Chapter 17 A Short Survey on Residual Stress Measurements by HDM and ESPI C. Pappalettere Abstract In this paper a review of progresses occurred along the years in measuring residual stresses by optical methods is presented. These methods allow to implement the hole drilling procedure for residual stress profile measuring, without applying the strain gage rosette. This approach presents several advantages such as easier and cheaper preparation of the test beside the possibility to avoid eccentricity issues and to increase the amount of available data in view of the fact that each pixel can be considered as a measure point. In particular the evolution of the Electronic Speckle Pattern Interferometry (ESPI) technique will be presented showing how set up, methodologies and calculation approaches have developed along the last three decades. Keywords ESPI • Hole drilling method • Residual stresses • Optical methods • Interferometry 17.1 Introduction The stress field existing in some materials without application of an external source of stress, such as loads or thermal gradients, is known as residual stress. These residual stresses (RS) are generated in almost all manufacturing processes such as machining, grinding, forming [1], rolling, casting, forging [2], welding [3–5], melting [6], cladding [7] heat treatment [8], etc. or may occur during the life of structures. The hole drilling method (HDM) is one of the mo st widely used techniques for measuring residual stresses. This technique consists in the localized removal of stressed material and in measuring the strain field consequent to the relieved stresses. The hole drilling method using strain gauge rosettes is a consolidated approach for stress determination and it follows the ASTM test standard. Even though strain gauges are usually used to measure these displacements, they have some disadvantages: the specimen surface has to be flat and smooth so that the rosettes can be attached, the surface of the material has to be accurately prepared, the hole has to be drilled exactly in the center of the rosette in order to avoid eccentricity errors, and time and costs associated with installing rosettes are consistent. Furthermore the amount of available data is limited: for each measurement, only three discrete readings are available (six in the case of some special rosettes), just sufficient to fully characterize the in-plane residual stresses. Due these considerations several attempts to replace extensimetric measurements by optical ones have been done along the years. For example the use of moiré interferometry for strain determination in RS measurements by HDM was investigated in many situations since McDonach et al. [9]. However, bonding a grating can also be time consuming. The feasibility of using holographic interferometry was shown by Antonov [10]. More recently Hung et al. [11] have used shearography in conjunction with a small ball indentation to determine RS while in [12] shearographic and speckle approach are compared. Also the possibility to release residual stresses by using local heat treatment, combined with electronic speckle pattern interferometry was attempted in [13–15] and more recently repeated on aluminum alloys in [16, 17]. However the most explored approach consists in combining ESPI with incremental hole drilling; the basic idea is to adopt the same procedure for RS measurement as described in ASTM E837 but replacing data obtained by the rosette with data obtained by speckle pattern. Speckle interferometry approach is widely adopted in experimental mechanics [18–23]. It is based upon the observation that whenever a coherent beam of light shines on a surface a random intensity pattern is produced as a consequence of the interference of the light diffused by surface itself. Displacements of the surface introduce changes in the local phase. Having this in mind it is easy to understand that if an hole is drilled in a material and stresses are relaxed speckle patterns change and if the speckle pattern is subtracted C. Pappalettere ( ) Politecnico di Bari, Dipartimento di Meccanica, Matematica e Management, Viale Japigia 182, 70126, Bari, Italy e-mail: c.pappalettere@poliba.it © The Society for Experimental Mechanics, Inc. 2018 L. Lamberti et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-63028-1_17 105
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