颗粒尺寸对颗粒增强型金属基复合材料动态特性的影响

利用有限元模型分析了颗粒增强型金属基复合材料 ( PMMCs ) Al/SiC的颗粒尺寸对复合材料在不同应变率下的动态特性的影响。采用有限元三维立方体单胞模型嵌入单个和多个球形增强颗粒,颗粒直径分别为16 μ m和7.5 μ m,多颗粒模型内部颗粒随机分布。基体材料假设为弹塑性,应变强化及应变率强化均符合指数规律。模拟结果表明:颗粒尺寸、颗粒体积含量及应变率对金属基复合材料的动态特性的影响是相互耦合的。颗粒体积含量一定时,颗粒尺寸越小,复合材料流动应力越高;颗粒含量越高,材料流动应力越高;应变率越高,材料流动应力越高。      

C/C复合材料压缩破坏的应变率效应研究

研究了碳布叠层/碳复合材料在四种不同应变率下的压缩性能, 对其在准静态、动态载荷下的压缩破坏机理进行了初步的探讨. 研究结果表明: C/C复合材料的压缩破坏强度具有较强的应变率效应, 与准静态(10^-4/s)相比, 复合材料的动态(1.5×10²/s)压缩强度可提高70%左右; 复合材料在准静态、动态载荷下力学性能的差异可归结为纤维与基体界面特性的应变率效应以及不同应变率下破坏模式的不同.     

高体积含量颗粒增强复合材料的一个细观力学模型Ⅱ: 弹塑性与损伤分析

在高体积含量颗粒增强复合材料细观弹性分析的基础上, 引入了细观塑性和细观损伤模型: 基体用服从Von Mises 屈服准则的理想弹塑性材料模拟, 用沿圆柱形基体轴线方向的平均应力(即对称面上的应力) 来判断基体的屈服, 并将基体的塑性部分简化为圆柱状轴对称区域。建立了基体和颗粒/ 基体界面统一的损伤准则, 该准则同时考虑了最大应变和三轴应力的影响, 通过对细观塑性和细观损伤在空间取向上的平均, 建立了材料宏观模量的折减法则。用该细观力学模型, 数值模拟了一种实际金属基复合材料的强度实验, 理论模型与实验结果吻合。      

颗粒增强金属基复合材料的热导率

本文研究了颗粒增强铝合金(LD₂)复合材料的热导率与颗粒的种类、含量、粒度和在基体中的分布情况之间的关系。制作了不同颗粒含量和粒度的SiCp/LD2和Al₂O₃p/LD₂复合材料试样,用定常热流法测定其热导率。将测试结果与用混合热阻模型推得计算式提供的理论值进行比较,根据测试值和理论值比较的结果,对该计算式提出考虑到颗粒粒度对复合材料热导率影响的修正。得到综合考虑了颗粒的种类、含量、粒度和在基体中均匀分布程度影响的颗粒增强金属基复合材料热导率的理论计算式。     

40vol%SiC_P/2024Al复合材料的动态压缩性能

利用分离式霍普金森压杆(SHPB)研究了40%体积分数的SiCP/2024Al复合材料和基体材料2024Al在不同应变率下的动态压缩性能。在高应变率动态压缩时该复合材料与2024Al均表现出应变率不敏感,复合材料屈服应力高于2024Al;与2024Al的应变硬化性能不同,复合材料表现出应变软化性能。利用扫描电镜(SEM)观察动态压缩后复合材料试件的微观组织,发现试件内部出现一些孔洞、微裂纹以及一些增强颗粒的破碎等损伤现象,并且在较高应变率下基体呈现出明显的热软化甚至发生局部熔化,由此判断,在高应变率下SiCP/2024Al复合材料宏观应变软化的机制为内部损伤及基体热软化。将SiCP/2024Al复合材料与2024Al经400℃下烧蚀3 h后自由冷却至室温,利用SHPB再次进行测试,与烧蚀前的测试结果相比,2024Al的性能明显下降,而复合材料的性能变化较小,表现出比基体材料更好的抗高温稳定性能。     

