US20100312496A1 - Devices, systems and methods of detecting defects in workpieces - Google Patents

Devices, systems and methods of detecting defects in workpieces Download PDF

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US20100312496A1
US20100312496A1 US12/745,983 US74598308A US2010312496A1 US 20100312496 A1 US20100312496 A1 US 20100312496A1 US 74598308 A US74598308 A US 74598308A US 2010312496 A1 US2010312496 A1 US 2010312496A1
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workpiece
transducer
signal
detecting
defect
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Peter Robert Armitage
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University of Exeter
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University of Exeter
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/04—Analysing solids
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/04—Analysing solids
    • G01N29/043—Analysing solids in the interior, e.g. by shear waves
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/04—Analysing solids
    • G01N29/12—Analysing solids by measuring frequency or resonance of acoustic waves
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/34—Generating the ultrasonic, sonic or infrasonic waves, e.g. electronic circuits specially adapted therefor
    • G01N29/348—Generating the ultrasonic, sonic or infrasonic waves, e.g. electronic circuits specially adapted therefor with frequency characteristics, e.g. single frequency signals, chirp signals
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/36—Detecting the response signal, e.g. electronic circuits specially adapted therefor
    • G01N29/42—Detecting the response signal, e.g. electronic circuits specially adapted therefor by frequency filtering or by tuning to resonant frequency
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/44—Processing the detected response signal, e.g. electronic circuits specially adapted therefor
    • G01N29/4445—Classification of defects
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00—Indexing codes associated with group G01N29/00
    • G01N2291/01—Indexing codes associated with the measuring variable
    • G01N2291/014—Resonance or resonant frequency
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00—Indexing codes associated with group G01N29/00
    • G01N2291/10—Number of transducers
    • G01N2291/106—Number of transducers one or more transducer arrays
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00—Indexing codes associated with group G01N29/00
    • G01N2291/26—Scanned objects
    • G01N2291/269—Various geometry objects
    • G01N2291/2694—Wings or other aircraft parts

Definitions

  • This invention relates to devices, systems and methods for use in detecting a defect in a work piece and in particular, but not exclusively, to detecting defects in parts of aircraft.
  • the present invention describes a number of embodiments, which serve to mitigate one or more of these problems.
  • the invention consists in the device for use in detecting a defect in a workpiece including:
  • a broad band transducer for contacting the workpiece;
  • a driving circuit connectable to the transducer to operate the transducer in an actuator mode;
  • a signal output circuit connectable to operate the transducer in a sensor mode; and
  • a control for selectively connecting the transducer to the driving circuit or the signal output circuit.
  • the device includes a data interface and the microprocessor may form both control and the interface.
  • the microprocessor may be linked to at least one of an analogue to digital converter; a digital-to-analogue converter; a data memory; a wireless connection and a communication link.
  • the driving circuit may include a pair of oscillators for generating signals of frequency F 1 and F 2 respectively and summer for summing the signals F 1 , F 2 to provide a driving signal for the transducer.
  • the invention may also include a defect detecting system including an array of devices as set out above in a central computer linked to the interface of each device by a communication link.
  • the computer may be arranged for generating control signals to be sent by the communication link to instruct the controls of the respective device to operate the associated transducer in an actuator or sensor mode.
  • the array may be in the form of a grid for extending across a workpiece in which case the computer may operate the devices in a time reversal mode and the location of the time reverse defect generated signals may be determined by determining the location of the sensor nearest the focus of the time reverse defect generated signals.
  • the devices may be embedded in the workpiece.
  • the system may additionally include a dedicated actuator, such as a narrow band actuator.
  • a dedicated actuator such as a narrow band actuator.
  • the system may additionally include a dedicated sensor.
  • the system may monitor the signal received by at least one device, which is in sensor mode, for one or more harmonics, such as a third harmonic, in the output signal and may produce an output in response to that detection.
  • harmonics such as a third harmonic
  • system may include electronic circuitry to monitor the signal frequency and may produce an output in response to that detection.
  • the invention consists in a system for detecting defects including a plurality of transducers each having an interface and a configuration device for configuring at least some of the transducers as either a sensor or an actuator, a communication link connecting the interfaces to a central computer, whereby the computer can instruct the configuration devices via the interfaces to configure their respective transducer.
