Comparison of the effectiveness of boron/epoxy and graphite/epoxy patches for repaired cracks emanating from a semicircular notch edge

The adhesively bonded composite patch repair technique has been used to restore or extend the service life of the cracked aluminium structural components because of its efficiency. In this study, the finite element method is used to analyse the performance of the different bonded composite patches at a semicircular lateral notch and the repair of cracks emanating from this kind of notch. The knowledge of the stress distribution in the neighbourhood of the cracks is important for the analysis of their repair according to the geometry of the patch. The effects of the mechanical and geometrical properties on the variation of the stress intensity factor in the crack tip were highlighted. The effects of the adhesive properties and of the patch size on the stress intensity factor variation at the crack tip in mode I were also highlighted. The comparison between the double and single patch repairs is also given in this study. The results obtained show that the stress intensity factor of the crack tip repaired by two composite patches, is reduced to a half compared to the one that is repaired only by one patch. The orientation of fibres possessing a higher rigidity perpendicularly to the crack propagation considerably influences the reduction of the stress intensity factor. The adhesive properties must be optimised in order to increase the performance of the patch repair or the reinforcement.     

The effects of disbonds on the pure mode II stress intensity factor of aluminium plate reinforced with bonded composite materials

Adhesively bonded composite patch repair has been widely used to restore or extend the service life of cracked structural components due to its effectiveness to mechanical repair technique. In this work, the finite element method is applied to analyse the performance of the bonded composite patch for repairing cracks emanating from semicircular notch root in pure mode II. The stress intensity factor was computed at the crack tip repaired using a boron/epoxy patch for different orientation of fibers, taking into account the disbond. In this case, the increase of a patch thickness reduces the negative effects of disbond. When this effect is significant between the patch and the plate, it reduces the repair effectiveness. The maximum reduction obtained by using a boron/epoxy of fibers in the x-direction is of the order of 20% more important compared to a patch having its fibers in the y-direction. The stress intensity factor exhibits an asymptotic behaviour as the disbond size increases.     

Analysis of the distribution of thermal residual stresses in bonded composite repair of metallic aircraft structures

In this study, the distribution of the thermal residual stresses due to the adhesive curing in bonded composite repair is analysed using the finite element method. The computation of these stresses comprises all components of the structures: cracked plate, composite patch and adhesive layer. In addition, the influence of these residual stresses on the repair performance is highlighted by analysing their effect on the stress intensity factor at the crack tip. The obtained results show that the normal thermal stresses in the plate and the patch are important and the shear stresses are less significant. The level of the adhesive thermal stresses is relatively high. The presence of the thermal stresses increases the stress intensity factor at the crack tip, what reduce the repair performance.     

Fracture analysis of Kevlar-49/epoxy and e-glass/epoxy doublers for reinforcement of cracked aluminum plates

Adhesively bonded composite patch repair is efficient means to regain load carrying capacity, alleviate the crack growth, and improve the service life of the damaged structure. In this paper, three dimensional finite element models are developed to examine the fracture behavior of a single edge V-notched Aluminum plate repaired with Kevlar-49/epoxy or e-glass/epoxy pre-preg patches on both sides. Contour integral method was used for evaluating the stress intensity factor (SIF), an indicator of the crack stability. The load transfer mechanisms, stress distribution, damage variable (D), and crack mouth opening displacement (CMOD), were also presented to estimate the effectiveness of composite patch repair. The influence of the patch material, crack length and the adhesive thickness has been investigated. Results have shown that the crack induced damage increased nonlinearly with a larger crack size. With the composite patch repairs, SIF is reduced to 1/7–1/10 of that of the bare plate and CMOD decreased by 79%. The damage variable is reduced significantly and the load capacity is increased. A thinner adhesive layer results in a higher percentage of load shared by the composite patch.     

Numerical analysis of the notch effect and the behaviour of notch crack in adhesively bonded composite laminates

In this study the finite element method is used to analyse the notch effect and the behaviour of notch cracks in adhesively composite laminate under tension by computing respectively the stress concentration factor at the notch tip which characterize the notch strength and the stress intensity factor at the crack tip which characterize the resistance to the crack propagation. The effects of the adhesive properties and fiber orientation on the variation of both stress concentration and stress intensity factors are highlighted. The obtained results show that the notch strength is reduced in the layer of the laminate of which the fiber orientation is in the applied load direction; the resistance to the crack propagation is also reduced in this type of layer. The stress intensity factor at the tip of notch crack exhibits an asymptotic behaviour as the crack length increases.     

