反向压力对板材粘性介质压力胀形的影响研究

为研究带有反向压力粘性介质压力胀形过程中接触条件对板材成形性的影响规律,利用DEFORMTM-2D结合韧性断裂准则对覆层板粘性介质压力胀形过程进行有限元分析.结果表明:接触表面无摩擦单纯依靠反向压力能够提高板材成形极限,随着接触表面摩擦系数增大,板材壁厚分布愈均匀,板材的破裂位置由试件顶端转移到凹模圆角处,板材成形极限显著提高.因此,在三维应力状态下有效控制板材所受法向压力和界面摩擦力可以提高板材成形性.     

VPF用粘性介质粘弹塑性本构模型及参数确定

为确定粘性介质压力成形所采用的粘性传力介质的材料模型及其参数,对粘性介质剪切蠕变-回复实验结果进行分析,提出采用粘弹塑性材料模型描述粘性介质变形行为.将粘性介质的总变形分解为弹性分量、粘弹性分量和粘塑性分量,对实验结果进行拟合分析,分别确定了粘性介质变形的各个应变分量与应力之间的关系,最终建立了所选用的粘性介质粘弹塑性模型,为数值模拟粘性介质变形行为提供了依据.     

板材软模成形数值模拟研究现状

分析了板材软模成形数值模拟的特点,对目前主要采用的软模成形数值模拟方法进行了阐述和评价,并对三类典型软模成形(液压成形,橡皮成形及粘性介质压力成形)过程数值模拟应用进行了综述,最后对板材软模成形数值模拟发展趋势进行了展望.     

半球形件粘性介质压力成形的数值模拟研究

粘性压力成形技术是金属板材成形领域里新近出现的一种先进的加工工艺.对双面施放粘性介质的板材胀形进行了模拟研究,在双面施放介质的板材粘性介质胀形时,排放孔位置分布对毛坯成形过程有明显的影响.排放孔位置分布的不同造成模腔内介质的速度场不同,进而导致板材不同部位的流动速度的方向大小发生变化,最终使成形件的厚度分布不同.     

VPF粘性介质粘弹性本构关系研究

为研究粘性介质的力学性能对板材变形的影响,通过剪切蠕变-回复和松弛试验分析了甲基乙烯基粘性介质的流变性能.实验结果表明粘性介质可以简化为线性粘弹性材料.建立了粘性介质的积分型粘弹性本构方程,并结合剪切蠕变-回复过程的有限元分析确定了方程中的材料参数.利用该本构方程对粘性介质压力胀形过程进行了有限元分析,模拟结果与试验结果对比表明,所建立的本构方程可以较好的预测板材的变形过程.     

薄壁筒形件粘性介质外压缩径失稳起皱分析

缩径成形受筒坯稳定性的影响,容易产生失稳起皱,很难成形出缩径量较大的零件.提出了采用粘性介质外压缩径工艺成形直径变化量较大的薄壁异形曲面筒形零件新方法.针对坯料端部约束和自由条件下的失稳起皱问题进行了试验和有限元模拟,分析了试件失稳起皱过程中几何形状和应力的变化规律.研究表明,约束坯料端部可以改善试件失稳起皱后的应力分布规律,有利于消除褶皱,在较高粘性介质压力条件下可以直接成形出缩径量为15%的试件.     

柔性压辊拉形方式及其夹钳对板材成形的影响

为提高大型板类件拉形精度,介绍了传统整体夹钳拉形机的特点,提出了新型柔性压辊拉形原理.以球形件为例,分别对两种夹钳结构的拉伸成形过程建立有限元模型,并进行数值模拟,对比分析了两种夹钳结构作用下的成形结果.研究表明:柔性压辊拉形方式所得的成形件,其应力、应变分布均匀,成形质量较整体夹钳方式明显提高;而采用传统整体夹钳所成形的部件,其局部应力集中现象相对严重.利用柔性压辊拉形理论,分别对采用平板式结构夹钳和拉延筋式结构夹钳成形球形件的过程建立有限元模型并进行模拟分析,根据板材拉形过程的流动理论,对比分析两种夹持方式下板材的流动状态.研究发现:采用拉延筋式夹钳进行拉伸成形,处于夹钳中的板材流动效果较好;而利用平板式夹钳的成形过程中,板材流动量较小.实验验证结果与模拟结果趋势一致.     

