The contrast is also better with the real-time tech- nique. ... sponsored by the FAA Center for Aviation Systems Re- liability ... April 3, 1992, Orlando, Florida. 3.
Journal of Nondestructive Evahtation, Vol. 12, No. 2, 1993
Synchronized Reference Updating Technique for Electronic Speckle Interferometry B r u n o Pouet, 1 T o m Chatters, 1 and Sridhar K r i s h n a s w a m y 1
Received April 20, 1992; revised October 17, 1992
In this paper, a video-based speckle interferometric method using a continuous reference updating technique is presented. Unlike conventional ESPI techniques, this method synchroJlizes the optical interferometrie detection system with the acoustic stressing of the test object, and includes continuous renewal of the reference image. It is shown that the susceptibility of the method to environmental noise caused by vibration, temperature gradients, or thermal currents is substantially lower than that of conventional techniques. The application of this technique to the detection of defects in adhesively bonded structures is demonstrated. KEY WORDS: Electronicspeckle interferometry;nondestructiveevaluationof disbonds.
1. INTRODUCTION
is made to interfere with a reference beam, and the resulting interference speckle pattern is recorded by a CCD camera. This recorded speckle pattern carries information corresponding to the surface topography of the test object. If a second speckle pattern is then recorded with a slightly different surface shape for the object (typically caused by stressing the object), it is possible to compare the pair of recorded interference speckle patterns to extract information corresponding to the relative object deformation. Various processing methods including analog or digital signal processing techniques can be used to extract the desired information about the relative object deformation. (3-7) This information is typically displayed in the form of an interference fringe pattern. For NDE applications, the method of stressing of the test object is suitably chosen so as to cause internal structural defects in the object to manifest themselves on the surface of the test object as regions of strain concentration. It is clear from the foregoing description, that ESPI can be quite susceptible to extraneous noise-inducing factors such as thermal currents, temperature gradients or object drift. This technique is affected by relative speckle phase changes that occur during the time of recording of each speckle pattern and, more importantly, between the time of recording of the pair of speckle
Video-based speckle interferometric methods such as electronic speckle pattern interferometry (ESPI) and shearography are increasingly being looked upon as potentially valuable tools for NDE, particularly by the aerospace industry. (I,2~ The attractiveness of these optical techniques to the NDE community arises in large part due to their non-contacting nature and relative speed of inspection procedure. However, for these optical interferometric methods to become widely used as NDE tools by the industry, they must be made to be robust enough to operate in noisy environments typically encountered in most maintenance facilities. In this paper, we address these issues for the case of detection of disbonds using ESPI in conjunction with acoustic stressing. Electronic speckle pattern interferometry is a realtime speckle technique that provides full-field information about the surface deformation of an object. The basic principle of ESPI is briefly recounted here for completeness. A speckle pattern produced by the coherent light scattered from a diffuse test object (object beam) Center for QualityEngineeringand FailurePrevention,Northwestern University, Evanston,Illinois60208. 133
0195-9298/93/0600-0133507.00/0 43 1993 Plenum Publishing CiIrporatilln
134 patterns that are to be compared for relative displacement information. Oftentimes, the optical phase changes corresponding to environmental noise are larger than the phase changes related to the relative object displacement information, possibly leading to the complete submerging of the signal by the noise. In this paper, we describe a continuous referenceupdating ESPI method with synchronized stressing which avoids the interference degradation induced by extraneous noise (temperature gradient, thermal currents or slow object drift), and which allows this technique to be used in a noisy environment. The proposed technique is compared with classical ESPI in terms of fringe stability, quality, and contrast. Conventional and synchronized reference-updating ESPI are compared for two temporal modulation schemes(S~--time-average and real-time modulations--using a reference specimen with a large simulated structural defect. Finally, adhesively bonded specimens are investigated for disbonds using the synchronized reference-updating technique, showing its practical application in nondestructive evaluation.
