Thermographic inspection

Last updated

Thermographic inspection refers to the nondestructive testing (NDT) of parts, materials or systems through the imaging of the temperature fields, gradients and/or patterns ("thermograms") at the object's surface. It is distinguished from medical thermography by the subjects being examined: thermographic inspection generally examines inanimate objects, while medical thermography generally examines living organisms. Generally, thermographic inspection is performed using an infrared sensor (thermographic camera).

Contents

Terminology

Thermography refers to the visualization of thermograms, and encompasses all thermographic inspection techniques regardless of the technique used. For instance, a temperature sensitive coating applied to a surface to measure its temperature fields is a thermographic inspection contact technique based on heat conduction, and no infrared sensor is involved.

Infrared thermography specifically refers to a nonintrusive, noncontact mapping of thermograms on the surface of objects using a detector that is sensitive to infrared radiation. [1]

There are many other terms widely used, all referring to infrared thermography; the adoption of specific term(s) depends on the author's background and preferences. For instance, video thermography and thermal imaging draw attention to the acquisition of a temporal sequence of images that may be displayed as a movie. Pulse-echo thermography and thermal wave imaging [2] [3] [4] [5] are adopted to emphasize the wave nature of heat. Pulsed video thermography, [6] [7] transient thermography, [8] [9] [10] and flash thermography are used when the specimen is stimulated using a short energy pulse. [11]

Characteristics

When compared with other classical NDT techniques such as ultrasonic or radiographic testing, thermographic inspection is safe, nonintrusive, and usually noncontact, allowing the detection of relatively shallow subsurface defects (a few millimeters in depth) under large surfaces (typically covering an area of 30 by 30 cm (12 by 12 in) at once, although inspection of larger surfaces is possible) and quickly (from a fraction of a second to a few minutes depending on the configuration).

Techniques

In addition, there are two mutually exclusive approaches in thermographic inspection:

  1. passive, in which the features of interest are naturally at a higher or lower temperature than the background and no energy is introduced to the system being inspected. For example, the surveillance of people on a scene using a thermal imaging camera.
  2. active, in which an energy source is required to produce a thermal contrast between the feature of interest and the background. For example, internal flaws in an aircraft part may be identified by exciting the part with ultrasonic energy; the flaw responds to the ultrasonic energy through frictional heating, which can then be detected with a thermal imaging camera.

Passive techniques

Typically, passive techniques display information from an infrared sensor on a monitor; these images can be visualized in black and white or in false color. Passive techniques are capable of detecting temperature differences as small as 0.01 °C above or below ambient temperatures.

Active techniques

Infrared thermgography techniques InfraredThermography Techniques.gif
Infrared thermgography techniques

Active techniques may be further subdivided depending on the type of energy imparted (typically, optical or acoustic), whether energy is applied externally or internally, and mode of excitation.

A wide variety of energy sources can be used to induce a thermal contrast between defective and non-defective zones that can be divided in external, if the energy is delivered to the surface and then propagated through the material until it encounters a flaw; or internal, if the energy is injected into the specimen in order to stimulate exclusively the defects. Typically, external excitation is performed with optical devices such as photographic flashes (for heat pulsed stimulation) or halogen lamps (for periodic heating), whereas internal excitation can be achieved by means of mechanical oscillations, with a sonic or ultrasonic transducer [12] for both burst and amplitude modulated stimulation. [13]

As depicted in the figure, there are three classical active thermographic techniques based on these two excitation modes: lock-in (or modulated) thermography and pulsed thermography, which are optical techniques applied externally; and vibrothermography, [14] which uses ultrasonic waves (amplitude modulated or pulses) to excite internal features. In vibrothermography, an external mechanical energy source induces a temperature difference between the defective and non-defective areas of the object. In this case, the temperature difference is the main factor that causes the emission of a broad electromagnetic spectrum of infrared radiation, which is not visible to the human eye. The locations of the defects can then be detected by infrared cameras through the process of mapping temperature distribution on the surface of the object. [14]

See also

Related Research Articles

<span class="mw-page-title-main">Infrared</span> Form of electromagnetic radiation

Infrared is electromagnetic radiation (EMR) with wavelengths longer than those of visible light and shorter than radio waves. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around 1 millimeter (300 GHz) to the nominal red edge of the visible spectrum, around 700 nanometers (430 THz). Longer IR wavelengths are sometimes included as part of the terahertz radiation range. Almost all black-body radiation from objects near room temperature is at infrared wavelengths. As a form of electromagnetic radiation, IR propagates energy and momentum, exerts radiation pressure, and has properties corresponding to both those of a wave and of a particle, the photon.

<span class="mw-page-title-main">Ultrasound</span> Sound waves with frequencies above the human hearing range

Ultrasound is sound with frequencies greater than 20 kilohertz. This frequency is the approximate upper audible limit of human hearing in healthy young adults. The physical principles of acoustic waves apply to any frequency range, including ultrasound. Ultrasonic devices operate with frequencies from 20 kHz up to several gigahertz.