服役条件下方钴矿基热电元件的界面应力分析

热电器件中, 界面可靠性是影响整体稳定和功率输出的关键因素。对于方钴矿(SKD)器件, 热电臂和电极通过扩散阻挡层(DBL)连接。在高温下, DBL与SKD、电极之间会发生反应并生成复杂的界面结构, 导致界面附近的热、电、力学性能发生变化。本研究根据实际界面结构建立了包含微观结构的有限元模型, 并将其用于分析方钴矿基元件的界面应力状态。采用单层模型对DBL材料参数进行了筛选, 发现热膨胀系数(CTE)和弹性模量(E)对第一主应力有显著影响。采用包含界面微结构的多层模型定量模拟了不同老化温度、时间下元件内部的应力分布, 结果表明在SKD/Zr和SKD/Nb中, CoSb₂反应层最为薄弱, 随着老化时间的延长, 反应层的厚度增加, 界面应力变大。同时, 元件的拉伸试验结果与计算结果吻合较好, 验证了模型的准确性与可行性。本研究为提升SKD/DBL元件的结构稳定性提供了指导, 同时也为精确模拟多层结构中的应力状态提供了研究思路。     

活性金属/陶瓷粘着功

把金属/陶瓷界面反应处理成表面相之间的反应,运用表面热力学基本原理研究了活性金属/陶瓷体系的粘着功。提出了一个活性体系的粘着功计算方法,并和文献实验结果进行了验证。研究结果表明,活性体系的最终粘着功由两部分组成:一部分是每单位面积的金属/陶瓷表面相之间反应自由能的变化,另一部分是陶瓷或固相产物与基体金属的粘着功。界面反应越剧烈,固相产物与金属粘着功越大,则活性体系最终粘着功越大,这就为改善金属/陶瓷润湿性和粘着功提供了理论依据。     

服役条件下方钴矿基热电元件的界面应力分析（英文）

热电器件中,界面可靠性是影响整体稳定和功率输出的关键因素。对于方钴矿(SKD)器件,热电臂和电极通过扩散阻挡层(DBL)连接。在高温下,DBL与SKD、电极之间会发生反应并生成复杂的界面结构,导致界面附近的热、电、力学性能发生变化。本研究根据实际界面结构建立了包含微观结构的有限元模型,并将其用于分析方钴矿基元件的界面应力状态。采用单层模型对DBL材料参数进行了筛选,发现热膨胀系数(CTE)和弹性模量(E)对第一主应力有显著影响。采用包含界面微结构的多层模型定量模拟了不同老化温度、时间下元件内部的应力分布,结果表明在SKD/Zr和SKD/Nb中,CoSb_2反应层最为薄弱,随着老化时间的延长,反应层的厚度增加,界面应力变大。同时,元件的拉伸试验结果与计算结果吻合较好,验证了模型的准确性与可行性。本研究为提升SKD/DBL元件的结构稳定性提供了指导,同时也为精确模拟多层结构中的应力状态提供了研究思路。     

纵波在含有线性粘滞粘接层的粘接结构中的传播

研究了纵波垂直入射上下基体为同种介质且含有线性粘滞粘接层的三层板状粘接结构时声波的反射和透射特性,基于粘接界面的准静态模型(QSM)并结合各向同性线性粘滞体的本构方程,导出了含有粘滞系数和体积弹性模量的纵波反射和透射系数表达式。首先将纵波的准静态模型解和精确解进行对比,研究了准静态模型的适用条件。其次在界面处于不同接触形式(完好连接和粘滞型弱粘接)的情况下,分别讨论了考虑和不考虑界面相对质量对声反射和透射特性的影响。接着分析了粘滞粘接层参数的变化对纵波反射和透射特性的影响。最后在不大于0.1 MHz-mm的较小频厚积范围内简要阐述了如何鉴别界面形式。结果表明,准静态模型适用于较小频厚积的检测;是否考虑界面的相对质量对纵波的反射和透射特性影响不大;在特定的频厚积下,利用纵波的反射或透射系数方法结合准静态模型可有效辨别界面形式。研究成果可为实验时采用纵波垂直入射检测粘接结构提供一定的理论参考。     

Through-thickness strain field measurement in a composite/aluminium adhesive joint

This paper presents an experimental procedure, which enables us to assess the shear strain field in an adhesive joint between composite and aluminium. In practice, this strain field is representative of the progressive stress transfer between a loaded structure and a composite patch used for reinforcement purposes. Digital image correlation (DIC) is used to measure the displacement field through the thickness of a patched specimen subjected to a tensile test. The shear strain field derives from the measured displacement field. The shear strain clearly decreases when the distance from the free edge of the adhesive increases, as predicted by numerical and analytical models of the joint. These measurements are used to estimate the in situ shear modulus of the adhesive. It is observed that the shear modulus decreases when the shear stress increases, thereby illustrating the non-linear response of the adhesive.     

基于VPF工艺的铝合金覆盖件回弹研究

为了研究VPF工艺对铝合金覆盖件成形过程中回弹的影响,对VPF工艺成形过程中铝合金双曲率覆盖件的回弹特征进行了数值模拟和实验研究,并且与传统刚模成形零件的回弹特征进行了对比.研究结果表明:VPF工艺成形的铝合金双曲率覆盖件的回弹过程中同时存在正、负回弹现象,而传统刚模成形的零件回弹过程中只存在正回弹,并且VPF工艺成形的铝合金双曲率覆盖件大部分区域的回弹量小于传统刚模成形的零件;随着成形深度的增加,VPF工艺成形的铝合金双曲率覆盖件正回弹降低,负回弹量增加.     

