Nondestructive terahertz imaging for aerospace applications

ABSTRACT The full potential of terahertz imaging systems for nondestruc tive aerospace imaging applications has not been realized due to the lack of data linking damage and defects to terahertz signatures coup led with the complex ity of modeling the signatures. Terahertz systems (0.1  2.0 THz) may be id eally suited for NDI applications because of the ability of THz radiation to penetrate through substances commonly found on the surfaces of aircraft structures while maintaining the optical resolution required to de tect defects. We will discuss several systems that we have used to study the signatures of a set of target samples with known defects. Keywords: Terahertz, non-destructive evaluation, im aging, aerospace, corrosion, composites 1. INTRODUCTION The THz spectral range has received considerable attention for the development of standoff imaging systems for security and non-destructive evaluation ap plications[1, 2]. This is du e to the relatively high transmis sion through most dielectrics and high reflectivity off of metallic surfaces that are material properties associated with the microwave region, coupled with the potential to develop traditional optical imaging systems like those at shorter wavelengths, such as in the infrared. A THz imaging system can provide a non-destructive, standoff imaging technique capable of detecting corrosion on metallic surfaces under obscurants and defects in composite materials on aerospace structures. However, there is a significant transition of phenomenology when spanning this frequency region[3, 4]. In addition to higher transmission occurring at the lowest frequencies and higher optical resolution at the highest frequencies, signatures associated with defects will significantly change due to the relative scale of the wavelength to the physical structure of the defect along with changes in the complex dielectric constant of both the obscurants and defects. Due to the lack of data linking defects to terahertz signatures and the complexity of modeling the signatures, the potential of terahertz imaging systems for nondestructive aerospace imaging applications will need to be studied in much more detail before viable systems can be designed for a wide range of applications. Some successful examples that illustrate the potential for NDE applications include products from the companies Picometrix and Teraview, both of which have developed THz NDE commercial systems for specific applications[5, 6]. We will discuss several systems that we have used to study th e THz signatures of a set of target samples with defects. These include a set of metallic samples with corrosion and a set of fiberglass composite samples with a variety of defects. This work is an extension of work that was begun at the Air Force Institu te of Technology by Stoik and Bohn in a study of the fiberglass composite structures[7, 8]. This data, along with the images of the metallic samples, will provide a test bed for the development of an imaging model that will be able to evaluate potential terahertz technologies for aerospace NDE applications, much like that has been done for security applications and the detection of concealed weapons[9].