Issue 37

N.R. Gates et alii, Frattura ed Integrità Strutturale, 37 (2016) 160-165; DOI: 10.3221/IGF-ESIS.37.22 164 of the parameter. This is especially evident in the comparison between fully-reversed axial and torsion loading conditions in Figs. 1(a) and 1(b). In order to further evaluate the characteristics of the proposed modified FS parameter, a limited number of fatigue tests were also performed in this study using specialized load paths meant to differentiate between the two different versions of the parameter. These tests featured loading conditions which result in the same damage value based on the original FS parameter, but different damage values based on the modified parameter. Differences in experimental lives were compared to predictions to determine which parameter more closely reflects the fatigue damage variation between loading paths. These tests were conducted using a triangular shaped load path in shear versus axial stress space. This path, along with its corresponding stress-time history, is shown in Fig. 2(a) based on normalized axial and shear stress variations. Figure 2 : (a) Triangular load path in terms of stress-time history and shear vs. axial stress path, and (b) experimental and predicted fatigue life for discriminating load path tests. In analyzing potential load paths, it was found that by changing the ratio of applied shear to axial stress, relatively large differences in predicted fatigue damage could be obtained between the original and modified FS parameters. Therefore, the same loading path was used in these tests, but with two different ratios of applied shear to axial stress, λ = τ / σ = 2 and λ = 0.5. Stress values were carefully selected so that each path would result in the same fatigue damage value according to the FS parameter. Additionally, experimental results from fully-reversed torsion tests with static tensile stress were also available for comparison at the same damage value. This additional loading path allows for the evaluation of damage predictions for cycles containing different load-time interaction between shear and normal stress components. The results of each of these tests are shown in Fig. 2(b) along with life predictions from both the FS and modified FS damage parameters. Results from the triangular load path tests (Tri), at both nominal stress ratios, along with results from torsion with static tension loading (STSA), are included. A k value of 1 and uniaxial fatigue life properties were used for all analyses. In this figure, gray columns represent the average experimental fatigue life from two duplicate tests, while error bars indicate the individual life of each test. From these results, it is clear that variations in experimental fatigue damage exist between the different loading conditions. While the FS parameter in its original form does not account for these differences, the modified form of the parameter reflects the observed differences in fatigue life relatively well for each loading path considered. Given the improvements in life predictions obtained when using the modified FS parameter to calculate fatigue damage, one final step was to analyze the remaining fully-reversed fatigue data generated for the 2024-T3 aluminum alloy tested in this study. These data, plotted against the original FS parameter in Fig. 3(a), are presented again in Fig. 3(b) with damage calculated using the modified form of the parameter. Similar to the 7075-T651 data, it can be seen from these figures that the modified FS parameter provides somewhat improved fatigue life correlations for the 2024-T3 data. This is, again, evidenced by the slightly tighter grouping of test data from all loadings conditions. Overall life prediction accuracy, however, is similar to that obtained using the original form of the parameter. S UMMARY AND CONCLUSIONS n this paper, the effects of mean stress and normal-shear stress interaction on multiaxial fatigue damage were investigated. Based on literature data for 7075-T651 aluminum alloy, the Fatemi-Socie critical plane damage parameter was found to produce non-conservative life predictions in cases where significant tensile mean stress was present. While increasing the k value can improve uniaxial mean stress data correlation, this results in worse predictions I (a) (b)