Abstract:
A system for the thermal imaging of sonically or ultrasonically excited subsurface defects in a structure. The system includes a hand-held sound source, a thermal imaging camera and a control unit. The sound source emits pulses of sound energy into the structure, and the camera generates images of defects in the structure that are heated by the sound energy. The control unit controls the operation of the sound source on the camera for timing purposes. The sound source includes a transducer that is positioned against the structure at a desirable location. The source further includes a pair of legs that are also positioned against the structure to define a plane in combination with the transducer. The length of each leg is adjustable relative to the length of the transducer so that the gun can be used against irregular surfaces. The legs include a rubber tip to further prevent the transducer from slipping on the structure. In an alternate embodiment, the gun includes three transducers that define a plane and act to stabilize the gun against movement.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part application of U.S. patent application Ser. No. 09/397,585, titled Infrared Imaging of Ultrasonically Excited Subsurface Defects in Materials, filed Sep. 16, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to a system for detecting defects in a structural component by exciting the component with sound energy and then infrared imaging the component and, more particularly, to a hand-held sound source gun that includes stabilizing legs, and is applicable to be used in a system for detecting defects in materials.  
           [0004]    2. Discussion of the Related Art  
           [0005]    Maintaining the structural integrity of certain components and structures is very important in many areas because of safety concerns and the like. Loss of structural integrity is typically caused by material defects, such as cracks, delaminations, disbonds, corrosion, inclusions, voids and the like, that may exist in the component or structure. For example, it is very important in the aviation industry that reliable techniques are available to examine the structural integrity of the aircraft skin and structural components of the aircraft to ensure that the aircraft does not suffer from structural failure when in flight. The structural integrity of turbine blades and rotors, and vehicle cylinder heads is also important in those industries. Therefore, techniques have been developed for the non-invasive and non-destructive analysis of different structural components and materials in various industries.  
           [0006]    One known technique for non-invasive and non-destructive testing for material defects includes treating the structural component with a dye penetrant so that the dye enters any crack or defects that may be present in the material. The component is then cleaned, and the structure is treated with a powder that causes the dye remaining in the cracks to wick into the powder. An ultraviolet (UV) light source is used to inspect the material to observe locations on the component that fluoresces as a result of the dye. This technique has the disadvantage, however, that it is highly inspector intensive and dependent because the person inspecting for the fluorescence must be skilled. Additionally, the dye does not typically penetrate tightly closed cracks or cracks that are not on the surface.  
           [0007]    A second known technique for inspecting a component for defects employs an electromagnetic coil to induce eddy currents in the component. The coil is moved around on the component, and the eddy current pattern changes at a crack or other defect. The complex impedance in the coil changes as the eddy current changes, which can be observed on an oscilloscope. This technique has the drawback that it is also very operator intensive, and also extremely slow and tedious.  
           [0008]    Another known technique employs thermal imaging of the component to identify the defects. Typically, a heat source, such as a flash lamp or a heat gun, is used to direct a planar pulse of heat to the surface of the component. The material of the component absorbs the heat, and emits reflections in the infrared wavelengths. Certain types of defects will cause the surface temperature to cool at a different rate over the defects than for the surrounding areas. A thermal or infrared imaging camera is used to image the component and detect the resulting surface temperature variation. Although this technique has been successful for detecting disbonds and corrosion, it is ordinarily not successful for detecting vertical cracks in the material, that is, those cracks that are perpendicular to the surface. This is because a fatigue crack looks like a knife edge to the planar heat pulse, and therefore no, or minimal, reflections occur from the crack making the cracks hard or impossible to see in the thermal image.  
           [0009]    Thermal imaging for detecting defects in a material has been extended to systems that employ ultrasonic excitation of the material to generate the heat. The article Rantala, J. et al, “Lock-in Thermography with Mechanical Loss Angle Heating at Ultrasonic Frequencies,” Quantitative Infrared Thermography, Eurotherm Series  50 , Edizioni ETS, Pisa 1997, pg 389-393 discloses such a technique. In this technique, ultrasonic excitation is used to cause the crack or defect to “light up” as a result of the ultrasonic field. Particularly, the ultrasonic waves cause the opposing edges of the crack to rub together causing the crack area to heat up. Because the undamaged part of the component is only minimally heated by the ultrasonic waves, the resulting thermal images of the material show the cracks as bright areas against a dark background field.  
           [0010]    The transducer used in the ultrasonic thermal imaging technique referred to above makes a mechanical contact with the component being analyzed. However, it is difficult to couple high power ultrasonic energy into some materials, particularly in the case of metals. Significant improvements in this technique can be achieved by improving the coupling between the ultrasonic transducer and the component.  
