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.

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.

Acoustic (ultrasonic) inspection is among the most widely used NDE techniques. Ultrasonic techniques have proven to foster early detection of defects and enhance the understanding of failure processes. Acoustic imaging techniques and for instance Scanning Acoustic Microscopy (SAM) provides very detailed information about material defects with high spatial and depth resolution. It enables studying elastic properties, characterization of interfaces and detecting small defects, for instance, cracks in metals, polymers and ceramics. This chapter will consider some special applications of acoustic imaging techniques concerning recent aging aircraft problems. Characterization of corrosion, corrosion protective coatings, as well as fatigue damage are in the scope of the research reported here.

Maintenance and reliability of aircraft is a major safety concern and economical factor. Many civilian and military aircraft have been in service for 35 years or more. Aircrafts that were originally designed for a service life of 20 years are currently considered for life extensions of up to 80 years. The cost of corrosion and fatigue related maintenance on these aging structures has increased dramatically. A study conducted in 1998 showed that the direct costs of corrosion maintenance to the United States Air Force were $775 million/year [1] . These costs continue to rise in spite of Air Force structure changes resulting in a 20% reduction in the overall fleet. In an effort to reduce these maintenance costs, several programs have been initiated for the development of methods to manage and control corrosion and fatigue damage in aging aircraft.

Thermal techniques are attractive for materials characterization. They provide sophisticated contrast mechanisms and fast, non-contact investigation of large inspection areas. Recent progress in the development of infrared cameras is the key for a wide variety of new applications. In particular, active thermal techniques provide useful information about thermal properties and related quantities as well as geometrical and structural information. “Active” thermography means that, for the purpose of testing, heat is deposited at the surface of the test object or generated within the test object. The surface temperature is monitored as a function of time during or after stimulation. Thermal quantities are determined from the infrared frame sequence. With “Passive” thermography, the temperature of an object is imaged without additional thermal stimulation.

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 250mum to 5mm in surface extent, and depths of 25mum to I nun. 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.

Corrosion is one of the limiting factors for extended use of aircraft. Early detection of corrosion using nondestructive techniques is an important task for progression in the understanding of corrosion processes, as well as maintenance of aircraft. The paper summarizes newer results on nondestructive characterization of corrosion using thermal techniques and presents concepts for prediction of remaining lifetime of corroded parts. Studying aluminum airframe structures under corrosion protective coatings, we employ Scanning Acoustic Microscopy (SAM) and developed Fan Thermography (hot air heating), which made it possible to detect sites of poor adhesion and localized corrosion. For quantification of corrosion defects, such as corrosion pits or cracks, and for detection of corrosion activity, white light interferometry and scanning vibrating electrode technique were used. hi this paper we concentrate on some results obtained by Fan Thermography and White Light Interferometry.

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.

Polymer coatings provide an excellent corrosion barrier for Al-skinned military aircraft. However, the degradation and damage of the coatings in their service life over time leads to the initiation of corrosion damage at the substrate level. Early detection and negation of such activity can provide extensive cost savings. Several Electrochemical techniques and Non Destructive Evaluation (NDE) show promise in detecting the onset of corrosion under such coatings. Current accelerated testing of aircraft coating systems for corrosion protection relies heavily on salt spray methods. Electrochemical techniques such as Electrochemical Impedance Spectroscopy (EIS) and Electrochemical Noise Methods (ENM) provide insight into the global properties of a coating system, and both techniques are being used on a limited basis. However, there is a need to investigate corrosion events with greater spatial resolution under coatings at the metal/coating interface. Such corrosion activity may be related to coating defects and variations in the surface chemistry of the underlying metal. The Scanning Vibrating Electrode Technique (SVET) has been developed to allow the investigation of localized corrosion activity with high spatial resolution. Such activity may be associated with coating defects or galvanic coupled regions of the metal surface. Electrochemical and NDE techniques were used to investigate the early stage of corrosion activity under protective coatings. Coatings in this investigation ranged from a simple epoxy amine to commercially used military aircraft polyurethane coatings. SVET testing of panels with intact high-resistance barrier coatings could not reveal corrosion damage under normal testing conditions because of little or no corrosion activity within the limited exposure time. Chemical, mechanical, and electrochemical means of accelerating the corrosion damage were utilized to obtain results in a reasonable time frame. Corrosion initiation and its progress under the coating were studied in detail and the results are discussed here. Complimentary high-resolution NDE techniques, such as Scanning Acoustic Microscopy (SAM) and Fan Thermography measurements were used to identify the corrosion sites. The overall objective of this investigation is to establish a correlation between the electrochemical and NDE techniques.

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: (i) studying different degradation processes in the polymers to estimate the coating performance in service; and (ii) 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 (SAM) 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).

In recent years increasing interest in magnetic materials especially amorphous and nano crystalline layers has arised. Due to the magnetostrictive and magnetoresistive properties magnetic materials obtained high importance in sensor, actuator and memory applications. Therefore new methods to characterize magnetic and mechanical properties of magnetic materials are required. Two new microscopic techniques will be presented in this paper.

This paper describes the initial phase of the development of a nondestructive, multisensor approach for detecting, quantifying and monitoring degradation of organic coatings applied to aluminum surfaces. Descriptions of the purposes and chemical compositions of layered coatings used on aircraft structures are provided. The discussion then concentrates on ultrasonic thickness measurements. One is the well-established pulse/echo scanning acoustic microscopy and, as a proposed alternative, continuous acoustic waves measurements with a probe in contact to the sample. Advantages and disadvantages of the two methods and their potential as in field applications are discussed.

Corrosion protective coatings are of significant importance to reduce the degradation of aircraft structures. When a conventional aircraft coating (Polyurethane topcoat and epoxy primer) is exposed to the environment, the coating starts to degrade and this degradation changes the thickness and the elastic modulus. This can be monitored by acoustic microscopy. The ultrasonic surface echo, which characterizes the coating degradation, contains information about the coating and the coating-substrate interface. In order to study the coating degradation process the coating and coating-substrate information has to be separated. Two approaches have been considered to separate this information. One is using a network analyzer. The ultrasonic response for a frequency sweep is determined and Fourier transformation allows the separation of the reflectivity and thickness information. The other approach compares the high frequency ultrasonic signal of the degraded coating with that of the pristine coating. Cross correlation techniques have been used to compare the signals. Progressing coating degradation results in a minor modification of the shape of the reflected ultrasonic signal, which was detected by a change in the cross correlation function.