurized water reactor (PWR) are composed of about 8,000 ～ 13,000 U-shaped tubes. These tubes act as the structural material and the thermal boundary. Furthermore, these tubes account for about 70% of the cooling surface area, transmitting thermal energy between the high-temperature (about 320°C), high-pressure (about 157 Kg_f/cm²) primary coolant derived from the reactor and the secondary coolant, which is at about 220°C and 60 Kg_f/cm², from the secondary system. These heat transfer tubes used in the steam generator within the PWR were fabricated from seamless tubes of stainless steel (SUS304L), Alloy 600 and Alloy690. In this study, numerical and experimental investigations are carried out on the U-bending process for fabricating heat transfer tubes from long straight SUS304L tubes. In the numerical simulation, 3-dimensional finite element analysis is performed using ABAQUS Explicit/Implicit. In detail, process parameters such as the angular speed, U-bending period, and bending angle taking into account elastic recovery after cold U-bending are considered. Additionally, experimental investigations are conducted to verify the suitability of the predicted U-shaped geometries in terms of the ovality and wall thickness of the U-shaped heat transfer tubes.     

Experimental based finite element simulation of cold isostatic pressing of metal powders

The present work addresses the various ingredients required for reliable finite element simulations of cold isostatic pressing (CIP) of metal powders. A plastic constitutive model for finite deformation is presented and implemented into an explicit finite element (FE) code. The FE implementation is verified so that numerical errors (both temporal and spatial errors) are kept under control. Thereafter, uniaxial die compaction experiments are performed required for determining the material parameters in the constitutive model. Subsequently they are applied for the simulation of a “complex” CIP process. The experimental observations of the complex CIP process were used to validate the overall method by comparing the FE results (final dimensions and average relative density) to the experimental observations. The numerical results (final dimensions and relative density) are in good agreement with the experimental observations.     

Experimental investigation on sliding and fretting wear of steam generator tube materials

In nuclear power steam generators, high flow rates can induce vibration of the tubes resulting in fretting wear damage due to contacts between the tubes and their supports. In this paper, the sliding and fretting wear tests were performed using Inconel 600HTMA and 690TT against STS 304, which are the steam generator tube materials. The sliding wear tests with a pin-on-disk type tribometer were carried out under various applied loads and sliding speeds at air environment. The fretting wear tests were carried out under various vibrating amplitudes and applied normal loads.

The result of sliding and fretting wear tests show that the heat-treated Inconel 690TT has better wear resistance than Inconel 600HTMA in air. The fretting wear regimes were plotted using the test results and the wear coefficient was calculated also. From the results, it was observed that the wear and tear by stick-slip has very strong effect on the fretting wear behavior.     

Parametric study and numerical analysis of empty and foam-filled thin-walled tubes under static and dynamic loadings

In this paper the crushing behavior of thin-walled tubes under static and dynamic loading is investigated. First, a finite element (FE) model for empty thin-walled tube was constructed and validated by available experimental and numerical data. The comparison between the FE results and the existing numerical solutions as well as the available experimental results showed good agreements. Next, a model for the foam was adopted and implemented in an in-house FE code. The implemented isotropic foam model was then used to simulate the behavior of foam-filled tubes under both static and dynamic loadings. Good agreement was observed between the results from the model with those obtained by analytical relations and experimental test data. The validated FE model was then used to conduct a series of parametric studies on foam-filled tapered tubes under static and dynamic loadings. The parametric studies were carried out to determine the effect of different parameters such as the number of oblique sides, foam density and boundary conditions on crushing behavior of rectangular tubes. The characteristic included deformed shapes, load–displacement, fold length and specific energy absorptions.     

Evaluation of slant crack propagation under RCF in railway rail

Crack propagation due to rolling contact fatigue (RCF) could be a significant potential challenge to the integrity of railway rails because it may lead to a serious disaster. Fatigue cracks subjected to cyclic rolling contact force experience a complex non-proportional mixed loading and complicated boundary condition. In the present research, complex crack opening/closure/sliding/locked behaviors as the cyclic contact loading movement is analyzed considering liquid lubrication action on rail surfaces as well as crack faces. Based on a series of FE analyses, the calculations of the effective SIF ranges for RCF cracks under certain contact loading and boundary conditions are proposed in the form of polynomial functions which will be appropriately used to predict RCF crack growth rate.     

