Static Properties of Fibre Metal Laminates

In this article a brief overview of the static properties of Fibre Metal Laminates is given. Starting with the stress-strain relation, an effective calculation tool for uniaxial stress-strain curves is given. The method is valid for all Glare types. The Norris failure model is described in combination with a Metal Volume Fraction approach leading to a useful tool to predict allowable blunt notch strength. The Volume Fraction approach is also useful in the case of the shear yield strength of Fibre Metal Laminates. With the use of the Iosipescu test shear yield properties are measured.     

Predicting the influence of temperature on fatigue crack propagation in Fibre Metal Laminates

A robust crack growth prediction tool has been developed for a class of hybrid skin materials known as Fibre Metal Laminates (FMLs) which has been thoroughly validated for fatigue loading cases at room temperature. This paper provides a brief overview of this predictive model and presents an investigation into its predictive capabilities at various temperatures. Amongst the temperature effects investigated are crack growth rate in the metal layers, delamination growth rate along the metal–fibre interfaces, and residual curing stresses within the laminate. Results from this investigation indicate that the present model accounting for these effects can accurately predict crack growth in FMLs at room temperature and elevated temperature, but is overly conservative for predictions at low temperatures.     

Delamination growth in Fibre Metal Laminates under variable amplitude loading

This paper presents the experimental and analytical investigation of the effect of variable amplitude (VA) load sequences on delamination behavior in Fibre Metal Laminates (FMLs). Delamination tests were performed and results are compared with linear damage accumulation predictions. Scanning Electronic Microscopy (SEM) was used to analyse the delaminated surfaces to study the delamination growth rate under VA loading in more detail. The correlation between test results and predictions highlighted the absence of load sequence and interaction effects in delamination growth rate under VA loading. This correlation is supported by the SEM observations.     

An Historic Overview of the Development of Fibre Metal Laminates

In this paper a brief overview of the history of Fibre Metal Laminates Arall and Glare is given as background information for the other, technical articles in this journal. The story of the development of Fibre Metal Laminates is rather a unique story in the history of aircraft materials: A university laboratory invented, developed and certified an aircraft material. Many parties were involved naturally, yet the very heart of the activity was the Structures and Materials Laboratory of the Faculty of Aerospace Engineering of Delft University of Technology in The Netherlands. At the break of the world's largest passenger transport aircraft, the Airbus A380, in which a substantial part of the fuselage will be made of Glare, the glass fibre-aluminium version of Fibre Metal Laminates, it is a good moment to tell some of its history.     

Application of a modified Wheeler model to predict fatigue crack growth in Fibre Metal Laminates under variable amplitude loading

This paper presents the investigation regarding fatigue crack growth prediction in Fibre Metal Laminates under variable amplitude fatigue loading. A recently developed constant amplitude analytical prediction model for Fibre Metal Laminates has been extended to predict fatigue crack growth under variable amplitude loading using the modified Wheeler model based on the Irwin crack-tip plasticity correction and effective stress intensity factor range (ΔK_eff). The fatigue crack growth predictions made with this model have been compared with crack growth tests on GLARE center-cracked tension specimens under selective variable amplitude loading as well as flight simulation loading. The accuracy of the model is discussed in comparison with the experimental fatigue crack growth data.     

Preparation and properties of Fibre–Metal Laminates based on carbon fibre reinforced PMR polyimide

A novel Fibre–Metal Laminates (FMLs) based on carbon fibre reinforced PMR polyimide were prepared using a hot press process in this paper. Pre-treatment on the titanium surface were conducted prior to laminating. Scanning Electron Microscope (SEM) were used to observe the morphologies of the titanium and the cross-sections of the FMLs. SEM results showed that micro-roughness structures were formed on the titanium surface after anodization. This structure enhanced the interlaminar bond strength between titanium and polyimide. Flexural and Interlaminar shear (ILSS) tests showed that the FMLs possess excellent flexural and interlaminar properties at both room temperature and elevated temperature. Thermostability tests proved that the FMLs based on carbon fibre reinforced PMR polyimide offered excellent thermal properties. It is shown that no delamination appears between titanium layer and the fibre-reinforced polyimide layer after 1000 times thermal shock.     

