Abstract—In this investigation variation of cyclic loading effect on fatigue crack growth is the studied. This study is performed on 2024 T351 and 7050-T74 aluminum alloys, used in aeronautical structures. The propagation model used in this study is NASGRO model. In constant amplitude loading (CA), effect of stress ratio has been investigated. Fatigue life and fatigue crack growth rate were affected by this factor. Results showed an increasing in fatigue crack growth rates (FCGRs) with increasing stress ratio. Variable amplitude loading (VAL) can take many forms i.e. with a single overload, overload band… etc. The shape of these loads affects strongly the fracture life and FCGRs. The application of a single overload (ORL) decrease the FCGR and increase the delay crack length caused by the formation of a larger plastic zone compared to the plastic zone due without VAL. The fatigue behavior of the both material under single overload has been compared. Keywords—Fatigue crack growth, overload ratio, stress ratio, generalized willenborg model, retardation, Al-alloys. I. INTRODUCTION ATIGUE crack growth behavior of metals depend upon a number of variables namely the mechanical properties and microstructure, specimen, environment, applied cyclic loading, stresses and strains acting at the crack tip. Most of fatigue research has been concentrated on examining the phenomena under constant amplitude fatigue cycling for aluminum alloys [1]-[4]. During service, mechanical and aeronautical structures are subjected mostly to complex cyclic loading. It is well known that load fluctuations lead to fatigue crack propagation, the rate of which depends on the interaction of loads or stresses. The simplest case for the spectrum loading is when single and multiple peak overloads are applied to constant amplitude loading. Research on variable applied loading (VAL), determined that appreciable crack growth retardation can occur following tensile overloading [5]-[7]. A numbers of models have been developed to account for crack growth retardation due to tensile overloads [8]-[11] namely Willenborg, Wheeler model, Gallagher modified Willenborg model [12]. Crack growth retardation due to tensile overloads has been explained by several theories. The most commonly discussed theories are M. Benachour is with the University of Tlemcen, IS2M Laboratory of Tlemcen, Faculty of Technology, BP 230, Tlemcen, Algeria (phone: +213 43 28 56 89; fax: +213 43 28 56 85; e-mail: mbenachour_99@yahoo.fr). M. Benguediab is with the university of Sidi Bel Abbes, LMSR Laboratory, BP 89, City Larbi Ben Mhidi, Sidi Bel Abbes, 22000, Algeria, (e-mail: benguediab_m@yahoo.fr ). N. Benachour, is with the Physics Department, University of Tlemcen, Faculty of Sciences, IS2M Laboratory, Algeria, (e-mail: nbenachour2005@ yahoo.fr). fatigue crack closure [13]; residual stresses [8]-[9], crack tip blunting and sharpening [14] and cyclic strain hardening and softening [15]. Really, all mechanisms are not dissociable. Overload retardation has been widely investigated in a range of engineering materials [4], [13], [16], [17] and many research’s were oriented to the study of several form of variable amplitude and associated parameter namely single or block overloading on fatigue behavior of aluminum alloys. Fatigue crack retardation due to variable amplitude loading spectra was studied in 7075 T6511 aluminum alloy by Corbly and Packman [18]. It was shown that the degree of retardation depend strongly on the relative amplitudes of the peak stress intensity, the number of stress applications N t at the peak stress intensity, the magnitude of the constant amplitude crack growth rate at the lower stress intensity range and the number of fatigue cycles N I at the lower stress intensity level after the last peak stress is applied. The influence of overload ratio has been investigated primarily in aluminum alloys for the aerospace industry. In the investigation of Vardar [19], overload ratios between 1.3 and 2.4 were considered in a 7075-T6 alloy under plane strain conditions. A linear correlation was found between the number of retardation overload cycles and the overload ratio. In fatigue crack growth investigation conducted by Bathias and Vancon [20] on 2024 and 2618 aluminum alloy, fatigue crack growth rate was retarded after application of one or several overload. In this study, it was demonstrated that the process of fatigue crack retardation by application of overloads results from the plastic deformation at the crack tip and the nature of the test specimen surface. Plastic zone diameter and the retardation relationships depend on toughness, on the metal cyclic strengthening and cyclic plastic deformation. The delayed retardation phenomena after single overload in three steels and two aluminum alloys were investigated by Matsuoka and Tanaka [21]. It was confirmed that the model proposed early by Matsuoka et al. [22] was in good agreement with the experimental data for these materials when the stress state at the overloading was satisfied with the small scale yielding condition. Effect of overload on fatigue crack growth studied by Robin and Pelloux [23] was performed on a 2124 T351 aluminum alloy. The results showed that crack retardation near the surface of the specimen was greater than in the plane strain region near the center and Wheeler’s and Willenborg’s models of were found to provide a fair approximation of the retardation phenomenon. Recently, evaluation of retardation in fatigue life due to application of a single overload was conducted by Hairman Effect of Single Overload Ratio and Stress Ratio on Fatigue Crack Growth M. Benachour, N. Benachour, M. Benguediab F World Academy of Science, Engineering and Technology International Journal of Mechanical and Mechatronics Engineering Vol:7, No:12, 2013 2542 International Scholarly and Scientific Research & Innovation 7(12) 2013 ISNI:0000000091950263 Open Science Index, Mechanical and Mechatronics Engineering Vol:7, No:12, 2013 publications.waset.org/9996857/pdf