Experimental and numerical investigation of laminated steel sheet in V-bending process considering nonlinear visco-elasticity of polymer layer

Laminated steel sheet consists of two steel sheets and a polymer layer which bonds them. During forming process, mechanical properties of polymer layer significantly influence the shape of final product. In this study, a continuum model in which nonlinear visco-elasticity is taken into account has been developed for polymer layer of laminated steel sheet and implemented in a commercial finite element program by material subroutines. Lap-shear test and T-peel test have been conducted to obtain parameters of this continuum model. Two different methods are compared to establish a better method for modeling the polymer layer deformation in lap-shear test simulation. One is cohesive zone element and the other is contact method. In order to assess calculation efficiency, both explicit and implicit procedures are used to simulate lap-shear test, and T-peel test is simulated by implicit procedure to evaluate accuracy. The result indicates that cohesive element is easier to solve convergence problem and implicit procedure may save much simulation time. T-peel test data can be used to describe the normal mechanical behavior of polymer layer in an acceptable range. Finally, V-bending forming process has been studied to investigate the effect of polymer layer on the springback and final deformation shape through experiment and numerical simulation. The result indicates that the comparison between numerical simulation and experiment is in good agreement. The finite element model can accurately predict the final shape after bending and springback.     

IMPROVED COHESIVE ZONE MODEL AND ITS APPLICATION IN INTERFACE CONTACT ANALYSIS

An improved interface cohesive zone model is developed for the simulation of interface contact, under mixed-mode loading. A new debonding initiation criterion and propagation of debonding law, taking into account the pressure stress influence on contact shear strength, is proposed. The model is implemented in a finite-element program using subroutine VUINTER of ABAQUS Explicit. An edge-notch four-point bending process and laminated vibration damping steel sheet punch forming test are simulated with the improved model in ABAQUS Explicit. The numerical predictions agree satisfactorily with the corresponding experimental results.     

Surface analysis of disbonded titanium/sol-gel/polyimide joints

Disbonded lap-shear specimens were analyzed to determine the locus of failure within bonded titanium (Ti) sol-gel polyimide joints. Bonded Ti alloys are being evaluated for use at an operating temperature of 175 °C. Determining the locus of failure for bonded Ti lap-shear specimens is part of a larger effort to develop durable, environmentally safe surface treatments for Ti alloys. Surface-treated Ti alloy (Ti-6Al-4V) plates are bonded in a standard lap-shear specimen configuration and exposed to temperature for specified intervals. The lap-shear bond joint consists of two etched Ti panels coated with a silicon and zirconium containing sol-gel, primed with a polyimide, and then bonded together with adhesive and supporting scrim material. The lap-shear specimens are tested for overall strength and failure modes. Specimens with adhesive failure modes were examined with x-ray photoelectron spectroscopy (XPS) to determine the composition of the bond joint failure layer. Analysis shows that the failure was located closer to the sol-gel/polyimide interface than to the Ti/sol-gel interface. Transmission electron micrographs (TEM) of the cross-sectioned joint confirmed the chemical distribution determined from the XPS data.     

A numerical strength analysis of metal matrix composites considering the interface and matrix damage evolution

A numerical micromechanical method is adopted here to investigate the tensile strength of metal matrix composites (MMC) by considering interface and matrix damage evolution. A cohesive zone model is employed to simulate the fiber/matrix interface damage. The damage in the matrix, which characterizes microvoid nucleation, growth and coalescence, is described in term of the Gurson-Tvergaard material model. These damage models are performed to a boundary value problem that involves a double periodic array of elastic continuous fibers in the elastic-plastic matrix subjected to transverse loads. The main attempt is made to investigate effects of interface strength and toughness on tensile strength of MMC.     

基于内聚力模型的锆-钛-钢复合板剪切试验模拟

基于双线性张力位移法则,运用ABAQUS内置内聚力单元对锆-钛-钢复合板的剪切试验进行了有限元模拟,分析了界面断裂过程的应力分布特征,研究了内聚力强度、断裂能对于界面层损伤的影响,并验证了剪切强度预测公式的正确性。研究结果表明:内聚力单元可以模拟复合板界面层的断裂过程,且与剪切试验结果良好吻合;剪断位移与内聚力强度呈正相关;随着断裂能的增加,剪断位移、特征位移和最大剪切力都增加;剪切强度预测公式可对不同热处理工艺下剪切强度进行预测。     

