WO2021205041A1 - A method and system for detecting and locating buried defects using three dimensional infrared thermography - Google Patents

A method and system for detecting and locating buried defects using three dimensional infrared thermography Download PDF

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Publication number
WO2021205041A1
WO2021205041A1 PCT/EP2021/059464 EP2021059464W WO2021205041A1 WO 2021205041 A1 WO2021205041 A1 WO 2021205041A1 EP 2021059464 W EP2021059464 W EP 2021059464W WO 2021205041 A1 WO2021205041 A1 WO 2021205041A1
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Prior art keywords
test object
thermogram
respect
exposure
angles
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PCT/EP2021/059464
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French (fr)
Inventor
Alaa Addin Mani
Joanna BAUER
Charlie O'MAHONY
Sarah Kate MARKHAM
Syed A.M. TOFAIL
Christophe Silien
Ehstham UL HAQ
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University Of Limerick
Wroclaw University Of Science And Technology
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Priority to PL443765A priority Critical patent/PL443765A1/en
Publication of WO2021205041A1 publication Critical patent/WO2021205041A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N25/00—Investigating or analyzing materials by the use of thermal means
    • G01N25/72—Investigating presence of flaws
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/8851—Scan or image signal processing specially adapted therefor, e.g. for scan signal adjustment, for detecting different kinds of defects, for compensating for structures, markings, edges
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/8851—Scan or image signal processing specially adapted therefor, e.g. for scan signal adjustment, for detecting different kinds of defects, for compensating for structures, markings, edges
    • G01N2021/8854—Grading and classifying of flaws
    • G01N2021/8861—Determining coordinates of flaws
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/8851—Scan or image signal processing specially adapted therefor, e.g. for scan signal adjustment, for detecting different kinds of defects, for compensating for structures, markings, edges
    • G01N2021/8854—Grading and classifying of flaws
    • G01N2021/8867—Grading and classifying of flaws using sequentially two or more inspection runs, e.g. coarse and fine, or detecting then analysing
    • G01N2021/887—Grading and classifying of flaws using sequentially two or more inspection runs, e.g. coarse and fine, or detecting then analysing the measurements made in two or more directions, angles, positions
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/8851—Scan or image signal processing specially adapted therefor, e.g. for scan signal adjustment, for detecting different kinds of defects, for compensating for structures, markings, edges
    • G01N2021/8854—Grading and classifying of flaws
    • G01N2021/8874—Taking dimensions of defect into account
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/8851—Scan or image signal processing specially adapted therefor, e.g. for scan signal adjustment, for detecting different kinds of defects, for compensating for structures, markings, edges
    • G01N2021/8887—Scan or image signal processing specially adapted therefor, e.g. for scan signal adjustment, for detecting different kinds of defects, for compensating for structures, markings, edges based on image processing techniques

Definitions

  • the present invention relates to a method and system for identifying and locating buried defects using three dimensional infrared thermography.
  • thermography is the detection of spatial and temporal distribution of heat patterns in objects. It often uses an infrared camera to detect these heat patterns.
  • Commercial use of thermography is numerous and also includes melt pool temperature monitoring in real time in metal additive manufacturing.
  • IRT IR thermography
  • a major industrial use of IRT is the method known as active thermography, which quantitatively delivers dimensional values and thermal diffusivity of not only the surface, but also features within the sample, as heat diffusing from the bulk is affected by variations in structure and material. This allows for defect detection and their size, shape as well as location identification within a sample, from the variation in thermal properties that these defects instigate.
  • Flash thermography involves exciting a sample with a heat source and recording its surface temperature over the time with a thermographic camera. The propagation of this heat wave through the sample is then influenced by the thermal properties of the sample’s materials. This flash heating creates variance to the systems thermal equilibrium, which highlights defects due to their differing thermal properties. It is noted, that density and heat capacity are the primary impacts in detectability, whereas thermal conductivity and diffusivity are more related to the time required to reach the surface, varying the time window of highest contrast and provide an information about internal structure of the material. Numerous techniques on flash thermography simulations utilize tomographic aspects also, but these techniques utilize multi-face imaging. Usually the object under investigation is symmetrically heated and an infrared camera revolves around the object to obtain multiple thermographic images of the object while being heated.
  • flash thermography gives detail about the location and the XY dimensions of the sample. This does not however tell us anything about information in the Z direction, meaning the depth of the defect and the thickness of it.
  • the issue with the 2D surface image of flash thermography is that the hotspot dimensions are dependent on numerous variables and are difficult to distinguish. Area of hotspot is dependent on defect size, but also on depth and thickness. Meaning large defects can appear small at large depths and vice versa.
  • Two main factors to the effectiveness of flash thermography are the defect size and depth of wave penetration (on the Z axis above). For the first, at a given depth, defects can be detected only when they are large enough. Typically, the size must be at least equal to the depth. The latter, heat wave propagation gives the depth of detection, as only defects reached by the heat wave will affect the surface temperature.
  • thermographic tomography is cumbersome as the object needs to be symmetrically heated and an infrared camera has to revolve around the object to take a plurality of thermographic images from various angles. In an industrial setting, this is inconvenient as the object under investigation (such as a part of an airplane) has to be dismantled and placed within a device which performs thermographic tomography.
  • the present invention relates to a method and system for identifying and locating buried defects using three dimensional infrared thermography, as set out in the appended claims. More specifically, the method and system identifies a buried defect in an article or a physical object and also provides a location of said buried defect within said article or physical object.