FE analysis of the behaviour of octagonal bonded composite repair in aircraft structures

Bonded composite repair has been recognized as an efficient and economical method to extend the fatigue life of cracked aluminium components. In this work, the finite element method is applied to analyze the central crack’s behaviour repaired by a boron/epoxy composite patch. The knowledge of the stress distribution in the neighbourhood of cracks has an importance for the analysis of their repair according to the patch geometry. The effects of mechanical and geometrical properties of the patch on the variation of the stress intensity factor at the crack tip were highlighted. The obtained results show that the stress intensity factor at the repaired crack with composite patch of height 2c/3 is reduced about 5% compared to cracks repaired with octagonal patch of size c. For patch height of c/3 the reduction is about 7%. The adhesive properties must be optimised in order to increase the repair performances and to avoid the adhesive failure.     

Progressive edge cracked aluminium plate repaired with adhesively bonded composite patch under full width disbond

In this study, the crack growth behaviour of an aluminium plate cracked at the tip and repaired with a bonded boron/epoxy composite patch in the case of full-width disbond was investigated. This effect is the imperfection which could result during the bonded patch of the repaired structure. Disbonds of various sizes and situated at different positions with respect to the crack tip as well as the effect of adhesive and patch thickness on repair performance were examined. An analysis procedure involving the efficient finite element modelling applied to cracked plate, adhesive and composite patch was used to compute the stress intensity factors. The crack growth rate is dominated by the stress intensity factor near the location and size of the pre-existing disbonds. The cracked plate and disbond propagation result in an increase in the patch deformation. The patch does not have an influence on the crack growth when the ratio 2a/d_R exceeds 0.8.     

Computation of the stress intensity factors for repaired cracks with bonded composite patch in mode I and mixed mode

In this study, the finite element method is used to analyse the behaviour of repaired cracks with bonded composite patches in mode I and mixed mode by computing the stress intensity factors at the crack tip. The effects of the patch size and the adhesive properties on the stress intensity factors variation were highlighted. The plot of the stress intensity factors according to the crack length in mode I, shows that the stress intensity factor exhibits an asymptotic behaviour as the crack length increases. In mixed mode, the obtained results show that the Mode I stress intensity factor is more affected by the presence of the patch than that of mode II.     

Comparison between rectangular and trapezoidal bonded composite repairs in aircraft structures: A numerical analysis

The optimization of the patch shape of bonded composite repair in aircraft structures is a good way to improve the repair performance. In this study, the three-dimensional finite element method is used to compare the repair performance of patches with rectangular and trapezoidal shapes in aircraft structures. The comparison is done by analysing the stress intensity factor (SIF) at the tip of repaired crack and the distribution of the adhesive stresses for the two patch shapes. The obtained results show that, when the crack length is ranged from 5 to 20 mm, the trapezoidal shape presents lower stress intensity factor at the crack tip, which is beneficial for the fatigue life and lower adhesives stresses, which is beneficial for the repair durability. These advantages disappear when the crack length reaches the value of 40 mm. It is also shown that the use of the trapezoidal shape reduce the mass of the patch, which can reduce the repair cost.     

Effects of the adhesive disband on the performances of bonded composite repairs in aircraft structures

In this study, the effects of the adhesive disband on the efficiency of bonded composites repair in aircraft structures were analyzed. The three-dimensional finite element method was used to achieve the objectives of the study. The stress intensity factor at the crack tip was chosen as fracture criteria. The analysis was extended to the single and double symmetric bonded composite patches. The obtained results show that the repair efficiency is negatively influenced if the adhesive disband growths perpendicularly to the crack. In the case of double symmetric patch, the presence of double adhesive disband highly decreases the repair efficiency and increases the risk of adhesion failure between the composite patch and the cracked aluminum structures.     

Study of composite patch repair by analytical and numerical methods

A combined analytical and numerical study of an isotropic cracked plate that was repaired by using a bonded composite patch was conducted. The analytical work was based on Rose's equations, whereas for the numerical investigation a three-dimensional finite element analysis was implemented. A number of cracked plates with different crack lengths and overall dimensions of the composite repair were considered. The composite patch was made of unidirectional laminates with different stacking sequences. Both, one- and two-sided patches were analysed. Results are presented for the stress intensity factor in the patched crack and the maximum stress reinforcement stress and adhesive strain. It was found that for the case of a two-sided reinforcement the results obtained by both methods were in good agreement. However, for the case of a single reinforcement the accuracy of the analytical method decreased due to the tendency to out-of-plane bending as a result of bonding a reinforcing patch to only one face of a plate, which is ignored in the analysis.     