           [0011]    Additionally, the known ultrasonic thermal imaging technique employs complex signal processing, particularly vector lock-in, synchronous imaging. Vector lock-in imaging uses a periodically modulated ultrasonic source and includes a processing technique that synchronously averages successive image frames producing an in-phase image and a quadrature image both based on the periodicity of the source. This results in images that are synchronous with the periodicity and eliminates unsynchronous noise from the image. The periodicity of the image can also be induced by an external stimulus, such as a modulated laser beam, heat lamps, etc. The processor receives the frames of video images and stores them synchronously with the induced periodicity, and then averages the stored frames with subsequently received frames to remove the noise. U.S. Pat. No. 4,878,116 issued Oct. 31, 1989 issued to Thomas et al discloses this type of vector lock-in imaging.  
           [0012]    U.S. Pat. No. 5,287,183 issued to Thomas et al Feb. 15, 1994 discloses a synchronous imaging technique that is a modification of the vector lock-in imaging disclosed in the &#39;116 patent. Particularly, the imaging technique disclosed in the &#39;183 patent extends the vector lock-in synchronous imaging technique to include a “box car” technique variation where the source is pulsed, and the images are synchronously averaged at various delay times following each pulse. The box car technique multiplies the video signal by zero except in several narrow time windows, referred to as gates, which are at a fixed time delay from the initiation of each ultrasonic pulse. The effect of these gates is to acquire several images corresponding to the states of component being imaged at the predetermined fixed delay times after the pulses. These different delay times are analogous to the different phases, represented by the sine and cosine functions of the periodic signal in the lock-in technique. During the acquisition of the gated images, the images corresponding to different delay times are combined arithmetically by pixel-by-pixel subtraction to suppress non-synchronous background effects.  
           [0013]    The ultrasonic excitation thermal imaging technique has been successful for detecting cracks. However, this technique can be improved upon to detect smaller cracks, as well as tightly closed cracks, with much greater sensitivity. It is therefore an object of the present invention to provide a defect detection system that uses sound energy to detect subsurface defects in materials, and particularly such a system that employs a hand-held sound source.  
         SUMMARY OF THE INVENTION  
         [0014]    In accordance with the teachings of the present invention, a system is disclosed for infrared or thermal imaging of ultrasonically or sonically excited subsurface defects in a structure. The system includes a hand-held sound source, a thermal imaging camera and a control unit. The sound source emits pulses of sound energy into the structure, and the camera generates images of defects in the structure that are heated by the sound energy. The control unit controls the operation of the sound source and the camera for timing purposes.  
           [0015]    The hand-held sound source provides a way of conveniently exciting many locations on a large structure, such as an aircraft fuselage. The sound source includes a transducer that is positioned against the structure at a desirable location. To prevent the transducer from “walking” on the structure when the pulses of sound energy are emitted, the source includes a pair of legs that are also positioned against the structure to define a plane in combination with the transducer. The length of each leg is adjustable relative to the length of the transducer so that the gun can be used against irregular surfaces. The legs include a rubber tip to further prevent the transducer from slipping on the structure. In an alternate embodiment, the gun includes three transducers that define a plane and act to stabilize the gun against the structure.  
           [0016]    Additional objects, advantages and features of the present invention will become apparent from the following description and the appended claims when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a block diagram of an imaging system according to the invention;  
         [0018]    [0018]FIG. 2 is a broken-away, side view of the transducer, specimen and camera of the imaging system shown in FIG. 1;  
         [0019]    [0019]FIG. 3 is a graph with power on the vertical axis and time on the horizontal axis showing the ultrasonic signal used in the known thermal imaging techniques that employ vector lock-in synchronous imaging;  
         [0020]    [0020]FIG. 4 is a graph with power on the vertical axis and time on the horizontal axis showing the pulsed ultrasonic signal used in the thermal imaging technique of the present invention;  
         [0021]    [0021]FIG. 5( a )- 5 ( d ) show consecutive images at predetermined time intervals of an open crack in a specimen that has been ultrasonically excited and imaged by an imaging system of the present invention;  
         [0022]    [0022]FIG. 6 is an image generated by the imaging system of the invention, showing a closed crack excited by ultrasonic energy;  
         [0023]    [0023]FIG. 7 is an image generated by the imaging system of the present invention, showing a delamination or disbond excited by the ultrasonic energy;  
         [0024]    [0024]FIG. 8 is a perspective view of a person holding an ultrasonic transducer against an aircraft component, and using the imaging system of the present invention to detect cracks in the component;  
         [0025]    [0025]FIG. 9 is a perspective view of a hand-held, sound source gun for exciting a structure, according to an embodiment of the present invention;  
         [0026]    [0026]FIG. 10 is a side view of the gun shown in FIG. 9;  
         [0027]    [0027]FIG. 11 is a front view of the gun shown in FIG. 9;  
         [0028]    [0028]FIG. 12 is a perspective view of a hand-held, sound source gun for exciting a structure, according to another embodiment of the present invention; and  
         [0029]    [0029]FIG. 13 is a front view of the gun shown in FIG. 12.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    The following description of the preferred embodiments directed to an ultrasonic and thermal imaging system including a hand-held sound source gun is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.  