  • the system and method of the present invention provides a remote and reliable way to locate and/or identify buried defects or physical objects.
  • a method for detecting and locating a buried defect in a test object comprises heating a surface of said test object by exposing said surface with one or more radiation sources for a first predetermined duration of time from a plurality of angles with respect to said surface of said object and after each exposure of said plurality of angles of exposure performing the following steps of: recording a thermogram of said surface for a second predetermined duration using an infrared image capture device, wherein said infrared image capture device is static with reference to said test object; determining a time within said second predetermined time duration where a maximum variation of temperature occurs with respect to a reference thermogram; and extracting a thermal image of said surface from said thermogram at said time.
  • the reference thermogram comprises a thermogram of a reference surface of a reference object, wherein said reference object is identical to said test object, wherein said reference object does not have any buried defect, wherein said reference surface of said reference object is identical as compared to said surface of said test object, and wherein said reference thermogram is recorded for said second predetermined duration after similarly exposing said reference surface as compared to said surface of said test object.
  • the plurality of exposure angles comprises, e.g. -30° to 30° in 1° steps, wherein said exposure angles are measured with respect to a normal to said surface of said test object.
  • the exposure angles can be within a full range of 0° to 360°.
  • each of said extracted thermal image (for each angle of exposure) of said surface is processed to determine the depth of said buried defect with respect to said surface.
  • each of said extracted thermal image (for each angle of exposure) of said surface is processed to determine the location of said buried defect with respect to said surface.
  • each of said extracted thermal image (for each angle of exposure) of said surface is processed to determine the dimensions of said buried defect.
  • a system for detecting and locating a buried defect in a test object comprises one or more radiation sources, a processor and an infrared image capture device.
  • said one or more radiation sources are configured for heating a surface of said test object by exposing said surface with for a first predetermined duration of time from a plurality of angles with respect to said exposed surface of said object.
  • the infrared image capture device is operatively coupled to said processor, and said processor is operatively coupled to said radiation sources.
  • the processor is configured for: after each exposure of said plurality of angles of exposure: recording a thermogram of said surface for a second predetermined duration using said infrared image capture device, wherein said infrared image capture device is static with reference to said test object; determining a time within said second predetermined time duration where a maximum variation of temperature occurs with respect to a reference thermogram; extracting a thermal image of said surface from said thermogram at said time.
  • the processor is further configured for processing each of said extracted thermal image of said surface to determine the depth of said buried defect with respect to said surface.
  • the reference thermogram comprises a thermogram of a reference surface of a reference object, wherein said reference object is identical to said test object, wherein said reference object does not have any buried defect, wherein said reference surface of said reference object is identical as compared to said surface of said test object, and wherein said reference thermogram is recorded for said second predetermined duration after similarly exposing said reference surface as compared to said surface of said test object.
  • said self-referenced thermogram can be made by selection of a clean surface portion or any known temperature object within the thermogram image frame as a temperature reference, to remove or take account of any thermal measurement drifting.
  • said plurality of exposure angles comprises e.g.
  • the processor is further configured for processing each of said extracted thermal image (for each angle of exposure) of said surface to determine the location of said buried defect with respect to said surface. In one embodiment the processor is further configured for processing each of said extracted thermal image (for each angle of exposure) of said surface to determine the dimensions of said buried defect.
  • said infrared image capture device comprises a forward looking infrared (e.g. FLIR) camera or any focal plane array (FPA) thermal detector.
  • FLIR forward looking infrared
  • FPA focal plane array
  • a temperature logger configured to measure the temperature of a surface.
  • the temperature logger is configured to compare the surface temperature recorded with the thermal camera and the recorded temperature of the logger.
  • FIG. 1 exemplarily illustrates a flowchart for identifying and locating buried defects using three dimensional infrared thermography in accordance with some of the embodiments of the present invention
  • FIG. 2 exemplarily illustrates a block diagram of a system for identifying and locating buried defects using three dimensional infrared thermography in accordance with some of the embodiments of the present invention
  • FIG. 3 exemplarily illustrates a system for identifying and locating buried defects using three dimensional infrared thermography in accordance with some of the embodiments of the present invention
  • FIG. 4 exemplarily illustrates temperature vs. time for showing the variation of temperature in the second predetermined time interval: a) temperature of the reference object (blue), b) temperature of the test object (red) and c) variation in temperature between the two (black) in accordance with some of the embodiments of the present invention;
  • FIG. 5 exemplarily illustrates the reference object surface (shown as column (a)) and the test object surface (shown as column (b)) being heated using a radiation source where the radiation source is at 0, 15, 30 degrees angle with respect to the normal of the surface respectively;
  • FIG. 6 exemplarily illustrates the reference object surface and the test object surface matrix, where the columns represent the reference object surface (labelled as no-defect), the test object surface where the test object has a 1 millimetre buried defect (labelled as 1mm length), the test object surface where the test object has a 2 millimetre buried defect
  • the rows represent the test object surface and the reference object surface heated using a radiation source where the radiation source is at 0, 15, 30 degrees angle with respect to the normal of the respective surfaces respectively;
  • FIG. 7 exemplarily illustrates a simple graphical representation of defect image features stretching with variation in radiation source angle, yellow being the incident radiation, orange being the thermal diffusion from the defect to the surface plot profiles of 1mm (millimetre) & 2mm defect at a) 0° and b) 15°.
