PSI - Issue 43

Jan Klusák et al. / Procedia Structural Integrity 43 (2023) 142–147 Author name / Structural Integrity Procedia 00 (2022) 00 – 000

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Fig. 5. Frequency dependence of 304L/4307 specimens No. 9 and 10.

3.3. Microstructure evolution Applied cyclic loading results in formation of deformation structures. To accommodate plastic strain, dislocations are forming deformation structures, characteristic for FCC metals with low stacking fault energy. Typically, at the beginning of the fatigue life, the dislocation density is increased, resulting in the steep stress amplitude increase. With further cycling, the plastic deformation starts to localize, which is accompanied by a decrease in stress amplitude. At later stages of the fatigue life, a plateau in the stress amplitude is reached as the plastic deformation almost exclusively occurs within formed slip bands, where the formation of the strain-induced martensite can occur, Tamura (1982), Müller -Bollenhagen (2010). Extensive formation of the strain-induced martensite can result in a further increase of the stress amplitude, at the end of the fatigue life. Character and the magnitude of these changes strongly depends on the applied load amplitude. A more detailed study of the microstructure evolution in 304L steel during cyclic loading can be found in Jambor et al. 2021, and Šmíd et al. 2021 . The changes in the resonance frequency observed during gigacycle fatigue tests are analogical to the changes of stress amplitude observed during low frequency loading. The initial increase in frequency is probably accompanied by an increase in temperature, as also shown in Müller Bollenhagen (2010). Note that in our case the temperature was not measured due to the water cooling. Figure 6 shows ECCI micrographs of the specimen from 1.4306 steel, after reaching 1  10 10 cycles without failure. There is clearly visible high dislocation density within grains due to cyclic loading. Dislocations are arranged in the bands along {111} planes. Moreover, presence of the defo rmation induced α’ martensite was revealed in the structure.

Fig. 6. Micrograph of specimen from 1.4306 steel after reaching of 1E10 cycles without failure. ECCI revealed strong dislocation activity, accompanied by formation of the strain induced α’ marte nsite. Loading direction is oriented vertically.