Title of Invention	"AN INTEGRATED HIGH QUALITY FLAW IMAGING AND BOND TESTING APPARATUS"
Abstract	A portable ultrasonic imaging and bond testing apparatus and a method for ultrasonic flaw imaging and bond-testing using said apparatus are disclosed. The invention is particularly suitable for producing high quality ply-by-ply images in layered structures like composites and in evaluating bond integrity between two dissimilar materials (with severe acoustic impedance mismatch e.g. metal/elastomer interfaces) in adhesive bonded structures.

Title of Invention

"AN INTEGRATED HIGH QUALITY FLAW IMAGING AND BOND TESTING APPARATUS"

Abstract

A portable ultrasonic imaging and bond testing apparatus and a method for ultrasonic flaw imaging and bond-testing using said apparatus are disclosed. The invention is particularly suitable for producing high quality ply-by-ply images in layered structures like composites and in evaluating bond integrity between two dissimilar materials (with severe acoustic impedance mismatch e.g. metal/elastomer interfaces) in adhesive bonded structures.

Full Text	PORTABLE ULTRASONIC FLAW IMAGING AND BOND-TESTING APPARATUS Field of the invention The present invention relates to imaging and bond testing apparatus. The present invention also relates to a method for ultrasonic flaw imaging and bond-testing using said apparatus. The invention is particularly suitable for producing high quality ply-by-ply images in layered structures like composites. Another important application of the invention is in evaluating bond integrity between two dissimilar materials (with severe acoustic impedance mismatch e.g. metal/ elastomer interfaces) in adhesive bonded structures. Background of the invention Ultrasonic imaging techniques is of importance in several fields such as flaw imaging with respect to any solid structure subject to high stresses, for example, metal alloys used in automobile bodies, ship, or aircraft manufacture; or for testing the strength or weakness of metal rails, or ball bearings, etc. Flaw imaging is also of importance in fields such as aerospace technology where the presence of a single flaw in the material of the vehicle has a great impact in terms of safety of the equipment and determines the success of the mission. Ultrasonic techniques are also of importance for the testing of bonds between two dissimilar materials in order to determine the strength/ weakness of the interface formed between such materials. Ultrasonic imaging techniques are also of importance in dating of geological formations in order to determine the ore content in a particular geological zone. In fact, assays or surveys carried out using ultrasonic imaging techniques often determine the viability of mining operations in a particular geological zone. US patent 4,742,713 discloses an ultrasonic flaw detecting system with a manual scanning unit comprising a probe, a probe position monitoring means, a probe energizing means, a reflected signal receiving means, and a data collecting/ record ing system, wherein the scanning unit, the ultrasonic flaw detector, and the data collection/ recording unit constitute a portable flaw detection and data collection apparatus. While manually manipulable scanning unit imparts flexibility to the system, it can not produce high quality ultrasonic images with accurate localization of defects. US patent 5,839,442, discloses a portable ultrasound imaging system, where the system includes a hand held scan head, housing an array of ultrasonic transducers with dynamic focussing and defect imaging capabilities. It also has an additional facility for Doppler imaging of moving fluids. However, while, this portable scanner system in its present form is suitable for medical ultrasonics, it does not address the problem related to scanning of industrial components. European Patent EP 0187154 Al 860716 discusses an ultrasonic imaging system, which is used for providing contact imaging of a component. A transducer is scanned across a work piece to collect image data from scan area comprising a plurality of pixels. Means for resiliently carrying the transducer with two separate degrees of freedom, maintain a tight acoustic coupling between the work piece surface and the transducer. The image data relating to each position in the scanned area describe the amplitude of a reflection and its depth from a reference. Means are provided for easily varying the size of the image area while maintaining the number of pixels constant. The system provides a real time visual display of the scanned area. An automatic calibration mode for the system is provided as another feature to remove the acoustic delay of the transducer coupling from the image data. However, a simple removal of acoustic delay effect of the transducer, by multiplying the time of flight (TOF) with a correction factor (generated during calibration with material of known TOF), though adequate for peak - detection based simple imaging technique is not acceptable in advanced images involving full RF capture. Because the simple technique is not capable of restoring the front wall echo, any small variation in the thickness of the couplant and small variability of the trigger point for capture of digitized data, introduce small but significant errors. Moreover, magnitude of this error may change as the transducer moves on the test piece from one scan position to another. Thus the assumption of 'tight coupling' is not strictly valid in an automated contact - mode scanning situation, aimed at high quality imaging of defects. US patent 5,524,627 discloses an ultrasonic imaging system including means for displaying the actual trajectory of the ultrasonic probe, acoustic coupling monitoring apparatus and means for incorporating necessary coupling - correction to every location of the probe. The acoustic-coupling monitoring apparatus in this case uses two criteria based on the amplitudes of the first back wall echo and the ratio of the first BWE to the second BWE, for ascertaining the degree of acoustic coupling at each transducer position. Finally image correction is implemented, based on amplitude normalization w.r.t. the degree of acoustic coupling. However, the above method is based only on the amplitude of pulse. It does not address the problem of changing pulse shape due to variation in couplant layer thickness. Moreover, in heterogeneous materials like composites, existence of an explicit second back wall echo itself is often problematic, because of high ultrasonic attenuation of the material. To meet the challenge of high quality imaging, Glenn, M. Light et al had developed a special pulse-echo ultrasonic squirter system (Rev. Progress QNDE, 8A (1989) p.929). The system though excellent in its performance, is not suitable for field inspection due to flow of uncontained pool of water in fairly large quantity. Thadd, C. Patton et al had subsequently developed a dripless bubbler system, which could successfully overcome the above problem using self-contained water in closed-loop system, as couplant (Rev.Progress QNDE, 13 (1994) p. 701). It is essentially a captured water column incorporating a self-contained water delivery and vacuum recovery system. The dripless bubbler was originally designed as a means for implementing low freq. (~ 1 MHz) ultrasonic inspection technique. However higher frequency inspection (~15 MHz) could also be done, with not too large reduction in amplitude due to the presence of underlying membrane. This equipment was specially designed for scanning of elevated rivets. Thus disadvantages of the dripless bubbler under a general field inspection situation are that (i) it requires additional water delivery and vacuum recovery system, which certainly adds to the cost and complexity of the equipment (ii) presence of membrane under the transducer tends to reduce the amplitude of the signal, within certain limits. Higher the frequency of the interrogating transducer, higher is the magnitude of such loss. Ultrasonic imaging of high quality is generally achieved by using piezoelectric transducers excited by high voltage pulses, while the transducer remains coupled to the test object through a column of water, either by immersing the test piece in a water filled tank (immersion mode) or by using a coupling water jet (the squirter mode). In contact mode of testing on the other hand, the required 'near field distance' between the transducer and the test object is maintained by attaching a solid cylindrical piece of polymeric material (called the delay-line) onto the transducer surface, which in turn is coupled to the test object simply through a thin layer of coupling fluid. Of the two generic prior art methods outlined above, the contact mode is preferable particularly in field conditions, because it can be operated with ease without any water mess and additional accessories. Also because of the simplicity of design, the contact mode is less expensive than the more widely used squirter system. However, in spite of these obvious advantages, contact method is usually not recommended in high quality imaging system because additional reflection of the ultrasonic wave generated by the front surface of the delay line, invariably interferes with the front wall echo (FWE) of the test object, thereby distorting the received ultrasonic signal. This signal distortion gives rise to the following problems in production of high quality image during testing. 1. Finding exact location of FWE and hence of all subsequent echoes becomes problematic, due to distorted shape of FWE (extent of this distortion is a function of the material properties of test object the couplant and the couplant thickness) 2. Application of standard signal processing techniques (like deconvolution for improvement of resolution) becomes unviable, as the reference signal itself is distorted. Hence there is a pressing need in the field of ultrasonics, to overcome the above limitations of contact mode of testing, for production of high quality images with ease under field condition. In many field inspection requirements, particularly in case of primary aerospace structures, not only standard ultrasonic testing and imaging but also inspection of adhesive bonded structures is of paramount importance. In testing of bond between two adherends of comparable acoustic-impedance, the conventional multiple reflection pulse echo technique is applied with reasonable success, using any ultrasonic flaw detector in pulse-echo mode. However, the defect sensitivity of the above technique decreases as the acoustic impedance mismatch between the two adherends increases, becoming too low in extreme cases of metal-elastomer joints. These special bond inspection problems have generally been tackled exclusively (i.e. exclusive to the domain of general inspection and imaging problems) by several investigators, using different versions of swept frequency tone burst techniques (as against pulse excitation techniques). For example, Gammel addresses the problem of testing metal/rubber interfaces by a time-delay spectroscopy technique, where a swept-frequency source was used (Rev.Progress QNDE, 9B (1990) p. 1295). The received signal was followed by a tracking filter