<span class="mw-page-title-main">Photoluminescence</span> Light emission from substances after they absorb photons

Photoluminescence is light emission from any form of matter after the absorption of photons. It is one of many forms of luminescence and is initiated by photoexcitation, hence the prefix photo-. Following excitation, various relaxation processes typically occur in which other photons are re-radiated. Time periods between absorption and emission may vary: ranging from short femtosecond-regime for emission involving free-carrier plasma in inorganic semiconductors up to milliseconds for phosphoresence processes in molecular systems; and under special circumstances delay of emission may even span to minutes or hours.

<span class="mw-page-title-main">Thermographic camera</span> Imaging device using infrared radiation

A thermographic camera is a device that creates an image using infrared (IR) radiation, similar to a normal camera that forms an image using visible light. Instead of the 400–700 nanometre (nm) range of the visible light camera, infrared cameras are sensitive to wavelengths from about 1,000 nm to about 14,000 nm (14 μm). The practice of capturing and analyzing the data they provide is called thermography.

<span class="mw-page-title-main">Nondestructive testing</span> Evaluating the properties of a material, component, or system without causing damage

Nondestructive testing (NDT) is any of a wide group of analysis techniques used in science and technology industry to evaluate the properties of a material, component or system without causing damage. The terms nondestructive examination (NDE), nondestructive inspection (NDI), and nondestructive evaluation (NDE) are also commonly used to describe this technology. Because NDT does not permanently alter the article being inspected, it is a highly valuable technique that can save both money and time in product evaluation, troubleshooting, and research. The six most frequently used NDT methods are eddy-current, magnetic-particle, liquid penetrant, radiographic, ultrasonic, and visual testing. NDT is commonly used in forensic engineering, mechanical engineering, petroleum engineering, electrical engineering, civil engineering, systems engineering, aeronautical engineering, medicine, and art. Innovations in the field of nondestructive testing have had a profound impact on medical imaging, including on echocardiography, medical ultrasonography, and digital radiography.

<span class="mw-page-title-main">Thermography</span> Use of thermograms to study heat distribution in structures or regions

Infrared thermography (IRT), thermal video and/or thermal imaging, is a process where a thermal camera captures and creates an image of an object by using infrared radiation emitted from the object in a process, which are examples of infrared imaging science. Thermographic cameras usually detect radiation in the long-infrared range of the electromagnetic spectrum and produce images of that radiation, called thermograms. Since infrared radiation is emitted by all objects with a temperature above absolute zero according to the black body radiation law, thermography makes it possible to see one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature; therefore, thermography allows one to see variations in temperature. When viewed through a thermal imaging camera, warm objects stand out well against cooler backgrounds; humans and other warm-blooded animals become easily visible against the environment, day or night. As a result, thermography is particularly useful to the military and other users of surveillance cameras.

<span class="mw-page-title-main">Transparent ceramics</span> Ceramic materials that are optically transparent

Many ceramic materials, both glassy and crystalline, have found use as optically transparent materials in various forms from bulk solid-state components to high surface area forms such as thin films, coatings, and fibers. Such devices have found widespread use for various applications in the electro-optical field including: optical fibers for guided lightwave transmission, optical switches, laser amplifiers and lenses, hosts for solid-state lasers and optical window materials for gas lasers, and infrared (IR) heat seeking devices for missile guidance systems and IR night vision.

Laser-ultrasonics uses lasers to generate and detect ultrasonic waves. It is a non-contact technique used to measure materials thickness, detect flaws and carry out materials characterization. The basic components of a laser-ultrasonic system are a generation laser, a detection laser and a detector.

<span class="mw-page-title-main">Ultrasonic testing</span> Non-destructive material testing using ultrasonic waves

Ultrasonic testing (UT) is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material tested. In most common UT applications, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz, and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize materials. A common example is ultrasonic thickness measurement, which tests the thickness of the test object, for example, to monitor pipework corrosion.

<span class="mw-page-title-main">Time-of-flight diffraction ultrasonics</span>

Time-of-flight diffraction (TOFD) method of ultrasonic testing is a sensitive and accurate method for the nondestructive testing of welds for defects. TOFD originated from tip diffraction techniques which were first published by Silk and Liddington in 1975 which paved the way for TOFD. Later works on this technique are given in a number of sources which include Harumi et al. (1989), Avioli et al. (1991), and Bray and Stanley (1997).

