Characterisation of thermal relaxations of polyaniline fibers by dynamic mechanical thermal analysis

Molecular motions and thermal transitions in polyaniline fibers (∅ = 50 μm), cast from emeraldine base powder dissolved in N,N′-dimethylpropylene urea (DMPU), have been investigated by dynamic mechanical thermal analysis (DMTA). For this purpose, a special picopendulum has been used, and a double sweep DMTA study has been performed. From these preliminary experiments, different relaxations observed have been discussed and compared to what is found in the literature.     

The evaluation of two-dimensional carbon-carbon composite material by dynamic mechanical analysis

Carbon composite materials, consisting of carbon cloth densified with a carbon matrix, have been examined by dynamic mechanical analysis (DMA). Flat panel and conic section structures were investigated over the temperature range ambient to 500°C. Reproducibility of the calculated tensile storage modulus values has been shown to be within 5% at any one temperature over the entire temperature range. This technique, which has been demonstrated to be non-destructive of the sample analyzed, defines differences in mechanical properties of the composite material formed into a conic section by comparison with those of the same material formed into a flat panel. Minor heterogeneity has also been shown within a single conic section as well as differences between pitch and CVD matrix samples. A sample orientation effect was found with the conic section samples. Details of the experimental technique and assumptions made during calculation of the mechanical properties are discussed.     

非均质材料等效力、热分析综述

实际工程问题中常会涉及非均质材料时间相关的力学、传热分析。这类问题的数值模拟具有重要的工程应用背景与理论探讨价值。一种直接的方式是分别考虑非均质材料组分的物理/几何特性,将问题在空间/时域离散后进行计算,这往往会导致计算量过大,甚至不可行。一个变通的策略是将非均质材料考虑成一种宏观均质材料,进行等效求解,从而大幅降低计算量。分别以粘弹性节理岩体及非均质线性瞬态热传导问题为研究对象,探讨了非均质材料时间相关的等效数值求解方法。     

Characterization of side-chain liquid crystal polymers using dynamic mechanical thermal analysis and dielectric thermal analysis

Ten members of a homologous series of side-chain liquid crystal polymers, the poly[[-(4-methoxybiphenylyl-4-oxy)--alkyl]methacrylate]s, have been characterized using dynamic mechanical thermal analysis (DMTA) and dielectric thermal analysis (DETA). For the undecyl and dodecyl homologues both techniques revealed a smectic-smectic transition not detected in previous studies either by differential scanning calorimetry or by optical microscopy. It is proposed that this corresponds to a monolayer-bilayer smectic A transition. A weak transition at ca. -30 °C is detected using DMTA for polymers with spacer lengths 4–8 and by analogy with poly(methyl methacrylate) may indicate the absorption of small amounts of water. DMTA also proved to be particularly effective in deconvoluting transitions and for the propyl homologue established the existence of a smectic phase suggested by optical microscopy.     

A model of the dynamic mechanical responses of wood, paper and some polymers to moisture changes

The dynamic mechanical responses of Scots pine, paper, PA6, cellophane, PVAc and PUR samples subjected to changes in the relative humidity of the surrounding air from 5 to 85% and vice versa have been analysed semiquantitatively on the basis of coupled non-linear rate equations for the moisture concentration in the sample as a function of the time. Important characteristics of the diffusion of moisture into and out of the samples have been studied by measuring the sample weight as a function of time. Moisture sorption results in sample swelling in Scots pine, paper, PA6, cellophane and PUR, for all of which mechanical loss peaks were detected. For PVAc, which does not bind moisture at load-bearing hydrogen bonding sites, no mechanical loss peak could be found. Characteristic of the mechanical loss at low vibration frequencies (0.01 to 1 Hz) is a peak immediately following a change in relative humidity for all studied sample materials except PVAc. This peak is almost certainly due to modulation of the number of load-bearing hydrogen bonds in the material. The relation between the width of the mechanical loss peak and the duration of the moisture sorption and desorption processes is interpreted according to an accepted model of two water molecule binding modes, one in which load-bearing hydrogen bonds are broken unimolecularly by water molecules and one in which further water molecules form clusters on the already unimolecularly bound water. © 1998 Chapman & Hall     

The elastic modulus of spruce wood cell wall material measured by an in situ bending technique

Using a novel in situ testing technique, the elastic modulus of wood cell wall material can be determined with great accuracy. The method relies on a focussed ion beam system (FIB) to prepare samples from individual structural components at a length scale which otherwise is hardly, if at all, accessible for testing. To determine the elastic modulus of cell wall material, cantilevers are cut with the FIB from wood cells for beam bending experiments inside the FIB or a scanning electron microscope (SEM). This type of sample preparation is site-specific and, at the same time, minimises the usual sample mounting problems. Once cut, the cantilever is tested by applying a known force with a piezoresistive AFM tip that is mounted on a micromanipulator. The resulting displacement is determined from SEM micrographs taken during the test. The cross-sectional area of the cantilever is determined for a number of positions along its length using the FIB as a cutting tool. Applying this method, we measured the elastic modulus of spruce wood cell wall material to be ∼28 GPa.     

