WO1987000288A1

WO1987000288A1 - Eddy current inspection device

Info

Publication number: WO1987000288A1
Application number: PCT/US1986/001326
Authority: WO
Inventors: Thomas Henry Melling; Inderjit Singh Labana
Original assignee: Lockheed Corporation
Priority date: 1985-06-26
Filing date: 1986-06-20
Publication date: 1987-01-15
Also published as: AU6132986A; EP0227803A1; IL78086A0

Abstract

An eddy current inspection device (11) detects material irregularities in electrically conductive composite materials and employs a metallic shoe (63) for passively modifying the eddy current penetration depth. The device also employs a sensor head (15) including a transmitter coil (35) and an oscillator (31) for pulsing the transmitter coil (35) at a selected frequency to induce an eddy current in the composite material. A receiver coil (41) having first and second sections is arranged such that one section is positioned outside the transmitter coil (35), the first and second sections being connected with their respective outputs opposite.

Description

Eddy Current Inspection Device

Technical Field

The present invention generally relates to an eddy current inspection device for detecting material irregularities in an electrically conductive composite material, and more particularly, to a non-destructive, portable inspection device which utilizes a simple and reliable arrangement for detecting material defects in composite materials.

Background Art

Eddy current testing utilizes the application of inductive techniques, the basic principles of which are well known. In practice, a probe coil carrying an alternating current generates a pulsating magnetic field, and when the coil is placed a nominal distance from a metal test object, so that the test object is located within the pulsating magnetic field, current flow is induced within and on the surface of the test object. The induced currents, called "eddy currents" because of the circular pattern, produce a secondary a.c. magnetic field that opposes and reduces the intensity of the coil's magnetic field. Typically, changes in the impedance of the exiting coil can then be analyzed for inspection purposes.

The classic application of the eddy current phenomenon is in non-destructive testing. A typical procedure for such testing involves placement of the probe coil adjacent to the flat surface of a metallic test object. The influence of several physical properties of the test specimen upon the impedence characteristics of the probe coil can then be calculated for various test frequencies. With appropriate selection of an operating frequency, as determined by theory or by experiment, the depth of eddy current penetration is controlled to "look" at the surface only or into the metal itself to locate discontinuities which upset the current flow, such as cracks, or inclusions. From the impedence change it is often possible to measure, quantitatively and independently of each other, such parameters as the conductivity dimensions, and magnectic permeability of the test object, as well as such features as the magnitude and direction of cracks seams, inclusions, or the like.

In known eddy current inspection devices, the sensing coil is normally a section or a leg of a balanced bridges network. Unbalanced conditions,, produced by the impedence changes in the test coil, are sensed and converted into an alarm signal depending on the nature of the defect to be detected. In applications where a very high strength-to-weight ratio is desired, as in the aerospace industry, composite materials are replacing the use of metallic compositions and alloys. A composite material, typically, comprises a reinforcement which can be continuous or discontinuous in a matrix of another material. The material often comprises several layers or laminates of composite which are adhesively bonded together.

A major problem that has arisen in this technology is the lack of uniformity in the quality of the cured composite, thus giving rise to the need for a reliable inspection procedure to assure quality control. Moreover, when composite materials are incorporated into the airframe and structures of modern aircraft, there exists a need for detecting barely visible impact damage (BVID) in a reliable and simple manner.

Disclosure of the Invention

It is, therefore, an object of the present invention to provide an eddy current inspection device for detecting irregularities in electrically conductive composite materials wherein a transmitter coil is pulsed to induce an eddy current path in the target material and a receiver coil and monitoring circuit are arranged to measure the magnitude of the decaying eddy current for a predetermined time subr sequent to the initiation of the eddy current to thereby detect defects in the material by comparison with a referrence output.

Another object of the present invention is to provide a passive means for varying the penetration depth of the induced eddy currents in addition to frequency variations.

Yet another object of the present invention is to pror vide an eddy current inspection device for detecting impact damage on air vehicles comprised of composite materials, such damage including delamination, porosity, broken fibers, inclusions, lack of bond as well as fiber disorientation, The principal feature of the present invention is the provision of a totally new approach to detecting defects in composite materials utilizing eddy current inspection. In accordance with the present invention there is provided a sensor head including a pulsed transmitter coil for inducing an eddy current path in the target material which has a path similar to the shape of the transducer coil. Also furnished is a selectively viewed receiver coil which is arranged in two sections, one inside and one outside the pulsed transmitter coil, the two sections being connected such that their outputs are opposite. When a defect is common to both sections of the receiver coil, the output is zero; however, if a defect is detected by a single section, an output will result which is processed for a predetermined interval after pulsing of the transmitter coil.