  • 1mm the width from centre for 0° is about 4.55mm and at 15° is about 6mm, giving about 25% increase in size to the side of the radiation source.
  • the width from centre for 0° is about 4.5mm and at 15° is about 7mm, giving a about 36% increase in size to the side of the heat source.
  • the present invention relates to a method and system for identifying and locating buried defects using three dimensional infrared thermography. More specifically, the method and system identifies of a buried defect in an article or a physical object and also provides a location of said buried defect within said article or physical object.
  • FIG. 1 exemplarily illustrates a flowchart for identifying and locating buried defects using three dimensional infrared thermography in accordance with some of the embodiments of the present invention.
  • the method for detecting and locating a buried defect in a test object comprises heating 101 a surface of said test object by exposing said surface with one or more radiation sources for a first predetermined duration of time from a plurality of angles with respect to said surface of said object. After each exposure of said plurality of angles of exposure, the following steps are performed.
  • the plurality of exposure angles comprises-30° to 30° in 1° steps, wherein said exposure angles are measured with respect to a normal to said surface of said test object.
  • thermogram of said surface for a second predetermined duration is recorded 101a using an infrared image capture device, wherein said infrared image capture device is static with reference to said test object.
  • a time within said second predetermined time duration is determined 101b where a maximum variation of temperature occurs with respect to a reference thermogram.
  • the reference thermogram comprises a thermogram of a reference surface of a reference object, wherein said reference object is identical to said test object, wherein said reference object does not have any buried defect, wherein said reference surface of said reference object is identical as compared to said surface of said test object, and wherein said reference thermogram is recorded for said second predetermined duration after similarly exposing said reference surface as compared to said surface of said test object.
  • the reference thermogram is used as a control for comparison.
  • FIG. 4 exemplarily illustrates temperature vs. time graph for showing the variation of temperature in the second predetermined time interval: a) temperature of the reference object (blue), b) temperature of the test object (red) and c) variation in temperature between the two (black).
  • the position (time) of maximum difference between the surfaces of the reference object and the test object which is caused by the defect in the test object occurs 2.89K at 52 seconds as shown in FIG. 4.
  • the difference between surface temperatures of the test object and the reference object is maximum at 52 seconds into the second time interval.
  • a thermal image of said surface from said thermogram is extracted 101c at said time, where there is a maximum difference between the surfaces of the reference object (i.e. 52 seconds in accordance with the above embodiment as shown in FIG. 4).
  • FIG. 5 exemplarily illustrates the reference object surface (shown as column (a)) and the test object surface (shown as column (b)) being heated using a radiation source where the radiation source is at 0, 15, 30 degrees angle with respect to the normal of the surface respectively.
  • the thermal image of the reference object shown as column (a) is different vis-a- vis the thermal image of the reference object shown as column (a), with various angles of exposure to the radiation source.
  • each of said extracted thermal image (for each angle of exposure) of said surface is similarly processed to determine the location of said buried defect with respect to said surface. Further, each of said extracted thermal image (for each angle of exposure) of said surface is similarly processed to determine the dimensions of said buried defect.
  • FIG. 6 exemplarily illustrates the reference object surface and the test object surface matrix, where the columns represent the reference object surface (labelled as no-defect), the test object surface where the test object has a 1 millimetre buried defect (labelled as 1mm length), the test object surface where the test object has a 2 millimetre buried defect (labelled as 2mm length) and the test object surface where the test object has a 3 millimetre buried defect (labelled as 1mm length).
  • the rows represent the test object surface and the reference object surface heated using a radiation source where the radiation source is at 0, 15, 30 degrees angle with respect to the normal of the respective surfaces respectively.
  • the defect within the test object is donut shaped where the red dot in the centre shifts in accordance with the angle of the radiation source.
  • the system may estimate the size, shape and depth of the buried defect by processing said thermal images of said surfaces of the test object and the reference object at various angles of the radiation source.
  • FIG. 7 exemplarily illustrates a simple graphical representation of defect image features stretching with variation in radiation source angle, yellow being the incident radiation, orange being the thermal diffusion from the defect to the surface plot profiles of 1mm (millimetre) & 2mm defect at a) 0° and b) 15°.
  • the width from centre for 0° is about 4.55mm and at 15° is about 6mm, giving about 25% increase in size to the side of the radiation source.
  • the width from centre for 0° is about 4.5mm and at 15° is about 7mm, giving a about 36% increase in size to the side of the heat source.
  • FIG. 2 exemplarily illustrates a block diagram of a system for identifying and locating buried defects using three dimensional infrared thermography in accordance with some of the embodiments of the present invention
  • FIG. 3 exemplarily illustrates a system for identifying and locating buried defects using three dimensional infrared thermography in accordance with some of the embodiments of the present invention.
  • the system for detecting and locating a buried defect 302 in a test object 30, comprises one or more radiation sources (202, 202a, 202b), a processor 201 and an infrared image capture device 203.
  • the one or more radiation sources (202a, 202b) are configured for heating a surface of said test object 301 by exposing said surface with for a first predetermined duration of time from a plurality of angles with respect to said exposed surface of said object (see FIG. 3 the radiation sources expose the surface of the test surface at different angles).
  • the infrared image capture device 203 is operatively coupled to said processor 201 , and said processor 201 is operatively coupled to said radiation sources (202a, 202b).
  • the infrared image capture device 203 comprises a forward looking infrared camera or FPA detector.
