Thermal Inspection comprises all methods in which heat-sensing devices are used to measure temperature variations in components, structures, systems, or physical processes. Thermal methods can be useful in the detection of subsurface flaws or voids, provided the depth of the flaw is not large compared to its diameter. Thermal inspection becomes less effective in the detection of subsurface flaws as the thickness of an object increase, because the possible depth of the defects increases.
Thermal inspection is applicable to complex shapes or assemblies of similar or dissimilar materials and can be used in the one-sided inspection of objects. Moreover, because of the availability of infrared sensing systems, thermal inspection can also provide rapid, noncontact scanning of surfaces, components, or assemblies.
Thermal inspection does not include those methods that use thermal excitation of a test object and a nonthermal sensing device for inspection. For example, thermally induced strain in holography or the technique of thermal excitation with ultrasonic or acoustic methods does not constitute thermal inspection.
Principles of Thermal Inspection
The basic principle of thermal inspection involves the measurement or mapping of surface temperatures when heat flows from, to, or through a test object. Temperature differentials on a surface, or changes in surface temperature with time, are related to heat flow patterns and can be used to detect flaws or to determine the heat transfer characteristics of a test body. For example, during the operation of a heating system, a hot spot detected at a joint in a heating duct may be caused by a hot air leak. Another example would be a hot spot generated when an adhesive-bonded panel is uniformly heated on one side. A localized debonding between the surface being heated and the substructure would hinder heat flow to the substructure and thus cause a region of higher temperature when compared to the rest of the surface. Generally, the larger the imperfection and the closer it is to the surface, the greater the temperature differential.
Heat Transfer Mechanisms: Heat will flow from hot to cold within an object by conduction and between an object and its surroundings by conduction, convection, and radiation. Within a solid or liquid, conduction results from the random vibrations of individual atoms or molecules that is transferred via the atomic bonding to neighboring atoms or molecules. In a gas, the same process occurs but is somewhat impeded by the greater distance between the atoms or molecules and the lack of bonds, thus requiring collisions to transfer the energy. When a gas or liquid flows over a solid, heat is transferred by convection. This occurs from the collisions between the atoms or molecules of the gas or liquid with the surface (conduction) as well as the transport of the gas or liquid to and from the surface. Convection depends upon the velocity of the gas or liquid, and cooling by convection increases as the velocity of the gas or liquid increases.
Radiation is the remaining mechanism for heat transfer. Although conduction and convection are generally the primary heat transfer mechanisms in a test object, the nature of thermally induced radiation can be important, particularly when temperature measurements are made with radiation sensors.
Thermal Inspection Equipment: The temperature sensors used in thermal inspection can be separated into two categories: noncontact temperature sensors and contact temperature sensors. Other equipment includes recording instruments and calibration sources.
Noncontact Temperature Sensors: Noncontact temperature sensors depend on the thermally generated electromagnetic radiation from the surface of the test object. At moderate temperatures, this energy is predominately in the infrared region. Therefore, noncontact measurements in thermal inspection primarily involve the use of infrared sensors.
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