磁致伸缩复合材料性能的影响因素分析

首先由退磁场效应分析得到有效磁场强度与外磁场强度的关系,继而根据复合材料细观力学中的Eshelby等效夹杂理论和Mori-Tanaka方法导出了颗粒磁场伸缩应变与复合材料磁致伸缩应变的关系,结合Terfenol-D颗粒磁致应变及弹性模量与有效磁场强度的关系,最终预测了环氧基Terfenol-D复合材料磁致应变及有效弹性模量与外磁场强度的关系,并分析了颗粒含量、形状及基体弹模对复合材料饱和磁致伸缩系数及弹性模量的影响.结果表明,磁致伸缩复合材料的饱和磁致伸缩应变随颗粒含量、纵横比增大而增大,随基体弹模增大而减小;有效弹性模量随颗粒含量、颗粒纵横比、基体弹模的增大而增大;颗粒的纵横比越大、含量越大,复合材料的饱和磁场强度越小,磁致伸缩应变随磁场强度的变化越快;复合材料的有效弹性模量亦随磁场强度的增大而增大,其影响程度在颗粒体积含量和颗粒纵横比较大时尤为显著.     

二氧化锡颗粒增强银基复合材料的电阻率

根据复合材料的传导理论ｇ－ＭｅＯ系金属基复合材料电阻率的计算模型，并与Ａｇ－Ｓ的实验结果进行了对比。结果分析表明，颗粒增强金属基复合材料的电阻率不仅与基体和颗粒材料的电阻率及颗粒的体积分数有关，还与颗粒粒度及在基体中的分布状态有关。     

石墨烯纳米片增强铝基复合材料的动态力学行为

采用压制-烧结-热挤压工艺制备石墨烯纳米片(GNFs)增强铝基(Al)复合材料,并对其进行压缩性能测试。结果表明:GNFs/Al复合材料是应变率敏感材料,当应变率从10^-3s^-1提高至3×10~3s^-1时,复合材料的强度明显提高;而当应变率继续提高至5×10~3s^-1时,由于材料内部发生热软化,复合材料的强度反而表现出少许下降。动态压缩后复合材料中铝基体发生动态再结晶,且应变率越高,动态再结晶越显著;增强相GNFs则发生扭曲变形后仍保持完整结构且与基体间保持原子间结合。因此,GNFs/Al复合材料具有良好的动态压缩塑性。     

动态载荷下C/C复合材料的压缩破坏行为

利用电子万能试验机以及Split Hopkinson Compressive Bar(SHPB)测试了2DC/C复合材料在准静态、动态载荷下的压缩性能,结合光学显微镜分析了其在不同应变率下的破坏形貌、讨论了应变率对压缩破坏形貌的影响。结果表明:与准静态(10-4/s)相比,动态载荷下(5×102/s)复合材料的压缩强度提高了55%,压缩刚度提高了66%,具有较强的应变率效应;在准静态载荷下,C/C复合材料沿40°角剪切破坏,断口上炭纤维破坏具有溃散及剪切破坏特征,而在动态载荷下,C/C复合材料破坏成大小不一的碎片,其炭纤维破坏具有劈裂特征。C/C复合材料破坏模式的不同可归结为基体及界面强度的应变率效应。     

2A16铝合金中应变率力学性能研究

为研究2A16铝合金的中应变率力学性能及热处理状态对其应变率敏感性的影响,利用电子万能试验机和高速液压伺服试验机对其(O状态和T4状态)进行常温下准静态和中应变率力学性能试验,得到不同应变率下的应力应变曲线,并基于修正的Johnson-Cook本构模型对其进行拟合。结果表明:在应变率10~(-4)~10~2s~(-1)内,热处理状态对2A16铝合金的应变率敏感性有较大影响,其中2A16-O状态铝合金的应变率敏感性较强,而2A16-T4状态铝合金的应变率敏感性较弱,但两种材料均具有较强的应变硬化效应;此外,修正Johnson-Cook本构模型的拟合结果与试验结果吻合很好,能够很好表征材料的动态力学行为。     

Numerical analysis of the stress–strain distributions in the particle reinforced metal matrix composite SiC/6064Al

The axisymmetric cell model consisting of interface, matrix and reinforced particle is used to simulate the tensile test of particle reinforced metal matrix composite for predicting the micro stress/strain field and macro tensile stress/strain curve. In simulation of the tensile test, the cohesive element model is selected to model interfacial crack growth. It mainly analyzed the effects of interfacial properties, reinforcement volume fractions and aspect ratios on the stress–strain states of particle reinforced metal matrix composite. The results show that the peak micro stress and plastic strain occur at the interface in which it is a certain angle from the tensile stress direction; with the interfacial fracture toughness and reinforcement volume fraction increasing, the flow stress increases firstly and then decreases. The tensile stress–strain properties of SiC/6064Al are good when the interfacial fracture toughness is equal to 60 J/m and the reinforcement fraction volume is equal to 20%. Smaller reinforcement aspect ratio leads to smaller micro stress in composites.     

