Safety criteria of aircraft industry require careful inspection of aircraft components for structural integrity since airworthiness of aging aircraft can be significantly affected by combination of corrosion and fatigue damage. Surface defects can be efficiently detected by visual or other surface inspection techniques. Detection of hidden defects, on the other hand, is still a challenging task. Therefore, it is essential to develop non-destructive methods that
can inspect different layers of the aircraft structures for internal defects before they become a safety concern. Ultrasonic probes with the dry-coupled substrates are highly efficient for all modalities of ultrasonic techniques including pulse-echo, pitch-catch, or through-transmission modes. The probes can be deployed in conjunction with
portable ultrasonic instruments for B- and C-scanning. The dry-coupled probes have already been tested on a number of aircraft for rapid inspections of the aircraft structures from the outside without any disassembly. However, adequate inspection for small pitting corrosion and incipient fatigue cracks in metallic structures or
delaminations in composite panels may require superior sensitivity and resolution of the applied ultrasonic technique. Several novel configurations of the dry-coupled probes with increased sensitivity and resolution will be presented. Ultrasonic imaging with single- or double-element dry-coupled probes will be demonstrated on the specimens with heavy pitting corrosion, machined planar and volumetric defects, and embedded internal flaws.
Ultrasonic NDI methods have an impressive record of applications on metallic and composite structures. However, limitations arise from the need for a wet couplant between the specimen and the transducer and the rather long inspection times necessitated by point-by-point scanning of large structures. To overcome these constraints, a dry-contact large-area ultrasonic imaging system is being developed for real-time high-resolution NDI applications. This system includes the following: a large ultrasonic source, either piezoelectric or laser-based, a polymer dry-couplant, and a commercially available real-time ultrasonic CCD camera displaying easy-to-interpret images rather than A-scans. Applications of this real-time high-resolution ultrasonic imaging system on metallic and composite structures, using either PZT or laser-based ultrasound generation as the source, are presented. Aluminum and unidirectional and woven composites have been investigated. Images acquired in both through-transmission and pulse-echo modes are presented. Images of artificial defects of different types and shapes in the investigated materials will be demonstrated. The latest developments of the imaging system, with laser-based ultrasound generation as the source, are also reported. The laser-based source provides an efficient solution for some applications of the imaging system. In this configuration, the ultrasound is generated in a 1in. diameter area by an expanded laser beam which heats a constrained absorbing polymer layer. The soft polymer layer is also used as dry couplant to transmit the ultrasound between the test sample and the imaging system.
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.
Some advanced aircraft materials or coatings are porous or otherwise sensitive to the application of water, gel, or some other ultrasonic couplants. To overcome the problems associated with the liquid coupling medium, dry-coupled rolling modules were developed at Northwestern University for the transmission of both longitudinal and transverse ultrasonic waves at frequencies up to 10 MHz. Dry-coupled ultrasonic modules contain solid core internal stators and solid or flexible external rotors with the flexible polymer substrates. Two types of the dry-coupled modules are under development. Cylindrical base transducer modules include solid core cylindrical rotors with flexible polymer substrates that rotate around the stators with ultrasonic elements. Dry-coupled modules with elongated bases contain solid core stators and flexible track-like polymer substrates that rotate around the stators as rotors of the modules. The elongated base modules have larger contact interfaces with the inspection surface in comparison with the cylindrical base modules. Some designs of the dry-coupled rolling modules contain several ultrasonic elements with different incident angles or a variable angle unit for rapid adjustments of incident angles. The prototype dry-coupled rolling modules were integrated with the portable ultrasonic inspection systems and tested on a number of Boeing aircraft structures.
Dry-coupled inspection techniques are very important for applications on components with non-uniform surfaces and for inspections of advanced materials or coatings that are porous or otherwise sensitive to the application of water, gel, or some other ultrasonic couplants. To overcome the problems associated with the liquid coupling medium, a number of polymer films have been developed to transmit the ultrasound through a dry interface. These materials are very flexible so even low pressure loading is sufficient to adapt the films to the irregular inspection surfaces. Several polymer films have been evaluated to develop dry-coupled substrates for transducer modules. The modules will be utilized to detect and characterize fatigue cracks and corrosion spots in the aircraft structures. Ultrasonic properties of the polymer films were measured and compared with the properties of plastic or rubber-like materials commonly used for ultrasonic applications. Experiments have been carried out to analyze propagation of longitudinal and shear waves in the films. Two different types of the ultrasonic modules with the flexible polymer substrates are being developed. The influence of the surface condition on the module performance was evaluated for both types of the modules.
As a part of an aircraft inspection, characterization of fatigue cracks and corrosion spots is a vital task to ensure airworthiness of airplane structures. Conventional techniques for crack or corrosion characterization are based on either echo-amplitude measurements or time-of-flight evaluation. Application of more advanced imaging or scanning techniques significantly improves the accuracy and resolution of ultrasonic measurements. However, these techniques usually require high-cost instrumentation and training. A novel adjustable transducer array for an ultrasonic scanning using low-cost portable data acquisition units has been developed at Northwestern University. The array is utilized to detect and characterize small incipient fatigue cracks and corrosion spots, as well as to monitor the sealant quality in the multi-layered airplane structures. The arrays are suitable for rapid adjustments of elements with different parameters over a wide range of spatial orientations based on the configuration of the aircraft structure, type of the flaw to be detected, and the inspection procedure. The adjustable transducer arrays will replace currently used single element transducer assemblies that are custom-made for each particular application. The new arrays require minimal time to be readjusted from one application to another. The arrays are compatible with the commercially available portable ultrasonic units.
This paper is dealing with the ultrasonic imaging techniques and instrumentation that are used for the inspection of aircraft and bridge structures. A concept of a modular inspection system with interchangeable components will be presented. The modular concept provides greater flexibility in the integration of various commercially available sensors and data acquisition units deployed by operators. Miniature scanning and interface modules were developed as components of the integrated system for wide area inspections using ultrasonic sensors. A novel ultrasonic imaging technique has been utilized for characterization of internal fatigue cracks. The technique is based on a linear scan acquisition and processing. Portable ultrasonic flaw detectors or thickness gages can be used to acquire the data. An image-processing module was developed to manipulate the acquired images and to display the inspection results. Several rapid scanning techniques have been developed to satisfy flaw detection and characterization criteria as well as the scanning and data acquisition module capabilities. The techniques and instrumentation were successfully tested on various aircraft and steel bridge structures.
Application of ultrasonic imaging to the inspection of multilayered airplane structures can provide comprehensive information on possible defects such as corrosion and cracks. A quantitative assessment of defects in internal layers of airplane structures using this technique can be accomplished form an external surface of the airplane skin. No disassembly is required for component evaluation. Therefore this ultrasonic technique can be extremely useful for cost-effective maintenance of airplanes. Three different ultrasonic techniques developed for inspection of the DC-9 and DC-10 airplanes are discussed in this paper. One of the techniques was used for evaluation of corrosion and stress corrosion cracks in a DC-9 tee cap, other techniques have been developed for fatigue crack characterization in a DC-9 rear spar and a DC-10 spar-cap/strap connection, respectively. All three techniques were developed to be applied with the same type of data acquisition and imaging system. However for each kind of flaw and structural configuration substantial modifications of the scanning and the data acquisition procedures have been made. {Peculiarities of each ultrasonic technique will be discussed. The ultrasonic technique for the inspection of the DC-9 tee cap has been approved by the FAA as an alternate means of compliance to meet the requirements of the DC-9 Service Bulletin.
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