Pouet, Chatters, and Krishnaswamy Argon LASER
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2. SYNCHRONIZED REFERENCE-UPDATING ESPI A schematic of the synchronized reference-updating ESPI setup is shown in Fig. (la). A coherent laser beam is passed through an acousto-optic modulator which is used as an optical shutter. The laser beam is split by means of a beam splitter into two legs each of which is expanded by beam expanders. One of the expanded beams is used as the reference beam by passing it through a ground glass diffuser. The other beam is used to illuminate the test object. The scattered beams from both the reference diffuser and the test object are collected by a CCD camera for recording into a digital imageprocessing computer system. The test object is acoustically stressed by a piezoelectric transducer driven by a function generator. In a departure from conventional ESPI, in the proposed technique the acoustic stressing system, the optical shutter and the CCD image acquisition system are all synchronized through an electronic synchronization system shown schematically in Fig. (lb). The vertical video synchronization signal, which is output by the CCD camera at the end of every frame acquisition, is stripped from the composite video signal, and this provides a common time base for all the systems in the setup. A series of interference speckle images is acquired into the image processor by the CCD camera at video framing rates. Since the CCD camera integrates the in-
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1. (a) Schematic of the experiment; and (b) synchronization system schematic.
tensity of light impinging on it over the duration of a frame, the intensity recorded by the CCD camera during the recording time T of the Nth frame may be expressed as:
IN(x) = A~(x;N) + A2(x;N)
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Here eN(t) is an optical shuttering function of the acoustooptic modulator. The reference and object beam amplitudes are denoted by AR(x;N) and Ao(x;N) which are assumed to be essentially time invariant during the CCD integration time of the Nth frame, but could in general vary from one frame to another due to low frequency noise. The quantity +s(X,t) = 4,o(X;b0 - 6R(x;N) + "rN(t)M(X)sincot = q~(x;N) + "rN(t)M(x)sineot is the difference in the phases of the reference and object beams, and consists of a time-invariant (again, within
Electronic Speckle Interferometry
135
one frame) speckle phase component denoted for simplicity by q~(x;h0, and a time-varying part which represents the signal of interest caused by the object displacement. The object is stressed sinusoidally at frequency co, the maximum object displacement amplitude is M(x), and "rN(t) represents an acoustic gate which can be either on (1) or off (0) during a frame. In subtractive ESPI, the signal M(x) representing the object deformation can be extracted by subtracting one image (say, the current Nath frame) from a previously recorded and stored image (say, the reference Noth frame). The quantity q(x; N1; No) = IINl(x) - IN0(X)l
(2)
is computed digitally and displayed on a video monitor in real time. It is conventional to assume that the speckle patterns corresponding to the current and reference images stay correlated. That is, it is assumed that the reference and object beam amplitudes AR(x;N) and Ao(x;N), as well as the phase term q~(x;b0 are the same for both the current Nlth frame and the reference Noth frame. In conventional ESPI, the reference image is typically acquired at the beginning of a test with the specimen in the undeformed state (i.e., No = 1, the first frame), and the current image is then subtracted from this reference as the test object is stressed. It is clear that in an industrial environment where thermal noise and slow object drift are present, the assumption that the speckle patterns stay correlated will be increasingly violated as the time between the current image and the acquired reference image becomes large. It is for this reason that the fringe stability and constant degrades with time in conventional ESPI, requiring that a new reference image be acquired before the test can be resumed. In the proposed technique, this problem is overcome by comparing the current image with the immediately preceding acquired image which serves as the reference image (i.e., No=NI-1). This process is repeated in real-time for every pair of images. It is reasonable to expect that the speckle images will stay correlated for two successive video frames if the ambient noise is of sufficiently low frequency compared to the video acquisition rate. Thus, fringe contrast is enhanced and fringe stability is assured with synchronized reference-updating. However, for the proposed technique to work, the deformation information recorded in consecutive frames must be different. This can be done in one of two modes by using the acoustic gate and the optical shutter. These are schematically described in Fig. 2a and b. (i) Time-Average Modulation. In this case (Fig. 2a), the optical shutter is always open and hence the laser
