Determination of thermal strains by moiré interferometry

Abstract

An existing method is extended to measure thermal strain distributions on an absolute basis. Free thermal expansion and stress-induced deformations are separated, allowing the determination of coefficients of expansion, stress-induced strains, normal stresses and shear stresses. The method is applicable to many steady-state and transient thermal-strain problems.

A crossed-line grating is replicated on the specimen at elevated temperature. A zero-expansion mold is used for the replication, so that the grating frequency at the replication temperature can be retained to null the moiré interferometer at room temperature. When the specimen is viewed in the moiré interferometer, the fringe patterns reveal theU andV displacements induced by any change from the replication temperature. In addition, carrier fringe techniques are introduced for steady-state problems to subtract off the free thermal expansion and produce fringe patterns of the stress-induced deformations themselves. The method is demonstrated by analysis of a bimaterial plate subjected to a uniform change of temperature.