Session III:
What Should We Sense for Materials State Awareness and How Should We Look for It?

MATERIALS STATE AWARENESS: A PROPULSION PERSPECTIVE

Kevin Smith, Pratt & Whitney


Materials state awareness is an emerging issue in the propulsion nondestructive evaluation (NDE) community. While the NDE community has had this issue on its long-range plans for some time, only recently has an adequate understanding been developed in the academic community to begin realistically considering the implementation of practical methods to effectively consider material state as part of the overall engine maintenance philosophy. NDE has highly developed technology to address cracking, especially surface-connected low-cycle fatigue cracks. Very sensitive crack detection by means of eddy current has been implemented through automated systems at the original equipment manufacturers and at the military depot for military engines. On-wing and semiautomated crack detection has also been successfully implemented in the commercial fleet as well. Inspection methods, especially in the face of unanticipated durability issues, have provided excellent economic and readiness benefits to the military and commercial aircraft operators. 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. A greater understanding of the state of the components



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Session III: What Should We Sense for Materials State Awareness and How Should We Look for It? MATERIALS STATE AWARENESS: A PROPULSION PERSPECTIVE Kevin Smith, Pratt & Whitney Materials state awareness is an emerging issue in the propulsion nondestructive evaluation (NDE) community. While the NDE community has had this issue on its long-range plans for some time, only recently has an adequate understanding been developed in the academic community to begin realistically considering the implementation of practical methods to effectively consider material state as part of the overall engine maintenance philosophy. NDE has highly developed technology to address cracking, especially surface-connected low-cycle fatigue cracks. Very sensitive crack detection by means of eddy current has been implemented through automated systems at the original equipment manufacturers and at the military depot for military engines. On-wing and semiautomated crack detection has also been successfully implemented in the commercial fleet as well. Inspection methods, especially in the face of unanticipated durability issues, have provided excellent economic and readiness benefits to the military and commercial aircraft operators. 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. A greater understanding of the state of the components 19

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20 Proceedings of a Workshop on Materials State Awareness has allowed the safe and effective use of the material and the design to a much greater extent as the NDE capabilities have developed. The ability to monitor the state of components coming out of production as well as monitoring their condition in the field has moved in lockstep with the continuing push toward improved design life and engine efficiency. 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 high- performance 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. A number of challenges exist in this area that will require the development of enabling technologies to practically implement effective strategies. To the extent that the state of fatigue damage prior to cracking can be understood, it will be possible to more effectively address high cycle fatigue issues in the field. When the grain-size distribution inside individual disks can be understood and these data used to actively control the process such that resultant properties fall within a tighter band than they do today, the design system and subsequent fleet issues will lead to production of lighter, more fuel- efficient designs. Residual stress, its changes over the life of the component, and its influence on NDE are all very significant in the design and fleet management of the engine system. Prognostics is an emerging technology that will benefit from understanding of the materials state. Currently, usage monitoring is being applied to the F135 engine. This technology allows the actual usage and damage accumulation of each serial number engine to be tracked separately, thereby also allowing the maintenance plan to be optimized for each engine. This approach varies radically from the current approach of using a nominal flight mission mix to manage an entire fleet that is not flying the same way. By removing the conservatism of the current approach, the economic, safety, and readiness benefits are obvious. End users will actually be in a position to manage an engine in an optimal manner based on its usage rather than on nominal assumptions. To practically implement the capabilities mentioned above, the ability to readily access the interior of the engine and to deliver a sensor, as well as the ability to quantify the result of the sensor response, are of paramount importance. The ability to deliver a sensor deep inside the engine reliably—that is, to place the sensor accurately, but also to be able to retrieve the sensor—is a key technology that needs to be developed. To make meaningful use of any nondestructive technique in a design or fleet management scenario, the capability needs to be quantified. Model-assisted probability of detection (MAPOD) has been used successfully by Pratt & Whitney for some years as an effective method of quantifying the effectiveness of nondestructive evaluation techniques. A second generation of these MAPOD techniques that is able to more effectively leverage the modularity of the approach and the data collected from various sources is a key to success. As the need to respond more quickly and efficiently to the demands of the end user continues, the ability to quickly and accurately quantify the capability of conventional and next-generation NDE techniques will become more important. Ultimately, the implementation of technologies that allow for the understanding of the state of the material in a gas turbine engine at new manufacture and during service will provide additional opportunities to extend the economic life of the engine safely and provide additional flexibility to the customer.

