Issue 43

F.Z. Seriari et alii, Frattura ed Integrità Strutturale, 43 (2018) 43-56; DOI: 10.3221/IGF-ESIS.43.03 49 Figure 4: Overload ratio effect on fatigue crack growth rate for unpatched 2024 T351 Al-alloy The results shown in Fig. 5 indicate that the fatigue life of repaired specimens was improved for all overload ratios. It noticed clearly that the fatigue lives for all overloads ratios were affected highly by composite patch repair (Fig. 5) compared to the unpatched plate (Fig. 3). Also, at low overload ratio (ORL=1.5) the same effect of overload ratio comparatively to the unpatched plate. At high overload ratio (ORL=2.4) with the presence of patch, the fatigue life is increased from 1.18  10 5 cycles for constant amplitude loading to 2.27  10 5 cycles for ORL=2.4 witch represents an increasing ratio in fatigue life of 2 times. Fig. 6 presents the evolution of the crack growth rate (da/dN) as a function of the crack length “a” for repaired samples under block loading with single overload for the stress ratio R=0.2. At low crack length (a < 7 mm), we show instantaneous delay for ORL=2.0 and 2.4. The decrease in fatigue crack growth rate (da/dN) occurs at a length of 5 mm and varies from 8.95  10 -8 m/cycle in CAL to 4.8  10 -8 m/cycle and 1.90  10 -8 m/cycle respectively for overload ratios “2” and “2.4”. At this stage, patch gives significant rigidity. For ORL=2.0 and at crack length “a=7.7 mm”, deferred delay was shown. But for ORL=2.4, the presence of retardation is characterized by deferred delay in various crack length (a= 8.66; 11.10; 14.1; 18.0 and 28.37). At crack length “a=28.37”, the maximum crack growth rate does not exceed the value of 1.12  10 -4 m/cycle at application of overload ratio and decrease to 6.7  10 -8 m/cycle. For ORL=1.8, instantaneous delay is predominant until crack length “a=34 mm” with low effect crack growth rate. For all overload ratios, fatigue crack growth rate keep to constant amplitude fatigue crack growth rate from crack length “a=37 mm” and patch repair effect of disappeared. In detail, Figs. 7 and 8 present respectively comparison in fatigue life and crack growth rates between repaired and unrepaired 2024 T351 Al-alloy plates for overload ratio ORL=2.4. In unrepaired plate under application of the spectrum with single overload, the number of delay cycles “N D ” due to overload is about 1.25  10 4 cycles and retarded crack length, “a d ”, is 2.97 mm. But in repaired Al-alloy plate by Boron/Epoxy, the total number of delayed cycles, “N D ”, is about 19.3  10 4 cycles. This result represents a ratio of improvement in fatigue life about 15.5 times in presence of patch repair comparatively to unrepaired plate. Also, the retarded crack length, “a d ”, is about 26.5 mm influenced conjointly by applied spectrum with single overload and patch repair. Additionally to the combined effect of overload ratio and patch repair given in Fig. 6, Fig. 8 gives comparisons between fatigue crack growth rates in repaired and unrepaired alloy plates under spectrum with single overload (ORL=2.4). In unrepaired plate, is noticed a presence of single instantaneous delay at crack length “a=10.9 mm”. Fatigue crack growth rate is increased from 9.02  10 -7 to 5.52  10 -8 m/cycle. At same crack length “10.9 mm” in repaired plate, the curves in Fig. 8 demonstrate that the crack growth rate decrease with differed delay case and leads to 2.26  10 -8 m/cycle. The ratio in delayed crack growth rate is 2.44 times. In general, a significant reduction in crack growth rate after 5 mm of crack length is shown. Crack growth rates ratio between unrepaired and repaired plates in constant amplitude phases varies from 2.5 to 16.6 times. The high percentage of the beneficial effect in fatigue crack growth rate is related to the effect of patch repair characterized by the reduction in stress intensity factor [43] compared to the effect of variable amplitude loading with single overload.