Patent Publication Number: US-7591583-B2

Title: Transient defect detection algorithm

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to a nondestructive testing apparatus and method of nondestructive testing. More particularly, it relates to an apparatus and method for nondestructive testing of an object surface to determine the location of surface and subsurface flaws or other features using transient infrared thermography. 
     2. Related Art 
     The presence and location of both surface and subsurface flaws can be determined by various methods or techniques, including through the use of transient thermography. Transient thermography techniques generally utilize variations in the transfer of heat through an object over time to identify surface and subsurface flaws. 
     One transient thermographic method records the temperature rise of each resolution element by capturing a series of image arrays using an infrared camera while heating the surface of an object to be tested, and analyzing the temperature rise as a function of time to identify whether a linear temperature increase versus the square root of the heating time, occurs. Such behavior is reported to be indicative of the existence of a flaw within the object. This method is also reported to be of limited applicability for testing objects with complex geometries, or those which are otherwise subject to non-uniform heating transfer through the object. 
     One method of transient thermography comprises heating the surface of an object and analyzing individual infrared images of the object surface as a function of time. Flaws, particularly subsurface flaws, may be identified in the images by “hot spots” or regions of higher intensity infrared radiation because the rate at which heat transfers through the flaws is less than the rate at which it transfers in regions which do not include flaws. One limitation associated with this method is that the transient thermographic analysis is reduced to analyzing individual images which each represent a single point of time. This technique is of limited applicability with operators because of the possibility of missing a flaw indication because it requires analysis of multiple images which may have only minor variations to identify a flaw. Further, it is not easily adaptable to automation. 
     Another reported thermographic NDT technique utilizes a video replay of the recorded images and visual identification of intensity variations or bright spots by an operator to detect flaws. This technique is also not readily automatable. Further, it is highly operator dependent, and defects may easily be missed if the operator&#39;s attention is drawn away while observing the video. This technique also generally does not provide quantitative information about the location and or depth of the flaw. 
     A method of transient depth thermography is described in U.S. Pat. No. 5,711,603 to Ringermacher et al. This method involves heating the surface of the object and recording successive thermal images of each resolution element or pixel of the surface over a period of time. The contrast of each pixel is determined for each successive thermal image of the surface by determining the mean pixel intensity for that thermal image and subtracting the mean pixel intensity from the individual pixel intensity. The location of a flaw within the object is based upon changes in the pixel contrast. 
     SUMMARY OF THE INVENTION 
     The invention provides an apparatus and method for detecting flaws in an object. The method includes the step of heating a portion of a surface of an object wherein the surface is defined by a plurality of individual surface elements. The method also includes the step of recording a plurality of thermal images of the portion over time with a thermal imaging device. Each of the plurality of thermal images is defined by a plurality of pixels. Each of the plurality of pixels has an individual pixel address and corresponds to one of the plurality of individual surface elements. The method also includes the step of determining a pixel intensity for each of the plurality pixels in each of the plurality of thermal images. The method also includes the step of integrating the pixel intensity of each of the plurality of pixels having the same individual address from respective thermal images to establish elements within an array of integrated pixel intensity. The method also includes the step of using the array of integrated pixel intensity to detect a flaw in the object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein: 
         FIG. 1  is a schematic representation of the apparatus of the invention; 
         FIG. 2  is a flowchart of the method of the invention; 
         FIG. 3A  is a schematic illustration a first thermal image; 
         FIG. 3B  is a schematic illustration a second thermal image taken after the first thermal image shown in  FIG. 3A ; 
         FIG. 3C  is a schematic illustration a third thermal image taken after the second thermal image shown in  FIG. 3B ; 
         FIG. 3D  is a graph of pixel intensity over time corresponding to the pixels shown in  FIGS. 3A-3C ; 
         FIG. 4  is a schematic illustration of a plot of integrated pixel intensity as a function of pixel address; and 
         FIG. 5  is a cross-section of the plot of  FIG. 4  taken along section line  5 - 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The exemplary embodiment of the present invention is an automated and nondestructive testing method and apparatus for performing transient thermographic analysis to determine the presence or absence of flaws, including both surface and subsurface flaws, in an object. The method may also be used to determine and/or generate an image of a flaw and its location in relation to the object surface. The method integrates temperature related data about the object in the form of integrated pixel intensity or amplitude from a series of thermal images of the object surface, each of which comprise an array of pixels, taken over a period of time or as a function of time. The integrated intensity of each of the various pixel locations which make up the thermal images can be analyzed to identify variations which are indicative of defects. 
