Acoustic Microscopy is used to study the structure and properties of polymer coatings. In a multi-layer coating system, the reflection of an ultrasonic signal takes place at each interface. For thin coatings, the reflected signals from different interfaces superimpose and appear as a single reflected signal. The amplitude and the fine structure of the reflected signal depend on the material properties of the sample. To separate the reflected signal from different interfaces of the multi-layer coatings, the pulse length has to be shorter than the time of flight of the ultrasonic pulse through each layer. However, usually ultrasonic pulses are longer. The approach used here is to model the acoustic signal for different interfaces and compare the model signal with the signal recorded from the degraded coating. Due to thermal and environmental effect, the properties like acoustic impedance, density and thickness of the polymer coatings will change with time. This results in minor modifications of the shape of the reflected signal from a degraded coating. By using a calibrated scale for different kinds of coating properties, coating characteristics can be determined. This paper will discusses the application of the above method to characterize the degradation of aircraft coatings.
Detection and quantification of corrosion damage in aircraft structures is essential for condition based maintenance strategies and for the extension of the life of the aircraft. The eddy current technique was found to be one of the most favorable methods for the determination of thickness loss due to corrosion because this technique is capable of detecting corrosion in several layers of a multi-layer structure. A limitation for the eddy current technique is the eddy current penetration depth. Decreasing the analyzing frequency can increase the eddy current penetration depth. Giant Magneto Resistive sensors are highly sensitive magnetic field sensors, they have better signal to noise ratio for very low frequencies than conventional coils systems. Moreover these sensors are very efficient over a broad frequency range. Hence they allow the use of the multi-frequency concept for multi-layer structures of higher thickness. Images of corrosion damage can be generated separately for different layers of a multi-layer structure by using deep penetrating GMR based eddy current probes and data acquired from the multi-frequency eddy current testing. This paper describes the design of deep penetrating GMR based eddy current probes and their application for generating images of corrosion in different layers with the help of a MAUS scanner.
The paper will give a brief overview on techniques that have been developed or are in progress for high resolution characterization of materials at the Center for Materials Diagnostics, University of Dayton. Acoustic microscopy is used to characterize coating systems and localized defects like corrosion pits. Significantly higher resolution is provided by Ultrasonic force microscopy, which allows the imaging of elastic inhomogenities in materials for example, studying nano-grain structures in copper films and nano precipates in aluminum alloys. Several optical high-resolution techniques have been developed or are in progress. These include interferometric imaging of the response of acoustic MEMS transducers, imaging of acoustic wave structures and early detection of crack initiation. Microellipsometric and NSOM imaging techniques are in development for imaging of surface structures significantly smaller than the optical wavelength. White light interference microscopy is frequently used to characterize surface topography with nanometer resolution for example, to quantify fretting damage or stress fields in front of fractures.
The detection and microscopic characterization of hidden corrosion has recently been a focus of several advanced NDE research efforts. A variety of approaches have been suggested, with laser ultrasonic (LU), scanning acoustic microscopy (SAM), thermography,and x-ray systems being four of the most promising NDE techniques. In this effort, a side-by-side comparison of each of these four techniques was conducted with the goal of assessing the detailed microscopic features of engineered and realistic hidden pitting corrosion reference samples. The reference samples included laser-etched cutouts and electro-chemically created surface pits ranging in size for 250 μm to 5 mm in surface extent, and depths of 25 μm to 1 mm. The effects of material loss/topography, corrosion-byproduct, and paint thickness levels were all addressed. Variations in measurement sensitivity, detectivity, and spatial resolution were studied, with particular attention being focused on the ability of the NDE technique to not only detect the hidden corrosion, but to provide any additional information regarding the microscopic nature of the corrosion area, its roughness, material loss levels, and pitting sharpness. In all cases, the NDE techniques provided an 'image' of the hidden corrosion areas, with some capability for assessing the internal structures of the pits from the measured signal levels or brightness levels of the measured image fields.
Polymeric aerospace coating systems are subject to environmental degradation from ultraviolet light, water exposure and thermal cycling. This paper discusses the current progress in a novel study to develop nondestructive evaluation (NDE) methods for monitoring coating degradation during service. In the current study, weathering tests were conducted for varying lengths of time. The examined specimens were single layer epoxies on aluminum alloy (AA2024-T3) substrates. Artificial weathering of the coated samples was conducted using simulated sunlight exposure (Xenon arc lamps), combined with heat and humidity. The coatings were characterized using spectroscopic and NDE techniques after each exposure interval. The NDE included infrared microscopy and scanning acoustic microscopy (SAM). IR absorption spectra as a function of UV radiation exposure were obtained by using attenuated total reflection- infrared spectroscopy (ATR-FTIR). These spectra provide quantitative measures of coating degradation and enabled a correlation with SAM measurements. Thus, potential acoustic parameters could be identified that can be used to track coating degradation. Degradation in the coating as indicated by the IR spectra and NDE data will be correlated with physical changes observed in the coating morphology.
The goal of this work is to develop a multi-sensor nondestructive evaluation (NDE) approach to characterize aluminum alloy airframe structures under polymeric corrosion protective coatings. Two main efforts are highly relevant: (1) studying different degradation processes in the polymers to estimate the coating performance in service; and (2) detecting and quantifying early stages of corrosion beneath an intact coating. To address these tasks we employed acoustic and thermographic NDE techniques, especially Scanning Acoustic Microscopy and Fan Thermography. SAM can be utilized to map either coating or interface properties (C-scans). The method revealed potential to determine the curing quality of the coatings. It was also possible to detect small corrosion pits under delaminated areas. Furthermore, we evaluated the reflections of surface waves, which are generated and detected by the same probe. This provided an additional tool to examine the substrate/coating interface. Thermography was applied to detect corrosion under the coatings and sites of delamination. Fan Thermography (hot air heating) made it possible, to observe sites of decreasing adhesion over longer time periods. Both acoustic and thermographic results were correlated to electrochemical mapping of corrosion activity which was obtained by Scanning Vibration Electrode Technique (SVET).
This paper describes the initial phase of the development of a nondestructive, multi-sensor approach for detecting, quantifying and monitoring degradation of organic coatings applied to aircraft aluminum frame structures. Two ultrasonic techniques are discussed: the well-established pulse/echo scanning acoustic microscopy, employing a 200 MHz transducer with a focusing lens and, as a proposed alternative, continuous acoustic wave measurements with a probe in contact to the sample. The High spatial and depth resolution of scanning acoustic microscopy provides the possibility to obtain information about coating inhomogeneities, e.g. density variations due to non uniform curing of the polymeric coating or interface voids, e.g. sites of weak adhesion. This is achieved by altering the probe/sample-distance, i.e. changing the focus point of the lens. Since the echoes from the topsurface and the interface can be separated, thickness measurements are possible, too. However, only down to a thickness of 10 - 15 micrometer. Here, continuous acoustic wave measurements can be considered to be a good alternative for acoustic measurements in the pulsed regime. The method enables very accurate thickness evaluation, but can not reach the excellent lateral resolution of scanning acoustic microscopy.