The article explores the use of step heating thermography for defect identification in glass fiber reinforced composites. Step heating methods have potential for increasing the maximum detectable defect depth as compared with pulse heating methods. Inspection of thick fiberglass composites containing flat-bottom holes and resistive embedded defects using step heating thermography is investigated using 1-D analytical models, axisymmetric finite element modes, and experimental methods. 1-D heat transfer models of embedded defects under step heating are developed based on results from previous research of pulse heating thermography. Finite element models are used to explore the range of validity of defect identification techniques based on correlation with 1-D analytical methods, and it is found that accurate results are obtained for a defect radius to thickness ratio greater than two. Experimental results demonstrate the validity of the 1-D models for high defect resistance cases. For low resistance defects, detection is more difficult due to reduction of signal strength. A simple procedure is proposed to simultaneously determine defect depth and thermal resistance from the early time surface temperature profile based a two-point correlation with one-dimensional resistive defect solutions.

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