  • the invention consists of a method of detecting micro-cracking or de-lamination in a workpiece including: inputting a sonic or ultrasonic impasse into the workpiece; detecting the resultant signal in the workpiece; monitoring the signal to detect the appearance of harmonics, such as the third harmonic of the impulse frequency and thereby detecting the presence of micro-cracking or de-lamination.
  • the method may also include monitoring the signal to detect the subsequent variation in the relative magnitude of the harmonics, such as the reduction in or disappearance of the third harmonic, and thereby detecting the imminent transition to structural failure.
  • the invention includes a method of detecting a defect in a workpiece including inputting into the workpiece two signals at respective frequencies F 1 and F 2 to create vibrations in the workpiece, detecting a resultant output at another location on the workpiece with wideband receiver and monitoring one or more of the resultant harmonics, overtones or sidebands to determine the presence or absence of a defect.
  • Another aspect of the invention includes a method of detecting a defect in a workpiece including:
  • step of monitoring includes feeding the detected signal through a phase lock loop including a voltage controlled oscillator VCO having the sonic input frequency as its input signal and determining the frequency shift from the VCO control voltage.
  • Another aspect of the invention includes a method of detecting a defect in a workpiece including inputting two sonic ultrasonic impulses into a workpiece, the impulses being of equal amplitude but 180° out of phase, detecting the resultant vibrations in the workpiece to generate respective output signals, summing the output signals and monitoring the amplitude of the summed signals for determining the presence or absence of a defect.
  • the invention consists in the broad band piezoelectric transducer including:
  • a piezoelectric element (a) a piezoelectric element; (b) an acoustic matching front plate; and (c) a tungsten particle containing epoxy resin backing block for providing acoustic attenuation over a range of operating frequencies.
  • the backing block may contain tungsten particles of 250, 25 and ⁇ 1 micron diameters. It may additionally or alternatively include a mounting rod extending through at least part of the backing block. Preferably a mounting rod is of a semi-rigid material.
  • FIG. 1 is a stylised view of a transducer unit
  • FIG. 2 is a picture of a transducer unit for use in the device of FIG. 1 ;
  • FIGS. 3 and 4 are views from above and below respectively of an actual unit of the type illustrated in FIG. 1 ;
  • FIGS. 5 to 8 illustrate a variety of electronic configurations for use in the device of FIG. 1 ;
  • FIGS. 9 to 12 illustrate a variety of configuration transducers devices linked to a central computer for use in different test modes
  • FIGS. 13( a ) to ( c ) show a number of device locations on the test workpiece, which incorporates a defect
  • FIGS. 14( a ) to ( c ) show the corresponding outputs
  • FIGS. 15( a ) to ( g ) illustrate the output of a transducer device in sensor mode for a workpiece which is stimulated by a transducer device in transmission mode with increasing loading on the workpiece;
  • FIGS. 16( a ) to ( d ) form a sequential description of time reversal defect detection
  • FIGS. 17( a ) to 17 ( d ) show waveforms connected with another embodiment
  • FIG. 18 is a graph illustrating non-linear stress v strain
  • FIG. 19 is a graph showing the resultant wave form distortion
  • FIG. 20 illustrates schematically transducer configurations on concrete samples
  • FIG. 21 is photographs of cracked and undamaged concrete samples
  • FIG. 22 shows test results from the samples of FIG. 21 ;
  • FIG. 23 is a photograph of a concrete sample prepared for testing
  • FIG. 24 illustrates test results from the sample of FIG. 23 ;
  • FIG. 25 shows the combination of two sine waves
  • FIG. 26 illustrates the inter-modulation products resulting from the use of combined sine waves
  • FIG. 27 illustrates a test configuration and test samples
  • FIGS. 28 and 29 are test results of test on the samples of FIG. 27 ;
  • FIGS. 31 and 32 show comparable results for a through core test and a surface core test respectively.
  • FIG. 33 is a schematic circuit of a detection device.
  • Ultrasonic techniques for detecting defects in workpieces are well known and a variety of transducers are utilised.
  • an ultrasonic transducer is passed over the surface of a workpiece sending ultrasonic pulses into, the workpiece and the reflected signal is detected.
  • a transmitter is placed at one location on a workpiece and one or more receivers on the workpiece detect the resultant signal.