纳米孪晶铜尺度效应的数值模拟

为了理解电解沉积纳米孪晶铜的拉伸变形行为,采用基于机制的应变梯度塑性理论对其拉伸变形进行数值模拟研究;提出孪晶薄层强化带的概念,并采用黏聚力界面模型模拟晶界的滑移和分离现象。采用的计算模型包含晶粒尺寸、弹性模量、塑性硬化指数、初始屈服应力和孪晶薄层分布等和尺度效应相关的一系列参数。计算结果有助于理解纳米孪晶铜的力学行为。     

Effects of interface adhesive and resin cohesive strength on tensile strength of resin sand based on numerical analysis

A plane-strain unit-cell finite element model was proposed to study the effects of resin/sand interface adhesive and resin cohesive strength on the overall tensile strength of resin sand, as well as the fracture modes. The main micro-scale characteristics of the numerical model were extracted from the micrograph of resin sand specimens by three-dimensional X-ray microscopy(3 D-XRM). The extended finite element method(XFEM) and cohesive behavior method were employed to explicitly describe the resin fracture and sand/resin interface debonding, separately. The corresponding experimental observation of micro-scale failure behavior based on the scanning electron microscopy(SEM) was presented for a comparison. The numerical results show that the initial failure of the model occurs at the sand/resin interface, followed by consequent resin failure. Dependent on the resin cohesive strength, the location of resin failure varies from the central zones to resin neck arc zones. A typical mixed mode fracture is observed, which is consistent with the corresponding micro-scale experimental observation. When the resin cohesive strength ranges between 8 and 12 MPa, the resin cracks occur at the central zone of resin bridges and propagate perpendicularly to the tensile direction until through cracks happen. At a higher range(between 12 and 16 MPa), interface cracks cross with resin cracks, bonding bridges of resin sand are broken. The interface adhesive strength has a more significant effect on the overall tensile strength of resin sand than the resin cohesive strength. The overall tensile strength of resin sand increases first then keeps stable with the increase of the resin cohesive strength. This work attempts to establish a numerical model which accurately describes the complicated mixed mode fracture of resin sand, which is beneficial to understand deeply the fracture mechanism of resin sand.     

Multi-scale modeling to predict sub-surface damage applied to laser-assisted machining of a particulate reinforced metal matrix composite

A multi-scale finite element model (FEM) is developed to predict the post machined sub-surface damage in a particle reinforced metal matrix composite (MMC) subjected to laser-assisted machining (LAM). The MMC studied is an A359 aluminum matrix composite reinforced with 20% by volume fraction silicon carbide particles. In this model, molecular dynamics (MD) simulations are carried out to parameterize traction–separation laws for the aluminum–silicon carbide interface. The parameterized traction–separation laws are then input into a finite element model, in the form of a cohesive zone model to subsequently simulate the sub-surface damage. In this manner, the multi-scale hierarchical model successfully bridges the atomic, micro and macro scales. Average values of the predicted quantities are compared with experimental results, and the favorable agreement confirms the validity of the multi-scale FEM.     

Cutting simulation capabilities based on crystal plasticity theory and discrete cohesive elements

A material microstructure-level (MML) cutting model based on the crystal plasticity theory is adopted for modelling the material removal by orthogonal cutting of the Titanium alloy Ti6Al4V. In this model, the grains are explicitly taken into account, and their orientation angles and slip system strength anisotropy are considered as the main source of the microstructure heterogeneity in the machined material. To obtain the material degradation process, the continuum intra-granular damage model and the discrete cohesive zone inter-granular damage model have been implemented. Zero thickness cohesive elements are introduced to simulate the bond between grain interfaces. The material model is validated by the simulation of a compression test and results are compared with experimental data from the literature. Simulation results demonstrate the ability of the MML cutting model to capture the influence of the material microstructure, in terms of initial grain orientation angles (GOA), on chip formation and machined surface integrity.     

Cohesive interface simulations of indentation cracking as a fracture toughness measurement method for brittle materials