Delamination and fatigue crack growth behavior in Fiber Metal Laminates (Glare) under single overloads

Delamination extension and fatigue crack growth behaviors under single overloads were investigated for GLARE 2-2/1-0.3 with fiber direction of 00/00. The results indicate that the stress intensity factor at the crack tip in metal layer while overload applied, K_tip,ol is a key controlling variable which influences fatigue crack growth and delamination behaviors. When K_tip,ol becomes bigger and exceeds a critical value, an obvious kink in the delamination shape is observed nearby the location of overload applied. Crack growth rate after application of overload could not return to its original level even the crack grows beyond the overload plastic zone. The reduction magnitude of the crack growth rate becomes bigger with the overload ratio (intrinsically K_tip,ol) increasing. These new results for the crack growth behavior have never been reported before, which can be well explained by the delamination extension behavior observed after overload applied.     

Manufacturing of GLARE Parts and Structures

GLARE is a hybrid material consisting of alternating layers of metal sheets and composite layers, requiring special attention when manufacturing of parts and structures is concerned. On one hand the applicable manufacturing processes for GLARE are limited, on the other hand, due to the constituents and composition of the laminate, it offers new opportunities for production. One of the opportunities is the manufacture of very large skin panels by lay-up techniques. Lay-up techniques are common for full composites, but uncommon for metallic structures. Nevertheless, large GLARE skin panels are made by lay-up processes. In addition, the sequences of forming and laminating processes, that can be selected, offer manufacturing options that are not applicable to metals or full composites. With respect to conventional manufacturing processes, the possibilities for Fibre Metal Laminates in general, are limited. The limits are partly due to the different failure modes, partly due to the properties of the constituents in the laminate. For machining processes: the wear of the cutting tools during machining operations of GLARE stems from the abrasive nature of the glass fibres. For the forming processes: the limited formability, expressed by a small failure strain, is related to the glass fibres. However, although these manufacturing issues may restrict the use of manufacturing processes for FMLs, application of these laminates in aircraft is not hindered.     

玻璃纤维-铝合金层板(GLARE)的残余应力

对确定纤维金属层板残余应力的几种方法的基本原理进行了介绍，用Ｘ光衍射法，解析法和腐蚀去层法确定了玻璃纤维－铝合金层板（ＧＬＡＲＥ）的残余应力，对以上几种测试方法的优缺点进行了分析，提出了确定纤维金属层板残余应力的建议。     

Post-stretching induced stress redistribution in Fibre Metal Laminates for increased fatigue crack growth resistance

Post-stretching is a potential method to change the unfavorable residual stress system in fibre metal laminates (FMLs). During post-stretching of the material, the metal layers will be strained into the plastic region of the stress–strain curve, while the fibre layers remain elastic. After unloading, the residual stress system due to curing will be reduced or even reversed dependent on the amount of stretching. In the past, the classical laminate theory (CLT) has been modified, incorporating the thermal expansion for the application to FMLs. Furthermore, a fatigue crack growth prediction model has been developed based on this extended theory. Here, this classical laminate theory is further extended to calculate the stress redistribution after stretching the uni-directional laminates (ARALL and GLARE). This paper present the method to calculate the residual internal stress distribution in the fibre direction.For the validation of the post-stretching formulations, fatigue crack growth tests on GLARE1-2/1, 3/2, 4/3, 5/4-0.3 have been performed. Good agreement has been obtained.     

Application Issues Using Fibre Bragg Gratings as Strain Sensors in Fibre Composites

Abstract: Fibre Bragg grating sensors have been developed as a strain measurement technique. With regard to practical applications, there is growing knowledge and experience. However, many application issues remain unsolved. These issues include signal interpretation, confidence in measurement accuracy, calibration, and zero-strain definition. This paper reports some issues which have been encountered in practical measurements and application trials. The investigation covers various aspects of strain measurement. The measurement accuracy in a composite laminate is found to be influenced by the formation of cracks in adjoined layers, interface between fibre sensor and host materials and nonuniform stress fields.     

