Introduction to Thermal Testing(AKA Thermal Inspection, Thermography, Thermal Imaging, Thermal Wave Imagingand Infrared Testing)(Photo courtesy of NASA!P"#$altechIPA$) Thermal N%T methods involve the measurement or mapping of surface temperatures as heat flo&s to, from andor through an o'(ect) The simplest thermal measurements involve ma*ing point measurements &ith a thermocouple) This type of measurement might 'e useful in locating hot spots, such as a 'earing that is &earing out and starting to heat up due to an increase in friction)In its more advanced form, the use of thermal imaging systems allo& thermal information to 'e very rapidly collected over a &ide area and in a non#contact mode) Thermal imaging systems are instruments that create pictures of heat flo& rather than oflight) Thermal imaging is a fast, cost effective &ay to perform detailed thermal analysis)The image a'ove is a heat map of the space shuttle as it lands)Thermal measurement methods have a &ide range of uses) They are used 'y the police and military for night vision, surveillance, and navigation aid+ 'y firemen and emergency rescue personnel for fire assessment, and for search and rescue+ 'y the medical profession as a diagnostic tool+ and 'y industry for energy audits, preventative maintenance, processes control and nondestructive testing) The 'asic premise of thermographic N%T is that the flo& of heat from the surface of a solid is affected 'y internal fla&s such as dis'onds, voids or inclusions) The use of thermal imaging systems for industrial N%T applications &ill 'e the focus of this material) Scientific Principles of Thermal TestingThermal Energy,nergy can come in many forms, and it can change from one form to another 'ut can never 'e lost) This is the -irst "a& of Thermodynamics) A 'yproduct of nearly all energy conversion is heat, &hich is also *no&n as thermal energy) When there is a temperature difference 'et&een t&o o'(ects or t&o areas &ithin the same o'(ect, heat transfer occurs) .eat energy transfers from the &armer areas to the cooler areas until thermal e/uili'rium is reached) This is the Second "a& of Thermodynamics) When the temperature of an o'(ect is the same as the surrounding environment, it is said to 'e at am'ient temperature)Heat Transfer MechanismsThermal energy transfer occurs through three mechanisms0 conduction, convection, andor radiation) $onduction occurs primarily in solids and to a lesser degree in fluids as &armer, more energetic molecules transfer their energy to cooler ad(acent molecules) $onvection occurs in li/uids and gases, and involves the mass movement of molecules such as &hen stirring or mi1ing is involved)The third &ay that heat is transferred is through electromagnetic radiation of energy) 2adiation needs no medium to flo& through and, therefore, can occur even in a vacuum) ,lectromagnetic radiation is produced &hen electronslose energy and fall to a lo&er energy state) 3oth the &avelength and intensity of the radiation is directly related tothe temperature of the surface molecules or atoms)Wavelength of Thermal EnergyThe &avelength of thermal radiation e1tends from 4)5 microns to several hundred microns) As highlighted in the image, this means that not all of the heat radiated from an o'(ect &ill 'e visi'le to the human eye6 'ut the heat is detecta'le) $onsider the gradual heating of a piece of steel) With the application of a heat source, heat radiating from the part is felt long 'efore a change in color is noticed) If the heat intensity is great enough and applied for long enough, the part &ill gradually change to a red color) The heat that is felt prior to the part changing color is the radiation that lies in the infrared fre/uency spectrum of electromagnetic radiation) Infrared (I2) radiation has a&avelength that is longer than visi'le light or, in other &ords, greater than 744 nanometers) As the &avelength of the radiation shortens, it reaches the point &here it is short enough to enter the visi'le spectrum and can 'e detected &ith the human eye)An infrared camera has the a'ility to detect and display infrared energy) 3elo& is an infrared image of an ice cu'e melting) Note the temperature scale on side, &hich sho&s &arm areas in red and cool areas in purple) It can 'e seen that the ice cu'e is colder than the surrounding air and it is a'sor'ing heat at its surface) The 'asis for infrared imaging technology is that any o'(ect &hose temperature is a'ove 48K radiates infrared energy) ,ven very cold o'(ects radiate some infrared energy) ,ven though the o'(ect might 'e a'sor'ing thermal energy to &arm itself, it &ill still emit some infrared energy that is detecta'le 'y sensors) The amount of radiated energy is afunction of the o'(ect9s temperature and its relative efficiency of thermal radiation, *no&n as emissivity)(Photo courtesy of NASA!P"#$altechIPA$)EmissivityA very important consideration in radiation heat transfer is the emissivity of the o'(ect 'eing evaluated) ,missivityis a measure of a surface9s efficiency in transferring infrared energy) It is the ratio of thermal energy emitted 'y a surface to the energy emitted 'y a perfect 'lac*'ody at the same temperature) A perfect 'lac*'ody only e1ists in theory and is an o'(ect