层合板层间断裂分析的组合界面单元及其特性

提出了一种由刚性元和零厚度的内聚力单元组合而成的新型界面单元,该界面单元嵌在板壳结构界面之间,可用来模拟界面损伤的起始和演化,能考虑板壳的平动和转动对分层损伤的作用。该界面单元具有有限厚度,八个结点,每个结点有五个自由度,通过刚性元将板壳单元结点的位移和结点力转换到内部零厚度的内聚力单元上,界面损伤通过内聚力单元的损伤演化体现出来。采用板壳单元和新型界面单元建立有限元模型,对混合弯曲(MMB)试验和双悬臂梁(DCB)弯曲试验进行了计算模拟,计算结果能很好地模拟结构的界面损伤过程。相比传统的用内聚力单元和三维实体单元组成的模型,建模方便,在精度相当的前提下,可以使单元尺寸增大一倍,减少裂尖内聚力区域(cohesive zone)内的单元数量,缩小计算规模,提高计算效率。     

界面对双向纤维增强复合材料力学性能的影响

基于均匀化理论与有限元方法,针对双向连续纤维增强复合材料(CBFRC),建立了微观代表体积单元(RVE)模型,并预测了界面对CBFRC宏观等效力学性能的影响。在建立的RVE模型中,采用表面内聚力本构关系描述纤维/基体之间的界面。研究结果表明:不同界面刚度下,CBFRC中的基体发生不同形式的初始损伤。与界面断裂能相比,界面刚度和界面强度对CBFRC面外抗拉强度的影响较大,而对CBFRC面内抗拉强度的影响较小,且界面的存在会降低CBFRC整体的抗拉强度。随着纤维体积分数的增加,CBFRC面内的抗拉强度也随之急剧增加,但CBFRC的面外抗拉强度反而有减小的趋势。本文中所提出的方法能够简单有效地对实际复杂的三维纤维增强复合材料进行优化设计。     

基于Voxel有限元网格对球形夹杂复合材料的应力分析

通过Voxel有限元网格对球形夹杂复合材料进行应力分析时,由于Voxel网格在两相界面呈现阶梯状,所以在两相界面附近的单元会表现出明显的应力集中现象。提出采用局部应力平均方法来处理由于Voxel有限元网格而引起的应力集中,并且考虑应力平均区域、应力平均加权函数以及网格密度的影响。结果表明:该局部应力平均方法能够有效地去除两相界面附近单元的应力集中,但应力平均区域不能过大也不能过小。通过计算发现采用2个Voxel网格深度的平均区域为最优,并且具有网格不依赖性。该方法也可以进一步用于球形夹杂复合材料的积累损伤演化分析。     

短纤维增强橡胶密封复合材料界面疲劳损伤行为

依据广义自洽方法,建立了包含芳纶纤维、界面相、橡胶基体和等效介质的代表性体积单元(RVE)模型。采用自定义材料子程序对内聚力疲劳累积损伤模型进行编译,分别在基体/界面相的界面和纤维/界面相的界面设置内聚力单元,研究界面相性能参数对纤维增强橡胶密封复合材料(SFRC)界面疲劳损伤行为的影响。探讨了界面相厚度和模量的确定方法,获得了不同界面相厚度和模量下SFRC界面脱粘起始位置以及脱粘起始疲劳次数。结果表明,较低的界面相模量能够抑制界面脱粘的产生;随着界面相厚度的增加,界面脱粘的起始疲劳次数增加,SFRC抗疲劳损伤能力得到提高。     

ZrO2增韧Al2O3颗粒增强Fe45复合材料拉伸断裂的有限元-离散元耦合方法仿真ZrO2增韧Al2O3颗粒增强Fe45复合材料拉伸断裂的有限元-离散元耦合方法仿真

采用有限元-离散元耦合方法（FEM-DEM方法）,进行了氧化锆增韧氧化铝颗粒增强Fe45复合材料（ZrO₂-Al₂O₃/Fe45）轴对称代表体元模型的拉伸断裂仿真分析。分析了FEM-DEM模型对单元尺寸的敏感性,结果表明采用,二阶实体单元加双零厚度内聚力单元的FEM-DEM模型降低了计算结果对单元尺寸的敏感性。ZrO₂-Al₂O₃/Fe45复合材料拉伸断裂的模拟结果表明,颗粒形状对裂纹的扩展会产生较大影响,复合材料的开裂首先在垂直于拉力方向的界面处发生,界面裂纹扩展至基体应力集中处之后基体发生开裂,裂纹由开裂的界面和基体裂纹共同组成。      

