A review on optical techniques for measurements at different scales

Full-field measurement of the 3-D displacement components of a deformable body submitted to arbitrary loading conditions and determination of the shape of a complex object are very important engineering problems. These aspects are deeply connected. In fact, determinations of the deformed shape resulting from the action of the applied loads passes through the measurement of 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. This paper will present some examples of measurements performed with optical techniques. Figure 1 shows the results obtained using double illumination speckle interferometry for the structural monitoring of electronic components subject to Joule heating. Single components (i.e., resistors and stabilizers) as well as an entire electronic board have been analyzed. The maximum displacement observed is of the order of a few microns while the size of the specimens ranged from 1 to 15 cm. The experimental set-up has been calibrated for the single specimens first and then used for the board assembly. Remarkably, the speckle set-up utilized in the experiments made it possible to identify critical zones where thermal distortions may cause mechanical failure of the board. The second example of application is concerned with the mechanical characterization of a hyperelastic membrane (see Figure 2). A combination of moiré techniques allowed the whole 3-D displacement components to be measured simultaneously when the membrane is subject to inflation. For this purpose, a dot grating has been printed onto the membrane and another grating has been projected on the specimen. Figure 2b shows the different modulations experienced by the two gratings. Displacement components can be extracted from the FT of the moiré patterns and then compared with the predictions of a finite element model. The difference between experiments and numerical data can be minimized by means of a global optimization algorithm which finally provides the unknown material constants. The characterization procedure based on the combination of moiré techniques has proven itself to be robust and efficient. Remarkably, it has been successfully applied to both isotropic and anisotropic specimens. The size of the specimens considered in this case is about 4 cm while the maximum displacement is about 0.5 cm. In the last example outlined in this abstract, the projection moiré technique has been utilized in order to measure out-of-plane displacements caused 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 50 x 70 cm and the maximum displacement is larger than 1 cm. Non-collimated light projection has been utilized in order to obtain the full-field information on the entire glazing surface. Remarkably, optical measurements resulted in excellent agreement with measurements carried out independently using a digital micrometer. Besides these three examples, an extensive review on other applications of optical techniques carried out at the Politecnico di Bari will be given in the paper.