A comparison of methods used to predict the vibrational energy required for a reliable thermosonic inspection

Thermosonics is capable of detecting cracks in several types of components. The component is excited with high-power ultrasonic vibrations, causing cracks to generate heat, which can be detected by an infrared (IR) camera. However, the excitation in a typical thermosonic test is non-reproducible and can lead to cracks being undetected if sufficient vibrational energy is not applied at the crack location. The vibrational energy dissipated as heat at the defect is directly related to the frequency and amplitude of the vibration, and this energy can be represented by a single parameter (Heating Index) computed from the vibration waveform. The Heating Index parameter is useful as it can be used to predict the vibration level required for a reliable thermosonic inspection. The aim of this work is to compare different vibration measuring devices that may be used to capture the vibration waveform required to compute the Heating Index. In this study, an aero engine turbine blade is inspected using a practical thermosonic setup, after which the vibration waveforms acquired from a laser vibrometer, microphone and strain gauge are processed. Results from this work will highlight the relative merits and limitations of these different vibration measuring devices for computing the Heating Index.