Deformation due to mechanical and thermal sources in generalised orthorhombic thermoelastic material

A dynamical two-dimensional problem of thermoelasticity has been considered to investigate the disturbance due to mechanical (horizontal or vertical) and thermal source in a homogeneous, thermally conducting orthorhombic material. Laplace-Fourier transforms are applied to basic equations to form a vector matrix differential equation, which is then solved by eigenvalue approach. The displacements, stresses and temperature distribution so obtained in the physical domain are computed numerically and illustrated graphically. The numerical results of these quantities for zinc crystal-like material are illustrated to compare the results for different theories of generalised thermoelasticity for an insulated boundary and a temperature gradient boundary.     

Deformation due to Mechanical and Thermal Sources in a Thermoelastic Body with Voids under Axi-symmetric Distributions

The Laplace and Hankel transforms have been employed to find the general solution of a homogeneous, isotropic, thermoelastic half-space with voids for a plane axi-symmetric problem. The application of a thermoelastic half-space with voids subjected to a normal force and a thermal source acting at the origin has been considered to show the utility of the solution obtained. To obtain the solution in a physical form, a numerical inversion technique has been applied. The results in the form of displacements, stresses, temperature distribution, and change in volume fraction field are computed numerically and illustrated graphically for a magnesium crystal-like material to depict the effects of voids in the theory of coupled thermoelasticity (CT) and uncoupled thermoelasticity (UCT) for an insulated boundary and a temperature gradient boundary.     

Deformation due to expanding ring load in modified couple stress thermoelastic diffusion in time and frequency domain

The two-dimensional problem of expanding ring load in a modified couple stress theory of thermoelastic diffusion with heat sources in time and frequency domains is investigated. The mathematical formulation prepared for thermoelastic diffusion solids with one and two relaxation times using Laplace and Hankel transforms. The displacements, stress components, temperature change, and chemical potential are obtained in a transformed domain. Numerical computation is performed for these quantities and the resulting quantities are shown graphically for the time and frequency domains. Comparisons are made with the results predicted by the two theories and different values of time and frequency. Particular cases of interest are also deduced.     

一种辐轮式低温支撑的热学与力学性能实验

介绍了一种特殊的辐轮式低温支撑,针对这种不同材质和辐条数量及其截面形状的辐轮式结构,分别从热学性能和力学性能两方面进行了实验研究,测量了聚醚醚酮（PEEK）和玻璃钢（GRP）两种材料的热导率以及由这两种材料组成的多种结构形式的支撑轮的热阻,对试样进行力学拉伸破坏实验,并对实验结果进行了分析与比较,给出了误差分析。实验结果表明,温度在200 K以下时PEEK材料热导率小于GRP材料,200 K以上时两者相当;支撑轮的热阻随着辐条数量和截面积的增加而减小,且实验测得的热阻是固体传热和辐射换热的综合结果;支撑轮的力学性能随着辐条数量和截面积的增加而提高,GRP材料的支撑轮径向承受载荷能力较强、轴向偏弱,而PEEK材料的支撑轮各方向上承载能力较为均匀,这主要是因为GRP材料是各向异性的。综合热性能和力学性能实验结果,PEEK材料6辐带肋结构的支撑轮是最佳选择。     

Change in impedance of a single-turn coil due to a flaw in a conducting half space

The problem of detection and characterization of a flaw in a conducting half-space using an eddy-current coil oriented parallel to the interface is examined. An expression is derived for a first order approximation for the change in complex impedance due to a flaw located within the conducting medium. The overall impedance is a function of the radius and lift-off distance of the test coil and the conductivity of the material. An analytical expression is derived for the change in impedance as a function of the electric fields at the position of the flaw. It is found to be an integral over the volume of the flaw of the electric fields found with and without the flaw being present. The limiting case of a degenerate point flaw may be examined in greater detail by allowing the field in the presence of the flaw to be approximated by the unperturbed field. For flaws small enough that the field does not vary much over its volume, the field may be even further approximated by using just the value of the field at the position of the centroid of the flaw. Plots are shown to illustrate the behavior of the change in impedance as a function of the radial range of the flaw and the depth of the flaw centroid, using previously derived expressions for the fields for the unflawed case.     

