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.