R. Krishna Kumar

A comparative evaluation of the theoretical failure criteria for workability in cold forging

Abstract This paper evaluates various theoretical failure criteria pertaining to workability in cold forging reported in the published literature for their reliability and sensitivity in predicting the occurrence of ductile fracture in metalworking. Finite element (FE) simulation of the published upsetting experiments on cylindrical test specimens was performed to determine the threshold values attained by various criteria at the fractured locations, for a wide variety of materials. A comparison of the experimental threshold values of different criteria, with those obtained through FEA of complex metalworking processes at fracture was also made. A statistical analysis of the results revealed that none of the criteria are truly friction and geometry-independent for universal application. Nevertheless, within a family of processes such as upsetting, the criteria depending on cumulative specific plastic energy adjusted suitably with the maximum tensile stress, are the most reliable ones in the estimation of workability limits.

Solid-state supramolecular chemistry of porphyrins. Stacked and layered heterogeneous aggregation modes of tetraarylporphyrins with crown ethers

New crystalline materials of mixed composition based on the interaction between tetraarylporphyrin and 18-crown-6 derivatives have been prepared and characterized by X-ray diffraction analysis. Free crown ether macrocycles (18-crown-6 and dibenzo-18-crown-6) associate to manganese- or zinc-tetraphenylporphyrin in aqueous solution through a bridging molecule of water which simultaneously coordinates to the axial site of the porphyrin metal core and hydrogen bonds to the oxygens of the crown ether. This ternary mode of self-assembly can lead to the formation of monomeric, oligomeric and stacked polymeric entities, depending on the symmetry of the crown structure and the preferred coordination geometry of the metal ion. Sodium or potassium 18-crown-6 chlorides were found to be excellent templates for the construction of non-interpenetrating β-molecular networks from zinc-tetra(4-carboxyphenyl)porphyrin building blocks. The resulting layered motifs incorporate the crown ether moieties within the interporphyrin cavities. These arrays are stabilized by strong hydrogen bonds between the self-complementary carboxylic groups as well as by ion pairing, as their formation is associated with proton transfer from one of the carboxylic groups to the chloride anion and expulsion of hydrochloric acid. Molecules of the methanol solvent, which coordinate axially to the central metal ions of the porphyrin and crown ether moieties in one layer while hydrogen bonding to the carboxylic groups of another layer, contribute to the tight packing of the molecular layers along the third dimension. The experimentally established geometries and packing modes of these aggregates provide useful information for further crystal engineering efforts of networked multi-porphyrin domains.

Simultaneous optimization of flame spraying process parameters for high quality molybdenum coatings using Taguchi methods

Flame-sprayed molybdenum coatings are extensively used in industrial applications to enhance the performance of engineering components such as pistons, piston rings and shafts. Improved resistance to thermal degradation, corrosion and wear can be achieved. In all these applications, the performance of the coating is dependent upon its cohesive and adhesive strengths, which are affected by the spraying process parameters employed during the coating deposition process. In the present study, an attempt is made to produce high quality coatings by optimizing the spraying process parameters using Taguchi techniques. Experiments are conducted using ASTM specimens to produce coatings which have high cohesive strengths. A new test specimen is proposed by modifying the ASTM specimen to give a fracture mechanics specimen which can quantify the adhesive strength of the coating. Microstructural studies are conducted on the sprayed coatings and the changes in the microstructure with different spraying conditions are correlated with the strength variations of the coating.

Thermo-mechanical finite element analysis of a rail wheel

Abstract The rail wheel, which is acted upon by mechanical forces also experiences thermal stresses due to braking, during service. The coupled nature of these forces is analysed using a three-dimensional elasto-plastic finite element model. Contact stresses at the rail–wheel interaction location are analysed using a global – local approach on a three-dimensional elasto-plastic finite element model. The paper also brings out the size and shape of the plastic zone at the contact region. Commercial finite element code ABAQUS has been used for the analysis and SDRC’s I-DEAS has been used for modelling. American Association of Railroad’s standards, available for the purpose of analytical evaluation of the rail wheel, has also been critically evaluated in this work.

Mixed chalcogen carbonyl compounds: II. Synthesis and characterization of Fe2Ru(μ3-Se)(μ3-Te)(CO)9 and (μ5-C5H5)CoFe2(μ3-Se)(μ3-Te)(CO)6

The new mixed metal complexes, each containing bridging selenium and tellurium ligands in the same molecule, Fe 2 Ru(CO) 9 (μ 3 -Se)(μ 3 -Te) and (η 5 -C 5 H 5 )CoFe 2 (CO) 6 (μ 3 -Se)(μ 3 -Te) have been prepared from the room temperature reactions of Fe 2 (CO) 6 (μ-SeTe) with Ru(CO) 4 (C 2 H 4 ) and (C 5 H 5 )Co(CO) 2 , respectively. Reaction of (η 5 -C 5 H 5 )CoFe 2 (CO) 6 (μ 3 -Se)(μ 3 -Te) with Pt(PPh 3 ) 4 forms the previously known (CO) 6 Fe 2 (μ 3 -Se)(μ 3 -Te)Pt(PPh 3 ) 2 and on treatment with NaOMe, followed by acidification, Fe 2 (CO) 6 (μ-SeTe) is formed.

