Preface

The BFRP crack-patching technique has been applied to the field repair of fatigue cracks in the aluminium alloy wing skins of Mirage III fighter aircraft. Finite-element procedures were used in patch design. The repair was qualified using fatigue-crack propagation studies on panels simulating the cracked and repaired area. A field support unit was designed to allow repairs to be carried out by air force personnel during routine maintenance of the aircraft. To date over 150 patches have been applied and nearly three years of operational history gained. While some crack growth was observed after repair of a few wings, the patch stopped further growth and no wing skin has required further repair.     

Patch repair design optimisation for fracture and fatigue improvements of cracked plates

The problem of improving fracture and/or fatigue life of structural components by means of patch repairs is addressed in the present paper by applying a biology-based method, known as the Genetic Algorithm. The optimum design procedure consists in evaluating the patch topology, to be applied to the highest stressed region, which maximises the fracture resistance or the expected fatigue life of the improved structural component by keeping constant the total patched area (constrained optimisation problem). The proposed procedure is implemented in a finite element code and some numerical simulations are carried out in order to assess its reliability. The method is applied to two simple cases of cracked plates under pure Mode I or Mode II: for such analysed configurations, the method allow to improve significantly the fracture or fatigue behaviour of structural components repaired by employing the optimised shape patches instead of a simple shape (e.g. square or rectangular) patches.     

Fatigue crack growth analysis of adhesively repaired panel using perfectly and imperfectly composite patches

Fatigue crack behavior of cracked aluminum panel repaired with the imperfectly bonded composite patch is analyzed. The imperfection is in the form of debond which could result during the bonding of patch or the service life of the repaired structure. Debonds, of various sizes and at different locations with respect to the crack front, are investigated. An analytical procedure, involving two-dimensional finite element method having three layers to model cracked plate, adhesive and composite patch, is used to compute the stress intensity factors of test coupons. From the computed stress intensity factors, the crack growth rates are obtained analytically by assuming that the relationship between the stress intensity factor and the crack growth rate after repair is the same as the fatigue crack growth relationship for cracked panel material. The fatigue crack growth rates obtained experimentally and analytically are in good agreement with each other and they vary linearly with crack length inside the patch. The experimental results are bounded between its analytical counterparts at the mid-plane and free edge surfaces of the cracked panel. The present analytical procedure can, thus, be used to characterize the effects of imperfectly and perfectly bonded composite patch repairs on the durability and damage tolerance of the repaired structure.     

复合材料修补结构腐蚀扩展特性实验研究

为了研究腐蚀环境对复合材料修补结构疲劳特性的影响,开展以某型飞机加速环境谱为基础的预腐蚀试验,并采用递进式的试验方法分别完成对未修补未腐蚀、修补但未腐蚀、又修补又腐蚀的试样疲劳裂纹扩展试验。利用试验数据计算了复合材料修补构型、腐蚀环境对裂纹尖端应力强度因子影响的修正系数,并确定了通过试验验证的修补结构疲劳裂纹扩展修正模型。该模型预测腐蚀环境下修补结构的疲劳寿命与试验值一致。试验数据和预测模型可为海军飞机复合材料修补结构的损伤容限设计提供参考。     

Delamination effects on cracked steel members reinforced by prestressed composite patch

Prestressed composite patch bonded on cracked steel section is a promising technique to reinforce cracked details or to prevent fatigue cracking on steel structural elements. It introduces compressive stresses that produce crack closure effect. Moreover, it modifies the crack geometry by bridging the crack lips and reduces the stress range at crack tip. Fatigue tests were performed on notched steel plate reinforced by CFRP strips as a step toward the validation of crack patching for fatigue life extension of riveted steel bridges. A debond crack in the adhesive–plate interface was observed by optical technique. Debond crack total strain energy release rate is computed by the modified virtual crack closure technique. A parametric analysis is performed in order to investigate the influence of some design parameters such as the composite patch Young’s modulus, the adhesive thickness and the pretension level on the adhesive–plate interface debond.     

