PSI - Issue 53
P.N.B. Reis et al. / Procedia Structural Integrity 53 (2024) 309–314 P.N.B. Reis / Structural Integrity Procedia 00 (2019) 000–000
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It is possible to observe that it is from the third load cycle, when the load exceeds the deformation value associated to the knee, that the hysteresis cycles become more expressive. Until then, the unloading phase is very close to the loading phase, suggesting some overall reversibility of the process. Therefore, due to microstructural changes in the composite combined with the viscoelastic behavior, this phenomenon suggests that there is a threshold load value from which an irreversible process begins. Furthermore, due to the repeatability observed in all the tests, it is possible to report that there is a threshold stress value of around 3.5 MPa for this material, above which the material shows an accumulation of inelastic deformation. The results obtained in this type of test were also compared with those obtained in the monotonic test and can be found in Fig. 6. It can be seen that the curve obtained by joining the peaks of the hysteresis cycles, represented by the circular symbols, follows the profile of the pure monotonic curve, although with a slight lower slope due to viscoelastic effects. In fact, although the displacement rate is the same for both tests, in progressive load-unload tests the material is more subjected to creep effects because the test takes more time to be completed than in a monotonic test.
Fig. 6. Comparison between results obtained by a monotonic tensile test and progressive load-unload test for similar displacement rate.
Finally, Fig. 7 shows the hysteresis cycles obtained from the constant loading amplitude cyclic tests. Similar to Fig. 5, Fig. 7a shows the hysteresis cycles obtained for the 9 load-unload cycles, while Fig. 7b shows all cycles but with a strain shift of 0.1% between them.
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Fig. 7. a) Hysteresis cycles obtained from the cyclic load-unload tests; b) Similar representation of the hysteresis cycles obtained from the cyclic load-unload tests but with gap between them.
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