Full-field measurement of 3D displacement components of a deformable body submitted to arbitrary loading conditions and determination of the shape of a complex object are very important problems in Experimental Solid Mechanics. These aspects are deeply connected. In fact, determinations of the deformed shape under the action of applied loads involves measuring the local geometric properties of the object surface. Displacements are the basic information from which analysts can derive the state of strain and the distribution of stresses, thus assessing the probability of failures and the corresponding mechanisms of damage. In general, two different approaches can be followed to carry out measurement on deformations and shape of solid bodies: (i) contact tactile methods (CTM) where the different positions of a mechanical device tip (stylus) following the deformed object surface are recorded with respect to the reference configuration; (ii) non-contact optical methods (NCOM) where a pattern of fringes formed because of the interference between light wave fronts is modulated by the deformations of the object surface: fringes correspond to iso-displacement loci. Fringe patterns are recorded by a CCD camera and hence properly processed with dedicated software. CTM have a number of limitations with respect to NCOM. In the first place, experimental data are available only for a limited set of points of the specimen surface. Secondly, the state of the surface could be modified by the devices utilized in the measurement process. Thirdly, measurement resolution may depend strongly on the geometry and size of contacting tip. Conversely, NCOM are able to provide full-field information with a high level of resolution which basically depends on the sampling of recording system and can be determined a priori. Finally, the state of specimen surface is not modified in any way by the execution of experimental tests. NCOM such as moiré, speckle and holography all rely on the principle that the object is illuminated by light wave-fronts propagating along the illumination direction and the light intensity modulated by the deformed body returns back along the viewing direction until it is collected by a sensor. The direction of illumination and the direction of viewing are defined by two vectors whose relative orientation and position in the space with respect to the displacement vector define a new vector called “sensitivity”. A definite strength point of NCOM is the possibility of performing measurements at very different scales. For instance, interferometry can be used in the micron range while fringe projection can be used for large deformations and large-scale specimens. While speckle and holography techniques are sensitive at the extent of half of the wavelength of the light  illuminating the object, moiré techniques are sensitive by a quantity proportional to the size of the pitch “p” engraved or projected onto the specimen surface. Full field information provided by phase distribution together with numerical super-resolution allow users to measure object details well beyond /2 or “p”. The paper presents examples of application of optical techniques to the determination of displacements and shapes. In the first example, double illumination speckle interferometry is utilized to monitor structural behavior of electronic components subject to Joule heating. Single components (i.e., resistors and stabilizers) as well as an entire electronic board are analyzed. The second example of application is the mechanical characterization of a hyperelastic membrane. By combining moiré techniques, it is possible to measure simultaneously 3D displacement components of the membrane subject to inflation. A hybrid method then allows hyperelastic properties to be retrieved. In the last example, projection moiré is utilized to measure out-of-plane displacements produced by aerodynamic loads on a landing light glazing of an Airbus A340 aircraft. This is an example of large-scale measurements since the specimen dimensions are 50x70 cm and the maximum displacement is larger than 1 cm.

Application of optical techniques to the determination of 3D displacements and object shapes / Casavola, Caterina; Lamberti, Luciano; Pappalettere, Carmine. - (2011).

Application of optical techniques to the determination of 3D displacements and object shapes

CASAVOLA, Caterina;LAMBERTI, Luciano;PAPPALETTERE, Carmine
2011-01-01

Abstract

Full-field measurement of 3D displacement components of a deformable body submitted to arbitrary loading conditions and determination of the shape of a complex object are very important problems in Experimental Solid Mechanics. These aspects are deeply connected. In fact, determinations of the deformed shape under the action of applied loads involves measuring the local geometric properties of the object surface. Displacements are the basic information from which analysts can derive the state of strain and the distribution of stresses, thus assessing the probability of failures and the corresponding mechanisms of damage. In general, two different approaches can be followed to carry out measurement on deformations and shape of solid bodies: (i) contact tactile methods (CTM) where the different positions of a mechanical device tip (stylus) following the deformed object surface are recorded with respect to the reference configuration; (ii) non-contact optical methods (NCOM) where a pattern of fringes formed because of the interference between light wave fronts is modulated by the deformations of the object surface: fringes correspond to iso-displacement loci. Fringe patterns are recorded by a CCD camera and hence properly processed with dedicated software. CTM have a number of limitations with respect to NCOM. In the first place, experimental data are available only for a limited set of points of the specimen surface. Secondly, the state of the surface could be modified by the devices utilized in the measurement process. Thirdly, measurement resolution may depend strongly on the geometry and size of contacting tip. Conversely, NCOM are able to provide full-field information with a high level of resolution which basically depends on the sampling of recording system and can be determined a priori. Finally, the state of specimen surface is not modified in any way by the execution of experimental tests. NCOM such as moiré, speckle and holography all rely on the principle that the object is illuminated by light wave-fronts propagating along the illumination direction and the light intensity modulated by the deformed body returns back along the viewing direction until it is collected by a sensor. The direction of illumination and the direction of viewing are defined by two vectors whose relative orientation and position in the space with respect to the displacement vector define a new vector called “sensitivity”. A definite strength point of NCOM is the possibility of performing measurements at very different scales. For instance, interferometry can be used in the micron range while fringe projection can be used for large deformations and large-scale specimens. While speckle and holography techniques are sensitive at the extent of half of the wavelength of the light  illuminating the object, moiré techniques are sensitive by a quantity proportional to the size of the pitch “p” engraved or projected onto the specimen surface. Full field information provided by phase distribution together with numerical super-resolution allow users to measure object details well beyond /2 or “p”. The paper presents examples of application of optical techniques to the determination of displacements and shapes. In the first example, double illumination speckle interferometry is utilized to monitor structural behavior of electronic components subject to Joule heating. Single components (i.e., resistors and stabilizers) as well as an entire electronic board are analyzed. The second example of application is the mechanical characterization of a hyperelastic membrane. By combining moiré techniques, it is possible to measure simultaneously 3D displacement components of the membrane subject to inflation. A hybrid method then allows hyperelastic properties to be retrieved. In the last example, projection moiré is utilized to measure out-of-plane displacements produced by aerodynamic loads on a landing light glazing of an Airbus A340 aircraft. This is an example of large-scale measurements since the specimen dimensions are 50x70 cm and the maximum displacement is larger than 1 cm.
2011
Application of optical techniques to the determination of 3D displacements and object shapes / Casavola, Caterina; Lamberti, Luciano; Pappalettere, Carmine. - (2011).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11589/25279
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