ENGINEERING INTEGRITY, VOLUME 45, SEPTEMBER 2018, pp.8.13.
orientation dependency of the material behavior. The mechanical size effect II is related to the geometry and material dependent extension of the highly stressed volume. Both mechanical size effects have to be implied by the elastoplastic finite element analysis of the stressstrain state. The mechanical size effect III results from the Masing and Memory behaviour in conjunction with the inhomogeneous stressstrain state. Depending on the load sequence, the material behaviour and the component geometry, the relation between external load ratio and internal stress-strain ratio may change throughout elasto-plastic cyclic loading.
ISSN 1365-4101/2018
Figure 4. Miniaturized specimen and special electro-mechanical test system with strain gauge for the experimental assessment of local material properties
In local strain-based fatigue design approaches it can be assumed, that the assumptions regarding the cyclic material behaviour affect the evaluation of the stress-strain state and the assessment of the fatigue life and also the consideration of the effects of the component size on fatigue life. In the following sections of this paper, selected features of the cyclic material behaviour and strategies to derive cyclic material parameters will be presented with respect to their influence on the fatigue life assessment. CHARACTERISATION OF THE CYCLIC MATERIAL BEHAVIOUR The cyclic stress-train behaviour is estimated using straincontrolled test on un-notched specimens, see Figure 3. State of the art strain measurements for the control of the tests employ a clip-on strain guage, which detects the elongation over a defined measuring length, Figure 3. Thus, the experimentally derived material properties represent a homogenized integral mean value of the strain and the material properties over the measurement length. In order to assess material with local property distributions, also specimens with miniaturized geometry may be used, applying a strain guage for the measurement of the local strain, Figure 4. The strain measurement with strain guages is also applicable to components with discontinous cross sections or for bending and torsional loading.
cyclic softening, and depending on the boundary conditions, also mean stress relaxation or ratchetting [20]. After a certain number of cycles, most of the common engineering materials show a cyclic stabilisation within the stress-strain behaviour. The extent to which the stress-strain behaviour will stabilise depends on the load magnitude and, for variable amplitude loading also on the load sequence. For a huge number of engineering applications in the high cycle and very high cycle fatigue regime, when elasto-plastic load amplitudes with macroscopic plasticity do not dominate the fatigue mechanisms, it is admissible to neglect the transient material behaviour. Several guidelines for fatigue testing, as for example the SEP1240 [21], assume a cyclically stabilised state, when the material has undergone half of the cycles to crack initiation.
Figure 5 shows the load magnitude dependency of the cyclic softening on the example of the cyclic deformation curves up to 5∙104 cycles and the stress-strain path for the first 10 cycles, for strain controlled fatigue tests with constant amplitude loading on smooth specimens made of 42CrMo4+QT quenched and tempered heat treatable steel. As is obvious, the assumption of a cyclical stabilization of the material behaviour will lead to a misinterpretation of the actual stress-strain state. The opposite is true for the cyclic stress-strain behaviour of the precipitation hardened ferritic-pearlitic steel 30MnVS6+Ti, depicted in Figure 6. The 30MnVS6+Ti steel exhibits a slight cyclic hardening and enters a phase of steady-state stress-strain behaviour with The cyclic stress-strain behaviour exhibits an initial phase, a negligible softening for strain amplitudes up to the highest which is marked by transient effects like cyclic hardening, magnitudes. The cyclic stress-strain behaviour, as well as the fatigue behaviour, is determined by the generation and the annihilation of dislocations, the formation of dislocation structures and strain localizations within the microstructure of the material. The changes in the dislocation structure, being induced by Figure 3. Standardized specimen geometry and test environment for the cyclic straining, depend strain-controlled tests
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