  • the processor 201 is configured for: after each exposure of said plurality of angles of exposure: recording a thermogram of said surface for a second predetermined duration using said infrared image capture device 203, wherein said infrared image capture device 203 is static with reference to said test object 301 ; determining a time within said second predetermined time duration where a maximum variation of temperature occurs with respect to a reference thermogram; extracting a thermal image of said surface from said thermogram at said time.
  • the processor 201 is further configured for processing each of said extracted thermal image of said surface to determine the depth of said buried defect 302 with respect to said surface.
  • the reference thermogram comprises a thermogram of a reference surface of a reference object, wherein said reference object is identical to said test object, wherein said reference object does not have any buried defect, wherein said reference surface of said reference object is identical as compared to said surface of said test object, and wherein said reference thermogram is recorded for said second predetermined duration after similarly exposing said reference surface as compared to said surface of said test object.
  • said plurality of exposure angles comprises e.g. - 30° to 30° in 1° steps, wherein said exposure angles are measured with respect to a normal to said surface of said test object.
  • the processor 201 is further configured for processing each of said extracted thermal image (for each angle of exposure) of said surface to determine the location of said buried defect with respect to said surface.
  • the processor 201 is further configured for processing each of said extracted thermal image (for each angle of exposure) of said surface to determine the dimensions of said buried defect.
  • the processor 201 can be further configured to ensure stability and calibrate thermal camera readings dynamically to prevent any variations that occur due to temperature drift. This can be achieved by using an optional self-referenced measurement using thermal data logger.
  • the thermal data logger is a device that can read a thermal sensor positioned in closed proximity to the surface/area under test or inspection within the frame of the camera or FPA detector. By having the comparative thermal logger, error in readings can be reduced. Through comparing the surface of the logger temperature with the thermal camera and the recorded temperature of the logger any errors due to temperature drift can be reduced. This can be calculated or determined using a suitable data analysis, e.g. mathematical statistics fitting, of the temperature distributions over time for both the logger and camera. The normal variation in the thermal camera was found to fit well, thus providing an effective dynamic calibration of the thermal camera.
  • the method/technique of the present invention provides one or more of the following advantages over known techniques:
  • Customization of the setup configuration e.g. the source type and the camera settings may be altered for use in range of devices including industrial and medical devices.
  • the above technique may also be combined with different imaging techniques for defect characterization.
  • the invention can be applied to a wide range of industries, such as additive manufacturing, quality control and inspection, reverse engineering, virtual simulations and others.
  • industries such as additive manufacturing, quality control and inspection, reverse engineering, virtual simulations and others.
  • applications include aerospace, aircraft and defence industry, architecture and construction, automotive, medicine, semiconductors and electronics, automatics, energy and power, heavy industry, mining, and others such as media, education, paintings, forensic, fashion and jewellery, and research.
  • the apparatus described in the present disclosure may be implemented in hardware, firmware, software, or any combination thereof.
  • the processing units, or processors(s) or controller(s) may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, electronic devices, other electronic units designed to perform the functions described herein, or a combination thereof.
  • ASICs application specific integrated circuits
  • DSPs digital signal processors
  • DSPDs digital signal processing devices
  • PLDs programmable logic devices
  • FPGAs field programmable gate arrays
  • processors controllers, micro-controllers, microprocessors, electronic devices, other electronic units designed to perform the functions described herein, or a combination thereof.
  • software codes may be stored in a memory and executed by a processor.
  • Memory may be implemented within the processor unit or external to the processor unit.
  • memory refers to any type of volatile memory or non-volatile memory.

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Abstract

The invention discloses a method and system for detecting and locating a buried defect in a test object, comprises heating a surface of said test object by exposing said surface with one or more radiation sources for a first predetermined duration of time from a plurality of angles with respect to said surface of said object. After each exposure of said plurality of angles, recording a thermogram of said surface for a second predetermined duration using an infrared image capture device, wherein said infrared image capture device is static with reference to said test object; determining a time within said second predetermined time duration where a maximum variation of temperature occurs with respect to a reference thermogram; and extracting a thermal image of said surface from said thermogram at said time.

Description

Title
A method and system for detecting and locating buried defects using three dimensional infrared thermography. Field
The present invention relates to a method and system for identifying and locating buried defects using three dimensional infrared thermography.
Background Thermography is the detection of spatial and temporal distribution of heat patterns in objects. It often uses an infrared camera to detect these heat patterns. Commercial use of thermography is numerous and also includes melt pool temperature monitoring in real time in metal additive manufacturing. Over the last decade, IR thermography (IRT) has established its place as a valuable technique for both metrology and medical applications. A major industrial use of IRT is the method known as active thermography, which quantitatively delivers dimensional values and thermal diffusivity of not only the surface, but also features within the sample, as heat diffusing from the bulk is affected by variations in structure and material. This allows for defect detection and their size, shape as well as location identification within a sample, from the variation in thermal properties that these defects instigate. This technique finds applications in aerospace, additive manufacturing, welding melts and carbon fibre construction. Combining IRT ability to visualize internal defects with imaging and heating a sample from multiple angles, a 3D reconstruction can be achieved to provide 3D-IR thermography (3D-IRT). Three-dimensional IRT imaging also has huge potential for biomedical applications, especially in new patient-oriented paradigm called P4 approach that seeks to make medicine more predictive, preventive, personalized and participatory.
