*2.1. Test Standards Used for Ultrasonic Material Characterization*

The ultrasonic technique is an important procedure for viscoelastic materials' characterization at high strain rates. It is broadly used for developing precise measurements of speed of sound and attenuation. These two variables are the bases for accurately evaluating elastic moduli, and for assessing mechanical properties at high frequencies. Layer thickness and the speed of sound are important linked parameters also to account for LEP system configuration. If one of the parameters is known, the other one can be determined by simple time-of-flight (TOF) measurement of ultrasound.

An ultrasound examination is based on the propagation of ultrasonic waves in the part to be examined and the follow-up of the transmitted signal (called transmission technique), or of the signal reflected or diffracted by any surface or discontinuity (called reflection technique). Both techniques can use a single probe that acts as a transmitter and receiver, or a double probe, or separate transmitter and receiver probes. In the same way, these two techniques can involve an intermediate reflection coming from one or more surfaces of the examined object.


by knowledge of this distance, the direction of wave propagation, and the position of the probe. Contact with the test object is generally preferred over separation by a liquid buffer or immersion coupling medium. Although it is applicable, in general terms, to discontinuities in materials and applications, other techniques like the time-of-flight diffraction (TOFD, ISO 16828 [34]) can be used for both detection and sizing of discontinuities provided is performed with necessary consideration of geometry, acoustical properties of the materials, and the sensitivity of the examination.

For speed of sound measurements, the objective is to determine the exact time interval needed for a signal to travel between the front and back surface of a test object with previously known thickness. Attenuation may be calculated from the ratio of the two amplitudes measured. The pulse echo technique uses a broad band frequency range for most engineering solids, from about 300 to about 400 MHz. Preferably, the test object must have smooth, flat, parallel opposing surfaces and minimum thickness (to avoid excess of attenuation). It should meet the limitations for precise signal analysis, like the absence of discontinuities like voids or other particles. In addition, adequate force on the transducer is required to squeeze out excess coupling medium. Note that direct, normal incidence reflections may not appear even if test object shape and boundaries meet the conditions when the material is anisotropic, orthotropic or contains microstructural gradients.
