【国外标准】 Standard Practice for Determination of Metal Purity Based on Elastic Constant Measurements Derived from Resonant Ultrasound Spectroscopy

5.1 Since the beginning of human history, currency has existed in the form of metal coins and bullion. Thieves learned that shaving some precious metal provided a method to change its value. Substitution of common metals for precious metals of higher value was commonplace until weighing methods became so accurate, that it became easily detected. Alloys were also used as substitutes until inexpensive spectrometers became available which ended the counterfeiting practice. The rapid rise in the value of gold inspired the unscrupulous to find a new method. Tungsten was widely used for light bulb filaments until regulations changed that market. The great abundance of tungsten now available, coupled with the almost identical density of gold, presented a new opportunity.5.2 RUS provides a method to create an unique electronic signature for each piece tested which is operator independent.1.1 This practice is intended for use with resonant ultrasound spectrometers capable of exciting, measuring, recording, and analyzing multiple whole body mechanical vibration resonant frequencies within parts exhibiting acoustical ringing in the acoustic or ultrasonic, or both, resonant frequency ranges.1.2 This practice uses Resonant Ultrasound Spectroscopy (RUS) to distinguish conforming parts, as determined from qualified training sets, from those containing significant anomalies in their elastic properties.1.3 The basic functions of a RUS monitoring system are to detect and classify resonance phenomena. Solid structure resonances are governed by the part’s dimensions, density, and elastic properties. When a material substitution occurs in a precious metal, the chosen metals have almost identical densities and unchanged dimensions, leaving only the elastic properties to affect the resonances.1.4 This practice can be used to replace destructive methods, which damage the test object through drilling or melting, or both.1.5 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.