Issue 50

A. Salmi et alii, Frattura ed Integrità Strutturale, 50 (2019) 231-241; DOI: 10.3221/IGF-ESIS.50.19 235 When the crack extends over ten or more grains, the influence of the material structure on the growth of the crack becomes negligible and the theory of mechanics of linear elastic fracture can be applied later [19]. In this simple form, the presence of a growth threshold of fatigue cracks and a limit greater than ∆K (stress intensity factor range) for the fracture are not shown, although, if appropriate, expressions taking into account these limits, as well as the influence of the load ratio of the cycle R = P min / P max can be easily found in the literature. L OAD RATIO INFLUENCE ig. 2 shows the curves representing the cracking rate versus the stress intensity factor range for the different load ratios studied. Fig. 2 indicates that the propagation rates vary according to the load ratio. The load ratio effect is very important, in fact, the propagation rate at R=0.3 is much higher than at R = 0.1 in the range 19.07 MPa√m<∆K<37.55 MPa√m. For example, for ∆K = 23.55 MPa√m, the propagation rate for R = 0.1 is about 9.89E-5 m/cycle, while it is more than twice as high for R = 0.3 (da/dN =1.11E-4 m/cycle). The same observations can be made if the different load ratios R = 0.3 and R=0.4 are compared. R = 0.4 and R = 0.5 and finally R = 0.5 and R = 0.7. Figure 2: Load ratio influence on crack propagation The impact of the load ratio R (0.1, 0.3, 0.4, 0.5 and 0.7) was verified. Indeed, the load ratio effect has a very significant impact on the crack propagation rate of the 2024 T3 aluminum alloy. To study the influence of the load ratio on the propagation time of a fatigue crack, Fig. 3 shows the propagation of a 5 mm crack as a function of the number of cycles for load ratios 0.1, 0.3, 0.4, 0.5 and 0.7 under a maximum stress of 118 MPa. Figure 3: Propagation time of a 5mm crack for different load ratios. F