SH波入射双相介质半空间浅埋任意位置圆形夹杂的动力分析

采用Green 函数及复变函数方法研究了SH 波入射到双相介质半空间时,浅埋任意位置圆形夹杂的动力响应问题。首先,利用“镜像”法构造满足直角平面自由边界条件的散射波场解答,进而求出该文所需的Green函数;其次,利用“契合”思想,将模型沿着垂直界面剖分为两个直角域,并利用界面连续性条件及Green函数建立待解外力系的第一类Fredholm 积分方程组;最后,通过具体算例给出了圆形弹性夹杂周边的动应力集中系数。结果显示:界面、自由边界、圆形夹杂、入射波数等因素均对动应力集中系数有影响。     

开孔氧化铝薄膜/铝合金基体热屈曲变形和应力分析

假设薄膜和基体界面处于理想结合状态,基于应变协调理论,采用有限元软件(ANSYS 8.0)分析了不同膜基比(hc/hs)和开孔对氧化铝薄膜/铝合金基体系统热屈曲变形、热应力的影响.结果表明,当矩形薄板发生热屈曲时,曲率和热应力均随膜基比非线性变化.随着膜基比的增加,曲率不断减小,而薄膜和基体中的热应力表现出不同的变化趋势,基体中的热应力随膜基比的增加而增加,薄膜中的热应力随膜基比的增加而减小.当hc/hs<0.005时,曲率受膜基比的影响非常大且曲线较陡;当hc/hs>0.005时,曲率随膜基比的增加而缓慢减小.开孔能在一定程度上缓解系统的热屈曲变形,但是这种缓解程度相对较小.无孔时系统中的热残余应力在面内基本上都是均匀分布的,而开孔时系统中的热残余应力分布不再均匀,特别是在小孔附近产生了严重的应力集中现象,膜基比越小则应力集中现象越严重.     

SH波对界面圆柱体夹杂下半圆脱胶的散射分析

本文采用有限元法研究近场界面圆柱夹杂脱胶结构对瞬态SH波的散射和动应力集中系数问题。用有限元法模拟弹性波在半无限介质中的传播,用ANSYS进行计算,给出了部分节点的位移、应力时程解和夹杂周边的动应力集中系数,讨论圆柱夹杂、介质参数以及波数对动应力集中和各点位移带来的不同影响,并进行对比分析。     

复合材料层板界面缺陷的有限元分析

基于对多种界面缺陷的一致表达,建立了能够反映复合材料层合结构界面缺陷的有限元模型。通过引入横向剪切变形函数来反映界面粘贴状况;对于界面局部缺陷问题,通过缺陷边缘处相邻单元的几何矩阵的简单匹配来满足缺陷边缘的连续条件。只需简单选择界面柔度系数,可方便地处理界面理想粘贴、弱粘贴和脱层三种界面粘贴状况。有限元形式简单,仅涉及6个节点自由度。算例验证了有限元模型的精度,讨论了各种参数时界面缺陷对结构的影响。     

用脉冲喷射电沉积法制备纳米晶镍镀层

采用脉冲喷射电沉积方法制备了纳米晶镍镀层,用扫描电子显微镜(SEM)、透射电子显微镜(TEM)和X射线衍射(XRD)等方法研究了镀层的生长形貌和微观结构,并考察了脉冲电流密度对镀层微观结构如晶粒尺寸、织构等的影响.结果表明:镀层内表面(基体一侧)具有比外表面(镀液一侧)更为精细的晶粒结构,说明随着厚度的增加,镀层中的晶粒逐渐粗化.随着电流密度从45 A/dm2增加到180 A/dm2,镀层中晶粒生长的择优取向由(111)织构逐渐转变为强(220)织构.当电流密度从45 A/dm2增加到120 A/dm2时,镀层平均晶粒尺寸逐渐减小;而进一步增加电流密度到180 A/dm2,镀层晶粒尺寸又会有轻微的增大.     

