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3-D Stress Measurement in Glass

IP.com Disclosure Number: IPCOM000020565D
Publication Date: 2003-Nov-27
Document File: 5 page(s) / 290K

Publishing Venue

The IP.com Prior Art Database

Abstract

ID614873 & ID615175

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Description of the invention

Known solutions

Measuring stresses in glass is important both in manufacturing and in product design optimalisation. The most common methods rely on the use of polarised light. All these methods are non-destructive. As stresses cause the glass to become birefringent, the relative retardation of polarised light travelling through a stress containing glass body is a measure for the stress present. However, most of these measurements give an average stress over the glass thickness or light path through the glass. For flat glass, only two methods are known to measure the stress profile over their thickness, the scattered light method and magneto-photo-elasticity. The latter method is very exotic, complicated and expensive and therefore rarely used. At the mo ment there is only one of these set-ups available in the world. The former method has been widely discussed in literature over the past decades, but practical problems have prevented it from being adopted in industry, or even industrial R&D.

Another method of measuring stresses in brittle materials in general is by quantifying the effect of stresses on (hardness, eg. Vickers) indentations. However, this only enables measurements at the surface of the sample and is destructive.

Problem definition

Insight in glass forming processes requires a detailed knowledge of the stress distribution. Measurement of stress in glass is difficult and complicated. Furthermore, most of them only give information either on a small part of the glass (e.g. the surface) or the net-result for the whole thickness of the glass (birefringing of stressed glass). Therefore, standard practice is to use finite element thermo - mechanical calculations together with the aforementioned methods, to gain insight. However, the need remains to have a direct measurement method of the 3-dimensional stress distribution

Measures and insights

To determine whether the minimum laser energy density depends on stress we use the following set-up: a glass plate (free of internal stresses) is stressed at a well known level using a four-point bending test. The load is either zero, or 80 N, corresponding to 0 and 8 Mpa tensile stress. The laser beam is directed towards the stressed region. Tensile stress is maximum at the surface of the glass plate....