Controlled Mesoporosity in SiOC via Chemically Bonded Polymeric “Spacers”

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.     

Material parameter optimization for interior and exterior fluid-structure acoustic problems

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.     

Defects in rails

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.     

Some aspects on the synthesis and characterization of dodecenyl succinic anhydride (DDSA)—a curing agent for epoxy resins

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.     

Comments on "An Efficient SS/TDMA Time Slot Assignment Algorithm"

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.     

Generic Overlapping Cracks in Polymers: Modeling of Interaction

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 K_th 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.     

Influence of ferrite-martensite microstructural morphology on the low cycle fatigue of a dual-phase steel

A CMnSi 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.     

Semi-implicit and explicit finite element schemes for coupled fluid/thermal problems

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.