understanding infrared thermography reading 6 chapter 3

Charlie Chong/ Fion Zhang

NDEQC-V16+1Thermal Inspection


1. IntroductionTHERMAL INSPECTION comprises all methods in which heat-sensing evices are used to measure temperature variations in components, structures,systems, or physical processes. Thermal methods can be useful in thedetection of subsurface flaws or voids, provided the depth of the flaw is notlarge compared to its diameter. Thermal inspection becomes less effective inthe detection of subsurface flaws as the thickness of an object increases,because the possible depth of the defects increases. Thermal inspection isapplicable to complex shapes or assemblies of similar or dissimilar materialsand can be used in the one-sided inspection of objects. Moreover, because ofthe availability of infrared sensing systems, thermal inspection can alsoprovide rapid, noncontact scanning of surfaces, components, or assemblies.Thermal inspection does not include those methods that use thermalexcitation of a test object and a nonthermal sensing device for inspection. Forexample, thermally induced strain in holography or the technique of thermalexcitation with ultrasonic or acoustic methods does not constitute thermalinspection.


2. Principles of Thermal inspectionThe basic principle of thermal inspection involves the measurement or mapping of surface temperatures when heat flows (1) from, (2) to, or (3) through a test object.

Temperature differentials on a surface, or changes in surface temperature with time, are related to heat flow patterns and can be used to detect flaws or to determine the heat transfer characteristics of a test body. For example, during the operation of a heating system, a hot spot detected at a joint in a heating duct may be caused by a hot air leak. Another example would be a hot spot generated when an adhesive-bonded panel is uniformly heated on one side. A localized debonding between the surface being heated and the substructure would hinder heat flow to the substructure and thus cause a region of higher temperature when compared to the rest of the surface. Generally, the larger the imperfection and the closer it is to the surface, the greater the temperature differential.


Heat Transfer Mechanisms. Heat will flow from hot to cold within an object by conduction and between an object and its surroundings by (1) conduction,(2) convection, and (3) radiation.

Within a solid or liquid, conduction results from the random vibrations of individual atoms or molecules that is transferred via the atomic bonding to neighboring atoms or molecules.

In a gas, the same process occurs but is somewhat impeded by the greater distance between the atoms or molecules and the lack of bonds, thus requiring collisions to transfer the energy. When a gas or liquid flows over a solid, heat is transferred by convection. This occurs from the collisions between the atoms or molecules of the gas or liquid with the surface (conduction) as well as the transport of the gas or liquid to and from the surface. Convection depends upon the velocity of the gas or liquid, and cooling by convection increases as the velocity of the gas or liquid increases.


Radiation is the remaining mechanism for heat transfer. Although conduction and convection are generally the primary heat transfer mechanisms in a test object, the nature of thermally induced radiation can be important, particularly when temperature measurements are made with radiation sensors.

Electromagnetic radiation is emitted from a heated body when electrons within the body change to a lower energy state. Both the intensity and thewavelength of the radiation depend on the temperature of the surface atomsor molecules. For a blackbody, the radiation wavelength and spectralemission power vary as a function of temperature, as shown in Fig. 1. At 300K (27 °C), the temperature of a warm day, the dominant wavelength is 10 μm(400 μin.), which is in the infrared region (Fig. 2). A surface would have to bemuch hotter for the dominant wavelength to fall in the visible region below 0.7 μm (30 μin.). Examples of this are red-hot steel in a forge, a light bulb filament, and the sun (indicated as solar radiation in Fig. 1 at a temperature of 5800 K). However, most subjects in thermal inspection methods will be at temperatures near room temperature and will emit in the infrared region.


Fig. 1 Spectral blackbody emissive power


Fig. 2 Spectrum of electromagnetic radiation


Material Heat Transfer Characteristics. The heat transfer mechanisms of conduction, convection, and radiation are affected by material heat transfercharacteristics, and thermal inspection depends on local variations in thematerial heat transfer characteristics. The material characteristics that affectconduction and convection are as follows:

Specific heat, c, is the amount of heat a mass of material will absorb for a given temperature interval

Density, ρ, is the mass per unit volume of the material Thermal conductivity, k, is the amount of heat that flows in a given

direction when there is a temperature difference across the material in that direction

Thermal diffusivity, α, is the speed at which the heat flows away from a region of higher temperature to the surrounding material

Convection heat transfer coefficient, h, is a measure of how efficiently heat is exchanged between a surface and a flowing gas or liquid

Temperature, T, is a measure of the heat energy (local thermal agitation) contained at each point in the test object


Thermal inspection depends on differences in these material characteristics to establish a measurable, and usually localized, temperature differential. For example, when a test body with variations in density and specific heat is heated or cooled from a state of uniform temperature, the change in temperature will occur more slowly in the regions with a higher density and/or specific heat. This difference in the rate of change of temperature within the body produces temperature differentials, which may be measurable. The important material characteristic in radiation heat transfer is the emissivity, ε, of a test surface. The emissivity indicates the efficiency of a surface as a radiator (or absorber) of electromagnetic radiation.

