Suresh T. Gulati

Design methodology for the mechanical reliability of optical fiber

An engineering methodology for the mechanical reliability of optical fiber is developed within a fracture-mechanics framework. The model expresses allowable in-service and installation stresses as a fraction of fiber strength in a fatigue environment for a range of n values and fiber types. Failure probability is incorporated into the model by the measurement of the fiber-strength distribution appropriate to the application. For long-length applications, strength distributions of hundreds to thousands of kilometers of fiber are needed. A 400-km strength distribution captures the beginnings of the truncated portion of the distribution.

NEW DEVELOPMENTS IN CATALYTIC CONVERTER DURABILITY

Abstract A number of new developments in the areas of material composition, cell geometry, wall porosity, substrate contour, washcoat formulation, thermal and acoustic insulation, and clamshell and heatshield designs have occurred in recent years to meet the more demanding performance and durability requirements of automotive catalytic converters. This paper reviews the major developments in ceramic and metal substrates, stable washcoat systems, nonintumescent mounting materials, edge-shielded intumescent ceramic mats, ceramic insulation rings, dual can and dual cone packaging designs, and perforated heatshields with convection cooling. These improvements offer higher conversion efficiency, lower pressure drop and extended durability. A brief comparison of ceramic and metal substrate durability, based on laboratory tests, is also included.

Application of a phenomenological fatigue model to optical fibers

A phenomenological formulation of Si-O bond dissociation is utilized to interpret stable crack velocity and static and dynamic fatigue phenomena. The resulting model has an exponential form and is applicable to a wide range of flaw sizes, service stresses, and test environments. Furthermore, it is readily reduced to the power law by retaining the first term of the series expansion of the exponential function. The model provides a sound physical basis for comparing different fiber compositions, service environments, and stress-time histories (static versus dynamic) from the fatigue point of view. The application of the model to silica and titania-doped silica optical fibers provides valuable insight into their relative fatigue behaviors and sheds further light on the fundamental mechanisms controlling such behavior.

HIGH TEMPERATURE FATIGUE IN CERAMIC WALL-FLOW DIESEL FILTERS

Under certain operating conditions when the combined stresses in a ceramic wall-flow diesel filter from mechanical, thermal, and vibrational loads exceed its threshold strength, the fatigue effects become important. This paper reviews the theory of static and dynamic fatigue, and presents fatigue data for Cornings high efficiency filter composition (EX-47, 100/17) in the temperature range 25/sup 0/ - 400/sup 0/C which is representative of the stressed peripheral region during regeneration. The measurement and analysis of fatigue data, together with the implication on long-term durability of cordierite ceramic filters, is discussed.

New tests for characterizing the durability of a ceramic catalytic converter package

New test methods were developed to characterize the high temperature durability of intumescent mats that are used to mount ceramic catalyst supports in stainless steel cans. The key attribute of these tests is the use of an electric resistance heating method to maintain a temperature gradient through the thickness of the mat when a cyclic or constant shear stress is applied to the mat interface. These tests are simple to perform and do not require expensive equipment or highly skilled operators. Using these new test methods, the durability of ceramic preconverters mounted with 4,070 gm/m{sup 2} intumescent mat was studied. The results of these tests indicate that a preconverter package with 4070 gm/m{sup 2} intumescent mat can perform satisfactorily in the close-coupled application where temperatures exceed 900 C. The mat performance can be quantified in terms of applied stress and test temperature by utilizing the experimental methods described in the present study.

Thin specialty glass for reliable thin film PV modules

Glass has long been used for photovoltaic (PV) module covers and thin-film (TF) module substrates and superstrates. These applications typically use float glass of soda-lime-silica composition and thickness ≥ 3.2 mm. Thin specialty glass is being considered as a replacement for substrates and superstrates for dual-glass laminated TF PV modules. This study focuses on module in-service stresses and their impact on mechanical reliability when using thin specialty glass. Substrate and superstrate thicknesses as low as 0.7 mm were analyzed using various support conditions. Hail impact tests were conducted according to IEC 61646 with standard ice balls (25mm diameter at 23 m/sec). Wind and snow loads as described in IEC 61646 were studied by finite element analysis. Reliability analyses were performed using experimental results, modeled stress levels, initial glass strength, and strength decrease over time (fatigue). This study concludes that thin specialty glass is reliable in the variety of configurations analyzed. Wind and snow load stresses increased systematically as glass thickness decreased. However, support structure configuration showed a greater influence on stress and reliability levels than glass thickness. High speed wind tests on full sized laminates with complete support structures provided validation of performance under realistic conditions. Thin specialty glasses are shown to be a reliable option for thin-film photovoltaic module substrates and superstrates.

Isostatic strength of extruded cordierite ceramic substrates

This paper provides elastic analysis of compressive stresses in the matrix and skin regions of automotive substrates during 3D- and 2D-isostatic strength testing. The matrix region is treated as transversely isotropic material and the skin region as isotropic material, each with their independent elastic properties. Such a solution helps quantify load sharing by the matrix and skin regions which, in turn, affect compressive stresses in each region. The analysis shows that the tangential compressive stresses in the skin and matrix differ significantly at the interface due to high stiffness ratio of skin versus matrix. The resulting strain in the skin is more severe for thin and ultrathin wall substrates and may lead to localized bending of interfacial cells thereby inducing premature failure. Methods to reduce compressive strain in both the matrix and skin without affecting performance-related advantages are discussed. The analysis points out that the key factors that affect isostatic strength include skin vs matrix stiffness, skin vs web thickness, cell configuration, cell density and substrate diameter.

Long-term reliability of large ULE mirror blanks

ULETM, the titania-silica binary glass with zero expansion coefficient, is an ideal material for large telescope mirror blanks due to the unique combination of its optical, thermal and mechanical properties--together with the ease of fabrication--which help meet performance and durability requirements in a cost-effective manner. Indeed, the 8m class Subaru and Gemini telescope mirror blanks have been fabricated successfully from ULE glass and will be in full operation in the not too distant future. This paper will focus on the stringent reliability requirements which the mirror blank must meet during fabrication, transportation, installation and operation atop high mountains with extreme environmental fluctuations. In particular, the paper will present strength and fatigue data for ULE glass as a function of surface finish. Such data are critical for selecting the appropriate surface finish to ensure mechanical reliability of the mirror blank at various stages of fabrication and during transportation. The use of Weibull statistical distribution for surface flaws combined with Power Law fatigue model helps arrive at a safe stress level which should not be exceeded to ensure the mechanical reliability of the mirror blanks. The safe stress level is verified through independent static fatigue tests on ULE discs with surface finish identical to that of the mirror blank. In this manner the mechanical reliability of large ULE mirror blanks can be ascertained at extremely low failure probabilities. The successful application of reliability model to both Subaru and Gemini mirror blanks will be illustrated.

Strength/flaw relationship for CRT panel glass (alkali strontium silicate glass)

— Four-point bend strength of samples of CRT panel glass was measured following sandblast, gritblast, and 150-J abrasions, after either a 48-hour soak in air or a 24-hour soak in water. Strength/flaw characteristics were determined by examining the fracture surface and relating strength to flaw depth and mirror radius. Fracture toughness was also measured. Based on the strength and the fracture toughness values, the flaw shape factor or flaw severity for each type of abrasion was calculated.