  • the transducers and receivers are designed very specifically for a particular purpose and tend only to be able to handle a narrow band of signals.
  • the techniques used can also be very workpiece specific. Problems have also been encountered when trying to use such techniques with composite materials.
  • transducers and methodology which is more flexible, and in particular, is capable of determining whether or not a component has a defect both quickly and easily.
  • the Applicants have developed a number of approaches in this connection, which are in part based on the insight that if they could configure a broadband ultrasound transducer, which could be operated either as a transmitter or a receiver then it would be possible to implement a wide range of techniques, which the user could select in accordance with the issues that needed to be dealt with.
  • the device is capable of being pre-programmed with the desired waveform data and consequently able to transmit a variety of complicated waveforms, some examples that have been evaluated are sine wave bursts of a specific duration, sine wave sum bursts and frequency sweeps.
  • the microprocessor 33 is connected to SRAM memory which is addressed by an independent counter, the ND converter 31 or the D/A converter 32 directly accesses this memory when the device is either sampling a received signal or transmitting a pre-programmed waveform, the counter being incremented for each data sample received or transmitted.
  • the counter can be made to count in reverse, that is decrement.
  • the microprocessor can implement a pre-determined series of count instructions to the counters, thereby allowing the data to be fetched or loaded in any part of the memory space. This method can be usefully employed when using time reversal techniques that require the data to be filtered for harmonic content.
  • FIGS. 1 and 2 illustrate aspects of a basic transducer assembly.
  • the transducer 10 comprises a piezoelectric crystal 11 mounted on a tungsten epoxy backing 12 .
  • a spring loaded threaded nylon rod 13 and nut 14 is used to secure the transducer into its housing and provide a force when the transducer is placed in contact with the work piece.
  • Electrical connections 15 also pass through the backing 12 , whilst a matching front plate 16 is mounted on the front surface of the crystal 11 .
  • the piezoelectric is preferably composed of a epoxy/PZT composite and the ratio of PZT to Epoxy can be changed to match the workpiece material.
  • the workpiece is made of aluminium, aluminium has an acoustic impedance of 12MRayls and the PZT/Epoxy element has also 12MRayls acoustic impedance, and likewise the tungsten/Epoxy backing material has an acoustic impedance of 12MRayls.
  • the tungsten, in the epoxy, has typically been made of spheres of a single size according to the frequency at which the transducer has been intended to be used. However, for lower frequencies, where large spheres are required, this has been particularly unsuccessful because only a limited number of spheres per unit volume could be inserted.
  • the Applicants have realised that they can overcome the restricted frequency range of such transducers by having a range of sphere sizes, which not only enables them to achieve the desired density of tungsten, but also provides spheres of requisite size for different frequencies.
  • a backing block containing tungsten particles of 250, 25, and less than 1 micron diameters has provided a particularly practical transducer.
  • the semi-rigid nature of the nylon rod 14 is also efficacious as this would not introduce resonators in the backing.
  • the transducer 10 is mounted in a frame 17 , with associated circuit boards 18 . Examples of such overall box assemblies are shown, indicated 19 , at FIGS. 3 and 4 .
  • FIG. 5 shows the circuitry for one such arrangement 19 , where analogue signals are utilized, throughout for the signal processing.
  • the transducer 10 can receive and transmit signals to relay controlled multiple pole switch at 21 . If it is acting as a receiver, then the switch is connected as shown in FIG. 5 and its output signal passes to a differential charge amplifier 22 and thence to another amplifier and level converter 23 , whose output is fed to a further switch 24 and thence to an interface 25 , which can be connected to a data bus. If the transducer is in transmit mode, switches 24 and 21 are switched to form a separate channel. In this case the transmission signal passes through an amplifier and level converter 26 and a power amplifier differential output 27 and thence to the transducer 10 through switch 21 . The position of the switches 21 , 24 is controlled from the interface 25 on output 28 and the interface also provides the necessary power supply at 29 . It may also adjust the gain of the amplifiers 23 , 26 on output 30 .
  • the transducer 10 can operate on a broadband and because it can be remotely configured as a transmitter or a receiver via the interface 25 , the Applicants have created an extremely flexible arrangement for use in defect detection.
