Fracture Mechanics Model for Analysis of Plain and Reinforced High-Performance Concrete Beams

In developing a one-dimensional analysis and design procedure for reinforced concrete structures, research is generally based on yield phenomena and the plastic flow of steel in tension and concrete in compression. The ability of concrete to resist tension is considered in the form of tension stiffening or is completely disregarded. This procedure does not account for the influence of structural size in changing the failure mode and the stress distribution across the uncracked or cracked ligament. The key factor affecting this stress distribution is found to be the strain-softening modulus. This paper presents an improved model that is based on the fundamental equilibrium equation for the progressive failure of plain concrete beams. The concrete stress-strain relationship in tension is derived by calculating the peak tensile stress and softening modulus for different depths of beams on the basis of the fracture parameters obtained with the size effect law. Thus, the proposed model uses the peak tensile stress and the softening modulus, which vary depending on the size of the beam. To study the effect of the strength of high-performance concrete (HPC) on the concrete tensile stress-strain relationship, the experimental load-deflection plots of different-sized beams are compared with those obtained by using the proposed analytical model for eight different mixes made with locally available fly ash and slag. The model is also extended for lightly reinforced concrete beams, and the results are compared with those in the literature and are found to be in good agreement.     

Silicide Precipitation in Alloy Ti-6AI-5Zr-0.5Mo-0.25Si

Silicide precipitation in the titanium alloy containing by wt pct 6Al-5Zr-0.5Mo-0.25Si (Alloy 685) has been investigated using electron diffraction. The solutionizing temperature for this alloy is 1323 K. It is observed that no resolvable silicide precipitates are present in the alloy as received, furnace cooled from 1323 K and aged at 823 K, or oil quenched from 1323 K and aged at 823 K. Specimen solutionized at 1323 K for 30 minutes followed by water quenching and aging at 923 K for 24 hours shows fine precipitates at boundaries of α′ platelets. Diffraction analysis shows that the fine precipitates belong to two different types of identifiable hexagonal silicides similar to those observed by Floweret al in the ternary Ti-5Zr-0.5Si alloy to be (TiZr)₅Si₃ and another of unknown stoichiometry. However, aging of the water quenched specimens for 24 hours at higher temperatures, 973 K and 1073 K, results in the precipitation of a silicide similar to the latter one. It is analyzed that this silicide has lattice parameters a = 0.70₂ nm and c = 0.36₈ nm. Since aging at 973 K and 1073 K gives rise to precipitation of the same silicide, it is concluded that this is the stable silicide in Alloy 685 in the temperature range investigated.     

Effect of silicide precipitation on tensile properties and fracture of alloy Ti-6Al-5Zr-0.5Mo-0.25Si

The effect of silicon in solid solution and in the form of suicides has been studied on the tensile properties and fracture behavior of alloy Ti-6Al-5Zr-0.5Mo-0.25Si (alloy 685). The heat treatment to hold silicon in solid solution consists of solutionizing at 1323 K for 0.5 hour under vacuum (∼10^-5 MPa), followed by water quenching, and the treatment to precipitate suicides involves subsequent aging of the solutionized and water quenched specimens at 1073 K for 24 hours. There is only marginal effect of the aging treatment on strength values; however, the ductility parameters are found to be drastically reduced. There are marked differences in the fracture behavior of the alloy in the as-quenched and the quenched and aged conditions. While the fracture surface of the unaged specimen shows characteristic dimples, there is a large number of facets on the fracture surface of the aged specimen. The facets in the central region are relatively smaller in size than those in the peripheral zone. The central facets show fluted features at higher magnifications; however, the peripheral facets are usually featureless. The faceted fracture in the aged condition is attributed to enhanced tendency for heterogeneous planar slip. The fracture characteristics correlate with the observed differences in the ductility in the two conditions.     

Room-temperature low-cycle fatigue behaviour of Ti-27Al-15Nb alloy

Low-cycle fatigue (LCF) behaviour of the alloy Ti-27Al-15Nb, in (α ₂+B2) heat-treated condition was studied in total axial strain control mode at different total strain amplitudes (Δɛ ₁/2) from ±0·65 to ±1·0% and room temperature. While there was little hardening of the material at the lowest strain amplitude (Δɛ _t/2: ±0·65%), pronounced hardening was observed at the higher strain amplitudes (Δɛ _t/2⩾0·83%). The cyclic stress, at high strain amplitudes, continuously increased from the beginning till fracture of the specimens. The LCF resistance of the material was found to be low and this was due to its poor ductility at room temperature. Dual slope was observed in the Coffin-Manson plot, with less slope of the upper segment than that of the lower one, as observed in several alloys. The fracture behaviour pointed to brittleness and faceted features were observed.     

