Design strength of optical glass

Abstract

Brittle materials such as glass do not possess a single characteristic strength. The strength of the material is dependent on the distribution of cracks or surface flaws. These factors, coupled with the inherent brittleness (cause of catastrophic or rapid failure) mean that extremely conservative design approaches are typically used for optical elements made of glass. Determining a design allowable for glass elements is critical for optical systems using relatively brittle glass types or for optical elements subject to relatively high stress levels. Rule-of-thumb tensile design strengths are typically at 1000 -1500 psi for nominal glass materials. This neglects the specific glass composition, subcritical crack growth, surface area under stress, and nature of the load -static or cyclic. Several methods to characterize the strength of optical glass are discussed to aid engineers in predicting a design strength for a given surface finish, glass type, and environment. These include estimating fracture toughness for a given glass, predicting inert strength using material test data, and lifetime predictions accounting for static fatigue and cyclic loading. Determining a design strength for a spaceborne optical element is discussed.

Structural integrity assessment of glass components in Concentrated Solar Power (CSP) systems

“…Recently, Santarsiero and Froli [6] formulated a new semi-probabilistic failure prediction method, called "Design Crack Method" (DCM). Doyle and Kahan [7] and Sutherland [8] proposed more advanced method for glass strength forecasting which takes into account the probability of failure and the surface damaging level.…”

“…Components stressed over a large volume (area) will activate a bigger number of flaws, and therefore In recent years, considerable research efforts have been paid to improve the understanding of the load-carrying behaviour of structural glass elements, and many new design approaches have been proposed based on the Weibull´s statistical failure probability function [3] to improve the safety and serviceability of the structural glasses [4][5][6][7][8]. Porter [4] proposed the Crack Size Design method (CSD), and Haldimann [5] made further development to the solution of this problem with the Lifetime Prediction Model (LPM).…”

Design and analysis of the International X-Ray Observatory mirror modules

“…The strength of a test specimen can be related to the strength of a glass segment as supported in the FMA, which have different stressed areas and therefore a different number of flaws, by the following equation [9]:…”

Integrated assessment procedure for determining the fracture strength of glass components in CSP systems

“…The total time-to-failure, ts, of a component under a constant static stress with a known flaw was expressed as [5]:…”

“…These large safety factors are somewhat arbitrary and not satisfactory, because it is not very clear what the true factor of safety really is. In recent years, considerable research efforts have been paid to improve the understanding of the load-carrying behaviour of structural glass elements, and many new design approaches have been proposed to improve the safety and serviceability of the structural glasses [2][3][4][5][6]. In 1972, Brown [7] proposed the "Load Duration Theory" (LDT), which combined the static fatigue theory of Charles & Hillings [8] with the statistical failure probability function proposed by Weibull [9].…”