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Session III: What Should We Sense and How Should We Look for It? 21 INTEGRATED STRUCTURAL HEALTH AND LIFE MANAGEMENT OF AIRFRAME STRUCTURES DEPENDENT ON CHARACTERIZING THE STATE OF THE MATERIAL AS A FUNCTION OF TIME IN SERVICE J.P. Gallagher, Independent Consultant Damage characterizes and defines the material state that directly relates to structural health and to remaining life. The aircraft structural integrity program 1 provides the framework and processes associated with the initial and continuing airworthiness certification of USAF airframe structures. Defined in this presentation are the framework and processes used to characterize the state of damage in aircraft structures as a function of time in service. Concepts for multiple methods for characterizing the damage state are presented, including the onboard usage-monitoring method (provided by virtual sensors), the off-board damage-monitoring method (based on maintenance/inspection data collection and storage of damage information), and the onboard damage-monitoring method (provided by damage event sensors).2 By focusing on the fatigue mechanism and aging issues, several types of damage distributions are identified. The presentation identifies several serious challenges associated with characterizing the current state of damage and its accurate projection into a future state. It identifies one particularly difficult challenge associated with the accurate measurement of damage using existing nondestructive inspection (NDI) capabilities. This well-known existing NDI reliability challenge provides important guidance for those investing in onboard damage-monitoring systems for airframe structures. The presentation provides some guidance on the way ahead. MATERIALS PROPERTY MEASUREMENT USING NONDESTRUCTIVE EVALUATION METHODS AT GE Shridhar Nath, Tom Batzinger, Waseem Faidi, Jian Li, Ed Nieters, Harry Ringermacher, and Nilesh Tralshawala, GE Global Research; and Thadd Patton, GE Aviation General Electric (GE) has a rich history and track record of solving problems with NDE. GE researchers develop new NDE technologies, manufacturer state-of-the-art NDE equipment and systems, and use NDE methodologies in a wide range of businesses. 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. 1 Department of Defense. 2005. Standard Practice, Aircraft Structural Integrity Program. MIL-STD-1530C. 2 J.P. Gallagher. 2007. A Review of Philosophies, Processes, Methods and Approaches that Protect In-Service Aircraft from the Scourge of Fatigue Failures. Proceedings of the 24th ICAF Symposium. Naples, Italy. May; L.M. Butkus et al. 2007. U.S. Air Force Efforts in Understanding and Mitigating the Effects of “NDI Misses.” Proceedings of the 24th ICAF Symposium. Naples, Italy. May; J.P. Gallagher et al. 2007. Demonstrating the Effectiveness of an Inspection System to Detect Cracks in Safety of Flight Structure. Proceedings of the 10th DoD/FAA/NASA Aging Aircraft Conference, Palm Springs, April; J.P. Gallagher. 2007. Damage Tolerant Aircraft Design and Its Relationship to Inspections. Presentation at the G.R. Irwin Memorial Conference. College Park, Maryland. March.

