Active thermography

Last updated January 11, 2025

Active thermography is an advanced nondestructive testing procedure, which uses a thermographic measurement of a tested material thermal response after its external excitation. This principle can be used also for non-contact^[1]infrared non-destructive testing (IRNDT) of materials.^[2]

The IRNDT method is based on an excitation of a tested material by an external source, which brings some energy to the material. Halogen lamps, flash-lamps, ultrasonic horn or other sources can be used as the excitation source for the IRNDT. The excitation causes a tested material thermal response, which is measured by an infrared camera. It is possible to obtain information about the tested material surface and sub-surface defects or material inhomogeneities by using a suitable combination of excitation source, excitation procedure, infrared camera and evaluation method.^[3]

Modern thermographic systems with high-speed and high-sensitivity IR cameras extend the possibilities of the inspection method. Modularity of the systems allows their usage for research and development applications as well as in modern industrial production lines.

Thermovision nondestructive testing of components can be carried out on a wide range of various materials. Thermographic inspection of material can be regarded as a method of infrared defectoscopy, that is capable of revealing material imperfections such as cracks, defects, voids, cavities and other inhomogeneities.^[1] The thermographic testing can be provided on individual components in a laboratory or directly on technology facilities that are in duty.

Theory

Active thermography uses an external source for measured object excitation, that means introducing an energy into the object. The excitation sources can be classified by the principles:

optical radiation or microwaves absorption,
electromagnetic induction,
elastic waves transformation (e.g. ultrasound),
convection (e.g. hot air),
plastic deformation transformation (thermoplastic effect during mechanical loading).

Various excitation sources can be used for the active thermography and nondestructive testing, for example laser heating, flash lamps, halogen lamps, electrical heating, ultrasonic horn, eddy currents, microwaves, and others. The measured object can be heated by an external source directly, e.g. by halogen lamps or hot air. The material inhomogeneities or defects cause then a distortion of temperature field. This distortion is detected as temperature differences on the material surface. Another possibility is to use thermophysical processes in the material, when mechanical or electrical energy is transformed into thermal energy due to defects and inhomogeneities. It creates local temperature sources, which cause temperature differences detected on the object surface by infrared techniques, such as in the case of ultrasound excitation.

Methods

A lot of methods were developed for active thermography for the nondestructive testing measurement evaluation. The evaluation methods selection depends on application, used excitation source and excitation type (pulse, periodic, continuous). In the simplest case, the response is evident from a thermogram directly. However, it is necessary to use advanced analysis techniques in most cases. The most common methods include Lock-In, Pulse or Transient (Step thermography) evaluation techniques, with continuous excitation used in some cases:^[4]

Lock-In thermography ^[5]^[6] (periodic excitation method). A modulated periodic source is used for the excitation. The phase and amplitude shift of the measured signal are evaluated and the analysis can be done by various techniques. Halogen lamps, LED lamps, ultrasound, laser ^[7] or an electric current are suitable excitation sources. It has the advantage that it can be used on large surfaces, and it puts a low thermal energy on the part being inspected. The disadvantage is a longer measurement time and dependence of detection capabilities on a geometrical orientation of defects (except of an indirect excitation such as ultrasound). The Lock-In method is suitable for testing components with a low thermal diffusivity and it has many modifications for various specific applications (such as Lock-In Ref, Lock-In Online, etc.).
Pulse thermography (pulse method).^[6] A very short pulse – usually in the units of milliseconds – is used to excite the object. The cooling process is then analyzed. A flash lamp is typically used as an excitation source. The advantage of this method is the speed of the analysis and a possibility to estimate the defects depth. The disadvantage is a limited depth of the analysis, a limited area that can be inspected (with regard to a usable power of excitation sources) and a dependence of detection capabilities on geometrical orientation of defects.
Transient thermography (step thermography, thermal wave method). In principle, the excitation and evaluation are similar to the pulse thermography, however, the pulse length is much bigger. Less powerful excitation sources are required compared to the pulse thermography. It is therefore possible to analyze larger areas and the measurement time is shorter than in the case of Lock-In thermography. As in the pulse thermography, the sensitivity of the method is limited by the geometrical orientation of defects. Halogen lamps are the suitable excitation source for this type of evaluation.
Continual excitation. The simplest method usable only in special applications.