and was analyzed in the frequency domain. Morio Shimiza et al on the other hand, had used a swept frequency source and time domain multiple reflection signal, for evaluation of debonding at metal/rubber interfaces (/. Spacecraft, Oct 1989, p.379). It may be noted that both the above techniques use special swept frequency sources, which are not generally available in (pulse-excitation based) ultrasonic flaw detectors. Therefore, development of a pulse-excitation based bond-testing technique and its integration into a portable contact-mode ultrasonic equipment (with advanced imaging capability), is highly desirable for integrated field inspection of complex structures. Exclusive Bond testers, based on low frequency ultrasonic excitation (typically 50 to 500 kHz) have also been developed by different inventors. For example, Fokker Bond Tester MK II has used frequency and amplitude of the first two harmonics generated by thickness vibration resonance of bonded joints. The input signal used in this equipment is a swept frequency sinusoid. (Proc. I.Mech. E., 201B (1987) p. 41) US Patent 4,215,583 has disclosed a bond testing apparatus, in which the contact transducer is activated with a sine-wave oscillator of known frequency. Waves reflected from the structure under test superimpose on incoming waves to create a standing wave, amplitude and phase of which are dependent on the quality of bond under inspection. Specific acoustic impedance of the material is monitored here for determining the bond condition. Another digital bond tester operating at 5 -30 kHz frequency range has been patented vide US patent no 6,018,999, which uses (tone burst) input signals to generate and detect vibration signal outputs. Systematic frequency selection corresponding to each test piece followed by the scanning operation, produces amplitude plots, phase plots and magnitude plots as simultaneous outputs. The above bond testers which operate on sinusoidal excitation in kHz range, form an exclusive class of equipment by themselves, as against the pulse echo imaging apparatus used in flaw imaging systems, operated generally in 0.5 MHz to 15 MHz frequency range. Objects of the invention The primary objective of the present invention is to develop a method and apparatus for integrated high quality flaw imaging and bond testing which obviates the above-mentioned disadvantages of the prior art. It is another object of the invention to develop an apparatus that is useful for integrated high quality flaw imaging and bond testing. It is another object of the invention to develop a method and portable apparatus for flaw imaging and bond testing that are useful in field conditions. It is a further object of the invention to develop a technique for high quality flaw imaging that is inexpensive and more efficient. Summary of the invention Overcoming the limitations of the prior art and meeting the objectives stated above, the techniques and the equipment have been developed in the present invention, comprising (i) Means to determine the optimum scan plan for a given scan area, (ii) An irrigated ultrasonic transducer (or probe) with attached delay which interrogates the test object, and is moved on its surface along two programmed axes (either x, y or r, 0) in predetermined steps, maintaining its perpendicularity to the test piece, (iii) A pulser circuit which energizes the transducer, either by spike excitation or by square wave excitation at every probe location, (iv) An amplifier circuit, which conditions (generally amplification plus rectification) the received signal, (v) The mechanical scanner system, which ensures accurate placement and movement of the transducer along two axes, (vi) The motor control circuit, which controls the automated movement of the mechanical scanner system (with the probe-holder) in programmed manner along r, 6/x,y axes, (vii) The gate forming circuit, which selectively transmits a portion of the ultrasonic signal to the peak-detector during initial scans, (aimed at capturing real-time images for quick detection of defects) (viii) The peak-detector, which detects maximum value of the signal situated within the selected time gate, (ix) The 12-bit slow ADC, which converts each detected peak value into a 12-bit digital data. (x) The time of flight gate which monitors the time-of-flight (TOF) of the signal, whenever a pre-set amplitude threshold is crossed and feeds the same into the 12 bit slow ADC. (xi) The multiplexer, which multiplexes the input to the 12-bit slow ADC, so that both amplitude and time of flight data are simultaneously digitized and fed into the system computer. (xii) The display system, which simultaneously produces real-time amplitude and time-of-flight image on the monitor of the system computer (a laptop PC) for quick detection of defects in the test object. (xiii) A quadrature-controlled 8-bit fast analog-digital converter, which digitizes the full RF signal during 'defect analysis scan' of selected defective zones of the test object, under dynamic on-fly conditions and stores the data in the main computer. (xiv) A synchronized master clock, which with the help of data bus, address bus and the system software operated on main computer, coordinates and activates all the operations in such a manner that movement of mechanical scanner (plus capture of its positional coordinates), transmission of energizing pulse (spike or square voltage) to the transducer on its arrival at each scan position, formation of gate, initiation and control of quadrature phase-shifted A/D conversion and sequencing of data transfer, all are achieved with perfect synchronization. (xv) The defect analysis means for off-line analysis of the scan data. This includes date compression, signal restoration (for eliminating the delay-line reflection and couplant thickness variation effects from RF signal). TOF calculation, signal processing, image processing, ply-by-ply imaging, amplitude interpolation, 3-D image display (with provision of interactive rehearsal for repair of defect) and consolidated scan output with hard copy option. (xvi) The bond testing module, which uses pulse ultrasonic spectroscopy technique for evaluation of bond integrity between two acoustically mismatched materials like metal /elastomer interfaces or skin-core structures with highly damped core material. (xvii) Means for calculating and displaying probability distribution plots of 'slenderness parameter' pertaining to the above bonded structures, determining the debonded area and making consolidated report on system computer with hard copy option. Thus according to the present invention, there is provided an integrated high ~~ quality flaw imaging and bond testing apparatus, said apparatus comprising a probe including an ultrasonic transducer for interrogating a test object and receiving ultrasonic signals, said probe being capable of being moved on its surface along two predetermined programmed axes and maintaining its perpendicularity with respect to said test object, a pulser means connected to said transducer for energising said transducer, an amplification and rectification means connected to said ultrasonic transducer for receiving said ultrasonic signals and subjecting them to amplification and rectification, peak detection means connected to said amplification and rectification means, for detecting the maximum value of the signal occurring within a predetermined time gate, a time of flight gate means connected to said peak detection means for monitoring the time of flight of said signal whenever a pre-set amplitude threshold is crossed, a first analog to digital converter connected between said peak detection means and a system controller for converting the peak values detected by said peak detection means into digital data for display as a colour coded image in said system controller, said time of flight gate means being operationally connected to a time-to- voltage converter configured to receive the time of flight gate data, said time to voltage converter being connected to the said first analog to digital converter for digitizing the time of flight values, said amplification and rectification means being also connected to a quadrature controlled flash analog to digital converter which is in turn connected to said system controller for digitizing and capturing full unrectified RF waveforms alongwith corresponding positional information. Brief description of the accompanying drawings Fig 1 a) Immersion mode of testing Fig 1 b) Contact mode of testing Fig 2 System block diagram Fig 3 a) Portable ultrasonic equipment; operational steps (imaging mode) Fig 3 b) Portable ultrasonic equipment; operational steps (bond testing mode) Fig 4 Quadrature controlled flash ADC: block diagram Fig 5 a] Off-line defect analysis: flow diagram Fig 5 b) Signal restoration technique: flow diagram Fig 6 Ultrasonic signal before and after restoration: Typical example Fig 7 a) Bond testing module: physical principle Fig 7 b) Bond testing module: typical output (well bonded case) Fig 7 c) Bond testing module: typical output (debonded case) Detailed description of the invention The present invention is of a portable contact mode ultrasonic imaging system cum bond-tester (a) for imaging objects of all materials in general and layered composite materials in particular, for detection and analysis of defects (b) for testing of bond-integrity between two acoustically mismatched materials e.g. metal/rubber adhesive bonded structures, skin-core structures with highly damped core material etc. The principles and operations of the above systems according to the present invention may be better understood with reference to the drawings listed above and the accompanying description. Fig. 1 (a) shows the probe configuration generally used in immersion mode of ultrasonic testing for producing high quality images of the test object, which demands excellent coupling condition. The test piece here is held just beyond the near-field of the scanning transducer and the space between the transducer and the test object is kept always filled with a stationary (in immersion mode) or a moving (in squirter mode) water column. In the simpler contact mode of testing (which generally is used in wide variety of applications, other than advanced imaging) used in the present invention, the distance between the transducer and the test object is maintained by attaching a solid cylindrical piece of polymeric material (the delay-line) onto the probe surface (shown in Fig. 1b) which in turn is coupled to the test object through a very thin layer of coupling fluid. Thus coupling of transducer to the test object is achieved here without associated water mess/special water-jet facility, as is evident from Figs. l(a) and l(b). However the simplicity of contact mode of testing also gives rise to associated problems, as explained below. Additional reflection of the ultrasonic wave from the front surface of the delay line invariably interferes with the echo generated by the front wall of the test object, thereby deteriorating the quality of the signal and consequently the quality of the image. Such signal degradation has been tackled in the present invention, by developing a