Application of photoelasticity for analysis of residual stresses in CDs

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 of this object. Relaxation is the main reason of redistribution of residual stresses and it can occur without external influence or if there 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. Modern 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 of the NDT of CDs at various stages of their manufacturing is offered.