Matched excitation energy comparison of the pulse and lock-in thermography NDE techniques

The defect detection capabilities of pulse transient thermography and lock-in thermography have been compared using equal excitation energies. A signal-to-noise ratio analysis has been performed on defect images obtained by the two techniques using a CFRP sample with flat-bottomed hole artificial defects of 4mm, 6mm and 12mm diameters at depths ranging from 0.25mm to 3.5mm. Similar limits of defect detection have been found for the two techniques. Lock-in thermography phase images have been found to exhibit anomalous switches between positive and negative phase values for different modulation frequencies and for different combinations of defect diameter and depth. This effect resulted in values of defect phase response that differed substantially from the one-dimensional thermal wave interpretation of lock-in thermography imaging. The consequences of this anomalous effect and its physical origins are discussed.