novel 'signal restoration technique' which has been described in details later in this section. Figs. 2 & 3 together illustrate a preferred embodiment of the ultrasonic test equipment, constructed and operative according to the teaching of the present invention (a) for imaging test object (initially for real-time defect detection and subsequently for more detailed defect analysis through full RF capture and off-line analysis) and (b) for testing of bond integrity at the interfaces between two acoustically mismatched materials e.g. metal/elastomer interfaces etc. The ultrasonic system as per the system block diagram of Fig.2 includes. /. The probe (1) (i.e. the ultrasonic transducer) mounted on the probe-holder of the mechanical scanner, which is driven along two axes by two stepper motors controlled by the motor control circuit (comprising units 2,3,4,5 of block diagram) in Fig.2, coupled finally to the main computer. //. The transmitter unit (6) gets activated through the master clock (7) and the timer synchronizer unit (8), every time the transducer attains a pre-defined position in course of its scanning of the test object. Consequently, the piezoelectric crystal of the transducer gets energized, either by a spike voltage or by a square voltage, as per user's selection effected via main computer. III. As per the preferred embodiment of the present invention, the output from the transducer can be processed in two different ways. During the initial set of scans (aimed only at quick real-time detection of defect zones of the test object) the ultrasonic signal received by the transducer (1) is amplified and rectified by unit No. (9) and then processed for detection of peak voltage and corresponding time of flight (TOF) for on-line imaging of the scanned area of test object. The real time image thus is based only on peak-detection of the rectified signal, occurring within user selected time gate. It may be noted that the full RF wave capture facility (shown in the flow diagram of Fig. 3a) is not activated at this stage of testing. It becomes operational only during subsequent scans of few selected defect zones of the test object, for full wave capture and off-line defect analysis. The sequence of operational steps mentioned above, is described in the flow diagram of Fig. 3a. IV. A detailed description of the real time imaging unit is given below. A real time display of the selected time-window is obtained, by passing the portion of the ultrasonic signal situated within the time-window, to a peak detector (10). The peak values thus detected are converted into 12 bit digital data by a 12 bit slow ADC (15) for display as colour coded image on the monitor of the main computer (6). Simultaneously, the time of flight value is measured by transforming the pulsed ultrasonic RF signals into TTL signal levels, by use of fast comparators. The comparator levels are set to be above user selected threshold levels. A new gate called 'time of flight gate' (11 of Fig.2) is formed by the next ultrasonic signal, which crosses the preset threshold value. The time of flight gate is then fed into a time-to-voltage converter circuit (13). This transforms 'time from the start of the gate to the first echo above threshold' into a linear ramp, whose output voltage is directly proportional to TOF gate time. This TOF value is also digitized by the 12-bit slow ADC (15) mentioned earlier. In order to produce simultaneous display of amplitude and TOF images in real-time, the input to the 12 bit ADC is multiplexed by a multiplexer unit (12), controlled by the multiplex controller (14) coupled to the main computer (6). The digitized data corresponding to the peak amplitude value, the time of flight value and the positional coordinates of the interrogating transducer are all read by the main computer and displayed simultaneously as two real-time colour-coded images on the monitor screen. The features of the equipment described so far are essentially similar to any conventional ultrasonic imaging equipment, except for the fact that it uses a simple easy-to-use contact-mode test configuration, and yet aims at producing high quality ply-by-ply images through full RF wave capture in defective zones, plus signal processing capabilities. It also aims at integrating bond testing capability with this portable imaging system. 1. It is a special feature of the preferred embodiment that the full unrectified RF signals generated during the defect-analysis scans of defect-detected zones, are digitized by a special quadrature-controlled 8 bit flash ADC (16 of Fig. 2) and read along with the corresponding transducer locations by the main computer. The data thus generated are stored in memory and used off-line for (i) advanced imaging/defect-analysis and (ii) bond-testing operations, as explained in the flow diagrams of Figs. 3a and 3b respectively. The details of these operations are described later in this section. Unique design features of the flash ADC (16) are explained below. The frequency of conversion required for full RF capture in an ultrasonic imaging equipment is determined (i) by the highest frequency signal generated by the ultrasonic transducer (i.e. by the nominal frequency and its band width) following Nyquist criteria and (ii) by the required thickness resolution of the object under inspection. However, using a high frequency ADC as per this requirement often becomes quite expensive. A more cost effective way of achieving the required speed of AD conversion as per the above criteria, by actually using a lower frequency ADC capable of phase reversal under quadrature sampling scheme, has therefore been developed. Data conversion is accomplished here under the dynamic condition of transducer movement, as described below. Time interval between two consecutive steps of the scanner is so chosen that at least two samples are generated on-fly within the minimum sampling time interval, ultimately required by the ultrasonic scanning system. Every alternate ultrasonic signal generated on-fly is then fed into the AD converter, (having its actual sampling frequency = 0.5 x 'ultimate frequency of sampling required by the equipment) after effecting phase reversal by 90°. Thus two sets of data, sampled at half the required frequency and phase-shifted by 90° w.r.t. each other, combine to give the full ADC performance, i.e. the effective sampling frequency is twice the actual sampling frequency used in the system. The details of this quadrature controlled flash ADC are shown in the block diagram of Fig.4. For synchronization and control of the conversion phases described above, the master system clock is used. The digitized data are fed into the system computer, recombined in memory and processed to form a composite data stream. Advantage of this quadrature sampling technique lies in reduction of (i) the actual sampling speed of the ADC hardware and (ii) speed requirement of each subsequent semiconductor device that follow the ADC, e.g. the FIFO memories shown in Fig.4. This results in effective reduction of the cost of ADC, without sacrificing its operational quality. VI. Another special feature of the present invention lies in a novel signal restoration technique, used for restoring the 'ideal reflected signal' free from the distortions (i) due to contact mode of testing (i.e. delay-line effect) (ii) due to couplant thickness variation. Thus, in a preferred embodiment, the apparatus of the present invention essentially comprises a probe (1) held in a self aligning probe-holder by mechanical means, provided with a synchronized pulser circuit (6,8) and carried by a programmable dual axes portable scanner driven by the motor control means (2,3,4,5), output of the said probe being connected to an amplification/rectification means (9), a peak detection means (10), a time of flight gate means (11) and a time-to-voltage conversion means (13). The peak-detection means (10) and the time-to-voltage conversion means (13) are connected to a multiplexer means (12,14) and finally through a 12-bit slow AD conversion means (15) to the system controller (6). Output of the said probe are also connected via unrectified signal option of the amplification/rectification means (9) to synchronized and quadrature-controlled flash ADC (16) to the system controller (6). In one embodiment of the invention, the said peak detector (10) and time of flight gate (11) with time to voltage converter (13) are provided with a multiplexer means (12,14) at the output in conjunction with positional information of probe (1), in order to produce simultaneous display of amplitude and TOF images in real time on the monitor of the system controller during initial scans of the test object. In another embodiment of the invention related to defect analysis and bond testing means, the amplified but unrectified signal from position synchronized probe (1) is taken through quadrature-controlled flash ADC means (16) to the system controller (6). The said quadrature-controlled flash ADC means (16) comprises a positional synchronization pulse generating means operatively connected to a phase generating means, said phase generator being operatively connected to a master clock-controlled selector means and to a flash analog to digital conversion means with its output connected to memory means, the phase generating means being connected to a sequencing means and a bus interface means, said bus interface means interacting with an instrument data bus, the entire circuit being connected to the system controller (6). The display means finally produces full RF wave-based images on the monitor of the system controller (6) for defect analysis and also for bond testing. The physical principles and the computational methods underlying this signal-restoration technique are described below. It may be recalled here that in contact mode testing, the experimentally determined apparent FWE' is actually formed by superposition of 'true FWE' and the 'delay-echo', time separated due to the couplant layer thickness (Ref. Fig. 1b). The objective of the signal restoration technique is therefore to compute the 'true FWE' from the 'apparent FWE' captured by the contact-mode ultrasonic test at every scan position. In principle, it should be possible to restore the true FWE by simple subtraction, if a 'reference delay-echo' is recorded by simply immersing the transducer with the attached delay-line in water and if the exact time separation due to couplant thickness could be ascertained in each case. However, in addition to this, a straight subtraction becomes problematic if the trigger points of the data acquisition system in the 'experimental waveform' and in the 'reference delay echo' are not perfectly matched. Moreover every single automated scan, even for a seemingly small 100 mm x 100 mm scan area, generates several thousand RF signals, each having 256/512/1024 data points. Handling such large size of data during actual inspection under field condition is impossible. The above problem has therefore been tackled by the computational steps summarized in the flow diagram of Fig.5 (b) and explained below. a) The RF waveforms captured during scanning of a selected defect zone are subjected to