The effect of patch geometry on the static and fatigue behaviors of defective aluminum panels repaired with a composite patch

In this study, 6.125-mm cracked Al 7075-T6 plates unpatched and repaired with 4-ply boron/epoxy composite patches of several geometries have been investigated under both static and fatigue loads. The stress distributions around the crack tip for these specimens were calculated using the finite element method. It was shown that Rose's model was not adequate to calculate the stress intensity factor for different patch geometries where patch dimensions were on the order of those of the cracked structure. A new definition based on the stress near the crack tip was introduced. Also, based on the experimental data, a new definition for relative repair efficiency was introduced, and the effects of patch geometry on the static tensile and fatigue behaviors of the repaired structures were examined experimentally. Combining the results of static tensile and fatigue tests, it was concluded that the geometry of the patch had large effects on the properties of the repaired structures, the effects not included in Rose's model.     

Design guidelines for composite patches bonded to cracked aluminum substrates

Design guidelines are proposed for boron fiber-reinforced epoxy composite patches bonded to cracked aluminum substrates. Stress analysis was performed on the composite patch bonded to an uncracked aluminum substrate. The load transfer via the adhesive and the stress levels in the aluminum substrate and the patch were determined. The local stress and stress distribution in the repaired V-shape side cracked substrates were obtained. The effects of crack length and patch geometries including the length and number of plies on the failure behavior of the repair were identified. The results were validated by static tensile tests. It was found that the theoretical calculations were in agreement with the experimental results. This indicates that the proposed analysis can be used as design guidelines for composite repairs of cracked structures. These guidelines coupled with a simple surface preparation technique developed in this work might aid composite patch installation in service.     

Nonlinear stress analysis of v-notched aluminum plates repaired with the wet patch technique

In this study, V-notched aluminum plates were repaired using the wet patch technique. Two different repair operations, single-side and double-side composite patches, were applied to the notched plates. Four layers of woven carbon fiber textile and with the same reinforcement angle were used as a composite patch. Previously, the epoxy was saturated onto the prepared carbon fiber textiles and glued to the plates, and then those plates were placed in a vacuum medium. The repaired plates were experimentally tested under tension load. A nonlinear stress analysis was carried out using the finite element methodology in addition to the experimental results. As major foci of the study, the effects of repair type and notch size on failure behavior and plastic stress distribution were examined. As a result, it was found that the notch depth affects the repair performance far more than the notch width. Debonding failure load between the patch and the plate in plates repaired with a single-side patch was found to be two times higher than the yield load. For the double-side patch, the yield load was close to the debonding load and this repair method increases the debonding load more than a single-side patch.     

双向受载裂纹板的复合材料补片胶接修补

建立和验证了碳纤维/环氧复合材料补片单面胶接修补双向受载的裂纹板的3D有限元模型,探讨了补片材料、铺层顺序、补片长度及双向载荷比等对裂纹应力集中强度因子的影响规律.结果表明,单面贴补的复合材料补片能有效地降低裂纹铝板的裂纹尖端的应力强度因子,同时胶接的复合材料补片使裂纹铝板的垂直裂纹方向的载荷和平行裂纹板方向的载荷发生偶合作用,应根据裂纹铝板的不同载荷情况,选择不同的补片进行修补.     

残余热应力对复合材料修理金属裂纹板影响分析

本文建立了复合材料胶接修理金属裂纹板的三板有限元模型;采用分析金属裂纹板裂纹尖端应力强度因子的方法,分析了残余热应力对复合材料修理金属裂纹板修理效果的影响。结果表明,残余热应力对单面修理结构的影响小于双面修理结构。残余热应力对单面修理结构的影响由于裂纹板宽度比值不同而不同。补片与母板的热膨胀系数相差越大,残余热应力对修补效果影响越大。     

Fatigue behavior of bonded composite repairs

In this study, recent efforts into the research and development of composite repairs bonded to defective aircraft structures are discussed. The fatigue crack growth (FCG) behavior of pre-cracked Al 7075/T6 substrates with bonded composite patches was investigated experimentally and analytically. Boron-epoxy patches with 2-, 4- and 6-plies were installed on Al substrates with single-side-crack. Tension-tension fatigue tests were also conducted on Al substrates to establish their fatigue behavior for comparing with the repaired specimens. A considerable increase in the fatigue life and a decrease in the stress intensity factor (SIF) were observed as the number of plies increased. An analytical model, based on Rose's analytical solution and Paris' power law, was developed to predict the FCG behavior of the repaired substrates. The analytical and experimental results are found to be in good agreement.     

