17 A Short Survey on Residual Stress Measurements by HDM and ESPI 107 Fig. 17.2 Set-up for HDM-ESPI adopted in the pioneering work by Furgiuele et al. [24] in the series of papers [30–34] as well as in [35] while in [36] repeatability of stress measurement in ESPI hole-drilling was studied. The system described in those papers is quite compact; the sample is illuminated just from one side by an optical fiber while a reference beam is sent directly to the camera CCD by a second optical fiber. A database of finite element solutions was built allowing to determine surface deformation for each combination of Young’s modulus and Poisson’s ratio, hole geometry and stress state. Full field least square technique was implemented to determine the stress state as the combination of ¢x, ¢yand£xy that minimizes the differences between the measured displacements and the modelled ones. The algorithm allowed also to take into account the rigid body motion that can occur during the measurement. Furthermore some sources of errors connected with the measurement such as the error on the determination of the illumination and observation angles or in the determination of the pixel size are taken into account. Further work in this sense has be done later on in [37] where effects of drilling parameters are considered for aluminum and stainless steel or in [38–43] where analogous analysis is carried out for Titanium grade 5. In [44–46] other sources of errors are considered such as the extension of the analysis area that is to say the effective region of the acquired interferogram that will be used for the analysis. In [48] was explored, with a good success, the possibility to perform remote drilling operation that means to afford issues connected with replacement of the sample and consequent fringe decorrelation. The issue was overcome by using a kinematic mount with two pins and two bolts and by implementing rigid body correction algorithms. Optimization of the entire measurement processes are analyzed in [49] by considering the problem of full exploitation of all available data and in [50] by considering the problem of the choice of the number and distribution of drilling steps. 17.4 Recent Progresses In the last decade the number of works where hole drilling method an ESPI are used in combination to measure residual stresses has increased very much with applications ranging from measurement on submerged arc welded parts [51], TIG welded structural elements [52], FHPP welded part [53], butt-welded joints [55], dissimilar-weld joint [55] 3D printed part by Fused Deposition Modelling [56–58], glass and plastics [59, 60] etc.. At the same time some alternatives approaches also started to be attempted, for example in [61] incremental grooving was used instead than drilling while in [62–63]
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