data compression by pattern matching technique. The waveforms having highest correlation coefficient defined by (Figure Removed) where x= {xi} and y = {yi} with i = 0,l,2.....N-l are designated as 'same'. A set of waveforms obeying the sameness criterion are averaged to produce a particular 'distinct waveform' with its corresponding 'repeat counf signifying its probability of occurrence. The most probable distinct waveform (MDSW) is determined for every scan set. Only distinct waveforms (typically 20/30 in number against a scan comprising several thousand waveforms) are stored in memory. This gives tremendous advantage in terms of memory space and computational efficiency. b) Similarly, the RF waveforms captured by immersing only the transducer and the attached delay-line in coupling fluid over a sufficiently long time window, is subjected to data compression by pattern matching technique. The distinct waveform with maximum repeat-count is designated in this case as the most probable delay-echo (MDE). VII. A straight subtraction of (MDE) the most probable-delay-echo from the most probable-distinct-waveform (MDCW) generated on a defect free calibration sample returns the 'true-front-wall-echo' (TFWE). VIII. For all distinct waveforms, generated during a scan of the test object the following procedure is followed: a) Every distinct waveform is cross-correlated with MDE and the position of the correlation peak 't' is determined on the time axis. b) Next the most probable delay echo (MDE) time shifted to t, is subtracted from the corresponding distinct waveform. c) The resultant waveform of the previous step is compared with the 'true-front- wall-echo' (TFWE) computed in (Hi) above, in terms of both shape and amplitude. d) MDE is then left-shifted on the time axis by steps of one time-interval of flash ADC in an iterative manner, till the above matching exercise is found satisfactory. e) The final subtracted waveform in each case gives the 'restored signal', which alone is used in all subsequent signal processing/imaging operations for maintaining high quality of image. J) Next, impulse response of restored signal is computed by Wiener filtering, using the expression (Formula Removed) where y(t), x(t), h(t) are restored experimental signals, restored reference signal and impulse response function respectively and X(□), Y(□), H(□) are the corresponding Fourier Transforms. The constant 'K1 is an arbitrary desensitizing parameter. The analytic signal of the impulse response is computed by using the expression Analytic[h(t)] = h(t)+jH[h(t)] where H represents Hilbert Transform g) Magnitude of analytic signal represents the signal envelop, which is time-sliced for producing ply-by-ply image, 3-D visualization of the defect zone, plus standard image processing. IX. Another special feature of the present invention lies in a bond testing module, based on a novel pulse spectroscopy technique, which is easily integrated with the system hardware described in Fig.2. This module is particularly useful for testing bond integrity between two severely mismatched acoustic surfaces, e.g. metal/rubber interfaces. The physical principle underlying this technique and associated equipment details are described below: a) A broadband transducer of suitable nominal frequency (as per the criteria explained later in this section) is used, first for scanning a metal-alone calibration piece and then the actual bonded structure. b) Multiple reflections in pulse-echo full RF mode are captured in the time domain, by using the quadrature controlled flash ADC described in Figs.2 and 3, with provision for user selectable sampling rate. X The record-length and the sampling rate of data acquisition are so adjusted that adequate frequency resolution is achieved, when these signals are viewed in frequency domain, by running Fast Fourier Transform programme through the main computer. XI. It is well known that the spectrum of the signal captured in pulse echo mode, consists of a series of equispaced dips (due to phase reversal on reflection) governed by the resonance frequencies of thickness vibration of the metal plate. XII. If the above plate is bonded with a low acoustic impedance flexible layer on the other side, then the resonance dips get affected due to the damping characteristics of this flexible layer. A typical spectrum with equispaced dips is shown in Fig. 7 (a) (top diagram) whereas the zoomed displays of selected dips, corresponding to the bonded and debonded cases are presented in the bottom parts of the same diagram. It may be noted that the long slender dip of the debonded case (i.e. metal alone) gets transformed into a broad and short dip in the bonded case. Thus a slenderness parameter 'K' is defined such that (Formula Removed) XIII. The value of K is high in case of debonding and low in case of well-bonded interface. XIV. Typical probability distribution of slenderness parameter 'K', corresponding to the calibration piece (metal alone) and metal/rubber bonded structures, are presented in Figs. 7b and 7c for well-bonded and debonded cases respectively. On the basis of the RF waveforms captured during the scan, the system computer computes and plots the above probability distribution curves, where the overlapped area between the calibration curve and the experimental curve signifies the area debonded as percentage of the total area under the experimental curve. (Ref. Fig. 7c). Nominal frequency of the interrogating transducer is carefully selected, such that the two probability distribution curves are well separated (i.e. without any overlap) in case of well-bonded joints (tested initially for system calibration). XV. Variation of K value against spatial coordinates of the bonded surface is plotted in form of a binary image (with user selectable threshold) by the main computer. The same is displayed on the monitor with hard-copy option. While the invention has been described here with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made We Claim: 1. An integrated high quality flaw imaging and bond testing apparatus, said apparatus comprising a probe including an ultrasonic transducer for interrogating a test object and receiving ultrasonic signals, said probe being capable of being moved on its surface along two predetermined programmed axes and maintaining its perpendicularity with respect to said test object, a pulser means connected to said transducer for energising said transducer, an amplification and rectification means connected to said ultrasonic trasducer for receiving therefrom said ultrasonic signals and subjecting them to amplification and rectification, peak detection means connected to said amplification and rectification means, for detecting the maximum value of the signal occurring within a predetermined time gate, a time of flight gate means connected to said peak detection means for monitoring the time of flight of said signal whenever a pre-set amplitude threshold is crossed, and an analog to digital converter connected between said peak detection means and a system controllen, for converting the peak values detected by said peak detection means into digital data for display as a colour coded image in said system controller, said time of flight gate means being operationally connected to a time-to-voltage converter configured to receive the time of flight gate data, said time to voltage converter being connected to the said analog to digital converter for digitizing the time of flight values, said amplification and rectification means being connected to a quadrature controlled flash analog to digital converter which in turn is connected to said system controller. 2. An apparatus as claimed in claim 1 wherein the said analog to digital converter is provided with a multiplexing means at the input thereof in order to produce simultaneous display of amplitude and time of flight (TOF) images in real-time coupled to the system controller. 3. An apparatus as claimed in claim 1 wherein said quadrature-controlled flash analog-digital converter comprises of a positional synchronization pulse generation means operatively connected to a phase generating means, said phase generator being operatively connected to a selector and to a flash analog to digital operatively connected to a selector and to a flash analog to digital conversion means with its output connected to a memory means, the phase generating means being connected to a sequencing means and a bus interface means, said bus interface means interacting with an instrument data bus, the entire circuit being connected to the system controller, for digitizing full unrectified RF signals generated during final scanning of defect zones. 4. An apparatus as claimed in claim 1 wherein the display means of the system controller displays resolution enhanced ply-by-ply images of the defect zone, using full RF signals captured by the quadrature controlled flash analog digital converter (ADC), in conjunction with synchronized positional information during final scan of the defect zone. 5. An apparatus as claimed in any preceding claim wherein said probe including said ultrasonic transducer is mounted on a mechanical scanner, said scanner being driven by a pair of stepper motors controlled by said system controller. 6. An apparatus as claimed in any preceding claim wherein a gate forming circuit is connected to said peak detection means for selectively transmitting a portion of the ultrasonic signals to said peak detection means during initial scans for capturing real-time images for quick detection of defects in the test object. 7. An apparatus as claimed in any preceding claim wherein said system controller is a computer. operatively connected to a synchronized master clock, which with the help of data bus and address bus coordinates all the operations in such a manner that movement of mechanical scanner including capture of its positional coordinates, transmission of energizing pulse to the transducer on its arrival at each scan position, formation of gate, initiation and control of quadrature pahse-shifted A/D conversion and sequencing of data transfer, are all synchronized. An apparatus as claimed in claim 2 wherein time of flight gate is connected to a time to voltage converter for transforming the 'time from the start of the gate to the first echo above threshold time of flight (TOF) into a linear ramp, whose output voltage is directly proportional to the time of flight (TOF) time. 9 An apparatus as claimed in claim 9wherein the output of said time to voltage converter is connected to said analog to digital converter for digitization of said time of night (TOF) value. An apparatus as claimed in claim 7 wherein said peak detection means and 10 time of flight gate with said time to voltage converter are connected to said multiplexer means at their outputs in conjunction with positional information of said probe in order to produce simultaneous display of amplitude and time of flight (TOF) images in real time on the monitor of the system controller during initial scans of the test object. 11 An integrated high quality flaw imaging and bond testing apparatus substantially as herein described with reference to the accompanying drawings.

