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nondestructive evaluation[‚nän·di‚strək·tiv i‚val·yə′wā·shən]
Nondestructive evaluation (NDE) is a technique used to probe and sense material structure and properties without causing damage. It has become an extremely diverse and multidisciplinary technology, drawing on the fields of applied physics, artificial intelligence, biomedical engineering, computer science, electrical engineering, electronics, materials science and engineering, mechanical engineering, and structural engineering. Historically, NDE techniques have been used almost exclusively for detection of macroscopic defects (mostly cracks) in structures which have been manufactured or placed in service. Using NDE for this purpose is usually referred to as nondestructive testing (NDT).
A developing use of NDE methods is the nondestructive characterization (NDC) of materials properties (as opposed to revealing flaws and defects). Characterization typically sets out to establish absolute or relative values of material properties such as mechanical strength (elastic moduli), thermal conductivity or diffusivity, optical properties, magnetic parameters, residual strains, electrical resistivity, alloy composition, the state of cure in polymers, crystallographic orientation, and the degree of crystalline perfection. Nondestructive characterization can also be used for a variety of other specialized properties that are relevant to some aspect of materials processing in production, including determining how properties vary with the direction within the material, a property called anisotropy.
Much effort has been directed to developing techniques that are capable of monitoring and controlling (1) the materials production process; (2) materials stability during fabrication, transport, and storage; and (3) the amount and rate of degradation during the postfabrication in-service life for both components and structures. Real-time process monitoring for more efficient real-time process control, improved product quality, and increased reliability has become a practical reality. See Materials science and engineering
Visual inspection is the oldest and most versatile NDE tool. In visual inspection, a worker examines a material using only eyesight. The liquid (or dye) penetrant visual method uses brightly colored liquid dye to penetrate and remain in very fine surface cracks after the surface is cleaned of residual dye. The magnetic particle visual method requires that a magnetic field be generated inside a ferromagnetic test object. Flux leakage occurs where there are discontinuities on the surface. Magnetic particles (dry powder or a liquid suspension) are captured at the leakage location and can be readily seen with proper illumination.
The eddy current method uses a probe held close to the surface of a conducting test object. X-rays provide a varied and powerful insight into material, but they are somewhat limited for use in the field. The acoustic emission technique typically uses a broadband piezoelectric transducer to listen for acoustic noise. The thermography technique uses a real-time “infrared camera,” much like a home camcorder, except that it forms images using infrared photons instead of visible ones. Contact ultrasonics technique is the workhorse of traditional and mature NDE technology. It uses a transducer held in contact with a test object to launch ultrasonic pulses and receive echoes. See Eddy current
Many noncontact measurements have been developed that enhance the mature technologies. These include noncontact ultrasonic transducers that involve laser ultrasonics, electromagnetic acoustic transducers, and air- or gas-coupled transducers. Thermal wave imaging uses a main laser beam to scan the surface of the object to be examined. Electronic speckle pattern interferometry is a noncontact, full-field optical technique for high-sensitivity measurement of extremely small displacements in an object's surface. “Speckle” refers to the grainy appearance of an optically rough surface illuminated by a laser. Development of microwave techniques are under way in such diverse applications as ground-penetrating radar for land-mine detection, locating delaminations in highway bridge decks, and monitoring the curing process in polymers. See Laser, Transducer