An approximate theoretical treatment of the narrow sandwich seal provides the necessary correction factor for calculating true thermal expansion mismatch between seal members from the observed birefringence. The shape factor, which physically represents the extent to which the Poisson strain is developed in the narrow direction, is a strong function of the aspect ratio (width/thickness) of the core member. Its value is 0 to 1 for aspect ratios of 0 to >20. The effect of the thickness ratio (of the seal members) and elastic moduli on the shape factor is relatively small. It is found that large errors can arise in the calculated value of the thermal expansion differential if the shape factor is not taken into account.
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Gulati et al. (1978) studied this question.
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