The interferometric strain rosette technique

An interferometric strain rosette can be used to measure three in-plane strains. The strain rosette consists of three microindentations produced on an object surface. Upon illuminating the indentations with laser light, each pair of indentations acts as a two-point source generating a pair of Young's interference fringe patterns. When the object is deformed, the distance between the indentations is altered. By measuring the change of spacing of the Young's fringes, the strain in the direction of the separation of the indentations is measured. Three indentations are at the vertexes of an equilateral triangle to constitute a strain rosette. Each pair of the microindentations enables measurement of an axial strain in the indentation separation. The rosette measures simultaneously three in-plane strains in the directions of the triangular sides. As three in-plane strains are measured, the in-plane shear and two normal strains can be found. Compared with a resistance strain rosette, the interferometric strain rosette has great features such as non-contacting and a short gage length. In addition, the interferometric strain rosette can measure large elastoplastic strains and is applicable to measurements at elevated temperatures. The theory with experimental evaluation is presented. Measurement sensitivity of the technique is discussed. Potential applications and limitations of the technique are to be described as well.