Blackbodies, the most efficient radiators and absorbers of electromagnetic radiation, have an emissivity of 1.0. All other bodies have an emissivity less than 1.0. Emissivity is a function of several variables, including color and surface roughness. Like other material heat transfer characteristics, variations in emissivity are important in thermal inspection. This is particularly true when surface temperatures are measured with infrared sensors. Variations in emissivity change the power of radiation emitted at a given temperature and thus affect infrared temperature measurements.


3. Surface PreparationThe surface condition of the test object is important for thermal inspection. Inspection results can be influenced by variations in surface roughness,cleanliness, foreign material (such as decals), and the uniformity andcondition of paint or other surface coatings. A good practice is to clean thesurface, remove or strip poorly adhering coatings (if present), and then applya uniform coating of readily removable flat-black paint. This will allow uniformheat transfer into (or from) the subject and will also produce a reasonablyuniform emissivity ε.


4. Establishing Heat FlowIn thermal inspection, the test object can be classified as either thermally active or thermally passive. Thermally active test objects generate thenecessary heat flow during their operation, while thermally passive testobjects require an external heat source or heat sink.

Thermally Active Test Objects. Some test objects can be inspected without the application or removal of heat because they are involved in a process thateither generates or removes heat. When a defect results in an abnormaltemperature distribution on the surface, no external heating or cooling isrequired. When the heat transfer process is transient, the timing of theinspection is important. An example of this would be a fluid-contaminatedhoneycomb panel on an aircraft that has just landed after a long flight at highaltitudes. Although the entire aircraft would be warming up from the coolertemperatures experienced at high altitude, the contaminated regions wouldnot warm as rapidly as the uncontaminated areas and therefore could bedetected as cool spots in the structure if the inspection were performedimmediately after landing.


However, if the inspection were delayed, the entire structure would reach an equilibrium temperature, and the contaminated regions would no longer be detectable. When the heat transfer process is in a steady-state condition, timing no longer becomes critical. An example is an electronic circuit board. Defective electronic components, in which the defect changes the electrical resistance of the component, will be either hotter or cooler than the same component properly operating on another circuit board. Another example would be a blocked tube in a heat exchanger. Temperatures along the tube would be different from temperatures along adjacent, unblocked tubes.


Electronic Circuit Board Thermogram


Thermal Excitation of Passive Test Objects. Thermally passive test objects require an external heat source or heat sink to establish the flow of heat to or from the test object. Generally, infrared or thermal measurement techniques become more sensitive as the average temperature of the subject increases. Consequently, the most common form of excitation is heating. However, in cases where additional heating could cause damage, cooling is used to create the required heat flow.

Key points:Generally, infrared or thermal measurement techniques become more sensitive as the average temperature of the subject increases. Consequently, the most common form of excitation is heating.


Precautions. One primary concern in heating or cooling the test object is that the thermal changes must not be intrusive. The rate of heating or cooling must be below the point of producing damaging thermal stresses. For example, a chilled glass placed in hot water may crack because the exterior is expanding rapidly while the interior is still cool and because the induced mechanical stress is sufficient to cause fracture. The degree of heating of metals must be controlled so as not to affect the heat treatment, and excessive heat can degrade the material properties of adhesives and matrices in reinforced resin composites.


Point heating can be generated with either a laser or a spherically focused infrared or visible light source to apply heat to a very localized area. For a very thin, homogeneous subject, the heat will flow outward in a circle, and the temperature change will be inversely proportional to the square of the distance from the center of the circle or the irradiated point. For a very thick, homogeneous subject, the heat can also flow in the thickness direction; consequently, the temperature change will be inversely proportional to the cube of the distance to the point of irradiation. Most subjects are somewhere between these two extremes. Because of the rapid change in temperature across the subject, the use of point heating requires rapid-response, continuous time-based measurements, which may be enhanced with pulsed heating techniques (Ref 1, 2, 3).


