On the homogenization of metal matrix composites using strain gradient plasticity

The homogenized response of metal matrix composites（MMC） is studied using strain gradient plasticity.The material model employed is a rate independent formulation of energetic strain gradient plasticity at the micro scale and conventional rate independent plasticity at the macro scale. Free energy inside the micro structure is included due to the elastic strains and plastic strain gradients. A unit cell containing a circular elastic fiber is analyzed under macroscopic simple shear in addition to transverse and longitudinal loading. The analyses are carried out under generalized plane strain condition. Micro-macro homogenization is performed observing the Hill-Mandel energy condition,and overall loading is considered such that the homogenized higher order terms vanish. The results highlight the intrinsic size-effects as well as the effect of fiber volume fraction on the overall response curves, plastic strain distributions and homogenized yield surfaces under different loading conditions. It is concluded that composites with smaller reinforcement size have larger initial yield surfaces and furthermore,they exhibit more kinematic hardening.

ONSET CONDITION OF STRAIN LOCALIZATION IN MATRIX OF SATURATED POROUS MEDIA

Based on governing equations of saturated porous media and Liapunov＇ s stability here, onset conditions matrix of porous media used by solid stress and Terzaghi＇s effective stress constitutive description under seepage flow state, are presented, which have different forms with different representation of the solid phase, matrix or skeleton, constitutive model of porous media. The main difference relates with how to describe the interaction between solid phase and liquid phase in constitutive model. The derived onset condition of strain localization under Terzaghi＇ s effective stress description can be used to interpret different failure types, piping effect, landslides and mudflows, by means of the type and the magnitude ratio of relative movement between solid phase and liquid phase. Examples here illuminate the onset condition of how to work.

TiO_(2)光催化高延性水泥基材料拉伸性能和光催化性能研究

随着大气污染给建筑物及居住环境带来的恶劣影响日益严重,TiO_(2)光催化水泥基材料广受关注。利用光催化剂纳米TiO_(2)颗粒制备光催化高延性水泥基材料(PC-ECC),研究其单轴拉伸应力作用下力学性能和光催化性能;结合微观力学模型及孔结构分析纳米TiO_(2)颗粒对PC-ECC延性的作用机理。结果表明:掺0~15%纳米TiO_(2)颗粒的PC-ECC在龄期28 d时均具有显著的应变硬化特征和优异的多缝开裂能力,拉伸应变在掺1%纳米TiO_(2)颗粒时出现最低值3.48%,之后升至4.55%;宏观拉伸应变随纳米TiO_(2)颗粒掺量变化趋势与应变硬化指标J_(b)’/J_(tip)一致,且后者与孔结构等微观结构紧密相关;掺5%纳米TiO_(2)颗粒的PC-ECC经72 h紫外光照后光降解亚甲基蓝效率达95%,符合一级反应动力学模型。

基于FBG的辊压机辊面非均匀应变分段重构研究

为实现对辊压机辊面内部损伤的监测,传统的重构方法不能较好地重构出结构损伤时应变局部突变的特点,文中提出了一种基于FBG传感的非均匀应变分段重构的方法。结合传输矩阵法推导了分段重构方法的可行性,改进差分进化算法对不同工况下的应变分布进行分段重构仿真与分析。结果表明:在正常工作状态下,分段与未分段重构方法都可以较好地重构非均匀应变;在损伤状态下,分段重构方法得到的应变均方根误差为4.505με,小于未分段重构方法的18.991με。说明分段重构方法具有一定的泛化能力,可以适应不同辊面工况下的非均匀应变重构。

High strain rate superplasticity of SiC whisker reinforced pure aluminum composites

A β SiC whisker reinforced pure aluminum composites expected to exhibit high strain rate superplasticity has been successfully fabricated by a new processing route consisting of pressure infiltration, extrusion with a low extrusion ratio and rolling. The composites exhibite a total elongation of 220%～380% in the initial strain rates within 1.0×10 -2 ～1.0×10 -1 s -1 and at 893～903 K. According to differential thermal analysis(DTA) and microstructure observation, it is concluded that an appropriately small amount of liquid phase is necessary to cause a good high strain rate superplasticity in aluminum matrix composites in addition to fine and uniform microstructure.

HIGH STRAIN RATE SUPERPLASTICITY IN A SiC_w/2024Al COMPOSITE MADE BY SQUEEZE CASTING

In this paper,the superplastic characteristics of the beta-SiC whisker reinforced 2024aluminum composite, fabricated by squeeze casting and hot-rolling after extrusion were investigated. The compsite had a fine grain size of about 2μm, and exhibited a strain rate sensitivity of about 0.35 and a maximum elongation of 350% at an initial strain rate of 1.1×10-1s-1 at 803K. In addition, the superplastic deformation mechanisme of the composite were also examined.

