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排序方式: 共有492条查询结果,搜索用时 15 毫秒
91.
Yigal Blum Gian Domenico Sorarù Aravind Parakkulam Ramaswamy David Hui Sara Maria Carturan 《Journal of the American Ceramic Society》2013,96(9):2785-2792
Silicon oxycarbides with controlled porosity in the mesopore range have been obtained through high‐temperature pyrolysis of newly developed reactive siloxane formulations. The starting gels have been synthesized via Pt catalyzed hydrosilylation reaction between polyhydromethylsiloxane (PHMS) and vinyl‐terminated polydimethylsiloxane (PDMS) of different molecular weights in the presence of tetravinyltetramethylcyclotetrasiloxane as a crosslinking enhancer. In our approach, the PDMS serves the double purpose of size‐controlling templating agent as well as solvent at the early stages of the synthesis. During the curing step, the vinyl‐terminated PDMS is chemically bonded to the preceramic network through the extremely efficient hydrosilylation reaction and “solidify.” Accordingly, its removal during pyrolysis occurs through decomposition of a solid phase with retention of the formed porosity. The structural and morphological evolution of the preceramic gels containing the molecular spacers have been investigated as a function of the thermal treatment temperature by N2 physisorption measurements, thermogravimetry, and SEM analyses. The results show that the pore size distribution of the resulting SiOCs depends on the molecular weight of the PDMS and is directly related to the molecular volume assumimg that the PDMS chains are entangled into spheroidal shapes. The total pore volume is related to the initial amount of templating PDMS assuming its complete decomposition during pyrolysis. 相似文献
92.
Harisankar Ramaswamy Saikat Dey Assad A. Oberai 《International journal for numerical methods in engineering》2020,121(24):5568-5589
We present an efficient adjoint-based framework for computing sensitivities of quantities of interest with respect to material parameters for coupled fluid-structural acoustic systems with explicit interface coupling. The fluid is modeled using the Helmholtz equation and the structure is modeled using the Navier-Cauchy equations. Sensitivities are used to drive a gradient based optimization algorithm to solve important problems in structural acoustics, viz noise minimization and vibration isolation. For each problem, we consider two different priors: one where the optimal solution has a smooth variation and another with a bimaterial distribution. These priors are imposed with the help of suitable regularization terms. The effectiveness of this approach is demonstrated on both interior and exterior structural acoustic problems. 相似文献
93.
94.
The performance of rails in service is greatly influenced by the presence of metallurgical defects. Hydrogen-induced shatter
crack is the most prominent among these and has been discussed in detail. Principal causes of rail failure have been outlined
and different types of defects leading to failure of rails during service have been documented. Wear of rails which accounts
for reduced life has been considered. Influence of microstructure on wear rate has been described. 相似文献
95.
Dodecenyl succinic anhydride (DDSA), a liquid anhydride curing agent for epoxy resins, was synthesised from C12 olefin and maleic anhydride by the “ene synthesis” reaction. C12 olefin was prepared by the dehydration of lauryl alcohol using Al2O3 (acidic) catalyst. The dehydration was carried out in a flow type reactor. The olefin collected was found to be a mixture of positional isomers of C12 olefin. Systematic studies were carried out by varying the dehydration temperature, mode of feeding and flow rate of lauryl alcohol, and the C12 olefin collected were converted into DDSA and mechanical properties of an epoxy resin–DDSA system were evaluated. The C12 olefins and DDSA were characterized, and the properties correlated with structure and composition. It was found that DDSA derived from C12 olefin with double bond near to the center of the chain gave better tensile strength than the DDSA derived from C12 olefin with terminal double bond. 相似文献
96.
The time slot assignment algorithm presented in the above paper needs certain improvements for it to be an efficient one. In this correspondence, the necessary improvements are incorporated and an improved SS/TDMA time slot assignment algorithm is presented. The new algorithm is compared to the old one and the computer simulation exhibits a better performance of the present algorithm. 相似文献
97.
This paper deals with modeling of the interaction in overlapping cracks that the authors have earlier identified to be generic
to a wide range of polymeric systems (Ramasamy and Lesser, J Polym Sci B Phys, 2003). A complex stress function method is
used for evaluating stress intensity factors for interacting cracks. The interaction between two parallel overlapping cracks
is considered first. It is shown for this case that the stress intensity factor can fall below the threshold value when there
is sufficient overlap, leading to arrest of crack growth at the overlapping tip. Then the interaction in a doubly periodic
infinite array of cracks is considered. The interaction in the array is found to be non-linear. However, at a given stress
level, the highest density of stable cracks is related to the threshold value for crack propagation Kth though a simple set of equations. It is also shown that in an infinite array of cracks, the energy release rate criterion
for crack growth is different from the stress intensity factor criterion due to a reduced stiffness of the material. 相似文献
98.
A CMnSi dual-phase steel has been heat-treated to yield three different microstructures — a dispersion of martensite in fine-grained ferrite (Type I), a continuous network of martensite around ferrite grains (Type II) and martensite islands encapsulated in a ferrite matrix (Type III) — and low cycle fatigue tests have been conducted. All three conditions have been found to exhibit cyclic hardening. The lower hardening in the Type I microstructure and its cyclic softening at lower strains are attributed to a more uniform initial distribution of dislocations because of the dispersion of the martensite. The Type I microstructure displays the best fatigue performance and is shown to obey the Coffin-Manson law. The deviation of the other two microstructures from this law is shown to be due to a transition from a ductile mode of fracture at lower strains to a brittle type of fracture at higher strains. 相似文献
99.
B. Ramaswamy T. C. Jue J. E. Akin 《International journal for numerical methods in engineering》1992,34(2):675-696
A comparative investigation, based on a series of numerical tests, of two purely explicit and one semi-implicit finite element methods used for incompressible flow computation is presented. The ‘segregated’ approach is followed and the equations of motion are considered sequentially. The fundamental concepts and characteristics of the formulations and the solution methodology used are described in technical detail. Various modifications to Chorin's projection algorithm are investigated, particularly with respect to their effects on stability and accuracy. The stability of the semi-implicit method is shown to be less restrictive when compared to the explicit methods as the Reynolds number increases. At large time steps the artificial viscosity is also reduced and higher accuracy is obtained. The performance of the methods discussed in this paper is illustrated by the numerical solutions obtained for the cavity flow and flow past a rearward-facing step problems at high Reynolds numbers, and free convection flow problem at high Rayleigh numbers. It is shown that the semi-implicit method needs fewer iterations than the explicit methods, and the accuracy of the present methods is guaranteed by comparison with the existing methods. 相似文献
100.