Corrosion is a major problem for airframe operators. For the aircraft industry in general, the direct costs of corrosion are estimated at $2.2 billion. As part of their strategy to control corrosion, airframe operators constantly seek to improve their ability to anticipate, manage and identify corrosion activity. Motivated by the need for an on-line real-time corrosion-monitoring tool for industry and aircraft a prototype system and analysis approach is presented. The tool employs ultrasonic Lamb waves along with a dispersion compensated synthetic aperture focusing technique (SAFT) to detect emerging pitting damage. In order to develop an automated detection approach the noise sources of the SAFT processed defect maps were examined and modeled. The random noise was found to be neither stationary nor normally distributed. Locally varying Weibull distribution parameters are used to characterize the image noise. An algorithm is developed to quantify the uncertainty in the corrosion detection and to allow assignment of a constant false alarm probability to any region of the monitored area.
Broadband signals are commonly used in ultrasonic spectroscopy to measure the frequency dependent attenuation characteristics of lossy solid media. Compared to narrowband signals, broadband signals are preferred since they do not require tedious frequency scanning and extensive data reduction efforts. Typically these broadband signals take the form of a pulse. Although the spectral range of a pulse is wide, the spectral resolution is limited by the duration of the signal. By employing signals with large time- bandwidth-products, the overall accuracy and resolution of ultrasonic spectroscopy can be improved. Expressions for the interaction of longitudinal waves, with large time- bandwidth-product, and isotropic materials are developed. The approach is effective for evaluating material with signals optimized for a frequency resolution and range of interest, but can also be used when thin materials (<EQ (lambda) ) are characterized by pulse signals. Using these expressions, the acoustical properties of wave-speed and attenuation can be determined when density and thickness are measured. Explicit account is made for diffraction corrections, multiple echo contributions, and interface scattering losses. The formalism is compared with the traditional analysis approach to illustrate the improved accuracy of the new technique, detailing where diffraction correction and multiple echo effects can become significant. Measured attenuation spectra are presented for common plastic materials as well as for a castable polyurethane commonly used in ultrasonic transducer fabrication.
Navy administrators estimate that hundreds of tons of hazardous material (HAZMAT) are being wastefully discarded due to premature disposal. Currently, HAZMAT items are coded when they are brought into the DoD supply system to indicate their storage lifetime. However, this process has come under criticism for generating shelf life codes that are too conservative and can not account for the varying storage conditions experienced by an individual item. Naturally, a detailed laboratory examination could determine when an time has reached the end of its useful shelf life, but this logistically clumsy approach is rarely undertaken, and thus HAZMAT is wastefully discarded. An ideal inspection method would be fast, reliable and non-invasive. We have investigated the development and use of an inexpensive nondestructive sensor to actively assess the shelf state of a ubiquitous HAZMAT, Silicon Alkyd Haze Gray paint. A simple sensor was designed to measure six features of the paint: ultrasonic velocity and attenuation, electric mobility and polarization and temperature and thermal diffusivity. To simulate the storage environment, an accelerated environmental degradation procedure was implemented to force the paint along realistic and prominent failure modes. During this degradation process, the material was monitored with the sensor and a set of standard laboratory measurement techniques. Pattern recognition techniques were applied to identify key characteristics of the data and to design a classifier to discriminate between different classes of the aged samples. Issues of sensitivity, uniqueness and indeterminacy in the problem were considered. Based on these results, a prototype sensor for shelf life management of hazardous silicon alkyd paint appears to be promising.
Ultrasonic elastic waves have traditionally been examined using single point measurement techniques. In this paper, we present a technique capable of recording out-of-plane ultrasonic displacement data over wide areas. The method employs 2D surface vibration data collected via electronic speckle pattern interferometry used in combination with laser modulation and optical phase stepping. Using this rapid, wide-area measurement method, images of ultrasonic Lamb waves on ideal and flawed plate structures are presented. For a single ultrasonic excitation frequency, multiple data records are collected as an externally excited traveling plate wave moves through the sample. When an optical phase shift is introduced between the data records, the data can be processed to yield quantitative displacement fields. The resulting processed images demonstrate evidence of Lamb wave reflection, transmission and scattering. Following the full field data collection, a novel inverse scattering algorithm was applied to reconstruct images of the scattering sources responsible for the measured displacement field data. The results of these investigations will be presented and discussed in the context of detecting hidden flaws in aging aircraft.
Despite its beginning as an imaging tool, the scanning acoustic microscope has become the tool of choice for obtaining quantitative information about the mechanical properties of a sample when microscopic spatial resolution is required. Essential points in the experimental use of the scanning acoustic microscope will be reviewed with emphasis placed on issues, materials and developments of concern to a medical transducer designer. A simple methodology will be discussed for measuring the elastic moduli of isotropic materials along with expressions to determine the error propagation of the technique.
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