Elasto-plastic contact stress analysis of joints subjected to cyclic loading

Rigorous elastic-plastic finite element analysis of joints subjected to cyclic loading is carried out. An incremental-iterative algorithm is developed in a modular form combining elasto-plastic material behaviour and contact stress analysis. For the case of the interference fit, the analysis sequentially carries out insertion of the pin and application of the load on the joint, covering possible initiation of separation (and/or yielding) and progressively the receding/advancing contact at the pin-plate interface. Deformations of both the plate and the pin are considered in the analysis. Numerical examples are presented for the case of an interference fit pin in a large plate under remote cyclic tension, and for an interference fit pin lug joint subjected to cyclic loading. A detailed study is carried out for the latter problem considering the effect of change in contact/separation at the pin-plate interface on local stresses, strains and redistribution of these stresses with the spread of a plastic zone. The results of the study are a useful input for the estimation of the fatigue life of joints. Copyright (C) 1996 Elsevier Science Ltd

Modeling Of Hydrolytic Polymerization In A Semibatch Nylon 6 Reactor

Hydrolytic polymerization of caprolactam to Nylon 6 in a semibatch reactor is carried out by heating a mixture of water and caprolactam. Evaporation of volatiles caused by heating results in a pressure build-up. After the pressure reaches a predetermined value, vapors are vented to keep the pressure constant for some time, and thereafter, to lower the pressure to a value slightly above atmospheric in a preprogrammed manner. The characteristics of the polymer are determined by the chemical reactions and the vaporization of water and caprolactam. The semibatch operation has been simulated and the predictions have been compared with industria data. The observed temperature and pressure histories were predicted with a fair degree of accuracy. It was found that the predictions of the degree of polymerization however are sensitive to the vapor-liquid equilibrium relations. A comparison with an earlier model, which neglected mass transfer resistance, indicates that simulation using the VLE data of Giori and Hayes and accounting for mass transfer resistance is more reliable.

Influence of strain rate and state-of-stress on the formation of ferrite in stainless steel type AISI 304 during hot working

The influence of strain rate and state-of-stress on the formation of ferrite in stainless steel type AISI 304L, 304 and 304 as-cast, during hot working has been studied. Compression and torsion tests were conducted in the temperature range 1100 to 1250 degrees C and strain rate range 0.001 to 100 s(-1) on these materials, Ferrite formation occurs during deformation at temperatures above 1150 degrees C and strain rates above 10 s(-1), in stainless steel type AISI 304L and 304. The tendency for the formation of ferrite is more in as-cast 304 than in wrought 304, In as-cast 304 the ferrite forms at lower temperatures and strain rates, The tendency for the ferrite formation is more in torsion than in compression.

Synthesis of epoxy-terminated polymers by radical polymerization using alpha-(t-butylperoxymethyl)styrene as chain transfer agent

Epoxy-terminated polystyrene has been synthesized by radical polymerization using alpha-(t-butylperoxymethyl) styrene (TPMS) as the chain transfer agent. The chain transfer constants were found to be 0.66 and 0.80 at 60 and 70 degrees C, respectively. The presence of epoxy end groups was confirmed by functional group modification of epoxide to aldehyde by treatment with BF3.Et(2)O. Thermal stability of TPMS was followed by differential scanning calorimetry and iodimetry. Thermal decomposition of TPMS in toluene follows first order kinetics with an activation energy of 23 kcal/mol. (C) 1996 John Wiley & Sons, Inc.

Glutathione peroxidase3 of Saccharomyces cerevisiae protects phospholipids during cadmium-induced oxidative stress

The present study was undertaken to determine the role of glutathione peroxidase3 (gpx3) in phospholipid protection in cells. Wild-type (WT) cells showed an overall increase in phospholipids upon 50 mu M cadmium (Cd)-treatment, whereas an untreated gpx3 Delta strain showed a drastic reduction in overall phospholipids which was further reduced with 50 mu M Cd. In WT cells, Cd-exposure increased the short chain fatty acids and decreased the unsaturated fatty acids and the magnitude was high in Cd-treated gpx3 Delta cells. Purified recombinant gpx3p showed higher activity with phospholipid hydroperoxides than shorter hydroperoxides. An increase in gpx activity was observed in Cd-treated WT cells and no such alteration was observed in gpx3 Delta. WT cells treated with Cd showed an increase in MDA over untreated, while untreated gpx3 Delta cells themselves showed a higher level of MDA which was further enhanced with Cd-treatment. Iron, zinc and calcium levels were significantly altered in WT and gpx3 Delta cells during Cd-treatment.