Finite element simulation of the micromachining of nanosized-silicon-carbide-particle reinforced composite materials based on the cohesive zone model

A finite element method based on the cohesive zone model was used to study the micromachining process of nanosized silicon-carbide-particle(SiCp) reinforced aluminum matrix composites. As a hierarchical multiscale simulation method, the parameters for the cohesive zone model were obtained from the stress-displacement curves of the molecular dynamics simulation. The model considers the random properties of the siliconcarbide-particle distribution and the interface of bonding between the silicon carbide particles and the matrix.The machining mechanics was analyzed according to the chip morphology, stress distribution, cutting temperature, and cutting force. The simulation results revealed that the random distribution of nanosized SiCp causes non-uniform interaction between the tool and the reinforcement particles. This deformation mechanics leads to inhomogeneous stress distribution and irregular cutting force variation.     

Simulation of dynamic ductile crack growth using strain-rate and triaxiality-dependent cohesive elements

Rate-sensitive and triaxiality-dependent cohesive elements are used to simulate crack growth under quasi-static and dynamic loading conditions. The simulations are performed for a middle-cracked tension M(T) specimen made of an aluminum alloy (6XXX series). To consider the effect of stress triaxiality and strain rate on the cohesive properties, a single plane strain element obeying the constitutive equations of a rate-dependent Gurson type model has been used. The single element is loaded under various stress biaxiality ratios and strain rates and the obtained stress-displacement curves are considered as traction separation law for the cohesive elements. These curves are used for analyzing the aluminum M(T) specimen. The qualitative effects of constraint, strain rate, inertia and stress waves on the energy absorption of the specimen and crack growth are discussed.     

Automatic generation of 2D micromechanical finite element model of silicon–carbide/aluminum metal matrix composites: Effects of the boundary conditions

Two-dimensional finite element (FE) simulations of the deformation and damage evolution of Silicon–Carbide (SiC) particle reinforced aluminum alloy composite including interphase are carried out for different microstructures and particle volume fractions of the composites. A program is developed for the automatic generation of 2D micromechanical FE-models with randomly distributed SiC particles. In order to simulate the damage process in aluminum alloy matrix and SiC particles, a damage parameter based on the stress triaxial indicator and the maximum principal stress criterion based elastic brittle damage model are developed within Abaqus/Standard Subroutine USDFLD, respectively. An Abaqus/Standard Subroutine MPC, which allows defining multi-point constraints, is developed to realize the symmetric boundary condition (SBC) and periodic boundary condition (PBC). A series of computational experiments are performed to study the influence of boundary condition, particle number and volume fraction of the representative volume element (RVE) on composite stiffness and strength properties.     

Yield stress of SiC reinforced aluminum alloy composites

This article develops a constitutive model for the yield stress of SiC reinforced aluminum alloy composites based on the modified shear lag model, Eshelby’s equivalent inclusion approach, and Weibull statistics. The SiC particle debonding and cracking during deformation have been incorporated into the model. It has been shown that the yield stress of the composites increases as the volume fraction and aspect ratio of the SiC particles increase, while it decreases as the size of the SiC particles increases. Four types of aluminum alloys, including pure aluminum, Al–Mg–Si alloy, Al–Cu–Mg alloy, and Al–Zn–Mg alloy, have been chosen as the matrix materials to verify the model accuracy. The comparisons between the model predictions and the experimental counterparts indicate that the present model predictions agree much better with the experimental data than the traditional modified shear lag model predictions. The present model indicates that particle failure has important effect on the yield stress of the SiC reinforced aluminum alloy composites.     

Mechanical and thermal stresses at shot particles during fatigue of Kaowool aluminum composites at 20°C

During fatigue of Kaowool fiber reinforced aluminum composites at 20°C, cracks are initiated at hollow Kaowool particles. The stress concentrations associated with these particles arise from two sources: (i) residual stresses due to differential thermal contraction of the Kaowool and aluminum and (ii) the applied cyclic fatigue stress. These stresses are calculated from a finite element model which incorporates plasticity of the aluminum matrix. In general, the mechanical stresses are considerably larger than the thermal stresses. The total stress, in both the aluminum matrix and the Kaowool particle, increases with decreasing particle wall thickness and the proximity of the particle to the surface. In general, the stress concentrations in the aluminum matrix are more critical than those in the Kaowool particles, and are predicted to exceed locally the yield strength of 339 aluminum for all values of wall thickness. The particles observed experimentally at the fatigue fracture origins are thin walled and close to the surface, in quantitative agreement with the predictions of the finite element model.     

