Issue 41

S.M.J. Razavi et alii, Frattura ed Integrità Strutturale, 41 (2017) 440-446; DOI: 10.3221/IGF-ESIS.41.55 442 d = 4 mm  =1.0 mm  20 90° d = 4 mm  =4.0 mm  20 Figure 1: Geometry of V-notched specimens and semicircular specimens. The specimens were tested under pure tension, pure torsion and multiaxial tension-torsion loading with different biaxiality ratios (λ = τ a /σ a =0.6 and 1.0). In particular, ten fatigue test series were conducted, according to the following subdivision: 1)Two series of tests on notched specimens (V and semicircular notches) under pure torsion fatigue loading (nominal load ratio R = -1); 2)One series of tests on V-notched specimens under pure tension fatigue loading (nominal load ratio R = -1). 3)Two series of tests on V-notched specimens under combined tension-torsion fatigue loading, under constant biaxiality ratio λ = 1, load ratio R = -1, load phase angle φ = 0° or φ = 90°; 4)Two series of tests on V-notched specimens under combined tension-torsion fatigue loading, under constant biaxiality ratio λ = 0.6, load ratio R = -1, load phase angle φ = 0° or φ = 90°; 5)Two series of tests on semicircular specimens under combined tension-torsion fatigue loading, under constant biaxiality ratio λ = 1, load ratio R = -1, load phase angle φ = 0° or φ = 90°; 6)One series of tests on semicircular specimens under combined tension-torsion fatigue loading, under constant biaxiality ratio λ = 0.6, load ratio R = -1, load phase angle φ = 0°. F ATIGUE TEST DATA ON V- NOTCHED AND SEMICIRCULAR SPECIMENS efore being tested, all specimens have been polished in order to both eliminate surface scratches or machining marks and to make the observation of the fatigue crack path easier. Fatigue tests have been carried out on a MTS 809 servo-hydraulic biaxial machine with a 100 kN axial load cell and a torsion load cell of 1100 N.m. All tests have been performed under load control, with a frequency ranging from 1 and 10 Hz, as a function of the geometry and load level. At the end of the fatigue tests, the notch root and the fracture surfaces were examined using optical and electronic microscopy. Some examples of fracture surfaces are visible in Fig. 2a and 2b for in phase and out of phase multiaxial loading. The results of statistical analyses carried out by assuming a log-normal distribution are summarised in Tab. 3. In particular, it is summarised the mean values of the nominal stress amplitudes at different number of cycles, the inverse slope k of the Wöhler curves and the scatter index T, which quantifies the width of the scatterband included between 10 and 90% probability of survival curves. All failures from 10 4 to 5x10 6 have been processed in the statistical analysis whereas the run-outs were excluded. Fig. 3 depicts fatigue data from V-notched specimens tested under tension, torsion and combined tension and torsion with different load phases. In this case the biaxiality ratio is equal to 1. It is visible from the figure that the out of phase loading causes a detrimental effect on the fatigue life of the V-notched specimens. This effect is not observed in the case of semicircular notches where the most damaging case is the in-phase loading. By changing the biaxiality ratio from 1 to 0.6 on V notched specimens tests, the effect of out of phase is again present. Summarising this aspect it seems that the phase displacement could increase or vice versa decrease the fatigue life of the component as a function of the notch geometry. B