一种高聚物粘结炸药和B炸药的本构关系研究

 利用万能材料试验机对一种高聚物粘结炸药（简称PBX炸药）和压装B炸药进行了准静态压缩实验,得到了两种炸药在不同应变率下的应力应变曲线。通过两种炸药在不同应变率下力学性能的研究,建立了两种炸药含应变率效应的本构方程。结果表明,建立的本构方程能较好地描述两种炸药在弹性阶段和强化阶段的力学性能。     

Micromechanical modelling of an arbitrary ellipsoidal multi-coated inclusion

The present work aims to provide a general framework to deal with an elementary heterogeneous problem, where the inhomogeneity consists of an n-layered inclusion composed of n concentric ellipsoids made of anisotropic elastic materials. The methodology is based on a combination of Green's function techniques with interface operators, illustrating the stress and strain jump conditions at the interfaces between two adjacent coatings, which are considered perfectly bonded. The model is validated in the case of double-coated spherical inclusions made of isotropic materials, where the obtained analytical results cover the exact solution of Hervé and Zaoui. The model can be applied, after adequate choice of scale-transition methods, to describe the overall behaviour of real composite materials with complex microstructures that are significantly influenced by the presence of interphase layers between constituents (fillers and matrix). Such composites are widely employed in automotive and aerospace industries. As a typical example one can consider a composite with an epoxy matrix reinforced by glass beads coated using a thin soft polymeric phase or syntactic foams particulate composites obtained by filling a polymeric matrix with hollow solid inclusions.     

Volume dilatation in a polycarbonate blend at varying strain rates

Impact loaded polymers show a variety of strain-rate dependent mechanical properties in their elastic, plastic and failure behaviour. In contrast to purely crystalline materials, the volume of polymeric materials can significantly change under irreversible deformations. In this paper, uni-axial tensile tests were performed in order to measure the dilatation in the Polycarbonate-Acrylnitril-Butadien-Styrol (PC-ABS) Bayblend T65. The accumulation of dilatation was measured at deformation speeds of 0.1 and 500 [mm/s]. Instrumented with a pair of two high-speed cameras, volume segments in the samples were observed. The change in volume was quantified as relation between the deformed and initial volumes of the segments. It was observed that the measured dilatations are of great significance for the constitutive models. This is specifically demonstrated through comparisons of stress-strain relations derived from the two camera-perspectives with isochoric relations based on single-surface observations of the same experiments.     

Linear and cross-linked polymeric solid-state dye lasers based on 8-substituted alkyl analogues of pyrromethene 567

The lasing properties of analogues of the commercial laser dye pyrromethene 567 (PM567) incorporated (dissolved or copolymerized) into polymeric matrices have been studied using both linear and cross-linked methyl methacrylate-based copolymers with different degrees of functionalization. All synthesized dyes have the chromophore core of PM567 but with an ω-acyloxypolymethylene chain at position 8. In general, the new materials exhibited laser emission with higher efficiencies and much higher photostabilities than those containing the commercial dye PM567 under the same experimental conditions. Lasing efficiencies of up to 40% were obtained under transversal pumping at 534 nm. The highest photostabilities, with the laser output remaining stable or dropping by less than 15% after 100 000 pump pulses at the same position of the sample, were reached in cross-linked materials with dyes covalently linked to the polymeric chains. When the polymeric samples were incorporated into a grazing-incidence grating oscillator, narrow-line-width operation with tuning ranges of up to 40 nm was obtained. These results show that efficient and photostable solid-state dye lasers competitive with their liquid counterparts can be developed by adequate chemical modifications in the dye molecules, as well as by the selection of appropriate polymeric formulations.     

Novel experimentally observed phenomena in soft matter

Soft materials such as colloidal suspensions, polymer solutions and liquid crystals are constituted by mesoscopic entities held together by weak forces. Their mechanical moduli are several orders of magnitude lower than those of atomic solids. The application of small to moderate stresses to these materials results in the disruption of their microstructures. The resulting flow is non-Newtonian and is characterized by features such as shear rate-dependent viscosities and non-zero normal stresses. This article begins with an introduction to some unusual flow properties displayed by soft matter. Experiments that report a spectrum of novel phenomena exhibited by these materials, such as turbulent drag reduction, elastic turbulence, the formation of shear bands and the existence of rheological chaos, flow-induced birefringence and the unusual rheology of soft glassy materials, are reviewed. The focus then shifts to observations of the liquid-like response of granular media that have been subjected to external forces. The article concludes with examples of the patterns that emerge when certain soft materials are vibrated, or when they are displaced with Newtonian fluids of lower viscosities.     