Stress-intensity factors in plates with a partially patched central crack

The fatigue and fracture performance of a cracked plate can be substantially improved by providing patches as reinforcements. The effectiveness of patches is related to the reduction they cause in the stress-intensity factor of the crack. Hence, an accurate evaluation of SIF in terms of various parameters is required for reliable patch design.In this paper, the influence of patch parameters on the opening-mode stress-intensity factor for a plate with a central crack is studied by employing transmission photoelasticity. Cracked plates made of photoelastic material are patched on one side as well as both sides by epoxy, phenolic and E glass-epoxy composite materials. The patch is located on the crack in such a way that the crack tip is not covered. Magnified isochromatic fringes are obtained by using a projection microscope of 50 magnification, converted into a polariscope. Irwin's method with extrapolation is employed to compute the stress-intensity factor from photoelastic data. The reduction in the stress-intensity factor is presented in graphic form as a function of pathch parameters, namely stiffness, width and length.     

复合材料胶接修补问题的试验研究和分析

用带中心裂纹的Ｌｙ１２ＣＺ铝合金板模拟飞机的损伤结构,对于不同的修补方式（单边和双边修补）,不同的补片材料（铝合金和复合材料）和几何尺寸以及脱胶等因素,进行了静强度和疲劳裂纹扩展试验． 实验表明． 胶结修补能明显地提高损伤结构的强度和疲劳寿命,同时实验还为修补实践的选材和几何参数的设计等提供了大量数据． 文中最后给出了修补结构疲劳裂纹的有限元计算估计,并与试验结果进行了比较和分析．     

Crack Patching: Analysis and Design

Bonded composite patches are frequently used to retard crack growth. This repair procedure is usually referred to as crack patching. The present paper outlines the various methods for the analysis and design of fiber composite patches in thin and thick structures. As illustrative examples the repair of fatigue cracks in the wing skins of Mirage III aircraft, of surface flaws in Macchi landing wheels, and of cracks in a truss is considered.     

腐蚀环境下复合材料修补件力学特性研究

采用复合材料补片胶接修补含裂纹LY12CZ铝合金板,开展了试验室大气环境和加速预腐蚀环境下复合材料修补件静强度拉伸和疲劳裂纹扩展对比试验研究.结果表明,复合材料补片均能显著提高损伤结构的拉伸强度和疲劳寿命,且短周期的预腐蚀环境对修补件两种力学特性的影响可以忽略不计.同时,基于Paris公式和Rose分析模型,建立了常规环境和预腐蚀环境下疲劳裂纹扩展寿命预测模型,通过与试验结果的对比证明了该模型的工程有效性.     

Fracture mechanics and crack contact analyses of the active repair of multi-layered piezoelectric patches bonded on cracked structures

In this paper, two-dimensional plane strain finite element analyses of the active repair for cracked structures by using multi-layered piezoelectric patches have been studied. The reductions of stress intensity factors and strain energy density factor at the crack tips are obtained. Also, the repair voltages for various conditions are obtained for estimating the repair performances. Lower repair voltage is a better choice because it is low-energy-consuming and safer for the operation. From the results of numerical fracture mechanics, it shows the crack contact conditions must be considered in the analysis. However, the friction on the crack has few effects on the repair performances for this mode-I dominated case. The better design choices for the piezoelectric patch are as follows: increasing the layer number, increasing the patch length, and reducing the patch thickness. In additions, it is not a good idea to use higher input voltage that is larger than the repair voltage because it will enlarge the crack open near the crack tip. Too long patch length has no advantage for the active repair.     

