Residual stresses appeared with time due to the imperfection of the material production process. Sometimes the redistribution of residual stresses is the reason of cracks initiation in the tested object on one of the manufacturing stages or this redistribution increases them up to the critical value, when a small external load results in breakage ofthis object. Relaxation is the main reason of redistribution of residual stresses and it can occur without extemal influence or ifthere is heating, static and cyclic loads. Relaxation results in change of the sizes and shapes of the tested object. The dimensional stability is especially important in modern high technologies, in particular in the use of compact discs for the storage of information.Modem CDs are produced from polycarbonate, possessing an effect of birefringence. It allows to make an estimation of stresses at CDs by the method of photoelasticity. A metallized coating put on one of a CD surfaces provides ideal conditions for registration of interference picture observed in the reflected light. When investigating the stresses in CDs with the use of reflective V-type plane polariscope the solving equations become similar to those used in a photoelastic coating method. The present work discusses the results of the research of the residual stresses in CDs with various operation time produced by various firms and by different technologies (punching, laser recording). A simple optical method ofthe NDT ofCDs at various stages oftheir manufacturing is offered.

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A full understanding of the plastic zone formation in a stress concentration region of homogeneous and polycrystalline bodies, has long been a goal of researchers in experimental mechanics. Although some analytical and numerical models have been proposed to predict the formation of plastic zones, no satisfactory correlation has been obtained with experimental results due to the complexity of measurement procedures. 1-3The purpose of this paper is to develop a simple experimental technique to investigate strains and stresses in a plastic zone in engineering materials by using contact holographic interferometers.Contact holographic interferometer is presented as an assembly of the object, surface section and holographic plate in an immediate vicinity to the object.4'5 This technique is used to determine in-plane and out-of-plane displacements of the tested object. If a hologram is recorded at a metallized grating surface it restores information peculiar to some optical methods: holographic interferometry, speckle photography, holographic moire and mirror-optical method. For the interpretation of interference patterns simple analytical equations are obtained. Contact holographic interferometers are used effectively to solve some elasto-plastic deformation measurement problems of isotropic and polycrystalline materials. It is specially designed to permit the accurate measurement of the in-plane component of strain in the immediate vicinity of stress concentrators in plates. Some research results of plane static and signvariable load elasto-plastic problems are considered.

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