A high-speed cooled infrared camera with a high sensitivity is commonly used for IRNDT applications. However, an uncooled bolometric infrared camera can be used for specific applications. It can significantly reduce acquisition costs of the measurement system.

The IR nondestructive testing system are usually modular. It means that various excitation sources can be combined with various infrared cameras and various evaluation methods depending on application, tested material, measuring time demands, size of a tested area, etc. The modularity allows universal usage of the system for various industrial, scientific and research applications.

Applications

IRNDT (infra-red nondestructive testing) method is suitable for detection and inspection of cracks, defects, cavities, voids and inhomogeneities in material, it is also possible to use the method for inspection of welded joints of metal and plastic parts, inspection of solar cells and solar panels, determination of internal structure of material etc.

The main advantage of IRNDT method is availability for inspection of various materials in wide range of industrial and research applications. IRNDT measurement is fast, nondestructive and noncontact. Restrictive condition for IRNDT method is inspection depth combined with dimension and orientation of defect/crack/inhomogeneity in material.

Inspection of laser welded plastic parts

Laser welding of plastics is a progressive technology of connecting materials with different optical properties. Classical methods for testing of welding performance and weld joints quality – such as the metallographic cut microscopic analysis or X-ray tomography – are not suitable for routine measurements. Pulse IRNDT analysis can be successfully used for weld inspection in many cases.

The images show an example of plastic parts inspection with a defective weld and with a correct weld. The gaps in the defective weld and the correct uninterrupted weld line are both well visible in the results of the IRNDT flash-pulse analysis.

Inspection of laser welded joints

IRNDT evaluation with an indication of weld imperfections and a correct weld of lap joint.

Laser beam welding is a modern technology of fusion welding. Currently finds its wide usage not only in the field of scientific research but also establishes itself in a variety of industries. Among the most frequent users belong the automotive industry, which due to its stable continuous innovation enables fast implementation of advanced technologies in their production. It is clear that laser welding significantly enhances engineering designs and thus brings a number of new products which previously could not be made by conventional methods.

The laser welding can produce quality welds of different types, both extremely thin and thick blanks. Weldable are common carbon steels, stainless steels, aluminum and its alloys, copper, titanium and last but not least, special materials and its combinations.

An integral part of the weldment production is a quality control. Unlike conventional non-destructive test methods, IRNDT is used not only after the laser welding process, but also during it. This makes possible to decide whether or not to the weldment comply with established quality criteria during manufacture process.

Solar cells testing

Active thermography, particularly lock-in thermography, is widely employed for inspecting solar cells.^[6]^[8] While effective, lock-in thermography often requires physical contact with the solar cell for excitation. However, techniques that involve periodic excitation using light sources allow for non-contact testing of electrode-free cells. Common methods such as Illuminated Lock-In Thermography (ILIT) and Open Circuit Voltage Illuminated Lock-In Thermography (VOC-ILIT) are used to investigate defects or issues like ohmic shunts, cracks, open or short circuits, and degradation in photovoltaic materials. Pulsed thermography, another method under investigation, provides a non-contact alternative with significantly reduced inspection times; however, it usually offers lower detectability than the ILIT method.

Related Research Articles

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.

Infrared thermography (IRT), thermal video 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.

Plastic welding is welding for semi-finished plastic materials, and is described in ISO 472 as a process of uniting softened surfaces of materials, generally with the aid of heat. Welding of thermoplastics is accomplished in three sequential stages, namely surface preparation, application of heat and pressure, and cooling. Numerous welding methods have been developed for the joining of semi-finished plastic materials. Based on the mechanism of heat generation at the welding interface, welding methods for thermoplastics can be classified as external and internal heating methods, as shown in Fig 1.

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. In commercial and general knowledge domains, it is commonly accepted that transparent ceramics or ceramic glass are varieties of strengthened glass, such as those used for the screen glass on an iPhone.

