Induction Thermography of Surface Defects

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. Values for the detection limits for various types of cracks and approaches to determine crack depths are given. The sensitivity for crack detection is comparable to magnetic particle inspection. A hidden defect in ferritic steel with a coverage of 140 mm was detected by lowering the induction frequency down to 1500 Hz. Cracks in silicon solar cells were detected. Defects of fibers were detected in carbon fiber-reinforced polymer (CFRP). Inductive excitation is complementary to flash excitation. Crack detection in railway components like rails and wheels is shown. 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 demonstrator for wheel testing was built up, which can detect surface defects in railway wheels with sensitivity comparable to magnetic particle testing. Standardization of thermography has gained progress in the last years and led to first standards on active thermography and induction thermography.