Phase-Locked Restored Pseudo Heat Flux Thermography for Detecting Delamination Inside Carbon Fiber Reinforced Composites

Thermogram reconstruction methods based on one-dimensional models are widely used in data processing for thermography inspections. However, the surface temperature variances caused by thermal diffusion will be compatible with those caused by delamination whose normalized aspect (diameter-to-depth) ratio is close to 1 when the defect itself is very thin, about 0.15 mm in this research. This phenomenon makes the detection capacity of these methods reduced at such defects with small aspect ratios. The paper proposes a new reconstruction method, phase-locked restored pseudo heat flux (RPHF), for thermography inspection using square-wave optical stimulations. The theoretical analysis shows the independence of the method upon the effect of thermal diffusion blur at defect-free areas. Square-wave thermography tests are conducted on a carbon fiber composite panel with artificial delimitations buried up to 4 mm deep. The method is implemented on a private computer to deconvolute the RPHF kernel from the transformed thermogram data. The data are separated into two sets with a one-period phase shift to each other sequentially; a phase-locked substation is applied between the sets. The global signal-to-noise ratios obtained with the proposed method are compared to those obtained with Busse's lock-in phase images and those with thermographic signal reconstruction. The phase-locked RPHF gives the best global signal-to-noise ratios for normalized aspect ratio at 1.1 when sufficient heat is applied. It's concluded that the thermal diffusion effect at defect-free areas should be considered in thermography inspection for defects with a normalized aspect ratio at 1.1.