Effects of patch layer and loading frequency on fatigue fracture behavior of aluminum plate repaired with a boron/epoxy composite patch

In this study, 6.125 mm cracked Al 7075-T6 plates with different patch layers of boron/epoxy composite have been fatigue tested. By using analytical and experimental methods, the variation of critical parameters in fracture mechanics, such as stress intensity factor (SIF), potential energy (PE) and energy release rate (J ^*), with patch layers were studied. According to the results of energy and driving force approaches, it was shown that both theories had good consistency using more ply numbers to repair cracked substrates. It was suggested that to investigate the effect of the patch layer on the air stream around the aircraft structure, in addition to fracture mechanics analysis, using a fluid dynamics simulation would be very helpful.     

Analysis and design of adhesively bonded joints for fatigue and fracture loading: a fracture-mechanics approach

An experimental–computational fracture-mechanics approach for the analysis and design of structural adhesive joints under static loading is demonstrated by predicting the ultimate fracture load of cracked lap shear and single lap shear aluminum and steel joints bonded using a highly toughened epoxy adhesive. The predictions are then compared with measured values. The effects of spew fillet, adhesive thickness, and surface roughness on the quasi-static strength of the joints are also discussed. This fracture-mechanics approach is extended to characterize the fatigue threshold and crack growth behavior of a toughened epoxy adhesive system for design purposes. The effects of the mode ratio of loading, adhesive thickness, substrate modulus, spew fillet, and surface roughness on the fatigue threshold and crack growth rates are considered. A finite element model is developed to both explain the experimental results and to predict how a change in an adhesive system affects the fatigue performance of the bonded joint.     

Effect of temperature on the probability and cost analysis of mixed-mode fatigue crack propagation in patched aluminium plate

By the present paper, a numerical model for cost and probability analysis of mixed mode fatigue crack propagation in a patched 2024-T3Aluminium plate is developed. The finite element method is used to analyse the temperature effect on the bonded composite repair performance of the Aluminium plate. Using a fine mesh, mid-plane finite element results are in good experimental results given by literature. Different patch shapes are proposed like; circle, rectangle, square, ellipse, regular octagon, and prolonged octagon. The stress intensity factors at the crack tip are chosen as fracture criteria in order to estimate the repair performance. According to the temperature variation, the lifespan of the repaired fatigue crack growth is estimated using Paris law. The repair done on a central inclined crack with a single-side composite patch show that, best results are given by prolonged octagon and patch is damaged under at elevated temperature. The life-cycle of the patched Aluminium plate is obtained by Monte Carlo simulation, and the probability of failure is quantified according to various geometrical and mechanical parameters. At elevated temperature, the most dominant input parameters are thicknesses of adhesive and patch. In sensitivity analysis effect of thickness of the adhesive layer reaches 35%.Finally, a code source has been developed and implanted in ANSYS software, to consider the manufacturing process and optimal maintenance costs to prove the performance of patch operation from both safety and economical point of view.     

Fatigue delamination growth under cyclic compression in glass/epoxy composite beam/plates

Results are reported on the fatigue growth of internal delaminations in glass/epoxy composite beam/plates subjected to constant amplitude cyclic compression. Because of compressive loading, these structures undergo repeated buckling/unloading of the delaminated layer with a resulting reduction of the interlayer resistance. A noteworthy feature of the problem is that the state of stress near the delamination tip is of mixed mode (I and II). The present combined experimental/analytical investigation for the glass/epoxy composites complements our earlier studies on delamination growth under cyclic compression in unidirectional graphite/epoxy specimens. Several configurations are studied with the delamination located at different depths (through the thickness) and with different applied maximum compressive displacements. The experimental data are correlated with the predictions from a combined delamination buckling/postbuckling and fracture mechanics model. A mode-dependent fatigue delamination growth law is used together with an initial postbuckling solution for the deformation pattern of the delaminated layer and the substrate, which does not impose any restrictive assumptions on the delamination thickness and plate length. The experimental data seem to be adequately correlated with the theory and the fatigue delamination growth is found again to be strongly affected by the relative location of the delamination through the plate thickness. Finally, a comparison of the cyclic growth rate in glass/epoxy specimens with the corresponding one in graphite/epoxy specimens of the same geometry and applied loading shows that the delamination would grow much faster in the graphite/epoxy specimens.     

