Issue 37

C. M. Sonsino et alii, Frattura ed Integrità Strutturale, 37 (2016) 200-206; DOI: 10.3221/IGF-ESIS.37.26 205 rupture, cannot be attributed to different crack propagations under pure bending and pure torsion, because deformation- controlled tests for the failure criterion of crack initiation also reveal the same tendency for pure axial strain and pure shear strain [5]. The significantly different stress levels under fully reversed bending are caused by the different highly stressed material volumes in the notches due to the particular stress concentrations. Under pulsating loading, as the mean- stress effect for bending is more pronounced than for torsion, the stress levels are quite close to each other. These differences make choosing the appropriate Woehler-line for the assessment of fatigue life difficult. However, as the local normal stresses under combined loading are more dominant than the local shear stresses, the lines for pure bending are taken as starting Woehler-lines for fatigue lifing: - R = -1,  ak = 147 MPa, N k = 1·10 6 , k = 5, k* = 22 - R = 0,  ak = 97.5 MPa, N k = 1·10 6 , k = 5, k* = 22 The mean-stress sensitivity, which is needed for amplitude transformations for R = -1 when mean stresses vary, is M = 0.51 resulting from the strength values at the knee points. Fatigue lifing is carried out for the applied Gaussian amplitude distribution with the sequence length L s = 1·10 5 according to the modified Palmgren-Miner hypothesis with k′= 8 against the allowable damage sum D al = 0.3 using the Woehler-line for pure bending with R = -1. The experimental and calculated fatigue-life lines for fully reversed and pulsating constant and variable amplitude loadings are displayed in Figs. 4 and 5. The application of the NSH reflects the life increase due to the non-proportional combined loading. Except in one case, all results are on the safe side, by up to a factor of about 3 in life. The unsafe result for fully reversed out-of-phase spectrum loading differs by a factor of about 2 from the experimental outcome. Regarding the stresses, the calculated values are up to a factor of about 1.25 on the safe side and, in the unsafe case, by a factor of about 1.10. In addition to this outcome, it can be observed that neither the slopes nor the knee points of the calculated fatigue-lines are in accordance with the experimental ones. The calculated results are driven by the properties of the starting Woehler- lines and the experimental results by the combination of local normal and shear stresses. Also, the comparison of the Woehler-lines for pure bending and pure torsion indicated the problem by their different slopes and knee points. This was also observed in other investigations [15] and particular modified Woehler-lines for overcoming these different influences on these properties were derived for the assessment. However, because of space limitations this will not be presented here. S UMMARY AND CONCLUSIONS he investigations carried out with component-like specimens of the cast aluminium alloy EN AC-42000 T6 (A 356 T6, G-AlSi7Mg0.3 T6), show that, under non-proportional normal and shear stresses fatigue life is increased in contrast to ductile steels where life is reduced due to changing principal stress directions. Because of the low ductility of this cast alloy (e < 10%) compared to ductile quenched and tempered and structural steels, normal stresses are considered to be the main damage driving property, suggesting the application of the NSH. Fatigue lives under uni- and multiaxial spectrum loadings were estimated by the modified Palmgren-Miner-Rule using the allowable damage sum D al = 0.3. For all investigated stress states under multiaxial constant and variable (Gaussian spectrum) amplitudes without and with mean stresses, the NSH was able to depict the life increase by the non-proportionality and, for most cases, delivered conservative but non-exaggerated results. Except for one case, all results were on the safe side by up to a factor of approximately 3 in life. The one unsafe result, for fully reversed out-of-phase spectrum loading, differed by a factor of approximately 2 from the experimental outcome. Regarding the appertaining stresses, the calculated ones are up to a factor of about 1.25 on the safe side and, in the unsafe case, by a factor of about 1.10. In design practice safety factors j σ for safety components of vehicles are in the range of about 1.7 to 2.2 [8] and cover the mentioned under- or overestimation of supportable stresses by the NSH. If, in final durability proof tests, the fatigue life should be insufficient, then the required duration can be adjusted by simple geometrical modifications, e. g. by lowering local stresses by the use of larger radii, if this is not possible, by increasing thickness. As, for most safety components, the dominant loading partition is bending rather than torque or axial loading, to compensate the above mentioned factor of 1.10 resulting from the unsafe calculation, an increase of thickness by the square root of this factor, i.e. 1.05, 5 % only, would be sufficient. In the case of the conservative factor of 1.25 under predominantly bending loading, thickness could be reduced by factor of 1.12. Because of the advantage of the T