Full Text

PORTABLE ULTRASONIC FLAW IMAGING AND BOND-TESTING APPARATUS
Field of the invention
The present invention relates to imaging and bond testing apparatus. The present invention also relates to a method for ultrasonic flaw imaging and bond-testing using said apparatus. The invention is particularly suitable for producing high quality ply-by-ply images in layered structures like composites. Another important application of the invention is in evaluating bond integrity between two dissimilar materials (with severe acoustic impedance mismatch e.g. metal/ elastomer interfaces) in adhesive bonded structures. Background of the invention
Ultrasonic imaging techniques is of importance in several fields such as flaw imaging with respect to any solid structure subject to high stresses, for example, metal alloys used in automobile bodies, ship, or aircraft manufacture; or for testing the strength or weakness of metal rails, or ball bearings, etc. Flaw imaging is also of importance in fields such as aerospace technology where the presence of a single flaw in the material of the vehicle has a great impact in terms of safety of the equipment and determines the success of the mission. Ultrasonic techniques are also of importance for the testing of bonds between two dissimilar materials in order to determine the strength/ weakness of the interface formed between such materials. Ultrasonic imaging techniques are also of importance in dating of geological formations in order to determine the ore content in a particular geological zone. In fact, assays or surveys carried out using ultrasonic imaging techniques often determine the viability of mining operations in a particular geological zone.
US patent 4,742,713 discloses an ultrasonic flaw detecting system with a manual scanning unit comprising a probe, a probe position monitoring means, a probe energizing means, a reflected signal receiving means, and a data collecting/ record ing system, wherein the scanning unit, the ultrasonic flaw detector, and the data collection/ recording unit constitute a portable flaw detection and data
collection apparatus. While manually manipulable scanning unit imparts flexibility to the system, it can not produce high quality ultrasonic images with accurate localization of defects.
US patent 5,839,442, discloses a portable ultrasound imaging system, where the system includes a hand held scan head, housing an array of ultrasonic transducers with dynamic focussing and defect imaging capabilities. It also has an additional facility for Doppler imaging of moving fluids. However, while, this portable scanner system in its present form is suitable for medical ultrasonics, it does not address the problem related to scanning of industrial components.
European Patent EP 0187154 Al 860716 discusses an ultrasonic imaging system, which is used for providing contact imaging of a component. A transducer is scanned across a work piece to collect image data from scan area comprising a plurality of pixels. Means for resiliently carrying the transducer with two separate degrees of freedom, maintain a tight acoustic coupling between the work piece surface and the transducer. The image data relating to each position in the scanned area describe the amplitude of a reflection and its depth from a reference. Means are provided for easily varying the size of the image area while maintaining the number of pixels constant. The system provides a real time visual display of the scanned area. An automatic calibration mode for the system is provided as another feature to remove the acoustic delay of the transducer coupling from the image data.
However, a simple removal of acoustic delay effect of the transducer, by multiplying the time of flight (TOF) with a correction factor (generated during calibration with material of known TOF), though adequate for peak - detection based simple imaging technique is not acceptable in advanced images involving full RF capture. Because the simple technique is not capable of restoring the front wall echo, any small variation in the thickness of the couplant and small variability of the trigger point for capture of digitized data, introduce small but significant errors. Moreover, magnitude of this error may change as the transducer moves on the test piece from one scan position to another. Thus the assumption of 'tight coupling' is not strictly
valid in an automated contact - mode scanning situation, aimed at high quality imaging of defects.
US patent 5,524,627 discloses an ultrasonic imaging system including means for displaying the actual trajectory of the ultrasonic probe, acoustic coupling monitoring apparatus and means for incorporating necessary coupling - correction to every location of the probe.
The acoustic-coupling monitoring apparatus in this case uses two criteria based on the amplitudes of the first back wall echo and the ratio of the first BWE to the second BWE, for ascertaining the degree of acoustic coupling at each transducer position. Finally image correction is implemented, based on amplitude normalization w.r.t. the degree of acoustic coupling. However, the above method is based only on the amplitude of pulse. It does not address the problem of changing pulse shape due to variation in couplant layer thickness. Moreover, in heterogeneous materials like composites, existence of an explicit second back wall echo itself is often problematic, because of high ultrasonic attenuation of the material.
To meet the challenge of high quality imaging, Glenn, M. Light et al had developed a special pulse-echo ultrasonic squirter system (Rev. Progress QNDE, 8A (1989) p.929). The system though excellent in its performance, is not suitable for field inspection due to flow of uncontained pool of water in fairly large quantity. Thadd, C. Patton et al had subsequently developed a dripless bubbler system, which could successfully overcome the above problem using self-contained water in closed-loop system, as couplant (Rev.Progress QNDE, 13 (1994) p. 701). It is essentially a captured water column incorporating a self-contained water delivery and vacuum recovery system. The dripless bubbler was originally designed as a means for implementing low freq. (~ 1 MHz) ultrasonic inspection technique. However higher frequency inspection (~15 MHz) could also be done, with not too large reduction in amplitude due to the presence of underlying membrane. This equipment was specially designed for scanning of elevated rivets.
Thus disadvantages of the dripless bubbler under a general field inspection situation are that
(i) it requires additional water delivery and vacuum recovery system, which
certainly adds to the cost and complexity of the equipment
(ii) presence of membrane under the transducer tends to reduce the amplitude
of the signal, within certain limits. Higher the frequency of the
interrogating transducer, higher is the magnitude of such loss.
Ultrasonic imaging of high quality is generally achieved by using piezoelectric
transducers excited by high voltage pulses, while the transducer remains coupled to
the test object through a column of water, either by immersing the test piece in a water
filled tank (immersion mode) or by using a coupling water jet (the squirter mode). In
contact mode of testing on the other hand, the required 'near field distance' between
the transducer and the test object is maintained by attaching a solid cylindrical piece
of polymeric material (called the delay-line) onto the transducer surface, which in turn
is coupled to the test object simply through a thin layer of coupling fluid.
Of the two generic prior art methods outlined above, the contact mode is preferable particularly in field conditions, because it can be operated with ease without any water mess and additional accessories. Also because of the simplicity of design, the contact mode is less expensive than the more widely used squirter system. However, in spite of these obvious advantages, contact method is usually not recommended in high quality imaging system because additional reflection of the ultrasonic wave generated by the front surface of the delay line, invariably interferes with the front wall echo (FWE) of the test object, thereby distorting the received ultrasonic signal. This signal distortion gives rise to the following problems in production of high quality image during testing.
1. Finding exact location of FWE and hence of all subsequent echoes becomes problematic, due to distorted shape of FWE (extent of this distortion is a function of the material properties of test object the couplant and the couplant thickness)
2. Application of standard signal processing techniques (like deconvolution for
improvement of resolution) becomes unviable, as the reference signal itself is
distorted.
Hence there is a pressing need in the field of ultrasonics, to overcome the above limitations of contact mode of testing, for production of high quality images with ease under field condition.
In many field inspection requirements, particularly in case of primary aerospace structures, not only standard ultrasonic testing and imaging but also inspection of adhesive bonded structures is of paramount importance. In testing of bond between two adherends of comparable acoustic-impedance, the conventional multiple reflection pulse echo technique is applied with reasonable success, using any ultrasonic flaw detector in pulse-echo mode.
However, the defect sensitivity of the above technique decreases as the acoustic impedance mismatch between the two adherends increases, becoming too low in extreme cases of metal-elastomer joints.
These special bond inspection problems have generally been tackled exclusively (i.e. exclusive to the domain of general inspection and imaging problems) by several investigators, using different versions of swept frequency tone burst techniques (as against pulse excitation techniques). For example, Gammel addresses the problem of testing metal/rubber interfaces by a time-delay spectroscopy technique, where a swept-frequency source was used (Rev.Progress QNDE, 9B (1990) p. 1295). The received signal was followed by a tracking filter and was analyzed in the frequency domain.
Morio Shimiza et al on the other hand, had used a swept frequency source and time domain multiple reflection signal, for evaluation of debonding at metal/rubber interfaces (/. Spacecraft, Oct 1989, p.379). It may be noted that both the above techniques use special swept frequency sources, which are not generally available in (pulse-excitation based) ultrasonic flaw detectors.
Therefore, development of a pulse-excitation based bond-testing technique and its integration into a portable contact-mode ultrasonic equipment (with advanced
imaging capability), is highly desirable for integrated field inspection of complex structures.
Exclusive Bond testers, based on low frequency ultrasonic excitation (typically 50 to 500 kHz) have also been developed by different inventors.
For example, Fokker Bond Tester MK II has used frequency and amplitude of the first two harmonics generated by thickness vibration resonance of bonded joints. The input signal used in this equipment is a swept frequency sinusoid. (Proc. I.Mech. E., 201B (1987) p. 41)
US Patent 4,215,583 has disclosed a bond testing apparatus, in which the contact transducer is activated with a sine-wave oscillator of known frequency. Waves reflected from the structure under test superimpose on incoming waves to create a standing wave, amplitude and phase of which are dependent on the quality of bond under inspection. Specific acoustic impedance of the material is monitored here for determining the bond condition. Another digital bond tester operating at 5 -30 kHz frequency range has been patented vide US patent no 6,018,999, which uses (tone burst) input signals to generate and detect vibration signal outputs. Systematic frequency selection corresponding to each test piece followed by the scanning operation, produces amplitude plots, phase plots and magnitude plots as simultaneous outputs.
The above bond testers which operate on sinusoidal excitation in kHz range, form an exclusive class of equipment by themselves, as against the pulse echo imaging apparatus used in flaw imaging systems, operated generally in 0.5 MHz to 15 MHz frequency range. Objects of the invention
The primary objective of the present invention is to develop a method and apparatus for integrated high quality flaw imaging and bond testing which obviates the above-mentioned disadvantages of the prior art.
It is another object of the invention to develop an apparatus that is useful for integrated high quality flaw imaging and bond testing.
It is another object of the invention to develop a method and portable apparatus for flaw imaging and bond testing that are useful in field conditions.
It is a further object of the invention to develop a technique for high quality flaw imaging that is inexpensive and more efficient. Summary of the invention
Overcoming the limitations of the prior art and meeting the objectives stated above, the techniques and the equipment have been developed in the present invention, comprising
(i) Means to determine the optimum scan plan for a given scan area, (ii) An irrigated ultrasonic transducer (or probe) with attached delay which
interrogates the test object, and is moved on its surface along two programmed
axes (either x, y or r, 0) in predetermined steps, maintaining its
perpendicularity to the test piece, (iii) A pulser circuit which energizes the transducer, either by spike excitation or by
square wave excitation at every probe location, (iv) An amplifier circuit, which conditions (generally amplification plus
rectification) the received signal, (v) The mechanical scanner system, which ensures accurate placement and
movement of the transducer along two axes, (vi) The motor control circuit, which controls the automated movement of the
mechanical scanner system (with the probe-holder) in programmed manner
along r, 6/x,y axes, (vii) The gate forming circuit, which selectively transmits a portion of the ultrasonic
signal to the peak-detector during initial scans, (aimed at capturing real-time
images for quick detection of defects) (viii) The peak-detector, which detects maximum value of the signal situated within
the selected time gate, (ix) The 12-bit slow ADC, which converts each detected peak value into a 12-bit
digital data.
(x) The time of flight gate which monitors the time-of-flight (TOF) of the signal, whenever a pre-set amplitude threshold is crossed and feeds the same into the 12 bit slow ADC.
(xi) The multiplexer, which multiplexes the input to the 12-bit slow ADC, so that both amplitude and time of flight data are simultaneously digitized and fed into the system computer.
(xii) The display system, which simultaneously produces real-time amplitude and time-of-flight image on the monitor of the system computer (a laptop PC) for quick detection of defects in the test object.
(xiii) A quadrature-controlled 8-bit fast analog-digital converter, which digitizes the full RF signal during 'defect analysis scan' of selected defective zones of the test object, under dynamic on-fly conditions and stores the data in the main computer.
(xiv) A synchronized master clock, which with the help of data bus, address bus and the system software operated on main computer, coordinates and activates all the operations in such a manner that movement of mechanical scanner (plus capture of its positional coordinates), transmission of energizing pulse (spike or square voltage) to the transducer on its arrival at each scan position, formation of gate, initiation and control of quadrature phase-shifted A/D conversion and sequencing of data transfer, all are achieved with perfect synchronization.
(xv) The defect analysis means for off-line analysis of the scan data. This includes date compression, signal restoration (for eliminating the delay-line reflection and couplant thickness variation effects from RF signal). TOF calculation, signal processing, image processing, ply-by-ply imaging, amplitude interpolation, 3-D image display (with provision of interactive rehearsal for repair of defect) and consolidated scan output with hard copy option.
(xvi) The bond testing module, which uses pulse ultrasonic spectroscopy technique for evaluation of bond integrity between two acoustically mismatched materials like metal /elastomer interfaces or skin-core structures with highly damped core material.
(xvii) Means for calculating and displaying probability distribution plots of 'slenderness parameter' pertaining to the above bonded structures, determining the debonded area and making consolidated report on system computer with hard copy option.
Thus according to the present invention, there is provided an integrated high ~~ quality flaw imaging and bond testing apparatus, said apparatus comprising a probe including an ultrasonic transducer for interrogating a test object and receiving ultrasonic signals, said probe being capable of being moved on its surface along two predetermined programmed axes and maintaining its perpendicularity with respect to said test object, a pulser means connected to said transducer for energising said transducer, an amplification and rectification means connected to said ultrasonic transducer for receiving said ultrasonic signals and subjecting them to amplification and rectification, peak detection means connected to said amplification and rectification means, for detecting the maximum value of the signal occurring within a predetermined time gate, a time of flight gate means connected to said peak detection means for monitoring the time of flight of said signal whenever a pre-set amplitude
threshold is crossed, a first analog to digital converter connected between said peak