Delamination shape effects in aramid-epoxy-aluminum (ARALL) laminates with fatigue cracks

An aramid reinforced aluminum-epoxy-laminate, ARALL, which contains a fatigue crack and a delamination zone is analyzed. It is assumed that the interlaminar shear forces between the aluminum and aramid/epoxy layers are transmitted along the delamination boundary. The aramid/epoxy layer of the laminate is considered a series of linear springs. The tensile stress in the aramid/epoxy layer and the stress intensity factor in the aluminum layer are found for various experimentally observed delamination shapes. A residual strength criterion based on the maximum tensile stress in the aramid/epoxy layer is applied and the analytical results are correlated with the available experimental data.     

Fatigue Behavior of Alumina–Zirconia Multilayered Ceramics

The influence of sustained and cyclic loading on the crack growth behavior of a multilayered alumina–zirconia composite exhibiting high internal compressive stresses is investigated. The study was conducted on precracked notched samples and focused on evaluating the static and cyclic fatigue resistance to crack extension beyond the first arresting interface (threshold) as well as the mechanisms involved during stable crack growth through the layered structure for each loading condition studied. Although it is found that the layered composite is prone to subcritical crack growth, the effectiveness of operative toughening mechanisms, i.e., compressive residual stresses as well as crack bifurcation and delamination at interfaces, is observed to be independent of the loading conditions. As a consequence, fatigue degradation of the multilayered ceramics studied is restricted to the intrinsic environmental-assisted cracking of the individual layers, pointing them out as toughened composites practically immune to variable stresses and much less static and cyclic fatigue sensitive than other structural ceramics.     

Molecular Structure of Polymer/Metal Interphases

The molecular structure of interphases in aluminum/epoxy and steel/epoxy adhesive joints was characterized using infrared spectroscopy. In one series of experiments, adhesive joints were prepared by curing beams of epoxy against aluminum or steel substrates. When the joints were cooled to room temperature, the residual stresses were sufficient for crack propagation along the interface. The adhesive and substrate failure surfaces were then analyzed with reflection-absorption infrared spectroscopy (RAIR), attenuated total reflection infrared spectroscopy (ATR) and X-ray photo-electron spectroscopy (XPS). When an epoxy/anhydride adhesive was cured against aluminum substrates primed with an aminosilane coupling agent, amide and imide groups were formed in the interphase. Chemical reaction between the primary amine of the primer and the anhydride of the curing agent precluded chemical bridge formation between the primer and adhesive. Metal cations from the 2024 aluminum substrate reacted with the anhydride to form carboxylate salts on the surface. When an epoxy/tertiary amine adhesive was cured against steel substrates, evidence of oxidation of the primary amine to imine was observed in the interphase.     

Molecular Structure of Polymer/Metal Interphases

The molecular structure of interphases in aluminum/epoxy and steel/epoxy adhesive joints was characterized using infrared spectroscopy. In one series of experiments, adhesive joints were prepared by curing beams of epoxy against aluminum or steel substrates. When the joints were cooled to room temperature, the residual stresses were sufficient for crack propagation along the interface. The adhesive and substrate failure surfaces were then analyzed with reflection-absorption infrared spectroscopy (RAIR), attenuated total reflection infrared spectroscopy (ATR) and X-ray photo-electron spectroscopy (XPS). When an epoxy/anhydride adhesive was cured against aluminum substrates primed with an aminosilane coupling agent, amide and imide groups were formed in the interphase. Chemical reaction between the primary amine of the primer and the anhydride of the curing agent precluded chemical bridge formation between the primer and adhesive. Metal cations from the 2024 aluminum substrate reacted with the anhydride to form carboxylate salts on the surface. When an epoxy/tertiary amine adhesive was cured against steel substrates, evidence of oxidation of the primary amine to imine was observed in the interphase.     

Mechanical behaviour of adhesively repaired pipes subject to internal pressure

Pipes can crack over time, particularly in areas with pipefittings and joints subject to high pressure and unsteady temperatures. Repair of these cracks requires labour, time, and expense and the cracked pipes are currently repaired with two methods. The first method is cutting out the damaged section of the pipe and adding an additional joint, which requires much time and labour. The second method is replacing the damaged pipe, which requires expensive materials. The aim of this study is to propose an alternative method that reduces or eliminates the use of labour, time, and materials, in order to quickly re-activate pipelines. For this purpose, the cracked steel pipes were repaired by using an adhesive, and the mechanical behaviours of the repaired pipes were investigated experimentally and numerically. In the first step of the study, artificial cracks were created on the pipes and the cracked pipes were repaired using adhesive and galvanized steel patches with different overlap lengths, overlap angles and thicknesses. Then, the repaired pipes were subjected to internal pressure in order to evaluate the effects of patch thickness, overlap angle and overlap length on the joint strength. Finally, the numerical analyses and experimental results show that the variation of the patch thickness, overlap length and overlap angle will change the stress distributions and strength of the adhesively repaired pipes.     

