（中国微米纳米10035）基于叠层定向结构调控的高导热柔性聚偏氟乙烯/膨胀石墨复合材料

利用二次成型工艺制备了PVDF/EG叠层定向结构复合导热材料,并采用X衍射、扫描电镜对PVDF/EG复合材料及形貌进行表征,同时,研究了PVDF/EG叠层定向结构复合导热材料不同方向的热扩散特性、热导特性及材料的机械力学特性.叠层定向结构沿内部薄膜面内方向热扩散系数是其竖直方向的4.1倍,在EG质量分数为15wt％,20 wt％and 25％25wt％时,叠层定向结构PVDF/EG复合材料热导率分别为1.595 W/(m·K)、2.87 W/(m·K)和3.9843 W/(m·K),具有较低的"渗漏"阈值,表明利用二次成型工艺对PVDF/EG复合材料定向结构调控,能达到提高复合材料热导率的目的.而且,PVDF/EG叠层结构复合导热材料具有较好柔韧性,适合作为界面热导材料.     

Automatic image analysis and morphology of fibre reinforced concrete

Automatic image analysis is an efficient tool to quantify the morphology of materials. Moreover, it can aid to understand their mechanical behaviour. Several applications of automatic methods are presented to investigate concrete reinforced by ribbon shaped amorphous cast iron fibres. Introducing ribbons into the plain matrix entrapped air voids. This affected the workability and, later on, the compressive strength of the fibre reinforced concrete (FRC). Both were improved by additions of superplasticizer in order to keep the water to cement ratio constant. The influence of the superplasticizer and fibre contents on the compactness of the FRC was characterized by the dimensional and the spatial distributions of the air voids. The orientations of fibres and microcracks were quantified by Fourier image transforms. Due to the casting procedure of the FRC, the fibres were located in “horizontal layers”, perpendicular to the casting axis. Whatever the direction of compression with respect to the layers of fibres, the microcrack network was getting more and more oriented in the direction of compression as stresses increased. The analysis of fibre and microcrack orientations suggests that, under uniaxial compression, the inelastic strain domain should be characterized by an anisotropic biaxial damage model, whose principal axes are the orthogonal and parallel directions to the layers of fibres.     

Effect of polyacrylic acid (PAA) binder system on particle orientation during dry-pressing

Effect of polyacrylic acid (PAA) binder system on the development of particle orientation during uniaxial pressing is reported. Granules of different properties were compacted using uniaxial pressing at various pressure and their degree of particle orientation (DPO) was determined. The increment trend of DPO varies with granule strength and shows higher value for binder system of higher flexibility. Anisotropic shrinkage related to particle orientation is shown by compact of higher relative density (RD) and DPO; whereas anisotropic shrinkage related to non-uniform packing density is shown by compact of lower RD and DPO. Anisotropic shrinkage remains in the former compact while isotropic shrinkage was obtained for the latter compact at sintering temperature of 1600 °C. Subsequent cold isostatic pressing increases DPO and sintering shrinkage ratio of uniaxially pressed compacts.     

A model for process-based crash simulation

Manufacturing of a bumper system from aluminium extrusions often involves series of forming operations performed in the soft W-temper condition, and then artificially age-hardening of the components to the material's peak hardness T6 condition. It is probable that proper finite element (FE) modelling of the crash performance of the resulting systems must rely upon a geometry obtained from an FE model following the process route, i.e., including simulation of all major forming operations. The forming operations also result in an inhomogeneous evolution of some internal variables (among others the effective plastic strain) within the shaped components. Results from tensile tests reveal that plastic straining in W-temper leads to a significant change of the T6 work-hardening curves. In addition, the tests show that the plastic pre-deformation causes a reduction of the elongation of the T6 specimens. In the present work, these process effects have been included in a user-defined elastoplastic constitutive model in LS-DYNA incorporating a state-of-the-art anisotropic yield criterion, the associated flow rule and a non-linear isotropic work hardening rule as well as some ductile fracture criteria. A first demonstration and assessment of the modelling methodology is shown by ‘through-process analysis’ of two uniaxial tensile test series. The industrial use and relevance of the modelling technique is subsequently demonstrated by a case study on an industrial bumper beam system.     

