Issue 52

W. N. Bouzitouna et alii, Frattura ed Integrità Strutturale, 52 (2020) 256-268; DOI: 10.3221/IGF-ESIS.52.20 257 I NTRODUCTION amage to metal aircraft structures is often caused by corrosion, erosion, normal stress, and accidents and mishaps, which made the component susceptible to cracking in service. The replacement of the cracked component can be very costly, thus it’s important that metal structural repairs be made according to the best available techniques to expand its service life and investigate the satisfactory performance of an aircraft in a rapid way reducing the immobilization time of the device structure. Numerous and varied crack repair techniques have been proposed for arresting or retarding crack propagation in structural components such as: i) the use of bonded composite patch repair, A Baker and al [1] explain the concept of using bonded composite materials as a mean to maintain aging metallic aircraft. The reinforcements of composite patch repairs reduce stresses in the cracked region and keeps the crack from opening and therefore from growing; ii) The infiltration method, which is based on reinforcing the cracked body by depositing closure material like epoxy resin along the crack path. [2] stated that crack growth retardation is promoted by through the infiltration of closure materials into a crack, and the closure material properties play a dominant role in promoting the crack closure and the retardation. iii) drilling a hole at the crack tip is one of the easiest and most accessible crack arresting procedures. Cracking resumes very shortly after drilling because of the high stress concentration associated with hole and this is why it remains temporary and relatively ineffective method [3, 4]; iv) a much superior approach is to stop drilling and then expand the stop hole, usually with a special sleeve to develop favorable compressive stresses that reduce or prevent crack opening, while this approach is often highly effective in stopping crack growth [4–6]. [5] studied the influence between the holes of different diameters and also between the expanded holes and non-expanded holes on arresting the crack propagation, finding that the number of cycles to initiate a new crack is three times longer than non-expanded holes; v) repair of corrosion types such as pitting or exfoliation in aluminum alloy structures generally involves the removal of visible damage by grinding followed by an extra confidence cut to ensure that all the corrosion is removed. The region is then treated, primed and painted. In the case of severe corrosion, panel thickness may be reduced below the allowable thickness and must be reinforced to make it airworthy [7–9]; vi) the overload repair is applied to the metallic structure in order to create a plastic zone over the crack-tip, this zone generates a residual compressive stress that leads to a decrease in the velocity of the crack growth, [10], that is explain by the closure of the crack caused by the plastic zone generated [11–14]; vii) another method based on the application of residual compressive stresses to reduce the crack growth rate is to press a steel ball in the crack-tip. [15,16], studied this phenomenon by leaving a Brinell-type dimple of a certain diameter and showed that the greatest arrest of crack growth due to the compressive stresses was caused by forming the dimple. It is obvious that each repair procedure has advantages and disadvantages; this paper focuses on the requirement of hybrid repair method by combining a bonded patch with drilled stop hole, that enhances the residual strength of the damaged structures. The combination between two techniques can offer a possibility of complementarity and eliminate the weaknesses of the two techniques. Few studies on this combination exist in the literature in fact, there are several studies concerning hybrid joints, we can mention the work of [17–19]. In this work, three-dimensional finite element analyzes are performed to compare the effectiveness of the hybrid repair technique with composite patches and the drilling hole to repair an aluminum plate containing U lateral notches. A volumetric approach is developed to obtain the notch stress intensity factor K ρ in mode-I condition [20]. Variation of notch bottom opening stress fields in the different methods was examined based on the notch stress intensity factor (NSIF). The plastic zone at the U-Notch front has been studied as part of a drill hole, bonded repair and hybrid method repair. In addition, the variation in peel stresses in the plate / adhesive interface area for single patch repair and hybrid repair has also been checked. Volumetric method (K ρ ) The stress field around the notch-tip in mode I condition and subsequent evaluation of the notch stress intensity factor K ρ are performed to describe the fracture criterion. The determination of the K ρ is made using the volumetric approach [20], this method was chosen because it is feasible to give the appropriate K ρ values before and after the repairs. It assumed that the fracture process needs a physical volume where the necessary fracture energy release rate is stored to launch the fracture phenomenon; it corresponds to the high stressed region in which the fracture notch effect is taking place. This leads to a volumetric approach; two parameters are describing this fracture criterion: the effective distance and the effective stress. A bi-logarithmic graph shown in Fig. 1, represent the stress distribution at the notch tip where the stress normal to the notch plane is plotted versus the distance, and the effective stress and distance are also shown. D