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22 Proceedings of a Workshop on Materials State Awareness The presentation on which this abstract is based discusses two examples being pursued at GE Global Research: (1) nonlinear ultrasound for characterizing low cycle fatigue and (2) thermoelectric magnetic field measurements for mapping residual stress in titanium. A quick overview of the NDE methods used to study aerospace composite materials is also discussed. Finally, a technology roadmap is presented that lays out a strategy that GE is pursuing in improving the prediction of the remaining life accuracy of engine components. EXAMPLES OF MATERIALS STATE AWARENESS PROBLEMS AND RESEARCH DIRECTIONS TO SOLVE THEM S.I. Rokhlin, Ohio State University Several research programs seem to represent applications of the materials state awareness concepts well: for example, how one can predict the effect of the evolution of the state of a material (including damage) in service and the resulting mechanical state. First, the problem of cold dwell fatigue (CDF) in Ti alloys, used in engine components, is reviewed. CDF results in a significant reduction (debit) of fatigue lifetime compared with continuous cycling fatigue. This debit decreases with increasing temperature. In spite of the importance of the CDF phenomenon for aircraft engine safety, its mechanism and life predictive capabilities have not been sufficiently addressed. There is an urgent need to develop an understanding of dwell-time fatigue–microstructure relations and dwell-time crack initiation: Why does CDF happen? What are the microstructural features that control it? How can the resulting mechanical state be predicted? How can dwell-fatigue-sensitive microstructures be nondestructively sensed? How can dwell fatigue life be prognosticated? This set of problems is addressed by an Ohio State University research program sponsored by the Federal Aviation Administration. The problems are being attacked by an interdisciplinary team: J. Williams and M. Mills (microstructures), S. Ghosh (microstructure-based mechanical modeling), and S.I. Rokhlin (microstructure and damage sensing and NDE). 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. It was found that the rates for small dwell fatigue cracks are one to two orders of magnitude higher than those for small cyclic fatigue cracks. This growth behavior of small cracks differs greatly from that of long cracks, which exhibit identical rates for dwell and cycling fatigue. Thus, the process of crack initiation and small crack growth controls the reduction of dwell fatigue life. A micromechanical predictive model has been developed that incorporates experimental input data of single colonies and single crystals: a matrix of elastic properties (anisotropy) obtained by time-resolved line-focused acoustic microscopy, and microscale plastic properties measured by microscale and nanoscale compressive tests. The key modeling results include an understanding of microstructural effects on crack initiation and small crack growth. The soft phase has less resistance to plastic flow than the hard phase does, and as a result the load redistribution is taken up by the hard phase, and high stress concentration at the interface of the hard and soft phases leads to crack initiation. The issue of how possibly one may sense nondestructively dwell-fatigue-sensitive microstructures is also briefly addressed. In addition, work at the University of Cincinnati and the AFRL (Nagy, Blodgett) on residual stress sensing to improve engine reliability is very briefly reviewed, as is Boeing (Bossi) work on a laser method of strength assessment of bonded joints, and work on characterization of

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Session III: What Should We Sense and How Should We Look for It? 23 bonded joints by angle beam ultrasonic spectroscopy (Ohio State University, S.I. Rokhlin, Adler Consultants, Adler). ISSUES AND IDEAS IN MATERIALS STATE AWARENESS FOR AEROSPACE STRUCTURAL JOINTS Thomas Farris, Purdue University Much progress has been made in characterizing materials state awareness in homogeneous materials and structures. 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. For instance, load transfer will depend on the current coefficient of friction that may evolve over time due to wear of the contacting surfaces. Environmental effects such as temperature will also influence the evolution of friction during the life of the joint. There may also be changes, induced by thermal exposure, to residual stresses generated by manufacture that are important to materials state awareness in joints. The wear itself may lead to a loss of stiffness of the joint that changes the nature of the load transfer. There are measurement techniques, such as infrared thermography, capable of capturing the detailed changes in the behavior in the joints in the laboratory. Some of these techniques are discussed and some successful applications demonstrated. Ideas for future research in state awareness for joints are also discussed. MATERIALS CORROSION FUNDAMENTALS, PREVENTION, AND DETECTION Matthew J. O’Keefe, Missouri Institute of Science and Technology 3 “Corrosion” is normally defined as the unwanted deterioration of a material, especially metals, by chemical or electrochemical reaction with the environment. In most cases this involves the oxidation of a metal or alloy through an electrochemical process. Not all oxidation processes are detrimental; for instance, the oxidation of aluminum in air provides a very thin (~10 nm) layer of aluminum oxide on the surface that protects, or passivates, the underlying aluminum metal from further oxidation under ambient conditions. However, in the presence of halogen compounds, such as chlorine and fluorine, the aluminum oxide is removed and pitting corrosion can occur. Although atomistic in mechanism, the impact of corrosion on the U.S. economy is enormous, estimated at $300 billion annually.4 Corrosion and metal wastage arising from oxidation as caused by exposure to the elements and reactivity between dissimilar materials cost the U.S. military about $20 billion each year.5 Both the U.S. Air Force and U.S. Navy spend 3 The Missouri Institute of Science and Technology was formerly known as the University of Missouri-Rolla. 4 Corrosion Costs and Preventative Strategies in the United States, DoD Report Number FHWA-RD-01-156, CC Technologies, Inc. Houston, Tex. 5 Corrosion Costs and Preventative Strategies in the United States, DoD Report Number FHWA-RD-01-156, CC Technologies, Inc. Houston, Tex.