Cracks in brittle solids induced by pyramidal indenters are ideal for toughness evaluation since the indentation stress fields decay rapidly from the contact center and any cracks will be eventually arrested. Thus, if the applied energy release rate can be determined analytically, the material toughness can be deduced by measuring the crack length. However, such a driving force calculation is a nontrivial task because of the complex stress fields; only a number of limit cases can be solved, such as the long half-penny cracks (at least two times larger than the contact size) in the classic Lawn–Evans–Marshall (LEM) model. Important questions such as the evolution from short cracks to median/radial and then to half-penny cracks, the form of the scaling relationship that relates fracture toughness to material hardness and indenter angles, the threshold load for indentation cracking, etc., cannot easily be answered without a detailed knowledge of the co-evolution history of the stress fields and crack morphology. To this end, a finite element model of four-sided pyramidal indentation adopting cohesive interface elements is developed to study the effects of indenter geometry, load, cohesive interface parameters, and material properties on the initiation and propagation of the median/radial/half-penny crack systems. The validity and artifacts of the cohesive interface model are carefully examined, and the crack morphologies under various indentation and material parameters are systematically studied. Numerical predictions lead to a quantitative evaluation of the threshold load for indentation fracture, and an improved method for the evaluation of material toughness from the indentation load, crack size, hardness, elastic constants, and indenter geometry, which compare favorably to a large set of experiments in the literature. It is also found that the toughness evaluation method is very sensitive to Poisson’s ratio – an observation that has previously received little attentions. An approximate analysis for short cracks is developed based on the fracture mechanics of annular cracks and the embedded-center-of-dilatation model for indentation-induced residual stress fields.     

Influence of alumina particles on the mechanics of machining metal matrix composites

A plane-strain thermo-elasto-plastic finite element model has been developed and used to simulate orthogonal machining of alumina/aluminium 6061 metal matrix composite using a tungsten carbide tool. Simulations were carried out employing temperature-dependent material physical properties. The interface failure mode between the aluminium matrix and alumina particles was incorporated in this model. The model is used to investigate the effective and shear stresses on the alumina particles. Detailed results of the cutting forces generated during the machining process are presented and a comparison has been made with the experimental results for a range of feeds. Of particular interest are the contact stress distributions and alumina particle's interface failure. Normal and shear stresses and cutting temperatures were investigated.     

Toward a new tribological approach to predict cutting tool wear

This work aims at improving the numerical modelling of cutting tool wear in turning. The key improvement consists in identifying a fundamental wear model by means of a dedicated tribometer, able to simulate relevant tribological conditions encountered along the tool–workmaterial interface. Thanks to a design of experiments, the evolution of wear versus time can be assessed for various couples of contact pressure and sliding velocities (σ_n, V_s) leading to the identification of a new wear model. The latter is implemented in a numerical cutting model to locally simulate tool wear along the contact with regard to each local tribological loading.     

滚动导轨智能组合单元结合部动态特性研究

以滚动导轨智能组合单元为研究对象,借助赫兹接触理论,分析计算滚动导轨副、滚珠丝杠副和角接触球轴承的刚度,建立了滚动导轨智能组合单元结合部的动力学理论模型.通过对滚动导轨智能组合单元结合部进行相应的简化,采用弹性单元模拟结合部特性,提出结合部特性仿真分析的有限元方法,建立该机构的有限元模型,并对其进行了模态分析和谐响应分析,得到该机构的动态特性.分析结果为滚动导轨智能组合单元的动态特性优化提供了理论依据.     

接触模型对搅拌摩擦焊接数值模拟的影响

在完全热力耦合搅拌摩擦焊接数值模型中采用两种接触模型--经典的Coulomb接触模型和修正的Coulomb接触模型,模拟了搅拌摩擦焊接过程,以分析不同接触模型对搅拌摩擦焊接过程数值模拟的影响.结果表明,对于低转速的搅拌摩擦焊接,两种模型的预测结果区别不大;但是对于高转速,由于界面摩擦剪切应力没有上限,采用经典的Coulomb接触模型无法模拟,需采用修正的Coulomb接触模型.搅拌头转速的增加不会改变搅拌摩擦焊接技术固态连接的本质.当采用高转速时,焊接构件上、下表面的变形趋于均匀.有利于得到均匀的显微结构.     

Mechanical characterization of friction stir spot microwelds

Microstructures and failure mechanisms of friction stir spot microwelds in 300-μm thin sheet of aluminum 1050 alloy were investigated. As an alternative to conventional soldering and welding in joining thin metals for electronic, medical and microdevices, friction stir welding may be utilized in order to limit the excessive heat damage. Transmission electron microscopy micrographs of the cross sections of friction stir spot microwelds in lap-shear specimens were examined. These microwelds showed the failure mode of nugget pullout under lap-shear loading conditions. The experimental observations suggested that under lap-shear loading conditions, the failure was initiated near the possible original notch tip in the stir zone and the failure propagated along the circumference of the nugget to final fracture. Microindentation hardness data of base metal, heat affected zone, thermal-mechanical affected zone and stir zone were obtained. The interface between the heat affected zone and the thermal-mechanical affected zone was the softest region, where the cracks of friction stir spot microwelds in the lap-shear specimens under the loadings initiated and leaded to fracture of the specimens.     