Flash thermography involves exciting a sample with a heat source and recording its surface temperature over the time with a thermographic camera. The propagation of this heat wave through the sample is then influenced by the thermal properties of the sample’s materials. This flash heating creates variance to the systems thermal equilibrium, which highlights defects due to their differing thermal properties. It is noted, that density and heat capacity are the primary impacts in detectability, whereas thermal conductivity and diffusivity are more related to the time required to reach the surface, varying the time window of highest contrast and provide an information about internal structure of the material. Numerous techniques on flash thermography simulations utilize tomographic aspects also, but these techniques utilize multi-face imaging. Usually the object under investigation is symmetrically heated and an infrared camera revolves around the object to obtain multiple thermographic images of the object while being heated.
Therefore, although flash thermography gives detail about the location and the XY dimensions of the sample. This does not however tell us anything about information in the Z direction, meaning the depth of the defect and the thickness of it. The issue with the 2D surface image of flash thermography is that the hotspot dimensions are dependent on numerous variables and are difficult to distinguish. Area of hotspot is dependent on defect size, but also on depth and thickness. Meaning large defects can appear small at large depths and vice versa. Two main factors to the effectiveness of flash thermography are the defect size and depth of wave penetration (on the Z axis above). For the first, at a given depth, defects can be detected only when they are large enough. Typically, the size must be at least equal to the depth. The latter, heat wave propagation gives the depth of detection, as only defects reached by the heat wave will affect the surface temperature.
However, the above techniques fail to detect all the features of the defects within an object and also fails to identify the depth of said defect from a particular surface of said object. Further, thermographic tomography is cumbersome as the object needs to be symmetrically heated and an infrared camera has to revolve around the object to take a plurality of thermographic images from various angles. In an industrial setting, this is inconvenient as the object under investigation (such as a part of an airplane) has to be dismantled and placed within a device which performs thermographic tomography.
Therefore, there is an unresolved and unfulfilled need for a method and system for locating buried defects in an object using three dimensional infrared thermography where said features of buried defect may be identified and located with reference to an exposed surface of said object.
Summary The present invention relates to a method and system for identifying and locating buried defects using three dimensional infrared thermography, as set out in the appended claims. More specifically, the method and system identifies a buried defect in an article or a physical object and also provides a location of said buried defect within said article or physical object. The system and method of the present invention provides a remote and reliable way to locate and/or identify buried defects or physical objects.
In one embodiment there is provided a method for detecting and locating a buried defect in a test object, comprises heating a surface of said test object by exposing said surface with one or more radiation sources for a first predetermined duration of time from a plurality of angles with respect to said surface of said object and after each exposure of said plurality of angles of exposure performing the following steps of: recording a thermogram of said surface for a second predetermined duration using an infrared image capture device, wherein said infrared image capture device is static with reference to said test object; determining a time within said second predetermined time duration where a maximum variation of temperature occurs with respect to a reference thermogram; and extracting a thermal image of said surface from said thermogram at said time.
In one embodiment the reference thermogram comprises a thermogram of a reference surface of a reference object, wherein said reference object is identical to said test object, wherein said reference object does not have any buried defect, wherein said reference surface of said reference object is identical as compared to said surface of said test object, and wherein said reference thermogram is recorded for said second predetermined duration after similarly exposing said reference surface as compared to said surface of said test object.
In one embodiment, the plurality of exposure angles comprises, e.g. -30° to 30° in 1° steps, wherein said exposure angles are measured with respect to a normal to said surface of said test object. In another embodiment the exposure angles can be within a full range of 0° to 360°.
In one embodiment each of said extracted thermal image (for each angle of exposure) of said surface is processed to determine the depth of said buried defect with respect to said surface.
In one embodiment each of said extracted thermal image (for each angle of exposure) of said surface is processed to determine the location of said buried defect with respect to said surface.
In one embodiment each of said extracted thermal image (for each angle of exposure) of said surface is processed to determine the dimensions of said buried defect. In another embodiment there is provided a system for detecting and locating a buried defect in a test object, comprises one or more radiation sources, a processor and an infrared image capture device. said one or more radiation sources are configured for heating a surface of said test object by exposing said surface with for a first predetermined duration of time from a plurality of angles with respect to said exposed surface of said object. The infrared image capture device is operatively coupled to said processor, and said processor is operatively coupled to said radiation sources. In one embodiment the processor is configured for: after each exposure of said plurality of angles of exposure: recording a thermogram of said surface for a second predetermined duration using said infrared image capture device, wherein said infrared image capture device is static with reference to said test object; determining a time within said second predetermined time duration where a maximum variation of temperature occurs with respect to a reference thermogram; extracting a thermal image of said surface from said thermogram at said time.
In one embodiment the processor is further configured for processing each of said extracted thermal image of said surface to determine the depth of said buried defect with respect to said surface.
In one embodiment the reference thermogram comprises a thermogram of a reference surface of a reference object, wherein said reference object is identical to said test object, wherein said reference object does not have any buried defect, wherein said reference surface of said reference object is identical as compared to said surface of said test object, and wherein said reference thermogram is recorded for said second predetermined duration after similarly exposing said reference surface as compared to said surface of said test object. In one embodiment, in the absence of a reference object, said self-referenced thermogram can be made by selection of a clean surface portion or any known temperature object within the thermogram image frame as a temperature reference, to remove or take account of any thermal measurement drifting. In one embodiment, said plurality of exposure angles comprises e.g. -30° to 30° in 1° steps, wherein said exposure angles are measured with respect to a normal to said surface of said test object. In one embodiment the processor is further configured for processing each of said extracted thermal image (for each angle of exposure) of said surface to determine the location of said buried defect with respect to said surface. In one embodiment the processor is further configured for processing each of said extracted thermal image (for each angle of exposure) of said surface to determine the dimensions of said buried defect.