detection means and a system controller for converting the peak values detected by said peak detection means into digital data for display as a colour coded image in said system controller, said time of flight gate means being operationally connected to a time-to- voltage converter configured to receive the time of flight gate data, said time to voltage converter being connected to the said first analog to digital converter for digitizing the time of flight values, said amplification and rectification means being also connected to a quadrature controlled flash analog to digital converter which is in turn connected to said system controller for digitizing and capturing full unrectified RF waveforms alongwith corresponding positional information.
Brief description of the accompanying drawings
Fig 1 a) Immersion mode of testing
Fig 1 b) Contact mode of testing
Fig 2 System block diagram
Fig 3 a) Portable ultrasonic equipment; operational steps
(imaging mode)
Fig 3 b) Portable ultrasonic equipment; operational steps
(bond testing mode)
Fig 4 Quadrature controlled flash ADC: block diagram
Fig 5 a] Off-line defect analysis: flow diagram
Fig 5 b) Signal restoration technique: flow diagram
Fig 6 Ultrasonic signal before and after restoration:
Typical example
Fig 7 a) Bond testing module: physical principle
Fig 7 b) Bond testing module: typical output (well bonded case)
Fig 7 c) Bond testing module: typical output (debonded case)
Detailed description of the invention
The present invention is of a portable contact mode ultrasonic imaging system cum bond-tester (a) for imaging objects of all materials in general and layered composite materials in particular, for detection and analysis of defects (b) for testing of bond-integrity between two acoustically mismatched materials e.g. metal/rubber adhesive bonded structures, skin-core structures with highly damped core material etc.
The principles and operations of the above systems according to the present invention may be better understood with reference to the drawings listed above and the accompanying description.
Fig. 1 (a) shows the probe configuration generally used in immersion mode of ultrasonic testing for producing high quality images of the test object, which demands excellent coupling condition. The test piece here is held just beyond the near-field of the scanning transducer and the space between the transducer and the test object is kept always filled with a stationary (in immersion mode) or a moving (in squirter mode) water column.
In the simpler contact mode of testing (which generally is used in wide variety of applications, other than advanced imaging) used in the present invention, the distance between the transducer and the test object is maintained by attaching a solid cylindrical piece of polymeric material (the delay-line) onto the probe surface (shown in Fig. 1b) which in turn is coupled to the test object through a very thin layer of coupling fluid. Thus coupling of transducer to the test object is achieved here without associated water mess/special water-jet facility, as is evident from Figs. l(a) and l(b). However the simplicity of contact mode of testing also gives rise to associated problems, as explained below. Additional reflection of the ultrasonic wave from the front surface of the delay line invariably interferes with the echo generated by the front wall of the test object, thereby deteriorating the quality of the signal and consequently the quality of the image. Such signal degradation has been tackled in the present invention, by developing a novel 'signal restoration technique' which has been described in details later in this section.
Figs. 2 & 3 together illustrate a preferred embodiment of the ultrasonic test equipment, constructed and operative according to the teaching of the present invention (a) for imaging test object (initially for real-time defect detection and subsequently for more detailed defect analysis through full RF capture and off-line analysis) and (b) for testing of bond integrity at the interfaces between two acoustically mismatched materials e.g. metal/elastomer interfaces etc.
The ultrasonic system as per the system block diagram of Fig.2 includes. /. The probe (1) (i.e. the ultrasonic transducer) mounted on the probe-holder of the mechanical scanner, which is driven along two axes by two stepper motors controlled by the motor control circuit (comprising units 2,3,4,5 of block diagram) in Fig.2, coupled finally to the main computer.
//. The transmitter unit (6) gets activated through the master clock (7) and the timer synchronizer unit (8), every time the transducer attains a pre-defined position in course of its scanning of the test object. Consequently, the piezoelectric crystal of the transducer gets energized, either by a spike voltage or by a square voltage, as per user's selection effected via main computer.
III. As per the preferred embodiment of the present invention, the output from the
transducer can be processed in two different ways.
During the initial set of scans (aimed only at quick real-time detection of defect zones of the test object) the ultrasonic signal received by the transducer (1) is amplified and rectified by unit No. (9) and then processed for detection of peak voltage and corresponding time of flight (TOF) for on-line imaging of the scanned area of test object. The real time image thus is based only on peak-detection of the rectified signal, occurring within user selected time gate. It may be noted that the full RF wave capture facility (shown in the flow diagram of Fig. 3a) is not activated at this stage of testing. It becomes operational only during subsequent scans of few selected defect zones of the test object, for full wave capture and off-line defect analysis. The sequence of operational steps mentioned above, is described in the flow diagram of Fig. 3a.
IV. A detailed description of the real time imaging unit is given below.
A real time display of the selected time-window is obtained, by passing the portion of the ultrasonic signal situated within the time-window, to a peak detector (10). The peak values thus detected are converted into 12 bit digital data by a 12 bit slow ADC (15) for display as colour coded image on the monitor of the main computer (6).
Simultaneously, the time of flight value is measured by transforming the pulsed ultrasonic RF signals into TTL signal levels, by use of fast comparators. The comparator levels are set to be above user selected threshold levels. A new gate called 'time of flight gate' (11 of Fig.2) is formed by the next ultrasonic signal, which crosses the preset threshold value.
The time of flight gate is then fed into a time-to-voltage converter circuit (13). This transforms 'time from the start of the gate to the first echo above threshold' into a linear ramp, whose output voltage is directly proportional to TOF gate time. This TOF value is also digitized by the 12-bit slow ADC (15) mentioned earlier.
In order to produce simultaneous display of amplitude and TOF images in real-time, the input to the 12 bit ADC is multiplexed by a multiplexer unit (12),
controlled by the multiplex controller (14) coupled to the main computer (6). The digitized data corresponding to the peak amplitude value, the time of flight value and the positional coordinates of the interrogating transducer are all read by the main computer and displayed simultaneously as two real-time colour-coded images on the monitor screen.
The features of the equipment described so far are essentially similar to any conventional ultrasonic imaging equipment, except for the fact that it uses a simple easy-to-use contact-mode test configuration, and yet aims at producing high quality ply-by-ply images through full RF wave capture in defective zones, plus signal processing capabilities. It also aims at integrating bond testing capability with this portable imaging system.
1. It is a special feature of the preferred embodiment that the full unrectified RF signals generated during the defect-analysis scans of defect-detected zones, are digitized by a special quadrature-controlled 8 bit flash ADC (16 of Fig. 2) and read along with the corresponding transducer locations by the main computer. The data thus generated are stored in memory and used off-line for (i) advanced imaging/defect-analysis and (ii) bond-testing operations, as explained in the flow diagrams of Figs. 3a and 3b respectively. The details of these operations are described later in this section.
Unique design features of the flash ADC (16) are explained below. The frequency of conversion required for full RF capture in an ultrasonic imaging equipment is determined (i) by the highest frequency signal generated by the ultrasonic transducer (i.e. by the nominal frequency and its band width) following Nyquist criteria and (ii) by the required thickness resolution of the object under inspection. However, using a high frequency ADC as per this requirement often becomes quite expensive.
A more cost effective way of achieving the required speed of AD conversion as per the above criteria, by actually using a lower frequency ADC capable of phase reversal under quadrature sampling scheme, has therefore been developed. Data
conversion is accomplished here under the dynamic condition of transducer movement, as described below.
Time interval between two consecutive steps of the scanner is so chosen that at least two samples are generated on-fly within the minimum sampling time interval, ultimately required by the ultrasonic scanning system. Every alternate ultrasonic signal generated on-fly is then fed into the AD converter, (having its actual sampling frequency = 0.5 x 'ultimate frequency of sampling required by the equipment) after effecting phase reversal by 90°. Thus two sets of data, sampled at half the required frequency and phase-shifted by 90° w.r.t. each other, combine to give the full ADC performance, i.e. the effective sampling frequency is twice the actual sampling frequency used in the system. The details of this quadrature controlled flash ADC are shown in the block diagram of Fig.4.
For synchronization and control of the conversion phases described above, the master system clock is used. The digitized data are fed into the system computer, recombined in memory and processed to form a composite data stream. Advantage of this quadrature sampling technique lies in reduction of (i) the actual sampling speed of the ADC hardware and (ii) speed requirement of each subsequent semiconductor device that follow the ADC, e.g. the FIFO memories shown in Fig.4. This results in effective reduction of the cost of ADC, without sacrificing its operational quality.
VI. Another special feature of the present invention lies in a novel signal restoration technique, used for restoring the 'ideal reflected signal' free from the distortions (i) due to contact mode of testing (i.e. delay-line effect) (ii) due to couplant thickness variation.
Thus, in a preferred embodiment, the apparatus of the present invention essentially comprises a probe (1) held in a self aligning probe-holder by mechanical means, provided with a synchronized pulser circuit (6,8) and carried by a programmable dual axes portable scanner driven by the motor control means (2,3,4,5), output of the said probe being connected to an amplification/rectification means (9), a
peak detection means (10), a time of flight gate means (11) and a time-to-voltage conversion means (13).
The peak-detection means (10) and the time-to-voltage conversion means (13) are connected to a multiplexer means (12,14) and finally through a 12-bit slow AD conversion means (15) to the system controller (6).
Output of the said probe are also connected via unrectified signal option of the amplification/rectification means (9) to synchronized and quadrature-controlled flash ADC (16) to the system controller (6).
In one embodiment of the invention, the said peak detector (10) and time of flight gate (11) with time to voltage converter (13) are provided with a multiplexer means (12,14) at the output in conjunction with positional information of probe (1), in order to produce simultaneous display of amplitude and TOF images in real time on the monitor of the system controller during initial scans of the test object.
In another embodiment of the invention related to defect analysis and bond testing means, the amplified but unrectified signal from position synchronized probe (1) is taken through quadrature-controlled flash ADC means (16) to the system controller (6).
The said quadrature-controlled flash ADC means (16) comprises a positional synchronization pulse generating means operatively connected to a phase generating means, said phase generator being operatively connected to a master clock-controlled selector means and to a flash analog to digital conversion means with its output connected to memory means, the phase generating means being connected to a sequencing means and a bus interface means, said bus interface means interacting with an instrument data bus, the entire circuit being connected to the system controller (6). The display means finally produces full RF wave-based images on the monitor of the system controller (6) for defect analysis and also for bond testing.
The physical principles and the computational methods underlying this signal-restoration technique are described below.
It may be recalled here that in contact mode testing, the experimentally determined apparent FWE' is actually formed by superposition of 'true FWE' and the
'delay-echo', time separated due to the couplant layer thickness (Ref. Fig. 1b). The objective of the signal restoration technique is therefore to compute the 'true FWE' from the 'apparent FWE' captured by the contact-mode ultrasonic test at every scan position.
In principle, it should be possible to restore the true FWE by simple subtraction, if a 'reference delay-echo' is recorded by simply immersing the transducer with the attached delay-line in water and if the exact time separation due to couplant thickness could be ascertained in each case. However, in addition to this, a straight subtraction becomes problematic if the trigger points of the data acquisition system in the 'experimental waveform' and in the 'reference delay echo' are not perfectly matched. Moreover every single automated scan, even for a seemingly small 100 mm x 100 mm scan area, generates several thousand RF signals, each having 256/512/1024 data points. Handling such large size of data during actual inspection under field condition is impossible.
The above problem has therefore been tackled by the computational steps summarized in the flow diagram of Fig.5 (b) and explained below.
a) The RF waveforms captured during scanning of a selected defect zone are
subjected to data compression by pattern matching technique. The waveforms
having highest correlation coefficient defined by
(Figure Removed)
where x= {xi} and y = {yi} with i = 0,l,2.....N-l
are designated as 'same'. A set of waveforms obeying the sameness criterion are averaged to produce a particular 'distinct waveform' with its corresponding 'repeat counf signifying its probability of occurrence. The most probable distinct waveform (MDSW) is determined for every scan set. Only distinct waveforms (typically 20/30 in number against a scan comprising
several thousand waveforms) are stored in memory. This gives tremendous advantage in terms of memory space and computational efficiency. b) Similarly, the RF waveforms captured by immersing only the transducer and the attached delay-line in coupling fluid over a sufficiently long time window, is subjected to data compression by pattern matching technique. The distinct waveform with maximum repeat-count is designated in this case as the most probable delay-echo (MDE).
VII. A straight subtraction of (MDE) the most probable-delay-echo from the most
probable-distinct-waveform (MDCW) generated on a defect free calibration sample
returns the 'true-front-wall-echo' (TFWE).
VIII. For all distinct waveforms, generated during a scan of the test object the
following procedure is followed:

