ISO 4790:1992 pdf download – Glass-to-glass sealings -Determination of stresses
If two glasses are sealed together, stresses can persist in them after cooling. The value of these stresses depends on the differences in the thermal, elastic and viscous properties of the glasses. In general, glasses become birefringent when they are subjected to stresses. For the relationship be- tween the stress and the optical path difference re- sulting from the birefringence
For positive stress-optical coefficients K! in the case of a tensile stress in the glass, the value for As shall have a positive sign (+), and in the case of a compressive stress in the glass a negative sign (-). For negative stress-optical coefficients K, the signs of As are the contrary.
In equation (1), σ represents the difference between the principal stresses σ2 and σ3. In the vicinity of the sealed area, the principal stress oz is small in com- parison with the principal stress 02，1 so that in equation (1)， σ may be treated as equivalent to principal stress σ2. The optical path difference, which occurs when light passes between the two light waves oscillating in the direction of principal stresses σ2 and σg, is caused by the differing speeds of propagation in birefringent test specimens.
5.2 Condition of glass pleces
The glass pieces shall be free of striae, gaseous and solid inclusions in the vicinity of the surfaces to be sealed. The surfaces of the glass pieces to be sealed shall be f1at, with a roughness less than that achieved by polishing with an abrasive of particle size P 240 ac- . cording to FEPA-Standard 43-GB.
5.3 Preparation of the test specimen
5.3.1 Seal two glass pieces completely together, on the surfaces prepared for sealing, to form one test specimen (see figure 1). During this procedure, any deformation of the flat sealed surface shall be avoided.
6.5 When the surfaces of the test specimens through which the light passes are not sufficiently smooth for the stress picture to be clear, it is advis- able to place the test specimen in an immersion fuid, for example water in the case of fine polished surfaces. The bottom of the immersion vessel itself shall not have any interfering birefringence and op- tical path difference.
6.6 When the ight is passed through a split-free test specimen in a perpendicular direction, measure the principal stresses lying parallel to both sides of the sealed surface as the optical path differences, one after another at a distance of e= 0,5 mm from the sealed surface and at the centre of the test specimen (see also figure 1). In the case of a test specimen with one non- transparent glass piece, the optical path difference can only be measured in the transparent half of the test specimen. In the case of a test specimen of which one-half consists of an agreed reference glass, possibly of the same type of glass, it is sufficient to measure the optical path difference of the birefringence in the reference glass half.
6.7 When the type of stress is not clearly revealed by measurement of the optical path difference, and in the case of a test specimen having splits, deter- mine by means of the stress testing equipment and by comparison with a tension or compression test piece, whether the optical path difference is the re- sult of tensile or compressive stress.