Time-dependent interfacial failure in metallic alloys

Time-dependent intergranular brittle fracture has now been studied experimentally in a number of alloy systems, and the generic features are becoming clear. Mobile surface-adsorbed elements are caused to diffuse inward along grain boundaries under the influence of a tensile stress, and this can lead to sub-critical crack growth by decohesion. Oxygen is found to play this role in nickel-base superalloys and intermetallics, as well as in a precipitation-strengthened Cu–Be alloy. Crack-growth rates lie in the range 10⁻⁷–10⁻⁴ m sec⁻¹. The same kind of cracking is found in steels treated so that free sulfur is able to segregate to the surface, as well as in Cu-Sn alloys, in which the embrittling element is surface-segregated Sn. The latter has been studied in bicrystals, and the importance of the variation in diffusivity with grain boundary structure has been documented. Hydrogen-induced cracking is a special case of an extremely mobile embrittling element and is responsible for much of the brittleness found in intermetallics. The effect of boron in retarding brittle behavior in Ni₃Al has been shown to result partly from its interaction with hydrogen. This is a prime example of how segregated solutes can be used to ameliorate the tendency for diffusion-controlled brittle fracture.     

The interplay of hydrogen-bonding and metal-coordination motifs in the formation of supramolecular patterns: Copper/zinc malonates–aminopyrimidine complexes

The X-ray crystal structures of 1 [2[AMPYH] · [(Cu (H₂O) (mal)₂)]:2H₂O] and 2 [2[AMPYH] · [ZnBr (Hmal) (mal)]:2H₂O ] [mal = malonate; Hmal = hydrogen malonate; AMPYH = 2-amino-4, 6-dimethylpyrimidinium] reveal the interplay of metal-coordination motifs and the hydrogen-bonding patterns . In both the structures, the 2-amino-4,6-dimethylpyrimidine cations are not directly coordinated to copper and zinc, but are hydrogen-bonded to the malonate ion. The protonated endocyclic nitrogen and the amino group of 2-amino-4,6-dimethylpyrimidine molecule interact with malonate ligand via a pair of N–HO hydrogen-bonds, (with graph set notation ) whereas the unprotonated endocyclic nitrogen and the amino group interact with the malonate ligand via N–H..O and C–HN hydrogen-bonds forming the same motif. The combination of hydrogen-bonding motif and metal-coordination motif creates a novel supramolecular motif. The interplay of the hydrogen-bonding motif and metal-coordination motif via hydrogen-bonds leads to similar supramolecular motifs in both the structures.     

Rating Curve Development at Ungauged River Sites using Variable Parameter Muskingum Discharge Routing Method

A physically based simplified discharge routing method, namely, the variable parameter Muskingum discharge-hydrograph (VPMD) routing method, having the capability of estimating the stage hydrographs simultaneously in channels with floodplains is presented herein. The upstream discharge hydrograph is routed using this VPMD method in different two-stage symmetrical trapezoidal compound cross section channel reaches. The performance of the VPMD method is evaluated by numerical experiments using the benchmark MIKE11 hydrodynamic model and the field data of the Tiber River in central Italy. The proposed method is capable of accurately routing the discharge hydrographs, corresponding stage hydrographs and synthesizing the normal rating curves at any downstream ungauged river site which is not affected by any downstream effects. This study can be helpful for various planning and management of river water resources in both the diagnostic and prognostic modes.     

Variable Parameter Muskingum Routing Considering Downstream Effects

Using a variable parameter Muskingum method, a procedure for discharge hydrograph routing in prismatic channels considering downstream effects is presented. The method involves a two step process: (1) a given routing reach is divided into a number of subreaches with each subreach having a representative unique stage–discharge relationship established by using a gradually varied flow profile estimation technique; and (2) the routing of a given inflow hydrograph through these subreaches successively using the variable parameter Muskingum method with parameter variation achieved using the established stage–discharge relationships pertinent to the subreaches and the assumptions of the routing method. The ability of this procedure to route floods accounting for downstream effects is demonstrated by routing a given hypothetical inflow hydrograph in three rectangular channels each with a reach length of 40?km, and for two different scenarios of downstream boundary conditions, one resulting in an M1 profile and another in an M2 profile with the control at the end of the 40?km reach. The M1 profile is formed due to the prescribed boundary condition at the outlet of the reach that the flow depth at that section is equal to twice the normal depth in the channel reach. The M2 profile is formed due to a free fall located at the outlet of the reach. The routing results obtained using this procedure are compared with the corresponding Saint-Venant solutions arrived at using the U.S. National Weather Service’s DAMBRK model, which is used as a benchmark. The performance of this discharge routing procedure compares well with the corresponding DAMBRK model solutions subject to the criterion ∣(1/S0)?y/?x∣<1 being satisfied.     