a) Every distinct waveform is cross-correlated with MDE and the position of the
correlation peak 't' is determined on the time axis.
b) Next the most probable delay echo (MDE) time shifted to t, is subtracted from
the corresponding distinct waveform.
c) The resultant waveform of the previous step is compared with the 'true-front-
wall-echo' (TFWE) computed in (Hi) above, in terms of both shape and
amplitude.
d) MDE is then left-shifted on the time axis by steps of one time-interval of flash
ADC in an iterative manner, till the above matching exercise is found
satisfactory.
e) The final subtracted waveform in each case gives the 'restored signal', which
alone is used in all subsequent signal processing/imaging operations for
maintaining high quality of image.
J) Next, impulse response of restored signal is computed by Wiener filtering, using the expression
(Formula Removed)
where y(t), x(t), h(t) are restored experimental signals, restored reference signal
and impulse response function respectively and X(□), Y(□), H(□) are the
corresponding Fourier Transforms. The constant 'K1 is an arbitrary desensitizing
parameter.
The analytic signal of the impulse response is computed by using the expression
Analytic[h(t)] = h(t)+jH[h(t)] where H represents Hilbert Transform
g) Magnitude of analytic signal represents the signal envelop, which is time-sliced for producing ply-by-ply image, 3-D visualization of the defect zone, plus standard image processing.
IX. Another special feature of the present invention lies in a bond testing module, based on a novel pulse spectroscopy technique, which is easily integrated with the system hardware described in Fig.2. This module is particularly useful for testing bond integrity between two severely mismatched acoustic surfaces, e.g. metal/rubber interfaces. The physical principle underlying this technique and associated equipment details are described below:
a) A broadband transducer of suitable nominal frequency (as per the criteria
explained later in this section) is used, first for scanning a metal-alone
calibration piece and then the actual bonded structure.
b) Multiple reflections in pulse-echo full RF mode are captured in the time
domain, by using the quadrature controlled flash ADC described in Figs.2 and
3, with provision for user selectable sampling rate.
X The record-length and the sampling rate of data acquisition are so adjusted that adequate frequency resolution is achieved, when these signals are viewed in
frequency domain, by running Fast Fourier Transform programme through the main computer.
XI. It is well known that the spectrum of the signal captured in pulse echo mode,
consists of a series of equispaced dips (due to phase reversal on reflection) governed
by the resonance frequencies of thickness vibration of the metal plate.
XII. If the above plate is bonded with a low acoustic impedance flexible layer on the
other side, then the resonance dips get affected due to the damping characteristics of
this flexible layer. A typical spectrum with equispaced dips is shown in Fig. 7 (a) (top
diagram) whereas the zoomed displays of selected dips, corresponding to the bonded
and debonded cases are presented in the bottom parts of the same diagram.
It may be noted that the long slender dip of the debonded case (i.e. metal alone) gets transformed into a broad and short dip in the bonded case. Thus a slenderness
parameter 'K' is defined such that
(Formula Removed)
XIII. The value of K is high in case of debonding and low in case of well-bonded interface.
XIV. Typical probability distribution of slenderness parameter 'K', corresponding to
the calibration piece (metal alone) and metal/rubber bonded structures, are presented
in Figs. 7b and 7c for well-bonded and debonded cases respectively. On the basis of
the RF waveforms captured during the scan, the system computer computes and plots
the above probability distribution curves, where the overlapped area between the
calibration curve and the experimental curve signifies the area debonded as
percentage of the total area under the experimental curve. (Ref. Fig. 7c).
Nominal frequency of the interrogating transducer is carefully selected, such that the two probability distribution curves are well separated (i.e. without any overlap) in case of well-bonded joints (tested initially for system calibration).
XV. Variation of K value against spatial coordinates of the bonded surface is plotted
in form of a binary image (with user selectable threshold) by the main computer. The
same is displayed on the monitor with hard-copy option.
While the invention has been described here with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made