Lipopeptide Antibiotics Produced by the Engineered Strain Bacillus subtilis GEB3 and Detection of Its Bioactivity

MALDI-TOF-MS technology was used for identification of lipopeptide antibiotics producedby GEB3 strain, a derivative of Bacillus subtilis 168 which was transformed by lpaB3gene. The result showed GEB3 only produced lipopeptide antibiotic surfactin. The analysisby LC-MS demonstrated that GEB3 produced standard surfactin isoforms with side chainlengths of 13,14 and 15 carbon atoms. The bioactivity detection of surfactin indicatedthat the surfactin produced by GEB3 had inhibition effect on plant pathogens Rhizoctoniasolani and Pyricularia oryzae.

PARTICULATE SIZE EFFECTS IN THE PARTICLE-REINFORCED METAL-MATRIX COMPOSITES

The influences of I,article size on the mechanical properties of the particulate metal matrix composite;are obviously displayed in the experimental observations. However, the phenomenon can not be predicted directly using the conventional elastic-plastic theory. It is because that no length scale parameters are involved in the conventional theory. In the present research, using the strain gradient plasticity theory, a systematic research of the particle size effect in the particulate metal matrix composite is carried out. The roles of many composite factors, such as: the particle size, the Young's modulus of the particle, the particle aspect ratio and volume fraction, as well as the plastic strain hardening exponent of the matrix material, are studied in detail. In order to obtain a general understanding for the composite behavior, two kinds of particle shapes, ellipsoid and cylinder, are considered to check the strength dependence of the smooth or non-smooth particle surface. Finally, the prediction results will be applied to the several experiments about the ceramic particle-reinforced metal-matrix composites. The material length scale parameter is predicted.

A study of interparticulate strain in a hot-extruded SiC_p/2014 Al composite

We report a correlative study of strain distribution and grain structure in the Al matrix of a hot-extruded SiC particulate-reinforced Al composite(SiC_p/2014 Al). Finite element method(FEM) simulation and microstructure characterization indicate that the grain structure of the Al matrix is affected by the interparticulate strain distribution in the matrix during the process. Both electron-backscattered diffraction(EBSD) and selected-area electron diffraction(SAED) indicated localized misorientation in the Al matrix after hot extrusion. Scanning transmission electron microscopy(STEM) revealed fine and recrystallized grains adjacent to the Si C particulate and elongated grains between the particulates. This result is explained in terms of recrystallization under an interparticulate strain distribution during the hot extrusion process.

Expression of strain tensor in orthogonal curvilinear coordinates

Based on an analysis of connotation and extension of the concept of the orthogonal curvilinear coordinates, we have deduced a platform of strain tensor expression of Cartesian coordinates, which turns out to be a function of Lame coefficient and unit vector. By using transform matrix between Cartesian coordinates and orthogonal eurvilinear coordinates, we have deduced a mathematical expression for correcting displacement vector differential in orthogonal curvilinear coordinates, and given a general expression of strain tensor in orthogonal curvilinear coordinates.

Continuous Deformation Monitoring by Polymermatrix Carbon Fiber Sensitive Layer

Composite made of short-cut carbon fiber mat and vinyl ester resin was observed to be an effective sensor for tensile strain up to 6 000με. Based on its strain sensitivity, a skin-like sensitive layer which can continuously cover the structural surface to sense strain in large area was developed. The sensitive layer was applied to continuously monitor the deformation of a simply supported beam. The result indicates that the fractional change in electrical resistance of the sensitive layer reversibly reflects the beam deformation in each section and describes the distribution of the average strain of the beam. The effect of temperature change on the monitoring was studied by monitoring tests conducted at different temperatures ranging from 20 to 80 ℃, which reveals temperature sensitivity in the sensitive layer and the temperature dependence of the piezoresistive behavior when the temperature exceeds 50 ℃. By the application of differential conaection principle, a method for temperature compensation was established and the gauge factor for the monitoring was dramatically increased. This method was verified experimentally.

High strain rate superplasticity of TiN_p/1N90Al composite prepared by powder metallurgy method

TiN_p/1N90Al composite was fabricated by powder metallurgy method with a reinforcement volume fraction of 15%. The tensile experiment, DSC and SEM were used to study the high strain rate superplasticity of the TiN_p/1N90Al composite. The DSC result shows that the incipient melting temperature of the TiN_p/1N90Al composite is 906K. The tensile tests were carried out over a range of deformation temperature from 918 to 928K and strain rate from 1.7 to 1.7×10 -3 s -1. The maximum elongation of 201% is realized at 923K with a strain rate of 1.7×10 -1 s -1. Otherwise all the elongations are higher than 100% in the strain rate range of 3.3×10 -2-6.7×10 -1 s -1 at the three deformation temperatures. The curves of m value of the TiN_p/1N90Al composite can be divided into two stages with the variation of strain rate at different deformation temperatures and the critical strain rate of 10 -1 s -1. When the strain rate is higher than 10 -1 s -1, the m values of the three curves are smaller than 0.3, but the m values of the three curves are about 0.37 when the strain rate is higher than 10 -1 s -1.

