Derivation of strain gradient length via homogenization of heterogeneous elastic materials

We present explicit upper bound estimates of the microstructural length used in simple gradient elasticity. Our model is a two dimensional composite made of circular hard inclusions randomly dispersed in a soft matrix. Both inclusions and matrix are described by isotropic linear elastic constitutive laws. The composite, however, is described by an isotropic gradient elastic law. The elastic modulus and the Poisson’s ratio are given by the exact classic analysis of Christensen. The in-plane microstructural length is estimated by energy optimization, based on solutions of the gradient elastic hollow cylinder. It was shown that the microstructural length decreases with the composition of the particles, taking high values at low particle composition. Naturally, the microstructural length is proportional to the particle diameter and increases with the stiffness of the particles. It was shown that there can be no microstructural prediction for particles that are softer than the matrix. This interesting result seems to be complementary to the result of Bigoni and Drugan who found that, for the couple-stress composite model, there can be no prediction for the microstructural length when the particles are stiffer than the matrix.     

Mechanical properties of novel aluminum metal matrix metallic composites: Application to overhead conductors

The mechanical behavior and microstructural evolution of aluminum metal matrix metallic composites fabricated under various process conditions were investigated to understand their process-structure–property relations. The novel techniques for arranging the matrix and reinforcement materials and controlling the processing atmosphere were applied to the extrusion process. The composites were comprised of matrix 1050 and reinforcement 6061 aluminum alloys with varying percent weight compositions and were arranged in a tailorable concentric annular pattern. The composites were shown to substantially increase compressive strength when the atmosphere of composite arrangement was evacuated prior to extrusion. Mechanical response of the composites were compared to the pre-extruded 1050 and 6061 aluminum alloys. The yield strengths of each composite, with varying percent weight compositions, were found to lie between those of matrix and reinforcement alloys, and abided by a simple rule-of-mixtures when considering weight composition. Highly elongated grains were oriented in the as-extruded composites along the extrusion direction and grains near the interface between two constituent alloys showed higher aspect ratio than in the interior region. The present study could lead to the optimum composite design for various industrial applications including all aluminum alloy overhead conductors with high strength and improved electric conductivity.     

Effect of Hot Rolling on the Properties of In Situ Ti-Aluminide and Alumina-Reinforced Aluminum Matrix Composite

In situ Ti-aluminide and alumina-reinforced aluminium matrix composite were prepared by the melt-cast route by adding 12 µm sized TiO₂ powder particles into molten aluminium at a temperature of 950°C. The effects of hot rolling temperature and percentage deformation on the microstructural architecture and mechanical properties (e.g., hardness and tensile strength) of aluminium matrix composite were studied in detail. Presence of different phases in the composite was identified with the aid of the Energy Dispersive X-Ray Analysis (EDXA). The percentages of reinforcements (Ti-aluminide and alumina) were estimated by image analysis. Fractographic investigations on the fractured surface of tensile specimens were also carried out with the aid of Scanning Electron Microscope (SEM). The composites exhibit fine distribution of reinforcements in the host matrix. It has been found that with the increase in rolling temperature and percentage deformation, hardness values and tensile strength of the composites improve significantly compared to the unreinforced aluminium.     

制备工艺对热压烧结SiC/SiC复合材料结构与性能的影响

采用纳米SiC和亚微米SiC粉料作为基体形成原料,通过热压烧结技术制备了SiC/SiC 复合材料.研究了粉料颗粒、烧结温度、烧结压力对复合材料显微结构和各种性能的影响.结果显示,采用纳米碳化硅粉体可有效降低烧结温度,促进复合材料的致密化过程,在1780℃、20MPa条件下可获得性能优良的复合材料.而采用亚微米SiC粉体,复合材料的致密化过程需要较高的温度,但随着密度的增加,基体与纤维之间的作用力增强,不利于性能的提高.     

New technology for fabrication of C/SiC composites reinforced by continuous carbon tows

Abstract

Carbon/silicon carbide composites were fabricated using the continuous synchronous composite (CSC) process, which is an improved technology based on conventional chemical vapour infiltration principles to fabricate ceramic matrix composites reinforced with carbon cloth or continuous tows which are not braided to a preform. In the CSC process, a gradient temperature field on the surface of the graphitic substrate, consisting of high (1000–1200°C), intermediate (900–1000°C), and low (700–900°C) temperature regions, was obtained by a bottom heating element. Since the rotation of the substrate accompanied simultaneously the preparation of the reinforcement phase and the deposition of the SiC matrix, micropores were well infiltrated in the intermediate temperature regions by diffusion transport, and macropores were well infiltrated in the high temperature regions with flow transport, respectively. Using methyltrichlorosilane (MTS) as a precursor, with hydrogen as a carrying gas and argon gas as a diluent, in the present studies, densification of C/SiC composites was uniform, and the highest deposition rate obtained was 0.168 mg cm^-2 min^-1), and the conversion efficiency of MTS varied from 31% to a maximum of 47%.     

