31 May 2001
Ultrasonics assess material properties
by Bob Felton
Sensors will monitor material quality and serviceability.
Ultrasonic sensors will someday be used on production lines to characterize microelectronic structures such as dielectric layers and the microstructural features of metals, alloys, and engineered surfaces, said researchers in the Ultrasonic Characterization Program at the National Institute of Standards and Technology (NIST).
"The idea," the researchers said, "is to establish models that relate microstructure to physical measurements so that by measuring appropriate physical properties, the salient microstructural features can be ascertained."
NIST researchers plan to investigate and develop models and ultrasonic methodology for measuring a range of physical characteristics, including elastic coefficients and related properties such as specific heat, thermal expansivity, and hardness. If the researchers are successful, plant engineers will use ultrasonic sensors to measure a host of material properties as components travel down the assembly line.
In the works
A project that aims to assess elastic coefficients and related physical properties has already succeeded in measuring the elastic properties of two varieties of mullite and the piezoelectric and elastic properties of langasite and quartz. A second project anticipates using nonlinear ultrasonics to characterize material hardness, fatigue, and adhesion; the technique works by measuring attenuation of waves as they pass through solids.
A similar project is developing methods for using sound waves to sense material changes arising from annealing, loading, and irradiation. "The accomplishment of these objectives," the researchers said, "will enable real-time evaluation of dislocation dynamics, providing fundamental information on deformation processes. It also will provide a basis for nondestructive ultrasonic evaluation of structural material integrity during production or service."
The researchers have so far succeeded in showing that "plastic deformation induces an increase in damping that partially recovers with time. Plastic deformation also permanently reduces the dislocation pinning such that the damping is changed with even small elastic loading."
Yet another project is investigating ways to "infer stress from precise measurements of the time of flight of shear waves propagating through the thickness of common structural elements formed by aluminum and steel plates." The sensor will work as a nondestructive, mobile strain gauge that measures stress levels without permanently affixing to the member.
Waves generally travel fastest in the rolling direction and slowest in the transverse direction. Stresses in the member, however, shift the fastest and slowest travel directions away from the rolling and transverse axes. The change in velocity is an index to the magnitude of the stresses. In experiments, researchers have found excellent agreement between the sensor and sample measurements made elsewhere at NIST.
The NIST research should result in sensors that have application across a broad range of activities and environments. The Nuclear Regulatory Commission funded a portion of the work, for example.
"As reactor pressure vessels age," the researchers explained, "and are embrittled by radiation, their anticipated safe life must be predicted from tests performed on the vessel itself during routine maintenance operations. The materials characterization studies in this project have revealed several new nondestructive techniques that can be applied to a pressure vessel in the field to predict its hardness, a quantity known to be related to the ductile-to-brittle transition temperature used by regulatory agencies as a measure of embrittlement." IT