In one embodiment, said infrared image capture device comprises a forward looking infrared (e.g. FLIR) camera or any focal plane array (FPA) thermal detector. In the context of the present invention any thermal detector or image capture device can be used to enable the invention.
In one embodiment there is provided a temperature logger configured to measure the temperature of a surface. In one embodiment the temperature logger is configured to compare the surface temperature recorded with the thermal camera and the recorded temperature of the logger.
Brief Description of the Drawings The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:-
FIG. 1 exemplarily illustrates a flowchart for identifying and locating buried defects using three dimensional infrared thermography in accordance with some of the embodiments of the present invention;
FIG. 2 exemplarily illustrates a block diagram of a system for identifying and locating buried defects using three dimensional infrared thermography in accordance with some of the embodiments of the present invention; FIG. 3 exemplarily illustrates a system for identifying and locating buried defects using three dimensional infrared thermography in accordance with some of the embodiments of the present invention; FIG. 4 exemplarily illustrates temperature vs. time for showing the variation of temperature in the second predetermined time interval: a) temperature of the reference object (blue), b) temperature of the test object (red) and c) variation in temperature between the two (black) in accordance with some of the embodiments of the present invention;
FIG. 5 exemplarily illustrates the reference object surface (shown as column (a)) and the test object surface (shown as column (b)) being heated using a radiation source where the radiation source is at 0, 15, 30 degrees angle with respect to the normal of the surface respectively;
FIG. 6 exemplarily illustrates the reference object surface and the test object surface matrix, where the columns represent the reference object surface (labelled as no-defect), the test object surface where the test object has a 1 millimetre buried defect (labelled as 1mm length), the test object surface where the test object has a 2 millimetre buried defect
(labelled as 2mm length) and the test object surface where the test object has a 3 millimetre buried defect (labelled as 1mm length). The rows represent the test object surface and the reference object surface heated using a radiation source where the radiation source is at 0, 15, 30 degrees angle with respect to the normal of the respective surfaces respectively; and
FIG. 7 exemplarily illustrates a simple graphical representation of defect image features stretching with variation in radiation source angle, yellow being the incident radiation, orange being the thermal diffusion from the defect to the surface plot profiles of 1mm (millimetre) & 2mm defect at a) 0° and b) 15°. For 1mm, the width from centre for 0° is about 4.55mm and at 15° is about 6mm, giving about 25% increase in size to the side of the radiation source. For 2mm, the width from centre for 0° is about 4.5mm and at 15° is about 7mm, giving a about 36% increase in size to the side of the heat source. Detailed Description of the Drawings
The present invention relates to a method and system for identifying and locating buried defects using three dimensional infrared thermography. More specifically, the method and system identifies of a buried defect in an article or a physical object and also provides a location of said buried defect within said article or physical object.
FIG. 1 exemplarily illustrates a flowchart for identifying and locating buried defects using three dimensional infrared thermography in accordance with some of the embodiments of the present invention. The method for detecting and locating a buried defect in a test object, comprises heating 101 a surface of said test object by exposing said surface with one or more radiation sources for a first predetermined duration of time from a plurality of angles with respect to said surface of said object. After each exposure of said plurality of angles of exposure, the following steps are performed. In an embodiment, the plurality of exposure angles comprises-30° to 30° in 1° steps, wherein said exposure angles are measured with respect to a normal to said surface of said test object.
A thermogram of said surface for a second predetermined duration is recorded 101a using an infrared image capture device, wherein said infrared image capture device is static with reference to said test object. A time within said second predetermined time duration is determined 101b where a maximum variation of temperature occurs with respect to a reference thermogram. The reference thermogram comprises a thermogram of a reference surface of a reference object, wherein said reference object is identical to said test object, wherein said reference object does not have any buried defect, wherein said reference surface of said reference object is identical as compared to said surface of said test object, and wherein said reference thermogram is recorded for said second predetermined duration after similarly exposing said reference surface as compared to said surface of said test object. In other words, the reference thermogram is used as a control for comparison.
FIG. 4 exemplarily illustrates temperature vs. time graph for showing the variation of temperature in the second predetermined time interval: a) temperature of the reference object (blue), b) temperature of the test object (red) and c) variation in temperature between the two (black). The position (time) of maximum difference between the surfaces of the reference object and the test object which is caused by the defect in the test object occurs 2.89K at 52 seconds as shown in FIG. 4. In other words, the difference between surface temperatures of the test object and the reference object is maximum at 52 seconds into the second time interval. Thereafter, a thermal image of said surface from said thermogram is extracted 101c at said time, where there is a maximum difference between the surfaces of the reference object (i.e. 52 seconds in accordance with the above embodiment as shown in FIG. 4).
