Proceedings of a Workshop on Materials State Awareness (2008) / Chapter Skim
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Session III: What Should We Sense for Materials State Awareness and How Should We Look for It?
Session III: What Should We Sense for Materials State Awareness and How Should We Look for It?
Pages 19-30
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From page 19... ...
The use of NDE to provide information about the condition of engine hardware both prior to installation and during service has a high value that is appreciated, certainly at Pratt & Whitney and probably at other original equipment manufacturers. Knowledge of the state of the product has been leveraged to apply effectively such damage tolerance philosophies as retirement for cause, engine structural integrity programs, and propulsion system integrity programs -- all of which have brought economic, readiness, and safety benefits to the military customer.
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From page 20... ...
NDE technology has been a key contributor in the evolution of aircraft propulsion, from the air-cooled radial engine, which was instrumental in the outcome of World War II, to the current generation of the highperformance military and commercial turbofan engines. The next quantum step in the development of product knowledge and its application to the fleet is awareness of the material state prior to crack formation.
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From page 21... ...
Materials characterization and materials property measurements are the new NDE paradigm, shifting from the traditional defect detection. Fundamental microstructural characteristics such as grain size, porosity, and texture and materials properties related to failure mechanisms such as fatigue and residual stress are of increasing interest to the NDE community.
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From page 22... ...
. Small dwell fatigue and cycle fatigue crack growth rates have been obtained in experiments in which crack initiation and evolution were monitored by ultrasonics and crack sizing by microradiography.
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From page 23... ...
There are state awareness issues that are unique to structural joints common in aerospace structures, such as lap joints, blade/disk attachments in engines, and bonded composite joints. The role that the evolution of friction plays in materials state awareness is also shown through the connection between contact stresses in joints and friction.
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From page 24... ...
It should be noted that corrosion is almost always a localized event due to the atomistic nature of the reactions. Prevention Methods to prevent each of the forms of corrosion vary widely, but in all cases corrosion is inhibited by isolating or removing one or more of the four components of an electrochemical cell, essentially creating an open circuit that prevents electron flow.
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From page 25... ...
. This need to replace traditional coatings impacts not only the corrosion community but also other technical areas, such as nondestructive inspection, as new materials and processes that must comply with all requirements may not behave or respond in a manner similar to that of the corrosion inhibitors that have been used in the past and provide the baseline for performance and inspection requirements.
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From page 26... ...
However, they can very effectively interact with critical surface flaws, including surfacebreaking fatigue cracks, impact damage, corrosion precursor pitting, and so forth. The relatively small wavelengths and wide bandwidths associated with these signals enable the production of high-spatial-resolution images of materials and structures.
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From page 27... ...
Surface Crack and Corrosion Precursor Pitting Detection Microwave and millimeter-wave signals do not penetrate inside highly conducting materials such as metals. However, these signals induce a surface current density in metals.
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From page 28... ...
Tiny corrosion precursor pitting can also be effectively detected while exposed or under paint. Information about the presence of such pitting can offer maintenance personnel critical information about the onset of corrosion, and because the dimensions of such pitting may also be effectively provided with these techniques, proper decisions with respect to resource-consuming repair activities can also be made.
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From page 29... ...
All of this must be done in the context of the engineering design and with careful attention to scale. Selecting an appropriate strategy, measurement method, and procedure for implementation for awareness of material state involves careful consideration of design for inspectability and design for detectability, as well as materials condition assessment.
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From page 30... ...
For example, corrosion resulting in a pit might be more problematic than uniform metal loss due to stress concentration caused by the localized loss of metal in the pit. Efforts of the author and others to develop methods to detect and track material degradation preceding detectable crack formation, as well as variations in material condition associated with precipitation hardening, include early work to monitor the influence of mobile dislocation populations on ultrasonic attenuation; efforts to use continuous monitoring of changes in ultrasonic attenuation to detect degradation preceding crack formation during cyclic loading of 7075 Al; combined continuous monitoring of changes in ultrasonic attenuation and acoustic emission for the early detection of life-limiting fatigue damage; continuous monitoring of ultrasonic plate waves to track damage development in fiber-reinforced laminated composite materials; and practical limitations with using nonlinear ultrasonic response to monitor precursor fatigue damage in metal alloys.
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