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• Depth prediction methods are used during cooling time in square-heating thermography. • Depth prediction methods suit materials with lower thermal diffusivity and deeper defects. • The logarithmic characteristic time and logarithmic depth show a linear relationship. • Two of the four depth prediction methods do not require a reference area. Square-heating thermography was used to predict the depth of glass fiber reinforced plastic (GFRP) flat-bottom holes during its cooling period. Four methods were proposed for depth prediction using a specified characteristic peak time. Square-heating thermography cooling theory was analyzed using numerical methods, the linear relationship between the logarithmic characteristic time and logarithmic depth of the GFRP's flat-bottom holes was obtained, and the slope and intercept of the linear curve were mainly related to the thermal diffusivity and the heating time. Finally, the flat-bottom hole depth was predicted using experimental cooling data from GFRP samples. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]