PSI - Issue 17

Pedro J. Sousa et al. / Procedia Structural Integrity 17 (2019) 835–842 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

836

2

made from calibration data. Since the main use of these cameras is for surveillance, security and automation procedures, the impact of large errors that cannot be accommodated by the electronic support systems may jeopardize the outcome with different degrees of severity. Therefore, it was necessary to characterize the behavior of the mechanical support for the imaging system using very high-resolution experimental techniques, while recreating the environmental conditions that these PCBs may be subjected to during their lifetime. For this type of analysis, two different technologies were considered: microscopic digital image correlation (DIC) and electronic speckle pattern interferometry (ESPI). The first requires that a random speckle pattern is present in the test subject, which, for the considered scales, is difficult to create. On the other hand, ESPI does not have such requirements, even though it is recommended to coat the whole part with dry powder to avoid specular reflections in the specimen that might saturate the image and obtain a uniform speckle. For the present work, ESPI was employed in order to measure the displacements of a printed circuit board relative to its mechanical support. The collected experimental data could be ultimately used to validate numerical simulations for the expected mechanical behavior of the components. Once validated, the numerical data could be used in different designs for the improvement of the final solution.

Nomenclature ESPI

Electronic Speckle Pattern Interferometry

NDT

Non-Destructive Testing

2. Electronic Speckle Pattern Interferometry

Electronic Speckle Pattern Interferometry (ESPI) is an important interferometric technique that uses coherent light to record interference patterns, which can then be used for full field measurements [1] – [3]. It is often used qualitatively in non-destructive testing (NDT), primarily for defect detection in many fields, ranging from aeronautics [4] to art [5], and it is also used quantitatively for displacement measurement [1] in a wide range of fields, such as structural monitoring [6], [7], structural dynamics [8] and biomechanics [9]. ESPI systems require very precise laser alignment to maintain coherence between its two laser paths. As such, they require highly controlled conditions that may be difficult to achieve in field applications. Even so, it is a high resolution method that can resolve dimensional variations of a twentieth of the laser wavelength [3], [10]. This methodology can have sensitivity to either out-of-plane or in-plane displacements using different set-ups. As a natural extension to the unidirectional setups, there are several approaches to obtaining ESPI results in 3D. Among these, some examples include the use of a combined out-of-plane and in-plane approach or of multiple (usually three or four) out-of-plane setups [1]. By using the former, it is possible to obtain the u and v displacements directly from the in-plane setups and, combining this information with the out-of-plane, obtain the w displacement. In contrast, when using the approach with multiple out-of-plane setups it is necessary to know the three (or four) results to be able to calculate displacements in these directions. Which means that using the first solution it is easier to use the combined setup to perform arbitrary 1D or 2D measurements in alternative to the 3D ones. For this work, a 3D ESPI solution based on the combined approach was developed, Fig. 1.