Analysis of the adhesive damage for different patch shapes in bonded composite repair of aircraft structures

In this study, the adhesive damage in bonded composite repair of aircraft structures is analyzed using a three dimensional finite element method. Four different patch shapes were chosen to analyze the adhesive damage: rectangular, trapezoidal, circular and elliptical. The modified damage zone theory was implemented in the FE code to evaluate the adhesive damage. The obtained results show that the rectangular shape is beneficial for the repair durability but it presents a disadvantage for the repair efficiency. The elliptical patch is found the optimal whereas the trapezoidal patch represents the worst shape among the four types analyzed in this paper.     

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.     

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.     

Effect of humidity absorption by the adhesive on the performances of bonded composite repairs in aircraft structures

In this study, an experimental work was done to analyse the effect of the humidity absorption on the mechanical properties of the adhesive Adekit A140 epoxy. The measured properties were introduced in a finite element model in order to analyse the effects of the humidity absorption on the performances of bonded composites repair in aircraft structures by computing the J integral around the crack tip and the distribution of the adhesive shear stresses. The obtained results show that the adhesive losses its rigidity by the humidity absorption which reduce the repair efficiency but increases the repair durability.     

Effect of humidity absorption by the adhesive on the performances of bonded composite repairs in aircraft structures

In this study, an experimental work was done to analyse the effect of the humidity absorption on the mechanical properties of the adhesive Adekit A140 epoxy. The measured properties were introduced in a finite element model in order to analyse the effects of the humidity absorption on the performances of bonded composites repair in aircraft structures by computing the J integral around the crack tip and the distribution of the adhesive shear stresses. The obtained results show that the adhesive losses its rigidity by the humidity absorption which reduce the repair efficiency but increases the repair durability.     

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.     

Nonlinear analysis of bonded composite patch repair of cracked aluminum panels

Analyses of adhesively bonded composite patches to repair cracked structures have been the focus of many studies. Most of these studies investigated the damage tolerance of the repaired structure by using linear analysis. This study involves nonlinear analysis of the adhesively bonded composite patch to investigate its effects on the damage tolerance of the repaired structure. The nonlinear analysis utilizes the three-layer technique which includes geometric nonlinearity to account for large displacements of the repaired structure and also material nonlinearity of the adhesive. The three-layer technique uses two-dimensional finite element analysis with Mindlin plate elements to model the cracked plate, adhesive and composite patch. The effects of geometric nonlinearity on the damage tolerance of the cracked plate is investigated by computing the stress intensity factor and fatigue growth rate of the crack in the plate. The adhesive is modeled as a nonlinear material to characterize debond behavior. The elastic-plastic analysis of the adhesive utilizes the extended Drucker-Prager model. A detailed discussion on the effects of nonlinear analysis for a bonded composite patch repair of a cracked aluminum panel is presented in this paper.     

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.     

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.     

Analysis of the plastic zone size ahead of repaired cracks with bonded composite patch of metallic aircraft structures

In this study, the three-dimensional and non-linear finite element method is used to estimate the performance of the bonded composite repair of metallic aircraft structures by analyzing the plastic zone size ahead of repaired cracks. Several calculations have been realized to extract the plasticized elements around the crack tip of repaired crack. The obtained results show that the presence of the composite patch reduces considerably the size of the plastic zone ahead of the crack. The effects of the adhesive properties and the patch thickness on the plastic zone size ahead of repaired cracks were analyzed.     

Numerical estimation of the mass gain between double symmetric and single bonded composite repairs in aircraft structures

In this study, the three-dimensional finite element method is used to analyse the advantages of the double symmetric composite repair compared to the single one in aircraft structures by analysing the mass again between the two techniques. This gain is defined as the ratio between the thickness of the double and single patches giving the same stress intensity factor (SIF) at the crack tip. The obtained results show that the mass gain resulting from the use of the double symmetric repair is important because of the double stress absorption. It also shown that, the adhesive properties must be optimised in order to improve the advantages of double symmetric composite repair.     

飞机用复合材料斜胶接修补结构的冲击损伤

高传力效率的斜面式胶接在飞机复合材料传力接头和修补中被广泛使用,但该结构的低速冲击损伤阻抗和损伤容限未在飞机结构设计中考虑。本文研究了低速冲击下的较厚的复合材料斜胶接板的力学性能及损伤失效。在胶接区域布置不同冲击点,寻找最敏感位置,在该位置进行冲击能量变化研究,通过冲击响应（冲击载荷、挠度、能量等）及冲击损伤两个方面获取其规律和失效机制。小能量和大能量冲击结果表明,胶接区域5个典型冲击位置中,中心位置冲击损伤最大,冲击敏感性最高,因此中心点为冲击损伤阻抗最小位置。中心点不同能量冲击时,冲击响应研究揭示了冲击过程中冲击载荷具有典型的4阶段行为。冲击载荷还具有双峰值力的现象。冲击后沿试样中心线切开的显微损伤图揭示了该结构有两种损伤模式,包括复合材料损伤及胶层损伤。复合材料的损伤包含90°和45°层基体的开裂和0°与90°层之间的层间损伤。胶层损伤出现在试样冲击点正下方背部的复合材料斜接尖端部位。进一步通过考虑复合材料层内、层间损伤及胶层损伤的渐进损伤模型对试验进行仿真研究,找出导致第Ⅱ阶段冲击载荷突降的主要原因为复合材料层间损伤,第Ⅳ阶段冲击载荷再一次突降是由于胶层出现了损伤。     

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.     

