Three-Dimensional Visualization of Subsurface Defect Using Lock-In Thermography

An analytical one-dimensional expression for the phase angle of thermal-wave diffusion in a solid layer provides a bridge to link the phase angle to the subsurface defect depth for nondestructive evaluation using lock-in thermographic imaging (LITI), which is helpful to reconstruct the three-dimensional (3-D) structure visualization of subsurface defects. A visualization method of 3-D subsurface defects is proposed from the phase angle by LITI, which consists of phase data pretreatment, smoothing, defect-depth calculating, interpolation, 3-D reconstruction, and morphological de-noising. The result indicates that the 3-D visualization of subsurface defects can be employed to demonstrate the defect structure in detail.