Non-Destructive Method of Thermal Imaging of a Surface - Essay Example

?Thermography Introduction: Thermal imaging or thermography is the mapping of temperature profile on the surface of the object or component. It makesuse of the infrared band of the electromagnetic spectrum. Any body above absolute zero temperature emits electromagnetic radiation. At ambient temperature and above, these radiations are predominantly in infrared band of electromagnetic spectrum. Using an infrared detector, it is possible to convert these infrared radiations into electrical signals, which can be then displayed on a monitor as a grey level image or colours representing different range. Thus a complete surface temperature map of the object can be obtained in a non-contact manner. With appropriate calibration, it is also possible to get the absolute temperature values of any point on the surface of the object. Standards are required for calibration and these standards are materials of known emissivity in the temperature range of calibration. Infrared refers to a region of spectrum between the visible and microwave. The infrared spectrum extends from 0.75 mm to 1000 ?m wavelength range. However for practical applications it is the 1 – 15 ?m wavelength band, which is used. The properties of infrared radiations are similar to those of other electromagnetic radiations like light. These radiations travel in straight lines; propagate in vacuum as well as in solid, liquid and gases. These radiations can be optically focussed and directed by mirrors and lenses. The laws of geometrical optics are valid for infrared radiations as well. The energy and intensity of infrared radiation emitted by an object primarily depends on its temperature and can be calculated using the analytical tools such as Wein’s law, Plank’s law and Stefan Boltzmann law. When a body is heated, there is an increase in the temperature and emitted energy. The spectrum of infrared radiation emitted by a heated object contains a continuous band of wavelength over a specific range. The fundamental equations or radiation laws that link the absolute temperature of the emitting object, peak radiation, the intensity and wavelength are the Plank’s Law, the Stefan-Boltzmann Law and the Wein’s Displacement Law. The Plank’s law describes the spectral distribution of radiation intensity from a black body and is mathematically expressed as: [Wm-2sr-1?m-1] ……………………….. (1) Where, W? = Blackbody spectral radiant emittance at wavelength ? (?m) c = 3x108 m/s is the velocity of light in vacuum h = 6.634 x10-34 Js is Plank’s constant k = 1.4 x10-23 J/K is Boltzmann’s constant T is absolute temperature of the blackbody Spectral radiant emittance of a blackbody at different temperatures is shown in Fig.1 [1]. Fig. 1: Spectral radiant emittance of a blackbody at different temperatures It can be seen in Fig. 1 that total energy radiated by a blackbody i.e. area under the spectral radiant emittance increases with increasing temperature of the blackbody. Further it can be seen that maxima of the spectral radiant emittance is shifting towards lower wavelength with increasing temperature of the blackbody. If one differentiates equation (1) with respect to ? and equates the differential to zero then one gets the relationship between the temperature of the blackbody and the wavelength corresponding to the maximum spectral radiance. This relationship is known as Wein’s law and is mathematically expressed as [2]: ……………………….. (2) Where, ?max is the wavelength corresponding to the maximum spectral emittance T is absolute temperature of the blackbody This equation supports left ward shift of the spectral emittance peak with increasing temperature of the blackbody as in Fig. 1. Integrating equation (1) with respect to ? between the limits ? = 0 to ? for a given temperature T of a blackbody, one gets total radiant power emitted into a hemisphere from the blackbody. This relationship is known as Stefan’s-Boltzmann law and is mathematically expressed as: Total emittance [W/cm2] …………….. (3) Where, ? = 5.6686x10-12 Wcm-2K-4 is Stefan-Boltzmann constant This law states that total radiant power is proportional to the fourth power of the source temperature. One can compute the power radiated by human body using equations (3). Let us take human body temperature as 300 K and an external surface area of say 2 m2; then the total radiated power is approximately 1 kilowatt. Detailed review of the principles of infrared imaging is presented in reference [3]. Infrared Imaging System A typical infrared imaging system consists of the infrared camera, control unit, image acquisition and analysis unit. The heart of the system is infrared scanner. This scanner unit converts the electromagnetic thermal energy radiated from an object into electronic video signals. These signals are amplified and transmitted to a display monitor via an inter-connecting cable. Infrared imaging system can be classified as qualitative or quantitative systems. A qualitative system displays only an isothermal map. This isothermal map is not corrected for emissivity variations, system non-linearity or atmospheric effects. Thermal measurements are not possible from image, as it does not include the temperature. In a quantitative system, the infrared signal is temperature calibrated using an internal blackbody reference. Appropriate correction factors are also applied such that the infrared image displayed has a temperature distribution approaching the true surface temperature distribution on the object. Performance Parameters of Infrared Systems Infrared images are primarily due to variations in temperature and / or emissivity within a scene or a target. The ability of an infrared system to produce a sharp and accurate image depends on its performance characteristics, which include the system’s ability to detect the infrared radiation and resolve the temperature difference spatially. To select an imaging system for a particular application or to determine whether a given system has adequate capabilities, it is necessary to have an understanding of the parameters that characterize the performance of the infrared system. The typical performance parameters generally considered include [4] (a) Temperature range (b) Absolute accuracy and repeatability (c) Frame repetition rate (d) Spectral range (e) Spatial resolution and thermal sensitivity (f) Environment During last two decades or so, there have been rapid advances in the capability of infrared systems due to developments in the field of electronics, instrumentation and computers. Infrared systems have progresses from liquid nitrogen cooled systems with limited dynamic range in which acquiring an image itself took a few minutes to sterling cooled focal plane arrays and uncooled microbolometer systems with temperature range of -233 K to 773 K extendable with filters to 2023 K. Present day systems have a temperature resolution as small as 0.05 K. The dynamic range of these systems is 14 bit and they have frame rates better than 30 frames per second. A variety of image processing and enhancement software are available to improve the contrast