Transient and thermal contact analysis for the elastic behavior of functionally graded brake disks due to mechanical and thermal loads

In this paper, the transient and contact analysis of functionally graded (FG) brake disk is presented. The analysis was carried out using ANSYS parametric design language (APDL). The FG brake disk is made of metal–ceramic material. The material properties vary in radial direction with the values from full-metal at the inner radius to that of full-ceramic at the outer radius. In the analysis, FG brake disk is in contact with one pure pad disk and coulomb contact friction is considered as heat source. The non-dimensional results are obtained for specific value of grading index (n = 1) by considering different material property divisions of 25, 50, 100 and 200. The results presented are for the pressure distribution, total stress, pad penetration, friction stress, heat flux and temperature during contact, for different values of contact stiffness factor, F_kn, which depends on the property gradation of FG brake disk with 200 material property divisions. The results show that the contact pressure and contact total stress increase with increasing values of F_kn, and hence it can be concluded that gradation of the metal–ceramic has significant effect in the thermomechanical response of FG brake disks.     

Predicting crack initiation in composite material systems due to a thermal expansion mismatch

Residual stresses due to curing and thermal stresses due to differences between the thermal expansion coefficients of the matrix and fiber may have a major effect on the micro-stresses within a composite material system and must be added to the stresses induced by the external mechanical loads. Such microstresses are often sufficient to produce micro-cracking even in the absence of external loads, example during the cooling process. In this investigation, a micro-mechnics approach is used in which the fibers of a composite material system are modeled as cylindrical inclusions that are embedded into a matrix plate. The model is then used to predict, analytically, the residual stresses due to a thermal expansion mismatch, e.g. during a cooling process. Additionally, some critical effects due to a load transverse to the direction of the fibers are examined. The analysis provides a better understanding of how residual stresses are developed and how they may be controlled in material systems where small strains are present. Moreover, the results are used to identify locations of possible crack failure and to derive a fracture criterion for crack initiation at the local level. Comparison with experimental evidence for matrix cracking in intermetallic composites caused by thermal expansion mismatch shows a good agreement. This revised version was published online in July 2006 with corrections to the Cover Date.     

一类高聚物电子封装材料的分层失效问题

对一类具有径向摄动的广义neo-Hookean特征的高聚物电子封装材料在回流焊过程中,由于湿热的联合作用而导致的"爆米花"式的分层失效行为进行了理论研究。给出了此类各向同性不可压材料在包含单一球形孔穴的条件下孔穴的增长和湿热应力之间的解析关系;讨论了孔穴失稳的临界相当应力对初始孔穴率以及径向摄动参数的依赖关系。计算结果表明,当高聚物电子封装材料具有某种缺陷存在时,其孔穴失稳的临界相当应力将会降低,即更易发生"爆米花"式的分层失效。     

高温下不同银含量微电子胶连点的力学性能及膨胀系数不匹配热应力

采用热机械分析仪和微压入测试系统对不同银含量微电子互连导电胶进行测试表征,并基于数值模拟分析其用于倒装芯片封装时胶层的不匹配热应力。结果表明,较高银含量固化导电胶的玻璃化转变温度滞后于较低银含量固化导电胶,且其热膨胀系数较低;随着温度的升高,导电胶的模量与硬度由玻璃态时的较高值降低到高弹态时的较低水平,且银含量较高时的刚度与强度较大;各不匹配热应力分量随温度的变化呈现出"蝌蚪状"或"半蝌蚪状",玻璃态时用于倒装芯片封装的导电胶未发生屈服。     

Numerical evaluation of inhomogeneity forces in a coating layer due to thermal loading

Abstract

In order to clarify the mechanical state at the interface between substrate and coating material, numerical analyses are implemented on model materials under thermal loads, and the possibilities of debonding are discussed from the configurational mechanics concept. The intensity of the interfacial singularity can be discussed in relation to the inhomogeneity force which is numerically evaluated in the course of thermal loading via finite element analysis. It is shown that interfacial roughness, a small perturbation at the interface, may have a dominant role in the evolution of inhomogeneity. Effects of initial defects, temperature gradient and additional external load are also evaluated. It is confirmed that the inhomogeneity force is a generalisation of the J-integral, i.e. the energy release rate in fracture mechanics, and this is also applicable to interfacial problems. A possible scenario of layer debonding is discussed in terms of the material inhomogeneity and the deformation characteristics of the thin coating layer under thermal loading.     

