Terahertz imaging is an emerging and very effective nondestructive evaluation (NDE) technique used for dielectric (nonconducting, i.e., an insulator) materials analysis and quality control in the pharmaceutical, biomedical, security, materials characterization, and aerospace industries. This new method can play a significant role in those industries for materials characterization applications where precision thickness mapping (to assure product dimensional tolerances within product and from product-to-product) and density mapping (to assure product quality within product and from product-to-product) are required.
- Basic schematic diagram of reflection-mode terahertz methodology
- Comparison of hand-measured thickness (cm) map and terahertz microstructure-independent thickness (cm) image.
After the Shuttle Columbia Accident in 2003, Columbia Accident Investigation Board recommendation R3.2.1 stated “Initiate an aggressive program to eliminate all External Tank Thermal Protection System debris-shedding at the source….” To support this recommendation, inspection methods for flaws in foam are being evaluated, developed, and refined at NASA. After the STS-114 flight in July of 2005, significant foam shedding was still observed. The ability to nondestructively detect and characterize crushed foam after that flight became a significant priority when it was believed that the staff processing the tank had crushed foam by walking on it or from hail damage when the shuttle is on the launch pad and in preparation for launch.
Additionally, density variations in the foam are also potential points of flaw initiation causing foam shedding. The innovation described here answered the call to develop a nondestructive, totally non-contact, non-liquid-coupled method that could simultaneously and precisely characterize thickness variation (from crushed foam due to worker handling and hail damage) and density variation in foam materials. It was critical to have a method that did not require fluid (water) coupling (ultrasonic inspection methods require water coupling).
Consider that there are millions of dollars of ultrasonic equipment in the field and on the market that are used as thickness and density meters. This product, when fully commercialized into a more portable form, and as the technology gets less expensive, will be able to replace the ultrasonic instruments for all structural plastic, ceramic, and foam materials. The new instruments will not require liquid coupling thereby enhancing their usefulness in field applications and possibly for high-temperature in-situ applications where liquid coupling is not possible. A potential new market segment can be developed with this technology. [External Link]
More Information
- Non-Contact Measurement of Density and Thickness Variation in Dielectric Materials [External Link], NASA Tech Briefs
- Simultaneous Noncontact Precision Imaging of Microstructural and Thickness Variation in Dielectric Materials Using Terahertz Energy [External Link] (NASA/TM-2008-214997), NASA Glenn Technical Reports Server
- NASA Researchers Receive R&D 100 Awards [External Link]