  • FIG. 6 shows the corresponding arrangement in which the signals to be transmitted or received are respectively analogue to digitally or digitally to analogue converted by the respective converters 31 , 32 , with the microprocessor 33 acting as the interface.
  • the transducer is connected to the amplifier module, which essentially substitutes for components 22 , 23 , 26 and 27 in FIG. 5 . It will be understood that the advantage of using digital signals is that it is easier to communicate these with a central computer.
  • FIG. 7 shows a further development in which the control circuitry includes sine wave generators 34 , 35 for generating respective frequencies in F 1 and F 2 . These can be summed together at 36 for embodiments where it is desirable for the transducer 10 to transmit two frequencies simultaneously.
  • FIG. 8 shows a similar “analogue” version for transmitting signals F 1 and F 2 and otherwise corresponds to FIG. 5 .
  • FIG. 9 illustrates a basic system configuration.
  • Two units 19 are applied to a workpiece 37 with the right hand one set to transmit and the left hand one set to receive. They are connected to a communication link 38 , e.g. a data bus which is in turn connected to a local computer 39 , which may be connected to other computers or a mainframe computer by an internet connection 40 .
  • a communication link 38 e.g. a data bus which is in turn connected to a local computer 39 , which may be connected to other computers or a mainframe computer by an internet connection 40 .
  • FIG. 10 illustrates an alternative arrangement which is essentially the same as FIG. 9 except that at 41 a unit 19 is used to operate either an actuator or a hammer schematically illustrated at 42 .
  • FIG. 11 exactly corresponds to FIG. 9 but recognises the possibility of transmitting two pulses.
  • FIG. 12 illustrates units 19 configured in an array across the workpiece.
  • the central unit 19 is set to transmit and the surrounding units are set to receive. It will be appreciated that a variety of arrays can be built up, for example five by five.
  • the basic configuration illustrated in FIG. 9 could be used in a typical industry way wherein the unit 19 set to transmit transmits a suitable resonant frequency an the receiving unit records the resonant decaying signal. If there is a defect there will be a shift in frequency as the amplitude decays.
  • FIG. 16 A person skilled in the art will also be familiar with the concept of time reversal, but for convenience this is set out in FIG. 16 .
  • the Applicants' ability to build extremely flexible arrays with transducers which can operate both as receivers and transmitters makes the achievement of time reversal much more than a theoretical possibility and enables the initial pilot signal (the source in FIG. 16 ) to be generated at any point in the array.
  • the Applicants have realised that if the time reverse retransmission is made at high power then considerable energy will be delivered at the point of origin of the source. In this way highly focused energy can be provided at any point on the surface of the test material underneath the array. If there should be a defect at the refocusing position itself, then harmonics will be generated and these will be observed by the surrounding units 19 as well as the unit at the refocusing point.
  • the relationship between the units 19 in their respective transmit (T) and receive (R) modes with the defect can be any of the instances illustrated in FIGS. 13 ( a ), ( b ) and ( c ). Corresponding outputs are shown in FIGS. 14 ( a ), ( b ) and ( c ). It will be particularly noted that in FIGS. 14 ( a ) and 14 ( b ) the third harmonic initially rises sharply under low impact, but then falls away as the second harmonic rises under higher impact. It is considered that the rising third harmonic indicates where the workpiece is in a condition of micro-cracking, but when delamination occurs then this falls away and the second harmonic rises rapidly. The Applicants have therefore developed an approach which enables the detection of micro-cracking and distinguishing this from the onset of de-lamination. They are not aware of any other system which can achieve this. The effect is also illustrated in the graphs of FIG. 15 .
  • FIG. 17 illustrates another mode of operation in which two impulses of equal frequency and amplitude but phase shifted 180° are transmitted.
  • FIG. 17 ( a ) shows the received signals from the workpiece which are generated by the impulses and
  • FIG. 17( b ) shows their summed output.
  • FIGS. 17 ( a ) and ( b ) illustrate the situation where there, is no defect.
  • FIGS. 17 ( c ) and ( d ) are the corresponding results where there is a defect. It will be noted that in FIG. 17( d ) there is a clear output, whereas in FIG. 17( b ) the signal is essentially null.
  • This process thus very quickly provides a readable output showing the existence of a defect, without the need for extensive frequency analysis using extensive computation and time.
