A laser speckle sensor for single-point measurement of strain

Single spot measurement of strain and displacement is very desirable in some applications, especially if the measurement spot can be very small. In this paper we introduce a single point strain measurement principle based on objective speckle correlation using just one CCD array sensor. The method yields one component of the strain in the plane of the surface of the specimen. It is insensitive to strain in the perpendicular direction, and to in-plane displacements of the surface. Measurements on thermal stress in a heated aluminium specimen have been made and are found to be consistent with two other independent strain measurements. The uncertainty of measurement can be as low as 5 microstrain. The dependence of the uncertainty on the diameter of the illuminated spot and on other factors is discussed.