EFFECT OF EXTRUSION PARAMETERS ON STRUCTURE AND PROPERTIES OF 2124 ALUMINUM ALLOY MATRIX COMPOSITES

Discontinuously reinforced aluminum matrix composites (DRA) have been attracting attention because of their amenability to undergo deformation processing by conventional metalworking techniques. Extrusion is used in processing of DRA composites for consolidation, redistribution of reinforcements, and shape forming. The important parameters that control the extrusion process are temperature and strain rate, which is a function of several equipment/extrusion parameters. Vacuum hot-pressed (VHP) 2124 Al/30 SiC_p composite billets were extruded at different ram speeds (1, 10, 100 mm sec^-1) and using different extrusion ratios (4:1, 10:1, and 20:1). The extruded samples were studied for their integrity, microstructure, and mechanical properties. The integrity of the extruded composite rod was very good at minimum extrusion speed of 1 mm sec^-1, whereas 100 mm sec^-1 extrusion speed resulted in extensive fir tree cracking. Extrusion of VHP billets, with necklace structure, resulted in elongated alternate stringers of matrix and SiC_p in the extrusion direction. Matrix stringer width and aspect ratio were found to vary with extrusion ratio. Because of the microstructural refinement, both the strength and ductility of the metal matrix composites (MMCs) were improved. Microhardness of the matrix stringers was found to be a function (power relation) of their width, irrespective of the location and extrusion ratio.     

Matrix motion of low-volume matrix W-Ni-Fe composites sintered in a temperature gradient

A low matrix volume (13%) composite liquid-phase sintered in a temperature gradient showed anomalous behaviour in that the spheroid size did not decrease as predicted with decreasing temperature, and the average spheroid size was larger than expected based on uniform temperature sintering experiments. Furthermore, similar composites with higher matrix volume (20 or 28%) had smaller average spheroid sizes at lower temperatures in the gradient. Copper was injected as a tracer into both presintered and liquid-phase sintered composites to study matrix flow during subsequent sintering treatments. The copper concentration decreased with distance from the hot zone for high volume matrix composites, but in the low volume matrix composites where the volume of solid tungsten spheroids was higher (greater resistance to matrix motion), the copper was distributed almost uniformly throughout the matrix. The spheroid and matrix compositions, densities, surface tensions, and most other properties were the same, whereas only the matrix volumes were different. This anomalous behaviour was attributed to some mechanism aided by capillary action since the smaller distances between spheroids in the low matrix composites increase the capillary forces by about 25%.     

Feasibility of underwater laser forming of laminated metal composites

Laminated metal composites are of great interest in various industries. Previous studies demonstrate undesired mechanical or microstructural changes in these composites during the laser forming process due to rapid temperature gradient. In this research, underwater laser forming is proposed to minimize this effect. This process could also be an effective method for on-site forming or repairing of large metal/composite sheets used in underwater applications, such as marine equipment, ships, and lake/sea-based offshore oil platforms. The underwater laser forming process is performed experimentally on a three-layered stainless steel/copper/stainless steel composite and compared with the results of in-air tests. Total forming time, bending rate, and microstructural changes are compared for both underwater and in-air conditions. The effects of forming parameters, such as the number of irradiations, laser beam velocity, diameter, and power, are also compared and discussed. It is shown that the bending angle per irradiation in underwater forming is significantly lower in comparison with in-air condition, but the production time is less due to the elimination of cooling time. Also, the microstructure of stainless steel at heat-affected zone was unchanged, and the hardness of upper layer experienced smaller changes when formed under water. The underwater laser forming process is demonstrated to be feasible and can be applied for underwater applications with a high degree of reliability.     

Application of model for work hardening behaviour of SiC reinforced magnesium based metal matrix composites

Abstract

An understanding of the work hardening behaviour of particulate reinforced metal matrix composites is crucial in optimising the parameters for deformation processing of these materials. In the present study, SiC reinforced magnesium metal matrix composites were produced using a liquid phase process. The microstructure of the composite was characterised and the mechanical properties were determined. The results of the ambient temperature tensile testing on the extruded Mg and Mg/SiC specimens revealed that an increase in the weight percentage of SiC particulates in pure magnesium increases the elastic modulus, does not affect the 0·2% yield strength, and reduces the ultimate tensile strength and ductility. A modified continuum model was applied to relate the work hardening behaviour of the composites to microstructural parameters and to predict the fracture strain of the composites. The model is shown to predict the fracture strain of the composites quite accurately for all the three weight fractions of reinforcements evaluated in the present study.     