α Transition of polyamide 6 in chemically bonded polyamide 6/polytetrafluoroethylene compounds studied by dynamic mechanical thermal analysis and dielectric thermal analysis

The α transition of polyamide 6 (PA 6) component in chemically bonded PA 6/polytetrafluoroethylene (PTFE) compounds is studied by dynamic mechanical thermal analysis (DMTA) and dielectric thermal analysis (DETA). It is found that DMTA shows better compatibility between two components than DETA does. It is also found that at the α transition temperature of PA 6 component (T _α^{PA 6}), the dynamic mechanical response of PTFE component is remarkable while its dielectric response is negligible. The effect of PTFE component on the segmental mobility of PA 6 component is discussed on the basis of DMTA, DETA and differential scanning calorimetry (DSC) results and the chemical bonding effect is found to play the dominant role. The measurement of apparent activation energy (ΔE _a) shows that the addition of PTFE component reduces the cooperativity of the α transition of PA 6 component.     

Thickness direction inhomogeneity of mechanical properties and fracture toughness as observed in aluminium 7075-T651 plate material

The variation of the mechanical properties, hardness and fracture toughness in the thickness direction of an aluminium 7075-T651 plate with a thickness of 25.4 mm is investigated. The results indicate that these properties show a distinct inhomogeneity in this direction. The influence of this inhomogeneity on fatigue crack growth rate and crack closure is investigated also. Explanations for this inhomogeneous behaviour are discussed, including observations on inhomogeneity of grain size and chemical composition.     

Glass transitions of hygrothermal aged pultruded glass fibre reinforced polymer rebar by dynamic mechanical thermal analysis

The changes that can occur in glass fibre reinforced polymer (GFRP) composites with ageing can affect its application, performance and lifetime. Hygrothermal ageing (i.e. accelerated ageing by moisture absorption and temperature change) is a very useful technique to evaluate durability as well as development of GFRP composites in a reasonable timeframe. Dynamic mechanical thermal analysis (DMTA) is essentially able to detect all changes in the state of molecular motion in polymer composites as temperature is scanned. In this work, pultruded GFRP rebars were accelerated aged in an alkaline aqueous environment at 60 °C for 1, 2, 3, 4 and 6 months to evaluate the changes in glass transition of viscoelastic GFRP rebars by DMTA. Five different glass transitions in an average temperate range from 110 to 165 °C were observed at storage modulus, loss modulus and damping factor traces of DMTA. It was also found that glass transition temperature (T_g) of the aged samples changed up to maximum 6 °C compared with that of controlled sample. This change in T_g with ageing time was believed to be due to moisture absorption by rebars.     

Internal friction of Cu-13.5Al-4Ni shape memory alloy measured by dynamic mechanical analysis under isothermal conditions

Cu-13.5Al-4Ni shape memory alloy (SMA) exhibits a β₁(DO₃) → β₁′ (18R) internal friction peak with high damping capacity and elevated martensitic transformation temperature in a dynamic mechanical analysis tan δ cooling curve. When the specimen is isothermally maintained at peak temperature, the damping capacity decreases significantly and reaches a steady value. The inherent and intrinsic internal frictions of Cu-13.5Al-4Ni SMA are extremely low because the β₁′ (18R) martensite has an ordered 9R structure with stacking faults rather than twinning with movable twin boundaries.     

Tensile properties of cork in the tangential direction: Variation with quality,porosity, density and radial position in the cork plank

The behaviour of cork under tensile stress in the tangential direction was studied using cork planks of two commercial quality classes and samples taken at three radial positions in the cork planks.     

正交各向异性材料结构热变形和热应力分析的无网格法计算模型及应用正交各向异性材料结构热变形和热应力分析的无网格法计算模型及应用

利用无网格伽辽金法（Element-free Galerkin,EFG）建立了正交各向异性材料结构热变形和热应力分析的计算模型,并推导了正交各向异性材料结构热弹性问题的EFG法离散控制方程。选择复合材料冷却栅管算例验证了计算模型和程序的正确性,利用该计算模型分析了具有不同材料方向角及热导率因子、热膨胀系数因子和主次泊松比因子的汽轮机叶轮,得到了其热变形总位移和Mises应力,讨论了材料方向角和上述正交各向异性材料因子对其热变形总位移和Mises应力的影响规律,给出了这些参数的合理取值范围,并选取一组参数与各向同性材料结构进行了热变形和热应力对比分析。结果表明,基于EFG法的热变形总位移和Mises应力的计算精度比有限元法高,材料方向角同时影响热变形总位移和Mises应力的大小和方向,而正交各向异性材料因子只影响热变形总位移和Mises应力的大小,不影响其方向。在复合材料结构设计过程中,合理选取材料方向角和正交各向异性材料因子可有效减小结构热变形和热应力。      

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.     