     In the context of the present invention, a pixel is the smallest unit or picture element of a thermal image. Pixels are generally rectangular in shape, but may comprise other shapes also. A thermal image is made up of a two dimensional array of pixels, with the number of pixels determining the size of the image. A surface element is an area of the surface of the object being imaged, which is generally rectangular, and which corresponds to and is represented by a single pixel. The array of pixels, therefore, corresponds to an array of surface elements, such that a thermal image comprising an array of pixels corresponds to an array of surface elements which in turn corresponds to a composite area of the surface of the object. The size of the composite area of the surface is related to the magnification factor associated with the thermal imaging device used to make the thermal image. 
     Referring to  FIG. 1 , in an exemplary embodiment of the invention, an apparatus  10  is operable to detect flaws in an object having a surface. Apparatus  10  is adapted to perform a form of transient thermographic analysis to determine the presence or absence of flaws, including both a surface and subsurface flaws, in an object. Apparatus  10  includes a heater  12  for heating a surface  14  of an object  16 . Apparatus  10  also includes a thermal imaging device  18  which is adapted to produce a series of thermal images  20  of the surface  14  as a function of time and provide the thermal images  20  as a signal output. Each of the thermal images  20  is made up of an array  24  of pixels  26 . Each pixel  26  has a unique pixel address in the array  24  and corresponds to a surface element in an array of surface elements that makes up a portion of the surface  14  of object  16 . Apparatus  10  also includes a recorder  34  that is in signal communication with the thermal imaging device  18 . Recorder  34  is adapted to capture the thermal images  20  for subsequent analysis. Apparatus  10  also includes means  36  for determining a pixel intensity for each of the pixels  26  in each of the thermal images. Apparatus  10  also includes means  38  for integrating the pixel intensity of the pixels  26  having the same address from each of the plurality of thermal images  20  for each pixel address. Means  38  is also operable to establish an array  42  of integrated pixel intensity. 
     The apparatus  10  may be used to detect surface and subsurface flaws in objects formed from most solid materials, including those made from metals, ceramics, glasses, engineering plastics, elastomers, composites and the like, and combinations thereof. The depth to which defects may be detected will depend upon the materials, the size of the defect and the particular elements of the apparatus and their design, features and capabilities. While applicant has observed the ability to detect defects in metal sheets of up to about 0.004 inches below the surface, and in rubber somewhat deeper, it is believed that defects located substantially deeper can be detected by appropriate design and selection of the elements of apparatus  10  and details of the method of its operation. For example, the amount of energy that reaches the defect area can be increased. One challenge in detecting defects in metal is the difficulty of getting energy into the test specimen. When the flash technique is used the heat the part, a portion of the energy is reflected off of the surface of the part. Heating techniques that are able to get more energy to the defect region will provide greater contrast. Alternatively, the surface finish of the part being inspected can be altered by changing the surface finish of the part being inspected. The surface of the part being inspected can be painted to change the surface finish. Alternatively, the parameters of the program can be changed to enhance defect identification. The following parameters can be changed to enhance defect contrast in some operating environments: the number of images considered, the frame rate, adjusting the numerical integration technique, adjusting the delay between flash and first image. 
     The heater  12  of the exemplary embodiment of the invention may include a photographic flash lamp, quartz lamp, infrared lamp, laser or other like device for delivering thermal energy to the surface  14  of object  16 . In a preferred embodiment, a plurality of photographic flash lamps was utilized as heater  12  and was adapted to deliver about 4800 watt-sec of energy to the surface  14  of object  16 . Heater  12  should be capable of heating the surface to a sufficient temperature to allow thermographic monitoring in accordance with the method described herein. 
     Many types of infrared cameras may be employed as the thermal imaging device  18 . The thermal imaging device  18  of the exemplary embodiment of the invention is preferably an infrared camera with a focal plane array infrared detector. The video device is preferably a high speed focal plane array camera, or similar device, with a variable frame rate. The variable frame rate range may vary from, for example from about 50 frames per second up to about 250 frames per second or greater. 