that a'sor's and reemits all of its energy) .uman s*in is nearly a perfect 'lac*'ody as it has an emissivity of 4):;, regardless of actual s*in color)If an o'(ect has lo& emissivity, I2 instruments &ill indicate a lo&er temperature than the true surface temperature)-or this reason, most systems and instruments provide the a'ility for the operator to ad(ust the emissivity of the o'(ect 'eing measured) Sometimes, spray paints, po&ders, tape or ed) -or e1ample, miro'olometers are theactive elements in some high#tech porta'le imaging systems, such as those used 'y firedepartments) 3enefits of thermal detectors are that the element does not need to 'e cooled andthey are comparatively lo& in price) Thermal detectors are used to measure the temperature in everything from home appliances tofire and intruder detection systems to industrial furnaces to roc*ets) Quantum (Photon) Detectors @nli*e thermal detectors, /uantum detectors do not rely on the conversion of incoming radiation to heat, 'ut convert incoming photons directly into an electrical signal) When photons in a particular range of &avelengths are a'sor'ed 'y the detector, they create free electron#hole pairs, &hich can 'e detected as electrical current) The signal output of a /uantum detector is very small and is overshado&ed 'y noise generated internally to the device at room temperatures) Since this noise &ithin a semiconductor is partly proportional to temperature, /uantum detectors are operated at cryogenic temperatures Ai) e) do&n to 77 K (li/uid nitrogen) or B K (li/uid helium)C to minimi>e noise) This cooling re/uirement is a significant disadvantage in the use of /uantum detectors) .o&ever, their superior electronic performance still ma*es them the detector of choice for the 'ul* of thermal imaging applications) Some systems can detect temperature differences as small as 4)478$) ?uantum detectors can 'e further su'divided into photoconductive and photovoltaic devices) The function of photoconductive detectors are 'ased on the photogeneration of charge carriers (electrons, holes or electron#hole pairs)) These charge carriers increase the conductivity of the device material) Possi'le materials used for photoconductive detectors include indium antimonide (InS'), /uantum &ell infrared photodetector (?WIP), mercury cadmium telluride (mercad, D$T), lead sulfide (P'S), and lead selenide (P'Se))Photovoltaic devices re/uire an internal potential 'arrier &ith a 'uilt#in electric field in order to separate photo#generated electron#hole pairs) Such potential 'arriers can 'e created 'y the use of p#n (unctions or Schott*y 'arriers) ,1amples of photovoltaic infrared detector types are indium antimonide (InS'), mercury cadmium telluride (D$T), platinum silicide (PtSi), and silicon Schott*y 'arriers)Detector CoolingThere are several different &ays of cooling the detector to the re/uired temperature) In the early days of thermal imaging, li/uid nitrogen &as poured into imagers to cool the detector) Although satisfactory, the logistical and safety implications led to the development of other cooling methods) .igh pressure gas can 'e used to cool a detector to the re/uired temperatures) The gas is allo&ed to rapidly e1pand in the cooling systems and this e1pansion results in the significant reduction in the temperature of a gas)Dechanical cooling systems are the standard for porta'le imaging systems) These have the logistical advantage of freeing the detection system from the re/uirements of carrying high pressure gases or li/uid nitrogen) Home - Education Resources - NDT Course Material - Thermal Equipment - Imaging TechnologImaging SystemsThermal imaging instruments measure radiated infrared energy and convert the data to corresponding maps of temperatures) A true thermal image is a gray scale image &ith hot items sho&n in &hite and cold items in 'lac*) Temperatures 'et&een the t&o e1tremes are sho&n as gradients of gray) Some thermal imagers have the a'ility to add color, &hich is artificially generated 'y the camera9s videoenhancement electronics, 'ased upon the thermal attri'utes seen 'y the camera) Some instruments provide temperature data at each image pi1el) $ursors can 'e positioned on each point, and the corresponding temperature is read out on the screen or display) Images may 'e digiti>ed, stored, manipulated, processed and printed out) Industry#standard image formats, such as the tagged image file format (TI--), permit files to &or* &ith a &ide array of commercially availa'le soft&are pac*ages)Images are produced either 'y scanning a detector (or group of detectors) or 'y using &ith focal plane array) A scanning system in its simplest form could involve a single element detector scanning along each line in the frame (serial scanning)) In practice, this &ould re/uire very high scan speeds, so a series of elements are commonly scanned as a 'loc*, along each line) The use of multiple elements eases the scan speed re/uirement, 'ut the scan speed and channel 'and&idth re/uirements are still high) Dultiple element scans do, ho&ever, result in a high degree of uniformity) The frame movement can 'e provided 'y frame scanning optics (using mirrors) or in the case of line scan type imagers, 'y the movement