Thermal instability of rotating nanofluid layer

In the present paper we have considered thermal instability of rotating nanofluids heated from below. Linear stability analysis has been made to investigate analytically the effect of rotation. The more important effect of Brownian motion and thermophoresis has been included in the model of nanofluid. Galerkin method is used to obtain the analytical expression for both non-oscillatory and oscillatory cases, when boundaries surfaces are free–free. The influence of various nanofluids parameters and rotation on the onset of convection has been analysed. It has been shown that the rotation has a stabilizing effect depending upon the values of various nanofluid parameters. The critical Rayleigh number for the onset of instability is determined numerically and results are depicted graphically. The necessary and sufficient conditions for the existence of over stability are also obtained.     

Mechanical property degradation of a Nicalon fiber reinforced SiNC ceramic matrix composite under thermal shock loading

The degradation of mechanical properties of a SiC fiber reinforced SiNC ceramic matrix composite due to thermal shock by water quenching have been investigated. Post thermal shock tensile tests were performed to determine the degradation of mechanical properties of this composite. In situ acoustic emission (AE) tests were also conducted. The tensile tests data and acoustic emission data were correlated. The AE signal indicated a sudden increase in AE events at critical points in the stress–strain relationship. The effects of thermal shock temperature and the number of thermal shock cycles on the mechanical properties, and on the AE responses were also evaluated. It was observed that an increase in either factor resulted in more AE responses. Fracture damage in the tensile test specimens was examined by Scanning Electron Microscopy. It was observed that the failure mechanism changed as the thermal shock temperature increased. The fracture surfaces of the specimens tested without thermal shock indicated an extensive fiber pullout while the thermally shocked specimens showed reduced fiber pullout.     

Mechanical,thermal and morphological properties of a bio-based composite derived from banana plant source

This paper focuses on the synthesis and testing of a novel bio-based composite structure in which banana fibres was infused with resin made from banana sap. The mechanical, thermal, morphological and biodegradation properties of the bio-composite were characterized and it was found that the material was suitable for general non-functional components. Mechanical tests indicated 15% increase in tensile strength, 12% improvement in tensile modulus and a 25% improvement in flexural modulus when compared to structures produced without banana sap. At elevated temperatures a decrease in the moduli was observed. The thermal stability of the biocomposite composite improved and this corresponded with an increase in the glass transition temperature. Morphological studies using scanning electron microscopy revealed improved compatibility between the fibre and banana sap matrix. This resulted in improved dynamic modulus values and low damping values. Finally, degradation tests revealed increased microbial activity on the banana sap composite. This was indicative of improved biodegradation rates.     

Cu-Fe合金的强磁场固溶时效行为

将Cu-15%Fe(质量分数)合金在强磁场中进行不同同溶时效处理,研究了合金的时效行为.结果表明,施加10 T强磁场可以促进第二相Fe枝晶的球化,而且Fe枝晶的形貌受强磁场的球化作用与高温缓慢冷却引起的粗化作用的影响.在Cu-15%Fe合金1000℃同溶处理中,施加10 T强磁场使基体中的Fe含量降低了0.39%.这表明,强磁场在一定程度上促进了Fe在Cu基体中的析出,获得与缓冷相类似的效果;施加10 T强磁场固溶处理并在10 T强磁场作用下经500℃时效处理后,基体中的Fe含量较低.其原因是,施加强磁场后Fe原子的析出规律受温度制度和析出相磁性转变的共同影响.施加强磁场改变了原子的激活能,进而影响了原子的扩散行为.     

Microstructure-based simulation of thermomechanical behavior of composite materials by object-oriented finite element analysis

While it is well recognized that microstructure controls the physical and mechanical properties of a material, the complexity of the microstructure often makes it difficult to simulate by analytical or numerical techniques. In this paper we present a relatively new approach to incorporate microstructures into finite element modeling using an object-oriented finite element technique. This technique combines microstructural data in the form of experimental or simulated microstructures, with fundamental material data (such as elastic modulus or coefficient of thermal expansion of the constituent phases) as a basis for understanding material behavior. The object-oriented technique is a radical departure from conventional finite element analysis, where a “unit-cell” model is used as the basis for predicting material behavior. Instead, the starting point of object-oriented finite element analysis is the actual microstructure of the material being investigated. In this paper, an introduction to the object-oriented finite element approach to microstructure-based modeling is provided with two examples: SiC particle-reinforced Al matrix composites and double-cemented WC particle-reinforced Co matrix composites. It will be shown that object-oriented finite element analysis is a unique tool that can be used to predict elastic and thermal constants of the composites, as well as salient effects of the microstructure on local stress state.     