Experimentelle Verifizierung einer neuer Modellgleichung für die voraussage des Elastizitätsmoduls eines Verbundwerkstoffes

Experimental Verification of a New Enquation for the Youngs Modulus of Composite Materials A recently proposed model equation for the prediction of Youngs modulus of elasticity of composite materials has been compared with extensive experimental data from the literature. As the derivation of the equation assumes a definite matrix-type microstructure with spheroidal inclusions, only composites containing particulate inclusions of different geometries were considered, including data on ceramic, glass and polymer matrix composites. A quantitative good agreement between theory and experiment was found, specially when the inclusion shape and orientation, being the microstructural parameters entering in the equation, were accurately known. The microstructural parameters involved in the equation can be obtained from real microstructural data via quantitative microstructural analysis and sterology, no fitting is involved. This fact makes the proposed equation substantial also for practical applications.     

Thermal expansion of composites with shape memory alloy inclusions and elastic matrix

Incorporation of shape memory alloy (SMA) inclusions into a continue matrix can make composites with various thermal dilatation behavior, and this depends sensitively on the microstructural parameters of the composite. A micro-mechanical method is proposed to relate quantitatively the overall thermal dilatation with the microstructures and the transformation characteristics of SMA materials. Composites with aligned and with two populations of perpendicularly oriented SMA inclusions are analyzed in detail in this paper. It is found that the composite with SMA fibers can have a large transformation temperature range during the heating process, and a linear shrinkage may take place during the cooling because of the large difference between the austenite finish and martensite start temperatures of the composite. Design aspects to minimize the overall thermal dilatation during a full thermal cycle are also discussed.     

厚截面树脂基复合材料的温度场研究 Ⅰ :模拟

研究了厚截面树脂基复合材料制造过程中的内部温度场发展变化。从含有非线性内热源的瞬态热传导方程出发,建立了用于分析复合材料热传导的有限元公式。以通用有限元软件包为基础,开发了能够模拟复合材料整个制造过程中复杂物理化学变化的软件。并用该软件对两种不同厚度树脂基复合材料的制造过程进行了模拟计算,发现现有的固化一般厚度复合材料的固化历程不适合固化厚截面复合材料。      

Microstructure and properties of SiCw/6061 aluminium alloy composites after compression at temperatures around solidus of matrix

Abstract

The effect of compressive deformation at temperatures around the solidus of the matrix on the microstructure and properties of SiC_w/6061 aluminium alloy composites was investigated. It was found that the temperature, strain rate, and amount of deformation affect whisker distribution and breakage, densification and uniformity of composites, and SiC_w/matrix alloy interfacial bonding. The microstructural evolution due to compression affects the properties of the composites, which is considered to be the most important aspect for evaluating high temperature plastic forming of the composites. The optimum parameters for compressive deformation were determined by analysing the microstructure and the properties of the composites.     

Microstructure-guided numerical simulations to predict the thermal performance of a hierarchical cement-based composite material

This paper presents a microstructure-guided numerical homogenization technique to predict the effective thermal conductivity of a hierarchical cement-based material containing phase change material (PCM)-impregnated lightweight aggregates (LWA). Porous inclusions such as LWAs embedded in a cementitious matrix are filled with multiple fluid phases including PCM to obtain desirable thermal properties for building and infrastructure applications. Simulations are carried out on realistic three-dimensional microstructures generated using pore structure information. An inverse analysis procedure is used to extract the intrinsic thermal properties of those microstructural components for which data is not available. The homogenized heat flux is predicted for an imposed temperature gradient from which the effective composite thermal conductivity is computed. The simulated effective composite thermal conductivities are found to correlate very well with experimental measurements for a family of LWA-PCM composites considered in the paper. Comparisons with commonly used analytical homogenization models show that the microstructure-guided simulation approach provides superior results for composites exhibiting large property contrast between phases. By linking the microstructure and thermal properties of hierarchical materials, an efficient framework is available for optimizing the material design to improve thermal efficiency of a wide variety of heterogeneous materials.     

Thermophysical properties of ceramic matrix composites produced by polymer pyrolysis

A unidirectional composite and a series of bidirectionally reinforced composites were fabricated using carbon fibre reinforcement in a silicon carbide matrix, which was produced by the pyrolysis of a polymer precursor. The thermal expansion over the temperature range 20–1000 °C has been measured and the thermal diffusivity measured over the temperature range 200–1200 °C. Thermal diffusivity data was converted to conductivity data using measured density and literature specific heat data. Metallographic examination has been carried out on the composites and the results are discussed in terms of the observed microstructural features.     

