A fundamental problem in composite diagnostics is the detection of material defects before and during the regular service of a component. Fibre-reinforced materials, such as carbon/epoxy composites, have been successfully employed as structural materials in the aerospace and aircraft industries. Severe damages to sandwich structures may occur as a consequence of microscopic disbonds of the fibre-matrix interface, broken fibres, delamination or cracks. These defects may be due to fabrication errors, unwanted impacts, excessive thermal or mechanical stress. In recent years, optical methods have been proposed for composite materials diagnostics. Instruments based on double exposure holography or shearography are now available and can be used for routine tests in an industrial environment. More recently, the technique of TV holography or ESPI - Electronic Speckle Pattern Interferometry - (for a typical ESPI system, see figure 2 at Diffusivity Measurements) has replaced holographic metrology in many applications requiring versatility, ease of use and a high number of tests. As regards composite materials diagnostics, a quantitative analysis of defects in composites by ESPI can be performed. A good visualization of the microdefects is produced by adding linear carrier fringes. The out-of-plane deformation field is obtained from a phase map created by a Fast Fourier Transform algorithm. High resolution devices, with short exposure times, together with flexible but optically stable fibres for guiding the laser light, make the system suitable for in situ inspection in an industrial environment.

As an example, figure 1 shows a fringe pattern, with carrier fringes, on an aircraft component. Small irregularities in the fringes point out the effects of a delamination.

Figure 1

Figure 2

Figure 2 shows the three-dimensional plot of the out-of-plane deformation due to the delamination.

Figure 3 depicts an interferogram on an aircraft component (rotor blade). A large disbond is clearly present in correspondence of the oval fringe pattern. In this case the deformation fringes have an irregular appearance, which indicates the defect area. Obviously, when the defects are so well developed, fringe manipulation is important only if a quantitative analysis is required. The range of measurable deformations (tested by means of a centrally loaded aluminium plate) is 0.05 - 10 micron

Figure 3

 .PAOLETTI, G.SCHIRRIPA SPAGNOLO, P.ZANETTA, M.FACCHINI and D.ALBRECHT
"Manipulation of speckle fringes for non-destructive testing of defects in composites"
Optics and Lasers Technology 26, 991-1004, (1994)

Figure 1

Figure 2

Figure 3

Figure 1 shows contouring fringes on a turbine blade. Figure 2 shows the corresponding phase map while figure 3 depicts the three-dimensional plot of object shape.

Figure 4

Figure 5

As another example, contouring fringes on a small ancient amphora can be seen in figure 4. Figure 5 shows the relative three-dimensional profile. The advantages of the simplicity and flexibility of fiber-optic illumination are combined with the easy data interpretation in terms of phase by Fast Fourier Transform.

D.PAOLETTI and G.SCHIRRIPA SPAGNOLO
"Fast Fourier transformed electronic speckle contouring for diffuse surfaces profilometry"
Optics and Lasers in Engineering 20, 87-96, (1994)

Figure 2 shows the slope contours for the centrally loaded disk with its edges rigidly clamped. To perform a quantitative analysis by Fast Fourier Transform, carrier fringes must be added.

Figure 2

Figure 3

Figure 3 shows the resulting phase map (proportional to the strain distribution) codified in pseudo-colors. The method can be extended to determine other slope components.

G.SCHIRRIPA SPAGNOLO and D.PAOLETTI
"Automated slope measurement by digital speckle shearography"
Journal of Optics (Paris) 26, 241-246, (1995).

Vibration Analysis

Top Page

Holographic and ESPI techniques are powerful tools to study vibrating objects. The introduction of digital image processing has led to interesting new possibilities in signal processing in this area. Despite any technical improvements, the limits to the quality of ESPI images and the complex technologies of the holographic systems are significant shortcomings. As an alternative approach we can use the local correlation degree of subjective speckles for describing the stationary motion of a vibrating object. The principle of local correlation of laser speckles consists in the evaluation of a local parameter, that estimates the speckle correlation after any modification of a surface element. As an example, a comparison between the performance of local correlation method vs ESPI system has been made.

Figure 1
Figure 1 shows the experimental setup. The vibrating object is a square aluminium plate (1 mm thick). The specimen is held fixed between two heavy steel black frames having a circular opening of 30 cm. Thus the structure behaves as a circular plate of radius 15 cm. The plate is excited with the help of a loudspeaker, emitting sound waves of sinusoidal form. To perform decorrelation method, a speckle image of the plate at rest is acquired and stored. Subsequent frames are subtracted from the first. A final average operation gives the correlation image. At the resonating frequencies of the plate, most pixels of the subtraction image become bright indicating the occurrence of speckle decorrelation. It is only in correspondence to antinodes and saddle points of the vibrating surface that the displacement has negligible tilt components and dark spots are observed. The mathematical interpretation of ESPI fringes is similar to that of holographic interferometry, as each fringe represents a line of constant displacement.

The vibration mode (2,0) is shown in figures 2a and 2b. Plate was vibrating at 327.8 Hz. Figure 2a is the ESPI interferogram while figure 2b is the correlation image. The central dark point in figure 2b corresponds to a saddle point, the other four indicate the position of the antinodes.

Figure 2a

Figure 2b

Figure 3a

Figure 3b

As another example the vibration mode (0,2) is shown in figures 3a and 3b. Plate was vibrating at 924 Hz. Figure 3a is the ESPI interferogram while figure 3b is the correlation image. The local correlation method can be considered less sensitive than time-averaged holographic or ESPI techniques, but for its simplicity it can be used as an alternative tool for in situ applications.

G.SCHIRRIPA SPAGNOLO, D.PAOLETTI and P.ZANETTA
"Local speckle correlation for vibration analysis"
Optics Communications 123, 41-48, (1996).

Bouncing Light Beams

Top Page

Bouncing light beams are a fascinating tool in optics demonstrations. They can be easily realized pouring a solution of sugar in water over pure water in a narrow tank.

The green light from an Argon laser bounces along the tank.

We can eliminate spurious reflection and scattering dissolving in water a dye (DASBTI), usually used as a saturable absorber. The light from the Argon laser at 514.5 nm is closed to the peak absorption wavelength of the dye. We select the consequent fluorescence by a narrow-band interference filter centred at 633 nm. The beam appears to grow thinner as it propagates along the tank because it is absorbed by the dye.

The bouncing light beam recorded using a dye and an interference filter.

These curved ray paths are a demonstration of the bending of light (and acoustical) beams in inhomogeneous media. They also depict the trajectory of material particles in potential fields. This opto-mechanical analogy, which can be traced back to Alhazen (about 1000 A.D.), was noted by Descartes (1637) and formalized by W. R. Hamilton in 1833.

D. AMBROSINI et al.
"Bouncing light beams and the Hamiltonian analogy"
European Journal of Physics vol. 18, pp. 284-289, (1997).

Click here to read this paper, courtesy of the Institute of Physics.

This article may be downloaded for personal use only. Any other use requires prior permission of the authors and the Institute of Physics.