Damage Assessment of Quasi-Isotropic Composite Material Using an Enhanced Thermography Nondestructive Technique

For quantifying defects encountered in the field it is desirable to have results from comparable calibration specimens or other forms of reference temperature profiles. Composite laminates exhibit a variety of complex damage states such as those seen under impact damage, and it is very difficult to fabricate calibration specimens that represent such damage configurations. In the absence of suitable calibration specimens, the interpretation test data and evaluation of the defect severity by the test technician is not possible. In this circumstance, results from validated numerical simulations will be valuable in interpreting the results obtained in the field. However, the experimentally obtained thermograms are available only in terms of an arbitrary scale that is not readily relatable to a standard temperature scale.;In this thesis, a flash thermographic technique along with a new image processing is used to quantify subsurface defects in a carbon-epoxy laminate with quasi-isotropic layup. In addition, the surface temperature evolution following the flash heating of the surface in the defective region is modeled using finite element technique. A normalization scheme is used to convert the experimental and numerical results into a form that can be easily compared and correlated. The validity of this normalization procedure is verified in this thesis. Variation of temperature with space and time for these defects were recorded and analyzed. In order to directly correlate the two results, the temperature profiles from experiments as well as numerical simulations were presented in the form of contour plots with time, space, and temperature plotted along x, y, and z directions. A three dimensional correlation between the experimental and numerical results were evaluated.;Experimental results were then evaluated. A sample profile length, starting at the center of the "hot-spot"and extending outwards, were evaluated. With this profile length, a contour plot of temperature evolution over time was created. The experimental contour plot was then compared to each numerical contour plot. Excellent correlation between the results from the experiments and numerical simulations for each of the defects considered. It was also demonstrated that it was possible to readily differentiate between the different defects and determine the correct defect dimensions.