     One example of an infrared camera that may be used in conjunction with the apparatus and method of the invention is the ThermaCAM SC 3000 Quantum Well Infrared Photodetector (QWIP) System made by FLIR Systems, Inc. This camera provides infrared imaging in the 8 to 9 um wavelength IR band by utilizing a Quantum Well Infrared Photodetector focal plane array IR detector having a sensitivity of less than 20 mK at 30° C. A wide range of lenses and other optics may be utilized to obtain the desired viewing area of the object for objects of varying size located at various distances from the camera. 
     The camera is adapted to provide analysis of dynamic or digitally stored images, and is adapted to provide image output through a standard video interface output to a computer for recording and storage of the images. Images are output as a 14-bit data stream for storage in a mass storage device of a computer. The camera may be used to record live IR video, or real-time high speed dynamic events; and is adapted to capture and store thermal images and data at extremely high rates of 50 to 60 Hz up to 900 Hz NTSC/750 Hz PAL. 
     The IR focal plane array detector is described as a quantum well infrared photodetector and has an image resolution of 320×240 pixels. The 14-bit digital output of the detector permits each pixel of the image array to be assigned an intensity or amplitude value between 1 and 16,383. Thus an individual frame may encompass a broad range of surface temperature and still be able to visualize minute thermal differences. The camera had a spatial resolution (IFOV) of about 1.1 mrad. An excellent description and survey of quantum well IR photodetectors and focal plane arrays has been published as “Quantum Well Infrared Photodetector (QWIP) Focal Plane Arrays by S. D. Gunapala and S. V. Bandara of the Center for Space Microelectronics Technology, Jet Propulsion Laboratory, Pasadena, Calif. in Semiconductors and Semimetals, Vol. 62, 1999. 
     The image recorder  34  of the exemplary embodiment of the invention is adapted to record or store the images output from the thermal imaging device  18 . Image recorder  34  is preferably mass storage in a computer, such as a hard drive, tape drive or similar device. Recorder  34  may also include a video capture card that is adapted to receive and/or buffer the images received as signal output from thermal imaging device  18  prior to storage in mass storage. 
     Means  38  for integrating the pixel intensity of the exemplary embodiment of the invention is preferably a computer or similar device adapted to access the intensity or amplitude information for each pixel address in each of thermal images  20 . Means  38  of the exemplary embodiment of the invention is also adapted to integrate the pixel amplitude or intensity at each pixel address across all or any portion or grouping of thermal images  20 . Integration may be performed using well-known numerical integration methods. Means  38  of the exemplary embodiment of the invention is also adapted to incorporate the integrated pixel intensity associated with each address as an element in an array  42  of integrated pixel intensity as a function of pixel address. 
     Means  38  of the exemplary embodiment of the invention is also adapted to analyze the array  42  of integrated pixel intensity to identify flaws or defects in the object based on variations among the integrated pixel intensity of elements within integrated pixel intensity array  42 . Means  38  of the exemplary embodiment of the invention is also preferably adapted to display various representations of integrated pixel intensity array  42 , such as by display on a computer monitor. Means  38  of the exemplary embodiment of the invention is also preferably adapted to perform sorting or selection of objects depending on whether flaws or defects are identified in the objects or not. Thus, apparatus  10  may be used in conjunction with the manufacture of objects as an automated inspection system or sorting system. 
     Referring to  FIGS. 1 and 2 , in an exemplary method of operation of the present invention, a method  100  includes the step  110  of heating the surface  14  of the object  16  with the heater  12 . The surface  14  of object  16  is heated to a sufficient temperature to allow thermographic monitoring. Typically, relatively thin objects only require minimal heating, about 5° C. or less above the ambient object temperature, with as little as about 1° C. or less above ambient object temperature even more preferable. Meanwhile, thicker objects may require significantly greater differential heating, for example, up to about 20° C. above the ambient object temperature has been reported by others using thermographic techniques as desirable for objects having a thickness of about 0.5 inches. Surface factors, including color, emissivity and thermal conductivity, are also important in determining the amount of heat energy to deliver to the surface of the object during heating, while the physical properties establish the upper heating limit at a temperature which will not damage the object  16  and/or surface  14 . 
