Thermographic Inspection of Composite Materials

Thermography is a non-contact NDT technique for inspection of materials in wide application areas, including corrosion detection in metals, and delamination, porosity and moisture detection in composite materials. Composites often are highly anisotropic in nature. This anisotropy coupled with low thermal diffusivity in thickness direction, severely restricts detection of deeper defects in composite materials. In the present work, a systematic methodology has been presented for detection of defects in composite materials. The material under consideration is carbon-epoxy, which has high thermal anisotropy because of carbon fibers. Carbon-epoxy laminates with Teflon inserts of varying depth were fabricated for experimental studies. Both one sided and two sided test methods were explored. Based on numerical solution of the heat conduction problem, the optimum combination of heat flux, heating time and observation time for ‘best defect detectibility’ was worked out in each case, assuming zero noise condition. Using these parameter windows as the guidelines, experiments were conducted and the results were compared with the corresponding theoretical predictions. Noise characteristics for each laminate-test configuration were studied experimentally. The same was used for determining the expected limits of defect detection in presence of normal experimental noise. Finally, standard data processing algorithms e.g polynomial fitting and phase imaging were used for enhancing contrast and visualizing the defects.