Line heating is similar to point heating and is usually accomplished with a linear heat source such as a quartz-tube lamp or other heat source with areflector that concentrates the heat into a line on the surface of the test object.For a thin panel, the heat will flow away from the line on both sides, and thetemperature change will be inversely proportional to the distance from the line.For a very thick, homogeneous subject, the temperature change will beinversely proportional to the square of the distance to the line of irradiation.Because the temperature changes rapidly with distance, line heating shouldbe monitored with rapid-response, continuously monitored, time-basedequipment (tracking the time to reach a given temperature by scanning thedetector outward from the line source). Another technique involves scanningthe linear heat source across the surface.


Area heating typically involves uniform heating on one surface of the test object. A bank of quartz heaters, a hot plate with a high thermal conductivitymaterial such as copper as a face sheet, or other similar devices can be usedas area heaters. If heat is applied to the same side of the object that issubsequently monitored for temperature differences, the heat source isusually switched off to prevent interference with the detector from the source.If the opposite surface is heated, the heat source usually will not interfere withthe temperature sensor and can be left on to provide a steady-state condition.The heat source can also be switched off to produce a transientmeasurement condition.

Vibration-Induced Heating. Exciting a specimen on a shaker table or with a high-power speaker will cause the specimen to respond with its natural modes of vibration. These vibrations will induce localized areas of stress and strain in the object. An anomalous area will respond differently to these induced stresses and, as a result, may be hotter or cooler than the surrounding areas if the anomaly is not at a vibration node (Ref 4, 5, 6).


Mechanically Induced Heating. Mechanically loading the specimen will cause heating in those areas that are plastically deformed. Becausedeformation of the specimen is normally destructive (except in those caseswhere deformation is part of the intended processing), mechanically inducedheating is probably best suited to research studies. Examples areinvestigations of fatigue and crack growth, analysis of the response of acomponent to loading, and optimization of deformation processes such asforging (Ref 7, 8, 9).

Electrically Induced Heating. Passing current through a specimen, or inductive heating, can be applied only to materials that are electrically conductive. The heat produced is a product of the square of the electrical current and the resistance. If the anomaly of interest locally changes the electrical resistance, the local current flow and the local temperature will also change. One shortcoming of this method is that electrically conductivematerials often have good thermal conductive properties, thus making thelocal changes in temperature short in duration (Ref 10).


Area cooling can be used when heating may be damaging or impractical. Partial immersion of a test object in a cold fluid such as ice water or liquid nitrogen may provide sufficient heat flow to reveal anomalous areas when monitoring the surface temperature as the specimen cools or returns to ambient. As with area heating, a steady-state temperature profile obtained by continued cooling of one surface of the specimen may be useful. For example, filling a cooler with ice and water would allow the exterior detection of cold spots caused by faults in the insulation or a defective lid seal. Total immersion of the test object in a cold fluid can also be considered a cooling method. In this case, the transient surface temperatures are monitored while the test object returns to ambient temperature.

Cold-thermal-wave excitation involves preheating the test object, followed by cooling with an air jet (see Example 2). The cold-thermal-wave approach is attractive for the thermal testing of metallic structures for a number of reasons. It is an inexpensive technique for fast thermal stimulation of large areas; preheating can be performed with low-rate elements such as hot-wire heaters, although in several on-line applications the part may already be above ambient temperature; and finally, the use of a cold thermal source eliminates self- mission noise problems.


5. Thermal Inspection EquipmentThe temperature sensors used in thermal inspection can be separated into two categories: noncontact temperature sensors and contact temperaturesensors. Other equipment includes recording instruments and calibrationsources.

■ Noncontact Temperature Sensors Noncontact temperaturesensors depend on the thermally generated electromagnetic radiation fromthe surface of the test object. At moderate temperatures, this energy ispredominately in the infrared region. Therefore, noncontact measurements inthermal inspection primarily involve the use of infrared sensors.

Infrared imaging equipment is available with a wide range of capabilities. The simplest systems are responsive to the near-infrared portion of the optical spectrum. These include night-vision devices and vidicon systems with silicon or lead sulfide sensors (Ref 11). Silicon sensors provide sensitivity fortemperatures above 425 °C (800 °F), while lead sulfide sensors respond totemperatures above 200 °C (400 °F).


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Hand-held scanners are portable imaging systems capable of responding in the far-infrared portion of the optical spectrum (wavelengths of 8 to 12 μm).This range is emitted by objects at or near room temperature. In general,handheld scanners have poor imaging qualities and are not suitable for theaccurate measurement of local temperature differences. However, they canbe useful for detecting hot spots, such as overheated components, thermalrunaway in an electronic circuit, or unextinguished fires (Ref 11).