Thermal effects on adhesively bonded composite repair of cracked aluminum panels

This study introduces the two-dimensional finite element analysis involving three layer technique to investigate the adhesively bonded composite repair of cracked metallic structure under thermo-mechanical loading. The thermal loading involves, in this study, the temperature drop such as seen during the bonding process. Three patch materials having different stiffnesses and coefficients of thermal expansion are investigated to analyze the thermal effects on the damage tolerance of the crack in the repaired structure and of the debond in the adhesive bondline. For the single sided repair, the patch material having the maximum mismatch in the coefficient of thermal expansion with that of the cracked aluminum plate provides the better damage tolerance capability for both the crack in the panel and the debond in the adhesive. On the other hand, for double sided repair, the patch material having the minimal mismatch in the coefficient of thermal expansion with that of the cracked plate provides the better damage tolerance capability.     

Multiplexed optical fiber sensors for air frame repair patch monitoring

This paper describes the use of two multiplexed in-line fiber etalon (ILFE) sensors for air frame repair patch monitoring. The ILFE sensors are multiplexed using a combination of coherence division and frequency division multiplexing. Cantilever beam tests are used to verify the ILFE strain sensor performance and to measure multiplexing cross talk. The multiplexed ILFE sensors are then used to monitor the load transfer in a boron/epoxy patch utilized to repair cracks that occur in aging aircraft. The experiments show a clear relationship between a normalized measure of the strain inside the repair patch and crack extension in an aluminum single-edge notch specimen.     

Edge-cracked stiffened panels analyzed by caustics

Externally bonded composite patches have been proven to be an effective method for repairing damaged aircraft structural components. They are ease in application and provide excellent in-service performance. The major function of a repair is to reduce the stress intensity factor at the crack tip. Calculation of stress intensity factor of a repaired crack has been performed by analytical and numerical methods. However, these methods are based on simplifying assumptions regarding material behavior and repair conditions. In the present paper an experimental determination of mode-I stress intensity factor (SIF), K_I, at the tip of an edge-crack or a V-notch reinforced with double bonded strips or with compression pre-stresses applied along the crack surfaces is undertaken by using the optical method of caustics. This method is simple in its application and has successfully been used for the solution of a host of crack problems of engineering importance.     

Linear coupled bending and extension of an unbalanced bonded repair

An unbalanced repair is a composite patch bonded to one side of a cracked structure for the purpose of preventing or reducing damage growth in the substrate. A single-sided repair offsets the load path within the structure, inducing out-of-plane bending. This bending increases the stress intensity in the underlying crack and causes adhesive peel stresses and bending of the repair which can, relative to a repair that is restrained against bending, lead to early failure. In this article the authors correct the analysis of Wang and Rose [Wang, C.H., Rose, L.R.F., 1997. On the design of bonded patches for one-sided repair. In: Proceedings of the 11th International Conference on Composite materials, Gold Coast, Australia, vol. 5, pp. 347–356] developed by using an energy analysis of a single-sided or unbalanced repair applied to a very long-crack, to comply with Maxwell’s reciprocal theorem and to account for transverse normal and shear stresses at the crack tip and the accompanying shear deflections. The authors then develop closed-form equations useful for bonded composite repair design and damage tolerance assessment of cracks of arbitrary length by developing a new method for interpolation between this long-crack limit and a short-crack limit based on the stress intensity and crack face displacements for an unreinforced crack. The interpolation method is then tested against an advanced line-spring model that has been created by using a 6th order generalized plane strain plate formulation in extension and a new 8th order formulation in bending, thus allowing for the inclusion of transverse shear and normal stresses. The closed-form equations are found to be accurate when compared to the line-spring model, and to provide reasonable results when compared to a three-dimensional finite element model of a bonded repair. Inaccuracies are shown to exist principally in the determination of the nominal stresses in the vicinity of the crack.     