<span class="mw-page-title-main">Electromagnetic acoustic transducer</span>

Electromagnetic acoustic transducer (EMAT) is a transducer for non-contact acoustic wave generation and reception in conducting materials. Its effect is based on electromagnetic mechanisms, which do not need direct coupling with the surface of the material. Due to this couplant-free feature, EMATs are particularly useful in harsh, i.e., hot, cold, clean, or dry environments. EMATs are suitable to generate all kinds of waves in metallic and/or magnetostrictive materials. Depending on the design and orientation of coils and magnets, shear horizontal (SH) bulk wave mode, surface wave, plate waves such as SH and Lamb waves, and all sorts of other bulk and guided-wave modes can be excited. After decades of research and development, EMAT has found its applications in many industries such as primary metal manufacturing and processing, automotive, railroad, pipeline, boiler and pressure vessel industries, in which they are typically used for nondestructive testing (NDT) of metallic structures.

Photothermal spectroscopy is a group of high sensitivity spectroscopy techniques used to measure optical absorption and thermal characteristics of a sample. The basis of photothermal spectroscopy is the change in thermal state of the sample resulting from the absorption of radiation. Light absorbed and not lost by emission results in heating. The heat raises temperature thereby influencing the thermodynamic properties of the sample or of a suitable material adjacent to it. Measurement of the temperature, pressure, or density changes that occur due to optical absorption are ultimately the basis for the photothermal spectroscopic measurements.

Infrared vision is the capability of biological or artificial systems to detect infrared radiation. The terms thermal vision and thermal imaging, are also commonly used in this context since infrared emissions from a body are directly related to their temperature: hotter objects emit more energy in the infrared spectrum than colder ones.

<span class="mw-page-title-main">Infrared and thermal testing</span>

Infrared and thermal testing refer to passive thermographic inspection techniques, a class of nondestructive testing designated by the American Society for Nondestructive Testing (ASNT). Infrared thermography is the science of measuring and mapping surface temperatures.

"Infrared thermography, a nondestructive, remote sensing technique, has proved to be an effective, convenient, and economical method of testing concrete. It can detect internal voids, delaminations, and cracks in concrete structures such as bridge decks, highway pavements, garage floors, parking lot pavements, and building walls. As a testing technique, some of its most important qualities are that (1) it is accurate; (2) it is repeatable; (3) it need not inconvenience the public; and (4) it is economical."

Maintenance of today's bridge infrastructure presents many challenges. Transportation engineering and maintenance personnel must maintain around the clock service to millions of people each year while maintaining millions of cubic meters of concrete distributed throughout their facilities. This infrastructure includes bridges. Presently only a limited number of accurate and economical techniques exist to test these structures for integrity and safety as well as insure that they meet original design specifications.

Terahertz nondestructive evaluation pertains to devices, and techniques of analysis occurring in the terahertz domain of electromagnetic radiation. These devices and techniques evaluate the properties of a material, component or system without causing damage.

<span class="mw-page-title-main">TeraView</span>

TeraView Limited, or TeraView, is a company that designs terahertz imaging and spectroscopy instruments and equipment for measurement and evaluation of pharmaceutical tablets, nanomaterials, ceramics and composites, integrated circuit chips and more.

Active thermography is an advanced nondestructive testing procedure, which uses a thermography measurement of a tested material thermal response after its external excitation. This principle can be used also for non-contact infrared non-destructive testing (IRNDT) of materials.

Welding of advanced thermoplastic composites is a beneficial method of joining these materials compared to mechanical fastening and adhesive bonding. Mechanical fastening requires intense labor, and creates stress concentrations, while adhesive bonding requires extensive surface preparation, and long curing cycles. Welding these materials is a cost-effective method of joining concerning preparation and execution, and these materials retain their properties upon cooling, so no post processing is necessary. These materials are widely used in the aerospace industry to reduce weight of a part while keeping strength.

A variety of non-destructive examination (NDE) techniques are available for inspecting plastic welds. Many of these techniques are similar to the ones used for inspecting metal welds. Traditional techniques include visual testing, radiography, and various ultrasonic techniques. Advanced ultrasonic techniques such as time of flight diffraction (TOFD) and phased-array ultrasonics (PAUT) are being increasingly studied and used for inspecting plastic pipeline welds. Research in the use of optical coherence tomography (OCT) and microwave reflectrometry has also been conducted.

