Strengthening of Glasses by Partial Leaching

A technique for strengthening certain alkali borosilicate glasses is described. Porous surface layers of a desired thickness are created on an article, taking advantage of the phase separation and leaching phenomena in such glasses. The semiporous composite is then heated to sinter the high-silica clad about the borosilicate core. On cooling, residual compressive stresses develop within the cladding. The mechanical properties of cylindrical specimens prestressed by this method were characterized by photoelastic measurements and moduius-of-rupture tests. Residual stresses as high as —410 MPa were observed and found to be in reasonable agreement with the form of a previously developed thermal-stress model. The roles played by thermal tempering and stress-relaxation effects are also discussed.     

OPSYN: A CAD BASED EXPERT SYSTEM FOR OPTIMUM STRUCTURAL SYNTHESIS

This paper presents an overview of an expert system, OPSYN, for optimum structural synthesis, with particular emphasis on the significance of the development environment for such systems. The knowledge base for this system includes rules for facilitating finite element modeling, optimum design modeling, and selection of optimization strategies and parameters. The environment for this development is an inference engine with both forward and backward reasoning capabilities, a detailed explanation facility, and an automated knowledge acquisition system with a knowledge base editor facility. The use of a computer aided design (CAD) interface with its significant data structure and graphics capabilities in the knowledge acquisition and knowledge representation process is demonstrated with this expert system.     

USE OF COUNTERPROPAGATION NEURAL NETWORKS TO ENHANCE THE CONCURRENT SUBSPACE OPTIMIZATION STRATEGY

The paper explores the use of artificial neural networks in a concurrent optimization strategy that derives from a decomposition based approach to design of large-scale engineering systems. These problems are characterized by complex couplings that render parametric design methods inappropriate as solution tools. Decomposition methods reduce the large dimensionality problem into a sequence of smaller, more tractable optimization problems, each with a smaller set of design variables and constraints. The decomposed subproblems are rarely decoupled completely, and design changes in one subproblem have a profound influence on changes in another subproblem. Essential components of decomposition based design methods are strategies to identify a topology for problem decomposition, and to develop coordination strategies which account for couplings among the decomposed problems. The paper examines the effectiveness of artificial neural networks as a tool to both account for the coupling, and to develop methods to coordinate the solution in the different subproblems to a converged optimal design     

Diffusion-Controlled Redox Kinetics in a Glassmelt

A diffusion model was developed to analyze the rate at which Fe²⁺/Fe³⁺ equilibration occurs in a Ca-Al-borosilicate melt in an air atmosphere at 1260°C. The resulting diffusion coefficient of oxygen determined from the model of 3.7 × 10⁻⁷ cm²/s is in agreement with the range of values reported for other glass-melts at this temperature and with the value determined from the viscosity of the melt using the Eyring model. The results support the general interpretation of the multivalent ion oxidation equilibrium as a diffusion-controlled process. The model thus provides a means to determine the diffusivity of oxygen in glassmelts from the kinetics of the oxidation reaction for a variety of redox ions.     

Influence of curing regimes on compressive strength of ultra high performance concrete

The present paper is aimed to identify an efficient curing regime for ultra high performance concrete (UHPC), to achieve a target compressive strength more than 150 MPa, using indigenous materials. The thermal regime plays a vital role due to the limited fineness of ingredients and low water/binder ratio. By activation of the reaction kinetics, the effectiveness of the binder is enhanced which leads to improvements in mechanical as well as durability properties. The curing cycle employed are ambient air curing, water curing and hot air curing. The specimens were exposed to thermal regime at (90°C/150°C/200°C) for duration of 24, 48 or 72 hours at the age of 3rd and 7th day followed with air curing or water curing till 28 days. The results showed a marked difference in compressive strength ranging from 217 to 142 MPa with change in curing regimes. The samples when thermally cured at the age of 3rd and 7th day produced an average ultimate strength of 217–152 MPa and 196–150 MPa, respectively.     

A LOOK AT TWO UNDERUTILIZED METHODS FOR OPTIMUM STRUCTURAL DESIGN

The paper presents a closer examination of two infrequently used optimization algorithms, in relation to their applicability in large-scale structural synthesis. The geometric programming approach and the Gauss constrained method are recognized as extremely efficient solution strategies. The former approach reduces the solution of a nonlinearly constrained problem to one with strictly linear constraints, and, under special conditions, to a solution of linear algebraic equations. The Gauss constrained method provides a robust approach for optimum design that does not require a step-size determination, and is thus useful in problems where functional evaluations are computationally demanding. An efficient implementation of these methods for structural synthesis can be attained by a recourse to well-documented approximation concepts.     

GA BASED SIMULATION OF IMMUNE NETWORKS APPLICATIONS IN STRUCTURAL OPTIMIZATION

Genetic algorithms have received considerable recent attention in the optimal design of structural systems. These algorithms derive a computational leverage from an intrinsic pattern recognition capability, whereby patterns or schemata associated with a high level of fitness are identified and evolved at a near-exponential growth rate through generations of simulated evolution. This highly exploitative search process has been shown to be extremely effective in searching for schema that represent an optimum, requiring only that an appropriate measure of fitness be defined. This exploitative pattern recognition process is also at work in another biological system - the immune system responsible for recognizing antigens foreign to the system and generating antibodies to combat the growth of these antigens. The paper describes key elements of how the functioning of the immune system can be modelled in the context of genetic search. It then provides an overview of the implications of this model in improving the convergence characteristics of genetic search, in particular, in the context of handling design constraints.     

Combined Effect of Flaw Distribution and Diameter Variation on the Statistics of Glass Fiber Strength

This paper examines the combined effects of flaw-severity distribution and fiber diameter distribution on the fracture statistics of glass fibers. The approach adopted is to assume specific forms (Gaussian and Weibull) for the diameter distribution, a Weibull distribution for the true strength, and then to calculate the measured strength distributions. Exact analytical results are obtained in the cases of flawless fibers, uniform size flaws, and when true strength and fiber diameter are Weibull distributed. Approximate analytical expressions have been derived for the general case. A procedure is described to obtain the parameters of the true strength distribution from the knowledge of measured strength distribution and the coefficient of variation of fiber diameters. Implications of these results for the fracture statistics of high-strength glass fibers are discussed.