IS0 4790:1992 pdf download – Glass-to-glass sealings -Determination of stresses.
6 Procedure 6.1 In order to verify that the sealing of the test specimen is without any defects (see 5.3.1), the symmetry of the stress picture should be checked in each half of the test specimen by means of stress- testing equipment (see figure 2). 6.2 In addition, both halves of the test specimen shall be tested for splits. Splits in one or both halves of the test specimens disturb the stress symmetry and this may affect the results of the measurements taken as described in 6.6. For such test specimens, only the sign of stress can be determined, as de- scribed in 6.7. 6.3 Take all measurements at room temperature between 15 “C and 35 “C. Any test temperatures differing from this range shall be stated in the test report. 6.4 Measure the thickness of the test specimen in the measuring area (see 6.6).
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 fluid, 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 light 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.