Development and numerical validation of a finite element model of the muscle standardized femur

The human femur is one of the parts of the musculo-skeletal system most frequently analysed by means of the finite element (FE) method. Most FE studies of the human femur are based on computed tomography data sets of a particular femur. Since the geometry of the chosen sample anatomy influences the computed results, direct comparison across various models is often difficult or impossible. The aim of the present work was to develop and validate a novel three-dimensional FE model of the human femur based on the muscle standardized femur (MuscleSF) geometry. In the new MuscleSF FE model, the femoral attachment of each muscle was meshed separately on the external bone surface. The model was tested under simple load configurations and the results showed good agreement with the converged solution of a former study. In the future, using the validated MuscleSF FE model for numerical studies of the human femur will provide the following benefits: (a) the numerical accuracy of the model is known; (b) muscle attachment areas are incorporated in the model, therefore physiological loading conditions can be easily defined; (c) analyses of the femur under physiological load cases will be replicable; (d) results based on different load configurations could be compared across various studies.     

A study on wear coefficients and mechanisms of steam generator tube materials

Reciprocating sliding wear tests were performed to evaluate wear properties of Inconel 600MA and 690TT steam generator (SG) tube materials against 405 and 409 ferritic stainless steels. With increasing normal loads and sliding amplitudes, the wear rate of tube materials increased but a wear transition occurred only in Inconel 690TT. Subsurface deformation strengthening seemed to be an important factor that determines the wear resistance of tube materials. After the wear test, the worn surfaces were observed to investigate the wear mechanism of tube materials using SEM. The results indicated that there are different mechanisms of wear particle removal between the tube materials. The differences are related to the degree of work hardening due to the differences in chromium content in the tube materials. Based on the present results, wear coefficient values for the life estimation of SG tubes were calculated according to the work-rate model at each test condition. The wear rate is lower for Inconel 690TT compared to that for Inconel 600MA. Finally, parameters that should be considered for evaluation of wear coefficients were discussed.     

Experimental and numerical studies of stress/strain characteristics in riveted aircraft lap joints

The fatigue property of riveted lap joint is greatly related to the riveting-induced residual stress, especially the stress distribution on the faying surface. However, an accurate study of the residual stress characteristics in the riveted sheet could be very difficult. In this paper, both numerical and experimental investigations were carried out on the stress/strain characteristics in riveted aircraft lap joints. A special specimen was designed for the test of strain variations on the faying surface of the sheet by microstrain gages. For the numerical simulation, the rivet squeezing process was analyzed using the explicit dynamic finite element (FE) method, whilst a general static FE analysis was employed for the elastic springback after the squeeze force was removed. A comparison of the strain variations between the experimental results and FE simulations shows a general good agreement, although there may be some difference for points measured near the hole surface. The FE analysis reveals that both compressive and tensile residual stresses could be introduced in the riveted sheet. Massive compressive residual stress can be created in the near-surface layer of the hole. However, the stress level is not always increased with increasing the squeeze force, and so is the improvement of fatigue life observed. Further study is still necessary to account for the fatigue life decreasing effect caused by a high squeeze force.

     

Friction and wear of Inconel 690 for steam generator tube in elevated temperature water under fretting condition

Recently, material of Inconel 690TT (thermal treatment) for the steam generator tubes in a nuclear power plant was substituted for the existing material of Inconel 600HTMA (high temperature mill-annealing). Inconel 690TT has more chromium than Inconel 600HTMA in order to improve the corrosion resistance. In this study, to evaluate the friction and wear characteristics of Inconel 690TT under fretting condition, the fretting tests as well as sliding tests were carried out in air and in elevated temperature water environment, respectively. Fretting tests of the cross-cylinder type were done under various applied normal loads, and sliding tests of pin-on-disk type were also carried out to compare with the results of the fretting test. In summary, the results of the fretting tests correlated with the results of the sliding tests. The wear mechanism of Inconel 690TT in air was delamination wear and the mechanism in water was affected by micro-pitting. Also, it was found that the fretting wear coefficients in water were increased with increase in the temperature of water.     