FEM simulation of viscous properties for granular materials considering the loading rate effect

The deformation and strength characteristics of sandy soils as a kind of granular materials are very complex. The experimental results show that when the strain rate suddenly changes in monotonic loading (ML) case, the stress–strain curve of sandy soils changes sharply and then gradually converges into the original inferred one that would be obtained by continuous ML at constant strain rate after having exhibited clear yielding. Similar behaviors are also observed when ML is restarted at a constant strain rate following a creep loading or stress relaxation stage. An elasto-viscoplastic constitutive model for granular materials is developed, which consists of three components. One of the most important features of the model is that it can take into account the effects of loading rate due to viscous properties on the stress–strain behavior. The stress ratio-axial strain–time relations from four drained plain strain compression (PSC) tests on the saturated Toyoura sand are successfully simulated by the finite element method (FEM) code incorporating the proposed constitutive model. It is shown that the FEM code can simulate the viscous behaviors of sand accurately under arbitrary loading history.     

The effect of a spew fillet on adhesive stress distributions in laminated composite single-lap joints

A laminated composite single-lap joint without a spew fillet, subjected to tensile loading, is investigated experimentally and numerically. By directly comparing the experimentally- and numerically-determined deformations of the single-lap joints with and without a fillet, the effect of a spew fillet on adhesive stress distributions is discussed. Moiré interferometry is used to measure the in-plane surface deformation of the overlap region of the test specimens. The deformation interactions of the laminated adherends, adhesive layer and a fillet are documented in the form of orthogonal components of the displacement fields (u and v). Two-dimensional, geometrically linear and nonlinear finite element analyses are performed to simulate the mechanical response of the laminated composite single-lap joint and the effect of a spew fillet. Experimental and numerical results indicate that the adhesive shear and peel strain (stress) concentrations can be reduced greatly by introducing a fillet at the end of the overlap, and these concentrations are affected by the geometrically nonlinear deformation of the single-lap joint.     

Viscous correction for the viscous potential flow analysis of capillary instability with heat and mass transfer

The linear analysis of capillary instability of the cylindrical interface between two viscous, incompressible fluids confined in a concentric annulus in the presence of heat and mass transfer is carried out. The viscous correction for the viscous potential flow theory is used for the investigation. Both fluids are taken as incompressible and viscous with different kinematic viscosities. To study the effect of shearing stresses, viscous pressure in the normal stress balance is included along with irrotational pressure. It is observed that heat and mass transfer has a stabilizing effect on the stability of the system and this effect enhances in the presence of irrotational shearing stresses. A comparison between viscous potential flow (VPF) solution and viscous correction for the viscous potential flow (VCVPF) is made and it is found that the VCVPF solution is more stable than the VPF solution.     

Application of a split-Hopkinson tension bar in a mutual assessment of experimental tests and numerical predictions

This paper shows how experimental test results from a split-Hopkinson tension bar (SHTB) and numerical simulations of the test set-up can be used for mutual verification. Firstly, a SHTB where the tension stress wave is generated by pre-stretching a part of the incident bar is briefly presented. This SHTB is used to carry out tensile tests of four aluminium alloys at high rates of strain, while tests at low to medium strain rates were performed in a servo-hydraulic tensile test machine. Using the test results, the parameters of an anisotropic thermoelastic-thermoviscoplastic constitutive relation and a one-parameter fracture criterion are identified for the materials at hand. Subsequently, the material model is used in explicit finite element analyses of the SHTB tests, including the entire experimental set-up and the stress wave propagation during the test. The numerical predictions were found to represent the observed behaviour in the experimental tests fairly well.     

Basis for viscoelastic modelling of polyethylene terephthalate (PET) near Tg with parameter identification from multi-axial elongation experiments

The mechanical response of Polyethylene Terephthalate (PET) in elongation is strongly dependent on temperature, strain and strain rate. Near the glass transition temperature T_g, the stress vs strain curves present a strain hardening effect vs strain under conditions of large deformations. At a given strain value, the strain rate has also an increasing influence on the stress value. The main goal of this work is to propose a visco-elastic model to predict the PET behaviour when subjected to large deformations and to determine the material properties from the experimental data. The visco-elastic model is written in a Leonov like way and the variational formulation is carried out for the numerical simulation using this model. To represent the non–linear effects, an elastic part depending on the elastic equivalent strain and a non-Newtonian viscous part depending on both viscous equivalent strain rate and cumulated viscous strain are tested. The model parameters can then be accurately obtained through the comparison with the experimental uniaxial and biaxial tests.