Description: Picometrix proposes to demonstrate the feasibility of using differential time domain terahertz imaging methods to enhance the contrast and detectability of features such as kissing disbonds and cracks that in conventional THz imaging only weakly reflect or scatter the THz pulses. The goal of the project is to develop methods of shearographic loading of the samples, and use the penetrating THz pulses to detect the subsurface deformation of the defects in the differential THz images with better contrast than traditional THz imaging. In a "kissing" disbond there is a region where the two sides of the material are not adhered, but the space between the two sides are essentially in perfect optical contact. When the space between the two interfaces is so optically "thin," the reflections of the THz pulses from the top and bottom surfaces cancel each other out. The defect signature is only weakly detectable compared to when the spacing is greater than the minimum THz wavelength (approx. 50-150 microns), the shearographic loading will microscopically deform defects, changing the small THz reflections in the loaded vs. unloaded state. The differential images should subtract all background clutter and highlight the microscopic subsurface distortion of the defects under loading.

Benefits: Time-domain terahertz reflection imaging is a proven NDE technology for imaging sub-surface features, flaws, and defects within space flight structures such as thermal protection systems (ablative resin honeycomb, TUFI, SOFI), inflatable space habitats, composite overwrap pressure vessels, radomes, and other dielectric components. THz pulses (0.1 to 3 THz) penetrates these materials, and can be used to generate sub-surface images. THz NDE can detect voids, disbonds, and damage such as tearing and micro-meteorite impact. Material examples include Kevlar, Zylon, and other non-conductive polymer matrix composites. Differential THz imaging should improve the detectability of defects in each of these applications.

Time-domain terahertz gages are used in industry to measure the thickness of multi-layer sheet materials such as plastic film, insulation, foam sheets, roofing, paper, and other similar products that are extruded in presses. THz gages are used to measure the thickness of aerospace coatings, when dry or wet. The proposed differential imaging method and shearographic loading techniques should enhance the detectability of interfaces and delaminations in the manufacturing of these materials. THz instrumentation has significant potential in the research and development, government, and industrial markets for use in on-line and off-line inspection, communications, and test and measurement.