Dynamic analysis of Coriolis flow meter using Timoshenko beam element

Coriolis mass flow meter (CFM) is used to measure the rate of mass flow through a pipe conveying fluid. In the present work, the Coriolis effect produced in the pipe due to a lateral excitation is modeled using the finite element (FE) method in MATLAB©. The coupled equation of motion for the fluid and pipe is converted to FE equations by applying Galerkin technique. The pipe conveying fluid is excited at its fundamental natural frequency. The time lag observed between symmetrically located measurement points which are equidistant from the point of excitation, is utilized to predict the mass flow rate. The results predicted by the present code is validated using the experimental, and numerical results published in the literature. The main contribution is the development of a FE model, using three node Timoshenko beam element to analyse the dynamics of fluid conveying pipes subjected to external excitation. The direction of the Coriolis force is perpendicular to the plane containing the velocity of flow vector and angular velocity vector of the pipe. Hence a three dimensional FE model is essential. This model can include curved geometry, damping, velocity and gyroscopic effects for three dimensional flexible tubes. The reduced integration used for overcoming shear locking in two node elements, will result in the formation of spurious modes leading to an incorrect prediction of natural frequencies and velocity. These modes will not occur while using three node elements. Influence of spatial as well as temporal discretisation on the time lag and frequency are also discussed. The sensitivity analysis shows that the time lag varies linearly with the mass flow rate.     

一种加筋板多裂纹应力强度因子的试验验证方法

给出一种加筋板多裂纹应力强度因子试验验证方法,是基于等幅载荷下裂纹扩展速率反推得到的.该方法不仅能确定复杂问题的应力强度因子,而且能验证确定加筋板多裂纹应力强度因子的类比法.进行LY12CZ铝合金加筋板多裂纹裂纹扩展试验,给出试验验证反推应力强度因子的方法及过程.并给出用类比法近似计算的结果和试验验证结果及平均值.同时也指出这一方法可解决复杂问题应力强度因子的确定,但裂纹扩展速率存在一定的分散性.得到的结果表明试验验证方法和类比法对于确定加筋板多裂纹应力强度因子是可用的.     

Effect of continuum damage mechanics on springback prediction in metal forming processes

The influence of considering the variations in material properties was investigated through continuum damage mechanics according to the Lemaitre isotropic unified damage law to predict the bending force and springback in V-bending sheet metal forming processes, with emphasis on Finite element (FE) simulation considerations. The material constants of the damage model were calibrated through a uniaxial tensile test with an appropriate and convenient repeating strategy. Holloman’s isotropic and Ziegler’s linear kinematic hardening laws were employed to describe the behavior of a hardening material. To specify the ideal FE conditions for simulating springback, the effect of the various numerical considerations during FE simulation was investigated and compared with the experimental outcome. Results indicate that considering continuum damage mechanics decreased the predicted bending force and improved the accuracy of springback prediction.     

Numerical study on thermal stress cutting of silicon wafers using two-line laser beams

We propose a method of cleaving silicon wafers using two-line laser beams. The base principle is separating the silicon wafer using crack propagation caused by laser-induced thermal stress. Specifically, this method uses two-line laser beams parallel to the cutting line such that the movements of the laser beam along the cutting line can be omitted, which is necessary when using a point beam. To demonstrate the proposed method, 3D numerical analysis of a heat transfer and thermo-elasticity model was performed. Crack propagation was evaluated by comparing the stress intensity factor (SIF) at the crack tip with the fracture toughness of silicon, where crack propagation is assumed begin when the SIF exceeds the fracture toughness. The influences of laser power, line beam width, and distance between two laser beams were also investigated. The simulation results showed that the proposed method is appropriate for cleaving silicon wafers without any thermal damage.

     

Effect of steering wheel heating system on hand thermal sensation

A numerical study is conducted to verify the effect of the steering wheel heating system for enhancing the thermal sensation of the hand. First, a numerical study was conducted to quantify the sensory index called thermal sensation. A five-layer skin model was developed, and the reliability of the model was confirmed by comparing it to previous research results. The temperature and its rate of changes for skin layers were predicted by the numerical study, while the relationship between the numerical results and the hand thermal sensation data from ISO/TS 13732-2:2001 were quantitatively analyzed. A one-dimensional unsteady heat transfer model dealing with a steering wheel was designed and developed and it was determined whether the wheel heating system is effective as an auxiliary personal heating system for electric vehicles in the cold winter. Consequentially, even a small amount of electrical energy could provide a meaningful effect to hand thermal sensation.