The stress intensity factor for a cracked stiffened sheet repaired with an adhesively bonded composite patch

The problem of a cracked, stiffened metallic sheet adhesively bonded by a composite patch is analyzed. The composite patch is assumed to be either an infinite orthotropic sheet or an infinite orthotropic strip normal to the crack. Due to the high stress concentration around the crack and on the interface, an elliptical disbond is assumed to exist around the crack. The crack is asymmetric with respect to the stiffener's locations as well as to the patch's center. The effect of thermal stresses in curing process is also considered. The fracture problem is solved by the displacement compatibility method, using the complex variable approach and the Fourier integral transform method.The problem is dealt with in two steps. First, starting with an uncracked, patched stiffened sheet, the stress at the prospective location of the crack is determined in a closed-form solution. The second step is to introduce a crack into the stiffened patched sheet. The multivalue of the analytical formulation is treated in detail to ensure proper implement in the computer. The results show that the effect of the stiffeners on the stress intensity factor is not significant for a crack fully covered by a patch.For the repairs by Boron/Epoxy patches, the difference in KI between the infinite sheet patch and the infinite strip model is only minor (less than 5 percent) in the absence of the curing thermal stresses and it becomes more pronounced when these stresses are taken into consideration. The stress intensity factor for a crack repaired by an infinite composite strip also can be estimated with a good or reasonable accuracy via a simplified analysis in which the patch is considered as an infinite strip in the first step and is treated as an infinite sheet in the second step of the solution procedure mentioned above.The latter simplified analysis is based on the approach originally proposed by Rose for a relatively simple repair configuration. For most cases, that approach seems to work well for the repair of a stiffened sheet by an infinite composite strip with the effects of thermal stresses and a disbond included. It should be emphasized that the present methodology can apply to the problem of a crack in a metallic stiffened sheet growing beyond the patch's boundary and also to the repairs by an infinite adhesively bonded composite strip parallel to the crack.     

Analysis of cracked steel members reinforced by pre-stress composite patch

Pre‐stress bonded composite patch is a promising technique to reinforce steel member damaged by fatigue. The effectiveness of this technique was verified by fatigue tests on notched steel plates. Results showed that the application of carbon fibre reinforced plastic (CFRP) strips and, eventually, the introduction of a compressive stress by pretension of the CFRP strips prior to bonding produced a significant increment of the remaining fatigue life. In this paper, the stress intensity factor in the notched plates is computed by a two‐dimensional finite element model in connection with the three‐layer technique in order to reduce the computational effort. Due to high stress concentration at the plate crack tip, debond is assumed at the adhesive–plate interface. The goal is to illustrate the influence of some reinforcement parameters such as the composite strip stiffness, the pre‐stress level, the adhesive layer thickness and the size of the debonded region on the effectiveness of the composite patch reinforcement.     

Crack opening displacement in plate with bonded repair patch

Mathematical techniques are extended to compute crack opening displacements in a cracked plate with an adhesively bonded composite patch. The plate and the patch are considered as orthotropic materials. The problem is reduced to the solution of integral equations. A software program is written to compute shear stresses in adhesive, stress intensity factors in the plate and the crack openings at the centreline of the crack. The effects of adhesive thickness, adhesive modulus, patch thickness and plate thickness on crack openings are investigated. A test program is carried out to obtain crack opening displacements in plate with bonded patch. A good agreement with analytical predictions is obtained. The effects of patches bonded on one or both sides of a plate on stress intensity factors are evaluated.     

