PSI - Issue 56

Radim Halamaa et al. / Procedia Structural Integrity 56 (2024) 111–119 Halama et al / Structural Integrity Procedia 00 (2023) 000 – 000

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hysteresis loop is highlighted in Fig.4(b). The line segment in the first quadrant of the loop determines the shear modulus G = 23651 MPa.

(a)

(b)

Fig. 4. (a) the torque waveform in response to the deformation load on the specimen; (b) the hysteresis loop corresponding to the 13th load cycle.

The result of the proposed technique applied to the torsional fatigue test is shown in Fig.5, where the amplitude of shear strain vs the amplitude of shear stress is presented. In the same graph, the linear part (elastic domain) is represented by the shear modulus. The results of the evaluation of the stress-strain data from the upper and the bottom parts of the specimen correlate very well with the conventional cyclic stress-strain curve (evaluated accordingly with the ASTM 2207 standard).

Fig. 5. Cyclic strain curve for the case of symmetric torsional loading obtained by DIC measurements on the curved part of the specimen.

The Fig.5 shows that the first few points with the lowest stress value lie in the elastic region of deformation. Consequently, a fairly good agreement with the results obtained by the conventional method on cylindrical specimens can be seen. 4.2. Tension-compression case From the test machine recordings in the push-pull case, the force versus time dependence is available as shown in Fig.6(a). The 11 th cycle, which corresponds to the half-life of the specimen, is highlighted in the same figure.