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24 Proceedings of a Workshop on Materials State Awareness almost $1 billion annually 6 on corrosion-related efforts for aircraft, with the majority of the costs related to inspection for corrosion that involves removing and reapplying corrosion-inhibiting coatings. Improvements in corrosion prevention, detection, and remediation have the potential to make a significant impact on military force readiness and life-cycle costs. Fundamentals At the most fundamental level, corrosion occurs due to the formation of an electrochemical cell. Every electrochemical cell consists of four parts: an anode in which oxidation, or electron loss, occurs; a cathode in which reduction, or electron gain, occurs; a first- class conductor, such as metals, to transport electrons from the anode to the cathode; and a second-class conductor, an electrolyte, which allows the movement of ions in the cell. The most common electrolyte is water. These same four components are present in every electrochemical cell, including batteries, electroplating operations, and corrosion, but in some cases the reaction is spontaneous (i.e., batteries, corrosion), while in other cases it is nonspontaneous (i.e., electroplating that requires an external power supply). Differences in the electromotive force (emf) potential of materials provide the thermodynamic factor to cause a reaction, but the kinetics, or speed, of the reaction cannot normally be predicted a priori. In the case of corrosion, the electron loss at the anode is often associated with dissolution of the material (metal) into the electrolyte, resulting in a loss of mass, or in oxidation of the metal. While all corrosion cells have the same components and nominally result in the same deterioration of the material, there are different types of corrosion as determined by the mechanism or appearance of the degradation. Although there are a number of ways to categorize corrosion, eight forms of corrosion are often described: galvanic, uniform, erosion, crevice, pitting, intergranular, selective leaching, and stress corrosion cracking. Each has similarities and differences, with each corrosion product unique to that system or form, but all result in the degradation of material through chemical and electrochemical attack. 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. Anodic inhibitors, cathodic inhibitors, barriers, insulators, and so on are all viable methods to prevent or minimize corrosion. Identifying what controls the corrosion is a key component in determining how to prevent the reaction. While proper materials selection and compatibility can often minimize corrosion, it is often not the main priority for designers more concerned with other properties, such as mechanical strength, that are needed for the intended application. Therefore, in many instances corrosion is addressed as a postassembly issue, and methods to mitigate corrosion have to be compatible with the overall product. Since almost all corrosion events start at the surface of the 6 Corrosion Costs and Preventative Strategies in the United States, DoD Report Number FHWA-RD-01-156, CC Technologies, Inc. Houston, Tex.

