Nonlinear air-coupled thermosonics for fatigue micro-damage detection and localisation

Abstract

Over the years, traditional active infrared thermography has played a pivotal role in ensuring that a component is free of any damage. However, whilst optical thermography is still not sufficiently sensitive to the presence of material micro-flaws, current vibro-thermography requires inspection solutions that are not always feasible, such as the use of coupling materials between the sensing probe and the monitored structure. This paper presents a valid alternative to current thermographic systems by developing and experimentally validating nonlinear aircoupled thermosonics for a contactless, rapid and accurate detection of fatigue micro-cracks in an aero-engine turbine blade. The proposed thermographic method combines the high sensitivity to micro-damage of nonlinear ultrasonic techniques with non-contact air-coupled ultrasonic transducers and thermographic equipment. Narrowband frequency sweeps were performed to identify local damage resonance frequencies in order to generate large vibrational amplitudes at the damage location and compensate for signal losses caused by the high acoustic impedance mismatch between the air and the sample. An infrared camera was then used to acquire the thermal response generated by frictional heat at the crack interfaces. Moreover, an image processing method based on a combination of morphological opening and a Savitzky-Golay smoothing filter was employed to enhance the quality of thermal images affected by anisotropic heating and thermal noise effects. Nonlinear aircoupled thermosonics experiments were validated with laser Doppler vibrometry scan measurements and compared with both flash and pulsed phase thermography. Thermal imaging results showed that the proposed nonlinear air-coupled thermosonics was the only thermographic technique able to detect fatigue micro-cracks, thus demonstrating its potential as an efficient and sensitive inspection tool for micro-damage detection in geometrically complex components.

Recent developments in vibrothermography

“…For overcoming this problem, one promising approach is finding LDR values of defects via FEM analysis and stimulating the material with these values or using multi-frequency excitation [80]. By using this method, different defects with different sizes can be detected by changing the vibration at related LDR frequency [81][82][83]. For distinguishing damaged regions from surroundings, multi-frequency excitation is a very useful method which enables that the resulting heat generation covers the whole length of the material [84,85].…”

A Review of NDT/Structural Health Monitoring Techniques for Hot Gas Components in Gas Turbines

“…In this work different techniques were compared. The non-contact nonlinear air-coupled thermosonics (NACT) technique was the most promising [163]. Zhang investigated the physical process of the chaotic excitation of a turbine blade to improve the fault detection capability of thermosonic technology [164].…”

In-plane local defect resonances for efficient vibrothermography of impacted carbon fiber-reinforced polymers (CFRP)

“…Generally, the component is excited at an LDR frequency of the defect and the corresponding vibration induced heat is diffused to the surface and detected using a high-sensitivity infrared camera [10][11][12][13][14][15]. The efficiency of LDR based vibrothermography can be enhanced through nonlinear ultrasound spectroscopy and selecting those LDR frequencies that indicate highest amplitudes of defect induced second harmonics [16][17][18]. Furthermore, applying a combination of an LDR and a modulation frequency that shows highest side band amplitudes (so called nonlinear wave modulation thermography) intensifies the defect's interaction with excited vibrations and raises the resultant heating [19].…”