         [0031]    [0031]FIG. 1 is a block diagram of an imaging system  10 , according to an embodiment of the present invention. The imaging system  10  is being used to detect defects, such as cracks, corrosion, delaminations, disbonds, etc., in a specimen  12 . The specimen  12  is intended to represent any structural component or material, such as an aircraft skin, that may include these types of defects. It is stressed that the specimen  12  does not need to be metal, but can be other materials, such as ceramics, composites, etc. The system  10  includes an ultrasonic transducer  14  having a piezoelectric element that generates sonic or ultrasonic energy within a certain frequency band. The transducer  14  can be any transducer suitable for the purposes described herein, such as the Branson 900 MA ultrasonic transducer. In one embodiment, the ultrasonic transducer  14  generates a pulse of ultrasonic energy having a substantially constant amplitude at a frequency of about 20 kHz for a period of time of about {fraction (1/2)} of a second and at a power level of about 1 kW. However, as will be appreciated by those skilled in the art, other ultrasonic frequencies, power levels and pulse durations can be used within the scope of the present invention.  
         [0032]    The ultrasonic energy from the transducer  14  is coupled to the specimen  12  through a coupler  16 . The coupler  16  is in mechanical contact with an end  18  of the transducer  14  and a front side  20  of the specimen  12 . FIG. 2 is a broken-away, side view showing the transducer  14  in contact with the coupler  16  and the specimen  12 . A support structure  26  is used to help maintain the transducer  14  in contact with the coupler  16 . In one embodiment, the coupler  16  is a thin piece of a soft metal, such as copper, to effectively couple the ultrasonic energy into the specimen  12 . Of course, other couplers consistent with the discussion herein can be used. For example, the coupler  16  can be a piece of cardboard or automotive gasket material. The coupler  16  can be any suitable piece of material that is typically softer than the end  18  of the transducer  14 , and is malleable to be deformed against the end  18  of the transducer  14  and prevent the transducer  14  from bouncing or walking along the specimen  12 . In one embodiment, the coupler  16  couples about 30 to 40 percent of the ultrasonic energy from the transducer  14  into the specimen  12 . It is noted, however, that the coupler  16  may not be needed in certain applications, such as testing for defects in a composite.  
         [0033]    A thermal imaging camera  22  is provided and spaced from a back side  24  of the specimen  12 , and generates images of the side  24  of the specimen  12  in association with ultrasonic excitations of the specimen  12 . The camera  22  can be spaced from the specimen  12  any suitable distance to provide images of as much of the specimen as desired in a single image. In other embodiments, the ultrasonic energy from transducer  14  and the image generated by the camera  22  can be provided at the same side of the specimen  12 . The thermal camera  22  can be any camera suitable for the purposes described herein, such as the Galileo camera available from Raytheon. In one embodiment, the camera  22  senses infrared emissions in the 3-5 micron wavelength range, and generates images at 100 frames per second. The camera  22  includes a focal plane array having 256×256 InSb pixels to generate the resolution desirable. In one embodiment, the side  24  of the specimen  12  is painted black to provide better contrast for infrared imaging.  
         [0034]    A controller  30  provides timing between the transducer  14  and the camera  22 . The controller  30  can be any computer suitable for the purposes described herein. When the detection process is initiated, the controller  30  causes the camera  22  to begin taking sequential images of the specimen  12  at a predetermined rate. Once the sequence of images begins, the controller  30  sends a signal to an amplifier  32  that causes the amplifier  32  to send a pulse to the transducer  14  to generate the pulsed ultrasonic signal. The ultrasonic energy is in the form of a simple pulse at the frequency being used. It is not necessary to employ any type of vector lock-in or synchronous imaging techniques between the pulse of energy and the imaging, as is currently done in the prior art. However, such signal processing techniques can be used to further reduce noise. It is stressed that the frequencies and pulse time periods being described herein are by way of non-limiting examples, in that different ultrasonic frequencies, pulse times, input power, etc. will vary from system to system and specimen being tested. After the end of the pulse, the controller  30  instructs the camera  22  to stop taking images. The images generated by the camera  22  are sent to a monitor  34  that displays the images of the side  24  of the specimen  12 . The images can then be sent to a storage device  36  to be viewed at another location if desirable.  
