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Most of the multi-layered aircraft structures including composite structures are still inspected primarily through various visual methods that require removal of multiple structural components to detect flaws in the internal layers of the structure. Some aircraft operators utilize for the multi-layered inspections more advanced NDI techniques such as X-ray. However, application of the X-ray technique still requires access to the bottom layers of the multi-layered structures for proper positioning of films or digital sensors. Additional time is also needed to comply with the safety rules for the X-ray inspection procedures. Hence, current inspection procedures for the multi-layered aircraft structures are fairly cumbersome, time-consuming and costly.
Application of the dry-coupled ultrasonic modules makes it possible to detect and characterize defects in the internal layers from outside aircraft skin without disassembly. The inspection technique is easy to use, and, at the same time, is sensitive enough to identify critical structural degradation caused by the defects. The dry-coupled inspection technique is also sufficiently rapid so that aircraft downtime is minimized. The modules are also suitable for concurrent flaw detection and sealant quality monitoring in the multi-layer aircraft structures. The concept of the dry-coupled transducer modules has already been tested on the DC-10 horizontal stabilizer (crack detection around fasteners).
Several current inspection procedures for aircraft multi-layered composite structures were reviewed to identify the areas for effective implementation of the dry-coupled ultrasonic techniques. Ultrasonic inspection techniques are being developed including flaw detection and characterization protocols for internal defects in various layers of the multi-layered structures. Modular dry-coupled ultrasonic transducers with exchangeable elements and digital encoding systems are being modified for applications on the multi-layered composite structures.
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