An energetic approach to the analysis of anisotropic hyperelastic materials

We demonstrate that for the class of anisotropic hyperelastic materials with stiffening behaviour (i.e., the stiffness increases for increasing strain), it is possible to find an approximation by means of the linear superposition of an anisotropic quadratic potential, generated by the true linear elasticity tensor of the target material, and a suitable correction potential that is isotropic and hyperelastic. The proposed method can be implemented into commercially available Finite Element software by use of featured options only. This approach is intended to provide the solution to a stress-strain problem, based entirely on energetic considerations, which ensures the convexity of the potentials, and provides a simple material characterisation procedure.     

Monte Carlo simulation of grain growth of single-phase systems with anisotropic boundary energies

The grain growth in single-phase systems with various anisotropic boundary energies is simulated using a modified two-dimensional (2D) Monte Carlo algorithm. Structural self-similarity both in the grain shape distributions and the grain boundary configurations is found regardless of the degree and type of anisotropy. Parabolic evolution kinetics is observed in most cases. The growth rate represented by the time rate of average grain area change is found to be a function of the grain boundary properties. In some cases, the normalized grain size distributions are not time-invariant, and therefore, current grain growth theories cannot explain conclusively the occurrence of the parabolic kinetics in these cases. The effect of anisotropy on the grain growth kinetics is discussed.     

The viscoelastic bending stiffness of fiber-reinforced composite Ilizarov C-rings

In order to optimize the design of unidirectional fiber-reinforced composite (Ilizarov) C-rings, the viscoelastic load relaxation behavior was analyzed under a point load. Initially, the deflection and bending stiffness were calculated from the Castigliano theorem and the Euler–Bernoulli bending theory for the elastic solution. The viscoelastic relaxation and creep behavior were then derived from the elastic solution by using the correspondence theorem. Besides the orthotropic mechanical properties of the composite, the asymmetric mechanical properties due to different tensile and compressive properties were also considered. With the exception of the deviation, which was affected by a relatively large thickness ratio to the radius of the C-ring, the calculated relaxation showed good agreement with the experimental result.     

Estimation of length and surface of anisotropic capillaries

Surface density (S_V) and length density (L_V) of myocardial capillaries have hitherto been estimated from their profile boundary length (B_A) and their numerical density (Q_A) on transverse sections by the simplifying assumptions of the Krogh model (perfectly anisotropic, straight, unbranched capillaries with constant cross-sectional area). As the capillaries actually are partially anisotropic, curved, branching cylinders with variable cross-sectional area, a geometrical bias arises from the model-reality discrepancies. We have applied and compared two methods to overcome these inconsistencies: (1) estimation of L_V and S_V by a more realistic model (the Dimroth-Watson distribution); (2) estimation of L_V and S_V from isotropic uniform random (IUR) sections. Twelve male Wistar rats were fixed by retrograde vascular perfusion. One pair of longitudinal and transverse sections, and six IUR sections per animal were selected at random from the left ventricular papillary muscles. Ultrathin sections were silver-impregnated and studied by light microscopic morphometry. Nearly identical estimates of L_V and S_V were found by both methods. The model-based estimation provides biologically meaningful anisotropy constants, but it presupposes knowledge of the anisotropy axis. The IUR method provides no measure of anisotropy, but it can be applied in tissues where the anisotropy axis is not known. Both methods are equally efficient and practically unbiased in S_V estimation, but the model-based estimation is far more efficient in L_V estimation.     

Anisotropy of Superconducting Single Crystal SmFeAsO0.8F0.2 Studied by Torque Magnetometry

Single crystals of the oxypnictide superconductor SmFeAsO_0.8F_0.2 with T _c≃45(1) K were investigated by torque magnetometry. The crystals of mass ≤0.1 μg were grown by a high-pressure cubic anvil technique. The use of a high-sensitive piezoresistive torque sensor made it possible to study the anisotropic magnetic properties of these tiny crystals. The anisotropy parameter γ was found to be field independent, but varies strongly with temperature ranging from γ≃8 at T≲T _c to γ≃23 at T≃0.4T _c. This unusual behavior of γ signals unconventional superconductivity.      

The grain shape dependence of springback of integrated circuit leadframes

Anisotropy is known to be an important factor in affecting springback and deformation behaviour of cold rolled integrated circuit (IC) leadframes. Less work has been carried out to study the effect of grain shape on springback of IC leadframes, though it is considered to be a source of plastic anisotropy. In this paper, a plane stress model based on the concept of relaxed constraints has been developed to investigate the effect of grain shape on springback of a cold rolled copper alloy. Comparisons between the predictions by the relaxed constraint model and the conventional full constraints model and the experimental results have been made. It is found that significant improvement has been obtained by using the new model.