         [0035]    The ultrasonic energy applied to the specimen  12  causes faces of the defects and cracks in the specimen  12  to rub against each other and create heat. This heat appears as bright spots in the images generated by the camera  22 . Therefore, the system is very good at identifying very small tightly closed cracks. For those cracks that may be open, where the faces of the crack do not touch, the heating is generated at the stress concentration point at the crack tip. This point appears as a bright spot on the images indicating the end or tip of an open crack. The ultrasonic energy is effective to heat the crack or defect in the specimen  12  no matter what the orientation of the crack is relative to the energy pulse. The camera  22  takes an image of the surface  24  of the specimen  12 , providing a visual indication of any crack in the specimen  12  no matter what the position of the crack within the thickness of the specimen  12 .  
         [0036]    The present invention provides improvements over the known ultrasonic and thermal imaging techniques because the ultrasonic pulses used to heat the cracks and defects are simple pulses having a substantially constant amplitude, and do not need to employ sinusoidal signal modulation as used in vector lock-in synchronous imaging. To illustrate this point, FIG. 3 shows a graph with power on the vertical axis and time on the horizontal axis depicting the waveform of the ultrasonic signal used in vector lock-in imaging. The ultrasonic signal is generated at a predetermined frequency, and modulated with a low frequency sinusoidal modulating wave that provides amplitude modulation at a predetermined modulation period. The ultrasonic frequency signal rises and falls in amplitude with the low frequency modulation wave. Typically, the ultrasonic excitation is performed over several seconds. The image generated by this imaging technique is not the actual image of the particular component being imaged, but is a difference image generated by the subtraction process of the synchronous imaging. A more detailed discussion of this type of vector lock-in synchronous imaging to reduce noise in these types of systems is discussed in the &#39;116 patent.  
         [0037]    [0037]FIG. 4 is a graph with power on the vertical axis and time on the horizontal axis showing the pulses used to provide the ultrasonic excitation in the present invention. The ultrasonic frequency signal within each pulse has substantially the same amplitude, and is not modulated by a lower frequency sinusoidal waveform. The images generated by the camera  22  are real images, and not difference images of the type generated in the vector lock-in synchronous imaging technique. This provides a significant improvement in image quality and control simplicity. Although one pulse is ordinarily sufficient, more than one pulse can be employed, separated in time by a predetermined time period, for signal averaging purposes to reduce noise. The technique of “box car” integration can be used as discussed in the &#39;183 patent. In this technique, a gate is used in each time window to identify an image for each pulse, where the gate is at a certain fixed time delay from the beginning of the pulse. During the acquisition of the gated images, the images corresponding to different delay times are combined arithmetically to suppress non-synchronous background effects.  
         [0038]    FIGS.  5 ( a )- 5 ( d ) show four sequential images  38  of an open fatigue crack  40  in a metal specimen  42 . FIG. 5( a ) shows the images  38  of the specimen  42  prior to the ultrasonic energy being applied. FIG. 5( b ) shows the image  38  of the specimen  42  14 ms after the ultrasonic energy is applied. As is apparent, a light (higher temperature) spot  44  (sketched as a dark region) appears at the closed end of the crack  40 , where the mechanical agitation causes the heating. FIGS.  5 ( c ) and  5 ( d ) show subsequent images  38  at time equal to 64 ms and time equal to 114 ms, respectively. The light spot  44  on the specimen  42  increases dramatically over this sequence, clearly indicating the location of the crack  40 .  
         [0039]    [0039]FIG. 6 shows an image  48  of a closed crack  50  in a specimen  52  after being energized by the ultrasonic pulse. In this embodiment, because the crack  50  is closed, the entire length of the crack  50  generates heat creating a light spot  54  along the entire length of the crack  50  and providing an indication of a closed crack. Because the ultrasonic energy is so effective in causing the closed crack  50  to heat up significantly relative to the background, very short closed cracks, for example on the order of {fraction (2/3)} mm, are readily ascertainable in the image  48 .  
