Induction thermography: principle, applications and first steps towards standardisation

Abstract

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

Induction Thermography for Surface Crack Detection and Depth Determination

“…As these head checks lay typically inclined to the surface, further numerical models have been set up for angular defects [12][13][14]. As induction thermography became an important NDT (non-destructive testing) method for detecting surface cracks in industrial applications [15], many efforts have been undertaken [16] to set up a standard [17].…”

Thermography and Shearography Inspection of Composite Hybrid Sandwich Structure Made of CFRP and GFRP Core and Titanium Skins

“…At 100 kHz frequency, titanium's ESD is around 2 mm and TPD 1.5 mm. For CFRP, ESD is 50 mm and TPD is 1.2 mm [7]. Here we have a mix of CFRP/GFRP core for which these properties are not known.…”

“…As measurement result, the time lag between inductive excitation and thermal response is evaluated, providing an extremely robust signal, which is insensitive to irregularities or impurities of the component surface. IT is usually used for metallic samples crack detection, but other materials can be investigated too [6,7]. Parameters of importance are electromagnetic skin depth (ESD) and thermal penetration depth (TPD) which depend on the material, as well as to the frequency of the inductor.…”

Three-dimensional thermographic imaging using a virtual wave concept

“…To heat up the sample, various methods are used including a flash-lamp or laserheating, [3][4][5][6] ultrasonic waves, 7 or inductive heating. 8 Various temporal excitation methods, such as pulses in pulsed thermography, 9 or frequency-domain signals with a singlefrequency lock-in technique, 10 are applied to heat the sample. Pulsed phased infrared thermography (PPT) combines simultaneously advantages of pulsed and lock-in thermography.…”