Hot Deformation Behavior of Squeeze Casting SiC_p/2 A50 Matrix Composites

The flow stress behaviors of squeeze casting SiCp/2A50 matrix composites were investigated by means of compression tests on a Gleeble 1500 therma1 mechanical simulator at isothermal constant strain rates ranging from of 0.001 to 1.0 with the testing temperature ranging from 350 to 500 ℃. The experiments showed that the relationship between stress and strain was obviously influenced by the strain rate and temperature. Dynamic recrystallization generally occurred at a higher temperature and a 1ower strain rate. A linear equation could be fitted between the Zener-Hollomon parameter Z and stress in the experiments. The mean value reciprocal of temperature at every true strain had a linear relation with natural logarithm of Z parameter, and the correlation coefficient, R=0.99, which was very significant by examination. The hot deformation activation energy of SiCp/2A50 matrix composites was 163.47 KJ/mol by calculation.

Algorithmic tangent modulus at finite strains based on multiplicative decomposition

The algorithmic tangent modulus at finite strains in current configuration plays an important role in the nonlinear finite element method. In this work, the exact tensorial forms of the algorithmic tangent modulus at finite strains are derived in the principal space and their corresponding matrix expressions are also presented. The algorithmic tangent modulus consists of two terms. The first term depends on a specific yield surface, while the second term is independent of the specific yield surface. The elastoplastic matrix in the principal space associated with the specific yield surface is derived by the logarithmic strains in terms of the local multiplicative decomposition. The Drucker-Prager yield function of elastoplastic material is used as a numerical example to verify the present algorithmic tangent modulus at finite strains.

Analysis of Adiabatic Shearing Failure Mechanism for Aluminum Matrix Composites Based on Experimental and Numerical Simulation

Adiabatic shear behavior and the corresponding mechanism of TiB2/Al composites were researched by split Hopkinson pressure bar （SHPB）.Results show that the flow stresses of the TiB2/Al composites exhibit softening tendency with the increasing of strain rates. All the composites fail in splitting and cutting with a 45 degree, and the phase transformed bands of molten aluminum are found on the adiabatic shear layers. The deformation behavior and shear localization of the TiB2/Al composites specimens were simulated by finite element code MSC.Marc. The Johnson-Cook model was used to describe the thermo-viscoplastic response of the specimen material. There was unanimous between the numerical result and the experimental result on the location of the adiabatic shear band. From the numerical simulation and experiment, it was concluded that the instantaneous failure of the composite was ascribed due to the local low strength area where the formation of adiabatic shear band was, and the stress condition had significant effect on the initiation and propagation of adiabatic shear band （ASB）.

Sensing Traction Strain Induces Cell-Cell Distant Communications

Mechanobiology has been a highly recognized field in studying the importance of physical forces in physiologies at the molecular,cellular,tissue,organ and body-levels.Beside the intensive work focusing on the fine local biomechanical forces,the long-range force which can propagate through a relatively distant scale(in hundreds of micrometers and beyond)has been an intriguing topic with increasing attentions in recent years.The collective functions at cell population level often rely on cell-cell communications with or without direct contacts.Recent progresses including our own work indicate that the long-range biomechanical force propagating across scales far beyond single cell size may reserve the capability to trigger coordinative biological responses within cell population.Whether and how cells communicate mechanically in a distant manner remains largely to be explored.In respiratory system,the mechanical property of airway smooth muscle(ASM)is associated with asthma attack with prolonged contraction during airway hyper-responsiveness.In this work,we found that ASM cells rapidly self-assembled into a well-constructed network on 3D matrigel containing type I collagen(COL I),which required the collective functions and coordination of thousands of cells completed within 12-16 hours.Cells were assembled with aligned actin stress fibers and elongated nuclei.The assembling process relied on the long-range mechanical forces across the matrix to direct cell-cell distant interactions.We further found that single ASM cells could rapidly initiate multiple buds precisely pointing to neighboring cells in distance,which relied on cell traction force and force strain on the matrix.Beads tracking assay demonstrated the long-range transmission of cellular traction force to distant locations,and modeling of maximum strain distribution on matrix by finite element method predicted the consistency with cell directional protrusions and movements in experiments.Cells could sense each other in distance to move directionally on both non-fibrous matrigel and in much more efficient way when containing COL I.Cells recruited COL I from the hydrogel to build nearly identical COL I fibrous network to mechanically stabilize the cell network.Our results revealed that ASM cells can sense the traction strain transmitted through matrix to initiate distant communications and rapidly coordinate the network assembly at the population level through active cell-matrix interactions.As an interesting phenomenon,cells sound able to’make phone call’via the role of long-range mechanical force.In summary,this work demonstrated that long-range biomechanical force facilitates the collective functions of ASM cell population for network assembly.The cells reacted to traction strain on the matrix for distant communications,which resulted in directional budding and movement.Fibrous COL I had important roles in facilitating the efficiency of force transmission to induce the assembly and stabilizing the cell network.This work has helped advance the understanding of the feature andfunction of long-range biomechanical force at the cell population level.The observed high mechano-sensitivity of ASM cells might suggest a re-enforced feedback of enhanced contraction by excessive ASM under asthmatic condition.