of the imager itself) Another method is to use a num'er of elements scanning in parallel (parallel scanning)) These scanners have one element per line and scan several lines simultaneously) Scan speeds are lo&er 'ut this method can give rise to poor image uniformity)Another &ay thermal images are produced is &ith focal plane arrays (-PA), &hich are also *no&n asstaring arrays) A focal plane array is a group of sensor elements organi>ed into a rectangular grid) A high magnification image of a portion of a miro'olometer focal plane array is sho&n to the right) The entire scene is focused on the array, each element cell then provides an output dependent upon the infrared radiation falling upon it) The spatial resolution of the image is determined 'y the num'erof pi1els of the detector array) $ommon formats for commercial infrared detectors are EF4 'y FB4 pi1els (EF4 columns, FB4 ro&s), and GB4 'y B;4) The latter format is nearly the resolution o'tained 'y a standard TH) Spatial resolution, the a'ility to measure temperatures on small areas, can 'e as fine as 5I microns) Temperature resolution, the a'ility to measure small temperature differences, can'e as fine as 4)58 $) The advantage of -PAs is that no moving mechanical parts are needed and that the detector sensitivity and speed can 'oth 'e slo&er) The dra&'ac* is that the detector array is more complicated to fa'ricate and manufacturing costs are higher) .o&ever, improvements in semiconductor fa'rication practices are driving the cost do&n and the general trend is that infrared camera systems &ill 'e 'ased on -PAs, e1cept for special applications) A micro'olometer is the latest type of thermal imaging -PA, and consists of materials that measure heat 'y changing resistance at each pi1el) The most common micro'olometer material is vanadium o1ide (H=J)) Amorphous silicon is another relatively ne& micro'olometer material)Applications e1tend from microelectronic levels to scanning &ide areas of the earth from space) Air'orne systems can 'e used to see through smo*e in forest fires) Porta'le, hand#held units can 'e used fore/uipment monitoring in preventative maintenance and fla& detection in nondestructive testing programs)Eui!ment for Esta"lishing Heat #lo$In some inspection applications, such as corrosion or fla& detection, the components 'eing inspectedmay 'e at am'ient temperature and heat flo& must 'e created) This can 'e accomplished 'y a varietyof means) .eating can 'e accomplished 'y placing the part in a &arm environment, such as a furnace, or directing heat on the surface &ith a heat gun or &ith flash lamps) Alternately, cooling can'e accomplished 'y placing the component in a cold environment or cooling the surface &ith a sprayof cold li/uid or gas) Image Ca!turing an% &nalysisI2 cameras alone or used &ith an e1ternal heat source can often detect large, near#surface fla&s) .o&ever, repeata'le, /uantifia'le detection of deeper, su'tler features re/uires the additional sensitivity of a sophisticated computeri>ed system) In these systems, a computer is used to capture a num'er of time se/uence images &hich can 'e stepped through or vie&ed as a movie to evaluate the thermal changes in an o'(ect as a function of time) This techni/ue is often referred to as thermal &ave imaging) The image to the right sho&s a pulsed thermography system) This system uses a closely controlled 'urst of thermal energy from a 1enon flash lamp to heat the surface) The dissipation of heat is then trac*ed using a high speed thermal imaging camera) The camera sits on top of the gray 'o1 in the foreground) The gray 'o1 houses the 1enon flash lamp and it is held against the surface 'eing inspected) The e/uipment &as designed to inspect the fuselage s*ins of aircraft for corrosion damageand can ma*e /uantitative measurements of material loss) It has also 'een sho&n to detect areas of &ater incursion in composites and areas &here 'onded structure have separated) Home - Education Resources - NDT Course Material - Thermal Techniques and SelectIndustrial !pplications of Thermal ImagingSome thermal imaging techni/ues simply involve pointing a camera at a component and loo*ing at areas of uneven heating or locali>ed hot spots) The first t&o e1ample applications discussed 'elo& fall into this category) -or other applications, it may 'e necessary to generate heatflo& &ithin the component andor evaluate heat flo& as a function of time) A variety of thermal imaging techni/ues have 'een developed to provide the desired information) A fe& of these techni/ues are highlighted 'elo&)Electrical an% Mechanical System Ins!ection,lectrical and mechanical systems are the 'ac*'one of many manufacturing operations) An une1pected shutdo&n of even a minor piece of e/uipment could have a ma(or impact on production) Since nearly everything gets hot 'efore it fails, thermal inspection is a valua'le and cost#effective diagnostic tool &ith many industrial applications)With the infrared camera, an inspector can see the change in temperature from the surrounding area, identify &hether or not it is a'normal and predict the possi'le failure) Applications for infrared testing include locating loose electrical connections, failing transformers, improper 'ushing