Determination of bacterial activity by use of an evanescent-wave fiber-optic sensor

A novel technique based on fiber-optic evanescent-wave spectroscopy is proposed for the detection of bacterial activity in human saliva. The sensor determines th e specific concentration of Streptococcus mutans in saliva, which is a major causative factor in dental caries. In this design, one prepares the fiber-optic bacterial sensor by replacing a portion of the cladding region of a multimode fiber with a dye-encapsulated xerogel, using the solgel technique. The exponential decay of the evanescent wave at the core-cladding interface of a multimode fiber is utilized for the determination of bacterial activity in saliva. The acidogenic profile of Streptococcus mutans is estimated by use of evanescent-waveabsorption spectra at various levels of bacterial activity.     

Stress distribution in the dento-alveolar system using digital photoelasticity

Past research has confirmed that the governing factors in cellular modelling and remodelling adhere to sound principles of engineering mechanics. Hence studies of stress distributions would provide better understanding of the functional adaptation of dental supporting structures. Photoelasticity is an established experimental tool to study whole-field stress distribution in structures subjected to forces. However, it has certain limitations that make its application in biological specimens tedious. In this investigation an advanced digital photoelastic system is used to visualize and study the nature of the stress distribution in dental supporting structures. These digital fringe patterns are analysed using a phase-shift technique. The present biomechanical study shows that dental supporting structures exhibit a characteristic stress distribution, promoting structural adaptation based on needs. Furthermore, the advantage of using a digital image processing system along with the circular polariscope is discussed.     

Supercritical direct extraction of neodymium using TTA and TBP

Extraction of a metal ion from its oxide using ligand assisted supercritical carbon dioxide (SC CO₂) comprises namely ionisation of metal oxide, in-situ chelation of metal cation with the ligand to form metal chelate/adduct and subsequently its extraction. Understanding of the mass transfer of chelate/adduct is very important in deciding the overall performance of the in-situ supercritical fluid extraction (ISCFE) process. For the present study neodymium (Nd) is selected as a model metal ion for its extraction from oxide using a mixed ligand system containing thenoyl tri-fluoroacetone (TTA) and tri-butylphosphate (TBP). Extraction studies have been performed at 35 MPa and 60°C for the prepared Nd-TTA-TBP adduct as well as for neodymium oxide (Nd₂O₃). The rate of dissolution starting from oxide and TTA-TBP adduct of Nd have been calculated and compared with the equilibrium values based on dissolution studies at the same conditions of temperature and pressure. During the extraction starting from oxide, the ligands TTA and TBP are also co-extracted with the adduct as these are highly soluble in SC CO₂. Mass transfer coefficient has also been estimated for the steady state during the dynamic extraction. It is observed that the rate of extraction and mass transfer coefficient increase with flow rate of SC CO₂.     

Analysis of fatigue crack growth in reinforced concrete beams

Mechanical behavior of reinforced concrete members is influenced by the action of unknown crack bridging reactions of rebars. Under cyclic loading, due to progressive growth of cracks, this bridging action contributes to the overall strength, stiffness and hysteretic behavior of the member. In this work, fatigue behavior of reinforced concrete beams are studied using a crack propagation law, developed using dimensional analysis for plain concrete with the effect of reinforcement being simulated through constraint exerted on the crack opening. The parameters considered in the model are fracture toughness, crack length, loading ratio and structural size. A numerical procedure is followed to compute fatigue life of RC beams and the dissipated energy in the steel reinforcement due to the shake down phenomenon under cyclic loading. Through a sensitivity study, it is concluded that the structural size is the most sensitive parameter in the fatigue crack propagation phenomenon. Furthermore, the residual moment carrying capacity of an RC member is determined as a function of crack extension by including the bond-slip mechanism.     

Crystallographic evaluation of sodium zirconium phosphate as a host structure for immobilization of cesium

Sodium zirconium phosphate (NZP) is a potential material for immobilization of nuclear effluents. The existence of cesium containing NZP structure was determined on the basis of crystal data of solid solution. It was found that up to ~9.0 wt% of cesium could be loaded into NZP formulations without significant changes of the three-dimensional framework structure. The crystal chemistry of Na_1−x Cs _x Zr₂P₃O₁₂ (x = 0.1–0.4) has been investigated using General Structure Analysis System programming. The CsNZP phases crystallize in the space group R-3c and Z = 6. Powder diffraction data have been subjected to Rietveld refinement to arrive at a satisfactory structural convergence of R-factors. The unit cell volume and polyhedral (ZrO₆ and PO₄) distortion increase with rise in the mole% of Cs⁺ in the NZP matrix. The PO₄ stretching and bending vibrations in the infrared region have been assigned. SEM, TEM, and EDAX analysis provide analytical evidence of cesium in the matrix.     