A study of the relationship between Magic Formula coefficients and tyre design attributes through finite element analysis

Pacejkas Magic Formula Tyre Model is widely used to represent force and moment characteristics in vehicle simulation studies meant to improve handling behaviour during steady-state cornering. The experimental technique required to determine this tyre model parameters is fairly involved and highly sophisticated. Also, total test facilities are not available in most countries. As force and moment characteristics are affected by tyre design attributes and tread patterns, manufacturing of separate tyres for each design alternative affects tyre development cycle time and economics significantly. The objective of this work is to identify the interactions among various tyre design attributes-cum-operating conditions and the Magic Formula coefficients. This objective is achieved by eliminating actual prototyping of tyres for various design alternatives as well as total experimentation on each tyre through simulation using finite element analysis. Mixed Lagrangian–Eulerian finite element technique, a specialized technique in ABAQUS, is used to simulate the steady-state cornering behaviour; it is also efficient and cost-effective. Predicted force and moment characteristics are represented as Magic Formula Tyre Model parameters through non-linear least-squares fit using MATLAB. Issues involved in the Magic Formula Tyre Model representation are also discussed. A detailed analysis is made to understand the influence of various design attributes and operating conditions on the Magic Formula parameters. Tread pattern, tread material properties, belt angle, inflation pressure, frictional behaviour at the tyre–road contact interface and their interactions are found to significantly influence vehicle-handling characteristics.

A Sensitivity Analysis of Design Attributes and Operating Conditions on Tyre Operating Temperatures and Rolling Resistance Using Finite Element Analysis

Abstract The hysteresis characteristics of tyre materials primarily contribute to operating temperatures and rolling resistance. The experimental results of these characteristics measured for different tyres at various operating conditions, pavement surface texture, and tyre wear clearly indicate a complex relationship among various design attributes and operating conditions. It is extremely difficult to develop analytical models that can be used to study these complex relationships and, therefore, one should rely completely on experimental results. However, experimental techniques to study these tyre characteristics are highly expensive and complex. In addition to this, the manufacture of an actual tyre for each design alternative will increase significantly the tyre development cycle and, in addition, have a severe impact on the overall economy. In this study, a three-stage sequential model using finite element (FE) technique is used to determine these characteristics for tyres with smooth and circumferential groove tread profiles. An FE algorithm is developed with Petrov-Galerkin Eulerian technique in cylindrical coordinates and is used to determine three-dimensional tyre-operating temperatures. A sensitivity analysis has been made with various operating conditions and tyre design attributes that will aid tyre designers to realize improved designs with reduced rolling resistance and acceptable temperatures.

Numerical modeling of stress induced martensitic phase transformations in shape memory alloys

Abstract Phenomenological models of shape memory behavior are based on either continuum hypothesis (macroscopic) or on volume averages over a representative volume element consisting of several grains. These constitutive models attempt to model the shape memory behavior using macro/micromechanics and thermodynamics. In general, these models share a common feature. They describe the martensitic phase transformation by a parameter representing the martensite volume fraction, and formulae an evolution law for the martensite volume fraction. Exploiting the similarity of these models to elastoplasticity, we describe a finite element formulation of a micromechanics based constitutive model. Several other models can be formulated in a similar way, and the present work can be seen as a testbed approach to study and evaluate the constitutive models on a common platform. We present numerical results for Au-47.5at%Cd and Ti-50.6at%Ni to validate the finite element formulation.

Simulation of dynamic compaction of metal powders

This article presents numerical studies on the deformation of particles during dynamic compaction of metal powders. The analysis of the process is based on a micromechanics approach using multiple particle configurations. The material considered is elastoviscoplastic with interparticle friction. Two-dimensional studies on particles in close packed arrangement were carried out using plane strain conditions for deformation and thermal response. The finite element method using an explicit dynamic analysis procedure was used for the simulations. The influence of speed of compaction, strain hardening, strain rate dependency, interparticle friction and size of the powder particles on the final shape and temperature variations within the particles were analyzed. The studies offer useful information on the shape and temperature variations within the particles. The results provide a better understanding of the dynamic compaction process at the micromechanics level.

Fracture mechanics approaches to coating strength evaluation

Abstract The incidence of mechanical failure of flame sprayed molybdenum coating on piston rings prompted development of reliable methods of characterising and improving bond strengths. A method of using the ASTM specimen with a small modification, to estimate fracture toughness properties of the coating is proposed. Flame spraying process parameters are optimised to produce coatings of high adhesive strength using Taguchi techniques by using the modified ASTM specimen. Experiments are conducted using four point bend specimen, to study the coating behaviour under complex stress conditions. The effect of coating thickness on the fracture resistance of the interface is studied. Coating strength is characterised in terms of fracture mechanics parameters fracture toughness and Jcrit; using modified ASTM and four point bend specimens, respectively. The Jcrit values are determined by virtual crack extension technique, in conjunction with finite element analysis solutions. Theoretical investigations by finite element analysis are conducted on the test specimens to gain information about the stresses responsible for failure initiation and the failure initiation zones.