大理岩动态拉伸强度及弹性模量的SHPB实验研究

提出了获取脆性材料动态拉伸强度及弹性模量的实验步骤及相关记录数据的分析方法。利用直径为100mm的分离式Hopkinson压杆径向冲击巴西圆盘和平台巴西圆盘试样,测试了大理岩在高应变率加载下的动态力学性能。应力波加载下动态劈裂拉伸圆盘在试样中心产生了约45/s的拉伸应变率。分析了实验的有效性并考虑了试样两个端面应力波波形差异的影响以提高实验结果的精度。结果表明准静态下的公式可适用于动态劈裂拉伸实验;大理岩的动态拉伸强度及弹性模量比静态时有明显的增加。     

Water cavitation of hydrogels

Using the cavitation rheology (CR) technique developed in our labs, we show that fluids with negligible interfacial tensions with a surrounding material can be used to induce an elastic, cavitation instability in that material. We do this by changing the cavitation media from air, which was demonstrated to induce cavitation at the tip of a syringe needle in previous studies, to water, which has a negligible surface tension with the surrounding poly(vinyl alcohol) hydrogel material. In this case, the critical pressure in which this instability occurs can be directly related to the elastic modulus of the surrounding network and is shown to be nearly independent of length scale. This independence of size scale has important implications in the use of CR for the characterization of mechanical properties from molecular to macroscopic length scales. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1423–1427, 2010     

投影的约化极小模和空间的夹角

Let M and N be nonzero subspaces of a Hilbert space H, and PM and PN denote the orthogonal projections on M and N, respectively. In this note, an exact representation of the angle and the minimum gap of M and N is obtained. In addition, we study relations between the angle, the minimum gap of two subspaces M and N, and the reduced minimum modulus of （I - PN）PM,     

Use of the mark-tracking method for optical fiber characterization

The mark-tracking method was used in the uniaxial tensile test to determine the elastic properties of optical fibers. The mark-tracking method is based on the follow-up of two markers on the specimen with the help of an image processing technique. It allows us to determine the true strain with respect to the small strains assumption (≤1%) or the finite strains (>1%) without any impact of the rigid solid movement or pulley fiber sliding on the measured strain. Both coated optical fiber and stripped fiber were subjected to the uniaxial tensile test and the cantilever beam bending test. The Young’ modulus results of the stripped fiber were found to be very similar for both tests. Thus, the mark-tracking method is adaptable to the tensile test of optical fibers and the elastic behaviors of both coated optical fiber and stripped fiber are found to be non-linear. Their Young's moduli are 22 and 79 GPa, respectively. These results revealed that those coatings play a mechanical role in fiber elongation.     

辉光放电聚合物结构及力学性质研究

采用低压等离子体聚合技术,通过调整工作气压和反式二丁烯(T₂B)与H₂流量比制备了辉光放电聚合物(GDP)涂层. 利用傅里叶变换红外吸收光谱和元素分析法讨论了工作气压和T₂B与H₂流量比对GDP涂层内部H原子含量及结构的影响,并且通过纳米压痕技术对GDP涂层的硬度及杨氏模量进行了表征.研究结果表明:降低工作气压和T₂B与H₂流量比,GDP涂层内sp³ C 关键词： 傅里叶变换红外光谱 纳米压痕 杨氏模量 硬度     

Activation behavior and dielectric relaxation in polyvinyl alcohol and multiwall carbon nanotube composite films

The dc and ac electrical transport property of Polyvinyl Alcohol-Multiwall Carbon Nanotubes (PVA-MWNT) composites has been investigated within a temperature range and in the frequency range 20Hz-1MHz. The temperature variation of dc conductivity gives the presence of two different activation energies. The dielectric properties of the samples have been explained in terms of electric modulus vector.The dielectric relaxation has been explained in terms of interfacial polarization occurring in between the insulating PVA matrix and MWNT conductive filler. The variation of the relaxation time with temperature also indicates the presence of two different activation energies.     

Determination of Young's modulus of epoxy coated polyethylene micro-cantilever using phase-shift shadow moiré method

Young′s moduli of various epoxy coated polyethylene terephthalate (PET) micro-cantilevers were determined from the deflection results obtained using the phase-shift shadow moiré (PSSM) method. The filler materials for epoxy coatings were aluminum and graphite powders that were mixed with epoxy at various percentages. Young′s moduli were calculated from theory based on the deflection results. The PET micro-cantilever coated with aluminum-epoxy coating showed increasing value of Young′s modulus when the ratios of the aluminum-epoxy were increased. The graphite-epoxy coating on the PET micro-cantilever also showed the same trend. The experimental results also show that Young′s modulus of the graphite-epoxy coating is higher than aluminum-epoxy coating in comparison at the same mixing ratio.     

Mechanical characteristics of Fe-based coating obtained by nanoindentation

In this study, clad layers of iron-based alloy with a nature of self-fluxing were melted on low carbon steel by plasma cladding process. Nanoindentation with atomic force microscopy (AFM) has been used to investigate the mechanical properties of the coating. Hardness and elastic modulus at ultra-low loads were first determined using the method proposed by Giannakopoulos and Suresh (G&S method). The true contact area and mechanical properties were then determined using atomic force microscopy (AFM) combined with the Oliver and Pharr method (new proposed method) as the correction group. The mechanical properties calculated by the two methods showed the same distribution while had deviation in specific values. The effect of surface roughness to the calculated mechanical properties was investigated. Both hardness and elastic modulus were found to exhibit certain surface roughness dependence. When root mean square (RMS) roughness ranged from 2.2 nm to 4.4 nm, hardness calculated by both the methods increased obviously and reached maximums around 4.1 nm. Elastic modulus calculated by G&S method at different RMS showed the same distribution with that of hardness, while reduced elastic modulus obtained by AFM was insensitive to the range of RMS.     

Multiscale modeling of the effect of carbon nanotube orientation on the shear deformation properties of reinforced polymer-based composites

A combination of molecular dynamics (MD), continuum elasticity and FEM is used to predict the effect of CNT orientation on the shear modulus of SWCNT-polymer nanocomposites. We first develop a transverse-isotropic elastic model of SWCNTs based on the continuum elasticity and MD to compute the transverse-isotropic elastic constants of SWCNTs. These constants are then used in an FEM-based simulation to investigate the effect of SWCNT alignment on the shear modulus of nanocomposites. Furthermore, shear stress distributions along the nanotube axis and over its cross-sectional area are investigated to study the effect of CNT orientation on the shear load transfer.