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illiumination is continuous (i.e., eu(t) = 1 for all frames). However, the acoustic gate is alternatively on during one frame and off for the next (i.e., "rN(t) --- 1 for the Nth frame and "rN-l(t) ---- 0 for the previous frame). Thus, the specimen is at rest during one frame, while it is acoustically stressed during the next. Since the acoustic stressing period 2w/o~ is much smaller than the image acquisition duration T, the video monitor display for any frame N simplifies to: q(x; N; N - 1)
= 12ARAo cos(*s).[1 - Jo{M(x)}]l
(3)
where Jo is the zeroth order Bessel function of the first kind. It should be noted that this is exactly the result that is obtained using conventional ESPI, except that in this case low frequency noise is essentially frozen out. (ii) Real-Time Modulation. Here, the acoustic gate is always open (i.e., "ru(t) = 1 for all frames), and thus the specimen is continuously acoustically stressed (Fig. 2b). However, the optical shutter is so chosen that only information regarding the maximum positive displacement is obtained during one frame, while only information regarding the maximum negative displacement is acquired during the next frame. This is achieved by pulsing the laser light in accordance with the following opitcal shutter function:
eu(t)
= a{t - [(22 + 1/2),rr/o~]} for k = 0,1,2...kmax en-l(t) = ~{t - [(2k + 3/2)q'r/~o]} for k = 0,1,2...kma ~
(4)
which represents a series of delta functions (pulses) with kmax being the maximum number of pulses that can be accommodated during one frame time T. In this case, the video monitor display for any frame N is given by:
136
Pouet, Chatters, and Krishnaswamy q(x; N; N - 1) = [4A~ Ao sin(qb,).sin{M(x)}[
(5)
This expression shows that the current technique is more sensitive that its counterpart in conventional ESPI because, in the latter method, the reference image corresponds to the test object at rest, and stroboscopic pulses are then synchronzied with either the maximum positive or the maximum negative object deformation. (5) Thus, the updating real-time modulation not only ensures enhanced fringe stability and contrast, but leads to increased sensitivity as well.
3. EXPERIMENTAL SETUP Experiments were performed using both modes of the method of synchronized reference-updating ESPI, and the results were compared with conventional ESPI. An argon laser (1W output power at a wavelength of 514 rim) was used as the coherent light source. The reference beam was produced by illuminating a ground glass with one leg of the expanded laser beam. The light scattered by the diffuse object/specimen and the diffuse reference was gathered by an RS-170 CCD camera and recorded into an Imaging Technology series 150 digital image processor which was capable of performing image subtraction at rates of 15 Hz. The result was sent directly to a TV monitor. The CCD camera operating with RS170 TV standard, has an integration time, T, of about 33 ms. The synchronization system (shown in Fig. lb) allowed us to synchronize the light pulsation, acoustic stressing and the CCD camera image refresh as required by either the real-time or time-average techniques. The acoustic stressing was done using an audio speaker for frequency range investigation of 1-20 kHz, and broadband piezoelectric transducers were used for frequencies beyond 20 kHz. Each corner of the specimen was clamped down on a holding frame. The audio frequencies were investigated by placing the speaker directly behind the specimen. With piezoelectric transducers, the acoustic signals were generated with a set of transducers mounted on the holding frame. Two different specimens were used in the investigation. The first specimen was an aluminum plate (1 x 10 x 12 inches) containing a flat-bottomed hole of 3 inches in diameter leaving a 1/32 inch thick membrane located at the center of the plate. The second specimen was a two-layered aluminum composite plate with a thick back plate (1/2 inch thick) and a thin front plate (1/32 inch thick) bonded together using an epoxy adhesive. Three artificial disbonds (3, 2, and 1 inches in diameter) were introduced into the bonding layer by placing three
thin sheets of teflon between the two plates prior to bonding.