Polymerization and Pressure-Induced Amorphization of C60 and C70

Scanning tunneling microscopy of solid films of C-60 and C-70 clearly demonstrate the occurrence of photochemical polymerization of these fullerenes in the solid state. X-ray diffraction studies show that such a polymerization is accompanied by contraction of the unit-cell volume in the case of C-60 and expansion in the case of C-70. This is also evidenced from the STM images. These observations help to understand the differences in the amorphization behavior of C-60 and C-70 under pressure. Amorphization of C-60 under pressure is irreversible because it is accompanied by polymerization associated with a contraction of the unit cell volume. Monte Carlo simulations show how pressure-induced polymerization is favored in C-60 because of proper orientation as well as the required proximity of the molecules. Amorphization of C-70, on the other hand, is reversible because C-70 is less compressible and polymerization is not favored under pressure.

Mechanism of “Autoacceleration” in the Thermal Oxidative Polymerization of R-Methylstyrene

Thermal oxidative polymerization of alpha-methylstyrene (AMS) has been studied at various temperatures(45-70 degrees C) and pressures (50-400 psi). Due to its high electron dense double bond, it undergoes thermal oxidative polymerization even at low temperatures fairly easily. The major products are poly(alpha-methylstyrene peroxide) (PMSP), and its decomposition products are acetophenone and formaldehyde. Above 45 degrees C the rate of polymerization increases sharply at a particular instant showing an ''autoacceleration'' with the formation of a knee point. The ''autoacceleration'' is supported from the fact that the plot, of R-p vs T shows a rapid rise, and the plot of ln R-p vs 1/T is non-Arrhenius. The occurrence of autoacceleration is explained on the basis of acetophenone-induced cleavage of PMSP during polymerization, generating more initiating alkoxy radicals, which subsequently leads to the rapid rise in the rate of polymerization. The mechanism of autoacceleration is supported by the change in. order, activation energy, and activation volume before and after the knee point.

Algal Photosynthetic Dynamics in Urban Lakes under Stress Conditions

Urban lakes form vital ecosystems supporting livelihood with social, economic and aesthetic benefits that are essential for quality life. This depends on the biotic and abiotic components in an ecosystem. The structure of an ecosystem forms a decisive factor in sustaining its functional abilities which include nutrient cycling, oxygen production, etc. A community assemblage of primary producers (algae) plays a crucial role in maintaining the balance as they form the base of energy pyramid in the ecosystem. Algae assimilate carbon in the environment via photosynthetic activities and releases oxygen for the next level of biotic elements in an ecosystem. Besides these, algal cells rich in protein serve as food and feed, used as manure and for production of biofuels. Understanding algal photosynthetic dynamics helps in assessing the level of dissolved oxygen (DO), food (fish, etc.), waste assimilation, etc. Algal chlorophyll content, algal biomass, primary productivity and algal photosynthetic quotient are some of the parameters that help in assessing the status of urban lakes. Chlorophyll content gives a measure of the growth, spread and quantity of algae. Unplanned rapid urbanization in Bangalore in recent times has resulted in either disappearance of lake ecosystems or deteriorated the lake water quality impairing the ecological processes. This paper computes algal growth, community structure, primary productivity and composition for three major lakes (T G Halli, Bellandur and Varthur lakes) under contrast levels of anthropogenic influences.

Stress-strain Curves and Corresponding Structural Parameters in Mulberry and Non-mulberry Silk Fibers

Mulberry fiber (Bivoltine) and non-mulberry fiber (Tassar) were subjected to stress-strain studies and the corresponding samples were examined using wide angle X-ray scattering studies. Here we have two different characteristic stress-strain curves and this has been correlated with changes in crystallite shape ellipsoids in all the fibers. Exclusive crystal structure studies of Tassar fibers show interesting feature of transformation from antiparallel chains to parallel chains.