ESTM-fabric/3266复合材料低速冲击响应及冲击后压缩行为研究

基于“离位”技术,分别开发两种新型聚醚砜(PES)点阵附载型(ES-L)和PES无规附载U3160织物型(ES-R)ES^TM-fabric织物,采用RTM工艺制备ES^TM-fabric织物增强3266中温环氧树脂基复合材料(ES^TM-fabric/3266),对其进行冲击阻抗及冲击后压缩测试,并利用荧光显微镜、SEM结果分析离位增韧机理,还对比研究未增韧U3160织物增强3266中温环氧树脂基复合材料的性能。低速冲击测试结果表明:相比未增韧U3160/3266(ES-U),ES^TM-fabric/3266的起始损伤阈值载荷显著提高,冲击损伤面积明显减少,裂纹扩展更加平缓,且以层内基体裂纹、纤维束内的纤维-基体脱粘和局部铺层断裂为主。ES-L的CAI值比ES-U增大了37%。ES-R层间出现均布式相反转结构,ES-L层间存在硬相区(富BMI连续相)和软相区(富PES连续相/3266相反转结构);ES-L的相结构能够更加有效地缓解应力集中、耗散冲击能量,从而使其表现出最佳的损伤阻抗和损伤容限性能。     

Tensile behaviour of borosilicate glass matrix-Nicalon (silicon carbide) fibre composites

Unidirectional composites consisting of a borosilicate glass (Corning 7740) matrix reinforced with Nicalon (silicon carbide) fibres were fabricated and tested in monotonic tension at temperatures ranging from room temperature to 650 °C. The ultimate tensile strength showed little dependence on temperature up to about 425 °C and failed by longitudinal splitting. There was a significant increase in strength at 540 °C and a slight decrease in strength when tested above this temperature, and the failure involved extensive fibre pull-out. The elastic modulus (stiffness) was found to decrease progressively with increasing temperature. The matrix consists of borosilicate glass within the plies and very fine grains of alpha (low) cristobalite in the inter-ply regions. The behaviour of the composite as a whole was found to be dependent upon the behaviour of the matrix at the temperature of testing.     

Fabrication of nanostructured Al/Cu/Mn metallic multilayer composites by accumulative roll bonding process and investigation of their mechanical properties

Multilayered Al/Cu/Mn composites were produced from aluminum 1100 strips, commercial copper foils and manganese powders, through accumulative roll bonding (ARB). The structural and microstructural evolution of the produced composites was studied by X-ray diffraction and scanning electron microscopy. Also, their mechanical properties at various ARB cycles were studied by microhardness and tensile tests. In this process after nine ARB cycles, a multilayered Al/Cu/Mn composite with homogeneously distributed, fragmented copper layers and Mn powders in the aluminum matrix was produced. Also, it was observed that with increasing strain by progression of the ARB process, the strength and microhardness of the produced composites increased.     

Influence of microstructure of fibre/matrix interface on mechanical properties of Al/SiC composites

Abstract

The relationship between microstructure and mechanical properties of composites of squeeze cast Al–Cu and Al–Cu–Mg reinforced with 25 vol.-%SiC whiskers was investigated. Tensile test results were compared with values calculated using a modified rule of mixtures (ROM). The results were found to be in good agreement for the Al–Cu matrix composite, whereas a relatively large discrepancy was observed for the Al–Cu–Mg matrix composite. It was concluded from microstructural observations that this difference resulted from a reduction of the whisker strength due to more pronounced decoration of the interfaces by oxides and spinels. For the Al–Cu–Mg composite, the effect of interfacial phases on the composite strength must betaken into account when the modified ROM is applied.

MST/1242     

Current-activated pressure-assisted sintering (CAPAS) and nanoindentation mapping of dual matrix composites

Titanium boride (TiB_w) whiskers are currently recognized as one of the most compatible reinforcements for titanium (Ti) that have positively affected its wear resistance and stiffness. The fracture toughness and ductility have, however, been reported to deteriorate at increased TiB_w volume fractions, mainly due to the interlocking of these brittle TiB whiskers. This article investigates the processing of dual matrix Ti–TiB_w composites, where microstructures are generated consisting of TiB_w–Ti composite regions separated by a ductile, predominantly Ti, outer matrix. This microstructural design has the potential to prevent the continuous TiB_w interlocking over the scale of the composite (at high TiB_w volume fractions), and promote improved toughness of the material. The processing of these unique composites using current-activated pressure-assisted sintering (CAPAS) is discussed in this article. The effect of processing temperature on the microstructure and hardness of Ti–TiB_w dual matrix composites is also discussed, together with a simultaneous imaging and modulus-mapping nanoindentation technique used to characterize the composites