Vacuum effects on fatigue crack growth in submicrometre‐thick freestanding copper films

To clarify vacuum effects on fatigue crack growth in freestanding metallic thin films, experiments were conducted on approximately 500‐nm‐thick copper films inside a field emission scanning electron microscope. Fatigue crack growth accompanied by intrusion/extrusion formation occurred in vacuum, and da/dN was smaller than in air in the middle‐ΔK region (ΔK ≈ 1.7‐3.1 MPam^1/2). Conversely, in the low‐ΔK region (ΔK ? 1.7 MPam^1/2), da/dN was larger in vacuum than in air. Further, fatigue crack growth in vacuum occurred below the fatigue threshold in air (ΔK_th,air). A nonpropagating crack after reaching ΔK_th,air continued to propagate in vacuum when the environment changed from air to vacuum. This indicates that fatigue crack growth resistance is smaller in vacuum than in air under the same effective driving force. The fatigue damage area near the crack paths in vacuum in the low‐ΔK region became wider, suggesting that the nucleation of fatigue damage was enhanced in vacuum.     

Robust topology optimization of phononic crystals with random field uncertainty

The uncertain spatial variation of material properties can remarkably affect the band gap characteristics of phononic crystals (PnCs). It is necessary to consider this issue when designing and manufacturing PnC materials/structures. This paper investigates a robust topology optimization method for designing the microstructures of PnCs by considering random‐field material properties. Herein, the spatial distribution of the material properties is first represented by a random field and then discretized into uncorrelated stochastic variables with the expansion optimal linear estimation method; stochastic band gap analysis is then conducted with polynomial chaos expansion. Furthermore, a robust topology optimization formulation of PnCs is proposed on the basis of the relative elemental density, where a weighted objective function handles the compromise of the mean value and standard deviation of the PnC band gap. The band gap response is analyzed, employing the finite element method for each sample of polynomial chaos expansion. In this context, the sensitivities of the stochastic band gap behaviors to the design variables are also derived. Numerical examples demonstrate that the proposed method can generate meaningful optimal topologies of PnCs with a relatively large width and less sensitive band gap. Additionally, the effects of the weight factors in the objective function and the variation coefficient of material properties are discussed.     

Rapid acetylation of native cellulose by TFAA and characterization of the products

Acetylation of cellulose (Whatman cellulose powder CF-11 and Egyptian cotton fiber) by the use of the TFAA “impelling” method was examined, and the decrease in the degree of polymerization (DP) during the reaction was determined. Cellulose triacetate can be readily prepared by the TFAA method. When Whatman cellulose powder was used, the degradation of cellulose during acetylation was not observed in reaction times up to 5 hr, and a slight decrease in the degree of polymerization was detected in samples after reaction for more than 12 hr. The number of chain scissions per cellulose molecule for Egyptian cotton fiber was similar in magnitude to that for Whatman cellulose at the reaction time of 8 hr, but the value for Egyptian cotton fiber was considerably larger compared with that for Whatman cellulose after 12 hr.     

河道梯形洼地流动特性实验研究

研究河床洼地的水流特性.在一个可调节坡度的实验水槽内设置一个接近于自然形状的洼地.应用电磁流速仪测量洼地内3个方向的流速.实验过程中,在洼地拥有铅直岸壁的一侧观测到柱状分离涡.为使洼地更接近实际,在洼地内沿铅直岸壁设置两个6 cm高的可透水石笼.设置石笼后的洼地流态变化较大,分离涡发生在石笼的对岸一侧.本实验结果为环境流体力学和河流动力学探讨洼坑水流规律提供了研究基础.     

Nylon 6 microfiber prepared by carbon dioxide laser heating

A nylon 6 microfiber was easily obtained through carbon dioxide laser heating. The laser heating was carried out in two steps: the first laser heating was performed under an applied tension of 36.7 MPa at a power density of 17.3 W cm^?2, and the second was performed under 0.18 MPa at 51.81 W cm^?2. The microfiber was obtained by the second laser heating of the fiber. The microfiber prepared under the optimum thinning conditions had a diameter of 1.9 μm and a birefringence of 46.2 × 10^?3. Its draw ratio, estimated from the diameter, was 9895× (so far, it has been impossible to achieve such a high draw ratio by drawing). A (200) reflection and a (002/202) doublet due to an α form were observed on the equator, but no (200) reflection due to a γ form was observed. The morphology of the crystallites existing in the microfiber was only the α form. Laser heating made the microfiber more easily than conventional technologies, such as conjugate spinning, melt blowing, and flash spinning. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1449–1453, 2004