In the aeronautical industry, composite structures under testing often have complex and variable geometries. In such cases, an optimal use of ultrasonic transducer arrays requires specific algorithms in electronic systems in order to achieve rapid and reliable inspections. To fulfil such requirements, a new real-time and adaptive technique is presented. The surface adaptive ultrasound (SAUL) technique is based on an iterative algorithm that does not require a prior knowledge of the geometrical properties of the inspected domain. All different parts of a given component can be controlled using the same transducer array, such as a conventional linear array with a flat shape. In this paper, the adaptive processing is demonstrated through acquisitions performed with different typical aircraft composite structures. In addition, we present a new surface reconstruction algorithm. This fast algorithm is efficient and can be coupled with the real-time adaptive processing to reconstruct SAUL images and, then, to improve the characterization of flaws in composite materials.
The ability to measure early-stage high-temperature hydrogen attack (HTHA) has been improved by the use of optimized ultrasonic array probes and techniques. First, ultrasonic modeling and simulations were performed to design a set of array probes. The data was then collected using phased array ultrasonic testing (PAUT) and full matrix capture (FMC) techniques. Damage visualization, characterization, and sizing was completed with PAUT, total focusing method (TFM), and adaptive total focusing method (ATFM) advanced algorithms. The detection and sizing capabilities were initially validated on steel calibration samples with micromachined defects and synthetic HTHA damage. Vessels with suspected HTHA damage were removed from service, inspected with multiple array techniques, and then destructively evaluated for a results comparison with metallographic images. This study concluded that the FMC/TFM/ATFM techniques and algorithms improve detectability, characterization, and sizing of early-stage HTHA damage as compared to PAUT.
Techniques based on frequency shifts and mode-shape analysis are being investigated to determine their feasibility for characterizing defects in adhesive-bonded joints in automotive structures. It is well known (Rayleigh–Ritz derivation) that introduction of a crack-like defect into a structure reduces its stiffness and results in a corresponding downward shift in resonance frequencies. Experimental and modeling results show that analysis of resonance-frequency shifts is much more complicated for bonded joints where the defect consists of a gap in the adhesive layer. Structures containing a defective joint sometimes exhibit higher-resonance frequencies than a structure with an undamaged joint. Such results have been observed in laboratory experiments on aluminum plates with adhesive-bonded T joints, and in finite-element simulations for a variety of structures. In all cases, the defects studied are gaps in the adhesive layer of the joints. While the reduction in mass associated with a gap in the adhesive layer is very small, the mass effect overwhelms the frequency-decreasing effect of the reduced stiffness when the defect is located in a low-stress region of the joint. Thus, the direction and magnitude of frequency shifts depend on the resonance mode and the location of the defect.
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