Cyanoethylation of cyclopentadiene: Isolation of penta- and hexa(β-cyanoethyl)cyclopentadiene and an unexpected acrylonitrile polymer

Summary The cyanoethylation of cyclopentadiene was examined under a variety of reaction conditions. Approximately 9% of cyclopentadiene was converted to the desired hexa(-cyanoethyl)cyclopentadiene when the amount of acrylonitrile to cyclopentadiene was increased from 2 to 7 equivalents under phase transfer conditions. The low conversion is due to competitive formation of various unreactive side-products. For example, termination leading to less-highly substituted products was confirmed by isolation of crystalline 1,1,2,3,4-penta(-cyanoethyl)cyclopentadiene. Another side product isolated and characterized by ¹³C NMR was an oligomer of acrylonitrile attached to hexa(-cyanoethyl)cyclopentadiene; the anion (at a -alkyl position) of the latter apparently acted as anionic initiator. Attempts to induce thermal polymerization and cyclotrimerization of the pendent cyano groups of hexa(-cyanoethyl)cyclopentadiene were unsuccessful even with added Lewis acid catalysts. Reduction to the hexaamine was only partially successful.     

Realization of an Eco-Friendly Solid Propellant Based on HTPB-HMX-AP System for Launch Vehicle Applications

An eco-friendly higher solid loaded HTPB propellant with improved energetics has been realized in small scale by incorporating 10–20% by weight of a high energy material namely, HMX. This propellant with 88% solid loading has given good processability, mechanical properties and reduced pressure index. Pollution due to HCl smoke in the rocket exhaust is reduced by 20 to 30% by weight. This propellant is expected to give a vacuum specific impulse of 293 seconds at an operating pressure of 70 bar and an area ratio of 10. Sub-scale motors of 2 kg size have been successfully static tested to evaluate the performance.     

Polystyrene-supported thiosemicarbazone–transition metal complexes: synthesis and application as heterogeneous catalysts

Functionalized polymers are found to be highly efficient in immobilizing transition metal ions. Crosslinked polystyrene supported Schiff's base complexes of metal ions such as Fe(III), Co(II), Ni(II) and Cu(II) are very effective as heterogeneous catalysts. The catalytic activity of these metal complexes has been studied in the decomposition of H₂O₂ and in the epoxidation of cyclohexene and styrene. The reactions show a first order dependence on the concentration of both the substrates and the catalyst. The influence of the degree of crosslinking of the polymer support on the rate of reactions has been studied. The metal complexes show low catalytic activity at low crosslink density (2% and 5%) but 10% crosslinked resins show higher activity. A possible mechanism for the reactions is suggested. © 1999 Society of Chemical Industry     

Survey of Canadian retail coffees for ochratoxin A

One-hundred and one specimens of coffee were gathered from retail outlets across Canada and analysed for ochratoxin A. Seventy-one specimens were roasted beans or roasted ground coffee, and 30 were instant (or 'soluble') coffees. All samples were extracted with methanol-sodium bicarbonate. The extracts were cleaned up either by immunoaffinity column chromatography or by a combination of solid-phase extraction and immunoaffinity column chromatography. Ochratoxin A was quantified by liquid chromatography (LC) with fluorescence detection. The minimum quantifiable level was 0.1 ng g^-1. Ochratoxin A was present, above the minimum quantifiable level, in 42 (59%) of 71 beans and ground coffee and in 20 (67%) of 30 instant coffees. The mean ochratoxin A level in the positive samples of beans and ground coffee was 0.6 ng g^-1, and the mean level in the positive samples of instant coffee was 1.1 ng g^-1.     

Volume Conservation Controversy of the Variable Parameter Muskingum–Cunge Method

The study analyzes the volume conservation problem of the variable parameter Muskingum–Cunge (VPMC) method for which some remedial solutions have been advocated in recent literature. The limitation of the VPMC method to conserve volume is brought out by conducting a total of 6,400 routing experiments. These experiments consist of routing a set of given hypothetical discharge hydrographs for a specified reach length in uniform rectangular and trapezoidal channels using the VPMC method, and comparing the routed solutions with the corresponding benchmark solutions obtained using the full Saint-Venant equations. The study consisted of 3,200 routing experiments carried out each in uniform rectangular and trapezoidal channel reaches. Each experiment was characterized by a unique set of channel bed slope, Manning’s roughness coefficient, peak discharge, inflow hydrograph shape factor, and time to peak. A parallel study was carried out using an alternate physically based variable parameter Muskingum discharge hydrograph (VPMD) routing method proposed by Perumal in 1994 under the same routing conditions, and the ability of both the VPMC and VPMD methods to reproduce the benchmark solutions was studied. It is brought out that within its applicability limits, the VPMD method is able to conserve mass more accurately than the VPMC method. The reason for the better performance of the former over the latter method is attributed to the physical basis of its development. It is argued that adoption of artificial remedial measures to overcome the volume conservation problem makes the VPMC method semiempirical in nature, thereby losing the fully physically based characteristics of the method. The paper also dwells on the problems of negative initial outflow or dip in the beginning of the Muskingum solution, and the negative value of the Muskingum weighting parameter. Besides, the effect of incorporating the inertial terms in the estimation of Muskingum parameters and their impact on the overall Muskingum routing solutions is addressed by conducting another set of 6,400 numerical experiments using both the VPMC and VPMD methods.