Issue 43

F.Z. Seriari et alii, Frattura ed Integrità Strutturale, 43 (2018) 43-56; DOI: 10.3221/IGF-ESIS.43.03 48 Figure 2: Applied spectrum form: (a) constant amplitude loading with single overload (b) with band overload. C n p’ q’  K th at R=0 1.71  10 -10 3.353 0.5 1 2.857 Table 3: Parameters of fatigue crack growth according to Nasgro equation. R ESULTS & D ISCUSSION his section provides and discusses the obtained results including the effects of variable amplitude loading characterized by single overload ratio and band overload on fatigue life and fatigue crack growth rates respectively in cracked unpatched plate and cracked patched composite plate. Also, we investigated the effect of patch repair under constant and variable amplitude loading and presented the result in the following subsections. Combined effect of overload ratio and patch repair on the crack growth ig. 3 presents the fatigue life response (crack length vs. cycle number) for unpatched 2024 T351 Al-alloy specimen, pre-cracked to a length of 3 mm, under spectrum block (Fig. 2a) characterized by the presence of single overload and stress ratio R=0.2. From the presented, we notice a no effect for OLR = 1.5 [42]. But from ORL=1.8 to 2.0, we notice a little effect on fatigue life. At high overload ratio (ORL=2.4), we show significant effect on fatigue life compared to the others overloads ratios and constant amplitude loading. The retardation cycles N d between applied spectrums with constant amplitude loading and with single overload ratio is about 1.4  10 4 cycles. Figure 3: Overload ratio effect on fatigue life for unpatched 2024 T351 Al-alloy T F