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COMPUTATIONAL METHODS FOR ENGINEERING SCIENCE
Edited by: B.H.V. Topping
Application of Speckle Interferometry to Damage Identification
J.V. Araújo dos Santos1 and H. Lopes2
1IDMEC/IST, Instituto Superior Técnico, Lisbon, Portugal
J.V. Araújo dos Santos, H. Lopes, "Application of Speckle Interferometry to Damage Identification", in B.H.V. Topping, (Editor), "Computational Methods for Engineering Science", Saxe-Coburg Publications, Stirlingshire, UK, Chapter 12, pp 299-330, 2012. doi:10.4203/csets.30.12
Keywords: damage identification, isotropic beam, laminated plate, speckle pattern interferometry, speckle shearography.
This chapter describes and presents the application of two experimental techniques, based on the speckle effect, to damage identification. The first technique, known as speckle pattern interferometry, allows the measurement of displacement fields, whereas the second one, usually called speckle pattern shearing interferometry or shearography, allows the direct measurement of displacement field derivatives or gradients. Nowadays, and due to the use of digital recording and digital image processing of speckle patterns, these two techniques are commonly known as electronic speckle pattern interferometry or TV holography and electronic speckle pattern shearing interferometry or digital speckle shearography, respectively. Both techniques are non-contact, full-field and high resolution measurement techniques. Therefore, they are well suited and appropriate to overcome several problems revealed by contact measurements, which can significantly alter the structure by, for example, adding mass to it. Also, and since the present techniques are full-field techniques and the spatial resolution depends only on the recording medium resolution, the number of measured points is much greater than the usual measurements with strain gauges, piezoelectric sensors or accelerometers.
Firstly, the description of the speckle pattern, which is formed when a rough and diffuse surface is illuminated by coherent light, namely a laser beam, is presented. The principles of speckle pattern interferometry are reviewed and their mathematical relations with out-of-plane displacements of a structure are formulated. Afterwards, the concepts associated with speckle shearography are presented. The way the derivatives of out-of-plane displacements and the measured phase maps are related is described next. The differences between this technique and speckle pattern interferometry are also highlighted. Since the phase maps obtained with both techniques are noisy and discontinuous, several filtering and unwrapping methods are reviewed and compared. The procedure and several algorithms to perform the necessary differentiations to obtain data for damage identification are also described. The acquisition of both static and dynamic measurements is discussed.
Several case studies of damage identification in one aluminium beam and two carbon fibre reinforced epoxy plates, using experimental measurements obtained with both techniques, are presented. The damage identification methods used rely on comparisons of spatial derivatives of modal displacement fields and modal rotation fields of the structure in the undamaged and damaged states. These studies show that electronic speckle pattern interferometry and digital speckle shearography allow the localisation of cuts in beams and delaminations in laminated composite plates with a good level of accuracy.
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