     In order to accurately locate and determine the size of a flaw, the object surface should be heated to the desired temperature in a sufficiently short period of time so as to inhibit heating of the remainder of the object. However, in order for the apparatus and method of the invention to be effective, the heat applied at the surface must be of a sufficient combination of magnitude, intensity and duration to penetrate to the depth in the object over which it is desirable to detect defects. Therefore, for a given object, relatively lesser amounts of heat energy will be applied to detect surface and near surface defects, and relatively greater amounts of heat energy will be applied to detect defects which are located in regions spaced further away from the surface at which the heat is applied. Typically heating occurs in a fraction of a second for thin objects, but may take substantially longer, perhaps on the order of minutes, for thicker objects. The heating time may also be varied depending on the thermal conductivity of the object, and particularly on the thermal conductivity in the surface region to which the heat is applied. 
     Once the object&#39;s surface is heated, the method  100  proceeds with to step  120 . At step  120 , a plurality of thermal images  20  of the surface  14  are recorded as a function of time using the thermal imaging device  18 . Each of the thermal images  20  comprises the array  24  of pixels  26 , each pixel  26  having a unique pixel address in the array  24  and corresponding to a surface element in the array of surface elements.  FIGS. 3A-3C  schematically shows three thermal images  20   a ,  20   b ,  20   c  defining arrays  24   a ,  24   b ,  24   c . The three thermal images  20   a ,  20   b ,  20   c  correspond to the same portion of the surface and differ in that each of the three thermal images  20   a ,  20   b ,  20   c  is taken at a different time. 
     Preferably, thermal imaging device  18  is an infrared camera which records and stores successive thermal images  20  of the surface  14  of the object, recording an amplitude or intensity value of each pixel of each image thereof. The number of images recorded depends upon the desired resolution of the resulting integrated array of pixels for that image, the rate at which images are acquired and other factors. 
     The number of images needed for defect detection is dependent on many variables. The camera&#39;s frame rate and the material the part is made from both play a major role in determining the optimal number of images used in the algorithm. Generally, the number of images used to detect defects in low emissivity parts such as metal is less than the number of images used to detect defects in high emissivity parts such as rubber. 
     At step  130 , a pixel intensity or amplitude for each of the pixels  26  in each of the thermal images  20  is determined. The plurality of thermal images  20  are stored in the mass storage of the computer as a corresponding number of arrays  24  of pixels  26 , each pixel  26  having an associated pixel intensity. In general, flaws (i.e., a bubble or void) will tend to slow the dissipation of heat from surface  14  of object  16 . Therefore, surface elements of the object  16  in which or under which a defect or flaw is located will tend to have higher temperatures than other surface elements. Thus, those pixels  26  of array  24  which are associated with surface elements which include a flaw or defect will tend to register a higher intensity or amplitude than other pixels  26  of array  24 . 
     At step  140 , the pixel intensity of pixels  26  having the same address from each of the plurality of thermal images  20  are integrated to establish elements within an array  42  of integrated pixel intensity. This step is partially illustrated in  FIG. 3A-3C . Each image  20  is associated with a unique time at which it was recorded. The intensity over time at a particular pixel  26  address may be represented by a polynomial function:
 
 f ( t )= X   N   t   n   +X   N−1   t   n−1   +X   N−2   t   n−2   + . . . X   N−n   t   N−n  
 
     The integral of this function may be represented generally by an equation: 
     
       
         
           
             
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     This integration is performed for each of the corresponding pixel addresses of the successive images  20 . 
