Issue 38

L. Cui et alii, Frattura ed Integrità Strutturale, 38 (2016) 26-35; DOI: 10.3221/IGF-ESIS.38.04 34 SWT m P E , 2 2                 (9) with Δ  as constant stress range and  m as mean strain. As an alternative to the approach described above, the Neuber-hypothesis can be used to determinate the maximum stress and strain by using cyclic tensile curves. However the hypothesis is not applicable on the experiments uA16kb60 and uA1kb61, because the high nominal stress leads to significant plastic deformation. By simulating the experiments with the material model, the mean strain cannot be described satisfactorily, which leads to a very conservative lifetime estimation (Fig.10). Thus, the mean strain is read from measured value at half-life cycle to represent the “saturated” material state. By considering the mean strain effect, the lifetime can be predicted within a scatter band of factor 2 (Fig. 10). To summarize, lifetime estimation without considering mean strain is conservative. The Neuber hypothesis has its strength in its easy applicability. But its application is limited to the elastic domain and it leads to strong conservatism in lifetime estimation. The application of equivalent stress and equivalent strain determined with FE analysis based on the material model of type Chaboche [6] appears relatively expensive in terms of parameter identification from current level of technology. However, it has more potential on describing complex loading because it can better represent the deformation under the complex loading. Figure 10: Correlation between estimated life N* i and measured life N i . C ONCLUDING REMARKS ervice-type experiments were carried out on notched specimens of the X12CrMoWVNbN10-1-1 rotor steel under cyclic creep fatigue loading at service temperature 600°C. The peak-values of deformation decrease quickly if the loaded nominal stress is higher than the material’s 0.2% yield strength at the testing temperature. At lower nominal stress, the peak-values decrease slowly and almost linearly until about 2000 cycles and then they increase again. According to metallographic investigations on the 4 tested specimens, cracks are perpendicular to the direction of loading and propagate from the surface of the notch root. Crack initiations are transgranular and clearly through extrusions on the surface. The main transgranular cracks ramified during its propagation. The lifetime of the 4 test specimens was estimated with a phenomenological lifetime prediction method by using Neuber hypothesis and equivalent stress/strain in the gauge part of the specimens. The uniaxial equivalent stress was determined with von Mises from finite-element-analysis according to constitutive material model of type Chaboche. The estimated life and experiment are predicted in a scatter band of factor 2, while the results are conservative by using Neuber hypothesis if the effect of mean strain on lifetime estimation is not considered. S