Cohesive mechanism of the FeCr/Ni Interface: A first-principles study

Based on previous experimental results, a series of FeCr/Ni interface models have been constructed and analyzed using a first-principles pseudopotential plane-wave method. Several parameters, such as the ideal work of separation (W), formation enthalpy (ΔH), cohesive energy (ΔE), and electronic structure were calculated in order to analyze the bonding performance and adhesion mechanisms of elements along an FeCr/Ni interface. The largest ideal work of separation was obtained for the Fe(100)/Ni(100) interface, which implies that this interface model presented the most stable structure among a series of crystal interface indices, e.g., (100), (110), and (111). With Cr doping, the W of the FeCr(100)/Ni(100) interface was increased by 101.571 mJ/m². The corresponding ΔH and ΔE values also indicated that the FeCr(100)/Ni(100) interface model was strengthened by doping with chromium. Furthermore, the overlap population ratio, R _LBOP (R _LBOP= 1.04), of FeCr(100)/Ni(100) was smaller than that of Fe(100)/Ni(100) (R _LBOP = 1.35), which implies that the toughness of the Fe(100)/Ni(100) interface can be improved by the presence of chromium impurities. Moreover, electronic structure analysis provided an understanding of the mechanical performance of the various Fe(Cr)/Ni interface models. Thus, our findings open a potential avenue for the comprehensive study of composite material designs.     

晶粒尺寸对纳晶双峰材料断裂韧性的影响

为了描述由纳晶基体和粗晶颗粒组成的纳晶双峰材料的断裂韧性,通过建立一个粘聚力模型来研究纳晶双峰材料的临界应力强度因子K_(IC)(表征材料断裂韧性)。考虑到纳晶双峰材料的一个典型情况:裂纹位于2个纳晶颗粒的交界面处,裂纹尖端与粗晶粒的晶界相交,假设粘聚区的尺寸等于纳晶颗粒的尺寸d。裂纹的钝化和扩展过程受位错和粘聚力的共同影响,刃型位错是从粘聚力裂纹的尖端发射,该过程对裂纹产生屏蔽效应。模型计算结果显示:当粗晶颗粒尺寸D确定时,K_(IC)随着纳晶材料晶粒尺寸d的增大而增大;当纳晶材料晶粒尺寸d确定时,K_(IC)随着粗晶材料晶粒尺寸D的增大而增大;相对于纳晶颗粒的尺寸,断裂韧性对粗晶晶粒的尺寸更加敏感。     

铝合金电阻点焊中电极点蚀的形成机制

建立了铝合金电阻点焊过程数值模拟的有限元分析模型,考察了焊接过程中电极与试件界面上接触半径的变化,以及电极尖端表面上电极压力、电流密度和温度的分布.结果表明:所考察的焊接条件下,接触半径在焊接过程中逐渐增大,电极端面的中部温度最高,而电极压力和电流密度均在接触区边缘集中.实验研究发现电极表面上最初的点蚀部位呈环形,其半径与接触区半径基本一致,由此推断,环状电极点蚀主要是接触区边缘明显的应力集中所致.为减少电极点蚀提高电极寿命,电极的形状设计应使电极与工件接触界面上的应力集中尽可能减小.     

Numerical simulation of plate rigid restraint cracking tests based on cohesive element model

Cohesive element is developed from the Dugdal-Barenblatt model in the field of fracture mechanics. The mechanical constitutive relation of cohesive element can be artificially assumed depending on the specific applications. It has been successfully applied in the study of crystal plasticity/ brittle fracture process and decohesion between delaminations. In this paper, tensile experiments of large steel plate with different length of pre-existing cracks are conducted. Based on commercial software ABAQUS, cohesive element is adopted to simulate the tensile tests, and appropriate parameter values are obtained by fitting displacement force curves. Using these parameters, a numerical method is presented by applying cohesive element to thermo-elastic-plastic finite element method ( TEP-FEM) to simulate plate rigid restraint cracking ( PRRC) tests. By changing constitutive relation of cohesive element, dimensions of the model and welding conditions, the influence of welding restraint intensity and welding conditions on the crack propagation are discussed, respectively. Three types of welding cold cracking are simulated. Significant influence of welding cold cracking on resistant stress in welding line is captured by this numerical method.     

用CA方法模拟过冷熔体中自由树枝晶的生长

基于溶质扩散和界面能的作用，建立了过冷熔体中自由树枝晶的生长模型，考虑了成分过冷、曲率过冷和界面能各向异性，用胞元自动机（CA）方法模拟了枝晶生长、界面扰动以及分枝的竞争演化，对枝晶尖端生长速度和过冷度的关系进行了模拟计算，并与L-G-K理论模型进行了定量的比较。