Total peroxidase activity and peroxidase isoforms as modified by salt stress in two cultivars of fox-tail millet with differential salt tolerance

The effect of NaCl on total peroxidase activity, induction of isoperoxidases and lipid peroxidation in 5-day-old seedlings of two contrasting genotypes of Setaria italica L. (Prasad, a salt tolerant cultivar and Lepakshi, a salt susceptible cultivar), was studied. Total peroxidase activity increased under NaCl salinity and the degree of elevation in the activity was salt concentration dependent. Nevertheless, a greater activity was recorded in the tolerant cultivar (cv Prasad) compared to the susceptible (cv Lepakshi) one in all days of sampling. Further, the pattern of isoperoxidases was modified during stress conditions as evident from the electrophoregrams. Although, five acidic isoforms were detected in both cultivars, differences were found between the cultivars. Furthermore, it was observed that acidic isoperoxidases were strongly expressed and an acidic isoperoxidase, A(3p) (27 kDa) is specifically found in the tolerant cultivar (cv Prasad) under NaCl stress. This isoform was partially purified and found to be thermostable with pr 5.5 and the optimum pH 7.4. A close correlation exists between the rate of lipid peroxidation in terms of malonaldehyde (MDA) content and total peroxidase activity per gram fresh weight with salt tolerance of the two cultivars. The tolerant cultivar (cv Prasad) had low MDA content and high total peroxidase activity than the susceptible variety (cv Lepakshi) during salinity stress. (C) 1999 Published by Elsevier Science Ireland Ltd. All rights reserved.

Catalytic oxidative polymerization of 1,1-diphenylethylene at ambient temperature and potential application of peroxide macroinitiator

The Co(II)TPP(Py) complex was used as an efficient dioxygen carrier for the radical polymerization of 1,1-diphenylethylene (DPE), which has a low ceiling temperature, at ambient temperature and low oxygen pressure. The mechanism of polymerization is discussed' on the basis of kinetic data, W-vis, ESR, and H-1 NMR studies. The rate of polymerization (RP) and number-average molecular weights (M) of poly(1,1-diphenylethylene peroxide) (PDPEP) are higher and the polydispersity is lower than in 2,2'-azobis(isobutyronitrile) (AIBN) initiated polymerization. PDPEP was further. used as a macroinitiator for the polymerization of MMA. The polymerization obeys classical kinetics. The K-2 value of the PDPEP has been determined from the slope of R-P(2) VS [M](2)[I], which reveals that it can also be used at higher temperature for the polymerization. An "active" PMMA was also synthesized, containing initiating segments in the polymer backbone.

Critical stress intensity factors for cracked hollow pipes under transient thermal loads

Finite element analyses of a long hollow cylinder having an axisymmetric circumferential internal edge crack, subjected to convective cooling on the inner surface are performed. The transient thermal stress intensity factor is estimated using a domain version of the J-integral method. The effect of the thickness of the cylinder, crack length, and heat transfer coefficient on the stress intensity factor history are studied. The variations of critical normalized stress intensity factor with crack length-to-thickness ratio for different parameters are presented. The results show that if a small inner surface crack begins to grow, its stress intensity factor will increase with increase in crack length, reach a maximum, and then begin to drop. Based on the results, a fracture-based design methodology for cracked hollow pipes under transient thermal loads is discussed.

Influence of temperature and strain rate on the flow stress of an FeAl alloy

The deformation behavior of an FeAl alloy processed by hot extrusion of water atomized powder has been investigated. Compression tests are performed in the temperature range 1073–1423 K and in the strain rate range 0.001–100 s−1 up to a true plastic strain of 0.5. The flow stress has been found to be strongly dependent on temperature as well as strain rate. The stress exponent in the power law rate equation is estimated to be in the range 7.0–4.0, decreasing with temperature. The activation energy for plastic flow in the range 1073–1373 K varies from 430 kJ mol−1 at low stresses to 340 kJ mol−1 at high stresses. However, it is fairly independent of strain rate and strain. The activation area has similarly shown a stress dependence and lies in the range 160–45b2. At 1423 K and at strain rates lower than 0.1 s−1 a strain rate sensitivity of 0.3 is observed with an associated activation energy of 375 kJ mol−1. The plastic flow in the entire range of temperature and strain rate investigated appears to be controlled by a diffusion mechanism. The results have revealed that it is possible to process the alloy by superplastic forming in the range 1373–1423 K at strain rates lower than 0.1 s−1.