Design of a contactless interferometric strain gauge

In this paper an interferometric sensor is presented, which measures a mechanical strain with a differential laser Doppler measurement without contact on a technical surface and was designed for the fatigue strength test. The compact sensor detects the displacements at two adjacent measuring points in the plane perpendicular to the optical axis (in-plane), which is defined by a receiving lens. The differential displacement divided by the constant distance of two measuring points corresponds to the strain between them. The application of this strain sensor in the fatigue strength test requires a dynamic of elongation of 0.001 to 3 parts per thousand. A mathematical model was developed for the design of an optimal optical structure and for the determination of the theoretical physical resolution limit of the sensor. In this article we present our model and an optimized sensor design.