热力载荷下多材料构件连接区的界面断裂分析

应用改进的虚裂纹闭合技术对热、力载荷作用下多材料构件连接区界面进行断裂分析。首先,通过对含橡胶夹层的复合材料层合板单腿弯曲（SLB）试件断裂分析,研究了在不同温度载荷作用下,橡胶夹层对试件能量释放率及其各型分量的影响。其次,对具有热流边界下,典型复合材料-橡胶-金属组成的多材料圆柱壳体连接裙结构进行了热力耦合断裂分析,结果表明裂纹总能量释放率随温度升高而增大。最后,针对该连接裙结构讨论了裂纹位置和橡胶层厚度对裂纹能量释放率的影响,指出适当增加橡胶层厚度可以降低裂纹能量释放率,但橡胶厚层度与界面韧性之间存在尺寸效应。     

On the axial and interfacial shear stresses due to thermal mismatch in hybrid composite sheets

A simple analytical model has been developed which allows the determination of the axial and the interfacial shear stresses which can occur in hybrid fiber composites as a consequence of the mismatch in coefficients of thermal expansion and Young's modulus. The configuration considered is a finite-width hybrid composite monolayer with alternating high- and low-modulus fibers. To account properly for the interfacial shear between fiber and matrix, a modified shear-lag model is used which permits extensional deformation due to thermal expansion of the matrix in the fiber direction. Typical stresses due solely to temperature changes are calculated, and these show steep boundary-layer edge stresses at free corners.     

Morphological,thermal and dynamic mechanical properties of Cathay poplar/organoclay composites prepared by in situ process

In order to improve the thermal stability and dynamic mechanical properties of Cathay poplar (Populus cathayana Rehd.) wood, a kind of organoclay, that is, organo-montmorillonite (OMMT), was introduced into its structure via an in situ process by sequentially impregnating poplar wood with sodium-montmorillonite (Na-MMT, in concentrations of 1.0%, 2.0%, and 4.0%) and didecyldimethylammonium chloride (DDAC, in a concentration of 2.0%). Consequently, the wood/organoclay composites were prepared. The X-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersed X-ray analysis (SEM-EDXA) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the morphological and chemical alterations of the composites. Also the effects of clay type and concentrations on the thermal stability and dynamic mechanical properties of the composites were studied. The results showed that didecyldimethylammonium ions were intercalated into the galleries of Na-MMT through cation exchange, partially separating the silicate layers. Thereafter, the inorganic Na-MMT transformed to OMMT during the in situ synthesis process, and the latter was successfully intercalated into the wood cell wall. The thermal degradation was alleviated in the wood/clay composites, among which the wood/OMMT composites exhibited the best thermal stability. According to dynamic mechanical analysis (DMA) results, the wood/OMMT composites showed an enhancement in energy storage and a diminution in energy dissipation compared to other groups. The improvements in the thermal stability and dynamic mechanical properties of the composites became more significant with the increasing clay content.     

The errors in measuring thermal diffusivity by a pulsed method due to random fluctuations of the form of the laser pulse

An experimental estimate is given of the errors in measuring thermal diffusitivity by a pulsed method, due to random changes in the time distribution of the radiation of the pulsed laser. __________ Translated from Izmeritel’naya Tekhnika, No. 11, pp. 37–39, November, 2006.     

高温氧化及热震对SiC/ZrB2-SiC/SiC涂层炭/炭复合材料力学行为的影响

为提高炭/炭(C/C)复合材料的高温抗氧化性能,同时分析涂层制备及高温氧化对涂层材料力学行为的影响,在C/C复合材料表面采用反应熔渗、料浆涂刷结合化学气相沉积工艺制备了SiC/ZrB2-SiC/SiC三层高温抗氧化涂层。利用SEM和XRD分析复合涂层的微观结构和相组成,考察涂层复合材料1500℃高温抗氧化和1500℃-室温的抗热震性能,研究高温氧化及热震对涂层C/C复合材料力学行为的影响。结果表明,复合涂层试样1500℃静态空气环境下具有优异的抗氧化及抗热震性能:1500℃氧化20 h后试样保持增重,1500℃至室温热震50次后增重为0.69%。因涂层制备过程中粉料的渗入反应,复合材料弯曲强度增长了7.08%。在经历1500℃氧化20 h和1500℃至室温50次热震后,涂层复合材料弯曲强度有所下降,且因材料界面结合力的减弱使得纤维拔出特征明显,材料塑性断裂特征增强。     

