Thermal stresses at high temperatures in stainless-steel rings by the moiré method

Abstract

The authors have to be commended for the fine job done in complet ing their previous inves t iga t ion 1 concerning the appl ica t ion of moird method for the solut ion of thermal-s t ress problems. However , a few minor r emarks seem to be in order for clar ifying some points. Reference 1 of the paper and of the discussion contains techniques appropr ia te to determine the rmal s t ra ins up to 400 ~ F. According to this method, a copy of a line or crossed grat ing is reproduced on the polished surface of the metal l ic specimen and a moir6 pa t t e rn is formed by superimposing a reference grat ing (RG) on the specimen. The method has two major d i sadvan tages der ived f rom the contac t of R G with the specimen. This contac t res t r ic ts the appl icat ion of the method to low tempera tu res where ei ther f rac tur ing or nonuniform deformat ion of RG is excluded and to s t eady heat-flow problems where the contac t of R G with the specimen does not influence its the rmal equil ibrium. By removing the reference grat ing from contac t wi th the specimen, we can e l iminate bo th sources of error re la ted to the reference grating (RG). The idea of removing R G from the specimen and producing an interference between R G and the image of the specimen grat ing (SG) is simple and i t has been ment ioned by many experimenters . Among others, Gield and D a n t u suggested this technique as a useful appl ica t ion of moir6 pa t t e rns to metrology. But , to the best of our knowledge, i t has been appl ied for the first t ime