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In this study, the finite element is used to analyze and compare the performances of the double- and single-sided composite repair in aircraft structures in mode I and mixed mode. The stress intensity factors in modes I and II at the crack tip are computed to perform the objective of the study. The effects of the different parameters of repair such...
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... There are only a few investigations available on repairing panels in mixed-mode condition by the linear elastic and nonlinear fracture mechanics [11][12][13][14][15][16][17]. Hosseini-Toudeshky performed fatigue crack growth tests of single-sided repaired thick and thin panels containing center inclined cracks with various patch lay-ups configurations and various composite patch thicknesses [12,13]. ...
... Ayatollahi and Hashemi [14,15] used a finite element analysis to investigate the effect of composite patching on the SIF reduction for an inclined center crack panel under different mixed loading case. Bachir Bouiadjra et al. [16] have conducted FEA to estimate SIF in single and double-sided repairs in mode I and mixed mode edge-cracked panels. They have shown that the adhesive and composite patch properties have a significant and beneficial effect on the symmetrical patch. ...
Material fracture by opening (mode I) is not the only failure criteria responsible for fracture propagation. Many industrial examples show the presence of mode II and mixed mode I + II. In the present work, numerical analyses of the three-dimensional and non-linear finite element method are used to estimate the performance of the bonded composite repair of metallic aircraft structures with a pre-existent damage by analyzing the plastic zone size ahead of repaired cracks under mixed mode loading, to assess the effect of the composite repair system on the plastic zone. The Von Mises stress is used to predict yielding of materials under this loading condition. The extension of the plastic zone, which takes place at the tip of a crack, strictly depends on many variables, such as the yield stress of the material, the loading conditions, the crack size and the thickness of the cracked component.
... The authors 21,22 have highlighted the advantage of the double-sided patch repair compared to single-sided patch repair on the reduction of the SIF. They indicated that the adhesive properties must be optimized in order to increase the advantage of the double-sided patch and to avoid adhesive failure. ...
The repairing process of structures using bonding of composite materials is an efficient and economic way of increasing service life of damaged structures. The finite element method is developed and applied to assist in the design, assessment and optimization of the proposed plans before they are implemented, therefore reducing cost and time. In this study, the finite element method is used to analyze the behavior of a repaired crack using single and double circular composite patches by determining the stress intensity factor reduction. The effects of geometrical and mechanical properties of the patch on the fracture parameters are highlighted. The results show that the thickness gain and the reduction of the asymptotic value of the SIF in the case of the double patch is 7% when compared to the single one. Thus, the gain in the thickness decreases when the thickness of the bonded patch increases up to a value eP = 0.8 mm and beyond this value, it starts increasing. However, an inverse behavior occurs for the mixed mode. In addition, whatever is the size of the strengthened crack, the gain in thickness exceeds 16% in mode I and 20% in mixed mode.
... It is known that the determination of fracture criteria such as SIF or energy release rate at the crack tip can give a precise idea on the performance of the bonded composite repair. Several authors computed the SIF at the crack tip of repaired cracks under mechanical loading among them: Jones and Chiu, 6 Madani et al., 7 Bachir Bouiadjra et al., [8][9][10][11][12][13] Achour et al., 14 Fekirini et al., 15 and Belhouari et al., 16 and Ouinas et al. [17][18][19][20] In this study, the effects of the thermal residual stresses on the variation of the SIF for repaired aluminum crack with boron/epoxy and graphite epoxy patches are analyzed using the finite element method. The effects of curing temperature, adhesive properties, and the geometrical properties on the SIF variation under thermo-mechanical loadings are studied. ...
In this study, the finite element method is used to analyze the effect of the thermal residual stresses resulting from adhesive curing on the performances of the bonded composite repair in aircraft structures. The stress intensity factor at the crack tip is chosen as the fracture criterion in order to estimate the repair performances. The effects of the curing temperature, the geometrical properties of the patch, and the adhesive properties on the variation of the stress intensity factor are analyzed. The obtained results show a significant negative effect of the thermal residual stresses on the repair efficiency and this effect can be reduced if the different repair parameters are optimized.
The present study concerns with the experimental and numerical investigation of crack stoppers ahead of an edge crack in panels subjected to tensile loading. Two different patches (rectangular and semi-annular patches) have been analyzed. The patches (aluminium and steel) are placed at different distance from the crack, symmetrically on both sides of the panel and at a finite distance ahead of the crack tip. A finite distance ahead of the crack tip reveals that depending on the distance, the crack tip could remain straight or curve. In such cases, the crack could either be arrested, run through or run around the reinforcements. Moreover, the degree of instability is reflected by an index parameter that accounts for the effect of load, geometry and material properties. Moreover, a geometrically nonlinear, two-dimensional (2D) finite element analysis (Comsol Multiphysics) has been employed to determine the local energy intensity. It would be of special interest to know whether the crack would run straight and arrest at the patch regardless of the other variables. The ultimate goal for straight crack path is to produce sufficient low local energy intensity. This gives a significant advantage because as the local energy intensity is increased, crack would tend to curve and lead to complete fracture of the patched specimens. It is equivalent of moving the patch closer to the crack tip. The predictions made from the strain energy density theory, as well as, there is a good agreement between finite element results and experimental findings.