面向一体化热防护的陶瓷基复合材料轻量化结构研究进展与挑战EI北大核心CSCD

高超声速飞行器技术是航空航天领域发展的重要方向,对国防安全起着重要作用。高超声速飞行器能在极端环境中安全服役的关键在于飞行器的热防护材料与结构。一方面,热防护材料与结构必须能够经受恶劣的气动热环境;另一方面,热防护材料与结构还要在承载的同时尽可能降低质量以提高飞行器有效载荷。因此,需要研发兼具耐高温、轻量化、承载特性的热防护结构。本文首先综述了C/SiC陶瓷基复合材料轻量化点阵结构及其制造方法,对其在室温、高温环境下的力学行为与传热行为的研究现状进行了总结,并具体讨论了基于C/SiC陶瓷基复合材料轻量化点阵结构的耐高温、轻量化、承载、一体化热防护结构研究进展情况。最后,在新设计理论与方法、新制造技术、服役特性、多功能一体化设计与实现四个方面对面向一体化热防护的陶瓷基复合材料轻量化结构的研究挑战进行了展望。本文为高超声速飞行器新型热防护结构的发展提供一定借鉴与思考。     

Tuning austenite stability in a medium Mn steel and relationship to structure and mechanical properties

We study here the underlying factors that govern the stability of austenite in a medium Mn (Fe–0.18C–11Mn–3.8Al) (wt-%) steel. In this regard, a novel heat treatment involving intercritical quenching and tempering was designed to obtain high total elongation (TEL) and high ultimate tensile strength (UTS) in the cold-rolled steel. And the UTS and TEL approached 920–1150?MPa and 35–65%, respectively. The product of TEL and UTS (PSE) exceeded 40?GPa%, with a maximum value of 60?GPa%. A detailed analysis of microstructure before and after tensile deformation revealed that the TRIP effect occurred and the stability of austenite was predominantly governed by the grain sizes of austenite rather than the orientation of austenite grains. The theoretical analysis of work hardening data suggested that the superior elongation of medium Mn TRIP steel is related to the high stability of austenite and the cooperative deformation of ferrite.     

Fatigue damage in a unidirectional SiC/Al composite due to push-pull and zero-pull loading

Axial fatigue behaviour due to fully reversible and zero-tension cyclic loads on specimens cut from a 5 mm thick panel of a unidirectional SiC/A1 composite has been investigated at room temperature. The panel contained 40 vol.% SiC fibres (SCS-2), sandwiched between 32 layers of A1 6061 foils, which were bonded together by hot-pressing. The loading was always parallel to the fibres. Steady hysteresis loops were observed in the stress-strain plot after about 3 cycles of loading. A plot of S/N curves showed that at load ratio R = 0 the fatigue strength of the composite was about 3 times higher than that of the monolithic matrix metal. At R = −1, however, the fatigue strength of the composite was even lower than that of the matrix metal. At both R = 0 and R = −1, the composite suffered large modulus losses (about 15%) at cycles well before the final failure. At R = 0 the modulus loss involved fibre breakage and matrix cracks, which were transverse as well as parallel to the loading direction, while at R = −1 it involved delamination cracks and barrelling of outer layers. Fractography after the final failure at R = 0 showed secondary cracks and fibre pull-out.     

Anisotropic oxidation due to aging in a triaxially braided composite and its influence on tensile failure

The service life of Polymer Matrix Composite Materials can be severely affected by long exposures to elevated temperatures. This study investigated the high temperature durability of a carbon/MVK10 triaxially braided textile composite. The goal was to monitor, analyze and quantify the evolution of physical and chemical aging processes to verify if Federal Aviation Administration (FAA) regulations for maximum service temperature ensure the safe use of such materials. Tensile tests were performed at the maximum service temperature on aged samples in two material directions to evaluate the progression of stiffness and Ultimate Tensile Strength (UTS) degradation. Aging induced damage was observed with the help of microscopic observations on the edges and cross-sections of tensile samples prior to loading. It was found that aging had a significant effect on UTS. It was also observed that the rate of deterioration was different for samples cut along different material directions, pointing towards anisotropic degradation mechanisms promoted by the braiding architecture. The investigated material had performances similar to existing high temperature composites, albeit at a lower temperature. It would therefore seem that FAA rules for setting the operating temperature are suitable for this material when aging effects are taken into consideration at the design stages.     

Measurements of the thermal conductivity of R22, R123, and R134a in the temperature range 250–340 K at pressures up to 30 MPa

This paper reports new, absolute measurements of the thermal conductivity of the liquid refrigerants R22, R123, and R134a in the temperature range 250–340 K at pressures from saturation up to 30 MPa. The measurements, performed in a transient hot-wire instrument employing two anodized tantalum wires as the heat source, have an estimated uncertainty of ±0.5%. A recently developed semiempirical scheme is employed to correlate successfully the thermal conductivity and the viscosity of these refrigerants, as a function of their density.