PSI - Issue 18

Matthias Hell et al. / Procedia Structural Integrity 18 (2019) 823–836 Hell et al. / Structural Integrity Procedia 00 (2019) 000–000

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The cyclic stress-strain relation and the strain-life curve are experimentally derived using constant amplitude tests at different load magnitudes (SEP1240 DIN 50100). The cyclic stress-strain behaviour exhibits an initial phase, which is marked by transient effects like cyclic hardening, cyclic softening, and depending on the boundary conditions, also mean stress relaxation or ratchetting [1]. After a certain number of cycles, a more or less significant cyclic stabilization of the stress-strain behaviour sets in. Many guidelines and standards assume the cyclic stabilization at half of the number of cycles to crack initiation. Depending on the selected material, this assumption has to be thoroughly questioned. Fig. 4 shows the cyclic deformation curves for the heat treatable steel 42CrMo4+QT. For low strain amplitudes, the assumption of a cyclic stabilization is valid, but the stability decreases with rising load magnitude. If the evolution of the cyclic stress-strain behaviour over the number of cycles is reduced to a single stress-strain curve, a misinterpretation of the actual stress-strain state within the component will be the consequence.

Fig. 4: cyclic deformation curves and stress-strain paths for 42CrMo4+QT steel

In contrast to the heat treatable steel 42CrMo4+QT, the precipitation hardened steel 30MnVS6+Ti shows a cyclic stabilization up to highest load magnitudes, Fig. 5.

Fig. 5: cyclic deformation curves and stress-strain paths during the first 10 load cycles for a precipitation hardened ferritic-pearlitic steel 30MnVS6+Ti.