Thermographic determination of delamination depth in composites

The determination of the depth and size of delaminations is important for assessing their impact on structural integrity. A common technique for depth determination from thermal responses relies on a calibration of the technique based on flat bottom hole in a NDE standard. This assumes a delamination will effectively block all the heat diffusion from the region above the delamination to the region below the delamination. For graphite fiber reinforced composites, where a thin delamination has a contact resistance, which is comparable with the thermal resistance of the layer above it, this assumption is inaccurate. This paper discusses thermographic depth profiling based on an analytical model for heat diffusion representing two layers connected by a contact resistance. The model is shown to accurately represent the thermal response obtained for flash heating of composite specimens with known delaminations. Using the model to fit the thermal responses enables an estimation of the depth of the delamination. The accuracy of the technique is determined from measurements on composite specimens with delaminations at known depths.