The effects of size and stacking sequence of composite laminated patch on bonded repair for cracked hole

The performance of a bonded repair for cracked holes has been studied using the three dimensional finite element method, linear elastic fracture mechanics and strain energy density theory. Increasing the composite patch size reduces the strain energy level at the crack tip; increasing the patch length normal to the crack is a better choice. The stacking sequences of the laminated patch have little influence on the strain energy distribution in the vicinity of the crack. To repair the cracked holes of aircraft components subjected to variable direction loading during flight, the orientations of the patch ply, 90° and ±45° with respect to the crack direction, are the optimum selection in bonded repairs.     

Damage tolerance of bonded aircraft structures

This paper presents a damage tolerance philosophy for bonded structures and repairs. It is proposed to assess the damage growth in bonded structures loaded mainly in shear with a generic strain elastic energy release rate concept. This concept has been validated on metal-to-metal and metal-to-composite bonding in metallic and hybrid structures.     

Numerical analysis of the stress intensity factors for repaired cracks from a notch with bonded composite semicircular patch

In this paper, we investigated the crack growth behaviour of cracked thin aluminium plate repaired with bonded composite patch. The finite element method is used to study the performance of the bonded composite reinforcement or repair for reducing the stress concentration at a semicircular lateral notch and repairing cracks emanating from this kind of notch. The effects of the adhesive properties and the patch size on the stress intensity factor variation at the crack tip in mode I were highlighted. The obtained results show that the stress concentration factor at the semicircular notch root and the stress intensity factor of a crack emanating from notch are reduced with the increase of the diameter and the number of the semicircular patch. The maximal reduction of stress intensity factor is about 42% and 54%, respectively, for single and double patch. However, the gain in the patch thickness increases with the increase of the crack length and it decreases when the patch thickness increases. The adhesive properties must be optimised in order to increase the performance of the patch repair or reinforcement.     

Comparison between double and single sided bonded composite repair with circular shape

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.     

Design of optimum patch shape and size for bonded repair on damaged Carbon fibre reinforced polymer panels

Carbon fibre reinforced polymer (CFRP) composite laminates have become popular for structural applications as they are lighter, stronger and tougher. But they are also susceptible to damage while in service. Damage in composite structures reduces its structural integrity and hence the service life. For improved service life, the damages need to be repaired so that structural integrity is restored. Adhesively bonded composite patch repair is one of the prominent technique used for restoring the structural integrity of the damaged part. Patch shape is one of the important parameter in composite repair performance and it needs to be investigated thoroughly. In the present work, a 3D finite element based study is carried out to investigate the influence of various patch shapes on repair efficiency. Damaged CFRP laminates are repaired by symmetrical patch adhesively bonded over the damaged area. The panel analyzed is of pure unidirectional and quasi-isotropic laminate sequences. The patch shapes considered are circle, rectangle, square, ellipse, octagon and oval. Stress concentration factor (SCF) is estimated before and after the repair to evaluate their efficiency. Also peel stress is considered for quantitative comparison. The SCF reduction and peel stress are compared for various patch shapes keeping constant patch volume. Stress based 3D-Hashin’s failure criterion is employed for predicting the strength at damage initiation along with failure modes in notched and repaired panel. Optimal patch shape is then brought-out based on higher repair efficiency. Finally, a genetic algorithm based approach in-conjunction with finite element analysis is used for the optimization of patch geometry and adhesive thickness in order to obtain higher repair performance.     

复合材料层合板胶接贴补修理渐进损伤分析

建立了复合材料层合板胶接贴补修理构型渐进损伤分析的三维有限元模型, 其中层合板和胶层分别采用正交各向异性损伤和各向同性损伤的连续介质损伤力学模型, 整个分析过程中同时考虑层合板和胶层的损伤形成和扩展以及它们之间的相互影响, 单向压缩载荷作用下的层合板贴补修理构型的试验数据验证了该模型的有效性, 采用该模型分析了不同的贴补修理参数对修补强度的影响。 结果表明: 当层合板补片较薄时, 补片损伤是导致修补结构失效的主要原因; 当补片较厚时, 胶层失效是导致修补结构失效的主要原因, 此时补片厚度增加并不能显著增大修补结构的极限强度。在复合材料贴补修理时需要对补片和胶层进行详细优化设计。      

Adhesive elements for stress analysis of bonded patch to curved thin-walled structures

 Bonded composite patching has been recognized as an efficient and economical method to extend the service life of cracked aluminum components. However, current analysis methods and empirical databases for designing composite bonded joint and patch repair are limited to flat plate and/or flat laminate geometries, and the effect of curvature on the performance and durability of composite bonded joints and repairs is not known. This paper presents a novel finite element formulation for developing adhesive elements for conducting quick stress analysis of bonded repairs to curved structures. Illustrative examples are presented to demonstrate the effect of curvature and the effect of patch location, i.e., internal and external patches, patch size and patch thickness on stresses in adhesive layer. Received: 24 April 2002 / Accepted: 10 October 2002