A photoelastic study of crack repair

Birefringent coatings have been employed to study the effectiveness of an adhesively bonded repair of a center-cracked tension panel. The repair was one sided, with photoelastic coatings applied to the opposite side. Photoelastic coatings were also applied over the patch. Analysis methods are presented to permit the stress intensity factor to be determined from the isochromatic fringe patterns recorded from both continuous andX- andY-edged coatings. The results showed that the one-sided adhesively bonded patch reduced the stress intensity factor; however, the repair did not markedly change the character of stress distributions. Fringe loops formed near the crack tips for both the cracked and repaired tension panels. The primary difference was in the size of the loops. The reduction inK _I due to repair was smaller than anticipated, but even small improvements in ΔK _I markedly enhance the life of a repaired panel. The Paris power law is used to show the relation between the reduction in ΔK _I and the improvement in the crack growth rateda/dN. Fringe patterns from the birefringent coatings applied to the patch provided a means not only to investigate the stresses in the patch but also to detect the initiation of the local debonding of the adhesive in the neighborhood of the crack. The birefringent coating on the patch is an approach for producing an optically “smart” repair.     

On irregularity-based damage detection method for cracked beams

A new damage detection technique using irregularity profile of a structural mode shape is proposed in this paper. The mode-shape of a cracked beam is first obtained analytically by using a general function. Its irregularity profile is then extracted from the mode shape by a numerical filter. The location and size of the crack in the beam can be determined by the peak value appearing on the irregularity profile. Two types of numerical filters, i.e., triangular and Gaussian, are examined. It has been found that the former filter is more effective in damage detection than the latter one. Numerical simulations suggest that the irregularity-based method requires a relatively low measurement resolution. Noise stress tests are carried out to demonstrate the effectiveness and robustness of this method under the influence of noise. As a validation, the proposed method is applied to detect crack damage in an E-glass/epoxy laminated composite beam. The successful detection of the crack in the composite beam demonstrates that the irregularity-based method is capable of assessing both the location and size of the crack and can be used efficiently and effectively in damage identification and health monitoring of beam-type structures.     

Investigation on 3D fatigue crack propagation in surface-cracked specimens

In the present work the fatigue crack growth in AISI304 specimens is investigated experimentally. In 3D finite element analysis the virtual crack closure technique is applied to calculate distributions and variations of the stress intensity factor along the surface crack front. It is confirmed that the stress intensity factor along the surface crack front varies non-uniformly with crack growth. Crack growth rate is proportional to the stress intensity factor distribution in the 3D cracked specimen. The fatigue crack growth in surface cracked specimens can be described by the Forman model identified in conventional compact tension specimens. For crack growth in the free specimen surface the arc length seems more suitable to quantify crack progress. Geometry and loading configuration of the surface cracked specimen seem to not affect the fatigue crack growth substantially.     

Study of multi-site damage of fuselage lap joints

The downturn in the world economy coupled with the cost of new aircrafts has meant that there are now ageing fleets whose continued airworthiness requires special attention to corrosion treatments, repair design, fatigue and fracture analysis, and improved crack detection techniques. To assist in this goal the present paper first summarises recent Australian efforts into the development of a simple experimental test specimen which is capable of reproducing the crack growth and failure mechanisms seen in the fuselage lap splice of a wide bodied transport aircraft. The development of a composite repair to overcome these phenomena is then discussed.     

机翼主梁钛合金模拟件谱载疲劳寿命实验研究

确定某型飞机机翼主梁结构的使用寿命是保证该机使用安全的关键。本文对全机第一关键危险部位--机翼钛合金主梁下缘螺栓孔模拟件进行随机谱和程序块谱载荷下的疲劳寿命试验,获得了模拟件的疲劳裂纹形成寿命和疲劳全寿命,并对其寿命进行了统计处理和对比分析。结果表明,程序块谱较随机谱有更长的疲劳寿命。这说明随机谱比程序块谱要严重,对钛合金主梁模拟件的疲劳寿命有显著的影响。该结论可为机翼钛合金主梁构件疲劳寿命预测及疲劳设计提供试验依据。     

焊趾表面裂纹的形态发展曲线与疲劳寿命预测

以作者建立的焊地椭圆表面裂纹应力强度因子数据库以及复杂应力场中焊践半随圆表面裂纹前缘应力强度因子分布计算的基本模式法为基础上,给出了复杂应力场中焊践表面表纹在疲劳扩展过程中形态变化规律及寿命的工程分析方法。