Fatigue crack growth behavior of cracked aluminum plate repaired with composite patch

In this study, we investigated the fatigue crack growth behavior of cracked aluminum plate repaired with bonded composite patch especially in thick plate. Adhesively bonded composite patch repair technique has been successfully applied to military aircraft repair and expanded its application to commercial aircraft industry recently. Also this technique has been expanded its application to the repair of load bearing primary structure from secondary structure repair. Therefore, a through understanding of crack growth behavior of thick panel repaired with bonded composite patch is needed. We investigated the fatigue crack growth behavior of thick panel repaired with bonded composite patch using the stress intensity factor range (ΔK) and fatigue crack growth rate (da/dN). The stress intensity factor of patched crack was determined from experimental result by comparing the crack growth behavior of specimens with and without repair. Also, by considering the three-dimensional (3D) stress state of patch crack, 3D finite element analyses were performed to obtain the stress intensity factor of crack repaired by bonded composite patch. Two types of crack front modeling, i.e. uniform crack front model and skew crack front model, were used. The stress intensity factor calculated using FEM was compared with the experimentally determined values.     

Influence of disbond on notch crack behaviour in single bonded lap joints

The behaviour of the adhesive bonded joints due to the imposed eccentric loading generates a very complex distribution of the stress in the structure. Good adhesion between substrate and adhesive ensures a successful and lasting assembly. In this study the finite element method is used to analyze the behaviour of a bonded lap joint of dissimilar materials. The effects of the mechanical properties of the joints on the shear stress variation with and without presence of a circular notch are investigated. The results show that the maximum shear stresses are located at a distance of about 18% that of the lap length whatever the type of material used. In addition, the stress intensity factor is amplified by the presence of the negative effect of disband whose increase is linearly proportional to the square of the stress intensity factor. It reached its maximum value for a crack length equal to two-fifths of the notch radius.     

Effect of water absorption on the adhesive damage in bonded composite repair of aircraft structures

In this study, the mechanical properties of the aged adhesive Adekit A140 epoxy were used in a finite element model to evaluate the effect of water absorption on the adhesive damage in bonded composite repair of aircraft structures. The damage zone theory was implemented in the finite element code in order to achieve this objective. In addition, the effect of the water absorption, by the adhesive, on the repair efficiency was analyzed by computing the stress intensity factor at the crack tip. The obtained results show that, when the water absorption increases the adhesive loses its rigidity, which reduces the repair durability. Besides, it leads to the increase of the stress intensity factor at the crack tip indicating a reduction in the repair efficiency.     

Static Tensile and Transient Dynamic Response of Cracked Aluminum Plate Repaired with Composite Patch – Numerical Study

In this study, the central cracked aluminum plates repaired with two sided composite patches are investigated numerically for their response to static tensile and transient dynamic loadings. Contour integral method is used to define and evaluate the stress intensity factors at the crack tips. The reinforcement for the composite patches is carbon fibers. The effect of adhesive thickness and patch thickness and configuration in tensile loading case and pre-tension, pre-compression and crack length effect on the evolution of the mode I stress intensity factor (SIF) (K_I) of the repaired structure under transient dynamic loading case are examined. The results indicated that K_I of the central cracked plate is reduced by 1/10 to 1/2 as a result of the bonded composite patch repair in tensile loading case. The crack length and the pre-loads are more effective in repaired structure in transient dynamic loading case in which, the 100 N pre-compression reduces the maximum K_I for about 40 %, and the 100 N pre-tension reduces the maximum K_I after loading period, by about 196 %.     

Influence of bimaterial interface on kinking behaviour of a crack growth emanating from notch

The mechanism of crack deviation by an interface modifies considerably the behaviour of bimaterials fracture. Their fracture resistance is highly affected by the difference of the elastic properties of the bonded materials. In this work, the finite element method is applied to analyze the behaviour of a crack emanating from semicircular notch root growing in interface ceramic/metal composites and perpendicularly to this interface. The obtained results showed that the crack grew to interface from harder material, its energy decreased at the approach of the interface, in this case was retarded; an inverse phenomenon occurs if the crack is propagated towards a lower strength material and its energy increases, it has tendency to accelerate. The effects of geometry on the crack deflection near the interface are also discussed.     

Numerical analysis of the beneficial effect of the double symmetric patch repair compared to single one in aircraft structures

This paper concerns a numerical study by the finite element method of the cracked structure repaired by single and double bonded composite patches. The stress intensity factor is used as fracture criteria. The obtained results showed the advantage of the double patch compared to single on the reduction of the stress intensity factor at the crack tip. The effects of the properties of the plate and the patch and the adhesive on the beneficial effect of the double patch are highlighted. The adhesive properties must be optimised in order to increase the advantage of the double patch and to avoid the adhesive failure. The patch properties have a significant effect on the beneficial effects of the double symmetric patch.