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Several popular nondestructive evaluation (NDE) test techniques, including eddy current, magnetic measurements, neutron diffraction, radiography, thermography, and ultrasonics, were designed to detect and characterize void-like non-uniformities that can alter structural integrity. As a result, the NDE procedure and test instrumentation were intentionally established to minimize the potential influence of subtle variations in material properties and damages that can influence the test results. Over the years, as the necessity for more defect-sensitive NDE capabilities has increased, problems with the resolution of relevant versus non-relevant indications have also grown. Such concerns are demonstrated by some of the complex signal processing and multifrequency tests in use to resolve signal differentiation difficulties.

Despite the problems associated with high-sensitivity NDE procedures, advancements in this area have not been without rewards. Two interrelated developments stand out. The demand for more complex signal processing has increased the utilization of computers in NDE; computer-aided NDE, in turn, has extended inspection capabilities beyond the detection and characterization of defects. Perhaps the most significant new opportunity provided by these developments is the potential for the nondestructive characterization of material properties and damages. More specifically, it seems that computer-enhanced NDE techniques offer the capabilities needed to monitor and exhibit variations in sensor responses, which contain important changes in material properties and damages.

The importance of the nondestructive characterization of material properties and damages has been enhanced by two factors: the demand for the fabrication and utilization of advanced materials and the necessity for the development of life-prediction technology for in-service structural and machine components.

With regard to the former, one particularly important role of NDE for composite applications includes material processing and the option for process-interactive control. Because of many processing parameters associated with fabricating composites, the likelihood of detrimental discontinuities being present is high, and in-process NDE can be a cost-effective option. The detection of potential defects early in the processing cycle would enhance system yield and material quality.

As for the latter, there is a growing need to predict the remaining life of aging plants and structural components that have been in operation for long periods at high temperatures. A defensible plant life-extension strategy demands methods for characterizing material conditions of in-service structural components, an accurate determination of the time- and service-dependent material properties and damages of components, and a quantitative life-prediction technology.

Because of the limited availability of test materials available from inservice structural components, the NDE technology offers an attractive means to obtain information regarding material properties and damages. Nondestructive evaluation methods have proven to be effective in assessing material properties and damages.

For instance, NDE techniques can first be employed to investigate plant components and uncover critical areas containing severely degraded materials. Next, test or miniature specimens can be machined from the critical areas for testing to develop material properties and damages.

For the effective application of nondestructive characterization to material properties and damages, it is of paramount importance to develop a fundamental understanding of how NDE signatures relate to material properties and damages. The research areas for the nondestructive characterization of material properties and damages may be summarized as follows: the NDE of advanced materials; the correlation of NDE signatures and material properties and damages; in-situ NDE for investigating fracture mechanisms and damage assessment; in-situ NDE for process and quality control during fabrication (including raw materials and final products); the development of NDE techniques for characterizing microstructures as well as mechanical and physical properties; the NDE of residual stresses, textures, and dislocation densities; the theoretical modeling of NDE for material characterization; smart materials and intelligent structure technologies; and the NDE of aging assessment, including fatigue characteristics, irradiation damage, and hydrogen embrittlement.

The three articles in this JOM-e, online-only presentation present examples related to the application of NDE methods to characterize material properties and damages.

First, B. Yang, P.K. Liaw, H. Wang, J.Y. Huang, R.C. Kuo, and J.G. Huang present “Thermography: A New Nondestructive Evaluation Method in Fatigue Damage.” A high-speed and high-sensitivity thermographic infrared (IR) imaging system has been employed for nondestructive evaluation of temperature evolutions during 10 Hz, 20 Hz, and 1,000 Hz fatigue testing of reactor pressure vessel steels. Five stages of temperature profiles were observed: an initial increase of the average specimen temperature, a region of temperature decrease, an equilibrium (steady-state) temperature region, an abrupt increase of the temperature, and a drop of temperature following specimen failure. Crack propagations and Lüder-band evolutions during fatigue have been observed. The relationship between the temperature, stress-strain state, and fatigue behavior is discussed. Both thermodynamic and heat-transfer theories are applied to model the observed temperature variation during fatigue. The experimental and predicted temperature evolutions were found to be in good agreement. Thermography provides an effective method to in-situ monitor the material stress-strain behavior during fatigue, which can open wide applications of thermography in detecting mechanical damage of materials and components in real time.

In the second article, V. Giurgiutiu provides an overview of “Embedded NDE with Piezoelectric Wafer-Active Sensors in Aerospace Applications.” The capability of embedded piezoelectric wafer active sensors (PWAS) to perform in-situ NDE is studied. Laboratory experiments were employed to prove that PWAS could satisfactorily perform lamb wave transmission and reception. Subsequently, crack detection in an aircraft panel with the pulse-echo technique is demonstrated. For large-area scanning, a PWAS phased array was employed to create the embedded ultrasonics structural radar. For quality assurance, PWAS are self-tested with the electromechanical impedance technique. The emerging technology requires a sustained R&D effort to achieve its full potential for applicability to full-scale aerospace vehicles.

In the third article, J. Kim, P.K. Liaw, and H. Wang present “The NDE Analysis of Tension Behavior of Nicalon/SiC Ceramic-Matrix Composites.” Nondestructive evaluation methods were used to study tension behavior of ceramic matrix composites (CMCs). Two types of NDE methods, ultrasonic testing (UT) and IR thermography, were employed to assess defects and/or damage evolutions before, during, and after mechanical testing. Prior to tensile testing, a UT C-scan and a xenon-flash technique were performed to develop initial defect information in light of UT C-scans and thermal-diffusivity maps, respectively. An IR camera was used for in-situ monitoring of progressive damages, and the IR camera was further employed to determine temperature changes during tensile testing. Moreover, scanning electron microscopy characterization was performed to study microstructural evolutions and failure mechanisms. In this article, NDE methods were used to facilitate the understanding of tension behavior of ceramic matrix composites (CMCs). The research also explores the feasibility of using NDE techniques to interpret structural performance of CMCs.

As these articles indicate, NDE techniques have been widely used in characterizing material properties and damages. Nevertheless, more applications in the nondestructive characterization of material properties and damages should come in the future with the influence of advanced materials processing and remaining-life prediction demands. In particular, the application of NDE methods during manufacturing can provide a closed-loop feedback process control for fabricating advanced materials. This kind of intelligent processing capability needs to be further developed to assure the quality of advanced material processing, reduce manufacturing costs, and therefore, guarantee the success of material production. Moreover, with the rapid progress in NDE signal processing and the enhancement in computational capacities, NDE responses can be further quantified to characterize material properties and damages.

This series of articles, with accompanying animations, will offer some background information regarding the NDE of material properties and damages. It is a challenging and growing field for materials scientists and engineers as well as physicists and mechanical engineers to develop and utilize novel nondestructive methods to quantify material properties and damages and to theoretically predict the results. Through this endeavor, the material behavior of ceramics, metals, polymers, alloys, and composites, including advanced materials, can be better understood, which will result in the effective utilization of materials.

Peter K. Liaw is a professor and Ivan Racheff Chair of Excellence at the Department of Materials Science and Engineering, the University of Tennessee, Knoxville, TN 37996-2200. He is the advisor to JOM from the ASM/TMS Mechanical Behavior of Materials Committee.