13-08-2013, 04:33 PM
Thermal imaging as inspection method
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ABSTRACT
Thermal imaging is a technique that produces a visible graph or thermographic image of thermal energy radiated from objects. The human eye can only see the narrow middle band of visible light that encompasses all the colours of light in the rainbow. Thermography utilizes a portion of the infrared band of the electromagnetic spectrum between approximately 1 and 14 microns. Thermal infrared images translate the energy transmitted in the infrared wavelength into data that can be processed into a visible light spectrum video display. Thermal infrared imagers are detector and lens combinations that give a visual representation of infrared energy emitted by all objects. In other words thermal imagers let you “see” heat. Thermography is a non-contact, non-destructive test method that utilizes a thermal image to detect, display and record thermal patterns and temperatures across the surface of an object. Thermography is widely used in industry for predictive maintenance, quality assurance and forensic investigations of electrical, mechanical and structural systems. Other applications include, tank and concrete inspection, nondestructive testing, condition monitoring, night vision and medical and veterinary sciences.
INTRODUCTION
Thermal or infrared energy is an energy, not visible because its wavelength is too long for the sensors in our eyes to detect. It is the part of the electromagnetic spectrum that we perceive as heat. Unlike visible light, in the infrared spectrum, everything with a temperature above absolute zero emits infrared electromagnetic energy according to the black body radiation law. Even cold objects such as ice cubes, emit infrared radiation. The higher the temperature of the object, the greater the infrared radiation emitted. The Infrared camera allows us to see what our eyes cannot.
All objects, cold or hot, radiate heat in the form of infrared energy. As an object increases in temperature, it radiates more energy, and the wavelength gets shorter. Infrared radiation, visible light and ultraviolet light are all forms of energy in the electromagnetic spectrum. The only difference is their wavelength or frequency.
What is Thermography?
Infrared Thermography is the technique that uses an infrared imaging and measurement camera to "see" and "measure" invisible infrared energy being emitted from an object.
Thermography is a non-contact, non-destructive test method that utilizes a thermal imager to detect, display and record thermal patterns and temperatures across the surface of an object. Infrared thermography may be applied to any situation where knowledge of thermal profiles and temperatures will provide meaningful data about a system, object or process.
What principle used in Thermography?
Since infrared radiation is emitted by all objects based on their temperatures, 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. Radiation also originates from the surroundings and is reflected in the object, and the radiation from the object and the reflected radiation will also be influenced by the absorption of the atmosphere
If the temperature an object gets hot enough however, above 525°C the energy from that object will radiate energy in the visible spectrum and we will see it. This is when we see an object like the burner on an electric stove “glowing” red. In fact any time an object will emit or reflect energy in the same frequency of our eyes we will see it.
Where Thermography is used?
Thermal imaging photography finds many uses. For example, firefighters use it to see through smoke, find persons, and localize hotspots of fires. With thermal imaging, power line maintenance technicians locate overheating joints and parts, a telltale sign of their failure, to eliminate potential hazards. Where thermal insulation becomes faulty, building construction technicians can see heat leaks to improve the efficiencies of cooling or heating air-conditioning. Thermal imaging cameras are also installed in some luxury cars to aid the driver.
Benefit of Thermography?
Thermography is a non-contact, non-destructive test method. It is capable of catching moving targets in real time. It finds defects in shafts and other metal parts. It is used to take measurement in areas inaccessible or hazardous for other methods. It find deteriorating components prior to failure. It helps compare temperatures over a large area. When compared with other classical nondestructive testing techniques such as ultrasonic testing or radiographic testing, thermographic inspection is safe, nonintrusive and noncontact, allowing the detection of relatively shallow subsurface defects under large surfaces and in a fast manner
THERMOGRAPHIC CAMERA
Radiation from an Object
Every object whose surface temperature is above absolute zero (-273 °C) radiates energy at a wavelength corresponding to its surface temperature. Utilizing our highly sensitive infrared cameras, it is possible to convert this radiated energy into a thermal image of the object being surveyed.
Infrared energy is just one part of the electromagnetic spectrum that encompasses radiation from gamma rays, x-rays, ultra violet, a thin region of visible light, infrared, microwaves, and radio waves. These are all related and differentiated in the length of their wave (wavelength). All objects emit a certain amount of black body radiation as a function of their temperatures. The higher an object's temperature is, the more infrared radiation as black-body radiation it emits. A special camera can detect this radiation in a way similar to an ordinary camera does visible light. It works even in total darkness because ambient light level does not matter. This makes it useful for rescue operations in smoke-filled buildings and underground.
Active and Passive Thermography
If material under inspection is heated with radiators (active thermography), the temperature of the surface will rise suddenly. The speed at which the heat front dissipates into the material depends on the thermal properties like density, heat capacity, thermal conductivity and the bonding quality between top surface layer and the base material. A defect in the sub-surface creates a barrier for the heat diffusion process and, therefore, the surface temperature above the defect will decrease more slowly than the temperature in other regions. The surface above such a defect will show a hot spot for a longer time as its vicinity covering good bonded material. The principle of this effect is shown schematically in Fig.3 .
Results of active and passive thermographic measurements
For first tests with active on-line thermography, different panels of laminating floor were placed on a conveyor belt and heated by a few degrees centigrade while moving along three infrared heaters.
The result can be seen in Fig. 3.2.1. & Fig.3.3.2., where a blister shows up in the thermographic image as circular hot (red) spot. But moreover some extended areas show also a warmer behavior as its vicinity. Destructive tests showed that these areas have a decreased adhesion bonding in comparison to the rest of the material.
Nondestructive Testing
Nondestructive Testing (NDT), also called nondestructive examination (NDE) and nondestructive inspection (NDI), is testing that does not destroy the test object. To detect different defects such as cracking and corrosion, there are different methods of testing available, such as X-ray and ultrasound.
While destructive testing usually provides a more reliable assessment of the state of the test object, destruction of the test object usually makes this type of test more costly to the test object's owner than nondestructive testing. That there is a tradeoff between the cost of the test and its reliability favors a strategy in which most test objects are inspected nondestructively; destructive testing is performed on a sampling of test objects that is drawn randomly for the purpose of characterizing the testing reliability of the nondestructive test.
Condition monitoring
It is the process of monitoring a parameter of condition in machinery, such that a significant change is indicative of a developing failure. It is a major component of predictive maintenance. The use of conditional monitoring allows maintenance to be scheduled, or other actions to be taken to avoid the consequences of failure, before the failure occurs. It is typically much more cost effective than allowing the machinery to fail. Serviceable machinery includes rotating machines and stationary plant such as boilers and heat exchangers.
CONCLUSION
When compared with other classical nondestructive testing techniques such as ultrasonic testing or radiographic testing, thermographic inspection is safe, nonintrusive and noncontact, allowing the detection of relatively shallow subsurface defects under large surfaces and in a fast manner.Thermography enables us to see and measure heat. Thermography is a method that utilizes a thermal image to detect, display and record thermal patterns and temperatures across the surface of an object. Thermography is the future in water damage and mold claims adjudication for the insurance industry.
IR is going to allow the industry to be more exact in the data that it obtains, and to be able to resolve any claims that may be occurring now, as well as in the future, more quickly and cost-effectively, saving time and money. A large part of the savings is in the fact that more often than not, when infrared is involved, the case never gets to court.