Polymer fatigue fracture diagrams: BPA polycarbonate

A fatigue fracture diagram for BPA polycarbonate has been created from fatigue lifetime data obtained from knit line notched samples. This fatigue fracture diagram maps out stress-temperature zones where fatigue fracture is dominated by crack growth through leading crazes and zones where fatigue fracture occurs through shear fracture at 45 degrees to the load direction. Both craze and shear planes coexist in the fatigue crack tip plastic zone, and both compete to determine the ultimate crack growth behavior. The shear planes preferentially develop (and fracture) at higher temperatures and stresses, but this fracture process is quite slow. Consequently, an inversion in the fatigue lifetime curve is observed, with longer lifetimes at higher stresses. This inversion is easily understood as a transition between a craze branch and a shear branch on the fatigue lifetime plot. When the fatigue lifetime curve is plotted for data at different temperatures, with the stresses normalized to the yield stress at the respective test temperatures, the craze branch data from different temperatures overlap. This overlap can be explained by the N = 2 power law dependence of crack growth in the discontinuous crack growth regime.     

复合材料补片尺寸和形状对铝板修补结构lamb波传播影响

借助于ANSYS软件建立包含铝板、复合材料层合板补片、胶层和压电陶瓷片(PZT)的压电和逆压电效应的模型,实现在结构中激励和接收lamb波信号,并以试验验证模型的正确性。利用损伤对结构中lamb波传播特性的影响,对复合材料补片修补后的金属损伤结构的损伤检测进行数值模拟研究,研究不同补片尺寸及形状修补8 mm孔对lamb波损伤检测的影响。结果表明,圆形、方形、菱形补片的仿真模拟结果显示不同的形状对A0和S0波包的影响差异较大,不同尺寸的菱形补片也对A0和S0波包有较大差异。     

Failure analysis of adhesive-patch-repaired edge-notched composite plates

In this study, failure behaviors of edge-notched composite plates which were repaired with using patch and adhesive were investigated experimentally and numerically. The composite plates had a (0°/90°)₃ anti-symmetric stacking sequence; the patches were obtained from the same plates and were bonded using an epoxy-based adhesive. The effects of the geometry of the notch were investigated using U-, V-, and square-shaped notches. We varied the notch geometry, investigated single-lap and double-lap repairs, and varied the patch fiber-stacking sequence. Simulations were carried out to analyze the three-dimensional failure progression using ANSYS. The failure loads of the repaired plates increased by 170–304% for single-lap repairs and 240–476% for double-lap repairs compared with the notched plates. The simulated and measured failure loads were in agreement within 0.2–23.6%.     

Effect of patch hybridization on the tensile behavior of patch repaired glass/epoxy composite laminates using acoustic emission monitoring

This paper investigates the tensile response of damaged glass/epoxy composite laminates repaired using hybrid external patches. Hybrid external patches based on glass and Kevlar woven fabrics bonded on both faces of the damaged parent laminate were considered. Five different kinds of plain weave woven fabrics with a different ratio between glass and Kevlar fibers (100/0, 75/25, 50/50, 25/75 and 0/100) were used as the external patches. The intention of using these hybrid patches was to combine the excellent tensile stiffness of Kevlar fiber with the superior resin adhesion property of glass fiber. The virgin and damaged specimens were taken as the reference specimens for comparison of residual mechanical properties and damage mechanisms. Damage evolution and the failure progression of the repaired glass/epoxy specimens were monitored using real-time Acoustic Emission (AE) monitoring technique. The Acoustic Emission (AE) results depict different damage profiles and link them with mechanical test results to reveal the load to a change in failure mechanisms during mechanical loading concerning the influence of each hybrid patches on the performance of repaired glass/epoxy specimens. Good correlation of the acoustic emission results with the photographic images of fractured specimens was obtained. Specimens repaired with the equal volume fraction of glass and Kevlar fibers in the external patches presented the most favorable residual tensile response by effectively releasing the stress concentration in the damaged area.     

半水煤气气柜腐蚀原因及处理措施

分析了半水煤气气柜腐蚀的原因,介绍了补焊法、水封法和粘补法等堵漏措施。针对铜皮、不锈钢薄板粘补法不成功的原因,提出了用铝箔带粘补,即在铝箔带初步粘补之后覆盖玻璃丝布,并刷环氧树脂3层,固化2～3h。此法操作简便、经济,安全系数高,可防止漏点进一步腐蚀,实际使用效果较好。