         [0040]    [0040]FIG. 7 shows an image  66  of a specimen  68 . In this image, a light spot  70  is shown, and is intended to represent the type of image generated from the thermal energy that is created by ultrasonically exciting a delamination or disbond. The thermal imaging technique of the present invention is particularly useful in identifying “kissing” disbonds.  
         [0041]    [0041]FIG. 8 is a perspective view of an operator  56  holding a hand-held transducer  58  against a specimen  60 , such as an aircraft fuselage. A thermal imaging camera  62  is directed towards the specimen  60  at a location that is separate from the point of contact of the transducer  58 . FIG. 8 illustrates that the system according to the invention can be used in the field for testing such components.  
         [0042]    For certain applications, the hand-held transducer  58  has limitations because it tends to “walk” or move on the structure when the pulse energy is emitted. Movement of the transducer  58  during the test reduces the coupling of the transducer  58  to the structure, thus reducing the amount of sound energy entering the structure and the quality of the resulting images. Thus, the ability to detect certain types of defects and possibly very small defects is limited.  
         [0043]    To overcome this limitation, the present invention proposes a modified hand-held, sound source gun  70  as depicted in FIGS.  9 - 11 . The gun  70  includes a housing  72  that includes the components for generating the sound signal, as would be well understood to those skilled in the art. A transducer horn  74  is threadably mounted to one end of the housing  72  and extends therefrom. Thus, the horn  74  can be removed from the housing  72 . One end of an electrical cable  68  is attached to the housing  72  at an end opposite from the horn  74 , and an opposite end of the cable  68  is connected to the control unit, as discussed above. Further, a pistol-type grip  76  extends from a bottom of the housing  72 , and a stabilizing grip  78  extends from a top of the housing  72  to allow an operator to firmly grip the gun  70  during testing. A trigger switch  66  on the grip  76  allows the operator to activate the sound source.  
         [0044]    The gun  70  further includes a bracket assembly  80  clamped to an end of the housing  72  proximate the horn  74 , as shown. The bracket assembly  80  can be clamped to the housing  72  in any suitable manner for the purposes described herein. The bracket assembly  80  includes a first leg  82  and a second leg  84  mounted thereto. The legs  82  and  84  are substantially parallel to the horn  74 , and are about the same length. The bracket assembly  80  includes a base plate  92  that has a particular shape suitable to position the legs  82  and  84  a certain distance apart, as shown. The first leg  82  includes a rubber tip  86  and the second leg  84  includes a rubber tip  88  opposite the housing  72 . The rubber tips  86  and  88  allow the gun  70  to be more firmly positioned against the structure to prevent slipping. The tips  86  and  88  can be made of other, non-slip materials, as would be appreciated by those skilled in the art. The operator places the tip  90  of the horn  74  and the tips  86  and  88  of the legs  82  and  84 , respectively, against the structure being tested. The combination of the horn  74  and the legs  82  and  84  define a plane that prevents the horn  74  from moving when it is activated.  
         [0045]    The first leg  82  also includes a tip  94  that is threadably attached to and extends through the bracket assembly  80  and is secured thereto by a lock nut  96 . Likewise, the second leg  84  includes a tip  98  that is threadably attached to and extends through the bracket assembly  80  and is secured thereto by a lock nut  100 . The leg  82  includes a pin  94  and the leg  84  includes a pin  96  that allow the legs  82  and  84  to be easily rotated. In this manner, the length of the legs  82  and  84  can be adjusted relative to the horn  74 . This allows the gun  70  to be used against irregular surfaces.  
         [0046]    [0046]FIGS. 12 and 13 show a perspective view and a front view, respectively, of a hand-held gun  110  that is a modification of the gun  70 , where like reference numerals identify the same components. As discussed above, the horn  74  is threaded into the housing  72  and can be removed therefrom. For the gun  110 , the horn  74  and the bracket assembly  80  have been removed, and replaced with a horn structure  112 . The structure  112  includes a threaded tip (not shown) that is threaded into the housing  72  in the same manner as the horn  74 .  
         [0047]    The structure  112  includes a base plate  120  and three horns  114 ,  116  and  118  symetrically disposed about the plate  120 , as shown. The horns  114 - 118  are attached to the plate  120  by any suitable technique and can be integral therewith. The horns  114 - 118  provide the three leg foundation that defines a plane, and prevents the horns  114 - 118  from walking when the gun  110  is activated. The power from the sound generating components is distributed to the horns  114 - 118  evenly, which then enters the structure from three different locations.  
         [0048]    The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.