Thermographic Inspection of Internal Defects in Steel Structures: Analysis of Signal Processing Techniques in Pulsed Thermography
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sensors Article Thermographic Inspection of Internal Defects in Steel Structures: Analysis of Signal Processing Techniques in Pulsed Thermography Yoonjae Chung 1, Ranjit Shrestha 2 , Seungju Lee 3 and Wontae Kim 4,* 1 Department of Mechanical Engineering, Kongju National University, 1223-24 Cheonan-daero, Seobuk-gu, Cheonan-si 31080, Korea; [email protected] 2 Department of Mechanical Engineering, School of Engineering, Kathmandu University, Dhulikhel, P.O. Box, Kathmandu 6250, Nepal; [email protected] 3 Department of Convergence Mechanical Engineering, Kongju National University, 1223-24 Cheonan-daero, Seobuk-gu, Cheonan-si 31080, Korea; [email protected] 4 Division of Mechanical & Automotive Engineering, Kongju National University, 1223-24 Cheonan-daero, Seobuk-gu, Cheonan-si 31080, Korea * Correspondence: [email protected]; Tel.: +82-41-521-9289; Fax: +82-41-555-9123 Received: 27 September 2020; Accepted: 21 October 2020; Published: 23 October 2020 Abstract: This study performed an experimental investigation on pulsed thermography to detect internal defects, the major degradation phenomena in several structures of the secondary systems in nuclear power plants as well as industrial pipelines. The material losses due to wall thinning were simulated by drilling flat-bottomed holes (FBH) on the steel plate. FBH of different sizes in varying depths were considered to evaluate the detection capability of the proposed technique. A short and high energy light pulse was deposited on a sample surface, and an infrared camera was used to analyze the effect of the applied heat flux. The three most established signal processing techniques of thermography, namely thermal signal reconstruction (TSR), pulsed phase thermography (PPT), and principal component thermography (PCT), have been applied to raw thermal images. Then, the performance of each technique was evaluated concerning enhanced defect detectability and signal to noise ratio (SNR). The results revealed that TSR enhanced the defect detectability, detecting the maximum number of defects, PPT provided the highest SNR, especially for the deeper defects, and PCT provided the highest SNR for the shallower defects. Keywords: internal defects; nondestructive testing; pulsed thermography; thermal signal reconstruction; pulsed phase thermography; principle component analysis; signal to noise ratio 1. Introduction Steel is one of the world’s most versatile materials with excellent qualities of strength, corrosion resistance, weldability, formability, workability, and attractiveness. Also, having a long-life cycle and being 100% recyclable, it is the ideal material for a wide range of applications, such as architecture and construction, automotive and transportation, aircraft, chemical, food and beverage, energy, medical, and a variety of other applications. Steel is used in the construction of reactor vessels and vessel liners, reactor building structure, pressure channels, fuel cladding, heat exchanger and condenser tubes, and fuel pool liners in the nuclear industry as well as industrial pipelines [1–3]. A major challenge seen in these systems is the wall thinning due to flow accelerated corrosion and liquid droplet impingement. Such degradation mechanism affects the structural health and can lead to disastrous failures of the structure; as well as a massive economic loss [4–6]. This calls for a reliable testing and evaluation method to ensure that the functionality of the system is not compromised due to thickness reduction. Sensors 2020, 20, 6015; doi:10.3390/s20216015 www.mdpi.com/journal/sensors Sensors 20202020,, 2020,, 6015x FOR PEER REVIEW 2 of 17 to thickness reduction. Recently, numerous nondestructive testing (NDT) techniques, such as x‐ray Recently,[7–9], microwave numerous [10–12], nondestructive ultrasonic testing [13–16], (NDT) magnetic techniques, flux leakage such as [17–19], x-ray [7 –acoustic9], microwave emission [10 –[20–12], ultrasonic22], and eddy [13– 16current], magnetic [23–25], flux have leakage been [17 used–19], for acoustic the measurement emission [20 of–22 pipe], and wall eddy thinning. current Each [23–25 of], havethese beentechniques used for have the measurement their own advantages of pipe wall and thinning. limitations. Each ofThe these lists techniques of NDT havetechniques their own are advantagescontinuously and increasing limitations. as Theresearchers lists of NDT around techniques the globe are continuouslyare striving increasingfor the development as researchers of aroundtechniques the and globe methods are striving to guarantee for the development market superiority of techniques in terms andof quality methods and to productivity. guarantee