Fabrication of a polymer composite with high thermal conductivity based on sintered silicon nitride foam

A novel route was developed to fabricate Si₃N₄/epoxy composite. In this route, the Si₃N₄ particles were constructed into the foamed shape by using protein foaming method, firstly. Then the Si₃N₄ foams were sintered to bond these Si₃N₄ particles together. Finally, the Si₃N₄/epoxy composite was fabricated by infiltrating the epoxy resin solution into the sintered Si₃N₄ foams. This route was proved to be an efficient way in enhancing the thermal conductivity of epoxy matrix at a low loading fraction. For example, the thermal conductivity of the as-prepared Si₃N₄/epoxy composite with a loading fraction of 22.2 vol% was up to 3.89 W m⁻¹ K⁻¹, which was about 17 times higher than that of neat epoxy.     

GF/PCBT复合材料熔融连接接头失效分析的实验研究与数值模拟

为了解决高黏度热塑性树脂难以制备高强度、大尺寸纤维增强热塑性复合材料构件的问题,采用真空辅助树脂传递模塑(VARTM)工艺与热压工艺相结合的方法,以环状对苯二甲酸丁二醇酯(CBT)在催化剂作用下聚合成的聚环状对苯二甲酸丁二醇酯(PCBT)为基体,制备了纤维体积分数达70%的连续玻璃纤维(GF)/PCBT复合材料层合板及熔融连接件,并测得其力学参数。采用数值模拟方法对连接界面层数分别为1、2、3层的A、B、C型3种不同方案的GF/PCBT复合材料熔融连接接头的承载能力和失效模式进行了预测。结果表明:不同的结构设计方案对GF/PCBT复合材料接头性能的影响较大,当连接长度在一定范围内时,接头区域主要发生界面分层失效,接头处复合材料的翘曲为界面裂纹加速扩展的主要因素,C型连接方式的接头结构承载能力相比于A型连接方式有明显提高;增加C型接头连接长度,试件承载能力提高,直至接头处界面分层失效和纤维、基体失效同时发生;继续增加连接长度,纤维与基体失效将成为接头区域的主要失效模式,此时承载能力无明显提升。     

Mechanical performances of a thermal activated composite

The Controlled Behavior Composite Material (CBCM) is a thermal activated composite material. The thermal activation is made thanks to carbon yarns connected to a power supply. Two sorts of CBCM plates are tested. A three-point bending test is performed to obtain the characteristic curves of the plates and a “free deflection” as well as a “blocking force” are defined as relevant parameters. Response times, mechanical powers and yields are calculated to evaluate the performances of the CBCM plates. The influence of some working (intensity per yarn) and design (orientation of the main reinforcement) parameters is studied. Organization in the anisotropy of the structure is a key point for the thermo-mechanical behavior of the CBCM. The loss of matrix rigidity due to the rise in temperature is quantified and the results show that it can be partially offset by the CBCM effect.     

Influence of ultrasonic dual mixing on thermal and tensile properties of MWCNTs-epoxy composite

Multiwalled carbon nanotubes (MWCNTs) reinforced epoxy based composites were fabricated by using an innovative ultrasonic dual mixing (UDM) process consists of ultrasonic mixing with simultaneous magnetic stirring. The effect of addition of varying amount of MWCNTs on thermal stability and tensile properties of the epoxy based composite has been investigated. It is found that the thermal stability, tensile strength and toughness of the epoxy base improves with the increase of MWCNTs addition up to 1.5 wt.% and UDM processing at certain capacity of the system. Tensile tests and thermal gravimetric analysis (TGA) were performed on each group of composites containing different amount of MWCNTs to determine their mechanical and thermal properties respectively. The dispersion of 1.5 wt.% MWCNTs fillers in epoxy nanocomposites was studied by transmission electron microscopy (TEM) as well as by field emission scanning electron microscopy (FESEM) applied on their tensile fracture surface.     

Increasing the through-thickness thermal conductivity of carbon fiber polymer-matrix composite by curing pressure increase and filler incorporation

