COMPUTER-CONTROLLED APPARATUS FOR INTERNAL-FRICTION AND YOUNGS MODULUS MEASUREMENTS AS A FUNCTION OF TEMPERATURE

A mechanical resonance apparatus has been developed which allows autom atic recording of internal friction Q(-1) and dynamic Young's modulus E as a function of temperature. The attainable accuracies Delta Q(-1)/ Q(-1) and Delta E/E are approximately equal to 3.5 x 10(-2) and 1 x 10 (-5), respectively. The vibrating sample is kept at resonance, during heating and cooling, by continuously readjusting the frequency of the driving signal supplied by computer-controlled frequency synthesizer. The frequency regulation is obtained by keeping either the amplitude o f the response signal at its maximum level or the phase difference bet ween the excitation and vibration signals at a fixed, selected value. In comparison with previous automatic control systems designed for sim ilar applications, the one described here is more versatile, being abl e to measure internal friction from the width of the resonance curve a s well as from the logarithmic decrement of freely decaying vibrations , and being able to regulate the vibration frequency by either the amp litude or the phase control method. The two automatically interchangea ble measuring procedures allow recording over wide ranges of the dissi pation coefficient (10(-5) less than or equal to Q(-1) less than or eq ual to 10(-2)),while the two frequency regulation methods permit, acco rding to the requirements, either higher data acquisition rates (phase control approach) or better accuracies (amplitude control approach). Tests carried out on a CuZnAl alloy show good performance of the appar atus during temperature-induced martensitic transitions, which are ass ociated with large relative changes of frequency and energy dissipatio n. (C) 1996 American Institute of Physics.