High- resolution infrared imaging systems are required for most part inspection applications. These systems use either pyroelectric vidicon cameras with image-processing circuitry or cryogenically cooled mechanical scanners to provide good-quality image resolution (150 pixels, or picture elements, per scan line) (Ref 11) and temperature sensitivity to 0.1 °C (0.2 °F) (Ref 12). One system is claimed to have a temperature resolution of 0.001 °C (0.002 °F) (Ref 12). In addition to good image resolution and temperature sensitivity, response times of the order of 0.1 s or less facilitate the detection of transient temperature changes or differentials. These imaging systems willuse either a gray scale or a color scale correlated to temperature ranges to depict the temperature distribution within the image.


Thermal wave interferometer systems combine modulated laser excitation with rapid phase and amplitude sensing that can be scanned across asurface to produce an image (Ref 1, 12). One application for this type ofsystem is the inspection of plasma-sprayed coatings. The system senses theinteraction between the thermal waves of the laser and the thermal variationsfrom coating defects and thickness variations.

Radiometers and pyrometers are devices for measuring radiation, or spot or line temperatures, respectively, without the spatial resolution needed for an imaging system. Radiometers, because they usually have slow response times, are most useful for monitoring constant or slowly varying temperatures. Pyrometers are used as noncontacting thermometers for temperatures from 0 to 3000 °C (32 to 5400 °F). Newer instruments can superimpose a line trace of the temperature on the visible-light image of the surface or scene being viewed (Ref 13). Radiometers and pyrometers are usually rugged, low-cost devices that can be used in an industrial environment for the long-term monitoring of processes.


■ Contact Temperature SensorsContact temperature sensors include material coatings and thermoelectric devices. Material coatings are relatively low in cost and simple to apply, butthey may have the disadvantage of providing qualitative temperaturemeasurements (the exception is coatings with liquid crystals, which can becalibrated to show relatively small changes in temperature). Anotherdisadvantage of coatings is that they may change the thermal characteristicsof the surface.


Cholesteric liquid crystals are greaselike substances that can be blended to produce compounds having color transition ranges at temperatures from - 20 to 250 °C (- 5 to 480 °F) (Ref 13). Liquid crystals can be selected to respond in a temperature range for a particular test and can have a color response for temperature differentials of 1 to 50 °C (2 to 90 °F) (Ref 13). When illuminated with white light while in their color response range, liquid crystals will scatter the light into its component colors, producing an iridescent color that changes with the angle at which the crystals are viewed. Outside this color response range, liquid crystals are generally colorless. Theresponse time for the color change varies from 30 to 100 ms (Ref 13). This ismore than adequate to allow liquid crystals to show transient changes intemperature (Ref 14). The spatial resolution obtainable can be as small as0.02 mm (0.0008 in.) (Ref 14). In addition, because the color change isgenerally reversible, anomalies can be evaluated by repeating the test asmany times as needed.


Techniques for applying liquid crystals are relatively straightforward once the proper blend of compounds is selected. Because liquid crystals function by reflecting light, they are more readily seen when used against a dark background. Therefore, if the specimen is not already dark, covering the surface with a removable, flat-black coating is strongly recommended before application. The crystals can then be applied by pouring, painting, spraying, or dipping. Care must be taken that the specimen or the coating is not attacked by the solvent base used with the liquid crystals. The applied film of liquid crystals must be of uniform thickness to prevent color irregularities caused by thickness differences rather than temperature differences. A good film thickness is about 0.02 mm (0.0008 in.) (Ref 13). Successive layers used to build up the film thickness should not be allowed to dry between coats. A coating of proper thickness will have a uniform, low-gloss appearance when viewed with oblique illumination.


Thermally quenched phosphors are organic compounds that emit visible light when excited by ultraviolet light. The brightness of a phosphor isinversely proportional with temperature over a range from room temperatureto about 400 °C (750 °F), as indicated in Fig. 3. Some phosphors exhibit achange in brightness of as much as 25% / °C (14% / °F). An individual phosphor should be selected to cover the temperature range used for a particular inspection. The coating is applied by painting a well-agitated mixture of the phosphor onto the surface to a thickness of about 0.12 mm (0.0047 in.).


Fig. 3 Relative brightness of four thermally quenched phosphors (U.S. Radium Radelin phosphor numbers) as a function of temperature

Brightness decrease with raising temperature?


Other Coatings. Heat-sensitive paints, thermochromic compounds, heat-sensitive papers, and meltable frosts and waxlike substances can also beused to indicate surface temperatures. These coatings are useful fordetermining when a surface has exceeded a certain temperature. A few of thecoatings, such as the photochromic paints and thermochromic compounds,have reversible changes that can be used to evaluate indications by retesting.Each of the coatings can be applied directly to the surface. After someexperimentation, the coatings could be used for specialized thermalinspections.