  • a ratio is generated from the respective maximum amplitude of the signals. The value of the ratio is indicative of the presence or absence of a defect.
  • the waveform at (a) is essentially triangular, whereas at (b) the waves are more in the form of a sine wave. It is believed that this is a result of the undamaged carbon fibres in the test piece being locked up by each other, hence producing the triangular wave but when damage has occurred, they are more free to move and the sine wave output results. The degree of transition between the two wave forms may provide an early indication of damage.
  • the wide bandwidth of the transducers in, for example the FIG. 9 configuration, enables the transducers to pick up sidebands.
  • the presence of these sidebands has also been determined to be an indication of damage and they can be quantified using a modulation index.
  • These sidebands are usually generated where two frequencies are transmitted. They can either be transmitted from a single transmitter 19 , using the circuitry described earlier, or two separate transmitters can be utilised.
  • a loudspeaker may also be used to bring the workpiece up to resonance and, then after the input is discontinued the frequency shift can be measured as the workpiece's vibration dies down.
  • a laser interferometer may be used to measure a resonant peak when a frequency sweep is applied to the loudspeaker.
  • This resonant frequency is applied to bring the workpiece up to a high amplitude oscillation.
  • the loudspeaker is then turned off and the frequency shift measured as the oscillation dies away.
  • Nonlinear acoustic methods seek to determine how an ultrasonic waveform changes when it propagates through or over the surface region of a medium. These changes are directly related to the stress strain relationship and the hysteretic properties of a material and are not unduly effected by the ray path. In damaged materials, particularly ones that have microcracking, the stress strain relationship does not obey Hooke's Law of elasticity, stress is not proportional to strain, it is not linear.
  • the simplest method in a practical system that measures nonlinear effects in a material using acoustic waves is to measure the harmonics generated when a pure tone (pure sine wave) is transmitted through or over the surface of a material. This is illustrated below in FIG. 20 where a transmitter 43 and a receiver 44 are mounted on a workpiece 45 .
  • the harmonics are measured by examining the power spectra of the received signal.
  • the transmitted frequencies (fundamental) amplitude is compared to that of the amplitudes of each of the harmonic frequencies.
  • These harmonics are expressed in terms of decibels (dB) down from the fundamental, that is the number of decibels below the fundamentals amplitude. These values can be converted to a distortion factor that is expressed as a percentage.
  • FIG. 21 shows the photographs of two concrete test cylinders (size 300 mm long, 150 mm diameter).
  • FIG. 22 shows the results obtained by sending a 50 kHz sine wave over the surface of these two cylinders of concrete.
  • the second harmonic generated in the severely cracked region is clearly visible and has a level of distortion above 1%.
  • the third and fourth harmonics are not so prominent but have values above 0.5%.
  • the undamaged concrete sample does not produce any clear harmonics and consists of noise predominately below 0.25%.
  • FIG. 28 shows a microdamaged drilled test core with an ultrasonic transmitter on the right and a receiver on the left.
  • the blue trace of the spectral plot in FIG. 24 shows that transmitting and receiving in a line through the concrete close to the crack produces relatively high levels of 2, 3 and 4, harmonics above 0.5%. Transmitting and receiving in a line away from the crack (shown in red) produces little harmonic content. If two sine waves of different frequency are added together the resulting power spectrum is unaltered, this is illustrated in FIG. 9 below.
  • a ( t ) 1 ⁇ 2[1 cos(2 a )]+[cos( ab )cos( a+b )]+1 ⁇ 2[1 cos(2 b )]
  • a ( t ) 1+cos( ab )cos( a+b ) ⁇ 1 ⁇ 2 cos(2 a ) ⁇ 1 ⁇ 2 cos(2 b )
  • FIG. 26 shows a graphical representation of this process.
  • the frequencies are; the second harmonics of f 1 and f 2 that is (2*f 1 ) and (2*f 2 ).
  • the sum and difference frequencies of f 1 and f 2 that is (f 1 +f 2 ) and (f 1 f 2 ).
  • the second harmonics are half the amplitude of the sum and difference frequencies. As there is a larger variation in the generation of the sum and difference frequencies these should provide greater sensitivity in the indication of non-linearity. If the sine wave sum is subject to nonlinearity that is of a higher order than a square law stress strain relationship then many'other multiples, sum and difference combinations result, these will all appear in the spectra.