of thermal images and for quantitative measurements. Most of the infrared systems have become handheld with weights less than 2 kg, rugged and field worthy. All the modern day cameras are compatible with personal computers and the windows based software. Infrared Techniques Infrared imaging basically exploits the non-equilibrium thermal state within a material for the detection of defects. This non-equilibrium state can be achieved through use of sources which can heat or cool the body. Such sources can be located within the material itself or can be external to it. Thus two approaches are generally recognized in thermal NDE – (a) Passive and (b) Active. Passive techniques involve applications where the material already contains its own internal source of heat. Majority of condition monitoring applications where the component themselves get heated up due to variety of reasons fall under this category. Let us take an example of a steel convertor i.e. LD converter. This is used for making steel from hot metal and operates at very high temperature approximately 1800 – 2000 oC. To carry out NDE of this vessel in service using thermography; there is no need of heating this vessel as the heat source is within the vessel itself. Similarly if one intends to carry out thermography of a running engine; again there is no need of employing an external heat source. Active techniques involve the application of an external thermal perturbation (heating or cooling) to the object as a whole or of a small area of interest within the object. While both heating or cooling can be applied, it is heating which is generally preferred since a variety of sources such as hot air guns, incandescent and flash lamps, lasers, plasma arcs, inductive heating, heating strips etc. are available. Let us take an example of a pressure vessel which has been in service for quite sometime and some defect might have cropped up during service. Now one wants to do condition monitoring of this vessel for residual life assessment; then one will require to heat this vessel by some means like flowing hot water so that thermal profile of its surface can be acquired or analyzed for presence of hot spots which are indicator of presence of discontinuity or defect in the wall of the pressure vessel. Advantages and Limitations of Infrared Imaging The unique advantages of infrared imaging are its (a) non-contact nature, (i) real time capability, (c) ability to provide full field images that help in visualizing the process and effects and (d) its direct applicability to engineering components. However, interpretation of thermal images requires skill and adequate knowledge of infrared physics. The main disadvantage of thermal imaging is that it is basically a surface phenomenon. Thus defects deep inside in thick objects are likely to be missed. Applications of Thermography in Industry Thermography has emerged as very useful method of non-destructive evaluation of engineering components and structures in chemical, oil and gas, nuclear and power industry to name a few. The main advantage of this method is its non-contact nature and its ability to produce complete picture. Because of its non-contact nature it can be used remotely and is therefore, very useful for components and structure operating in toxic and radioactive environments. In subsequent sections, some of the interesting applications of this NDE technique is briefly discussed. Thermography of Laser Welding of Polymer [5] This is a very interesting application. Welding of polymers is a real challenge and is relevant of many applications like in electronic, medical devices, aerospace, automotive etc. to name a few. Laser welding of polymers is a through – transmission joining process. In this process the bonding is produced between two polymer, one is transparent and other is absorbing to the laser radiation. The laser energy is transmitted through the transparent polymer and gets absorbed at the interface between the transparent and the absorbing polymer. This causes melting and joining at the interface. However, process optimization is very critical as the temperature at the interface must remain between melting and degradation temperatures. It is very difficult to monitor this temperature by any method and infrared thermography remains the only option. This application is presented in detail in reference [5]. Thermograph of laser welding between PMMA and ABS/PC couple is presented in Fig. 2 below. It can be seen from this figure that infrared imaging is very effective tool in this application and very useful in optimizing and monitoring the laser welding process. Fig. 2: Infrared Image of Laser Welding Between PMMA and ABS/PC Couple Thermography for Fatigue Limit Determination [6] Fatigue is a mode of failure of material / component under cyclic loading. Through this mechanism, materials fail at loads much below its yield strength. This mode of failure is very prominent in automobile industry and hence it is very useful to know fatigue limit of materials. Fatigue tests are slow test and therefore a fast test method to determine fatigue limit of materials is very useful. Thermography provides a means of rapid determination of fatigue limit of materials and components and therefore, has a potential to contribute rapid development of new components from design to acceptance level. This methodology is presented in reference [6]. This method is known as Risitano method and uses thermography to obtain thermal map over the surface of the component and to determine fatigue limit of the component from this thermal image. Conclusions: Based on the analysis of the underlying physics of thermography and interesting applications of the same it can be concluded that infrared thermography is an excellent non-contact and non-destructive method of thermal imaging of a surface. This method is capable of defect detection on the surface and within the surface by careful analysis of distortions in the thermal image produced by these defects. Besides, actual temperature can also be calculates. This technique is not only a routine NDE method but also a very important real time diagnostic instrument for recording and analyzing evolution of thermal energy during a process like welding, fluid flow etc. and therefore, very useful for process monitoring. References: [1] http://scienceworld.wolfram.com/physics/PlanckLaw.html [2] Astarita T., Cardone G., Carlomagno G. M., Meola C. “A survey on infrared thermography for convective heat transfer Measurements”, Optics & Laser Technology 32 (2000) 593-610 [3] Baldev R., Venkataraman B. And Babu Rao C. Journal of NDE, Sept. 1992, pp 1 – 13 [4] Venkataraman B. And Baldev R. “Performance Parameters for Thermal Imaging Systems”, Insight vol 45, no. 8, Aug 2003, pp 531 – 535 [5] Speka M., Mattei S., Pilloz M., Ilie M. “The infrared thermography control of the laser Welding of amorphous polymers”, NDT&E International 41 (2008) 178–183 [6] Rosa G. L. and Risitano A. “Thermographic methodology for rapid determination of the fatigue limit of materials and mechanical components”, International Journal of Fatigue 22 (2000) 65–73 Read More