PSI - Issue 19

L.C. Araujo et al. / Procedia Structural Integrity 19 (2019) 19–26 L.C. Araújo et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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The tests were conducted in the laboratories of the Department of Mechanical Engineering of the University of Brasília. The equipment used was a servo-hydraulic axial-torsional fatigue testing machine (Fig. 2 (a)) equipped with 100 kN and 1100 Nm load cell operating between 5 and 15 Hz and capable of applying combined loading in phase and out of phase. During the tests the ambient temperature was controlled between 20 and 23 °C. All the tests were conducted in load control with fully reverse sinusoidal load, that is, with loading ratio R of -1. The values of the shear stress and normal stress ratio used were, / , of 0, 0.5, 1, 2 and ∞ . The tests with combined loading were conducted in-phase, that is, with phase angle, , of 0. The complete rupture of the specimen was considered as failure, as seen in Fig. 2 (b). The stop criterion was when a specimen did not fail when reaching 2 × 10 6 cycles.

Figure 2. Axial-torsional equipment used, (a) axial-torsional test system, (b) test mount detail with a broken specimen.

6. Comparison of predictions with the experimental results The values of the fatigue limit obtained from the √ parameter with Eq. 12 and 13 are shown in Table 2. They were estimated considering two volumes of material to exemplify the possibility of considering the size of the component for mechanical designs. The first volume refers to the useful section of the specimen used during the tests and the second represents the crankshaft from which the material for producing the specimens was removed. Table 2. Results of statistics of extreme value theory applied to inclusion’s √ . Prediction Volume Volume ( 3 ) (MPa) (MPa) √ ( ) at 90° √ ( ) at 45° Cumulative distribution (%) 1 Specimen 2.4 × 10 3 360 301 18.5 27.6 99.893 2 Crankshaft 7.9 × 10 9 311 260 63.5 65.4 99.9999996 Fig. 3 shows the fatigue limit prediction obtained from the Crossland ’s model and the experimental results of uniaxial, torsional and combined in-phase loads. The √ parameter values used in the calibration of the models for the comparisons made below correspond to the volume of the specimen shown in Table 2. Error bands of 5% where drawn on the graphs for comparison. At least two tests were performed at each stress levels, because of this there is overlap in the experimental points, which is evidenced where a failure and a run-out at the same stress level was obtained.