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.     

Load–displacement behavior of glass fiber/epoxy composite plates with circular cut-outs subjected to compressive load

An experimental study of the behavior of woven glass fiber/epoxy composite laminated panels under compression is presented. Compression tests were performed on to 16 fiber-glass laminated plates with and without circular cut-outs using the compressed machine. The maximum load of failure for each of the glass-fiber/epoxy laminated plates under compression has been determined experimentally. A parametric study was performed as well to investigate the effects of varying the centrally located circular cut-out sizes and fiber angle-ply orientations on to the ultimate load. The experimental work revealed that as the cut-out size increases, the maximum load of the composite plate decreases. Also, it has been observed that cross-ply laminates possess the greatest ultimate load as compared to other types of ply stacking sequences and orientations.     

Composite panels containing multiple through-the-width delaminations and subjected to compression. Part II: Experiments & verification

The objective of this investigation was to perform compression tests on laminated composites for verifying the analytical model and the finite element analysis previously developed by the authors for analyzing compression response of composite panels containing multiple delaminations. T300/976 graphite/epoxy composite was selected for the study. Specimens with various ply orientations were fabricated from both flat and cylindrical composite panels containing one to two pre-implanted through-the-width delaminations. Specimens were tested by uniaxial compression, and strain gauges were utilized to record the strain history as a function of the applied load from initial loading to final failure. Numerical simulations were performed according to the test conditions. Comparisons were then made between the predictions and the measured test data. Overall, the predictions agreed with the data very well. Parametric studies were also performed using the finite element analysis to demonstrate the effect of the size, location and number of the delaminations on the compression response of laminated composites.     

A strength-based multiple cutout optimization in composite plates using fixed grid finite element method

The design of interior cutouts in laminated composite panels is of great importance in aerospace, automobile and structural engineering. Based on the Tsai–Hill failure criterion of the first ply, this paper presents a newly developed Fixed (FG) Grid Evolutionary Structural Optimization (ESO) method to explore shape optimization of multiple cutouts in composite structures. Different design cases with varying number of cutouts, ply orientations and lay-up configurations are taken into account in this study. The examples demonstrate that the optimal boundaries produced by FG ESO are much smoother than those by traditional ESO. The results show the remarkable effects of different opening numbers and various lay-up configurations on resulting optimal shapes. The paper also provides an in-depth observation in the interactive influence of the adjacent cutouts on the optimal shapes.     

复合材料织物层合板挖补修理软件实现与试验验证

针对单向拉伸载荷作用下复合材料织物层合板胶接挖补修理结构,改进现有解析模型,建立适用于无附加层、附加1层和附加2层结构的阶梯型挖补修理结构和斜切型挖补修理结构的解析分析模型。给出求解算法,定义准确度用于评价数值计算精度,最终实现开发一套界面友好的复合材料胶接挖补修理设计与分析软件。该软件可以求解单向拉伸载荷作用下,复合材料胶接修理结构内部的剪应力场/剪应变场分布,评价搭接板受载情况,并预测结构失效载荷与失效模式。研究中采用T300/CYCOM-970织物作为母板与补片材料,METLBOND1515-4M作为胶层材料,设计进行了一系列阶梯型及斜切型挖补修理验证试验。试件失效载荷与软件计算结果吻合良好,阶梯型最大相差5.7%,斜切型最大相差14.0%。该软件可以对复合材料织物层合板胶接挖补修理进行高效、准确的初步辅助设计与分析。     

Experimental investigations on fatigue crack growth of repaired thick aluminium panels in mixed-mode conditions

In this paper, experimental fatigue crack growth of thick aluminium panels containing a central inclined crack of 45° repaired with single-side glass/epoxy composite patch are performed. It is shown that, the technique of single-side repair using glass/epoxy composite patch is effective in the crack growth life extension of the thick panels in mixed-mode conditions. It is also shown that the crack-front of the propagated cracks of the repaired panels has a curvilinear shape which is the effect of the existed out-of-plane bending due to the asymmetry conditions in the single-side repaired panels. It is indicated that the crack propagation path at patched surface is different from the un-patched surface of the panels. In the primary stages of the crack growth, the crack surfaces through the thickness, in the vicinity of the mid-plane propagate without surface twisting. There are considerable differences between the obtained crack growth path at patched and un-patched surfaces of the panels which mean that the crack propagation surfaces have three-dimensional patterns. Using the various thin patch lay-ups has minor effects on the crack re-initiation life of the repaired thick panels. It is shown that using various four layers patch lay-up configurations, the crack propagation life of the cracked panels may increase by the order of 30–85%. The most fatigue crack growth life extension belongs to the repaired panel with the patch lay-up of [90]₄.     