  • FIG. 11 shows a photograph of the transmitter and receiver placed against a test sample cube of concrete (size 50 ⁇ 50 ⁇ 50 mm). Two concrete test samples were selected and are shown in this figure, one has a crack running through its entire length, the other is undamaged.
  • the transmitter comprises a single piezoelectric wide band actuator that is continuously sending the sum of two sine waveforms at preprogrammed frequencies.
  • FIG. 28 shows the time and spectrum plots for the received waveform having passed through each of the concrete test cubes.
  • the difference in the amplitude of the two frequencies results from the ultrasonic attenuation of concrete being frequency dependent, losses are greater at higher frequencies.
  • the data for the damaged sample is shown in blue, and the good sample shown in red.
  • the damaged sample shows clearly that harmonics and intermodulation products have been generated by the crack.
  • the upper side band (f 1 +f 2 ) at 160 KHz is below 0.5% for the good sample and rises above 1% in the damaged sample.
  • the second harmonic of f 2 at 240 kHz changes from, 56 dB (0.16%) in the good sample and rising above 0.5% in the damaged sample.
  • the effect of the combinations of the harmonics and intermodulation products are very noticeable in the frequency range 200 to 350 kHz.
  • 2f 2 240 kHz
  • 2f 1 +f 2 200 kHz
  • f 1 +2f 2 280 kHz
  • 2f 1 +2f 2 320 kHz
  • the result is the formation of peaks and troughs within this range, corresponding to the interaction of their frequencies and phases, this effect can mask the changes between the good and bad samples.
  • the correct choice of the two frequencies f 2 and f 1 is an important factor.
  • FIG. 29 illustrates this by showing the spectra resulting from two different frequency combinations, f 1 +f 2 is reduced by the effect of the third harmonic of f 1 (3*f 1 ).
  • FIG. 30 shows a spectral plot taken over the surface of a mildly damaged region of the concrete test cylinders, shown previously in FIG. 21 .
  • This test was performed at the two frequencies 40 and 70 kHz.
  • the two fundamental frequencies are not sufficiently separated to form clear spectral peaks, however the intermodulation products and in particular the second multiple of f 1 +f 2 that is 2*(f 1 +f 2 ) shows a very clear peak above 1% distortion in the damaged region.
  • FIG. 31 shows a dual frequency being applied to the microdamaged drilled core.
  • the dual frequency ultrasonic waveform was transmitted through the sample at two locations, one along the crack, shown in blue and the other away from the crack shown in red. The difference between the two locations is very clear.
  • the cracked region produces harmonics and intermodulation products well above 0.5% distortion factor and f 1 +f 2 is above 1%.
  • the less cracked region has all levels below 0.5% and for frequencies above 250 kHz is below 0.25%.
  • FIG. 32 shows the same core but this time tested on one side only, the receiver positions are indicated by the letter R and the transmitter positions by letter T.
  • the red trace corresponds to a position away from the crack and the blue near to the crack. There is less difference between the two positions at low frequency however at higher frequencies, above 300 kHz, the cracked region does produce a significantly higher levels of intermodulation produces particularly at 3f 2 +f 1 (310 KHz).
  • the transmitter 43 may be powered by a signal generator 46 and the receiver 44 may transmit its output to a monitor/comparator 47 which measures the amplitude of the harmonics and/or intermodulation products to compare them with one or more predetermine levels (e.g. the 0.5% and 1% levels previously indicated).
  • the monitor comparator 47 may then produce either pass or fail signals or pass, check or fail signals which can be displayed by indicator 48 for example using a red and green light display or a red, amber, green display, as shown at 49 .

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GB0723622.7 2007-12-04
GBGB0723622.7A GB0723622D0 (en) 2007-12-04 2007-12-04 Devices, systems and methods of detecting defects in workpieces
US1313007P 2007-12-12 2007-12-12
PCT/GB2008/004001 WO2009071889A2 (en) 2007-12-04 2008-12-04 Devices, systems and methods of detecting defects in workpieces
US12/745,983 US20100312496A1 (en) 2007-12-04 2008-12-04 Devices, systems and methods of detecting defects in workpieces

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