Phosphor thermometry is an optical method for surface temperature measurement. The method exploits luminescence emitted by phosphor material. Phosphors are fine white or pastel-colored inorganic powders which may be stimulated by any of a variety of means to luminesce, i.e. emit light. Certain characteristics of the emitted light change with temperature, including brightness, color, and afterglow duration. The latter is most commonly used for temperature measurement.

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.

Eddy-current testing is one of many electromagnetic testing methods used in nondestructive testing (NDT) making use of electromagnetic induction to detect and characterize surface and sub-surface flaws in conductive materials.

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 centre 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 and erosion. Ultrasonic testing is extensively used to detect flaws in welds.

An infrared thermometer is a thermometer which infers temperature from a portion of the thermal radiation sometimes called black-body radiation emitted by the object being measured. They are sometimes called laser thermometers as a laser is used to help aim the thermometer, or non-contact thermometers or temperature guns, to describe the device's ability to measure temperature from a distance. By knowing the amount of infrared energy emitted by the object and its emissivity, the object's temperature can often be determined within a certain range of its actual temperature. Infrared thermometers are a subset of devices known as "thermal radiation thermometers".

Phased array ultrasonics (PA) is an advanced method of ultrasonic testing that has applications in medical imaging and industrial nondestructive testing. Common applications are to noninvasively examine the heart or to find flaws in manufactured materials such as welds. Single-element probes, known technically as monolithic probes, emit a beam in a fixed direction. To test or interrogate a large volume of material, a conventional probe must be physically scanned to sweep the beam through the area of interest. In contrast, the beam from a phased array probe can be focused and swept electronically without moving the probe. The beam is controllable because a phased array probe is made up of multiple small elements, each of which can be pulsed individually at a computer-calculated timing. The term phased refers to the timing, and the term array refers to the multiple elements. Phased array ultrasonic testing is based on principles of wave physics, which also have applications in fields such as optics and electromagnetic antennae.

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

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).

Rail inspection is the practice of examining rail tracks for flaws that could lead to catastrophic failures. According to the United States Federal Railroad Administration Office of Safety Analysis, track defects are the second leading cause of accidents on railways in the United States. The leading cause of railway accidents is attributed to human error. The contribution of poor management decisions to rail accidents caused by infrequent or inadequate rail inspection is significant but not reported by the FRA, only the NTSB. Every year, North American railroads spend millions of dollars to inspect the rails for internal and external flaws. Nondestructive testing (NDT) methods are used as preventive measures against track failures and possible derailment.

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

An 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.

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.

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.

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."

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.

Carbon fiber testing is a set of various different tests that researchers use to characterize the properties of carbon fiber. The results for the testing are used to aid the manufacturer and developers decisions selecting and designing material composites, manufacturing processes and for ensured safety and integrity. Safety-critical carbon fiber components, such as structural parts in machines, vehicles, aircraft or architectural elements are subject to testing.

