Table of Contents ICAC'98
Detection of disbonds and voids in thick composite, sandwiched plates by real-time phase-shifted electronic holographyDan Borza
INSA Rouen - LMR, Mechanics Department
76131 Mont-Saint-Aignan, France
Corresponding Author Contact:
|TABLE OF CONTENTS|
Holography, along with other coherent optical techniques, has already been proved to be an effective technique in non-destructive testing of composites. One of the difficulties of holographic non-destructive testing is finding the stressing technique able to produce singularities in the strain state at the surface of the object under test. Furthermore, the practical implementation of holographic testing needs careful optimisation of stressing techniques in order to differentiate between different defect types. While preserving the basic functionalities of classical holographic systems, the use of phase-shifted electronic holography digital processors enlarge the range of possible stressing procedures, by its intrinseque real-time nature. Another interest lies in the possibility to couple several different modules to the same phase-shifting image processor. Our laboratory's experience shows that coupling a holographic module and a fringe-projection module to the same testing system results in an increased displacement measuring range as well as in new possible stressing techniques. The result is a more-effective testing, extending the range of this non-destructive technique. Real-time observation during the thermal stressing of thick composite plates, for example, allows for detecting defects otherwise difficult to detect. Several practical examples are presented.
|Fig 1: HNDT of a honeycomb panel|
Optical testing of composites, through its different techniques, has two very general particularities: it has a full-field, non-contact character, and may detect a great number of defects.
The most widely known of these techniques, holographic interferometry, has already proved to be a technique of great potential in non-destructive testing of mechanical components and materials, especially composites.It is already a useful tool in detecting local anomalies such as flows, delaminations, voids, inclusions, imperfect gluing, broken glass or carbon fibers, and many others. Holographic interferometry detects, as interference fringes visible on the object surface, the deformations, in the micrometer range, at the surface of a stressed object. The principle of holographic NDT is very general: in the presence of local anomalies (defects) under the surface, the state of strain at the surface produced by a uniform object stressing will be locally modified. The fringe pattern will thus indicate, like in Figure 1, regions with defects.
Although holographic recording media are able to produce high resolution images, today's industrial non-destructive testing often needs true real-time capabilities and short testing times. Recording a hologram on a silver halide emulsion and developing it is a time-consuming operation, while other recording media, such as thermoplastics, are expensive without really being reusable for many times or presenting the high resolution of silver halide.
During the last years, there is a strong tendency to replace these technique by different digital speckle techniques. The main interest lies in eliminating any recording media and directly using a CCD camera feeding the frame grabber of a computer.
|Fig 2: Electronic holography|
Like the original ESPI (Electronic Speckle Pattern Interferometry) systems, electronic holography is based on the general principle illustrated in Figure 2. A part of a laser output beam is illuminating the object under test. The CCD camera simultaneously receives the wavefront diffused by the object and a second, reference beam coming directly from the beamsplitter following the laser. The current image I(x,y) is described by the relation;
I0(x,y) is the object image;
C(x,y) is the local contrast;
(x,y) is the local phase;
is an arbitrary phase shift.
In electronic holography, several (usually four) successive images are recorded, while the phase of the object beam is modified in equal steps by a piezo-electric actuator
The local modulo2 phase (the wrapped phase) of the object image can then be calculated by using the relation:
This operation can be done either in real-time by using a dedicated holographic processor  or at a later stage, by software . After obtaining the wrapped phase, a phase unwrapping program  eliminates the 2 jumps and the unwrapped phase is obtained.
In electronic holography with normal object illumination, the phase is directly related to the out-of-plane displacement d(x,y) at the object surface, so one can thus obtain the real-time interferogram which can be used in NDT.
The technique has the same general characteristics as classical holography, but without the use of any recording medium for the hologram itself. The displacement sensitivity is of the order of magnitudeof the laser light wavelength, according to the approximate relation:
where denotes the laser light wavelength (0.532 µm for a green YAG laser).
Figure 3 illustrates the result of NDT over a composite plate. It is a metal - plastic sandwich, with voids and imperfect gluing.
