Shown here is a rebound test of round stock using the Leeb principle. Source: Wilson Instruments, an Instron company
Nondestructive testing (NDT), or nondestructive evaluation (NDE), is a method of materials testing that assesses the characteristics of a component without altering or destroying it. NDT is important in the materials testing industry where quick, dependable information on finished or raw material is needed. This may occur during the production stage, during the service life of a material or product, or as a diagnostic tool in the event of material failure.
NDT is contrary to destructive testing such as stress or bend testing where critical material properties are determined through achieving specimen failure. While destructive testing is intrinsically more revealing, it is costly to a manufacturing operation due to material loss and, for obvious reasons, it is not suitable for in-service material testing.
Application uses for NDT cover a comprehensive range of material and industries such as automotive, aerospace, construction and a multitude of manufacturing types. A classic example of NDT at work is testing for weld defects at manufacture and periodically during service life of the welded material. Other typical applications include crack detection in aircraft skins, surface flaws in pipes or bar, and evaluation of heat treating of product.
NDT also has become a valuable tool in the research and development field where quick, reliable data can provide vital information on materials.
Typical types of NDT include ultrasonics, eddy current, rebound and ultrasonic contact impedance. Hardness testing by diamond or ball indentation also is-although sometimes arguably-considered a form of NDT, as in most cases the material only is indiscernibly affected. The force applied in a hardness test correlated either with the depth of indent or with area of indent provides a measurement of hardness. Typically the properties and usefulness of the material is not compromised. Microhardness testing is the best example of a nondestructive hardness test.
This image depicts a MacroVickers test across a heat-affected zone of welded tubing. Source: Wilson Instruments, an Instron company
Common NDT Tests
Ultrasonic testing (UT) uses high-frequency sound energy to evaluate a variety of materials to produce vital information. It is typically used for flaw detection, dimensional measurements and surface thickness. A typical system would include a pulser/receiver, transducer and display device. The pulser produces high-voltage electrical pulses to the transducer, which in turn generates ultrasonic energy. The energy is introduced to and flows through the test piece in the form of waves. Flaws are detected as a discontinuity on the wave path, and the resultant electrical signal is displayed on the display device. UT is one of the more widely used forms of NDT and has several advantages, including minimal material preparation, instant results, high accuracy and access to only a single side of the workpiece. UT is used on steels, concretes, woods and composites.Eddy current testing uses the principle of electromagnetism as the basis for conducting examinations. In eddy current testing, a current is introduced to the material. Changes in the current based on the material provide valuable workpiece information, such as flaws and surface cracks. Eddy current testing also can be used to determine material hardness, as well as material and coating thickness determination.
Rebound testing uses the Leeb principle. In this method, an impact device uses a spring to propel an impact body through a guide tube toward the test piece. As it travels toward the test piece, a magnet contained within the impact body generates a signal in a coil encircling the guide tube. After the impact, it rebounds from the surface inducing a second signal into the coil. The instrument calculates the hardness value using the ratio of the voltages and analyzes their phases to automatically compensate for changes in orientation.
Because the device is electronic in nature, most Leeb instruments are designed to automatically convert from the Leeb number to a more conventional hardness scale. By using a variety of different conversions to suit the different materials, a wide range of metallic parts can be tested. The main limitations are that the parts must have a good finish and a minimum weight of 5 kilograms. Leeb testers are portable and can be used at different angles as long as they are perpendicular to the test surface.
Ultrasonic contact impedance (UCI) testing uses a diamond indenter similar to that used in a classic Vickers microhardness test. While conventional Vickers requires optical evaluation of the area of an indent, the UCI method electronically detects by a shift of an ultrasonic frequency. UCI testers use a spring to apply approximately a 5-kilogram force to a Vickers indenter that is attached to the end of a resonating rod. As the resonating rod and Vickers indenter penetrate the test sample, a frequency shift will occur in the rod. The amount of the frequency shift is measured and can be related to the depth of penetration of the Vickers indenter into the sample. The results are electronically converted to other hardness scales such as Vickers and Rockwell. UCI instruments can test a wide variety of materials using different conversions, as long as the test sample has a smooth surface and is at least 0.5 inch thick.
NDT is a valuable and growing tool in the evaluation and maintenance of materials. In our rapidly changing and highly mobile environment, dependable, efficient and effective tools are needed to meet the quality demands of the manufacturing, R&D and inspection sectors. NDT technologies will continue to play a major role in these areas.