Synthesis and thermal analysis of aluminium nitride filled epoxy composites and its effective application as thermal interface material for LED applications

The improvement of heat conduction in any electronic devices has become a predominant issue in which effective heat dissipation is crucial to enhance the performance of packaged devices. This paper elucidates the application of thermally conductive particles filled composites as thermal interface material for LEDs. Present work aims on reducing the junction temperature and thermal resistance of the device under test with heavily filled ceramic-epoxy composite as the interface material between the device and metal substrate. Silane treated aluminium nitride (AlN) powder was studied for its feasibility as the filler material. The thermal conductivity values obtained by hot disc method (ISO/DIS 22007-2.2) were 0.66, 0.54 and 0.44 W/mK for 60, 50 and 40 wt% AlN filled epoxy composites respectively which were described well by thermal transient measurement of LEDs. The junction temperature and total thermal resistance of the thermal set up was reduced significantly with increased filler loading. The least junction to ambient thermal resistance (R_thJ-A) was achieved for 60 wt% followed by 50 and 40 wt% AlN filled TIM with the values of 24.8, 31.98 and 34.64 K/W respectively. Characteristics of the AlN filled composites for LED applications are discussed extensively in terms of thermogravimetric and thermo-mechanical analysis.     

Study the dynamic crack path in brittle material under thermal shock loading by phase field modeling

For a wide variety of quasi-brittle materials, the constitutive microplane models of damage are capable of describing the anisotropic development and growth of microcracks when materials exhibit inelastic response. Damage development in solids leads to the degradation of the macroscopic material stiffness and results in different response in loading and unloading. On the other hand, the constitutive microplane models of plasticity describe the anisotropic plastic sliding that originates macroscopic permanent deformation and remains upon unloading. For realistic modeling of these materials, in which both damage and plasticity mechanisms can evolve simultaneously, the microplane damage and plasticity models can be coupled in a systematic and robust manner. This work presents a theoretical formulation of a consistent framework to couple both microplane damage and plasticity models for triggering inelastic behavior (damage and plastic effects) in engineering materials. Throughout the derivation, it is specifically shown that the proposed derivation complies with the thermodynamical restrictions with regard to the assessment of the local energy dissipation based on the Clausius–Duhem inequality. Finally, the algorithmic treatment of the developed constitutive framework is outlined for its incorporation into incremental-iterative solution procedures using Newton–Raphson schemes and examined by means of simple benchmark examples.     

Thermodynamic analysis of the reasons for the difference between the values of the thermal conductivities of gases as measured by steady-state and transient methods

It is shown that the effective thermal conductivities of a gas measured by steady-state and transient methods are not equal.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 43, No. 5, pp. 804–807, November, 1982.     

Oxidized wax as compatibilizer in linear low-density polyethylene-clay nanocomposites: x-ray diffraction and dynamic mechanical analysis

Oxidized paraffin wax was used as a compatibilizer in composites of linear low-density polyethylene and layered nano silicate clays. X-ray diffraction analyses were carried out to investigate the crystalline morphology of five types of clays, oxidized wax, and their composites with LLDPE. The composites exhibited different X-ray diffraction and dynamic mechanical behaviour in the presence of different clays. Generally, the composites retained the partially crystalline behaviour of LLDPE, and no exfoliation was observed. Increased amount of wax did not change the morphology in most cases. The incorporation of clay resulted in an observable increase in the storage modulus of LLDPE. These values also increased with the addition of oxidized wax for most of the composites. The loss modulus increased with the amount of clay, irrespective of its nature. In most cases these values also increased with the incorporation of wax. The composites with 10% clay and 10% oxidized wax showed the highest storage and loss moduli, irrespective of the nature of the clay. The tan delta values did not change considerably with the addition of clay or wax.     

Postbuckling of nanotube-reinforced composite cylindrical shells under combined axial and radial mechanical loads in thermal environment

A postbuckling analysis is presented for nanocomposite cylindrical shells reinforced by single-walled carbon nanotubes (SWCNTs) subjected to combined axial and radial mechanical loads in thermal environment. Two types of carbon nanotube-reinforced composite (CNTRC) shells, namely, uniformly distributed (UD) and functionally graded (FG) reinforcements, are considered. The material properties of FG-CNTRCs are assumed to be graded in the thickness direction, and are estimated through a micromechanical model. The governing equations are based on a higher order shear deformation shell theory with a von Kármán-type of kinematic nonlinearity. The thermal effects are also included and the material properties of CNTRCs are assumed to be temperature-dependent. A boundary layer theory and associated singular perturbation technique are employed to determine the buckling loads and postbuckling equilibrium paths. The numerical illustrations concern the postbuckling behavior of perfect and imperfect, FG-CNTRC cylindrical shells under combined action of external pressure and axial compression for different values of load-proportional parameters. The results for UD-CNTRC shell, which is a special case in the present study, are compared with those of the FG-CNTRC shell.