The study of inspection on SiC coated carbon–carbon composite with subsurface defects by lock-in thermography

An investigation into the effect of size on the quantitative estimation of defect depth in a SiC coated carbon–carbon (C/C) composite has been undertaken by lock-in thermography. A dedicated 3-D thermal modeling has been introduced, and an efficient numerical algorithm based on finite-difference splitting method in time domain (FDSM-TD) is applied to solve the thermal model. The heat transfer partial differential equation (PDE) and mathematic morphological algorithms are used to filter the phase angle data noise. The diameter of a defect had an appreciable effect on the observed phase angle which consequently has significant implications with regard to estimating the defect depth. Phase angle contrast measurements for a range of defects in a 6.0 mm SiC coated C/C composite specimen indicate that an optimal excitation frequency of 0.525 Hz is available for defect detection. Results obtained with an excitation frequency of 0.525 Hz are used to discuss the limitations of determining the defect size and depth.