Terahertz imaging of aircraft composites
Recent development of terahertz systems producing frequencies between microwave and infrared has exciting potential for imaging composite materials as they are transparent at these frequencies. EU support of the project 'Development and optimization of THz NDT on aeronautics composite multi-layered structure' (DOTNAC) facilitated delivery of two new systems to inspect glass fibre laminates or composite structures. The pulsed and continuous wave terahertz systems were tested on a two-dimensional scanner for sample inspection and on a three-dimensional platform for inspection of real aircraft components. In addition, all samples were tested with conventional techniques for comparison. The terahertz techniques were able to detect delaminations and defects in dielectric laminates such as glass fibre laminates. They also detected problems with bonds in sandwich structures and coating issues on conductive substrates such as carbon fibre-reinforced plastic. Detection of porosity in glass fibre laminates requires further research. The frequency-modulated continuous-wave system is completely electronic consisting of three scanning heads of different frequencies (from 100 to 300 gigahertz). The three-dimensional image has depth resolution varying from two to six millimetres. It enables fast scanning and high detection sensitivity. The time-domain system uses a pulsed laser in a fibre-optic pump probe with significantly improved depth resolution for better accuracy. Cross-range resolution is similar to that of the frequency-modulated continuous-wave system. Performance assessments support the competitive position of the two systems compared to conventional techniques as well as the value of using both as complementary techniques. The DOTNAC concept is equally applicable to in-process inspection and continuous health monitoring. Technology has now paved the way to substantial savings in time and cost of aircraft development, production, operation and maintenance. It will also provide an important competitive edge to EU producers and suppliers of non-destructive inspection equipment.
Keywords
Non-destructive inspection, composite, terahertz, pulsed, continuous wave, aircraft, depth resolution, in-process, continuous health monitoring
