Issue 43

F.Z. Seriari et alii, Frattura ed Integrità Strutturale, 43 (2018) 43-56; DOI: 10.3221/IGF-ESIS.43.03 44 the fracture-critical D6AC steel wing pivot fitting of the F-lli bomber [4]. This technique offers significant advantages over traditional repair methods “riveting, fastening, and welding” [5]. In literature, several studies have investigated this technique, both numerical and experimentally, for different specimen’s dimensions, and which have the following advantages: provides a high structural efficiency and extends the life of cracked structural components at an economical cost, and reduce the stress field near the crack, leads to retardation or complete arrest of the crack growth [3, 5-7], hence, as a result, crack growth is delayed and the service life of the repaired structures is extended. Fortunately, the presence of humidity presents harmful effect on fatigue behavior of adhesive joints bonded with aluminium [8]. In patched zone, the composite patch and aluminum alloy present a bi-material, also the interface assembly presents shielding in repaired material [9] and increase the fatigue and decrease the fatigue crack growth rate. Sabelkin et al. [10] performed an experimental fatigue investigation on 2024 T3 aluminium alloy reinforced with composite patch Boron/Epoxy under constant amplitude loading. The results shown that bonded composite patch repair increase fatigue life about fivefold in the case of stiffened panels while it increased about ten fold in the case of un- stiffened panels. In absence of patch repair Negru and al. [11] have predicted the fatigue life in 2024 T3 aluminium alloy notched specimens (v-notch and semi circular notch) using three method based on finite element method. In the fatigue investigation conducted by Salehi-Khojin et al. [12] on 6.125 mm cracked 7075 Al-alloy without patch repair and repaired with 4-ply boron/epoxy composite patches of several geometries under constant amplitude loading (R=0.1). The results show that the propagation lifetime of the four kinds of patched (repaired) specimens is improved greatly compared to the unpatched Al-alloy plate and the fatigue crack growth depends on dimension of patch along the path of crack. The study of fatigue crack growth under constant amplitude loading in repaired aluminium alloy plate 6061 T6 by carbon-epoxy composite patch has shown that the fatigue life has significantly increased. The applied patch has provided about a 100-110% improvement in the fatigue life and a 30-35% decrease in the stress intensity factor [13]. Performance for a composite repair patch to prolong the service life of pre-cracked of some aluminium alloys plates under fatigue conditions in a corrosive environment and constant amplitude loading was conducted by Schubbe et al. [14]. The Boron- Epoxy used for repairs of all Al-alloys plates, showed a positive life improvement comparatively to Graphite-Epoxy. Several experimental research on fatigue crack growth of patched structures were oriented to the fatigue tests under constant amplitude loading including effect of stress ratio, amplitude of loading, number of ply’, geometry of patches…i.e. [15-19]. It is recognized that aeronautics structures were subjected to variable amplitude loading, essentially characterized by overload and underload [20]. Research on variable applied loading (VAL), determined that appreciable crack growth retardation can occur following tensile overloading for unpatched specimens [21, 22]. In recent study, Wang et al. [23] have investigated the effect of single and block loading on fatigue crack growth rates of 2024 T4 Al-alloy unpatched CT specimen. It is concluded that single tensile overload introduced in the constant amplitude loading causes a significant retardation of crack growth and similar retardation of crack growth occurs for three-step sequence loading. In repaired of crack specimens, few investigations were conducted to shown the simultaneous effects of patch repair and variable amplitude loading (single overload, stepped sequence loading, underload…). Albedah et al. [24] have investigated experimentally the associated effects of patch repair of cracks of plate in aluminum alloy 7075 and stepped variable amplitude loading on fatigue life. A small improvement on fatigue life is noticed in decreasing of loading amplitude blocks. Recent study on fatigue crack growth of patched 2024 T3 V-notch specimen was conducted by the same authors [25] under applied of two stepped block (increase/decrease and decrease/increase). A retardation effect was observed for decreasing blocks loading in unrepaired (unpatched) specimens. However, this retardation effect is attenuated by the presence of the patch which leads to lower fatigue life for repaired (patched) specimens. In the case of a bonded repair, different mechanisms may be involved in prediction of fatigue crack growth under variable amplitude loading (VAL) [26, 27]. It is recognized that peak loads might cause debonds under the patch, resulting in an increased crack growth rate. It was concluded that the effects of peak loads on bonded repairs presents a similar effects of peak loads on unrepaired structures; the crack shows retardation, and no debonds after application of the peak loads were found [27]. Fatigue crack growth of fibre reinforced metal laminates (Glare) under constant amplitude loading following a single overload was studied experimentally by Wu and Guo [28]. In this investigation, the mechanisms for the effect of a single overload on the crack growth rates and the delamination growth rates were identified. Fatigue tests on bonded Glare repairs were performed at room temperature in C-5A Galaxy fuselage fatigue spectrum [29]. As can be concluded from obtained result with fatigue spectrum, small load cycles are less important for repaired specimens than for unpatched specimens. The present work is aiming at performing a combined experimental fatigue crack growth data of 2024 T351 Al-alloy, analytical integrated models of variable amplitude loading (Generalized Willenborg model) and composite patch repair (Ratwani model) for investigation of fatigue behaviour of repaired edge cracked plate. The study of the fatigue behaviour is conducted on Al-alloy edge-cracked plate reinforced by a one-side composite patch. Applied spectrum characterized by constant amplitude loading, single or band overloading were investigated.