Differential Terahertz Imaging Methods for Enhanced Detection of Subsurface Features, Flaws, and Damage

In Phase II, Picometrix proposes to design, construct, test, characterize, and deliver a prototype differential time domain terahertz shearographic imaging system. The differential imaging methods developed in Phase I will be improved and the methods implemented in turn-key software and hardware. In Phase I the feasibility of using differential time domain THz imaging methods to enhance the contrast and detectability of features such as disbonds was demonstrated. Kissing disbonds and cracks may only weakly reflect the THz pulses in conventional THz imaging. The Phase I project developed the methods of shearographic loading of the samples, and used penetrating THz pulses to detect the subsurface deformation of the defects with better contrast than traditional THz imaging. In a disbond there is a region where the two sides of the material are not adhered, and the space between the two sides are essentially so close that THz interface reflection pulses from the non-adhered region may be partially cancelled out.. The defect signature may be only weakly detectable compared to when the spacing is greater than the minimum THz wavelength. The differential images null background clutter and highlight the subsurface distortion of the defects under loading.