用动力学法测杨氏模量实验及其实验装置的研制

介绍了新研制的用动力学法杨氏模量的实验装置,它具有功耗低,灵敏度高的特点;教学中测量悬线不同位置时的共振频率,用作图法拟合出其精确值,以减少由于悬线不在节点引起的系统性误差,测量精密度比传统的静态拉伸法高一个量级以上。     

Elastic constants of lead-bismuth alloys

A study of ultrasonic velocities and internal friction has been carried out in Pb-Bi alloys in the concentration range of 0 to 49.5 atomic % Bi using the composite oscillator technique. From the velocity and density data a set of elastic constants namely, Young’s modulus, rigidity modulus, bulk modulus and Poisson’s ratio are estimated. The results are interpreted in terms of the phase changes occurring in the alloy system. Internal friction is found to be more sensitive than the elastic constants to the phase changes.     

Zum Fließverhalten kurzglasfasergefüllter Thermoplastschmelzen im stationären und instationären Bereich

Zusammenfassung Durch Zugabe von Glasfasern wird das rheologische Verhalten von Kunststoffschmelzen verändert. Kurzglasfasergefüllte Styrol-Acrylnitril-Copolymere mit verschiedenen Füllgraden wurden mit dem Rotations-und dem Kapillarrheometer untersucht. Sowohl im stationären Bereich als auch im instationären Bereich (Anlauf- und Abklingverhalten) sind Unterschiede zum ungefüllten Material festzustellen: Mit Zunahme des Faseranteils steigt der Schubmodul, und das viskoelastische Überschwingen wird geringer als bei ungefüllten Systemen. Erhöhung von Temperatur, Schergeschwindigkeit und Faseranteil führen zu einer Verkürzung der Relaxationszeiten, Steigerung des hydrostatischen Druckes erwartungsgemäß zu deren Verlängerung.

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Summary The rheological behaviour of polymer melts is changed by addition of glass fibres. Experiments were conducted with styrene-acrylonitrile copolymers filled with various amounts of short fibres on a rotational and a capillary rheometer, both under steadystate and transient conditions. It is observed that by adding fibres the shear modulus increases but the stress overshoot decreases. With increase of temperature, shear rate and fibre content relaxation times become shorter whereas with higher hydrostatic pressures they grow longer, as is to be expected.

Mit 9 Abbildungen und 2 Tabellen     

Elastic properties of poly(hydroxybutyrate) molecules

Elasticity of various poly(hydroxybutyrate) (PHB) molecules of regular and irregular conformational structure was examined by the molecular mechanics (MM) calculations. Force - distance functions and the Young's moduli E were computed by stretching of PHB molecules. Unwinding of the 2(1) helical conformation H is characterized at small deformations by the Young's modulus E = 1.8 GPa. The H form is transformed on stretching into the highly extended twisted form E, similar to the beta-structure observed earlier by X-ray fiber diffraction. The computations revealed that in contrast to paraffins, the planar all-trans structure of undeformed PHB is bent. Hence, a PHB molecule attains the maximum contour length in highly straightened, but slightly twisted conformations. A dependence of the single-chain moduli of regular and disordered conformations on the chain extension ratio x was found. The computed data were used to analyze elastic response of tie (bridging) molecules in the interlamellar (IL) region of a semi-crystalline PHB. A modification of the chain length distribution function of tie molecules tau(N) due to secondary crystallization of PHB was conjectured. The resulting narrow distribution tau(N) comprises the taut tie molecules of higher chain moduli prone to overstressing. The molecular model outlined is in line with the macroscopically observed increase in the modulus and brittleness of PHB with storage time.     

TURNING-OVER OF THE ELECTRIC DIPOLE IN A POLYMER*

The transition from the biexciton to the exciton can turn over the direction of the electric dipole of a polymericmolecule. This turning-over action combined with the photoinduced polarization reversion can be used as a switch. Theswitching speed is governed by the relaxation time of the turning-over process, which can be determined by a dynamicalsimulation.     

Gaussian approximation in the theory of MR signal formation in the presence of structure-specific magnetic field inhomogeneities

A detailed theoretical analysis of the free induction decay (FID) and spin echo (SE) MR signal formation in the presence of mesoscopic structure-specific magnetic field inhomogeneities is developed in the framework of the Gaussian phase distribution approximation. The theory takes into account diffusion of nuclear spins in inhomogeneous magnetic fields created by arbitrarily shaped magnetized objects with permeable boundaries. In the short-time limit the FID signal decays quadratically with time and depends on the objects' geometry only through the volume fraction, whereas the SE signal decays as 5/2 power of time with the coefficient depending on both the volume fraction of the magnetized objects and their surface-to-volume ratio. In the motional narrowing regime, the FID and SE signals for objects of finite size decay mono-exponentially; a simple general expression is obtained for the relaxation rate constant deltaR2. In the case of infinitely long cylinders in the motional narrowing regime the theory predicts non-exponential signal decay lnS approximately -tlnt in accordance with previous results. For specific geometries of the objects (spheres and infinitely long cylinders) exact analytical expressions for the FID and SE signals are given. The theory can be applied, for instance, to biological systems where mesoscopic magnetic field inhomogeneities are induced by deoxygenated red blood cells, capillary network, contrast agents, etc.     

Old and new from SOR

From the long history of Successive Over Relaxation (SOR) between the end of second world war and today three points are considered: (1) Classical results of Young and Varga are described. (2) It is shown how results on semiiterative methods can be used to derive these classical results in a unifying way and to compare SOR with other iterative methods. (3) In the last 15 years the application of SOR to compute the stationary distribution of a homogeneous Markov chain has been discussed. These results are reported, considering especially the term extended convergence introduced by Kontovasilis, Plemmons and Stewart.     

金属丝的杨氏模量测定实验的改进

对金属丝杨氏模量测定仪重新设计.将金属丝水平放置,在中点处悬挂重物,通过测量金属丝伸长后的转角,进而测量金属丝的杨氏模量.     

基于AVR单片机的万能材料试验机控制系统

采用AVR单片机AT90S8515作为控制核心,通过步进电机带动丝杠运动,利用光栅编码器、高精度力传感器、2个引伸计分别测量了位移、力及试件横、纵变形,在老式电子万能材料实验机上建立了新型计算机控制系统,可方便测定弹性模量和泊松比.