Passive IRT is used in quality control and process monitoring applications. Temperature plays a crucial role in any industrial process. Thus, temperature measurement and monitoring during and after the industrial process is critical to achieve optimal results, such as steel rolling or sinterization. However, the computation of temperature from infrared images is not only based on measured radiation; it also depends on the internal camera calibration, as well as on the emissivity of the object radiating energy.
Active IRT is mostly used in non-destructive testing applications, where an external stimulus is applied to the specimen in order to induce relevant thermal contrasts between regions of interest. It is applied to the inspection of materials for subsurface defect detection and to detect areas of the specimen with different properties below the surface. Some subsurface anomalies are very subtle. Therefore, the signal levels associated with them can be lost in the thermographic data noise. In these cases, different post- processing methods can be used to improve the signal to noise(SNR) content of thermographic data.
In vibro thermography, the heat generated by the friction of the discontinuities, cracks or even delamination’s is induced by the effect of mechanical excitation (20–50Hz) applied externally to the structure. These discontinuities are excited under specific mechanical resonances. Depending on the variation of the frequency of mechanical excitation, the local thermal gradients that indicate the presence of the defect can appear or disappear.
Ultrasound thermography (UT) is a variation of vibro thermography. While the structure under study is vibrated in vibro thermography, in UT, a piezoelectric effect introduces ultrasonic waves that propagate through the material. A high-frequency ultrasound signal is generated at 40kHz and is additionally modulated with another lower frequency signal. The test configuration is as follows a horn (sonotrode) injects ultrasound waves into the material; low frequency waves make spreading possible, while high frequency vibration produces heat by the friction of particles.
Thermoinduction thermography creates eddy currents within the material to be inspected by circulating a current at certain frequencies along an induction coil. The current density where the defects are located is different, producing heat on the surface.
Pulsed thermography involves briefly heating the specimen with a short pulse of thermal stimulation and then recording the temperature decay curve. The temperature of the material varies rapidly after the initial thermal pulse, while the thermal front propagates by diffusion through the surface. The presence of a discontinuity reduces the diffusion rate, so that, by observing the temperature of the surface, the discontinuities appear as areas of different temperatures with respect to the surrounding sound areas. Therefore, deeper discontinuities will be observed later and with a smaller contrast.