Thermoelectric devices are widely used for measuring temperature. Typical thermoelectric devices are thermocouples, thermopiles, and thermistors.


Thermocouples consist of a pair of junctions of two different metals. As the temperature of one of the junctions is raised, an electromotive force (voltage) relative to the other (reference) junction is produced that is proportional to thetemperature difference between the two junctions. Thermocouples are usually used in a bridge circuit, with the reference junction maintained at a known and stable temperature. Thermocouples can be placed in contact with the surface of the subject or can be used near the surface to measure the air temperature.

Thermopiles are multiple thermocouples used electrically in series to increase the output voltage. Although thermopiles have a greater output (resulting in greater sensitivity) than individual thermocouples, they also have a slower response time because of the increased mass. Thermopiles are used as a sensing element in radiometers.

Thermistors are electrical semiconductors that use changes in electrical resistance to measure temperature. Thermistors are usually used in a bridge circuit, with one of the thermistors maintained at a known and stable temperature.


■ Recording EquipmentWhen high-resolution imaging systems are used, recording equipment, such as a videotape recorder, is extremely useful for analyzing transient effects or for reviewing techniques. Videotape recordings showing the time-varying response of a location on the specimen can be used to estimate the size and depth of the anomalies (Ref 2, 5, 15). Such recorded data can be processed with digital data-processing techniques to enhance the detection of temperature differences and to suppress spurious noise signals (Ref 4) (see the section "Digital Image Enhancement" in this article). Photographic techniques can also be used to record the thermal images of specimens. When coatings are used, the recording equipment usually consists of color photographs of liquid crystals, heat-sensitive paints, thermochromic materials, and wax sticks. Black-and-white photographs are adequate for most heat-sensitive papers, melting paint materials, and thermally quenched phosphors. For time-based measurements with liquid crystals, a video recorder can provide an excellent recording of color changes as the test progresses. The recording equipment for thermoelectric devices is usually time-based chart recorders or digital recorders.


■ Temperature Calibration SourcesTemperature calibration sources are needed for all devices used to measure temperatures. Some systems have built-in or internal calibration sources. External sources can vary from very simple devices, such as a container of ice and water or boiling water, to thermocouple-controlled heated plates that can be adjusted to the desired temperature.


6. Inspection MethodsSteady-state methods are used to detect anomalies where temperatures change very little with time. Many of the thermally active objects or processes can be observed under steady-state conditions. For example, a steam line that regularly carries steam can be inspected for insulation defects by looking for hot spots. These types of anomalies usually produce large differences in temperature, and the resultant images or contact coating indications are easily interpreted.

Steady-state methods are more challenging with the thermal excitation of passive objects. Uniform heating and cooling are essential, and the heating and cooling rates must be adjusted to allow the temperature differences caused by the defects to be maintained.


A honeycomb panel can be inspected for liquid intrusion or disbonds by heating one side and cooling the opposite side. The cooled side is viewed for the inspection, and either infrared imaging or contact coatings can be used.

Areas of liquid intrusion will produce warmer temperatures than thesurroundings, while disbonds will produce cooler temperatures. In general, anomalies must be large, must be close to the surface, or must create large temperature differences to be detectable with steady-state methods. With active heating or cooling, steady-state measurements will be more effective when made on the surface where the heat transfer is by convection.


IR Applications - Aerospace


Time-based methods are used to detect anomalies where temperatures change during the inspection. Temperature differences may develop and then disappear as a subject changes from one overall temperature to another (Ref 2, 15). Thechange may be actively or passively produced. For example, a hot forging exiting a forging press or die will cool to room temperature and will produce a temperature difference at a forging lap for only a short time.

An aircraft structure with hidden corrosion can be actively heated from one side and then allowed to cool. Areas with trapped water will cool more slowly and create a temporary warm spot, while thinned or corroded areas will coolslightly faster than the rest of the structure and produce a temporary cool spot.In general, time-based methods can provide the maximum detectionsensitivity and can permit inspection from one side.

For infrared or photothermal imaging methods utilizing active heating, inspection can be performed after the heat source is turned off, thus eliminating the interference in the image from the heat source itself.


Multiple time-based measurements are required for quantitative interpretation of thermal images (Ref 5). For example, a video recording will capturetransient temperature differences produced with surface coating methods on a continuous basis and can be analyzed frame by frame to evaluate the indications as they appear and disappear.

