Residual stress anisotropy in high pressure sodium lamp seals

Residual stresses in the seal material connecting the metallic feedthrough and the ceramic body of high pressure sodium lamps have been investigated. Anisotropy of the residual stresses were examined experimentally with Vickers indentation and were also modelled with the finite element modelling method. The two techniques resulted in good qualitative agreement. No microcrack formation or other microscale change happened during ageing, but relaxation of the stresses took place. Application of the technique based on counting the cracks initiated by Vickers indentation is described. The method is capable of showing relative changes in the stress level of samples after various ageing times. The method is also capable of testing systems with characteristic sizes of even less than 1 mm. (Some figures in this article are in colour only in the electronic version)