Densification behavior of aluminum alloy powder mixed with zirconia powder inclusion under cold compaction

Densification behavior of composite powders was investigated under cold compaction. Experimental data were obtained for aluminum alloy powder mixed with zirconia powder inclusion under triaxial compression. The Cap model with constraint factors was implemented into a finite element program (ABAQUS) to simulate compaction responses of composite powders during cold compaction. Finite element results were compared with experimental data for densification behavior of composite powders under cold isostatic pressing and die compaction. The agreement between experimental data and finite element calculations from the Cap model with the constraint factors was good for composite powders with low volume fractions of inclusions.     

Dynamic ductile fracture in aluminum round bars: experiments and simulations

The application of rate-dependent cohesive elements is validated in simulation of ductile fracture in aluminum round bars under dynamic loading conditions. Smooth and notched round bars made of AA6060-T6 are tested and simulated under quasi-static and dynamic loadings. The smooth round bar is modeled using finite elements that obey Gurson–Tvergaard–Needleman (GTN) formulation as the constitutive equation. Comparing with experimental results, corresponding GTN parameters and rate-dependent plasticity of the alloy are obtained. A single strain rate-dependent GTN element with the obtained parameters is examined under different values of stress triaxiality and loading rates. The resulting stress-elongation curves represent the traction separation law (TSL) for cohesive elements and the variations of the maximum traction and the energy absorbed are investigated. The notched round bars are modeled by axisymmetric continuum and cohesive elements. The undamaged bulk material is elastic-visco plastic and the cohesive elements obey the TSL defined from the single element calculations. The experiments are simulated by these models in which the cohesive elements are rate sensitive and automatically obtain the values of the total strain rate from their adjacent continuum elements to update the values of the cohesive strength during the analysis. The results of the analysis, including maximum load, time of failure and diameter reduction are validated with the experimental results. The effects of element size, rate-dependent plasticity of the material and stress triaxiality are also discussed.     

透平膨胀机钎焊叶轮超速试验后的检验

钎焊是高速透平膨胀机铝合金闭式叶轮制造中的一种重要方法，有限元分析（FEA）方法可以了解闭式叶轮在离心力场下的应力及变形的3D分布，并预测轮盘与轮盖钎接缝上的应力大小和变化趋势，及叶轮的最大应力发生区域和最大变形产生位置，有限元分析结果表明，叶轮超速试验后，应注意轮盖内孔钎接区域附近微裂纹的检测，同时在轮盖内孔及外缘处测量超速后的残余弹性变形。     

Modelling of microstructure evolution during hot rolling of AA5083 using an internal state variable approach integrated into an FE model

Hot rolling, a critical process in the manufacturing of aluminum sheet products, can significantly impact the final properties of the cold rolled sheet. In this research, a mathematical model was developed to predict the through-thickness thermal and deformation history of a sheet undergoing single stand hot rolling using the commercial finite element (FE) package, ABAQUS^™. A physically based internal state variable microstructure model has been incorporated into the FE simulation for an AA5083 aluminum alloy to predict the evolution of the material stored energy and the subsequent recrystallization after deformation is complete. The microstructure predictions were validated against experimental measurements conducted using the Corus pilot scale rolling facility in IJmuiden, the Netherlands for an AA5083 aluminum alloy. The model was able to predict the fraction recrystallized as well as the recrystallized grain size reasonably well under a range of industrially relevant hot deformation conditions. A sensitivity analysis was carried out to determine the influence of changing the material constants in the microstructure model and deformation conditions on the predicted recrystallization behaviour. The analysis showed that the entry temperature was the most sensitive process parameter causing significant changes in the predicted driving force for recrystallization, nucleation density, fraction recrystallized, and recrystallized grain size.     

Study on the tribological properties between the aluminum alloy 7050-T7451 and the YG8 cemented carbide

The ball–disc friction experiments are performed by applying the YG8 cemented carbide ball and the aluminum alloy 7050-T7451 plate under high speed and high load, and then the wear depth of the groove was observed using white light interferometer. The average contact normal stress was calculated by the depth of the groove. The relationship between friction coefficient and average contact normal stress as well as sliding velocity is fitted with Minitab. It shows the friction coefficient decreases with the increasing of the sliding velocity and the average contact stress. The results provide certain theoretical basis to build the friction model and finite element model of machining Al alloy.