Thereafter, each of said extracted thermal image (for each angle of exposure) of said surface is processed 102 to determine the depth of said buried defect with respect to said surface. FIG. 5 exemplarily illustrates the reference object surface (shown as column (a)) and the test object surface (shown as column (b)) being heated using a radiation source where the radiation source is at 0, 15, 30 degrees angle with respect to the normal of the surface respectively. A person skilled in the art would appreciate that as exemplarily illustrated in FIG. 5 the thermal image of the reference object shown as column (a) is different vis-a- vis the thermal image of the reference object shown as column (a), with various angles of exposure to the radiation source. A person skilled in the art would further appreciate that exposure at 30 degrees angle shows a thicker section of the material being heated at the right bottom as compared to the left bottom of the test object. The position, length, breadth and depth of the defect (from the bottom) may be estimated based on the comparison of the surface temperatures of said reference object and the test object when heated by the radiation source at various angles, density of the material, surface emissivity of the material, heat capacity of the material and thermal conductivity of the material. Also, each of said extracted thermal image (for each angle of exposure) of said surface is similarly processed to determine the location of said buried defect with respect to said surface. Further, each of said extracted thermal image (for each angle of exposure) of said surface is similarly processed to determine the dimensions of said buried defect.
FIG. 6 exemplarily illustrates the reference object surface and the test object surface matrix, where the columns represent the reference object surface (labelled as no-defect), the test object surface where the test object has a 1 millimetre buried defect (labelled as 1mm length), the test object surface where the test object has a 2 millimetre buried defect (labelled as 2mm length) and the test object surface where the test object has a 3 millimetre buried defect (labelled as 1mm length). The rows represent the test object surface and the reference object surface heated using a radiation source where the radiation source is at 0, 15, 30 degrees angle with respect to the normal of the respective surfaces respectively. In FIG. 6 it appears that the defect within the test object is donut shaped where the red dot in the centre shifts in accordance with the angle of the radiation source. Thus, upon calibration of the system using known buried defects (as shown in FIG. 6), the system may estimate the size, shape and depth of the buried defect by processing said thermal images of said surfaces of the test object and the reference object at various angles of the radiation source. For example, FIG. 7 exemplarily illustrates a simple graphical representation of defect image features stretching with variation in radiation source angle, yellow being the incident radiation, orange being the thermal diffusion from the defect to the surface plot profiles of 1mm (millimetre) & 2mm defect at a) 0° and b) 15°. For 1mm, the width from centre for 0° is about 4.55mm and at 15° is about 6mm, giving about 25% increase in size to the side of the radiation source. For 2mm, the width from centre for 0° is about 4.5mm and at 15° is about 7mm, giving a about 36% increase in size to the side of the heat source.
FIG. 2 exemplarily illustrates a block diagram of a system for identifying and locating buried defects using three dimensional infrared thermography in accordance with some of the embodiments of the present invention and FIG. 3 exemplarily illustrates a system for identifying and locating buried defects using three dimensional infrared thermography in accordance with some of the embodiments of the present invention. The system for detecting and locating a buried defect 302 in a test object 30, comprises one or more radiation sources (202, 202a, 202b), a processor 201 and an infrared image capture device 203. The one or more radiation sources (202a, 202b) are configured for heating a surface of said test object 301 by exposing said surface with for a first predetermined duration of time from a plurality of angles with respect to said exposed surface of said object (see FIG. 3 the radiation sources expose the surface of the test surface at different angles). The infrared image capture device 203 is operatively coupled to said processor 201 , and said processor 201 is operatively coupled to said radiation sources (202a, 202b). Suitably the infrared image capture device 203 comprises a forward looking infrared camera or FPA detector.
The processor 201 is configured for: after each exposure of said plurality of angles of exposure: recording a thermogram of said surface for a second predetermined duration using said infrared image capture device 203, wherein said infrared image capture device 203 is static with reference to said test object 301 ; determining a time within said second predetermined time duration where a maximum variation of temperature occurs with respect to a reference thermogram; extracting a thermal image of said surface from said thermogram at said time.
The processor 201 is further configured for processing each of said extracted thermal image of said surface to determine the depth of said buried defect 302 with respect to said surface.
The reference thermogram comprises a thermogram of a reference surface of a reference object, wherein said reference object is identical to said test object, wherein said reference object does not have any buried defect, wherein said reference surface of said reference object is identical as compared to said surface of said test object, and wherein said reference thermogram is recorded for said second predetermined duration after similarly exposing said reference surface as compared to said surface of said test object.
In an embodiment, wherein said plurality of exposure angles comprises e.g. - 30° to 30° in 1° steps, wherein said exposure angles are measured with respect to a normal to said surface of said test object.
The processor 201 is further configured for processing each of said extracted thermal image (for each angle of exposure) of said surface to determine the location of said buried defect with respect to said surface. The processor 201 is further configured for processing each of said extracted thermal image (for each angle of exposure) of said surface to determine the dimensions of said buried defect.
The processor 201 can be further configured to ensure stability and calibrate thermal camera readings dynamically to prevent any variations that occur due to temperature drift. This can be achieved by using an optional self-referenced measurement using thermal data logger. The thermal data logger is a device that can read a thermal sensor positioned in closed proximity to the surface/area under test or inspection within the frame of the camera or FPA detector. By having the comparative thermal logger, error in readings can be reduced. Through comparing the surface of the logger temperature with the thermal camera and the recorded temperature of the logger any errors due to temperature drift can be reduced. This can be calculated or determined using a suitable data analysis, e.g. mathematical statistics fitting, of the temperature distributions over time for both the logger and camera. The normal variation in the thermal camera was found to fit well, thus providing an effective dynamic calibration of the thermal camera. The method/technique of the present invention provides one or more of the following advantages over known techniques:
□ Non-destructive and non-contact even remote measurements are possible for detection of the structure and nature of deep buried defects.