     For example, as shown in  FIGS. 3A-3D , three pixels  26   a ,  26   b ,  26   c  correspond to the same portion of the surface and differ in that each of the three pixels  26   a ,  26   b ,  26   c  is taken at a different time. Similarly, three pixels  27   a ,  27   b ,  27   c  correspond to the same portion of the surface and differ in that each of the three pixels  27   a ,  27   b ,  27   c  is taken at a different time.  FIG. 3D  is a graph in which pixel intensity is defined along the y-axis and time is defined along the x-axis. Point  52  of the graph in  FIG. 3D  corresponds to both of pixels  26   a  and  27   a  in  FIG. 3A .  FIG. 3A  shows the pixel intensity of the graph of  FIG. 3D  reflects this condition. Point  54  of the graph in  FIG. 3D  corresponds to the pixel  26   b  in  FIG. 3B . Point  56  of the graph in  FIG. 3D  corresponds to the pixel  27   b  in  FIG. 3B .  FIG. 3B  shows the pixel  27   b  is more intense than pixel  26   b  and the graph of  FIG. 3D  reflects this condition. Point  58  of the graph in  FIG. 3D  corresponds to both of pixels  26   c  and  27   c  in  FIG. 3C .  FIG. 3C  shows that both of the pixels  26   c ,  27   c  are substantially less intense, in comparison with the pixels  26   a ,  27   a  in  FIG. 3A , and the graph of  FIG. 3D  reflects this condition. 
     The intensity time data plotted in  FIG. 3D  could be utilized in conjunction with well-known regression techniques to determine a polynomial expression which could in turn be integrated. However, to perform a reasonable approximation, the integral may also be obtained by applying numerical integration techniques on the time-based pixel intensity data associated with that pixel address which is obtained from the pixel array  24  information from the recorded images  20 . Generally, a curve  60  shown in  FIG. 3D  is consistent with a portion of the surface without a flaw and a curve  62  is consistent with a portion of the surface with a flaw. 
     At step  150 , the array  42  of integrated pixel intensity is used to detect a flaw in the object  16 . Step  150  may be performed in any one of a number of ways. One way of performing step  150  is using the array  42  of integrated pixel intensity to detect a flaw in the object  16 . For example, as best shown in  FIG. 4 , the array  42  provides a three dimensional representation of data. The exemplary array  42  reveals peaks  64 ,  66 ,  68 ,  70 ,  72 ,  74 ,  76  of varying heights from the X-Y plane. The peaks  64 ,  66 ,  68 ,  70 ,  72 ,  74 ,  76  constitute variations of the integrated pixel intensity values between the elements of the array  42  and correspond to flaws in object  16 . 
     Referring to  FIGS. 2 and 5 , another method of performing step  150  is by comparing the integrated pixel intensity of individual elements of the array  42  to a predetermined threshold  78 . Elements having an integrated pixel intensity greater than the predetermined threshold  78  of pixel intensity are identified as a flaw. In the exemplary array  42 , elements corresponding to peaks  64 ,  66 ,  68  correspond to a portion of the surface  14  having a flaw. The predetermined threshold  78  of integrated pixel intensity may, for example, be determined empirically from results obtained from samples of the same object  16  made from the same material which are known not to contain defects. The results from such samples may be used to develop a background level of integrated pixel intensity empirically which is representative of samples which do not contain defects. Alternatively, the predetermined threshold  78  value may be a mean value of integrated pixel intensity using standard techniques of calculating the mean, such as by summing all of the values of integrated pixel intensity from one or more images and dividing by the number values summed. Alternately, the predetermined threshold  78  may be a median value of pixel intensity using standard techniques of calculating the median. 
     The array  42  can be defined by a variety of colors wherein variations in the color reveal flaws in the surface  14 . The visual image may be viewed by an operator and defects may be identified using objective criteria, such as predetermined thresholds applied to the display as described above, or subjective criteria such as any number of determinations which may be made by an operator while viewing the display. Similarly, a fourth variation of the integrated pixel intensity among the elements of the array of integrated pixel intensity may also be displayed by creating a visual image of the array in which variations in integrated pixel intensity are correlated to variations in a gray-scale. Still similarly, a fifth alternative method of using variation of the integrated pixel intensity among the elements of the array of integrated pixel intensity is by creating a visual image of the array in which the integrated pixel intensity of an element is plotted as a function of the element address. 
     It will be appreciated that in order to view all of the surface  14  of an object  16  it may be desirable to index one of the object  16  or the apparatus  10  so as to expose all of the surface  14  to viewing or inspection by the apparatus. As such, the method  100  may also include a step  160  of indexing at least one of the object  16  and the thermal imaging device  10  to expose different portions of the surface  14  to the thermal imaging device  10 . It is preferred that the steps of method  100  be executed automatically in response to the execution of a software control algorithm in a computer. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. The invention is defined by the claims.