High temperature ceramic radomes (HTCR) – A review

An electromagnetically transparent, structurally robust and environmentally resistant enclosure of radar antenna for ground based systems to modern avionics in military aircraft and missiles is called as radome. Radome materials are classified based on: (i) type of function - surface-based or flight-mode and (ii) speed of operation - subsonic, supersonic to hypersonic. The desired properties of these materials are low dielectric constant and low loss factor in addition to its capacity to withstand the high temperature of operation. Composite laminates of glass or aramid fibre reinforced polymeric resins are radome material candidates for applications in subsonic range. However, ceramics are the only viable option for military aerospace applications such as a fighter jet travelling at Mach 3 or an advanced hypersonic missile speeding up to Mach 5. This review outlines the hand-full of ceramic materials already in application as radome materials like high-purity-alumina, pyroceram, slip-cast-fused-silica, their processing technology, electromagnetic and mechanical properties, advantages and disadvantages with respect to advanced military vehicles. Use of silicon nitride based radome materials, that has exceptional mechanical strength and thermal stability up to 1400 °C is illustrated with respect to reaction bonded silicon nitride, hot pressed silicon nitride, silicon oxynitride, sialon and their composites. Design of new generation radome materials was conceptualized and discussed as applicable to silicon nitride and related ceramics, wherein incorporation of varied degree of porosity improves electromagnetic properties, simultaneously, maintaining the required mechanical strength. Multilayer and graded porosity and its influence on electromagnetic properties were briefly discussed. Si₃N₄ ceramics having controlled porosity leading to optimum electromagnetic and mechanical properties produced through systematic processing is proposed as the futuristic high temperature radome material for supersonic applications.     

Solid–liquid extraction of terbium from phosphoric acid solutions using solvent‐impregnated resin containing TOPS 99

BACKGROUND: In this work, the solid‐liquid extraction of terbium from phosphoric acid solutions using solvent‐impregnated resin containing TOPS 99, an equivalent of di‐2‐ethylhexyl phosphoric acid, has been investigated. The parameters studied include equilibration time, acid concentration, amount of resin, metal concentration, kinetics, temperature, loading, elution, regeneration and recycling. RESULTS: FT‐IR results confirm the physical interaction of the extractant with the resin. The extraction of terbium with TOPS 99 impregnated Amberlite XAD 4 resin was acid dependent and transfer of metal follows a cation exchange mechanism. The loading capacity of TOPS 99‐impregnated resin for terbium was calculated to be 23.8 mg g^?1 resin. Controlling mechanism of the adsorption was found to be a chemical reaction following pseudo‐second‐order kinetics. The endothermic nature of extraction was confirmed by temperature studies. Among the various eluants studied, H₂SO₄ was the best. Regeneration and recycling of the resin indicated the resin can be used for continuous cycles. CONCLUSIONS: Terbium was successfully extracted from phosphoric acid using TOPS 99 extractant impregnated into Amberlite XAD4 with a maximum loading of 23.8 mg g^?1 resin and fully recovered with 1 mol L^?1sulfuric acid. The resin was subjected to seven cycles of extraction and elution without any loss of performance. Further studies showed that terbium could be separated from lutetium. Copyright © 2010 Society of Chemical Industry     

Variation in HLA-DM expression influences conversion of MHC class II alpha beta:class II-associated invariant chain peptide complexes to mature peptide-bound class II alpha beta dimers in a normal B cell line

The maturation of invariant chain (Ii):MHC class II complexes into peptide-loaded alpha beta dimers occurs by proteolytic removal of Ii chain and binding of antigenic peptides derived from exogenous and endogenous Ags. A fragment of the Ii chain (class II-associated invariant chain peptide (CLIP) remains associated with class II alpha beta and is an intermediate in this process. Conversion of alpha beta:CLIP complexes into alpha beta:peptide complexes is facilitated by HLA-DM. Two unique mAbs, specific for I-Ab bound to human CLIP and I-Ab bound to DR alpha peptide, were used to assess the formation of these peptide:class II complexes in a human B lymphoblastoid cell line (B-LCL) (Swei) transfected with I-A(b). In multiple independent Swei:I-Ab transfectants, the amount of human CLIP (hCLIP):I-Ab expressed was inversely proportional to the amount of DR alpha 52-68:I-Ab; quantitative differences in HLA-DM expression accounted for this phenotype. In the low DM transfectant, a substantial proportion of I-Ab, but not DR molecules, was altered structurally and unable to present native protein Ags. Addition of DM transgenes to the DM-low cells resulted in an increase in DR alpha 52-68:I-Ab coupled with a decrease in hCLIP:I-Ab complexes and restoration of exogenous protein Ag presentation. The DR5 molecules in Swei cells, which have a lower affinity for hCLIP than I-Ab, were not affected by low DM expression, suggesting that the amount of DM required for conversion of CLIP:class II to peptide:class II may depend on the affinity of the class II molecules for CLIP or DM.