4. RESULTS AND DISCUSSION Figures 3a-d show the fringe patterns recorded with the four different techniques investigated (real-time and time-average modulation for conventional and synchronzied reference updating techniques) for the cases of the flat-bottomed hole specimen excited at its first resonant frequency of 1.37 kHz. Because of the difference in thickness between the membrane portion and the surrounding plate, the resonant frequencies corresponding to the flat-bottomed hole were completely different from those of the plate modes. This allowed us to investigate the resonance of the flat-bottomed hole without identification problems arising from the superposition of different plate mode vibrations of the whole specimen. The experimental circumstances (acoustic stressing level, etc.) were kept the same for the four experiments. All results shown are without any data processing in order to point out the intrinsic quality of each technique and to facilitate comparison. Two features are brought out by these pictures. First, the fringe number is larger with the updating real-time technique as would be expected from its higher sensitivity. The contrast is also better with the real-time technique. Black fringes corresponding to zero intensity can
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Fig. 3. (a) Conventional time-average ESPI, (b) conventional realtime ESPI, (c) synchronized reference-updating ESPI in time-average mode, and (d) synchronized reference-updating ESPI in real-time mode.
Electronic Speckle Interferometry be seen, which are not observed with time-averaging. Second, we can see that the image quality is better with the reference updating technique. The noise that leads to speckle decorrelation and the consequent contrast reduction associated with the conventional technique are eliminated. Similar results were observed for multimodal exciation of the same defect at other frequencies. Figure 4 shows one such result using the synchronized reference-updating method for the resonant mode at 110 kHz. The results corresponding to the composite aluminum specimen with disbonds are given in Figs. 5a-c which show the fringe patterns corresponding to the resonance of each disbond. Here, the CCD camera was focused on each disbond in order to better resolve the fringes. The resonance frequencies of the disbond depend on the disbond size (larger disbond diameters resonate at a lower frequencies, for fixed plate thickness). The stressing frequency used for each disbond did not correspond to the first resonant mode but rather to a higher mode. The unsymmetric nature of some of the fringe patterns originates from experimental parameters which can not be completely controlled during the fabrication process. Figure 5d shows an overview of the plate under high frequency acoustic stressing. In this picture, the three disbonds are visible simultaneously. The frequency used corresponds to forced excitation of the three disbonds but not to the maximum/ideal resonance for each disbond. At the same time, it is also possible to see some fringes corresponding to the vibration-induced deformation of the entire thin ply of the two-ply sandwich plate. The fringes corresponding to the disbonds are brighter than the fringes of the plate modes, the induced displacement for the disbonds being larger. Furthermore, the disbond identification is done
137
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without any ambiguity when the frequency corresponds to resonance of each disbond as seen in Figs. 5a-c. It should be pointed out that in all these experiments, unlike conventional ESPI where speckle decorrelation invariably occurs sooner or later, the fringe patterns obtained with the synchronized reference updating technique were very stable. As an added advantage, this method, with its constant synchronized renewal of the reference, automatically recovers from any momentary loss of information caused by gross object motion. Finally, it is worth mentioning that the synchronization approach described in this paper can also be profitably used in conjunction with other interferemetric techniques as shearography.
5. CONCLUSION
Fig. 4. Higher resonant mode: 110 kHz.
A synchronzied reference updating ESPI technique is proposed as a way to minimize the susceptibility of optical NDE methods to environmental noise. The meehod requires synchronization of the optical detection system with the object stressing system, and involves continuous renewal of the reference image. Two modes of operation--a real-time and a time-average mode--are described. It is shown that the technique improves fringe stability as well as enhances fringe contrast. Application to optical NDE of disbonds is demonstrated.
138 ACKNOWLEDGMENT
This work was carried out in the course of research sponsored by the FAA Center for Aviation Systems Reliability, operated by Ames Laboratory, USDOE, for the Federal Aviation Administration under Contract No. W7405-ENG-82 for work by Iowa State University and Northwestern University.
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