Improved technique for characterization of texture in thick plates. (News Briefs).

Commercial metal alloys used for structures are usually produced by rolling or extrusion into the form of plates, bars, or beams. These fabrication processes leave their mark on the material in the form of a texture or preferred orientation of the grains in the microstructure. In many applications, such as the nondestructive inspection and materials characterization of structural components, a knowledge of this texture is of vital importance.

It can be measured in the laboratory by x-ray techniques or by ultrasonic methods that involve precision machining of a specimen cut from the part. However, the information on texture is often needed in the field, at depths below the penetration of x rays or on parts that cannot be cut up. For several years, NIST has been developing ultrasonic measurement procedures for deducing the texture of materials using specimens carefully machined from the part. We have developed a texture measurement technique by combining electromagnetic acoustic transducers (EMATs), which excite and detect ultrasonic waves that propagate near or on the surface of a plate, with theoretical models for the elasticity of polycrystalline aggregates.

It is nondestructive and applicable to large objects found in the field, at rolling mills, or in foundries. It is being used to measure surface residual stresses that distort parts being machined from thick plates, to characterize ultrasonic reference blocks used to calibrate ultrasonic inspection systems, and to monitor the condition of surface layers used to extend fatigue life or improve wear resistance.

CONTACT: George Alers, [phone]; [e-mail].