The low through-thickness thermal conductivity limits heat dissipation from continuous carbon fiber polymer-matrix composites. This conductivity is increased by up to 60% by raising the curing pressure from 0.1 to 2.0 MPa and up to 33% by incorporation of a filler (?1.5 vol.%) at the interlaminar interface. The 7-μm-diameter 7-W/m K-thermal-conductivity continuous fiber volume fraction is increased by the curing pressure increase, but is essentially unaffected by filler incorporation. The thermal resistivity is dominated by the lamina resistivity (which is contributed substantially by the intralaminar fiber-fiber interfacial resistivity), with the interlaminar interface thermal resistivity being unexpectedly negligible. The lamina resistivity and intralaminar fiber-fiber interfacial resistivity are decreased by up to 56% by raising the curing pressure and up to 36% by filler incorporation. The curing pressure increase does not affect the effectiveness of 1-mm-long 10-μm-diameter 900-1000-W/m K-thermal-conductivity K-1100 carbon fiber or single-walled carbon nanotube (SWCNT) as fillers for enhancing the conductivity, but hinders the effectiveness of carbon black (CB, low-cost), which is less effective than K-1100 or SWCNT at the higher curing pressure, but is almost as effective as K-1100 and SWCNT at the lower curing pressure. The effectiveness for enhancing the flexural modulus/strength/ductility decreases in the order: SWCNT, CB, K-1100.     

Flammability,thermal and physical-mechanical properties of cationic polymer/montmorillonite composite on cotton fabric

The flammability, thermal and mechanical properties on cotton fabric were improved by being finished with the composite containing montmorillonite. To this aim, polymer dimethyl diallyl ammonium chloride-allyl glycidyl ether (P_DMDAAC-AGE) was prepared and its structure characterized by Fourier transform infrared (FT-IR) and Nuclear magnetic resonance (¹H NMR). The quaternary ammonium salt copolymer/montmorillonite composite (P_DMDAAC-AGE/MMT) was obtained by polymer intercalation method. The X-ray diffraction (XRD) indicated that the MMT interlayer spacing increased after the polymer intercalation. Composite materials were loaded onto the cotton fabrics by a dip-pad-dry method. The thermo gravimetric analysis (TGA), vertical flame test and limiting oxygen index (LOI) results showed that the thermal and flammability properties of the cotton fabric were improved after it was finished with the composite. Tensile testing revealed an increase on mechanical properties of the finished fabric, but the physical properties hardly changed from the bending length and whiteness results. Scanning electron microscope (SEM) and energy disperse X-ray spectroscope (EDX) results verified the improvement of those properties due to the presence of montmorillonite in the composite.     

Flow Instability of Molten GaAs in the Czochralski Configuration

The flow and heat transfer of molten GaAs under the interaction of buoyancy, Marangoni and crystal rotation in the Czochralski configuration are numerically studied by using a time-dependent and three-dimensional turbulent flow model for the first time. The transition from axisymmetric flow to non-axisymmetric flow and then returning to axisymmetric flow again with increasing centrifugal and coriolis forces by increasing the crystal rotation rate was numerically observed. The origin of the transition to non-axisymmetric flow has been proved to be baroclinic instability. Several important characteristics of baroclinic instability in the CZ GaAs melt have been predicted. These characteristics are found to be in agreement with experimental observations.     

On the behaviour of a particulate metal matrix composite subjected to cyclic temperature and constant stress

The paper describes a method of characterising the behaviour of an idealised particulate metal matrix composite composed of elastic particles and an elastic–perfectly plastic matrix subjected to constant macro stress and a cyclic temperature history. The computational method, the Linear Matching Method, was originally developed for structural life assessment studies, and allows a direct evaluation of the load ranges for which differing modes of behaviour occur in the steady cyclic state; shakedown, reverse plasticity and ratchetting. A simple homogenised model is considered, consisting of spherical particles embedded in a cubic matrix array. The resulting solutions are presented as non-dimensional equations derived from numerical solutions for two composites, alumina and silicon carbide particles embedded in an aluminium matrix.     

The annealing phenomena and thermal stability of severely deformed steel sheet

However, there are many works on annealing process of SPDed non-ferrous metals, there are limit works on annealing process of SPDed low carbon steel. Therefore, in this study the annealing responses after constrained groove pressing (CGP) of low carbon steel sheets have been investigated. The sheets are subjected to severe plastic deformation at room temperature by CGP method up to three passes. Nano-structured low carbon steel sheets produced by severe plastic deformation are annealed at temperature range of 100-600 °C for 20 min. The changes of their microstructures after deformation and annealing are studied by optical microscopy. The effects of large strain and annealing temperature on microstructure, strength and hardness evolutions of the nano-scale grained low carbon steel are examined. The results show that annealing phenomena can effectively improve the elongation of SPDed sheets with preserving the hardness and mechanical strength. Also, the thermal stability of microstructure and mechanical properties can be observed through annealing temperatures up to 400 °C and temperature of 400 °C is achieved as an optimum annealing temperature in which both strength and elongation are increased and hardness inhomogeneity of the sheet is minimum. Annealing at temperatures of higher than 400 °C leads to abnormal grain growth.     