Composite sandwich materials with glass fibre-reinforced plastic (GFRP) skins and a foam core have been widely used in civil engineering. However, the interfacial delamination is the main failure mode in practice, especially at elevated temperatures. Temperature-induced interfacial shear stress can be generated because of the different coefficients of thermal expansion of GFRP skin and foam core, which can weaken the interfacial bond strength of sandwich materials. In this study, to investigate the distribution of temperature-induced interfacial strain, an analytical model was developed by using the infinitesimal method. In the meantime, a series of foam-core composite sandwich materials were tested via a kind of non-direct test method at different temperatures to validate the accuracy of the proposed analytical model. Finally, the comparison between experimental and analytical results demonstrates that the proposed analytical model can predict the interfacial strain distribution of sandwich structures at elevated temperatures.
Foam core sandwich materials are being widely used in structural engineering due to their advantages such as lightweight, high strength, and corrosion resistance. But the interfacial bond strength of foam core sandwich materials is weakened at elevated temperatures. In practice, the influence of high temperature cannot be ignored since the composites with foam are sensitive to the change in the environment. In this study, a series of sandwich double cantilever beams were tested at different temperatures to evaluate the effect of high temperatures on the interfacial fracture of the foam core sandwich materials. The temperature range studied was from 30 to 90℃, noting the glass transition temperatures are 85.7℃ for the composites and 72.1℃ for the foam core, respectively. In the meantime, the mode I interfacial crack propagation and its corresponding interfacial strain energy release rate were analyzed. Furthermore, an analytical model, considering the change in temperature, was proposed to predict the strain energy release rate of mode I interfacial facture of the foam core sandwich materials. The analytical results were found to be well matched with the experimental results.
In this study, the three-dimensional finite element method is used to determine the stress
intensity factor in Mode I and Mixed mode of a centered crack in an aluminum specimen repaired by a
composite patch using contour integral. Various mesh densities were used to achieve convergence of the
results. The effect of adhesive joint thickness, patch thickness, patch-specimen interface and layer sequence
on the SIF was highlighted. The results obtained show that the patch-specimen contact surface is the best
indicator of the deceleration of crack propagation, and hence of SIF reduction. Thus, the reduction in rigidity
of the patch especially at adhesive layer-patch interface, allows the lowering of shear and normal stresses in
the adhesive joint. The choice of the orientation of the adhesive layer-patch contact is important in the
evolution of the shear and peel stresses. The patch will be more beneficial and effective while using the
cross-layer on the contact surface.
Aircraft repair is gaining importance for extending the fatigue life of aging aircrafts and also for improving its structural integrity. Among various repair techniques, bonded composite repair is mostly preferred. In this paper, a three-dimensional finite element analysis is carried out to study and compare the performance of single- and double-sided patch on center-cracked aluminum panel. The effects of different parameters such as patch layup, patch thickness and patch material on the stress intensity factor is highlighted. It is shown that the mechanics of double-sided repair is completely different from the single-sided repair. Also, it is found that double-sided repair is more efficient than single-sided repair. In case of single-sided repair, alternative patch material like unbalanced lamina or transversely graded material is proposed. It is observed that there is significant reduction of stress intensity factor with transversely graded patch in case of single-sided repair.
The determination of the stress intensity factor at the crack tip is one of the most widely used methods to predict the fatigue life of aircraft structures. This prediction is more complicated for repaired cracks with bonded composite patch. This study was performed in order to establish an analytical model for the stress intensity factor for repaired crack emanating from central holes. The model was developed by modifying that of Kujawski initially developed for cracks emanating from notch without repair. The obtained results were compared with those calculated with the finite element method and the model of Jones et al. It was found that our model gives a good approximation of the stress intensity factor for repaired crack emanating from central elliptical and circular holes if the results of the finite element analysis are taken as references.
In this study, a new approach is applied to compare the performances of single sided and double-sided symmetric composite patch with circular shape for repairing cracked aircraft structures. This is an approach that consists to evaluate the mass gain eventually obtained by the use of double symmetric composite patch if the two patch configurations give the same stress intensity factor at the crack tip. The three-dimensional finite elements method is used to compute the stress intensity factor. The obtained results show that the use of the double patch technique leads to a significant reduction of the stress intensity at the crack tip. The mass gain eventually given by the double patch technique can be very significant and this gain depends on the patch shape and the adhesive properties.