用纳米压痕法分析溶剂对聚醚醚酮(PEEK)力学性能的影响

Poly (ether ether ketone)(PEEK) is a high-performance semi-crystalline thermoplastic polymer.Exposure of the polymeric surface to solvents can have a strong effect like softening/swelling of polymeric network or dissolution.In this study,nano-indentation analysis was performed to study the effect of acetone on the surface mechanical properties of PEEK using different exposure time.The experiments were performed with a constant loading rate (10 nm/s) to a maximum indentation displacement (1000 nm).A 30-second hold segment was included at the maximum load to account for any creep effects followed by an unloading segment to 80% unloading.The indentation hardness and the elastic modulus were computed as a continuous function of the penetration displacement in the continuous stiffness mode (CSM) indentation.The experimental data showed that the peak load decreased from ~5.2 mN to ~1.7 mN as exposure time in solvent environment increased from 0 to 18 days.The elastic modulus and the hardness of PEEK samples also displayed a decreasing trend as a function of exposure time in the solvent environment.Two empirical models were used to fit the experimental data of hardness as a function of exposure time which showed a good agreement with the experimental values.     

Shungite - a carbon-mineral filler for polymeric composite materials

Shungite rock material after fine grinding was investigated as a filler for polymeric composites. Shungite carbon structural and physico-chemical peculiarities predetermine shungite filler behaviour in polymeric matrixes. Specific interaction between the network of globules (the main structural units of shungite carbon of nanoscale size) and the network of mineral phase structural units in every shungite filler particle leads to the amphiphylic surface properties of the filler. Influence on rheological properties of different polymeric composite materials was demonstrated as an example of advanced potential possibilities of shungite filler. The metastable structure of shungite carbon and its reactivity seemed to be the key to understanding the observed effect.     

Structural relaxation and anharmonicity in polymer electrolytes: Effect of the thermal history

Summary The effects of the storage at room temperature of PEO-KSCN polymer electrolytes have been studied by differential scanning calorimetry (DSC) and dynamical mechanical analysis. It has been revealed that, over the explored time interval, the annealing causes small variations in the anharmonic and relaxation properties of the samples, which are to be ascribed to changes in the relative amount of the phases building up the structure. The elastic and anelastic characteristics show a well-defined dependence on the degree of crystallinity of the polymer, which grows slightly with increasing annealing time. The application of a simplified version of a quasi-harmonic model and of the Kolrausch-Williams-Watts stretched exponential function permits to describe the temperature behaviour of the elastic modulusE′ and to obtain an anharmonicity parameter characterizing the polymeric system. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Transverse excitations in liquid Ga

The transverse acoustic excitation modes were detected by inelastic x-ray scattering in liquid Ga in the Q range above 9?nm^?1 although liquid Ga is mostly described by a hard-sphere liquid. An ab initio molecular dynamics simulation clearly supports this finding. From the detailed analysis for the S(Q,ω) spectra with a good statistic quality, the lifetime of 0.5 ps and the propagating length of 0.4–0.5?nm can be estimated for the transverse acoustic phonon modes, which may correspond the lifetime and size of cages formed instantaneously in liquid Ga. The microscopic Poisson’s ratio estimated from the dynamic velocities of sound is 0.42, indicating a rubber-like soft elastic property of the cages.     

Random networks of fibres display maximal heterogeneity in the distribution of elastic energy

Above a small length scale, the distribution of local elastic energies in a material under an external load is typically Gaussian, and the dependence of the average elastic energy on strain defines the stiffness of the material. Some particular materials, such as granular packings, suspensions at the jamming transition, crumpled sheets and dense cellular aggregates, display under compression an exponential distribution of elastic energies, but also in this case the elastic properties are well defined. We demonstrate here that networks of fibres, which form uncorrelated non-fractal structures, have under external load a scale invariant distribution of elastic energy (epsilon) at the fibre-fibre contacts proportional to 1/epsilon. This distribution is much broader than any other distribution observed before for elastic energies in a material. We show that for small compressions it holds over 10 orders of magnitude in epsilon. In such a material a few 'hot spots' carry most of the elastic load. Consequently, these materials are highly susceptible to local irreversible deformations, and are thereby extremely efficient for damping vibrations.     

Thermal expansion of the C15 Laves-phase materials TaV2 and TaV2H x

The thermal expansion of the C15 Laves-phase material TaV₂H _x was measured over the temperature range of 130–350?K for x?=?0, 0.34 and 0.53. A primary objective of the study was to see if the anomalous temperature dependence of elastic constants found in an earlier study is due to, or associated with, unusual thermal expansion. The expansion was typical of transition metals, and quite close to theoretical results for related Laves-phase materials. Grüneisen parameters were determined from the measurements, and these were also typical of transition metals, and close to calculated values for related materials. Both the thermal expansion and the Grüneisen parameters of TaV₂ decreased somewhat with the addition of hydrogen. No anomalous behavior of the expansion was found that might account for the highly anomalous temperature dependence of elastic moduli reported earlier. This absence is consistent with the attribution of the elastic constant behavior to an electronic structure effect.     

Elastic properties of the premelting phases of the perfluoroalkanes C16F34 and C20F42

The elastic properties of the solid state of linear oligomers are closely related to their special intra-and inter-molecular interactions. This is proved for the perfluoroalkanes C₁₆F₃₄ and C₂₀F₄₂. The elastic tensors of both materials have been measured with Brillouin spectroscopy as a function of temperature. Both systmes show an extremely weak shear stiffnessc ₄₄, indicating the existence of a smectic-B like structure and an unexpected strong elastic anisotropy inversion in the premelting phase. Moreover a Curie-Weiss-law behavior was found for the stiffness constantc ₃₃ indicating the existence of a soft longitudinal acoustic phonon mode. The transition temperatureT _c is for C₂₀F₄₂ below the melting point. The elastic properties are purely static. The observations are tentatively attributed to an intermolecular premelting process.     