Fatigue and Damage Tolerance of Glare

Methods have been developed to describe the fatigue initiation and propagation mechanisms in flat panels as well as mechanically fastened joints and to determine the residual strength of large flat panels. Glare shows excellent crack growth characteristics due to the mechanism of delamination and fibre bridging. The fatigue insensitive fibres restrain the crack opening and transfer load over the crack in the metal layers. During the initiation phase fibre bridging does not occur and the behaviour is dominated by the metal initiation properties. Mechanically fastened joints introduce additional effects such as secondary bending, load transfer and aspects related to the fastener installation. The residual strength of Glare is dependent on the amount of broken fibres and the delamination size and can be described with the R-curve approach.The impact resistance of Glare is related to the aluminium and glass/epoxy properties and is significantly higher than the impact resistance of monolithic aluminium. The same has been proven for fire resistance. Depending on the Glare grade and thickness, the outer aluminium layer will melt away, whereas the other layers will remain intact due to carbonisation of the glass/epoxy layers and delamination of the laminate. The air in the delaminations will act as insulation, keeping the temperatures at the non-exposed side relatively low.     

Lamb Wave Interaction with Delaminations in CFRP Laminates

In this paper, we investigate Lamb wave interaction with delamination in an infinite carbon fiber reinforced plastics (CFRP) laminate by a hybrid method. The infinite CFRP laminate is divided into an exterior zone and an interior zone. In the exterior zone, the wave fields are expressed by wave mode expansion. In the interior zone, the wave fields are modeled by the finite element method (FEM). Considering the continuity condition at the boundary between the exterior and interior zones, the global wave fields can be calculated. Lastly, numerical examples show how a delamination in the laminate influences the mode conversion of different incident wave modes.     

Effect of Temperature on the Tensile Strength and Failure Modes of Angle Ply Aramid Fibre (KRP) Tubes Under Hoop Loading

A comprehensive study was undertaken to characterise Kevlar reinforced plastic (KRP) angle ply filament wound tubes at different temperatures. Quasi-static burst tests were performed on tubes of 25°, 55° and 75° winding angle. The tubes were burst under internal radial pressure with minimum end constraints. An experimental rig and two conditioning tanks were designed and built to test the specimens at three temperatures; –46°C (low temperature) and +20°C (room temperature) and +70°C (high temperature). For each test the internal pressure and the strains in both circumferential and longitudinal directions were recorded on suitable digital processing equipment.For a particular batch of tubes tested at three different temperatures, an increase in ultimate hoop strain and a decrease in hoop modulus of the 55° tubes with increasing temperatures was recorded; the temperature effect was less pronounced on the corresponding properties of 25° and 75° tubes. The use of a non-structural thin liner during the tests led to a higher ultimate strength of 55° tubes but had negligible effect on the behaviour of 25° and 75° tubes. The 75° tubes failed in a catastrophic fibre fracture under all test conditions. The mode of failure of 55° changed from weeping at 70°C to fibre fracture at –46°C. The 25° tubes failed by weeping with matrix cracking. The matrix cracking was particularly severe when a liner was used.     