     

Investigation of the fracture characteristics of the interfacial bond between bone and cement: experimental and finite element approaches

This study integrated the finite element method, fracture mechanics, and three-point bending test to investigate the fracture characteristics of the interfacial bond between bone and cement. The fracture tests indicated that the interfacial fracture toughness of the bone/cement specimens was 0.34 MN/m^3/2, with a standard deviation of 0.11 MN/m^3/2, which was in good agreement with the experimental data available in the literature. A finite element model of the experimental testing specimen was used to predict the critical stress intensity factor (SIF) at the fracture load by the proposed fracture analysis method. The critical SIF of the opening mode of the interface crack was 0.392 MN/m^3/2, which was slightly higher than the fracture toughness obtained in the experiment. Additionally, considering the coupled effects of the crack opening mode and shearing mode, the critical effective SIF was 0.411 MN/m^3/2, with a phase angle of 17.2°. Comparisons of the results obtained from the bending test and numerical analysis made it obvious that the fracture characteristics of the bonded interface between the bone and cement could be accurately predicted by the proposed model. With this analysis model, a realistic investigation on the debonding behavior of cemented artificial prosthetic components is highly expected.     

Analysis of lateral extrusion of gear-like form parts

The analysis of lateral extrusion process was carried out. A three dimensional FE model was developed to analyze the effects of some important geometrical parameters such as initial billet dimensions, gap height and frictional condition on the required forging load, the material flow pattern and effective plastic strain distribution. The FE code of DEFORM-3D was employed. A series of experimental tests on commercial lead billets were carried out to verify the FE results. The simulation work has been performed by the rigid-plastic FE method. The results obtained using the numerical solutions have been compared with the experimental data for each case study in terms of required forming load and material flow pattern in different regions. Comparison between FE and experiment results showed good agreement. Both the simulation and experimental results highlight the major role of above mentioned parameters on the required forming load and material flow pattern. The results showed that the gap height has the greatest effect on the forming load and material flow. The results presented in this paper could be used as basic data in the design of the lateral extrusion process.     

Study on design of progressive dies for manufacture of automobile structural member using DP980 advanced high strength steel

Advanced high-strength steel (AHSS) is widely used in automobile manufacturing to reduce the weight of vehicles, thereby improving fuel efficiency. However, the high yield and tensile strength of AHSS leads to a serious springback problem in the cold sheet metal forming process. This phenomenon has delayed the implementation of AHSS in vehicle parts due to the resulting negative impact on part accuracy. In this study, parameter optimization and multi-stage die compensation were conducted with Finite element (FE) analysis to develop a progressive forming process for automobile structural members using DP980. The FE simulation used the Yoshida-Uemori model to predict the springback phenomenon accurately. The key parameters that significantly influence the springback behavior were optimized using FE simulation and the Taguchi method. The simulation results were used to determine the die and mold compensation. After the parameter optimization and multi-stage die compensation, the final part was obtained with acceptable dimensional accuracy.

     

An analysis of the extrusion of bimetallic tubes by numerical simulation

This paper presents a numerical analysis of a high temperature extrusion process applied to bimaterial tubes made of two pre-bonded metallic materials with different plastic properties. The analysis has been carried out for different geometries and material combinations by using a FE code. An elasto-plastic behaviour with strain rate dependence has been assumed. A parameter related to the void growth rate at the bimaterial interface has been considered for fracture prediction during the extrusion operation.The influence of the different extrusion variables, namely the yield stress ratio, the location and relative thickness of the layers and the die angle, has been assessed. As a conclusion of the analysis, the yield stress ratio between the two materials turns out to be the most critical parameter of the process. Besides, the thickness fluctuations of the harder layer in the final product have been analysed for several material combinations. Those fluctuations can lead to an unacceptable geometrical heterogeneity under certain conditions. The results of the FE calculations are in reasonable agreement with the scarce data available from similar industrial operations.In short, the paper shows that use of FE calculations can be a pre-screening tool for the selection of material couples and extrusion conditions for the production of sound bimetallic tubes, before being engaged in very costly full-scaled trials.     

矩形管冷弯成形的辊花设计及成形规律

依据矩形管冷弯成形的多道次孔型轧制特点,考虑轧前圆管坯与轧后矩形管存在的拓扑等价关系,建立了两者之间的形心映射数学模型,实现了矩形管冷弯成形过程的辊花设计,并进行了轧制实验。轧后产品满足GB/T 6728-2002的要求。基于弹塑性有限元法建立了矩形管冷弯成形的仿真模型,研究了冷弯成形的金属流动、纵向延伸和残余应力分布,确定了变形过程开裂危险区位置。