Image interpretation has proved to be the most difficult part of many thermal inspection applications. Strong indications with large temperature differencesare the easiest to interpret. Strong indications will usually provide a truerimage of the anomaly than weak indications. An anomaly close to the surfacewill produce a stronger indication than an identical anomaly far from thesurface, and the resultant indication will more accurately portray the size andshape of the source. Thermal images reflect the heat flow in the structure. Ifthe structure has an area with a subsurface support or cavity, the heat willflow faster or slower, respectively, into these areas. The edges of thestructure will experience more convective and radiative heat transfer than theremainder. Support of the structure will also affect the image because thesupport is usually another source of heat transfer.


Thermal images can also be influenced by other factors. The heating source, as well as nonuniformities in the emissivity of the specimen caused byvariations in surface roughness and color, can produce indications.Consequently, the first step in image interpretation is to look at the strength ofthe image and closely examine the surface and structure of the specimen aswell as the uniformity of the heating source. When no obvious surface orstructural correlation exists between the specimen and the image, an internalanomaly should be suspected. If available or possible, digital imageenhancement, such as:

■ filtering, ■ image averaging, or ■ reference image subtraction, (self or standard referencing)

should be used to eliminate nonrelevant indications. An indication that persists is probably a valid indication of an anomaly.


Digital image enhancement can be used to improve the quality of thermal images. Spatial filtering can be used to smooth the temperature data byeliminating the high-frequency noise caused by heating devices and externalsources. One technique involves mathematically replacing each pixel valuewith the average value of its four nearest neighbors (Ref 4). Signal-averagingtechniques can also be used to reduce measurement noise if the image doesnot vary too rapidly with time. Averaging individual pixel values from 100successive image frames will eliminate 90% of the noise and will increasesensitivity (Ref 4).

Space-domain and time-domain subtraction functions can also be used to enhance flaw detection. Space-domain subtraction functions can eliminate temperature differentials from repetitive sources of noise (such as local variations in surface emissivity), patterns resulting from a nonuniform (or line or spot) heat source, and local variations in heat loss unrelated to anomalies (such as convection patterns or conduction into supporting structures).


If self-referencing is not possible, a defect-free reference specimen with similar characteristics can be used to provide a reference image in a spacedomain subtraction algorithm (Ref 15). For example, an image taken when a component is new or when a process is functioning properly can be subtracted from an image taken at a later date to indicate damage, flaw propagation, or changes in the process (Ref 4).

If self-referencing is not possible, a defect-free reference specimen with similar characteristics can be used to provide a reference image in a spacedomain subtraction algorithm (Ref 15). For example, an image taken when a component is new or when a process is functioning properly can be subtracted from an image taken at a later date to indicate damage, flaw propagation, or changes in the process (Ref 4).


Fig. 4 Example of time-domain image processing. (a) Image over a defect (10 × 10 mm and 0.5 mm deep, or 0.4 × 0.4 in. and 0.2 in. deep) obtained 3 safter initial heating. (b) Image over the same defect obtained 5 s after initialheating. (c) Image of defect after subtracting image (a) from image (b).Courtesy of P. Cielo, National Research Council of Canada


7. Quantitative MethodsIn general, thermographic methods do not lend themselves to the rapid derivation of quantitative data on anomalies. One exception is the case of thincoatings, in which anomaly size and shape correspond very closely to thesize and shape of infrared images or contact coating indications fromoptimized inspections. Similarly, quantitative estimates of very thin coatingthicknesses are possible with proper time-based techniques (Ref 1).

Quantification of anomaly size following the detection by thermal methods isusually more readily accomplished by application of a second nondestructiveevaluation (NDE) method. For example, determining the size and depth of adelamination detected in a graphite/epoxy laminate by thermal methodswould be best achieved by ultrasonic techniques. A crack detected in a rocketpropellant tank would be best quantified by other methods, such as multiple x- ray films or x-ray computer-aided tomography.


Rocket Propellant Tank


Rocket Propellant Tank - Engine


There are some cases in which quantification from thermal indications is always desirable. For example, in a process producing hot specimens, rapidfeedback may be necessary because time is too short to wait for thespecimens to cool for a subsequent inspection. Another example would be a coating process that needs a rapid thickness measurement to ensure proper thickness. Most quantitative methods are still under development. Nearly all the quantification methods must be tailored to specific applications. Although some methods can be approached from a theoretical basis, most require an empirical development effort involving reference standards, multiple timebased measurements, and interpretive methods or rules (Ref 5, 13).