Particle swarm-based structural optimization of laminated composite hydrokinetic turbine blades

Composite blade manufacturing for hydrokinetic turbine application is quite complex and requires extensive optimization studies in terms of material selection, number of layers, stacking sequence, ply thickness and orientation. To avoid a repetitive trial-and-error method process, hydrokinetic turbine blade structural optimization using particle swarm optimization was proposed to perform detailed composite lay-up optimization. Layer numbers, ply thickness and ply orientations were optimized using standard particle swarm optimization to minimize the weight of the composite blade while satisfying failure evaluation. To address the discrete combinatorial optimization problem of blade stacking sequence, a novel permutation discrete particle swarm optimization model was also developed to maximize the out-of-plane load-carrying capability of the composite blade. A composite blade design with significant material saving and satisfactory performance was presented. The proposed methodology offers an alternative and efficient design solution to composite structural optimization which involves complex loading and multiple discrete and combinatorial design parameters.     

Process Factors and Edgewise Compressive Properties of Scarf-repaired Honeycomb Sandwich Structures

Bonded repairs were conducted on flat and edge-closed composite sandwich panels that had undergone different levels of initial damage, and edgewise compression behaviors of repaired panel were tested. Experimental results indicate that these repair techniques can restore the compression performance of damaged panels effectively. The repaired specimens recovered an average of over 83 % of their strength. A k-sample Anderson-Darling test was used to analyze the influence of various parameters, including curing temperature, curing pressure, and repair configurations. After a thorough comparison, it was concluded that a high-temperature, high-pressure treatment can improve the mechanical performance of repaired panels, but the improvement is closely related to the structural complexity of the repaired region. A double-side repair scheme could be used to prevent the degradation of mechanical performance caused by the additional bending moment. The conclusions drawn in the present study provide further insight into the mechanical performance of repaired sandwich panels under edgewise compressive loads. These data facilitate the improved design methodology on bonded repair of composite sandwich structures.     

Analysis of cracks in aging aircraft structures,with and without composite-patch repairs

In this paper, the problems of composite-patch repair of (i) center and edge-cracked panels loaded in the far-field; and (ii) cracks emanating from pin-loaded fastener holes, are examined in thorough detail. The effects of various non-dimensional design parameters on the reduction in the stress-intensity factors near the crack-tip are determined, and are presented in the form of design charts. Both analytical and numerical methods are employed in this study. In the analytical method, the cracked metallic plate was considered to be infinitely large, and the composite patch was modeled as a long orthotropic strip of finite height (in the direction perpendicular to the crack axis). Next, by using the Finite Element Alternating Method (FEAM), a more general analysis capability that can treat arbitrary shapes of the cracked metallic sheet, as well as of the composite patches, is developed. This general FEAM is applied to: (i) composite patch repairs of cracks emanating from loaded fastener holes (the MSD problem); (ii) composite patch repairs of semi-elliptical surface flaws in thick plates; and (iii) composite patch repairs of quarter-elliptical surface flaws emanating from fastener holes. Problem (i) is two-dimensional in nature while problems (ii) and (iii) are fully three-dimensional. In all these cases, the effects of various design parameters on the crack-tip (front) stress-intensity factors are fully discussed.     

