A survey on theory, characteristic quantities and the experimental technique of induction thermography is given. Induction thermography is used for surface defect detection in forged parts of ferromagnetic steel at typical frequencies of 100–300 kHz. The sensitivity for crack detection is comparable to magnetic particle inspection. A hidden defect in ferritic steel with a coverage of 140 μm was detected by lowering the induction frequency down to 1500 Hz. Defects of fibres were detected in carbon fibre reinforced polymer (CFRP). Inductive excitation is complementary to flash excitation. By increasing the induction frequency up to 52 MHz, surface heating of CFRP can be realised. Cracks in silicon solar cells were detected. The crack tip is revealed with very high contrast. A new field is crack detection in railway components like rails and wheels. In rails, a larger defect could be detected from a test car moving at a speed of up to 15 km/h. A fully automated wheel testing system was built up as a demonstrator, that can detect defects with comparable sensitivity to magnetic particle testing. Standardisation of thermography has gained progress in the last years and will lead to standards on active thermography, flash excited and induction thermography in the next future

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A survey on theory, characteristic quantities and the experimental technique of induction thermography is given. The induction frequencies range from 1500Hz to 52MHz. Induction thermography is used for surface defect detection in forged parts of ferromagnetic steel. The sensitivity for crack detection is comparable to magnetic particle inspection. Hidden defects in steel can be detected, if necessary by lowering the induction frequency. Defects of fibres were detected in carbon fibre reinforced polymer. In silicon solar cells, cracks were detected with good contrast. A new field is crack detection in railway components like rails and wheels.

Within the scope of a standardisation research project, a flash thermography round robin test that evaluates reliability, comparability and efficiency of different testing situations was performed. Data recorded at metal test specimens with flat bottom holes (FBHs) were analysed by calculating the signal-to-noise ratio (SNR) of the defect signatures in the thermograms as well as in the phase images as a function of defect parameters. A new multi-parameter probability of detection (POD) model was developed, where an â versus a continuous signal analysis was based on the linear relationship between the SNR and a multi-parameter a. This linear relationship was verified by comparison to data obtained from an analytical model that is considering lateral thermal heat diffusion as well as to data obtained by numerical simulation

A high power ultrasonic burst excitation technique was used to study the potential of the ultrasound excited thermography (vibro-or sono-thermography) to detect and to estimate the depth position of cracks in ceramics as well as voids in glass fibre reinforced polymer. By verification using high-frequency (50 MHz) ultrasound techniques it was proven that sono-thermography can detect relatively deep defects in ceramics. The depth of the defect can be estimated using a theoretical model based on analytical calculations dealing with friction heat sources equally distributed over the crack flanks stimulated by the ultrasonic vibrations of the specimen. In carbon fibre reinforced polymers, the thermographic indications of defects were correlated with 3D X-ray cross sections of the specimen.