Norbert Meyendorf

Positron annihilation spectroscopy to study nanoprecipitations in aluminum alloys

Positron Annihilation Spectroscopy has been used to study the microstructure in aluminum alloy AA7075-T6 after RRA (retrogression and re-aging) heat treatment. Nano-precipitates act as traps for thermalised positrons. The measured positron lifetime is sensitive to the local electron density at the annihilation site while the Doppler broadening of the annihilation line is sensitive to the chemical environment of the annihilation site. The combination of both methods was used to study changes of nano-precipitates, during the Retrogression and Re-aging (RRA) process. Results are compared to previous published isochronal annealing experiments of an Al Cu model alloy.

GMR-based eddy current probes for RRA treated Al-7075 T6 and other high-temperature applications

Sensors for online monitoring of the heat treatment of aluminum alloys have been developed. The correlation between heat treatment parameters of Al 7075, the microstructure of the materials and NDE measurements has being studied systematically. By establishing correlations to the process parameters, it was assumed to be possible to devise a technique that provides not only insight into the aging process but also act as a quality control method for process verification. Multi-frequency eddy current allows compensation for environmental influences along with the ability to perform measurements at elevated temperatures and is therefore suitable for process monitoring.

International center for nano-materials reliability

Over the last two centuries, engineers have learned how to determine material parameters that describe the properties of construction materials and the behavior of the materials under loading conditions. These parameters are measured from small specimens that are exposed to specific laboratory test conditions, sometimes far away from the real loading conditions for the materials in use. Engineering experience is required to use these material parameters for design and to guarantee material lifetimes of several decades for complex structures and systems, like for example, aircraft. Nano-structures having completely different chemical and physical property behavior than large scale devices possess, due to the extremely large surface to volume ratio along with quantum mechanical effects. Due to cost constraints and the speed of invention and business, there will not be unlimited time as in the past to create the necessary materials testing and characterization procedures and to develop the criteria for design and construction that guarantees reliability and a long lifetime for these new materials. A concept for an international scientists network concerning characterization modeling and reliability of micro and nano materials will be presented. Objectives for the network are to assure safety and reliability of future products, to develop and harmonize noninvasive testing procedures, to develop training programs for nano characterization and modeling specialists, and for instance, to accelerate the process of application of nano materials.

High-resolution nondestructive evaluation at the Center for Materials Diagnosis

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.

Determination of Local Stress Intensity Factor at Crack Tip Using Image Correlation Techniques

The extraordinary mechanical properties of high strength aluminum alloys such as AA7075-T6 are caused by coherent nanoprecipitations. These nanoprecipitations generate local stress fields and interact with moving dislocations and propagating microcracks. In this paper, image correlation techniques are used to determine the local strain and stress field in the vicinity of fatigue crack tips during the loading of compact tension (CT) specimen. The fatigue crack tip was sharpened with decreasing fatigue loading after fatigue cracks initial appearance. Images of the crack tip were taken using atomic force microscopy/ultrasonic force microscopy (AFM/UFM) and white light interference microscopy (WLIM) before and after mechanical loading of the specimen. Both techniques are applicable for measuring the out-of-plane displacement during the loading process. In addition, image correlation techniques can be used to determine the in-plane displacement resulting from mechanical loading. This information is used to calculate the local stress intensity factor in the vicinity of the crack tips.

Development of GMR eddy current sensors for high-temperature applications and imaging of corrosion in thick multilayer structures

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.

Nondestructive ultrasonic characterization of multilayer coatings

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.

Zinc coating layer thickness on steel wires

Barkhausen noise measurements were used to determine the thickness of several micrometer thick zinc coatings on steel wires. Barkhausen noise is a broadband frequency signal ranging from several KHz to MHz. By comparing the attenuation of the noise through the zinc layer for different frequencies, it is possible to determine the thickness of the Zn coatings. An advantage of the method is that the attenuation of the signal can be calculated if the conductivity of the coating layer is known. Therefore, no calibration procedures are required.

Electric potential noise-A new way to characterize residual stress

This paper presents a new method to measure stresses in ferromagnetic wires. The methodology is based on observations of high frequency electric potential noise generated when an alternating electric current was used to periodically magnetize the material. The magnetically induced electric potential noise is related to the discontinuous processes that occur during magnetization, and can also be detected as magnetic Barkhausen noise.

Early detection of corrosion in aircraft structures

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. In this paper we concentrate on some results obtained by Fan Thermography and White Light Interferometry.