We Claim:
1. An integrated high quality flaw imaging and bond testing apparatus, said
apparatus comprising a probe including an ultrasonic transducer for
interrogating a test object and receiving ultrasonic signals, said probe being
capable of being moved on its surface along two predetermined programmed
axes and maintaining its perpendicularity with respect to said test object, a
pulser means connected to said transducer for energising said transducer, an
amplification and rectification means connected to said ultrasonic trasducer for
receiving therefrom said ultrasonic signals and subjecting them to amplification
and rectification, peak detection means connected to said amplification and
rectification means, for detecting the maximum value of the signal occurring
within a predetermined time gate, a time of flight gate means connected to said
peak detection means for monitoring the time of flight of said signal whenever a
pre-set amplitude threshold is crossed, and an analog to digital converter
connected between said peak detection means and a system controllen, for

converting the peak values detected by said peak detection means into digital
data for display as a colour coded image in said system controller, said time of flight gate means being operationally connected to a time-to-voltage converter configured to receive the time of flight gate data, said time to voltage converter being connected to the said analog to digital converter for digitizing the time of flight values, said amplification and rectification means being connected to a quadrature controlled flash analog to digital converter which in turn is connected to said system controller.
2. An apparatus as claimed in claim 1 wherein the said analog to digital converter
is provided with a multiplexing means at the input thereof in order to produce
simultaneous display of amplitude and time of flight (TOF) images in real-time
coupled to the system controller.
3. An apparatus as claimed in claim 1 wherein said quadrature-controlled flash
analog-digital converter comprises of a positional synchronization pulse
generation means operatively connected to a phase generating means, said
phase generator being operatively connected to a selector and to a flash analog
to digital operatively connected to a selector and to a flash analog to digital conversion means with its output connected to a memory means, the phase generating means being connected to a sequencing means and a bus interface means, said bus interface means interacting with an instrument data bus, the entire circuit being connected to the system controller, for digitizing full unrectified RF signals generated during final scanning of defect zones.
4. An apparatus as claimed in claim 1 wherein the display means of the system
controller displays resolution enhanced ply-by-ply images of the defect zone,
using full RF signals captured by the quadrature controlled flash analog digital
converter (ADC), in conjunction with synchronized positional information
during final scan of the defect zone.
5. An apparatus as claimed in any preceding claim wherein said probe including
said ultrasonic transducer is mounted on a mechanical scanner, said scanner
being driven by a pair of stepper motors controlled by said system controller.
6. An apparatus as claimed in any preceding claim wherein a gate forming circuit
is connected to said peak detection means for selectively transmitting a portion
of the ultrasonic signals to said peak detection means during initial scans for
capturing real-time images for quick detection of defects in the test object.
7. An apparatus as claimed in any preceding claim wherein said system controller
is a computer.
operatively connected to a synchronized master clock, which with the help of data bus and address bus coordinates all the operations in such a manner that movement of mechanical scanner including capture of its positional coordinates, transmission of energizing pulse to the transducer on its arrival at each scan position, formation of gate, initiation and control of quadrature pahse-shifted A/D conversion and sequencing of data transfer, are all synchronized.
An apparatus as claimed in claim 2 wherein time of flight gate is connected to a
time to voltage converter for transforming the 'time from the start of the gate to
the first echo above threshold time of flight (TOF) into a linear ramp, whose output voltage is directly proportional to the time of flight (TOF) time.
9 An apparatus as claimed in claim 9wherein the output of said time to voltage
converter is connected to said analog to digital converter for digitization of said
time of night (TOF) value.
An apparatus as claimed in claim 7 wherein said peak detection means and
10 time of flight gate with said time to voltage converter are connected to said multiplexer means at their outputs in conjunction with positional information of said probe in order to produce simultaneous display of amplitude and time of flight (TOF) images in real time on the monitor of the system controller during initial scans of the test object.
11 An integrated high quality flaw imaging and bond testing apparatus substantially as herein described with reference to the accompanying drawings.

Documents:

685-del-2001-abstract.pdf

685-del-2001-claims.pdf

685-DEL-2001-Correspondence Others-(29-03-2011).pdf

685-del-2001-Correspondence-Others-(14-12-2010).pdf

685-DEL-2001-Correspondence-Others-(19-11-2009).pdf

685-del-2001-Correspondence-Others-(28-04-2011).pdf

685-del-2001-correspondence-others.pdf

685-del-2001-correspondence-po.pdf

685-del-2001-description (complete).pdf

685-del-2001-drawings.pdf

685-del-2001-form-1.pdf

685-del-2001-form-13.pdf

685-DEL-2001-Form-15-(19-11-2009).pdf

685-del-2001-form-19.pdf

685-del-2001-form-2.pdf

685-del-2001-form-3.pdf

685-del-2001-gpa.pdf

685-del-2001-petition124.pdf

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Patent Number

218347

Indian Patent Application Number

685/DEL/2001

PG Journal Number

21/2008

Publication Date

23-May-2008

Grant Date

31-Mar-2008

Date of Filing

20-Jun-2001

Name of Patentee

THE ADDITIONAL DIRECTOR (IPR)

Applicant Address

DEFENCE RESEARCH AND DEVELOPMENT ORGANISATION (DRDO), MINISTRY OF DEFENCE, GOVERNMENT OF INDIA, B-341, SENA BHAVAN, DHQ P.O., NEW DELHI-110001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	LAHIRI JHUMUR	DEFENCE RESEARCH AND DEVELOPMENT, LABORATORY, P.O. KANCHANBAGH, HADERABAD 500 058, INDIA.
2	KULKARNI VILAS GOPAL	VIVACE SONICS PVT.LTD.,1-3-29/3 NANDANVAN COLONY,4th STREET,HABSIGUDA,HYDERABAD 500 007,INDIA
3	ZACHARIA SANJITH GEORGE	DEFENCE RESEARCH AND DEVELOPMENT LABORATORY,P.O KANCHANBAGH HYDERABAD 500 058,INDIA
4	BISWAS RATHINDRA NATH	E 20/1,LAB QUARTERS,KANCHANBAFH,HYDERABAD 500 058,INDIA
5	SEKHAR PONNURU PADMA	VIVACE SONICS PVT.LTD.,1-3-29/3 NANDANVAN COLONY,4th STREET,HABSIGUDA,HYDERABAD 500 007,INDIA

PCT International Classification Number

G01N 29/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA