Analysis of the compression process and dimensional optimization of self‐expanding stent of a nickel titanium alloy

The vascular stents are important devices introduced into the vessel to protect the lumen from unfriendly stenosis so that the blood can flow naturally. During its implantation to the vessel, the stent should be compressed to a delivery system with very small dimension to accomplish the minimal invasive operation. At this stage, the stent will withstand very large stresses which will cause large damages in the structure. In this paper, the compression process of the stent was analyzed by finite element analysis method with software ABAQUS. The stress, strain and martensite volume fraction distributions were investigated at the end compression. Results show that the stent fillets are the dangerous areas for the potential failure. Subsequently, the dimensional optimization was performed to decrease the maximum concentration stress. After the optimization, the maximum stress can be decreased by 14.2%, which means the stent's work life can be increased.     

多维加速度传感器的设计及优化

研制了一种基于单惯性质量块的一体化结构三维加速度传感器。依据传感器的工作原理,采用有限元工具对传感器静态和动态特性进行了分析,给出了弹性体的应变分布、固有频率及振型等,按各轴向灵敏度及其一致性和固有频率等目标对传感器结构尺寸进行了优化设计。有限元分析计算和实验结果表明,所研制的传感器在三个正交方向上都有较高的灵敏度,各加速度分量互干扰小。     

Simulation of the cabling process for Rutherford cables: An advanced finite element model

In all existing large particle accelerators (Tevatron, HERA, RHIC, LHC) the main superconducting magnets are based on Rutherford cables, which are characterized by having: strands fully transposed with respect to the magnetic field, a significant compaction that assures a large engineering critical current density and a geometry that allows efficient winding of the coils. The Nb₃Sn magnets developed in the framework of the HL-LHC project for improving the luminosity of the Large Hadron Collider (LHC) are also based on Rutherford cables. Due to the characteristics of Nb₃Sn wires, the cabling process has become a crucial step in the magnet manufacturing. During cabling the wires experience large plastic deformations that strongly modify the geometrical dimensions of the sub-elements constituting the superconducting strand. These deformations are particularly severe on the cable edges and can result in a significant reduction of the cable critical current as well as of the Residual Resistivity Ratio (RRR) of the stabilizing copper. In order to understand the main parameters that rule the cabling process and their impact on the cable performance, CERN has developed a 3D Finite Element (FE) model based on the LS-Dyna® software that simulates the whole cabling process. In the paper the model is presented together with a comparison between experimental and numerical results for a copper cable produced at CERN.     

A study on the parametric optimization of drawing metal stamping process for aluminum alloy tailgate parts using response surface methodology

The tailgate-extension in a vehicle is a part that connects the upper and lower parts of the tailgate. Since this part does not require high strength, aluminum must be used in order to reduce the weight of the vehicle. However, when choosing aluminum, it is difficult to satisfy the shape quality with the existing manufacturing process. Therefore, this study was conducted for the purpose of optimizing the draw metal stamping process for the development of tailgate extensions for vehicles using aluminum. The response surface methodology was utilized to optimize the draw metal stamping process. The process design parameters were established as blank holding force, coefficient of friction and die speed. Finally, the reliability of the optimization process and finite element analysis was secured by conducting field experiments to review the derived optimal process conditions.     

Effect of process parameters and talc ratio on hot plate welding of polypropylene

This study analyzes the influence of different talc ratios on weld strength of polypropylene joined with hot plate welding process. It further determines the optimum welding parameter settings to achieve the optimum weld strength and observes the effect of process parameters, namely plate temperature and heating time on the joint quality. Process parameters were considered as variables and their effect, interactions and relative significance were investigated by utilizing design of experiment. Simultaneously, a mathematical predictive model of the weld strength was developed in terms of welding parameters. The model can predict effectively weld strength with a 95% confidence level.     

