PSI - Issue 13

Milan Uhríčik et al. / Procedia Structural Integrity 13 (2018) 1571 – 1576 Milan Uhríčik / Structural Integrity Procedia 00 (2018) 000 – 000

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designed to fulfill the resonance condition that means the natural frequency must be approximately the same as the frequency of the test equipment ± 10 Hz, which is optimized for the measurements at the resonant frequency of about 20.5 kHz (Dresslerová et al. 2014). For the plastic deformation, this is a uniaxial deformation when the test bars are subjected to a compressive deformation. The test bar was placed in a steel device (Fig. 5) to prevent its offset during the deformation and was loaded in one direction with a force, to reduce about 1 mm. After the load, internal damping courses were measured. The shape of the test bar is that the plastic deformation occurs preferably in its shaft where the specimen is tapered.

Fig. 5. A device designed to perform the plastic deformation on test bars.

Fig. 4. An ultrasonic resonance device for internal damping measurement.

Figures 6-8 show the temperature-dependent measurements of internal damping of austenitic steels AISI 304, AISI 316L and AISI 316Ti in the initial state, where the internal damping peaks can be seen in the temperature range 80 100 °C. Figures also include a second measurement of internal damping, indicating that peaks no longer appear or they are smaller in comparison to the first course of measurement. This fact is related to the fact that during the first measurement all the relaxation processes, related to the mobility of dislocations, have occurred, as well as the release of internal energy.

Fig. 7. Results of internal damping measurement of AISI 316L in the initial state.

Fig. 6. Results of internal damping measurement of AISI 304 in the initial state.

Fig. 8. Results of internal damping measure ment of AISI 316Ti in the initial state.

During the measurement, the test bars were heated to a final temperature of 400 °C with subsequent cooling to room temperature. The further heating above 400 °C made no sense because all the processes were carried out within the temperature range of 30-100 °C. After this interval, a sharp decrease of internal damping occurs and at 200 °C internal damping values change very little. The highest internal damping value is achieved within the temperature range of 80-100 °C, at the local maximum, where the deformation-induced martensite is probably transformed into the austenite. All three materials reached a maximum damping within the temperature range of 80-100 °C. The highest internal damping value (0.00138) was observed for the test bar from AISI 316Ti. In the reversed damping process, during the cooling, the similar peak did not appear, as at the beginning. The internal damping in the reverse direction either reaches lower values or returns to the initial value.