A ground based wind turbine blade inspection system and method consists of a thermal imaging camera configured to detect propagating defects by acquiring thermal imaging data from a wind turbine blade when it is substantially at thermal equilibrium with respect to surrounding air and analyzing the thermal imaging data with a processor to identify thermal effects associated with latent defects caused by internal friction due to cyclic gravitational stresses and wind loads during normal turbine operation. The system permits latent defects to be identified using a ground-based in situ inspection before they become visually apparent, which allows repairs to be made economically while the blade is in place.