Electrodynamics of the vortex lattice in untwinned YBaCuO by complex impedance measurements

We report complex impedance measurements in an untwinned YBaCuO crystal. Our broad frequency range covers both the quasi static response and the resistive response of the vortex lattice. It allow us to characterize the irreversibility line without the need of any frequency dependent pinning parameters. We confirm the validity of the two modes model of vortex dynamic, and extract both the surface critical current and the flux flow resistivity around the first order transition T_m. This latter is identified by the abrupt loss of pinning and by an unexpected step of (T) at T_m. Received 22 November 2002 / Received in final form 17 February 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: alain.pautrat@ismra.fr RID="b" ID="b"UMR 6508 associée au CNRS     

降温速率对低温保存动脉粘弹性的影响

使用DMA测定了3种降温速率低温保存的家兔颈总动脉的蠕变曲线.结果表明:1.5℃/min降温速率低温保存后,血管的粘弹性最接近新鲜对照组;而对应降温速率1.5,5,10℃/min,血管粘弹性依次降低.动脉的粘弹性极有可能是评估其低温保存效果的潜在评价指标.     

Creep strain recovery of Fe-Ni-B amorphous metallic ribbon

Creep strain recovery and structural relaxation of the amorphous metallic glass Fe₄₀Ni₄₁B₁₉ after longtime loading at different annealing temperatures below the glass transition temperature have been studied using anisothermal differential scanning calorimetry (DSC) and dilatometry (TMA). It has been demonstrated that structural relaxation effects depend on the stress-annealing temperature of the amorphous ribbon. The structural relaxation states of the amorphous ribbon annealed at different temperatures under and without applied stress have been compared. The activation energy spectra were calculated from the anisothermal dilatometric measurements using the modern method based on the Fourier transformation technique. The influence of the annealing temperature on the shape of creep strain recovery spectra has been analyzed.      

基于多孔介质理论的混凝土徐变力学行为的有限元分析

水分是影响混凝土徐变的最重要因素之一,但通常为了计算的简便往往忽略了水分的影响,造成计算结果有很大的误差.文章根据混合物理论结合罚参数法、Galerkin加权残值法以及徐变应力分析的隐式解法,推导得到了混凝土徐变两相多孔介质模型.建立了有限元计算模型,给出了求解算法并编制了有限元程序.计算和试验比较分析表明,该理论和计算方法得到的结果遵循徐变相关性质,说明了该理论和计算模型的合理性.最后,指出了模型中存在的不足以及进一步的研究方向.     

Influence of humidity cycling parameters on the moisture‐accelerated creep of polymeric fibers

Accelerated creep is a curious and poorly understood transient moisture effect. The creep rates of most hydrophilic materials increase greatly with moisture content. However, when these same materials are subjected to creep loads in cyclic humidity environments, they often exhibit much higher creep rates than in a constantly humid state. This is called accelerated creep. Previous experimenters reported that accelerated creep was less likely to occur in polymeric fibers. We demonstrate experimentally that this happened only because of their choice of humidity cycling parameters. New results are given for Kevlar, lyocell, nylon‐6,6, and ramie fibers. Other paper scientists have argued that the absence of accelerated creep in single fibers supports a explanation based on fiber network effects for accelerated creep in paper. We argue here that accelerated creep is a more general phenomenon consistent with sorption‐induced stress‐gradient explanations. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2048–2062, 2001     

Creep and Long-Term Strength of Press-Powder Composites

The mechanical state of a press-powder filler for pyroautomatics system units of aerospace equipment during their long-term storage is investigated. With the use of the endochronic approach, the rheological properties of a dispersedly reinforced composite are simulated based on the conceptions of linear and nonlinear creep. Analytical expressions of constitutive relations of creep and strength for the composites are obtained. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 6, pp. 803–818, November–December, 2005.     

Reargument over E ～ j relation of high temperature superconductors

The E ∼ j relation of an HTSC slab is solved strictly within the Bean’s model in two different situations where the system does and does not reach equilibrium state under the magnetic relaxation respectively, with both forward and backward hopping taken into consideration (the backward hopping means the hopping from the barriers with low energy to high ones). Our results suggest and rigorously prove that the In E ∼ In j curves show only positive curvature on the side of slab where the directions of current-created field and the applied field are the same, while they show both positive and negative curvatures at a certain field range on the other side where the directions are opposite. The relationship of the positive and negative curvatures with the critical current, applied field and temperature is also discussed.     

AFM nanoindentation of viscoelastic materials with large end‐radius probes

We used atomic force microscopy (AFM) nanoindentation to measure mechanical properties of polymers. Although AFM is generally acknowledged as a high‐resolution imaging tool, accurate quantification of AFM nanoindentation results is challenging. Two main challenges are determination of the projected area for objects as small as AFM tips and use of appropriate analysis methods for viscoelastic materials. We report significant accuracy improvements for modulus measurements when large end‐radius tips with appropriate cantilever stiffnesses are used for indentation. Using this approach, the instantaneous elastic modulus of four polymers we studied was measured within 30 to 40% of Dynamic Mechanical Analysis (DMA) results. The probes can, despite their size and very high stiffnesses, be used for imaging of very small domains in heterogeneous materials. For viscoelastic materials, we developed an AFM creep test to determine the instantaneous elastic modulus. The AFM method allows application of a nearly perfect stepload that facilitates data analysis based on hereditary integrals. Results for three polymers suggest that the observed creep in the materials has a strong plastic flow component even at small loads. In this respect, the spherical indenter tips behave like “sharp” indenters used in indentation studies with instrumented indenters. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1573–1587, 2009