On the rigid to floppy transitions in calcium silicate glasses from Raman scattering and cluster constraint analysis

Calcium silicate glasses xCaO???(1???x)SiO₂ exhibit a threshold in Raman line-shapes which can be related, on the basis of Maxwell constraint counting, to the onset of network rigidity as the concentration of calcium oxide x is decreased. The present results are more deeply characterized by a size-increasing cluster approximation that allows to perform Maxwell mechanical constraint counting beyond the usual meanfield treatment. This permits to discuss under which structural conditions an elastic intermediate phase should be obtained in the future.     

A Change of Phase Morphology in Poly Lactic Acid/Poly Methyl Methacrylate Blends Induced by Graphene Nano Sheets

Blending of polymeric materials is an effective way to obtain materials with specific properties, since the properties of these multiphase polymeric materials are not only affected by the properties of the component polymers but also by the morphology formed. The research described here was focused on investigation of the morphology of polymer blends of poly lactic acid (PLA) and poly methyl methacrylate (PMMA) and the PLA/PMMA blends containing various amounts of graphene nano plates, (GNP). In this work, the blends were prepared by solution casting and the morphologies of these nano filled polymer blends were studied. By adding graphene nano plates into the PLA/PMMA blends, the morphology changed for all compositions. It was very interesting to note that the GNP were found to be preferentially located in one of the polymer phases, different for the different loadings, and its location determined the final morphology of the PLA/PMMA blends. The morphology of the blends was observed by SEM and the composition-morphology dependence responses were investigated using a Fourier transform infra-red (FTIR) spectroscopy technique.     

Crack motion in viscoelastic solids: The role of the flash temperature

We present a simple theory of crack propagation in viscoelastic solids. We calculate the energy per unit area, G(v), to propagate a crack, as a function of the crack tip velocity v. Our study includes the non-uniform temperature distribution (flash temperature) in the vicinity of the crack tip, which has a profound influence on G(v). At very low crack tip velocities, the heat produced at the crack tip can diffuse away, resulting in very small temperature increase: in this “low-speed” regime the flash temperature effect is unimportant. However, because of the low heat conductivity of rubber-like materials, already at moderate crack tip velocities a very large temperature increase (of order of 1000 K) can occur close to the crack tip. We show that this will drastically affect the viscoelastic energy dissipation close to the crack tip, resulting in a “hot-crack” propagation regime. The transition between the low-speed regime and the hot-crack regime is very abrupt, which may result in unstable crack motion, e.g. stick-slip motion or catastrophic failure, as observed in some experiments. In addition, the high crack tip temperature may result in significant thermal decomposition within the heated region, resulting in a liquid-like region in the vicinity of the crack tip. This may explain the change in surface morphology (from rough to smooth surfaces) which is observed as the crack tip velocity is increased above the instability threshold.     

类固态颗粒物质的剪切弹性行为测量

利用能以极慢变形率直接剪切颗粒固体的实验装置,测量了(玻璃珠)样品对大幅度循环剪切的力-位移曲线,以及一个循环周期后的塑性位移残留.发现随着循环频率的降低,样品会从有限塑性残留的弹塑行为转变到几乎没有塑性的纯弹性行为,同时伴随有率相关性.该转变在剪切力幅度高达样品破坏值的90%时依然存在,但需要极小的变形率(10~(-5)Hz)或惯性数(10~(-8)).这意味着无论是高频小幅度的声波扰动,还是极低频大幅度的直接剪切,静态颗粒固体都可做出纯弹性的力学响应.在足够慢的状态变化范围里,它仍是属于经典弹性理论范畴的一类材料.这个弹性区域一直未被报道和关注,可能是观测它时需要样品的变形率远比通常此类研究中所采用的慢变形还要小许多(大约两个数量级)的缘故.理论上本文测量结果支持描述颗粒固体宏观动力学的基本方程组,不能只有弹塑和率无关行为,它们必须在极慢变形极限下退化为经典弹性理论,并且在这个转变过程中表现出率相关特性.     

Force distributions and force chains in random stiff fiber networks

We study the elasticity of random stiff fiber networks. The elastic response of the fibers is characterized by a central force stretching stiffness as well as a bending stiffness that acts transverse to the fiber contour. Previous studies have shown that this model displays an anomalous elastic regime where the stretching mode is fully frozen out and the elastic energy is completely dominated by the bending mode. We demonstrate by simulations and scaling arguments that, in contrast to the bending dominated elastic energy, the equally important elastic forces are to a large extent stretching dominated. By characterizing these forces on microscopic, mesoscopic and macroscopic scales we find two mechanisms of how forces are transmitted in the network. While forces smaller than a threshold F_c are effectively balanced by a homogeneous background medium, forces larger than F_c are found to be heterogeneously distributed throughout the sample, giving rise to highly localized force chains known from granular media.