Processing speckle images to improve quality of Residual Stress measurements by HDM+ESPI

Industrial processes which lead to the creation of the final components are potential sources of residual stresses. Any kind of manufacturing process, in fact, as welding, forging, casting ecc. introduces a given stress state in the component that it is not easy to foresee. Many experimental techniques have been developed along the years in order to have an accurate evaluation of the residual stress state. Equipments and techniques have strongly evolved along the years and new approaches are also taken into account currently. Among the innovative procedure for residual stress evaluation, one of the most interesting one is HDM-ESPI that can be considered a direct evolution of the well consolidated hole-drilling method. This is a powerful approach consisting in detecting, by an optical interferometric technique, the displacements field around an hole drilled in order to get local stress relaxation. However quality of the recorded images can affect the reliability of the measurement. In this paper some considerations on pixel quality are presented along with some indications about the proper choice of the internal radius of analysis. It will be demonstrated that, from a practical point of view, the quality of the image can be diminished by the production of some fine dust that sticks to the surface altering the speckle pattern. As a consequence it will be seen as a proper choice, of the pixels to be used for residual stress evaluation can affect the final quality of the measurement.