刚性基体高应变复合材料在空间可展结构中的应用及发展

梳理了空间可展结构的分类及发展趋势,从铰接可展结构-带簧铰链、杆状可展结构、面状可展结构及体可展结构四个方面调研了刚性基体高应变复合材料在空间可展结构中的应用现状,从力学和黏弹性特性方面提出了其在可展结构中的应用优势及需要考虑的问题。高模量、低密度、低膨胀系数的刚性基体高应变复合材料是空间可展结构的应用方向,而刚性基体高应变复合材料可展结构的设计方法,以及将其用于大尺寸可展结构的低成本和高精度制造工艺仍有很大的探索空间。

多壁碳纳米管/天然橡胶复合材料的大应变传感特性研究

制备了一种多壁碳纳米管改性天然橡胶复合材料,制备过程中通过加入表面活性剂十六烷基三甲基溴化铵(CTAB)改善碳纳米管在橡胶基体中的分散性能,降低了碳纳米管含量的渗流阈值,更有效地构建导电网络。通过实验研究发现复合材料的拉伸强度和刚度随碳纳米管含量增加而上升;材料的电阻/应变敏感因子在碳纳米管含量4%时最高;材料在循环载荷作用下,电阻和应变保持同步变化,并在一定循环次数后达到稳定,表现出高灵敏、高稳定的响应特性,证明该复合材料适用于大变形构建的应变监测与传感。

New Formula for Geometric Stiffness Matrix Calculation

The standard formula for geometric stiffness matrix calculation, which is convenient for most engineering applications, is seen to be unsatisfactory for large strains because of poor accuracy, low convergence rate, and stability. For very large compressions, the tangent stiffness in the direction of the compression can even become negative, which can be regarded as physical nonsense. So in many cases rubber materials exposed to great compression cannot be analyzed, or the analysis could lead to very poor convergence. Problems with the standard geometric stiffness matrix can even occur with a small strain in the case of plastic yielding, which eventuates even greater practical problems. The authors demonstrate that amore precisional approach would not lead to such strange and theoretically unjustified results. An improved formula that would eliminate the disadvantages mentioned above and leads to higher convergence rate and more robust computations is suggested in this paper. The new formula can be derived from the principle of virtual work using a modified Green-Lagrange strain tensor, or from equilibrium conditions where in the choice of a specific strain measure is not needed for the geometric stiffness derivation (which can also be used for derivation of geometric stiffness of a rigid truss member). The new formula has been verified in practice with many calculations and implemented in the RFEM and SCIA Engineer programs. The advantages of the new formula in comparison with the standard formula are shown using several examples.

考虑页岩基质不同孔隙特征的表观渗透率模型

页岩基质含气量大、供气时间长,其渗透率随地层孔隙压力变化的演化直接影响气藏的长效产气能力。基质中微纳米孔隙发育,有机孔隙和无机孔隙的力学性质和流体传输机理各异,有效应力和流态变化都将影响其表观渗透率。此外,有机质中的吸附气使岩体发生吸附变形也不同程度地影响表观渗透率。基于孔弹性理论,考虑有机孔隙和无机孔隙中不同岩石力学参数、流体流动机理以及有机孔隙吸附变形的特点,建立了适用于页岩基质的表观渗透率模型,分析了不同孔隙参数对表观渗透率的影响,揭示了表观渗透率演化的主控因素。结果表明,在定压注气和采气过程中,页岩基质表观渗透率的演化均与时间有关,主要受稀薄气体效应、有效应力变化以及有机孔隙周围局部吸附膨胀或解吸附收缩影响。表观渗透率演化过程的主控因素随时间变化,同时与有机孔隙、无机孔隙的体积占总孔隙体积的比例有关。