The Evolution of NDE
The inspection of materials and components, with the objective of locating and quantifying defects and degradations in material properties that would lead to failure, is essential to the safe utilization of engineering structures. Such inspections are, thus, an integral part of the engineering needed to ensure product quality and reliability. A wide variety of noninvasive measurement techniques are employed, including ultrasonics, radiography, and electromagnetics, which are familiar from other fields of noninvasive measurements such as medicine, geophysical prospecting, sonar, and radar. Depending on the manner of their utilization, inspection with such techniques is often referred to as nondestructive testing (NDT) or nondestructive evaluation (NDE).
Nondestructive testing (NDT) has been practiced for many decades. The technological advances that occurred during World War II and the subsequent defense effort spurred initial rapid developments in instrumentation. The primary purpose was the detection of defects. As a part of “safe life” design, it was intended that a structure should not develop macroscopic defects during its life, with the detection of such defects being a cause for removal of the component from service. In response to this need, increasingly sophisticated techniques using ultrasonics, eddy currents, x-rays, dye penetrants, magnetic particles, and other forms of interrogating energy emerged.
In the early 1970s, two events occurred which caused a major change. The continued improvement of the technology, in particular its ability to detect small flaws, led to the unsatisfactory situation that more and more parts had to be rejected, even though the probability of failure had not changed. However, the discipline of fracture mechanics emerged, which enabled one to predict whether a crack of a given size would fail under a particular load if a material property, fracture toughness, were known. Other laws were developed to predict the rate of growth of cracks under cyclic loading (fatigue). This formed the basis for the new philosophy of “fail safe” or “damage tolerant” design. Components having known defects could continue in service as long as it could be established that those defects would not grow to a critical, failure-producing size.
A new challenge was presented to the NDT community. Detection was not enough. One needed to also obtain quantitative information about flaw size to serve as an input to fracture mechanics-based predictions of remaining life. These concerns, which were felt particularly strongly in the defense and nuclear power industries, led to the creation of a number of research programs around the world and the emergence of quantitative nondestructive evaluation (QNDE) as a new discipline. The Center for Nondestructive Evaluation (CNDE) at Iowa State University (growing out of a major research effort initiated at the Rockwell International Science Center); the Electric Power Research Institute in Charlotte, North Carolina; the Fraunhofer Institute for Nondestructive Testing in Saarbrucken, Germany; and the Nondestructive Testing Centre in Harwell, England, can all trace their roots to those changes. It should be noted that CNDE is closely tied to the National Science Foundation, being a member of the Industry/University Cooperative Research Program since 1985.
In the ensuing years, many important advances have been made. Quantitative theories have been developed to describe the interaction of the interrogating fields with flaws. Models incorporating the results have been integrated with solid model descriptions of real part geometries to simulate practical inspections. These tools allow NDE to be considered, as a part of the design process, on an equal footing with other failure-related engineering disciplines. Quantitative descriptions of NDE performance, such as the probability of detection (POD), have become an integral part of statistical risk assessment. Measurement procedures initially developed for metals have been extended to engineered materials, such as composites, in which anisotropy and inhomogeneity become important issues. The rapid advances in digitization and computing capabilities have totally changed the faces of many instruments and the type of algorithms that can be used in processing the resulting data.
High-resolution fusion imaging systems and the information from multiple measurement modalities in characterizing a flaw have emerged. An increasing interest is found not only in detecting, characterizing, and sizing defects, but in characterizing the materials in which they reside. Goals can range from the determination of fundamental microstructural characteristics such as grain size, porosity, and texture (preferred grain orientation) to material properties related to such failure mechanisms as fatigue, creep, and fracture toughness.
We are currently at another turning point in society’s needs that dictate that a new set of institutional relationships be developed. The industries that played the major role in driving the emergence of QNDE defense and nuclear power have been on the wane. Increases in global competition have dramatically changed the product development and business cycles. Finally, the aging of our civil infrastructure, from roads to buildings to aircraft, has presented a new set of measurement and monitoring challenges. Among the new applications of QNDE spawned by these changes is the increased emphasis on the use of QNDE to improve the productivity of manufacturing processes. Included are the characterization of materials during their development cycles, increasing both the amount of information about failure modes and the speed with which it can be obtained and thus reducing the development cycle time; and the development of in-line measurements for process control. Continued development of flaw detection and characterization techniques is also required for use at the end of manufacturing and during service. The use of advanced simulation tools to design for inspectability and the integration of such tools into quantitative strategies for life management will be a key element in increasing the engineering applications of NDE throughout the life cycle. As the globalization of business continues, companies will increasingly seek to develop uniform practices for use throughout the world. In the area of QNDE, this will drive an increased emphasis on standards, enhanced educational offerings, and simulations that can be easily communicated electronically.
Communication of research results occurs in a variety of forums, including both journals and meetings associated with many societies. Perhaps, foremost among the research meetings is theReview of Progress in Quantitative Nondestructive Evaluation, a week-long meeting now in its twenty-sixth year and attended by approximately 450 scientists and engineers from around the world, dedicated to both the communication of fundamental research results and to creating an environment that will foster the first steps of technology transfer.