基于离散元法的脆性岩石细观蠕变失稳研究

为从细观角度探究脆性岩石的蠕变失稳过程及失稳机理,该文基于三维颗粒流程序（PFC3D）考虑岩石的时效变形损伤过程,引入岩石细观单元时效损伤的应力腐蚀模型,建立了基于离散元方法的岩石时效变形损伤破裂模型,并通过单轴压缩及单轴蠕变的室内实验和数值模拟对比验证了所建立的时效变形损伤破裂模型的合理性。数值模拟再现了岩石的初始蠕变、稳态蠕变和加速蠕变三个蠕变阶段,同时模拟结果表明,在单级加载条件下,随着应力水平提高,稳态蠕变应变率显著增大,岩石蠕变失效时间逐渐缩短,初始轴向应变、初始侧向应变和初始体应变不断增大,且细观裂纹扩展形式与单轴压缩破坏形式基本相同,都是以拉伸裂纹为主,裂纹的增长速率随着时间增加而不断增大,尤其在第三蠕变阶段裂纹增长速率迅速增大;在分级加载试验过程中,模型的轴向应变、侧向应变和体应变以及裂纹最终扩展形态与单级加载基本相同;此外将三维蠕变模拟结果与二维模拟结果进行对比,结果显示三维模型拟合程度更高。     

Identification of Constitutive Material Model Parameters for High-Strain Rate Metal Cutting Conditions Using Evolutionary Computational Algorithms

Advances in plasticity-based analytical modeling and finite element methods (FEM) based numerical modeling of metal cutting have resulted in capabilities of predicting the physical phenomena in metal cutting such as forces, temperatures, and stresses generated. However, accuracy and reliability of these predictions rely on a work material constitutive model describing the flow stress, at which work material starts to plastically deform. This paper presents a methodology to determine deformation behavior of work materials in high-strain rate metal cutting conditions and utilizes evolutionary computational methods in identifying constitutive model parameters. The Johnson-Cook (JC) constitutive model and cooperative particle swarm optimization (CPSO) method are combined to investigate the effects of high-strain rate dependency, thermal softening and strain rate-temperature coupling on the material flow stress. The methodology is applied in predicting JC constitutive model parameters, and the results are compared with the other solutions. Evolutionary computational algorithms have outperformed the classical data fitting solutions. This methodology can also be extended to other constitutive material models.     

Strain gradients and continuum modeling of size effect in metal matrix composites

Summary Constitutive modeling for the particle size effect on the strength of particulate-reinforced metal matrix composites is investigated. The approach is based on a gradient-dependent theory of plasticity that incorporates strain gradients into the expression of the flow stress of matrix materials, and a finite unit cell technique that is used to calculate the overall flow properties of composites. It is shown that the strain gradient term introduces a spatial length scale in the constitutive equations for composites, and the dependence of the flow stress on the particle size/spacing can be obtained. Moreover, a nondimensional analysis along with the numerical result yields an explicit relation for the strain gradient coefficient in terms of particle size, strain, and yield stress. Typical results for aluminum matrix composites with ellipsoidal particles are calculated and compare well with data measured experimentally.     

原位自生TiB2颗粒增强2024-T4铝基复合材料断裂行为数值模拟

利用SEM结合原位观测技术观察了颗粒体积分数为4.17%的原位自生TiB₂颗粒增强2024-T4铝基复合材料（TiB₂/2024-T4）的损伤断裂行为。试验结果表明,TiB₂颗粒偏聚带中的铝合金基体比颗粒稀疏区域中的铝合金基体率先发生断裂。根据这一试验现象建立了三种含随机颗粒偏聚带的二维体胞有限元模型,并施加拉伸载荷和周期性边界条件,推导了平面应力状态下的径向返回算法,结合Rice-Tracey局部失效准则模拟了颗粒偏聚带中微裂纹的萌生及扩展过程。数值分析结果表明：就单个颗粒来说,颗粒两极附近基体损伤最严重。颗粒偏聚导致损伤在颗粒附近基体中迅速累积,并发展成为基体微裂纹,且随着颗粒偏聚程度加剧,材料断裂应变下降。另外,体胞模型应力-应变曲线的非线性部分低于实测曲线,说明除了本文模型反映的载荷传递强化机制外,还需要进一步考虑颗粒对基体的间接强化机制。     

