Concept and Objectives:
Modern aircraft structures have to comply with severe requirements: they have to be light as well as safe. These requirements lead to an increased use of composite materials in the aircraft industry. However, new materials also require new techniques in order to inspect aircraft components during production in a non-destructive way. The primary goal of the DOTNAC project is to develop a safe, contact-free, high resolution, and potentially on-site NDT tool based on terahertz (THz) waves, which will be easy to integrate in industrial facilities, and allowing the detection of surface, subsurface and in-depth defects in a variety of composite materials used in aeronautics. The developed NDT tool will fill in the performance gaps that are still present amongst the established NDT techniques and will therefore be an extremely useful tool in NDT in terms of sensor fusion.
The THz spectrum covers a frequency spectrum from the far-IR region to the mid-IR region (centre frequency between 100 GHz and 10 THz). Over the past several years, there has been a significant interest in the potential of THz detection for imaging. There are two major factors contributing to this interest:
(1) Terahertz radiation is readily transmitted through most non-metallic and non-polarized media, thus enabling THz systems to see-through materials,
(2) THz radiation is non-ionizing and poses no health risk to the system s operator.
Nowadays very little research is being performed using THz radiation for air transport NDT, and little is known on how typical composite material defects such as de-lamination, porosities and inclusions can be detected by THz waves. Because of the rapidly increasing use of composites, especially in critical parts, nearly any industry (civilian and military) that fabricates and/or maintains composite structures can benefit both in terms of cost savings and part integrity.
DOTNAC Results in brief:
Terahertz imaging of aircraft composites
Non-destructive inspection of aeronautical composites is an important way to ensure high quality and lack of defects. EU support facilitated development of two novel systems for non-invasive, non-contact and high-resolution evaluation.
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.