Interferometric Strain Rosette for Studying Fatigue and Creep of Notch Roots at High Temperatures

This paper presents a novel laser-based technique, the interferometric strain rosette, for measuring three in-plane strains. An interferometric strain rosette consists of three micro-indentations on a specimen surface and can be of two separate forms. The two forms are 45° and 60° rosettes. The three indentations for either form can be grouped into three pairs. Upon illuminating the indentations with laser light, each pair of the indentations behaves as a two-point source generating a pair of Young’s interference fringe patterns. As the strains in the specimen cause the indentation separations to change, the fringes move. The shift of the fringes is monitored with linear photodiode arrays and collected real-time via a microcomputer system. Three strain components in the directions of the indentation pairs, which serve as the strain gages, can thus be obtained. The technique is noncontacting and can be extended to measurements at high temperatures. Tests conducted on a notch root at 650°C verify the applicability of the technique in high temperature environments. The computer-controlled system allows a study of the time-dependent behavior of materials. Viscoplastic behavior of a nickel based superalloy, Inconel 718, is investigated through rate-dependent, cyclic and creep testing.