Analysis of the mechanical performance of a cardiovascular stent design based on micromechanical modelling

Stents are very commonly used in the treatment of coronary heart disease. They are permanent vascular support structures that offer a preferred alternative to bypass surgery in certain situations. The purpose of this work is to examine the mechanical behaviour of a stainless steel balloon expandable stent design using computational micromechanics in the context of the finite element method. Deployment and cardiac pulsing loading conditions are considered. Classical phenomenological plasticity theory (J₂ flow theory) and physically based crystal plasticity theory are used to describe the stent material behaviour. Parametric studies are carried out using both constitutive theories with a view to determining important stent deployment characteristics such as recoil and foreshortening. Comparisons of the results obtained using both theories illustrate differences, with the crystal plasticity theory models showing closer agreement to published performance data. The implications of this for stent design are discussed.     

Comparison between RVE and full mesh approaches for the simulation of compression tests on cellular metals

Metal foams are materials of recent development and application that show interesting combinations of physical and mechanical properties. Many applications are envisaged for such materials, particularly in equipments of passive safety, because of their high capacity of energy absorption under impact conditions. The damage analysis in metallic foams is a complex problem and must be performed in a finite strain context. Considering that compression is the dominant loading in impact situations, a finite deformation simulation including damage effects of a compression test on a cellular metal sample is shown in this work. The main objective of the paper is to compare simulations considering periodic boundary conditions, by means of a representative volume element (RVE) approach, with results obtained using full meshes. It is shown that, when the imposed deformation is high, the use of RVE does not describe in a proper manner the deformation that occurs at the walls of cells. This characteristic of RVE approach results in a too stiff behavior when considering load‐displacement relations. A comparison with experimental results is also presented.     

An analytical model for the prediction of hot roll temperatures in a hot rolling process

Temperature variations in the hot roll during a hot rolling process were analysed by solving heat conduction equations for boundary conditions using an analytical method. The analysis was conducted in a steady‐state regime, taking into account the effects of process parameters such as the contact surface, roll velocity and various cooling boundary conditions. Assuming the periodicity of the process, the development of a solution in the Fourier series was employed to solve the governing equations. The temperature and its gradient distributions in the roll depth were analytically expressed according to the process parameters. The accuracy of the predicted results was examined through comparison with predictions presented in the literature (finite element solutions and measurements). Results showed that an increase in the rolling speed leads to a shorter contact time, which decreases the temperature field in the work‐roll.     

Global optimization of a feature-based process sequence using GA and ANN techniques

Operation sequencing has been a key area of research and development for computer-aided process planning (CAPP). An optimal process sequence could largely increase the efficiency and decrease the cost of production. Genetic algorithms (GAs) are a technique for seeking to ‘breed’ good solutions to complex problems by survival of the fittest. Some attempts using GAs have been made on operation sequencing optimization, but few systems have intended to provide a globally optimized fitness function definition. In addition, most of the systems have a lack of adaptability or have an inability to learn. This paper presents an optimization strategy for process sequencing based on multi-objective fitness: minimum manufacturing cost, shortest manufacturing time and best satisfaction of manufacturing sequence rules. A hybrid approach is proposed to incorporate a genetic algorithm, neural network and analytical hierarchical process (AHP) for process sequencing. After a brief study of the current research, relevant issues of process planning are described. A globally optimized fitness function is then defined including the evaluation of manufacturing rules using AHP, calculation of cost and time and determination of relative weights using neural network techniques. The proposed GA-based process sequencing, the implementation and test results are discussed. Finally, conclusions and future work are summarized.     

Experimental and numerical analysis of aluminum-aluminum bolted joints subject to an indentation process

The increasing interest of the industry (especially automotive, aviation and marine) in the fastener joints (riveted, bolted, etc.) between metallic materials, has re-opened the study on the possibility to improve the performance of the drilled structure using plastic deformation processes.Indentation process, performed before the drilling operation, creates circumferential compression stresses around the hole which increase significantly the mechanical performance of the drilled structures.In this paper, static and the fatigue performances of aluminum–aluminum (AW 6082-T6) single-lap bolted joints are studied. In particular, the study compares the mechanical strength of only drilled single-lap bolted joints (OD specimens) and single-lap bolted joints subject to an indentation process (IP specimens). In order to determine the cycles to failure and the corresponding Wöhler diagram, several fatigue tests are performed. The analyses allow to determine the mechanical performance and the failure mode of the analyzed joints.Several numerical analysis, conducted in ANSYS environment on three-dimensional models of the single-lap joint, are focused on the evaluation of the residual stress on the indented plate and, in particular, to compare the stress distribution on both type of analyzed joints.     

