Patent GB-1018002-A (GB)
Title: Improved process and apparatus for examining and/or measuring stresses in glass and other transparent materials
IPC: G01N21/21
Filed: 1963-09-20
Abstract: 1,018,002. Photo-elastic stress indicators; testing photo-electrically. COMPAGNIE DE SAINT-GOBAIN. Sept. 20, 1962 [Sept. 20, 1961], No. 35861/62. Headings G1A and G1W. [Also in Division G2] A process for examining and/or measuring the stresses in a glass or other transparent specimen A, comprises passing a beam of light through the specimen and examining and/or measuring the interference fringes produced by diffusion of the beam along its path through the specimen either through a polarising analyser, if the incident beam is unpolarised, or directly if the incident beam is polarised. In the apparatus shown a beam of light from a mercury lamp 1 is passed through a condenser 2 and converging lens system 3, the focal point B of which is preferably at the centre of the specimen A. The light from the lens system 3 passes through a yellow filter 4, to increase the contrast of the observed image, through an adjustable polariser 5 and to the specimen A through a prism 6 which the light enters orthogonally Fig. 2, not shown. Light diffused at right angles from the beam in the specimen A is observed through a prism 7 and passes to a converging lens system 9 which forms a real image of the path of the light beam in the specimen at a slit 10. The prisms 6 and 7 are cemented to the specimen A optical contact being ensured by use of an index liquid. An adjustable analyser 11 is also provided in the path of the diffused light. The image of the slip 10 is viewed through slots 14 (Fig. 3, not shown) in a wheel 13, the slots scanning the slit 10 as the wheel rotates, and a photomultiplier may be used to produce electrical signals in response to the variations in light intensity of the image when scanned by the slots 14, the signals being displayed on an oscilloscope (not shown). If the glass specimen has a parabolic stress distribution with stresses parallel to opposite faces of the specimen as compressive stresses at the faces decreasing to neutral lines, and with the central region of the specimen under tensile stress then the path difference introduced by these stresses between the components of a polarised light beam varies as shown in (Fig. 5, not shown) rising to a maximum at the first neutral line, and falling to a minimum at the second neutral line, then interference fringes will be produced at all points where the curve C cuts axes, parallel to the abscissae axis, for phase differences of whole numbers of wave lengths. If a supplementary delay e.g. a movable quartz wedge or rotatable quarter-wave plate is introduced into the system the curve C will be shifted e.g. to the position of the curve C<SP>1</SP> and the interference fringes can be made to oscillate about two fixed points which will correspond to the neutral lines. The stresses in the layer of the specimen between the neutral lines can be found by counting the fringes between the corresponding points on the oscilloscope display. The curve of Fig. 5 may be materialised by using a quartz wedge in the primary beam instead of observing the diffused light, and depending on the angles involved between the light rays and the specimen the polariser or analyser may be dispensed with. Instead of focusing the light on the point B, a parallel beam of light may be used if desired.