不同加载边界下混凝土巴西劈裂过程及强度的DIC实验分析

平板、垫条、圆弧压模及平台试样等不同加载方式下, 脆性材料巴西劈裂实验结果的差异一直是人们所讨论的问题. 本文设计了混凝土平板直接加载与采用垫条加载实验, 采用理论、高速相机与数字图像相关法(digital image correlation, DIC)相结合的方法, 对试样表面应变场演化、起裂位置及裂纹扩展过程进行分析, 探讨了不同的加载边界对试样应变集中演化及劈裂拉伸强度结果的影响. 结果显示: (1)混凝土拉伸非线性特性导致的应变集中演化对不同加载条件非常敏感, 平板加载时应变集中较早在加载端起始并向中心演化, 即使满足巴西实验\"中心起裂\"条件, 其强度仍低于垫条加载情况, 两者差别达17.9%; (2)如果软垫条加载接触角合理设计, 与平板直接加载相比, 其应力场更稳定, 有利于保证圆盘试样的应变集中及断裂均从中心起始, 更好满足巴西劈裂实验条件; (3)仅校验巴西实验 \"中心起裂\"有效性条件是不充分的, 设计时必须谨慎考虑. 研究结果对脆性材料巴西劈裂实验设计、测试分析具有重要参考意义.      

Synergistic flame retarded poly(methyl methacrylate) by nano-ZrO2 and triphenylphosphate

The synergistic flame-retarded systems consisting of nano-ZrO₂ and triphenylphosphate(TPP) for poly(methyl methacrylate)(PMMA) are reported. The synergistic effects were studied by cone calorimeter test, thermal gravimetric analysis (TG), Raman spectra, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The synergistic effect of nano-ZrO₂ with TPP could be clearly observed by cone calorimeter test. The Raman spectra, SEM, and XPS results provide evidence that nano-ZrO₂ can efficiently promote the formation of charred layers composed of varying amounts of graphite and amorphous carbon. The possible mechanisms for synergy are discussed.     

The Effect of a Low-Frequency Structure on Passive Scalar Transport in the Flow Over a Surface-Mounted Rib

The unsteadiness of the flow over a surface-mounted rib involving passive scalar transport is studied using large-eddy simulation (LES) at a Reynolds number of 3000 (based on the rib height, \(h\), and the free-stream velocity, \(U_{0})\). The purpose of the present study is to gain further insight into the physical origin of the flow instability and its effect on passive scalar transport. Fourier spectral analysis of the velocity at different positions suggests that, in addition to the K-H instability in the shear layer (St ≈?0.42), two lower frequencies (St ≈?0.06 and 0.09) also exist. It is observed that the low-frequency instabilities accompany the shedding process of vortical structures. One low frequency, at \(\text {St}\approx 0.06\), is related to the pumping motion of the recirculation bubble, while the other, at \(\text {St}\approx 0.09\), is associated with the flapping mode of the shear layer. Through comparisons of velocity and temperature fields and the spectra of scalar fluctuations, it is found that the passive scalar is transported by the convection of vortical structures.     

Two-level stabilized nonconforming finite element method for the Stokes equations

In this article, we present a new two-level stabilized nonconforming finite elements method for the two dimensional Stokes problem. This method is based on a local Gauss integration technique and the mixed nonconforming finite element of the NCP ₁ — P ₁ pair (nonconforming linear element for the velocity, conforming linear element for the pressure). The two-level stabilized finite element method involves solving a small stabilized Stokes problem on a coarse mesh with mesh size H and a large stabilized Stokes problem on a fine mesh size h = H/3. Numerical results are presented to show the convergence performance of this combined algorithm.     

CHARACTERIZATION OF COVERGENT SUBDIVISION SCHEMES

Subdivision schemes provide important techniques for the fast generationof curves and surfaces.A recusive refinement of a given control polygonwill lead in the limit to a desired visually smooth object.These methodsplay also an important role in wavelet analysis.In this paper,we use arather simple way to characterize the convergence of subdivision schemesfor multivariate cases.The results will be used to investigate the regularityof the solutions for dilation equations.     

A facile approach for fabrication of underwater superoleophobic alloy

Superoleophobicity developed by creating roughness at multiple scales or lowering the surface energy has drawn extensive attention for technological applications. Currently, most methods for fabrication of superoleophobic surfaces employ either the complicated manufacture of micro/nano structures or delicate chemical decorations. Here, the sharp wetting transition from an oleophilic state to an oleophobic one has been practically realized, and the mechanism successfully interpreted. The underwater superoleophobic surfaces are newly realized and successfully controlled by subtly tuning the surface morphology of the alloy by practical corrosion plus a naturally obtained, stable, high-energy inorganic coating layer which induces the sharp wetting transition from oleophilic state to oleophobic state. The contact angle (CA) variations are quantitatively analyzed based on the wetting model by employing a roughness geometrical structure and a corrosion mechanism, which agree well with the measured results. Additionally, the dependence of the roughness on the corrosion procedure is modeled microscopically. Our method uncovers a facile fabrication protocol for optimum underwater superoleophobic surfaces by modifying the microstructures of alloy surfaces in the manufacturing process, which furthermore offers significant insights into the design and creation of other novel antifouling materials.     

Degree of approximation associated with some elliptic operators and its applications

The Jackson-type estimates by using some elliptic operators will be achieved. These results will be used to characterize the regularity of some elliptic operators by means of the approximation degree and the saturation class of the multivariate Bernstein operators as well.     

用白恩斯坦多项式逼近只有第一类间断点的有界函数

本文考虑在[0,1]上只具有第一类间断点的函数f(x),用它的n阶白恩斯坦多项式来逼近,给出了点态的逼近阶。所得结果较大地改进了Cheng Fuhua的工作,并且反映出当f(x)具有较好的点态性质时,逼近阶则随之而相应提高。