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Session III: What Should We Sense and How Should We Look for It? 25 material and work inward, coatings and surface treatments are frequently used to prevent the onset of unwanted electrochemical reactions. The most common corrosion coatings employ inhibitors that preferentially and electrochemically protect or passivate the surface of the underlying substrate. Examples include Zn and Cd on steel, hexavalent chromate (Cr[VI]) compounds on aluminum alloys, and polymeric sealants on any number of materials. Each inhibitor prevents corrosion in a certain way, such as barriers that prevent the ingress of moisture and thus eliminate the electrolyte, but the overall effect is to stop or slow the deterioration process. However, many of the coatings that have been very effective for many decades must now be replaced owing to other constraints, such as environmental and health issues related to Cd and Cr(VI). 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. Detection As is the case in many health-related matters, early detection of corrosion is a key aspect to maintaining the integrity and performance of the material subject to corrosion. Historically, visual examination and inspection have been used as the main detection method for corrosion. While very effective in assessing visible corrosion, in many ways these constitute an “after event” method that relies on a deteriorated condition to signal the occurrence of an undesirable or unacceptable event. In addition, it is left to the experience and judgment of the inspector to assess the extent and severity of the damage and how that impacts the mission. There are a number of technical challenges that could improve corrosion detection methods and assessment, including the following: • Characterizing and categorizing the response and sensitivity of the known types of corrosion for various materials systems using a variety of approaches and techniques. • Developing methods to profile systems over the entire life cycle, including before use, so as to predict the timing and extent of corrosion events that occur during deployment. • Resolving discontinuities at smaller dimensional feature sizes, even down to nanometer lengths if possible, to determine not only the presence and extent of the damage but also providing information that can be used to determine the source of the degradation. • Integrating sensors and inspection data to develop circuit models that simulate electrochemical cells and reactions that lead to kinetic rate models which predict the time-dependent degradation process and enable managing of the corrosion process through preventive maintenance. • Developing procedures and methods to assess accurately the status of materials in “blind” areas that are often inaccessible after final assembly and often have an unknown status during service and operation.

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26 Proceedings of a Workshop on Materials State Awareness • Investigating materials and processes to provide other means of detection, such as optical or electrical property changes as a result of corrosion, to supplement the nondestructive inspection methods. Collectively these challenges are quite formidable, but by addressing each in a systematic and coordinated manner, significant progress can be made in the detection, prevention, and management of materials corrosion. MICROWAVE AND MILLIMETER-WAVE NONDESTRUCTIVE TESTING AND EVALUATION TECHNIQUES AND APPLICATIONS: A COMPREHENSIVE OVERVIEW Reza Zoughi, Missouri Institute of Science and Technology Microwave and millimeter-wave signals occupy the specific frequency bands of ~300 MHz to 30 GHz and 30 GHz to 300 GHz, corresponding to the wavelengths of 1,000 mm to 10 mm and 10 mm to 1 mm, respectively. These waves possess certain advantageous attributes, which make them suitable for nondestructive testing and evaluation (NDT&E) of a wide array of materials and structures. Microwave and millimeter-wave signals can easily penetrate inside dielectric materials and composites and interact with their inner structures. This interaction may be at the molecular level making them suitable for materials characterization, or it may take the form of reflections from undesired boundaries produced as a result of inferior manufacturing or in-service stresses such as disbonds and delaminations. These signals do not penetrate inside electrically conducting materials, such as metals and multidirectional graphite composites. However, they can very effectively interact with critical surface flaws, including surface- breaking 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. These signals can be launched and received using a wide variety of probes (particularly when inspecting a material in their near fields), each with own unique characteristics that can significantly influence measurement accuracy and robustness. Signals at these frequencies can be launched and received using different “wave polarizations” (i.e., relative orientation of electric field vector). The proper choice of signal polarization can result in better detection of targets whose preferred orientation coincides with the wave polarization and more effective detection of small flaws near structural features that have a preferred orientation. Inspection systems at these frequencies are usually small, handled, portable, battery-operated, robust, rapid, online, real-time, and require no operator knowledge in the field of microwave and millimeter-wave engineering. The following is a list and brief description of applications for which microwave and millimeter-wave NDT&E techniques have provided capable and robust, and in some cases unique, NDT&E solutions.