Residual fatigue strength assessment of concrete considering tension softening behavior

In this study, the residual strength of plain concrete beams under fatigue loading is assessed. The quasi-brittle nature of the material is considered by including the effect of tension-softening taking place in the fracture process zone. A two step approach is followed. In the first step, the effective critical crack length for unstable fracture to occur is determined by using two different methods, namely a modified LEFM based fatigue crack propagation law and the crack resistance method. In the second step, the moment carrying capacity as a function of increasing effective crack length is obtained in order to assess the residual strength of the member. A parametric study is performed by considering three different softening laws: linear, bilinear and power laws. It is seen that the bilinear softening law matches close to the experimental predictions of other investigators.     

Energy Release Rate due to Friction at Bimaterial Interface in Dams

The interface between concrete dam and rock foundation is one of the most important regions governing the strength and stability of gravity dams. Many researchers have attempted to extend the fracture mechanics approach to study this rock concrete interface assuming stress free crack surfaces. In a real-life situation, because of the combined compression and shear loading, the crack faces come in contact resulting in a sizeable contact zone near the crack tip. Thus, frictional contact of the crack surfaces cannot be neglected. The frictional contact alters the stress singularity to become either weaker or stronger than the inverse square root singularity observed in homogeneous crack problems. Consequently, the strain-energy release rate as conventionally defined, either vanishes or becomes unbounded and thus cannot be used as a fracture parameter. In this work, an attempt is made to include the effect of friction associated with the sliding of crack surfaces and compute the energy dissipated during crack propagation. It is shown that the total energy release rate decreases with crack length when friction is accounted for between the rock-concrete interface in gravity dams.     

Issues on Puddle Formation During Rapid Solidification of Fe–Si–B–Nb–Cu Alloy Using Planar Flow Melt Spinning Process

The present investigation deals with the processing of rapidly solidified 10?C40?mm wide, 30?C50???m thick and 10?C30?m long continuous amorphous ribbons of Fe?CSi?CB?CNb?CCu alloy using a planar flow melt spinning (PFMS) equipment. As the formation of quality amorphous ribbon depends on the melt puddle formed between the nozzle and the rotating copper wheel, the effect of different processing parameters such as nozzle wheel gap, ejection pressure etc. on the shape and stability of puddle has been studied using a high-speed imaging system and correlated with ribbon formation. Simultaneously, a two-dimensional numerical model has been developed to simulate the puddle formation during the PFMS process. The model is based on a computational fluid dynamics technique, called volume of fluid using transient heat transfer and fluid flow equations with the evolution of free boundaries. Different simulations have been performed to study the effect of process parameters on the puddle formation. The simulated results have been verified with the experimental observations considering same set of process parameters. It has been observed that the present simulations have produced exact replica of the experimental observations. Further parametric analysis as well as analysis of simulated result has been carried out to understand how the different process parameters affect the formation of quality amorphous ribbons during PFMS process.     

Fracture stability and residual strength assessment of reinforced concrete beams

In conventional analysis and design procedures of reinforced concrete structures, the ability of concrete to resist tension is neglected. Under cyclic loading, the tension-softening behavior of concrete influences its residual strength and subsequent crack propagation. The stability and the residual strength of a cracked reinforced concrete member under fatigue loading, depends on a number of factors such as, reinforcement ratio, specimen size, grade of concrete, fracture properties, and on the tension-softening behavior of concrete. In this work, a method is proposed to assess the residual strength of reinforced concrete beams subjected to cyclic loading. The crack extension resistance based approach is used for determining the condition for unstable crack propagation. The effect of reinforcement is modeled as a closing force counteracting the effect of crack opening produced by the external moment. The effect of percentage reinforcement and specimen size on the failure of reinforced beams is studied. Finally, the residual strength of the beams are computed by including the softening behavior of concrete.