Another important feature of the invention is the provision of locating the detection circuitry within the sensor head of the device so that an output signal from the sensor head provided to s. signal processing section of the device is immune from disruption by eddy current inducing RF signals and induced eddy currents.

Still another important feature of the present invention is the utilization of a pulse and sense timing arrangement which senses defects by detecting changes in the total energy reflected back by the material upon excitation by the transmitter coil.

Yet another important feature and a significant advantage of the present invention is the utilization of a passive means for varying the penetration depth of the inspection in addition to frequency, variations. The passive means comprises a metallic shoe adapted to fit over the sensor head, the shoe having a known conductivity and permeability selected such that the depth penetration is peduced by a factor of 10. In accordance with these and other objects, advantages, and features of the present invention there is provided an eddy current inspection device for detecting material irregularities in an electrically conductive composite material. The device comprises a sensor head including a transmitter coil; an oscillator for pulsing the transmitter coil at a selected frequency to induce an eddy current in the composite material; a receiver coil having first and second sections arranged such that one section is positioned inside and the other section is positioned outside the transmitter coil, the first and second sections being connected with their respective outputs opposite; and a sample and hold circuit for receiving an output signal from the receiver coil a predetermined period after the pulsing of the transmitter coil for a predetermined time interval and for providing an output signal indicative thereof. Connected to the sensor head by a suitable connector is a processing section including a differential amplifier for comparing the sample and hold circuit output signal with a reference signal and an indicator for providing an indication when said sample and hold circuit output signal exceeds the reference signal.

Further in accordance with the present invention there is provided a shoe adapted to fit over the sensor head for varying the penetration depth of the induced eddy current, the shoe having a known conductivity and permeability selected to reduce the penetration depth by a known factor, Brief Description of the Drawings

Figure 1 is a perspective view of the eddy current inspection device of the present invention illustrating the sensor head in the stored position and the battery pack open;

Figures 2A-2B are top and side views, respectively, of the sensor head illustrating the controls and displays as well as the metallic shoe employed to passively modify skin depth; Figure 3 is a block diagram illustrating the component parts of the sensor head;

Figure 4 is a block diagram illustrating the component parts of the signal processing section of the device;

Figures 5A-5C illustrate the winding configuration of the transmitter coil and receiver coil of the sensor head; Figure 6 illustrates the skin depth for frequencies between 10 Hz and 10⁸Hz in plane conductors of various materials; and

Figures 7-8 are schematic diagrams of the circuitry of the device.

Best Mode for Carrying Out the Invention

Figure 1 is a perspective view illustrating an eddy current inspection device, generally indicated at 11, in accordance with the present invention. The device 11 comprises a sensor head 15 which includes processing circuitry.

Casing 13 is provided with a receptacle for receiving and retaining the sensor head 15 which is shown in the stored position in Figure 1. Suitable controls 17 are also provided including an on/off switch, voltmeter, a pressr-tor-test button as well as course and fire threshold setting dials. An earphone/ headset jack is provided as an alternative mode of operation in addition to the use of an audio speaker and a battery pack 19 is incorporated in casing 13 for receiving a rechargeable, replaceable battery. Referring to Figures 2A-2B, the sensor head 15 of the device 11 is shown and has a polycarbonate membrane switch 21 or the like. The switch assembly 21 includes calibration switch 22 as well as frequency selection switches 23. An LED bar graph 25 is also furnished to indicate the output signal level of the head 15. O-rings 27 provide a secure grip and assist in holding together the complementary halves of the sensor head casing. A suitable cable 29 provides the output signal from the sensor head 15 to the signal processing section of the device 11.

Figure 3 is a block diagram illustrating the component parts of the sensor head 15. Because of the high frequency utilized by the device 11, the RF portion of the electronics is built into the head 15, thus providing an arrangement which minimizes the sensitivity of the device to spurious signals.

