106 C. Pappalettere Fig. 17.1 Example of ESPI correlation fringes obtained during the hole-drilling process. Fringes are obtained by subtracting the speckle pattern recorded at a given drilling increment from the reference pattern recorded before starting the drilling procedure. Fringes represent the loci of points of isodisplacement along the direction of the sensitivity vector from a reference pattern correlation fringes are obtained as those shown in Fig. 17.1. In this paper the evolution of the ESPIHDM technique along the years is presented showing that systems have become increasingly compact and easy to operate, calculation methods have become more robust and faster and the field of application of the technique has increased more and more. 17.2 Origin of ESPI-HDM Pioneering works in the adoption of ESPI combined with Hole Drilling methods are those of Furgiuele et al. [24]. The optical set-up adopted allowed to determine three components of the displacement in the plane of the specimen and two oblique to the plane. In order to verify the reliability of the method the system was tested on a steel specimen on which a well know stress state was introduced. The centers of the fringes were detected manually that is to say that it was a task of the user to track them on the images and to assign them the proper order of fringe. This allowed to determine the displacement on the pixel along the fringes and to obtain stresses based upon calibration coefficients connecting the stress state to the displacement occurring as a consequence of the drilling procedure. In spite of the complexity of the optical configuration adopted and of the difficulties in speckle pattern elaboration authors obtained very promising results that paved the way to further developments. Analogous results, in terms of accuracy, were obtained 2 years later in [25] where, instead of a CCD camera as a recording media, photographic emulsions were adopted (Fig. 17.2). 17.3 Development of the Technique Attempts to simplify the optical system with respect to pioneering works were firstly done in [26] where a system based upon a couple of optical fibers was used to implement a double Lendeertz configuration showing potentialities of this technique to be implemented in a easy to operate set-up. A similar system will be also used later for industrial application such as RS measurement in CD disc [27]. A simplified set up was also adopted later where an ESPI out-of-plane interferometer was built however, in that case, the chemical etching procedure was used instead of hole drilling to get stress relaxation along the depth. In the direction of implementing the technique in a robust a portable device it is noticeable the work done in [28], in this paper by the use of a conical mirror, an optical configuration was obtained so that the sensitivity vector is directed along the radius for each point of the analyzed specimen. The system was also tested on the field by measuring residual stresses on a pressure vessel while the same interferometric configuration was adopted in combination with indentation instead of hole drilling in [29] even if a numerical model for quantitative evaluation of RS is missing. Very huge work from the side of system development, data analysis development and automation, error sources evaluation and results validation was done
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