Fatigue crack growth analysis of stiffened cracked panel repaired with bonded composite patch

Fatigue crack growth behavior in a stiffened thin 2024-T3 aluminum panel repaired with one-sided adhesively bonded composite patch was investigated through experiments and analyses. The patch had three plies of unidirectional boron/epoxy composite. 2024-T3 aluminum stiffeners were riveted as well as bonded on the panel. Stiffeners were oriented in the loading direction and were spaced at either 102 mm or 152 mm with a crack centered between them. Also, un-repaired cracked panel with and without stiffeners were studied. Experiment involved tension-tension fatigue at constant amplitude with maximum stress of 120 MPa and stress ratio of 0.05. Bonded composite patch repair increased fatigue life about five-fold in the case of stiffened panels while it increased about ten fold in the case of un-stiffened panels. Fatigue life also increased with decrease of the distance between the stiffeners for both repaired and un-repaired panels. A three-dimensional finite element method was used to analyze the experiments. Residual thermal stresses, developed during patch bonding, requires the knowledge of temperature at which adhesive becomes effective in creating a bond between the structure and patch in the analysis. A simple method to estimate the effective curing temperature range is suggested in this study. The computed stress intensity factor versus measured crack growth relationships for all panel configurations were consistent and in agreement with the counterpart from the test material. Thus, the present approach provides a means to analyze the fatigue crack growth behavior of stiffened structures repaired with adhesively bonded composite patch.     

铺层角度偏差对曲面复合材料结构固化变形的影响分析

本文阐述了铺层角度偏差对某变厚度曲面结构复合材料固化变形的影响。对铺层角度偏差的来源进行了归纳,采用考虑热膨胀和固化收缩的固化变形计算模型,对该变厚度的复合材料曲面层合板结构的固化变形进行了计算,计算结果与试验结果较为吻合,表明了计算模型的准确性。采用均匀试验设计方法,得到了该曲面结构铺层角度偏差在5°以内变化时的实验方案,对实验设计的计算结果进行了回归分析,结果显示,对于该曲面复合材料结构,总体上铺层角度偏差对固化变形的影响不大,相对的,-45°的铺层偏差对固化变形的影响较大,90°的铺层偏差对固化变形的影响较小。     

Bolted patch repair of composite panels with a cutout

The present investigation concerns the analysis of bolted patch repairs of flat composite panels by using a complex potential–variational method. The validity of the current analysis predictions is established by comparison against experimental measurements and previous predictions. The experimental investigation used two patch repairs, with different bolt patterns, of a cutout in an aluminum skin under uniaxial loading. The previous predictions were made for a patch-repaired composite skin with 16 bolts under uniaxial loading. The same patch repair configuration is analyzed here under more complex loading conditions and with two bolts missing, leading to a non-symmetric bolt arrangement. Also, the influence of patch geometry and bolt pattern on the effectiveness of the repair is investigated by considering an elliptical cutout in the skin.     

Multiobjective stacking sequence optimization for laminated composite structures

A Pareto-based multiobjective evolutionary algorithm is presented for stacking sequence optimization of composite structural parts. Special attention has been paid to engineering design guidelines for stacking sequence design. These guidelines are included into the formulation of the optimization problem as constraints or additional objectives. A new initialization strategy is proposed following mechanical considerations. The method is applied to the optimal design of a composite plate for weight minimization and maximization of the buckling margins under three hundred load cases that make also the originality of this work. It is shown that the introduction of new ply orientations compared to the classical 0°, ±45° and 90° plies can lead to significantly improved optimal designs.     

Design and Manufacture of Conical Shell Structures Using Prepreg Laminates

The design and manufacture of unstiffened composite conical structures is very challenging, as the variation of the fiber orientations, lay-up and the geometry of the ply pieces have a significant influence on the thickness imperfections and ply angle deviations imprinted to the final part. This paper deals with the manufacture of laminated composite cones through the prepeg/autoclave process. The cones are designed to undergo repetitive buckling tests without accumulating permanent damage. The aim is to define a process that allows the control of fiber angle deviations and the removal of thickness imperfections generated from gaps and overlaps between ply pieces. Ultrasonic scan measurements are used to proof the effectiveness of the proposed method.     