Reference standards are as necessary for thermal methods as they are for other NDE methods. Standards ensure consistent performance of the thermalactivation methods and the temperature-sensing materials or equipment.Reference standards are essential for quantitative techniques. A referencestandard should closely represent the thermal and surface characteristics ofthe components or items inspected. For example, if the subject is a solderjoint, the reference should be a solder joint with the same contact area, heatsink, wire sizes, and other characteristics. The standard must also provide ananomaly-free condition and a second unacceptable condition associated witha size or the presence of an anomaly. A standard for quantitative methodsrequires a progression of anomaly sizes and locations. The standard needs tocover the potential accept/reject conditions that can exist in the componentsto be inspected. This typically necessitates a test program to establish theanomaly sizes and locations that are unacceptable for proper operation of thecomponents to be inspected (Ref 16).


Multiple time-based measurements are needed to establish correlations between a response to thermal activation and anomaly size and location (Ref 5, 16). For pulsed or modulated thermal excitation, the time delay of the temperature response can be recorded as a relative phase change, which may be related to a parameter such as coating thickness (Ref 1) or delamination depth (Ref 3). Alternatively, variations in isotherm position with time can be monitored as a function of anomaly size and depth to establish known time-based response characteristics. Figure 5 shows how the effusivity (which is a function of the pulsed heat input and the time-dependent temperature change) directly over a 20 × 20 mm (0.8 × 0.8 in.) void in a carbon-epoxy laminate varies with time for void depths of 0.3, 1.12, and 2.25 mm (0.012, 0.044, and 0.088 in.).


Fig. 5 Experimental apparent-effusivity curves, after correction, of the carbon-epoxy laminate sample in the regions with embedded defects of 20 × 20 mm(0.8 × 0.8 in.) area at three depths. A, 0.3 mm (0.012 in.); B, 1.12 mm (0.044in.); C, 2.25 mm (0.088 in.). Source: Ref 2


Quantitative interpretation rules are needed before the time-based data can be used in an evaluation. A simple example is the phase change response asa function of coating thickness of yttria-stabilized zirconium on a nimonicsubstrate, as shown in Fig. 6. For an acceptable coating thickness of 0.45 to0.6 mm (0.018 to 0.023 in.), the data show that a 0.5 Hz modulation of thethermal excitation will supply the necessary phase sensitivity and thatacceptable coating thickness will be indicated by phase angles ranging from 5to 13°. This information then provides the accept/reject criteria for testsconducted under these conditions.

For a three-variable situation in which void thickness, width and depth beneath a surface are determined, at least three time-based measurements are needed (Ref 5). The resulting calculations relating defect geometry and location are complicated and can be performed only with computerassistance.


Fig. 6 Variations in the phase of temperature changes versus the coating thickness of a yttria-stabilized zirconia coating on a nimonic substrate. The phase of the temperature changes are relative to the modulation (0.5 and 5 Hz) of the thermal excitation. Source: Ref 1


Photothermal RadiometryThe photothermal measuring method is based on intensity modulated or pulsed heating of surfaces by laser light. During excitation, the surface temperature is monitored by an infrared detector. The generated heat diffuses into the sample. The detector measures the resulting surface temperature, which depends on material parameters and sample geometry. A lockinamplifier extracts the phase information of the infrared signal. Using calibration measurements, the phase can be linked to certain material parameters like hardness or coating thickness. Therefore, the system needs to be calibrated with destructive measurements on sample material and geometry. The measurement duration depends on the type of application:

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8.ApplicationsThe applications discussed below provide examples of how thermal methods have been used. For ease of discussion, the applications have been divided into the following categories:

· Hot and cold equipment· Process control· Liquid intrusion· Disbonds, delaminations, and voids· Electronic devices· Research


Hot and Cold Equipment. Many types of equipment that conduct or generate heat during operation are likely candidates for thermographicmethods. Heating ducts, steam lines (Ref 13), radiators (Ref 17), heatexchangers (Ref 15), exhaust systems/chimney stacks (Ref 13), andrefrigeration systems are heat transfer devices for which thermal inspectiontechniques can be used during periodic inspections for leaks, cloggedpassages, and missing or defective insulation. Furnaces, ovens, salt baths,autoclaves, reaction stacks, and hot manufacturing equipment (such aspresses and rolling mills) may also require periodic inspection forunnecessary heat losses. Thermographic techniques are also used to inspectcryogenic tanks. Other equipment, such as bearings, slides, brakes,transmitting antennas, and electrical equipment, generates heat duringoperation. Localized hot areas are usually a symptom of a mechanical orelectrical malfunction, and early detection provides the opportunity to replacethe defective components during regularly scheduled maintenance or beforemore serious damage occurs.


Process Controls. Thermal inspection methods are appropriate for certain processes in which the products are above room temperature as they exit aprocess. Examples are heat-set and heat-shaped plastics, hot-worked metalcomponents, hot coating processes, and weld components. Components canbe monitored as they exit the process. Abnormal temperatures would indicatean out-of-control process, and corrections can be made to prevent theproduction of large numbers of defective parts. An example of an activelyheated process control method is the technique for monitoring phosphatecoating thickness using infrared absorption characteristics (Ref 18).