market In this superioritycontext, infrared in terms thermography of quality and (IRT) productivity. plays a dynamic In this context,role to improve infrared the thermography quality and (IRT)productivity plays a dynamicof a scheme role from to improve creation the to qualitycompletion. and productivity of a scheme from creation to completion. IRT is is an an optical optical measurement measurement technique, technique, evolving evolving rapidly rapidly with withthe development the development of high of spatial high spatialresolution resolution and sensitivity and sensitivity detectors detectors and improved and improved computation computation power [26–28]. power [ IRT26–28 has]. IRTbeen has gaining been gainingincreased increased recognition recognition in recent in years recent because years because of its peculiar of its peculiar advantages advantages over its over counterparts. its counterparts. These Theseadvantages advantages include include high‐speed high-speed contactless contactless functioning, functioning, higher higherlevel of level safety, of safety,and portability and portability with a withpotential a potential to encompass to encompass a large a inspection large inspection area [29–31]. area [29 –IRT31]. utilizes IRT utilizes an infrared an infrared camera camera to extract to extract and andanalyse analyse a thermal a thermal pattern pattern based based on the on principle the principle that all that objects all objects above above absolute absolute zero zero(−273 ( °C)273 emit◦C) − emitinfrared infrared energy. energy. The infrared The infrared radiation radiation emitted emitted by an by object an object is sensed is sensed by the by infrared the infrared camera camera and andtransformed transformed into intoan electronic an electronic signal, signal, which which is then is then processed processed to produce to produce a thermal a thermal image image [32]. [ 32As]. Asshown shown in Figure in Figure 1, 1there, there exist exist several several types types of of thermographic thermographic approaches which cancan bebe classifiedclassified based on the procedure for testingtesting thethe structuralstructural components,components, the origin of the source ofof excitationexcitation energy, andand thethe temperaturetemperature didifferencesfferences ofof thethe constituents.constituents. Based on the experimental procedure, thermography can be classifiedclassified intointo activeactive andand passivepassive thermography.thermography. PassivePassive thermographythermography doesdoes not require any external source to excite surface thermal gradients as it is naturally at a higher or lowerlower temperature comparedcompared toto thethe background. In contrast, active thermography utilizes an external heating source to produce thermal contrast in the region of interest.interest. Based on the exciting method, active thermography can be further classifiedclassified asas pulsedpulsed thermographythermography (PT),(PT), lock-inlock‐in thermographythermography (LIT),(LIT), vibrothermographyvibrothermography (VT), (VT), and and step step heating heating thermography thermography (SHT) (SHT) [33 –[33–35].35]. PT andPT and LIT LIT are theare two the primarytwo primary conventional conventional practices. practices. In both In techniques, both techniques, thermal thermal energy isenergy delivered is delivered into the objectinto the in whichobject thein which heat propagates the heat propagates by diffusion by diffusion through through the material. the material. Then, the Then, thermal the thermal response response recorded recorded by the infraredby the infrared camera iscamera observed is observed to reveal theto reveal presence the of presence the defect. of Thethe experimentdefect. The can experiment be performed can be in twoperformed distinct in ways. two Indistinct transmission ways. In mode, transmission an infrared mode, camera an infrared and the excitation camera and source the excitation are kept opposite source theare samplekept opposite under investigation.the sample under In contrast, investigation. in reflection In contrast, mode, in an reflection infrared mode, camera an and infrared the excitation camera sourceand the are excitation positioned source identically are topositioned the sample identically being investigated to the sample [36,37]. being Furthermore, investigated various [36,37]. signal processingFurthermore, and various filtering signal techniques processing can be and used filtering to access techniques defects information, can be used including to access the defects small internalinformation, discontinuities including the and small material internal characteristics. discontinuities and material characteristics. Figure 1. ClassificationClassification of common infrared thermography techniques used in NDT. In the past few decades, research has demonstrated