Frequency Influence in Microwave Subsurface Holography for Composite Materials Testing

In recent years, microwave methods for nondestructive testing (NDT) as an alternative to conventional ultrasound techniques, are becoming more common in the aerospace industry. Microwave sensors are applied to the examination of composite materials and structures which have many advantages over, and are replacing, traditional metal alloys. Composites typically have a better strength-to-weight ratio, and can withstand adverse weather conditions and corrosive environments. Such materials include polyurethane foam (PUF) insulation, silica fiber tiles (which were used for thermal protection in the American Space Shuttles and the Soviet spacecraft “Buran”), and honeycomb fiberglass which is frequently used in airplanes. The American and Soviet reusable launch system, and other space rockets, which fly on cryogenic fuel components, use PUF insulation to prevent both boiling-away of these components during prelaunch preparation, and the formation of surface ice which can damage the construction. The Space Shuttle Columbia catastrophe showed that PUF insulation coatings require diligent quality control during manufacture and use. Due to the high level of acoustic wave attenuation in porous composites, conventional ultrasonic diagnostic methods are ineffective for these materials, and holographic subsurface radar may be the best alternative. This work describes the development of a special experimental setup consisting of a vector network analyzer, used for signal generation and reception, and an electromechanical scanner which precisely moves the studied samples. The operating frequency band of the vector network analyzer allows experiments across a wide waveband up to 24 GHz. For registration and reconstruction of complex multifrequency microwave holograms, specialized software was developed. The software has special functions for improving quality of reconstructed holograms and suppressing reflections from samples borders. The setup was used for investigation of the operating frequency influence on subsurface imaging results. All of the samples contained intentional model defects typical of composite materials. Comparison of blind experimental imaging results for these test samples with maps of defects (revealed post-experiment) showed good agreement, proving effective detection and delineation of hidden flaws.