Description: We propose to demonstrate key elements of feasibility for a single-sided time-domain terahertz reflection holographic tomographic imaging (TD-THz RHT) nondestructive evaluation (NDE) system which will provide high quality three dimensional images of the interior of complex aerospace composite structures. Time-domain terahertz imaging in the 0.1 to 3 THz spectral range is currently being used to characterize defects in Space Shuttle insulation and related materials. The principal imaging technique has utilized 2D raster scanning of a THz transmitter and receiver to generate a two-dimensional image from the reflected signal intensity. The proposed STTR will use existing Picometrix THz imaging hardware, but incorporate measurements of the scattered THz fields, enabling full 3D reflection-mode reconstruction of non-metallic materials on a metallic substrate. It will use holographic information consisting of phase and amplitude data collected through a 2D sampling plane. The specific reconstruction algorithm to be developed is a novel model-based reconstruction algorithm. This will algorithm will maximize the 3D reconstruction accuracy for the data available. Various options for image angles and sample geometries will be explored. The final imaging system will incorporate 2D or 3D scanning hardware and multiple angle data collection to maximize 3D image quality in rapid scanning applications.

Benefits: The commercial potential of the technology from a successful Phase I development effort extends well beyond the NASA Polymer matrix composites are used in automobile and ships and many other consumer and industrial products. TD-THz reflection holographic tomography (TD-THz RHT) 3D imaging applications can include inspection of automobile dashboards, imaging inspection for delamination of printed circuit boards, inspection of pipe insulation, as well as with manufactured parts such as pure plastic and paper products. TD-THz RHT imaging benefits homeland security applications under development such as personnel and luggage inspection for concealed weapons and explosives (in luggage, shoes, and etc.). TD-THz RHT imaging and spectroscopy can inspect items in shipment such as mail, cardboards packages, and plastic and wood crates.

The capabilities provided by a TD-THz holographic tomography NDE imager will be valuable in characterizing new spacecraft materials in complex 3-dimensional forms. Material examples include SOFI and other foam materials, TUFI and other thermal protection systems (TPS), Phenolic Impregnated Carbon Ablator (PICA) and adhesive systems, and other non-conductive polymer composite structures. Example NDE applications where these materials are used include inspection of soft shell fan containment, thermal protection systems, and composite overwrap pressure vessels (COPV). The technique is fully applicable to single-sided imaging and can locate and quantify internal structure, regions of damage, porosity, cracking, delaminations, intrusion, and other sub-surface properties.