Experimental fatigue crack growth and crack-front shape analysis of asymmetric repaired aluminium panels with glass/epoxy composite patches

In this study, we investigate the experimental fatigue crack-growth behaviour of centrally cracked aluminium panels in mode-I condition which have been repaired with single-side composite patches. It shows that the crack growths non-uniformly from its initial location through the thickness of the single-side repaired panels. The propagated crack-front shapes are preformed for various repaired panels with different patch thicknesses. It is shown that there are considerable differences between the crack-front shapes obtained for thin repaired panels with various patch thicknesses. However, the crack-front shapes of thick repaired panels are not significantly changed with various patch thicknesses. Furthermore, effects of patch thickness on the crack growth life of the repaired panels are investigated for two typical thin and thick panel thicknesses. It shows that the crack growth life of thin panels may increase up to 236% using a 16 layers patch. However, for thick panels, the life may extended about 21–35% using a 4 layers patch, and implementing 8 and 16 layers patches has not a significant effect on the life extension with respect to the 4 layers patch life.     

Optimum buckling design of composite stiffened panels using ant colony algorithm

Optimal design of laminated composite stiffened panels of symmetric and balanced layup with different number of T-shape stiffeners is investigated and presented. The stiffened panels are simply supported and subjected to uniform biaxial compressive load. In the optimization for the maximum buckling load without weight penalty, the panel skin and the stiffened laminate stacking sequence, thickness and the height of the stiffeners are chosen as design variables. The optimization is carried out by applying an ant colony algorithm (ACA) with the ply contiguous constraint taken into account. The finite strip method is employed in the buckling analysis of the stiffened panels. The results shows that the buckling load increases dramatically with the number of stiffeners at first, and then has only a small improvement after the number of stiffeners reaches a certain value. An optimal layup of the skin and stiffener laminate has also been obtained by using the ACA. The methods presented in this paper should be applicable to the design of stiffened composite panels in similar loading conditions.     

Compressive behaviour of CFRP laminates repaired with adhesively bonded external patches

Composite patches can be used to reinforce and repair both cracked composite and metallic aircraft structures. The repair of a composite structure with a composite patch may use mechanical fastening, which often introduces undesirable stress concentrations or adhesive bonding, external or flush patches. To ensure a reliable and durable bond, various parameters such as the quality of surface preparation and the design of the composite patch (size, shape, stiffness) are very important. This paper describes the testing of bonded external patch repaired CFRP laminates loaded in compression. It is found that the critical failure mechanism is fibre microbuckling in the 0° plies accompanied by matrix cracking and delamination, triggered by failures at the adhesive/adherend interface. A three-dimensional finite element analysis is performed to estimate the stress field in the repaired region. The calculated stresses are then used with the maximum stress and average stress failure criteria to predict damage initiation, mode and location. Carefully designed external patch repairs can recover more than 80% of the undamaged compressive strength.     

Strength improvement to composite T-joints under bending through bio-inspired design

This paper shows that a bio-inspired design methodology is an effective method to strengthen composite T-joints under bending loading. The ply angles in the laminate of a carbon/epoxy T-joint were tailored using an optimisation program mimicking the evolutionary process of adaptive growth in which the wood microfibril orientation in and around the tree branch-trunk joint is tailored to the prevailing bending loading condition. A single objective optimisation program with four ply angle input variables was used to compute the optimal design of the ply stacking pattern which minimised the interlaminar tensile stress in composite T-joints where delamination damage is initiated. FEA and experimental testing were performed to compare the structural properties of the bio-inspired T-joint against a base-line T-joint with a quasi-isotropic ply stacking pattern. The bio-inspired T-joint exhibited a higher bending failure initiation load (improved by 40%) and elastic strain energy capacity (increased by 75%) than the base-line T-joint.     

Parametric study of scarf joints in composite structures

Bonded scarf or stepped repairs are used in composite structures when high strength recovery is needed or when there is a requirement for a flush surface to satisfy aerodynamic or stealth requirements. Scarf repairs are complex to design and require the removal of significant parent structure, particularly for thick skins.

A parametric finite element (FE) model has been developed to allow a broad study into the effect of various parameters on the performance of a scarf joint. The stress distribution along the bondline has been investigated, and the sensitivity of peak stresses determined with respect to changes in scarf angle, adhesive thickness, ply thickness, laminate thickness, over-laminate thickness and lay-up sequence. Furthermore, the adhesive stresses resulting from joining matched and mismatched laminates was investigated. The benefit of load by-pass of a repair was also examined using a 3D model of a circular patch. The results of this investigation provide further insight into the stresses that develop in scarf repairs of composite structures under load. This insight may lead to improved design and analysis techniques of scarf joints in composite structures.