Issue 38

T. Morishita et alii, Frattura ed Integrità Strutturale, 38 (2016) 281-288; DOI: 10.3221/IGF-ESIS.38.38 283 E XPERIMENTAL RESULTS AND DISCUSSION Failure Life ab. 1 and Fig. 3 show the test results and the failure life. The failure life is affected largely by not only strain path (strain non-proportionality) but also strain waveform (creep damage). In the PP test, the failure life is reduced with the decrease in the strain rate under the same strain range, which may be resulting from larger creep damage caused by the lower strain rate. On the other hand, in the CI test, the failure life is increased when the strain rate is reduced from 0.2%/s to 0.01%/s, but the failure life is decreased by the further reduction in the strain rate from 0.01%/s to 0.002%/s. The former may be resulting from the reduction in the intensity of strain non-proportionality caused by stress relaxation due to creep and the latter can by the increase of creep damage. Strain path Waveform Strain rate (%/s) Holding time (s) Δε eq (%) N f (cycles) Push-pull PP-FF 0.2 0 0.5 2,757 1.0 680 PP-SS 0.01 0.5 1,980 1.0 501 PP-SS* 0.002 0.5 1,780 0.7 945 1.0 323 PP-TH 0.2 180 0.5 2,389 1.0 543 600 0.5 2,442 1.0 523 PP-CH 180 0.5 1,718 1.0 598 600 0.5 1,267 1.0 486 Circle CI-FF 0.2 0 0.5 870 1.0 240 CI-SS 0.01 0.5 1,452 1.0 378 CI-SS* 0.002 0.5 682 0.7 325 1.0 164 CI-TH 0.2 180 0.5 1,680 1.0 268 600 0.5 1,190 1.0 302 Table 1 : Test conditions and results. Fig. 4 shows a correlation of the failure life by von Mises’ equivalent strain range  eq . In the figure, a bold solid line is drawn based on the data in the PP-FF test which is the life curve for life evaluation and two thin lines show a factor of 2 band. The bold solid line is given by 6.0 f 12.0 f eq Δε     BN AN (1) where the coefficients A and B are defined as 3.5  B / E and  f 0.6 according to the definition of the universal slope method [7]. In this study, B is put as the curve to be fit with the data in the PP-FF test. This figure shows clearly a large reduction in the failure life in the CI test compared with the PP test according to the non-proportional loading effect. T