Issue 37

M. Margetin et alii, Frattura ed Integrità Strutturale, 37 (2016) 146-152; DOI: 10.3221/IGF-ESIS.37.20 150 presented by Findley (N f_find ) [3] and McDiarmid (N f_mcd ) [4] Eqs 9 and 10. For the calculation of the fatigue life time, material parameters for regression line of fatigue curves were used (Tab. 2). Material parameters used in Eqs 9 and 10 are shown in Tabs. 1, 2 and 3.        b * τ eq a fin n f f max τ τ k σ τ 2N (9)            b ' mcd τ eq a n,max f f u k τ τ σ τ 2N 2σ (10) k fin [-] τ f* [MPa] k mcd [MPa] 0.131 555 244 Table 3 : Material parameters for Eqs 9 and 10 n. τ a [MPa] σ a [MPa] N f_exp N f_com N f_find N f_mcd 1 159 204 650800 643902 485504 291483 2 180 204 172700 202990 154283 94670 3 167 204 597300 415031 313706 189936 4 209 163 105300 96899 65261 42766 5 183 163 536120 449372 283251 181345 6 196 163 198810 205010 133788 86699 7 209 122 193800 165247 113010 77586 8 201 122 301140 267920 178905 122110 9 193 122 375630 441567 287341 194911 10 185 122 996040 740543 468583 315769 11 209 82 231490 226301 178946 129725 12 201 82 572500 375732 291264 210195 13 193 82 952600 635892 482358 346420 14 185 82 2000000* 1098588 813741 581393 Table 4 : Experimental data. Looking at the table, it's evident that estimations using Findley's or McDiarmid's criteria are too conservative for the specimen material and the loading levels we used. At the same time, comparing these two criteria, Findley's criterion has smaller deviation from the experimentally acquired data. Figure 2 shows comparison of calculated and experimentally acquired life times for the proposed hypothesis. For each specimen, N f_cal are listed in the chart shown in a probabilistic form for the regression line and the lower and the upper prediction reliability intervals of the material parameters (Tab. 2). Chart shows that the proposed hypothesis correlates well with the experimentally acquired values of the fatigue life time. Majority of the experimentally acquired life times are placed within the reliability interval of the estimated life times. Experimentally acquired life times of specimens number 3,10,12, 13 and 14 were outside of the reliability interval. For these cases, the criterion provided conservative results. The specimen number 14 was put aside after going through 2 .10 6 loading cycles. Based on the experimental verification of the proposed hypothesis (Figure 2) and on its comparison with the well known hypotheses (Tab. 4), following can be stated:  The hypothesis correlates well with the experimentally acquired data - the majority of measured life times are placed within the prediction interval of the calculated life times.  In case the hypothesis doesn't provide correct results (experimentally acquired life time is outside of the prediction interval of the calculated life time), the results are on the conservative side of the calculation.