Oscillator 31 is provided so as to oscillate at any one of three selected frequencies depending upon the position of selector switch 23. Alternatively, switch 23 comprises a potentiometer for providing infinite frequency variation. The oscillator output signal triggers coil driver 33 so that transmitter coil 35 is pulsed at the selected oscillator frequency for a predetermined time interval sufficient to establish an eddy current path in the target material to be inspected, for example, about 3 to 4 microseconds. The output of coil driver 33 is also used to trigger a monostable or "one-ssot" multivibrator 37 for triggering sample and hold circuit 39 a predetermined period after the pulsing of transmitter coil, for example, about 3 to 4 microsecond delay. The sample and hold circuit 39 tracks the output from receiver coil 41 for a predetermined time interval, for example, about 20 microseconds. When a hold signal is received by sample and hold circuit 39 upon the passage of the predetermined time interval, the output from circuit 39 is maintained essentially constant even though there may be changes in. the input signal from receiver coil 41. The output signal from circuit 39 is amplified by amplifier 43 before being outputted to the processing section of the device 11. The sample and hold circuit 39 is arranged to measure the magnitude of the decaying eddy current for a predetermined time interval subsequent to the initiation of the induced eddy current path set up in the material to be inspected. Further, the receiver coil 41 is arranged to be critically damped by a suitable choice of the input resistance of the sample and hold circuit 39 to enable the measurements to be made in a minimum of time.

Referring to Figure 4, a processing section 45 of the device 11 is illustrated for receiving the output signal from sensor head 15. The signal from the sensor head 15 is inputted to a differential amplifier 47, the other input of amplifier 47 being connected to an auto-zero circuit 49 having a calibration switch 51. When the calibration switch 51 is depressed, the output from the auto-zero circuit 49 starts to rise until it is equal to the output signal from sensor head 15.

The resultant output from amplifier 47 enters a signal conditioning circuit 53 from which a positive only signal is generated. This signal output from circuit 53 is used to drive visual display 55, audio alarm 57 and peak value detector 59. The output of peak value detector 59 is also employed to drive visual display 55,

The visual display 55 can take a variety of forms including a digital or analog display. Audio alarm 57 is triggered when the signal from signal conditioning circuit 53 exτ ceeds the threshold setting of threshold 61. The difference between the two inputs to audio alarm 57 selects the frequency of the audible alarm 57.

Figures 5A-5C illustrate the configuration and positioning of the transmitter coil 35 and receiver coil 41 in sensor head 15. The receiver coil 41 is arranged in two sections, one inside and one outside of the transmitter coil 41, all coils being wound in the same direction. The two sections of receiver coil 41 are connected such that their outputs are opposite. Thus, if a defect is common to both sections then the outut is zero; however, when a defect is detected by one section, but not the other, an output results. This arrangement is used because it is easier to measure the difference between two signals rather than to measure a small change in a large signal, this being the case if the two sections were connected so as to complement one another.

The depth of penetration of the induced eddy current in the material to be inspected is controlled by the frequency of the pulse driving transmitter coil 35.

Referring to Figure 6 , skin depth or penetration depth of the induced eddy current path for the material to be inspected is shown as a function of frequency and material. Examination of the graph illustrated in Figure 6 reveals that for a given frequency, skin depth is much larger for carbon based materials than for metals. Note that at a frequency of 1 MHz, the skin depth for aluminum as compared to graphite is as follows:

Thus, a 1/10" thick slab of aluminum is equivalent to 1" of graphite based material. The above-noted relationship, between penetration depth, frequency, and material, provides a means for passively modifying skin depth, i,e. reducing the effective penetration depth of the induced eddy current path, without varying the frequency. The advantage of this approach is the elimination of expensive and complicated electronics need for the high frequencies required to examine relatively thin sheets of composite materials employed by the aerospace industry in their aircraft. Further, this approach eliminates or reduces the possibility of the unintentional or erroneous detection of assemblies beneath the composite material to be inspected such as hydraulic conduits and the like.

Thus, in accordance with the present invention and with particular reference to Figure 1, there is provided a metalHe shoe 63 adapted to fit over sensor head 15 in a manner so as to be retained thereon. The shoe 63 fits snuggly on head 15 so as to reduce power loss at the interface between head 15 and shoe 63. Further, it is necessary to minimize the impedance mismatch. The preferred material of the shoe 63 is aluminum.