References

  1. ↑ Maldague X. P. V.; Jones T. S.; Kaplan H.; Marinetti S.; Prystay M. (2001). "2: Fundamentals of Infrared and Thermal Testing, Part 1. Principles of Infrared and Thermal Testing". In X. Maldague (technical); P. O. Moore (eds.). Nondestructive Testing Handbook. Vol. 3, Infrared and Thermal Testing (3rd ed.). Columbus, Ohio: The American Society for Nondestructive Testing. ISBN   1-57117-044-8.
  2. ↑ Favro, L. D.; Han, X. (1998). "Thermal Wave Materials Characterization and Thermal Wave Imaging". In Birnbaum, G.; Auld, B. A. (eds.). Sensing for Materials Characterization, Processing and Manufacturing, ASNT TONES. Vol. 1. American Society for Nondestructive Testing. pp. 399–415. ISBN   978-1571170675.
  3. ↑ Han, X.; Favro, L. D.; Kuo, P. K.; Thomas, R. L. (1996). "Early-Time Pulse-Echo Thermal Wave Imaging". In Thompson, D. O.; Chimenti, D. E. (eds.). Review of Progress in Quantitative Nondestructive Evaluation. Vol. 15. Boston, Massachusetts: Springer. pp. 519–524. doi:10.1007/978-1-4613-0383-1_66. ISBN   978-1-4613-0383-1 . Retrieved 2022-11-10.
  4. ↑ Favro, L. D.; Han, X.; Wang, Y.; Kuo, P. K.; Thomas, R. L. (1995). "Pulse-echo thermal wave imaging". In Thompson, D. O.; Chimenti, D. E. (eds.). Review of Progress in Quantitative Nondestructive Evaluation. Boston, Massachusetts: Springer. pp. 14:425–429. CiteSeerX   10.1.1.1028.3194 . doi:10.1007/978-1-4615-1987-4_50. ISBN   978-1-4615-1987-4.
  5. ↑ Favro, Lawrence D.; Han, Xiaoyan; Kuo, Pao-Kuang; Thomas, Robert L. (March 28, 1995). Imaging the early time behavior of reflected thermal wave pulses. Thermosense XVII: SPIE's 1995 Symposium on OE/Aerospace Sensing and Dual Use Photonics. Vol. 2473. Orlando, Florida: Society of Photo-Optical Instrumentation Engineers (SPIE). pp. 162–166. doi:10.1117/12.204850.
  6. ↑ Milne J. M.; Reynolds W. N. (March 20, 1985). The Non-Destructive Evaluation of Composites and other Materials by Thermal Pulse Video Thermography. Thermosense VII: Thermal Infrared Sensing for Diagnostics and Control. Vol. 520. Cambridge, United States: Society of Photo-Optical Instrumentation Engineers (SPIE). pp. 119–122. doi:10.1117/12.946141.
  7. ↑ Reynolds, W. N. (1986). "Thermographic methods applied to industrial materials". Canadian Journal of Physics. Canadian Science Publishing. 64 (9): 1150–1154. Bibcode:1986CaJPh..64.1150R. doi:10.1139/p86-200. ISSN   0008-4204.
  8. ↑ Almond, D. P.; Lau, S. K. (1994). "Defect sizing by transient thermography. I. An analytical treatment". Journal of Physics D: Applied Physics. 27 (5): 1063–1069. Bibcode:1994JPhD...27.1063A. doi:10.1088/0022-3727/27/5/027. ISSN   0022-3727. S2CID   250814247.
  9. ↑ Almond, D. P.; Lau, S. K. (1993-06-21). "Edge effects and a method of defect sizing for transient thermography". Applied Physics Letters. AIP Publishing. 62 (25): 3369–3371. Bibcode:1993ApPhL..62.3369A. doi:10.1063/1.109074. ISSN   0003-6951.
  10. ↑ Saintey, M. B.; Almond, D. P. (December 1995). "Defect sizing by transient thermography. II. A numerical treatment". Journal of Physics D: Applied Physics. 28 (12): 2539–2546. Bibcode:1995JPhD...28.2539S. doi:10.1088/0022-3727/28/12/023. ISSN   0022-3727. S2CID   250751536.
  11. ↑ Parker, W. J.; Jenkins, R. J.; Butler, C. P.; Abbott, G. L. (1 September 1961). "Flash Method of Determining Thermal Diffusivity, Heat Capacity, and Thermal Conductivity". Journal of Applied Physics. 32 (9): 1679–1684. Bibcode:1961JAP....32.1679P. doi:10.1063/1.1728417. ISSN   0021-8979.
  12. ↑ Renshaw, Jeremy Blake; Chen, John C.; Holland, Stephen D.; Thompson, R. Bruce (December 2011). "The sources of heat generation in vibrothermography". NDT & E International. Center for Nondestructive Evaluation Publications. 44 (8): 736–739. doi:10.1016/j.ndteint.2011.07.012.
  13. ↑ Irana, Egor. "Clip-on Thermal Scopes". vtoptics.com. Retrieved 6 November 2019.
  14. 1 2 Parvasi, Seyed Mohammad; Xu, Changhang; Kong, Qingzhao; Song, Gangbing (3 April 2016). "Detection of multiple thin surface cracks using vibrothermography with low-power piezoceramic-based ultrasonic actuator—a numerical study with experimental verification". Smart Materials and Structures. 25 (5): 055042. Bibcode:2016SMaS...25e5042P. doi:10.1088/0964-1726/25/5/055042. ISSN   0964-1726. S2CID   113264322.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.