Issue 51

N. Benachour et alii, Frattura ed Integrità Strutturale, 51 (2020) 45-51; DOI: 10.3221/IGF-ESIS.51.04 49 hydraulic machine "MTS 810" with capacity 100 KN. Fatigue tests were conducted at R-ratio (R = 0.1) with variation of the levels of amplitude loading (P max =2.755 ; 3.31 ; 3.99 KN). Figure 5: V-notch specimen in four points bending tests in 2024 T351 Al-alloy R ESULTS AND DISCUSSION he effect of mean stress on fatigue crack is characterized either by variation of R-ratio or amplitude of loading. This work presents the effect of the amplitude loading on fatigue crack initiation in 2024 T351aluminum alloy. Tab. 2 summarizes the lifetimes of initiation crack for a crack initiation length between 170 and 180  m. The considered lengths are in the order of the average grain size of studied material. The applied cyclic loading  P is characterized by the amplitude loading where the maximal amplitude loading for the three tests are respectively P max =3.99 KN, 3.31 KN and 2.755 KN. It should be noted that the lifetime of the crack initiation decreases with increasing in the amplitude loading for the same order of magnitude of the detected length. The transition in the amplitude loading  P from 2.48 to 2.98 KN has reduced the number of cycles of 22.000 cycles to 12,800 cycles. This represents a decrease of 1.72 times. According to the high applied load, the lifetimes in crack initiation are negligible comparatively to fatigue life in crack growth stage where the percentage in lifetime varies from 3% to 6.6% [26]. The greatness in results was confirmed by Ranganathan et al. [14] under a high load (3%). Fatigue tests P max (KN)  P (KN) a i (  m) Ni (cycles) S1 2.755 2.480 175 22000 S2 3.310 2.980 170 12800 S3 3.990 3.591 180 3000 Table 2: Effect of amplitude loading on fatigue initiation life at R=0.1 for 2024 T351 Al-alloy In order to establish fatigue criterion to predict crack-initiation at the tip of a notch in aged hardening aluminum alloy from experimental results, an attempt is conducted for critical length from notch tip “d=175  m” where the evolution in opening stress xx     for three amplitude loading determined analytically and numerically (Figs 3 and 4). Analytical results of opening stress xx     are used for curves given the fatigue initiation criterion. The curve giving the opening stress versus number of initiation cycles at critical distance d=175  m is shown in Fig. 6. The criterion for fatigue crack initiation thus obtained is given by Eqn. 2 where “ xx     ” is expressed in MPa and “N i ” in cycles. This criterion remains valid for cycle numbers between 10 3 and 3  10 4 cycles.   0.209 2761.1 xx i N   (3) T