A MODEL TO PREDICT THE FATIGUE LIFE OF FIBRE-REINFORCED TITANIUM MATRIX COMPOSITES UNDER CONSTANT AMPLITUDE LOADING

A model based on micro-mechanical concepts has been developed for predicting fatigue crack growth in titanium alloy matrix composites. In terms of the model, the crack system is composed of three zones: the crack, the plastic zone and the fibre. Crack tip plasticity is constrained by the fibres and remains so until certain conditions are met. The condition for crack propagation is that fibre constraint is overcome when the stress at the location of the fibre ahead of the crack tip attains a critical level required for debonding. Crack tip plasticity then increases and the crack is able to propagate round the fibre. The debonding stress is calculated using the shear lag model from values of interfacial shear strength and embedded fibre length published in the literature. If the fibres in the crack wake remain unbroken, friction stresses on the crack flanks are generated, as a result of the matrix sliding along the fibres. The friction stresses (known as the bridging effect) shield the crack tip from the remote stress, reducing the crack growth relative to that of the matrix alone. The bridging stress is calculated by adding together the friction stresses, at each fibre row bridging the crack, which are assumed to be a function of crack opening displacement and sliding distance at each row. The friction stresses at each fibre row will increase as the crack propagates further until a critical level for fibre failure is reached. Fibre failure is modelled through Weibull statistics and published experimental results. Fibre failure will reduce the bridging effect and increase the crack propagation rate. Calculated fatigue lives and crack propagation rates are compared with experimental results for three different materials (32% SCS6/Ti-15-3, 32% and 38% SCS6/Ti-6-4) subjected to mode I fatigue loading. The good agreement shown by these comparisons demonstrates the applicability of the model to predict the fatigue damage in Ti-based MMCs.     

纤维/金属层状复合材料的研究及应用进展

玻璃纤维增强铝合金复合材料(GLARE层板),是由铝合金薄板(金属层厚度为0.3～0.5mm)与玻璃纤维/树脂预浸料(纤维层厚度为0.2～0.3 mm)多层交替铺放,在120～175℃范围内热压罐成形制造的先进航空材料。GLARE层板由金属与纤维层构成并将其各自的优点结合起来,因而具有极高的疲劳强度(较2024铝增加3倍以上)和损伤容限(较2024铝增加1～2倍),以及较大的减重潜力(较2024铝减重15%～30%),在新一代航空、航天等高技术领域的广阔应用前景,成为了大型航空制造业的关注热点。首先阐述了GLARE层板的研究历史,然后从GLARE层板的制造加工、拉伸性能、冲击性能、抗疲劳性能、环境耐受性能及回收利用6个方面,详细论述了GLARE层板的研究进展及应用。最后,总结了GLARE层板技术,并对其发展趋势进行了展望。     

Optimisation of fibre steering in composite laminates using a genetic algorithm

An optimization procedure using a genetic algorithm has been applied to define the optimum orientation of fibres in a uni-directional laminate in which the fibres were allowed to vary continuously across the domain. The domain was divided into two-dimensional finite elements and anisotropic properties corresponding to a carbon fibre laminate with all layers aligned in the zero element axis direction were applied to the laminate. The orientation of the material axis on each element was then prescribed as an independent variable for the genetic algorithm.     

木芯复材拉挤成型柱轴压性能及其工程应用

基于拉挤成型工艺与FRP夹芯构件设计理念,提出一种新型木芯复材拉挤成型柱(Pultruded FRP-wood Column,PFWC),其截面形式为外围的玻璃纤维(GFRP)和内部的南方松木芯(Wood-core)。PFWC生产工艺具备连续化、工业化、可控化及质量稳定等优点,且已成功运用于南京玉竹楼。PFWC的轴压试验研究表明:中柱和长柱的破坏模式分别为面板压溃破坏且纵向纤维撕裂及两侧面板局部屈曲,临界屈曲承载力随着构件长细比的增加逐渐减小。当PFWC长细比小于60时,欧拉临界屈曲承载力计算值大于试验值,但当长细比大于60时,欧拉临界屈曲承载力与试验值吻合较好。且随着长细比的增大,理论值与试验值之间的误差减小。     

Experiments on curved sandwich panels under blast loading

In this paper curved sandwich panels with two aluminium face sheets and an aluminium foam core under air blast loadings were investigated experimentally. Specimens with two values of radius of curvature and different core/face sheet configurations were tested for three blast intensities. All the four edges of the panels were fully clamped. The experiments were carried out by a four-cable ballistic pendulum with corresponding sensors. Impulse acting on the front face of the assembly, deflection history at the centre of back face sheet, and strain history at some characteristic points on the back face were obtained. Then the deformation/failure modes of specimens were classified and analysed systematically. The experimental data show that the initial curvature of a curved sandwich panel may change the deformation/collapse mode with an extended range for bending dominated deformation, which suggests that the performance of the sandwich shell structures may exceed that of both their equivalent solid counterpart and a flat sandwich plate.