Dynamic moduli measurements are of interest in materials research, not only as a source of data on elastic behaviour, but also for the insight they provide into structure-property relationships in general. Many existing problems in engineering and science are connected with the precise determination of dynamic elastic moduli.

From numerous experimental measurements [3-7] of static and dynamic elastic moduli on elastic and viscoelastic materials, was found a very good coincidence of these moduli for the first and a large divergence for the latest.

The present paper experimentally examines, using ultrasounds, the dependence of elastic moduli on the frequency and on the viscoelastic behaviour and the temperature of the material. This behaviour of material's elastic moduli is appeared as the dynamic moduli, according to the relative theory, depend on the frequency of the used elastic wave and on the relaxation time of each material. In this way, first is examined the influence of the frequency on the elastic moduli of elastic and viscoelastic materials and furthermore the influence of material temperature on the same moduli.

From this experimental study useful conclusions are taken, as follows:

The NDT method of ultrasounds in a powerful method for the determination, of material's elastic moduli. But it is very important to know that, as these moduli are the dynamic moduli, which differ from the corresponding static, they can't be used instead of the latest for static problems.

The difference of elastic constants, determined this way, from the corresponding static constants is so greater as higher is the frequency of the used elastic wave and as much as more viscoelastic is the material.

The rate of change of the moduli of elasticity is higher for low frequencies (less than 4 MHz) and lower, tending to zero, for higher frequencies (more than 10 MHz). This variation is so small as more elastic and so high as more viscoelastic is the material. So, the dependence of elastic moduli on the relaxation time is verified experimentally here.

Furthermore, by the same way is examined the influence of material's temperature on elastic moduli.

Finally, from these experimental results was proved, that according to the theory, the change of materials elastic properties versus the temperature is a mirror reflection of the change of the same properties versus the frequency of the used ultrasonic elastic wave. REFERENCES

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