Adaptive Finite Element Simulations for Macroscopic and Mesoscopic Models of Steel

During heat treatment and other production processes, gradients of temperature and other observables may vary rapidly in narrow regions, while in other parts of the workpiece the behaviour of these quantities is quite smooth. Nevertheless, it is important to capture these fine structures during numerical simulations. Local mesh refinement in these regions is needed in order to resolve the behaviour in a sufficient way. On the other hand, these regions of special interest are changing during the process, making it necessary to move also the regions of refined meshes. Adaptive finite element methods present a tool to automatically give criteria for a local mesh refinement, based on the computed solution (and not only on a priori knowledge of an expected behaviour). We present examples from heat treatment of steel, including phase transitions with transformation induced plasticity and stress dependent phase transformations. On a mesoscopic scale of grains, similar methods can be used to efficiently and accurately compute phase field models for phase transformations.     

Neutron Stress Measurement of W‐Fiber Reinforced Cu Composite

Stress measurement methods using neutron and X‐ray diffraction were examined by comparing the surface stresses with internal stresses in the continuous tungsten‐fiber reinforced copper‐matrix composite. Surface stresses were measured by X‐ray stress measurement with the sin²ψ method. Furthermore, the sin²ψ method and the most common triaxal measurement method using Hooke's equation were employed for internal stress measurement by neutron diffraction. On the other hand, microstress distributions developed by the difference in the thermal expansion coefficients between these two phases were calculated by FEM. The weighted average strains and stresses were compared with the experimental results. The FEM results agreed with the experimental results qualitatively and confirmed the importance of the triaxial stress analysis in the neutron stress measurement.     

Simulations of a stress and contact model in a chemical mechanical polishing process

A two-dimensional axisymmetric quasi-static contact finite element model for the chemical mechanical polishing process (CMP) was established. The von Mises stress on the wafer surface was investigated. The findings indicate that the profile of the von Mises stress correlated with that of the removal rate. The larger the elastic modulus of the pad or the smaller the elastic modulus of the carrier film, the larger is the maximum von Mises stress. The thicker the pad or the thinner the film, the smaller is the maximum von Mises stress. The larger the load exerted on the carrier, the greater is the maximum von Mises stress.     

Sythhesis of coral‐like and spherical nanoparticles of barium titanate using factorial and Taguchi experimental design

High purity barium titanate (BaTiO₃) nanoparticle and its coral like architectures were synthesized in a laboratory scale from BaO and TiCl₄ by microwave hydrothermal processing using a mixture of 0.1:2 volume ratio of Triethanalamine and distilled deionized water. The products were characterized by XRD, LLS, SEM and wet chemical analysis. The size of barium titanate particles was in the range of 5–25 nm with average particle size of 15.5 nm. The two dimensional coral – like product had the diameter of (3–18 nm and length of 140–420 nm). The experiments were designed to fulfill the 2³ factorial and L₄ Taguchi designs. Data analysis was performed using MINITAB software.     

Simulation‐based optimization of the local material state in the field of cyclically highly stressed case hardened construction details with notch effect*

Steel components very often show construction details, such as cross holes and rounded shaft shoulders which lead to local stress concentrations of the multiaxial stress state in case of mechanical loads (notch effect). Under cyclic loading these stress concentrations (hot spots) can cause crack initiation, crack propagation and finally failure of structural components. The fatigue strength of cyclically loaded components can be considerably increased by the heat treatment case hardening. The shape of the construction detail has a significant influence for the sub‐processes of the case hardening. This can be related to the carbon diffusion process during carburizing and the local heat transfer during quenching. As a result, the local material state following a case hardening process is often not optimal with respect to phase composition and residual stress field. In order to optimize the hardening process a heat treatment simulation based on the Finite Element Method was coupled with procedures for sensitivity analysis and optimization. Taking into account the operational loading conditions for the component, it was possible to adapt technological parameters of the case hardening process for the specific shape of the construction detail, leading to a substantially increased fatigue strength and therewith improvement of the efficiency of the case hardening process itself.     