As seen, thermal stressing may allow, as in the case of classical holography, detection of internal defects. To further increase the localisation and dimensions of existing defects, we are applying a repetitive thermal stressing. The results are illustrated in Figure 4.
|Fig 3: NDT of a sandwich plate by electronic holography with thermal stressing||Fig 4: NDT of the same sandwich plate with repetitive thermal stressing|
By using different illuminating and recording schemes, as will be shown later, the same phase stepping process may lead to the full-field in-plane displacement field, or to the object topography. It also may allow out-of-plane displacements to be measured with a lower sensitivity, as sometimes necessary with some composites or in the case of shape memory materials and structures.
|Fig 5: Directly obtained interference fringes, wrapped phase and unwrapped phase|
As mentioned earlier, obtaining the wrapped phase and unwrapping it may be done through dedicated software, at any later stage following the image acquisition process. Figure 5 illustrates the main screen of our home developed program calculating the phase either from four or from eight (double exposure case) speckle images.
The program operates almost identically in all the above mentioned measuring techniques. The second possibility of obtaining the wrapped phase is using a dedicated digital holographic processor, continuously displaying, in real time, the wrapped phase . The advantage of displying the phase fringes is a better visual understanding of the instantaneous deformation of the object and a more effective defect identification in NDT.
It may also lead to the implementation of stressing methods well suited for the different materials being tested .
In our laboratory, a single RT 7000 holographic processor of this kind allows the use of several connected measuring modules, either commercially available or home made (Figure 6).
|Fig 6: Modular phase stepping measuring system|
The fringe-projection system makes use of an adjustable Mach-Zender interferometer, as used to measure refractive index changes in gases and in interference microscopes to image transparent samples. It is shown in Figure 7.
|Fig 7: Fringe projector|
The complete fringe-projection module connected to the phase stepping system is shown in Figure 8. The phase difference with respect to a reference plane, as obtained in the fringe-projection technique, is related to the depth zi of the current object point by the relation
|Fig 8: Fringe-projection module|
That is characteristic for the shape of the object, as used, for example, in reverse engineering, to get CAD data for an existing object.
If the object shapes are measured at two different instants, their difference will show the deformations occuring between the two states. Thus, the NDT capabilities of the whole system are largely increased, since in the fringe-projection technique the sensibility is much lower (mm...cm range) than that of holographic systems (µm range).
The low displacement sensitivity allows the visualization of shape modifications, while eliminating the influence of any small local defect from the test results. It is used mainly as a structural test.
As an example, a similar sandwich plate was tested to check its behaviour in presence of large deformations imposed by a mechanically produced compression stressing. Figure 9 represents, in order, the two initial images in each series of shape measurements, with the projected fringes visible on the object, their unwrapped phase difference and respectively the surface reconstruction of the deformation field.
The deformation (maximum value 25 mm) occured as a consequence of the mechanically produced buckling. The buckling is non-symmetric and finally leads to destruction, by physical separation between the composite and the substrate on which it is glued.
Fig 9: NDT by phase-stepped fringe projection with mechanical loading
For the fringe-projection system, as well as for the speckle-based interferometric techniques using phase stepping, calibration of the phase step is essential. Our system uses as default the standard, in-line visual calibration based on the real-time visibility of the phase image. This calibration is completely satisfactory for NDT purposes, which do not make use of the the quantitative geometrical reconstruction of the object surface. For more precise results, as imposed in shape measurement, we also developed a new calibration method and the associated software, based on the fringe displacement control. Their description will be published soon.
Phase-stepped electronic processors are a very convenient means for the NDT of composite materials. They allow using different full-field, non-contact optical techniques of various sensitivities. Real-time observation is a practical advantage over classical holography in industrial testing. Implementation of new stressing techniques, as repetitive thermal stressing, becomes possible since the interferometric images are obtained in real-time. A large spectrum of local defects are visualized by holography or speckle correlation techniques, while structural behaviour and shape may also be studied by complementary experiments using fringe projection.