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 2 Kotovshchikov, I O (2019-11-01). "Active thermography as a contemporary method for ensuring the quality of composite material products". IOP Conference Series: Materials Science and Engineering. 683 (1): 012076. doi: 10.1088/1757-899X/683/1/012076 . ISSN 1757-8981.
↑ Skala, J.; Svantner, M.; Tesar, J.; Franc, A. (2016-12-01). "Active thermography inspection of protective glass contamination on laser scanning heads". Applied Optics. 55 (34): D60. doi:10.1364/AO.55.000D60. ISSN 0003-6935.
↑ Švantner, Michal; Muzika, Lukáš; Moskovchenko, Alexey; Pereira, Celeste M.C.; Das, Shumit (November 2022). "Repeatability study of flash-pulse thermographic inspection of carbon-fiber composite samples". Infrared Physics & Technology. 126: 104350. doi:10.1016/j.infrared.2022.104350.
↑ Budzier, Helmut; Gerlach, Gerald (2018), Ida, Nathan; Meyendorf, Norbert (eds.), "Active Thermography", Handbook of Advanced Non-Destructive Evaluation, Cham: Springer International Publishing, pp. 1–19, doi:10.1007/978-3-319-30050-4_13-1, ISBN 978-3-319-30050-4 , retrieved 2024-11-20
↑ Choi, Manyong; Kang, Kisoo; Park, Jeonghak; Kim, Wontae; Kim, Koungsuk (2008-03-01). "Quantitative determination of a subsurface defect of reference specimen by lock-in infrared thermography". NDT & E International. 41 (2): 119–124. doi:10.1016/j.ndteint.2007.08.006. ISSN 0963-8695.
1 2 3 Muzika, L.; Švantner, M.; Kučera, M. (2018-06-20). "Lock-in and pulsed thermography for solar cell testing". Applied Optics. 57 (18): D90 –D97. doi:10.1364/AO.57.000D90. ISSN 2155-3165.
↑ An, Yun-Kyu; Min Kim, Ji; Sohn, Hoon (2014-07-01). "Laser lock-in thermography for detection of surface-breaking fatigue cracks on uncoated steel structures". NDT & E International. 65: 54–63. doi:10.1016/j.ndteint.2014.03.004. ISSN 0963-8695.
↑ Bauer, Jan; Breitenstein, Otwin; Wagner, Jan-Martin (2009-08-01). "Lock-in Thermography: A Versatile Tool for Failure Analysis of Solar Cells". EDFA Technical Articles. 11 (3): 6–12. doi:10.31399/asm.edfa.2009-3.p006. ISSN 1537-0755.

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[:0-1] 1 2 Kotovshchikov, I O (2019-11-01). "Active thermography as a contemporary method for ensuring the quality of composite material products". IOP Conference Series: Materials Science and Engineering. 683 (1): 012076. doi: 10.1088/1757-899X/683/1/012076 . ISSN 1757-8981.

[2] Skala, J.; Svantner, M.; Tesar, J.; Franc, A. (2016-12-01). "Active thermography inspection of protective glass contamination on laser scanning heads". Applied Optics. 55 (34): D60. doi:10.1364/AO.55.000D60. ISSN 0003-6935.

[3] Švantner, Michal; Muzika, Lukáš; Moskovchenko, Alexey; Pereira, Celeste M.C.; Das, Shumit (November 2022). "Repeatability study of flash-pulse thermographic inspection of carbon-fiber composite samples". Infrared Physics & Technology. 126: 104350. doi:10.1016/j.infrared.2022.104350.

[4] Budzier, Helmut; Gerlach, Gerald (2018), Ida, Nathan; Meyendorf, Norbert (eds.), "Active Thermography", Handbook of Advanced Non-Destructive Evaluation, Cham: Springer International Publishing, pp. 1–19, doi:10.1007/978-3-319-30050-4_13-1, ISBN 978-3-319-30050-4 , retrieved 2024-11-20

[5] Choi, Manyong; Kang, Kisoo; Park, Jeonghak; Kim, Wontae; Kim, Koungsuk (2008-03-01). "Quantitative determination of a subsurface defect of reference specimen by lock-in infrared thermography". NDT & E International. 41 (2): 119–124. doi:10.1016/j.ndteint.2007.08.006. ISSN 0963-8695.

[:1-6] 1 2 3 Muzika, L.; Švantner, M.; Kučera, M. (2018-06-20). "Lock-in and pulsed thermography for solar cell testing". Applied Optics. 57 (18): D90 –D97. doi:10.1364/AO.57.000D90. ISSN 2155-3165.

[7] An, Yun-Kyu; Min Kim, Ji; Sohn, Hoon (2014-07-01). "Laser lock-in thermography for detection of surface-breaking fatigue cracks on uncoated steel structures". NDT & E International. 65: 54–63. doi:10.1016/j.ndteint.2014.03.004. ISSN 0963-8695.

[8] Bauer, Jan; Breitenstein, Otwin; Wagner, Jan-Martin (2009-08-01). "Lock-in Thermography: A Versatile Tool for Failure Analysis of Solar Cells". EDFA Technical Articles. 11 (3): 6–12. doi:10.31399/asm.edfa.2009-3.p006. ISSN 1537-0755.

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