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Session III: What Should We Sense and How Should We Look for It? 27 Materials Characterization Once exposed to an electric field, dielectric materials become polarized (“material polarization”). The degree to which this material polarization takes place is a function of the frequency of the electric field and molecular makeup of the material (e.g., its physical and chemical states). Material polarization is macroscopically manifested through a parameter called (relative to free space) complex dielectric properties (or constant) denoted by (εr = ε′r – jε″r). The real part, known as the relative permittivity, indicates the ability of the material to store microwave energy, and its imaginary part, known as the relative loss factor, indicates the ability of the material to absorb microwave energy. Therefore, the study of the dielectric properties of a material as a function of frequency can yield valuable information about the state and properties of the material. The dielectric properties of materials undergoing chemical changes such as curing (e.g., resins, special coatings, and so forth) change as a function of cure state, which can be detected and monitored to evaluate the state of cure. The dielectric properties of materials composed of several different constituents are a function of the dielectric properties and volume fraction of each constituent. Therefore, material changes, such as an addition of porosity due to microcracking during in-service conditions in thermal barrier coatings, can be detected and comprehensively evaluated. Stratified Composite Evaluation Microwave and millimeter-wave signals are sensitive to the presence of boundaries within a structure as they propagate through it. This is due to the fact that these signals partially reflect and transmit through boundaries of materials with dissimilar dielectric properties. Consequently, the presence of voids, disbonds, delaminations, and so forth can be easily and effectively detected. The thickness and location of such flaws within the structure can also be evaluated, resulting in critical information about their relative severity and impact on the in- service operation of a composite structure. In addition to the above applications, there are numerous structures and applications that may fall under this category of inspection: namely, detection and evaluation of corrosion under paint and/or dielectric composite laminates, accurate thickness evaluation of special coatings and paints, and thick composite inspection. Since the overwhelming majority of these inspections are conducted in the near field of a microwave probe, measurement accuracies in the range of a few micrometers are easily achievable at frequencies around 10 GHz (corresponding to a wavelength of 30,000 micrometers). Moreover, when operating in the near field, the frequency of operation, standoff distance, and the type of probe used provide for multiple degrees of freedom in choosing the most effective set of measurement parameters. 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. Therefore, when operating in the near field, the reflection properties of the metal surface

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28 Proceedings of a Workshop on Materials State Awareness markedly change as a result of the presence of a surface-breaking fatigue crack. Evaluating the changes in the reflection properties of the metal surface not only renders a tight crack detected but also provides information about its dimensions (width and depth) and crack ends (critical for repair purposes). This technique is metal-independent and applies to crack detection under coatings (i.e., no need to remove paint) and filled cracks. Moreover, different probes can offer experimental features that may be uniquely useful for particular applications. 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. High-Resolution Imaging Whether a structure is in the near field of a microwave and millimeter-wave probe or in its far field, a number of imaging techniques are available for producing high-spatial-resolution images of various structures. When operating in the near field of a probe, spatial resolution is no longer a function of wavelength and is a function of the probe dimensions, geometry and the electric field distribution. Since there are various available probes that may be strategically used, one may produce direct raster scan (C-scan) images of objects under inspection. Antennas focus these waves and produce small inspection footprints on an object. These antennas may be in the form of small horns or lenses which, when combined with high frequencies (i.e., 150 GHz), can provide footprints on the order of a couple of millimeters. Imaging methods such as synthetic aperture, holographical, and a multitude of back-propagation techniques can be used to produce high-resolution images in all three dimensions. Some of the current research activities in this area involve the development of portable, on-shot, and real-time imaging systems (e.g., microwave and millimeter-wave “cameras”). Microwave and millimeter-wave NDT&E techniques are not known as “standard” methods by the community, in particular in their early stages of development more than two decades ago. However, much has been gained since then, which has culminated in bringing these viable techniques to the forefront and serious consideration by NDT&E practitioners, users, and engineers. Much of what has been accomplished has been corroborated and improved using complex analytical and numerical electromagnetic modeling. This has been an important issue reaffirming the intricate science on which these techniques are founded. The advent and increased utility of dielectric-based composite structures have necessitated new and innovative inspection methodologies, since many of the “standard” methods are not capable of inspecting these structures. Most of the microwave and millimeter-wave hardware and systems developed for NDT&E purposes were custom-designed for a specific purpose, and there were not many off- the-shelf systems available. The telecommunications bonanza has significantly helped microwave and millimeter-wave NDT&E in that many of the required components are readily available at very high frequencies, are made to be compact, and are relatively inexpensive. Among other factors, this will aid in the availability of more inspection systems in the future. The fusion of data from other inspection modalities with those from microwave and millimeter- wave sensors is also expected to help bring these methods to the forefront. High-resolution imaging techniques at these frequencies are currently receiving significant attention and are