The implementation of a constitutive model with weighted dynamic recovery and its application

An explicit updating algorithm has been developed, based on the backward Euler integration methods for the modified Armstrong–Frederick type of nonlinear kinematic hardening constitutive model with weighted dynamic recovery. This has been achieved by considering the consistency of the updated stresses, back stresses and the yield function, resulting in three sets of nonlinear equations which are then linearised using the Newton method. The developed algorithm and a linearised version have been implemented in ABAQUS as user material subroutines. Numerical simulations show that as long as the size of the incremental plastic strain is small so that k₂ⁱp0.1, both the linearised and the nonlinear algorithm converge to the same solution. Numerical analysese have been conducted to predict the strain response near the fuel ventilation hole on a wing pivot fitting of fighter aircraft, and good agreement has been observed between the experimental and numerical results.     

Fabrication,process simulation and testing of a thick CFRP component using the RTM process

The present article introduces the case of a CFRP con-rod beam, and describes many aspects regarding its production with the Resin Transfer Moulding (RTM) process.The objective was to find the best process parameters of the injection and curing stages in order to manufacture the 20 mm thick CFRP part. The results are analysed in terms of the aesthetic aspect, the porosity and the mechanical properties of the final component.For the resin injection stage, results obtained from production experiences are presented, which have been performed with different set-ups, and simulations of the resin flow are used to analyse them. The results show that the resin flow during injection could be rather unpredictable, probably because of the fibre rearrangement and race tracking effects. Improvements in terms of aesthetic aspect and porosity of the part could be achieved by a process which included final compaction of the cavity by means of compressed air.Regarding the curing stage, the article presents the simulation results of a curing cycle, and it’s validation through DSC analysis of specimens obtained from the finished component.Finally, results of tensile mechanical tests are provided, performed on finished components produced by RTM and compared to others produced with the method of hand lay-up of pre-impregnated plies and curing in autoclave (Prepreg + Autoclave). The results confirm that it is possible to achieve components through RTM with comparable mechanical performance to those produced with the Prepreg + Autoclave process.     

On a numerical strategy to simulate nanotube‐reinforced composite materials

The application of carbon nanotubes as the reinforcing phase in composite materials is considered. A literature review in regards to the simulation approaches that have been done in order to study the behavior of nanotube‐reinforced composites from several aspects is provided. After that, a new approach for investigating the mechanical properties of the composites reinforced with randomly oriented fibers using the finite element method is proposed. The main idea is to create regular shaped islands around the distributed fibers and to connect these islands to each other and to the representative volume element. Two application examples show the flexibility and the reliability of the proposed method. It has been shown that this approach can handle both aligned and arbitrary distributions of the nanotubes.     

Analysis of the Vertical and Lateral Interactions in a Multisheet Array of InAs/GaAs Quantum Dots

The vertical and lateral interactions in a multisheet array of InAs/GaAs quantum dots are analyzed by finite element method(FEM).It is shown that due to the effects of vertical interaction,nucleation prefers to happen above buried quantum dots(QDs).Meanwhile,the effects of lateral interaction adjust the spacing of lateral neighboring QDs.The vertical coupling becomes strong with deceasing GaAs spacer height and increasing number of buried layers,while the lateral coupling becomes strong with increasing InAs...     

Analysis and characterization of a mechanical sensor to monitor stress in interconnect features

A mechanical rotating stress sensor fabricated in copper has been characterized in 100 nm single damascene technology. Geometrical variations to the structure produce a distinctive behaviour which can be used to fit the actuating stress. Existing analytical models were tested and shown to be unable to describe the structure due to geometric non-linearities not considered by these one-dimensional solutions. A model based on the large strain finite element method was developed to include this non-linearity and fully describe the sensor design for all geometrical variations. The stress determined from the Cu rotating sensors is comparable to measurements performed using high intensity X-ray diffraction on similar samples. Furthermore, the simulation methodology is validated for calibrated Al sensors. All of the studied samples show an excellent fit with the developed finite element analysis, demonstrating the validity of the model to predict smaller geometries, showing that the sensor can be utilized in future integration schemes and applied to other material systems.