Liquid Intrusion. Water or fuel intrusion in honeycomb structures is a significant problem for aircraft maintenance. Inspections can be performed immediately after flight to detect such liquids as the structure warms to ambient temperature. Water intrusion in roofs can also be detected in the evening as ambient temperature drops. This method has been reported to be successful in detecting leaks and retained moisture in insulation (Ref 19). The level of fluids in a sealed tank can also be determined thermographically. This is accomplished by either heating or cooling the tank and then noting thelocation of a sharp differential in resulting tank temperature.


Disbonds, Delaminations, and Voids. Modern fabrication techniques rely heavily on the use of bonding for structures and protective coatings. Thermaltechniques are good candidates for the detection of disbonds, delaminations,and voids in thin laminates, honeycomb to thin face sheets, and protectivecoatings (Ref 15). As the specimen thickness increases, thermal inspectionbecomes less effective because the possible depth of the defect may begreater. Thermal inspection may also be difficult with bonded metal structures,such as adhesive-bonded aluminum. The radiative heating of aluminum isrelatively inefficient because of the low surface absorptivity, and the thermal-mission signal is low because of the low emissivity. For such reasons, as wellas to avoid reflective noise, the aluminum parts must normally be black-ainted prior to thermal inspection. Moreover, the high thermal diffusivity ofaluminum requires a high thermal-power injection to produce a visible thermalpattern before thermal uniformization is reached within the structure.Nevertheless, the possibility of evaluating adhesive-bonded aluminumstructures at the rapid pace afforded by thermal inspection is attractive,particularly for online applications (Ref 20). The following two examplesdescribe the thermal inspection of adhesive-bonded aluminum after black-ainting.


Example 1: Thermal Inspection of Adhesive-Bonded Aluminum Sheets(Ref 15). Figure 7 shows the detection by a transmission configuration of a 1.5 × 1.5 cm (0.6 × 0.6 in.) lack-of-adhesive defect on a lap joint with nearly 1 mm (0.04 in.) thick skin. The hotter portions on the left and right of the thermal image correspond to the single-sheet material, which was heated at a faster rate than the adhesive joint. The heating rate must be sufficiently fast to avoid significant thermal propagation from the hot single-sheet to the adhesive point. Properly shaped shading masks on the lamp side can be used to avoid single-sheet over-heating when the sample geometry is repetitive. A line heating and sample displacement configuration of the type described in Ref 21 can also be used for the fast-response testing of longitudinal joints.


Fig. 7 Thermographic inspection of adhesive-bonded aluminum sheets. (a) Experimental configuration. (b) Thermal image of a 15 × 15 mm (0.6 × 0.6 in.) disbonding in an image area of 40 × 50 mm (1.6 × 2.0 in.). Courtesy of P. Cielo, National Research Council of Canada


Example 2: Cold-Thermal-Wave Technique for the Thermal Inspection of an Adhesive-Bonded Aluminum Structure(Ref 15). Rapid back heating procedures are difficult to apply when analyzing complex structures of the type shown in Fig. 8. In this case, a cold-thermal-ave approach was applied by which the whole structure was previouslywarmed up uniformly by nearly 10 °C (20 °F) above ambient, and an ambient air jet was used to rapidly cool the inspected face. The thermal image shows a relatively warmer central area corresponding to the adhesive-bonded strip (oriented vertically in the thermograph). The cooler area in the center of the strip corresponds to a lack-of-adhesive unbond.


Fig. 8 Cold-thermal-wave technique for inspecting a preheated, adhesive-bonded aluminum structure. (a) Experimental configuration. (b) Thermalimage of a 10 × 10 mm (0.4 × 0.4 in.) disbonding. Courtesy of P. Cielo,National Research Council of Canada


Electronic devices typically fail by local overheating, corrosion, or poor solder joints. Infrared microscopes have been used to inspectmicroelectronics for overheating, and laser-generated heat has been used toinspect circuit-board solder joints. More conventional thermal inspectionmethods have been used to inspect solder joints during cool-down afterfabrication or during circuit operation. Solar cells have also been screened fordefects with thermal techniques.

Research. Several investigators have used the heat generated by permanent deformation to track the onset of failure in tensile overloading and crack propagation in fatigue testing (Ref 7, 8, 9). Samples of components can be loaded to failure under thermographic monitoring to pinpoint the origin and propagation of deformation (Ref 7).


End Of Reading Three

understanding infrared thermography reading 6 chapter 3

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