Due to unique relationship between aluminum and graphite materials at 1 MHz, the transmitter coil 35 of head 15 is, preferably, driven at about 1.2 MHz, giving a skin depth for graphite, without passive modification of about 0.06 inches. However, as will be explained hereinafter, detectability of defects will depend on the threshold setting of the device 11. Thus, the shoe thickness is best determined experimentally utilizing the skin depth chart of Figure 6 as a guideline. The two other preferred frequency settings of head 15 are about 0.6 MHz and about 0.25 MHz which provide skin depth ratios of 1:1.4:2, respectively. Therefore, skin depth trimming can be performed at the lower frequencies once the maximum thickness of the shoe 63 is set for 1.2 MHz. Alternatively, different shoes 63 having different thicknesses can be employed for the different frequency settings. To operate the device 11, an operator selects the frequency to be employed by positioning selector switch 23 to the appropriate position on head 15. Thereafter, the threshold 61 is set to the full off position and the device 11 is switched on. Display 55 should be fully scaled. The head 15 is then placed on a sample piece of composite material and the calibration button 15 is depressed. This should reduce the display scale to zero or close to zero. If more than the first indicator of display 55 is lit, calibration is improper and the process must be updated.

After calibration, the head 15 is positioned over a small known defect, having a given area, which is to be detected. Rotation of the threshold 61 to increase sensitivity is commenced until the audio alarm 57 begins to sound.There- after, the threshold setting is locked and the device 11 is now ready to detect defects equal to or greater than the defect upon which the unit was calibrated.

In operation, the sensor head 15 is scanned across the surface to be mounted while the operator listens for the audio alarm 57 to sound. Depending on the threshold setting, one or more indicators may light up without the audio alarm sounding. This indicates a mismatch exists between the material being scanned and the sample used to calibrate. However, the mismatch is smaller than the smallest size defect the device 11 has been set to detect.

When head 15 goes over a large defect, the audio alarm 57 will sound and display 55 will show an increased indication in scale. The larger the defect, the greater the numer of indicators of display 55 that will light. The frequency of audio alarm 57 will also increase for large defects.

As a general guide, selection of a low RF frequency will result in the device scanning a greater depth of material than a higher frequency, If metal pipes or other objects are present under the surface of the material under test, a high frequency is preferred to insure the device does not detect the metal pipes.

Figure 7 is a schematic diagram of the processing section of the device 11 and is considered self-explanatory. Additionally, Figure 8 schematically illustrates the sensor head circuitry.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various alternations in form and detail will be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. An eddy current inspection device for detecting material irregularities in an electrically conductive composite material, said device comprising: a) a transmitter coil, b) an oscillator for providing an oscillator output signal at a selected frequency; c) a coil driver triggered by said oscillator output signal for providing a coil driver output which pulses said transmitter coil at said selected frequency for a predetermined time interval sufficient to establish an eddy current in the composite material to be inspected; d) a monstable multivibrator, triggered by said coil driver output simultaneously with said pulsing of said transmitter coil for providing an output signal a predetermined time interval after the triggering thereof; e) a receiver coil for providing an output indicative of the magnitude of the energy of the eddy current induced in the composite material; f) a sample and hold circuit triggered by said output signal from said monostable multivibrator for receiving said output from said receiver coil for a predetermined time interval after the pulsing of said transmitter coil and providing a sample and hold output signal; g) a differential amplifier for comparing said sample and hold output signal with a reference signal; and h) an indicator for providing an indication when said sample and hold circuit output signal exceeds said reference signal.

2. A device according to Claim 1 wherein said receiver coil comprises first and second sections arranged such that one section is positioned inside and the other section is positioned outside said transmitter coil, said first and second sections being with their output opposite, all coils being wound In the same direction.

3. A device according to Claim 2 further comprising a metallic shoe adapted to fit over said transmitter coil for passively varying the penetration depth of the induced eddy current, said shoe having a known conductivity and permeability selected to reduce the penetration depth by a known factor.

4. A device according to Claim 3, wherein said metallic shoe is constructed from aluminum.

5. A device according to Claim 4, wherein said oscillator is adapted to provide an output signal of about 1 MHz.

6. A device according to Claim 5, further comprising an LED display driven by the output from said differential amplifier.

7. A device according to Claim 6, wherein said indicator is an audio alarm.