□ Enhancement of the defect visibility/contrast in comparison to the surrounding material.
□This method/technique can be applied to different type of materials ranging from polymers and metals for non-destructive testing for industrial inspection applications to biological tissue for medical applications.
□ Compactness and foldability are major advantages based on the fact of using a simple setup with one radiation source (movable) and fixed camera position in contrast to thermal tomography.
□ Customization of the setup configuration, e.g. the source type and the camera settings may be altered for use in range of devices including industrial and medical devices.
□The above technique may also be combined with different imaging techniques for defect characterization. It will be appreciated that the invention can be applied to a wide range of industries, such as additive manufacturing, quality control and inspection, reverse engineering, virtual simulations and others. For example the non- limitative list of applications include aerospace, aircraft and defence industry, architecture and construction, automotive, medicine, semiconductors and electronics, automatics, energy and power, heavy industry, mining, and others such as media, education, paintings, forensic, fashion and jewellery, and research.
Further, a person ordinarily skilled in the art will appreciate that the various illustrative logical/functional blocks, modules, circuits, and process steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, or a combination of hardware and software. To clearly illustrate this interchangeability of hardware and a combination of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or a combination of hardware and software depends upon the design choice of a person ordinarily skilled in the art. Such skilled artisans may implement the described functionality in varying ways for each particular application, but such obvious design choices should not be interpreted as causing a departure from the scope of the present invention. The process described in the present disclosure may be implemented using various means. For example, the apparatus described in the present disclosure may be implemented in hardware, firmware, software, or any combination thereof. For a hardware implementation, the processing units, or processors(s) or controller(s) may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, electronic devices, other electronic units designed to perform the functions described herein, or a combination thereof.
For a firmware and/or software implementation, software codes may be stored in a memory and executed by a processor. Memory may be implemented within the processor unit or external to the processor unit. As used herein the term “memory” refers to any type of volatile memory or non-volatile memory.
In the specification the terms "comprise, comprises, comprised and comprising" or any variation thereof and the terms include, includes, included and including" or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.
A person skilled in the art would appreciate that the above invention provides a robust and economical solution to the problems identified in the prior art. The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.

Claims

Claims:
1. A method for detecting and locating a buried defect in a test object, comprising: heating a surface of said test object by exposing said surface with one or more radiation sources for a first predetermined duration of time from a plurality of angles with respect to said surface of said object; after each exposure of said plurality of angles of exposure: recording a thermogram of said surface for a second predetermined duration using an infrared image capture device, wherein said infrared image capture device is static with reference to said test object; determining a time within said second predetermined time duration where a maximum variation of temperature occurs with respect to a reference thermogram; extracting a thermal image of said surface from said thermogram at said time; processing each of said extracted thermal image of said surface to determine the depth of said buried defect with respect to said surface.
2. The method of claim 1, further comprising processing each of said extracted thermal image of said surface to determine the location of said buried defect with respect to said surface.
3. The method of claim 1 or 2, further comprising processing each of said extracted thermal image of said surface to determine the dimensions of said buried defect.
4. The method of any preceding claim, wherein the reference thermogram comprises a thermogram of a reference surface of a reference object, wherein said reference object is identical to said test object, wherein said reference object does not have any buried defect, wherein said reference surface of said reference object is identical as compared to said surface of said test object, and wherein said reference thermogram is recorded for said second predetermined duration after similarly exposing said reference surface as compared to said surface of said test object.
5. The method of any preceding claim, wherein said plurality of exposure angles comprises -30° to 30° in 1° steps, wherein said exposure angles are measured with respect to a normal to said surface of said test object.
6. A system for detecting and locating a buried defect in a test object, comprising: one or more radiation sources configured for heating a surface of said test object by exposing said surface with for a first predetermined duration of time from a plurality of angles with respect to said exposed surface of said object; an infrared image capture device operatively coupled to a processor, said processor operatively coupled to said radiation sources, said processor configured for: after each exposure of said plurality of angles of exposure: recording a thermogram of said surface for a second predetermined duration using said infrared image capture device, wherein said infrared image capture device is static with reference to said test object; determining a time within said second predetermined time duration where a maximum variation of temperature occurs with respect to a reference thermogram; extracting a thermal image of said surface from said thermogram at said time; processing each of said extracted thermal image of said surface to determine the depth of said buried defect with respect to said surface.
7. The system of claim 6, wherein said processor is further configured for processing each of said extracted thermal image of said surface to determine the location of said buried defect with respect to said surface.
8. The system of claim 6 or 7, wherein said processor is further configured for processing each of said extracted thermal image of said surface to determine the dimensions of said buried defect.
9. The system of any of claims 6 to 8, wherein the reference thermogram comprises a thermogram of a reference surface of a reference object, wherein said reference object is identical to said test object, wherein said reference object does not have any buried defect, wherein said reference surface of said reference object is identical as compared to said surface of said test object, and wherein said reference thermogram is recorded for said second predetermined duration after similarly exposing said reference surface as compared to said surface of said test object.
10. The system of any of claims claim 6 to 9, wherein said plurality of exposure angles comprises -30° to 30° in 1° steps, wherein said exposure angles are measured with respect to a normal to said surface of said test object.
11. The system of any of claims 6 to 10, wherein said infrared image capture device comprises a forward looking infrared camera or any thermal focal point array.
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