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Infrared Testing (IRT)Thermographic testing is used in those cases, when it is expected that the defect of tested piece will affect the temperature distribution on the surface. The inhomogeneous distribution of temperature can be reached by many ways. The object can be the heat source – e.g. at place of excessive heating, or at place of increased electric resistance. The object may accumulate heat nonuniformly (change of specific heat capacity) - e.g. at places of cavities or inclusions; or the heat can be transferred nonuniformly (heat conduction) – usually at places, where the material is internally damaged..
Thermography is often used in aircraft maintenance – it is a very good method for testing of presence of water in honeycomb structures. Another very often application is testing of electric switchboards (testing of malfunctioning contacts). Thermography in metallurgy is used for automated testing of hot-worked semiproducts (at places of potential defects the material cools down with different speed). Last but not least, the thermography is a very efficient tool for testing of rotational devices that allows detection of areas with increased friction..
Physical BackgroundEvery object (at common temperatures) is emitting infrared radiation from its surface. Normal objects at room temperatures emit the spectrum with maximum around 2 – 15 µm. These wavelengths are not visible by human eye, however they are part of the so-called optical radiation. The laws of geometrical optics apply on them and by use of special sensors it is possible to measure them. Thermal cameras allow observation in real time - how the intensity of IR radiation on surface of tested object is changing. The necessary condition is the thermal imbalance of the object (it cannot be uniformly heated). If the searched defect is not working as a heat source, the object is usually brought out of heat equilibrium by thermal pulse and it is measured how the absorbed heat is transferred to the material. Analogically it is possible to divide methods of IRT on active and passive.
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