Defect detection and characterization in composite materials using square pulse thermography coupled with singular value decomposition analysis and thermal quadrupole modeling

In this study, square pulse thermography is used to detect and characterize defects in carbon/epoxy composite plates. Defects are polymeric disks inserted between plies at different depths in the samples. A practical experimental set-up is developed: temperature measurements are only required on the front face of the samples and the heating source consists in using standard halogen lamps. A transient thermal modeling based on thermal quadrupoles and several adimensional parameters provides a one-dimensional analytical solution. Singular value decomposition analysis is used to differentiate sound and defect areas. An inversion procedure is carried out to estimate unknown parameters without any prior knowledge on sample properties or defect locations. Especially it is possible to estimate both depth and thermal resistance of the defect with reduced uncertainties. Results on the influence of the analysis and heating durations are presented. A comparison of the thermal contact quality relative to the different defects is finally performed.