Self-assemblies of linearly aligned diamond fillers in polysiloxane/diamond composite films with enhanced thermal conductivity

The relocation of diamond fillers was performed in polysiloxane-based composite films under different electric fields. The microscale diamond filler particles were dispersed by sonication in a prepolymer mixture of polysiloxane, followed by high-speed mixing. The homogeneous suspension was cast onto a polyamide spacer of microscale thickness and subjected to three different electric fields: AC, DC, and switched DC, before the mixture became cross-linked. Analysis revealed that self-assemblies of linearly aligned diamond fillers (LADFs) were fabricated in the composite film, connecting the film planes as bridges with different thicknesses depending on the applied electric field. Composites with assemblies of LADFs exhibited enhanced thermal conductivity and electrical insulation, and are attractive for application as thermal interface materials in the semiconductor industry.     

On the mechanical behavior of a functionally graded micro-beam subjected to a thermal moment and nonlinear electrostatic pressure

In the present work, mechanical behavior of a functionally graded cantilever micro-beam subjected to a nonlinear electrostatic pressure and temperature changes has been studied. It has been assumed that the top surface is made of pure metal and the bottom surface from a metal–ceramic mixture. The ceramic constituent percent of the bottom surface varies from 0% to 100%. In addition to the Volume Fractional Rule of material, exponential function has been used for representation of continuous gradation of the material properties through micro-beam thickness. Attention being paid to the ceramic constituent percent of the bottom surface, five different types of FGM micro-beams have been investigated. Nonlinear integro-differential thermo-electro mechanical equation based on Euler–Bernoulli beam theory has been derived and solved using Step-by-Step Linearization Method and Finite Difference Method. The effects of temperature changes and the electrostatic pressure on the deflection and stability of FGM micro-beams having various amounts of the ceramic constituent have been studied and normal stress distributions in the cross section along the beam thickness have been given and compared with a classic metal beam.     

复合材料波纹梁冲击试验与数值模拟

为了探究复合材料波纹梁的吸能性能,针对铺层形式分别为[(±45)3/(0,90)/(±45)3]、[(±45)8]和[(±45)7]的3种复合材料波纹梁元件,进行了动态冲击试验,得到了吸能载荷-位移曲线,并对其损伤破坏形貌进行了分析。以连续损伤力学为基础,结合改进的Hashin损伤判定准则以及损伤演化规律,提出了针对波纹梁耐撞性损伤分析的刚度退化模型,并基于有限元软件平台开发了适用于波纹梁渐进损伤分析的子程序。对3种不同结构形式的波纹梁进行了渐进失效数值分析,模拟得到了能量评估参数比吸能(SEA)和平均载荷值,并将模拟结果与试验结果进行了对比分析。比较分析了不同薄弱环节复合材料波纹梁的吸能能力。结果表明:波纹梁在冲击载荷作用下发生了渐进压溃失效;平均压溃载荷的相对误差不超过12%,能够满足工程应用要求;薄弱环节的设置需综合考虑复合材料性能和铺层方式等因素。     

Precipitation and thermal instability of M23C6 carbide in cast Ni-base superalloy K452

During long-term thermal exposure M₂₃C₆ dendritically precipitates from the supersaturated γ matrix and then changes in shape from flower-like dendrite to irregular block to regular polyhedron. The reduction in the interfacial energy is responsible for the morphological transition, but the stable morphology of the carbide is principally determined by the anisotropy of the interfacial energy between the carbide and the γ' in the M₂₃C₆-γ' cell. In addition, M₂₃C₆ has a negligible influence on the tensile property of the thermally exposed alloy even though it can block the mobile dislocations and stacking faults to some extent.     

Growing polystyrene chains from the surface of graphene layers via RAFT polymerization and the influence on their thermal properties

The polystyrene (PS) macromolecular chains were grafted on the surface of graphene layers by reversible addition-fragmentation chain transfer (RAFT) polymerization. In this procedure, a RAFT agent, 4-Cyano-4-[(dodecylsulfanylthiocarbonyl) sulfanyl] pentanoic acid, was used to functionalize the thermal reduced graphene oxide (TRGO) to obtain the precursor (TRGO-RAFT). It can be calculated that the grafting density of PS/graphene (PRG) composites was about 0.18 chains per 100 carbons. Successful in-plain attachment of RAFT agent to TRGO and PS chain to TRGO-RAFT was shown an influence on the thermal property of the PRG composites. The thermal conductivity (λ) improved from 0.150 W m⁻¹ K⁻¹ of neat PS to 0.250 W m⁻¹ K⁻¹ of PRG composites with 10 wt% graphene sheets loading. The thermal property of PRG composites increased due to the homogeneous dispersion and ordered arrangement of graphene sheets in PS matrix and the formation of PRG composites.