and 'earing lu'rication, overloaded motors or pumps, coupling misalignment, and otherapplications &here a change in temperature &ill indicate an undesira'le condition) Since typical electrical failures occur &hen there is a temperature rise of over I48$, pro'lems can 'e detected &ell in advance of a failure)The image on the right a'ove sho&s three electrical connections) The middle connection is hotter than the others) $onnections can 'ecome hot if they are loose or if corrosion causes an increase in the electrical resistance)Electronic Com!onent Ins!ectionIn electronics design and manufacturing, a *ey relia'ilityfactor is semiconductor (unction temperature) %uringoperation, a semiconductor generates heat and this heat &illflo& from the component) The heat &ill flo& from thecomponent in all directions, 'ut &ill flo& particularly &ellalong thermally conductive connectors) This leads to anincrease in temperature at the (unctions &here thesemiconductor attaches to the 'oard) $omponents &ith high(unction temperatures typically have shorter life spans)Thermal imaging can 'e used to evaluate the dissipation ofheat and measure the temperature at the (unctions)Corrosion Damage (Metal Thinning) I2 techni/ues can 'e used to detect material thinning of relatively thin structures since areas &ith different thermal masses &ith a'sor' and radiate heat at different rates) In relatively thin, thermally conductive materials, heat &ill 'e conducted a&ay from the surface faster 'y thic*er regions) 3y heating the surface and monitoring its cooling characteristics, a thic*ness map can 'e produced) Thin areas may 'e the result of corrosion damage on the 'ac*side of a structure &hich is normally not visi'le) The image to the right sho&s corrosion damage and dis'onding ofa tear strapstringer on the inside surface of an aircraft s*in) This type of damage is costly to detect visually 'ecause a great deal of the interior of the aircraft must 'e disassem'led) With I2 techni/ues, the damage can 'e detected from the outside of the aircraft)#la$ DetectionInfrared techni/ues can 'e used to detect fla&s in materials or structures) The inspection techni/ue monitors the flo& of heat from the surface of a solid and this flo& is affected 'y internal fla&s such as dis'onds, voids or inclusions) Sound material, a good &eld, or a solid 'ond &ill see heat dissipate rapidly through the material, &hereas a defect &ill retain the heat forlonger)A ne& techni/ue call vi'rothermograph or thermosonic testing &as recently introduced 'y researchers at Wayne State @niversity for the detection of crac*s) A solid sample is e1cited &ith 'ursts of high#energy, lofre/uency acoustic energy) This causes frictional heating at the faces of any crac*s present and hotspots are detected 'y an infrared camera) %espite the apparent simplicity of the scheme, there are a num'er of e1perimental considerationsthat can complicate the implementation of the techni/ue) -actors including acoustic horn location, horn#crac* pro1imity, horn#sample coupling, and effective detection range all significantly affect the degree of e1citation that occurs at a crac* site for a given energy input) 3elo& are t&o images from an I2 camera sho&ing a 4)4I4< thic* 747I aluminum plate sample &ith a prefa'ricated crac* 'eing inspected using a commercial vi'rothermography system) The image on the left is the I2 image &ith a pre#e1citation image su'tracted) A crac* can 'e seen in the middle of the sample and (ust to the right of the ultrasonic horn) Also seen is heating due to the horn tip, friction at various clamping sites, and reflection from the hole at the right edge of the sample) The image on the right is the same data &ith image processing performed to ma*e the crac* indication easier to distinguish)Images $ourtesy of Wayne State @niversity Home - Education Resources - NDT Course Material - Thermal Image InterpretationDost thermal imagers produce a video output in &hich &hite indicates areas of ma1imum radiated energy and 'lac* indicates areas of lo&er radiation) The gray scale image contains the ma1imum amount of information) .o&ever, in order to ease general interpretation and facilitate su'se/uent presentation, the thermal image can 'e artificially colori>ed) This is achieved 'y allocating desired colors to 'loc*s of grey levels to produce the familiar colori>ed images) This ena'les easier image interpretation to the untrained o'server) Additionally, 'y choosing the correct colori>ation palette the image may 'e enhanced to sho& particular energy levels in detail)Dany thermal imaging applications are /ualitative in nature) The inspection simply involves comparing the temperatures at various locations &ithin the field of vie&) The effects of the sun, shado&s, moisture and su'surface detail must all 'e ta*en into account &hen interpreting the image, 'ut this type of inspection is straightfor&ard) .o&ever, great care must 'e e1ercised &hen using an infrared imager to ma*e /uantitative temperature measurements) As mentioned previously, the amount of infrared radiation emitted from a surface depends partly upon the emissivity of that surface) Accurate assessment of surface emissivity is re/uired to ac/uire meaningful /uantitative results)