Direct numerical simulation of ductile spall failure

Large-scale direct numerical simulations of void growth and coalescence from 3-dimensional distributions of void nucleating particles are used to investigate the effect of material strain hardening and strain rate sensitivity on spall response. The computational model spans multiple particle spacings in the in-plane directions, and several finite elements span the initial particle diameters in the mixed-zone Arbitrary Lagrange–Eulerian (ALE) simulations. The matrix material is represented by traditional plasticity models in which material failure is not permitted. The 1000\(+\) particles are represented by the same material model as the surrounding matrix except the particles have low tensile strength to permit fracture, which is used to simulate particle cracking or decohesion. Voids grow and coalesce naturally in the ALE framework, and the simulations produce dimpled failure surfaces similar to those observed experimentally in spalled samples. The strain hardening and strain rate sensitivity of the matrix material are altered to explore their influence on the void growth and coalescence processes and on the simulated free surface velocity. The details available from the computational model permit association of the longitudinal stress evolution with features on the free surface velocity profile.     

Modeling fracture of fiber reinforced polymer

In the present paper 3D rate sensitive constitutive model for modeling of laminate composites is presented. The model is formulated within the framework of continuum mechanics based on the principles of irreversible thermodynamics. The matrix (polyester resin) is modeled by employing a 3D rate sensitive microplane model. For modeling of fibers (glass) a uni-axial constitutive law is used. The fibers are assumed to be uniformly smeared-out over the matrix. The formulation is based on the assumption of strain compatibility between matrix and fibers. Total stress tensor is additively decomposed into the contribution of matrix and fibers, respectively. To model de-lamination of fibers, the matrix is represented by periodically distributed initial imperfection over the pre-defined bands, which are parallel to fibers. Physically, this assumption accounts for the matrix-fiber interface in a smeared way. The input parameters of the model are defined by the mechanical properties of matrix and fibers (elastic properties, strength and fracture energy), the volume fraction of fibers and by their spatial orientation. The model is implemented into a 3D finite element code. To assure mesh objective results crack band method is employed. The model is first calibrated using a few basic test results. Subsequently, the model is validated with several numerical examples for specimens loaded in uni-axial tension, uni-axial compression and shear. Comparison between numerical and test results shows that the proposed model is able to predict the resistance and failure mode of complex fiber-reinforced composite for different orientation of fibers and different loading conditions with sufficient accuracy. Finally, based on the qualitative type of the finite element analysis, it is demonstrated that the strain rate dependency becomes more important when the angle between the fiber and load direction increases.     

Corrugation Reinforced Composites: A Method for Filling Holes in Material‐Property Space

Material‐property space is filled with holes representing desirable combinations of properties, such as high strength and high necking strain. One way to fill those holes is to use architectured materials. In this work, Finite Element Modeling (FEM) simulations are performed to evaluate composites with a corrugated reinforcement architecture across a range of volume fractions and corrugation heights for a model copper‐steel system. The corrugated reinforcement geometry shows large improvements in necking strain, which increases with corrugation height, without sacrificing strength, and fills a desirable region in material‐property space. Additionally, it is found that the necking strain of a matrix material can be increased by adding a less ductile reinforcing material provided it has a highly corrugated geometry. The improvement in necking strain seen in these composites is attributed to a boost in work hardening that results from an evolving reinforcement alignment as the corrugation unbends.     

Strengthening mechanism of load sharing of particulate reinforcements in a metal matrix composite

A 15 v% SiC particle reinforced Al-2618 matrix composite was selected to study strengthening mechanisms under different heat treatments to produce specimens in hard or soft matrices. The investigation showed that the conventional micro-mechanism models play a minor role in strengthening the composite by further addition of the SiC particles. A load sharing mechanism of the particulate reinforcements is suggested to explain the experimental yield strength increase. An analytical model based on Eshelby equivalent inclusion approach and Mori–Tanaka mean field extension was established by introducing numerical matrix and composite secant moduli to simulate the stress–strain curve of the composite. The same modeling work was also carried out by FEM analysis based on the unit cell model using a commercial ANSYS code. The modeling results by both models on evolution of the load carried by the SiC particles during straining provide strong evidences to back up the strengthening mechanism of the load sharing. However, the modeling work exposes that the load transfer mechanism plays a dominant role only for the composite with hard matrix and the reason for load transfer is mainly the mismatch strain between particulate reinforcement and matrix rather than commonly believed friction at their interfaces. Nevertheless, an experiment was used to estimate average stress level in the SiC particles by observation of the numbers of broken particles in the composite with different strains, which also offers a good support to the modeling work.