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Rock strain can be studied utilizing continuously and peripherally cemented photoelastic sheets and frozen-stress ring photoelastic gages in integrating mounts. Standard errors of estimate of strain differences, as calculated from normal incidence, range up to 20 μin./in. Standard errors of estimate of principal strains taken separately range up to about 30 μin./in. Strain levels as low as several tens of μin./in. can be measured with confidence. Strain levels calculated from photoelastic observations and elasticity theory typically differ from each other by 10 per cent. Results are consistent among continuously bonded and integrating mounts, rectangular and circular sheets, different loading schemes, and different lighting and viewing arrangements. Frozen ring photoelastic gages are relatively easily read and indicate directions of principal strains. Work with rock specimens gives results in accord with those obtained from work with aluminum bars. Photoelastic coatings are very sensitive indicators of fractures and other types of anisotropy. This method is applicable to many types of investigation.
photoelasticity, photoelastic coatings, residual stresses, strain optical constant, reflection polariscope, rock (material), rock mechanics, stress analysis
Pincus, H J
Professor of geologyPersonal member ASTM, The Ohio State University, Columbus, Ohio