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Session III: What Should We Sense and How Should We Look for It? 29 expected to flourish even more in the near future. More companies are looking to expand their product lines into new areas, and these techniques certainly are at the top of the list. In conclusion, microwave and millimeter-wave NDT&E methods are finding more applications and are being increasingly considered for many critical inspection applications. WE FIND WHAT WE SEEK John C. Duke, Jr., Virginia Polytechnic Institute and State University Selecting an appropriate method for materials condition assessment involves three fundamental considerations: identifying imperfections of interest, identifying the requirements of the assessment procedure, and recognizing constraints imposed by the application. 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. If one considers, for purposes of discussion, a solid composed of a collection of similar aluminum atoms, it is observed that these atoms organize themselves in a distinctive, face- centered cubic array. Depending on the scale of the engineering design, the associated collection might be organized with a similar characteristic array, but atoms might be missing (voids) or regions might be aligned differently (grains), other atoms might be present (impurities or alloy additions), alignment deviations (dislocations) might exist, regions might have slightly different spacing due to residual or thermal stresses, and near the surface the atoms might be bonded to oxide atoms with a form of bonding that is different from the bonding in the bulk material. Also, atoms at the surface are not completely surrounded by other atoms as in the bulk. If this collection of atoms was formed by casting, or forging, or rolling, or sintering, or some combination of these, the organization might be different. If one were to assess the condition of such a collection of atoms, the assessment would depend in part on the environment within which the collection of atoms exists: the temperature, the pressure, the chemical potentials at the surface, the electric and magnetic fields, and so forth. If the condition of the collection of atoms is determined at a particular instant, that condition might in fact be changing, so that if it was reassessed at another time it would be different. If the change is undesirable, it would be associated with degradation of the material. The purpose of the assessment of the condition might be to provide data for the overall assessment of the condition of a structure or system that contains the collection of atoms. However, it might be used as input to a more demanding evaluation as regards the future implications of the present condition on the performance of the system: a prognosis of future performance. In either case, it is recognized that if the condition is changing, then assessing the rate at which it is changing is perhaps even more important. Often, however, the rate of materials degradation depends directly on the environment; if the environment changes, the rate of degredation is also likely to change. So a material that at present is experiencing very little degradation might degrade rapidly if the environment changes (thermal, mechanical, chemical, radiation, and so forth). This fact suggests that monitoring the environment might offer an earlier

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30 Proceedings of a Workshop on Materials State Awareness indication of impending materials state changes than what is provided by knowledge of the instantaneous condition. However, in general, the nature of materials degradation is such that localization is more problematic than more extensive overall degradation. 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. The issue of measurement scale and critical flaw size is discussed in this context. In addition, the potential of electromagnetic, mechanical, and thermal assessment of surface and near-surface condition is discussed with regard to point and multipoint (array) measurements for assessing work hardening, alloy variation, nonuniform cyclic, residual stress mapping, and alloy variation. Finally, the notion of developing components from sensible material “particles” that facilitate state awareness polling or reporting is proposed as a way to overcome practical physical measurement limitations on the atomic scale. The importance of sustainable design in this context is emphasized.