Analysis of step-heating thermography methods for defect depth prediction

In this study, five theoretical equations of specific characteristic peak times are proposed to predict the defect depth for step-heating thermography. These equations were derived for test samples with surfaces under adiabatic and convective heat transfer conditions, where, as demonstrated from experiments, the effect of convection on depth prediction can be significant for materials with low thermal conductivities. Two glass fiber reinforced plastic (GFRP) samples with flat-bottom holes at various depths were inspected by step-heating thermography, and the defect depths were predicted by these five methods. Based on the experimental results, the prediction accuracy and the performance of these depth prediction methods are discussed.