Detection of Substrate Cracks Using Laser Holography and Thermal Pulse Propagation

A novel nondestructive inspection (NDI) technique is proposed for detecting surface flaws beneath a thin coating. The basic technique relies on the abililty of laser holography to detect very small surface displacements due to thermal expansion effects caused by a propagating thermal pulse. The technique is closely related to the method of infrared imaging which employs an infrared video camera to detect temperature changes due to an expanding heat pulse. The technique being proposed will have greater resolution than infrared imaging due to the very high sensitivity of the holographic detection mechansim which will allow for the detection fine surface cracks. A further advantage of the Laser Holography Thermal Pulse Propagation (LHTPP) techcnique is that it can be used for all material systems including ferromagnetic, non ferromagnetic, insulators and conductors. In addition LHTPP readily lends itself to computer automation. Currently existing image analysis codes can be directly applied to the storage, enhancement and analysis of the holographic data. This will allow for the development of convenient field units for on site testing as well stationary laboratory setups.