Design of Composite Wing Access Cover Under Impact Loads

The paper describes a design concept and prototype development study for a composite wing access cover panel which may be subjected to high velocity impact loads from burst tyre fragments. A number of design variants were fabricated in unidirectional (UD) carbon fibre reinforced polyetheretherketone (CF/PEEK) using the vacuum forming method. Emphasis in the paper is on the use of impact simulations at the design stage to assess various design concepts for the composite panel using finite element (FE) simulation techniques. These predict a damage state in the structure after impact loading, which allows the panel concepts to be ranked for impact resistance. This led to a successful design concept, which was verified by structural impact tests, in which the CF/PEEK access panel had an impact resistant liner of high performance polyethylene fabric.     

含脱粘损伤的复合材料加筋板压缩破坏渐进损伤数值分析

本文提出了基于连续损伤力学的复合材料层合加筋壁板渐进损伤分析模型。该模型采用界面单元模拟筋条和壁板之间的连接界面,连接界面和复合材料层板分别采用Quads准则和Hashin准则作为失效判据,基于内嵌连续损伤状态变量的材料刚度退化方案,采用非线性有限元方法,研究了复合材料加筋壁板在压缩载荷下的破坏过程,分析了结构相应失效模式的细观失效机理。数值分析结果与实验数据吻合较好,证明该方法的合理性和有效性,并详细探讨了界面单元关键参数和层板铺设角度对加筋壁板结构力学响应的影响规律,得到了一些富有价值的结论。     

Test and Modelling of Impact on Pre-Loaded Composite Panels

Currently test and simulation of low and high speed impact of Aerospace composite structures is undertaken in an unloaded state. In reality this may not be the case and significant internal stresses could be present during an impact event such as bird strike during landing, or takeoff. In order to investigate the effects of internal loading on damage and failure of composite materials a series of experimental and simulation studies have been undertaken on three composite types having different fibres, resins and lay-ups. For each composite type panels have been manufactured and transversely impacted under the condition of ‘unloading’ or ‘pre-loading’. For preloading a rig has been constructed that can impose a constant in plane strain of up to 0.25% prior to impact. Results have clearly shown that preloading does lower the composite impact tolerance and change the observed failure modes. Simulation of experiments have also been conducted and have provided an encouraging agreement with test results in terms of both impact force time histories and prediction of the observed failure mechanisms.     

Creep of Textile-Reinforced Composite Under Static and Cyclic Loads

Strength of Materials - The paper considers the creep of a textile-reinforced thermoplastic composite with a glass filler under the action of a combined short-term cyclic and long-term static...     

复合材料加筋壁板冲击损伤特性及损伤容限研究进展

根据近年来复合材料层合板和加筋壁板的冲击损伤特性及损伤容限方面的研究文献,从试验研究和理论分析两方面综述了其研究进展。将含冲击损伤复合材料层板压缩强度分析方法分为五类:宏观唯象法、子层屈曲法、开口等效法、软化夹杂法、累积损伤法,并分析了各类方法的特点和存在的问题。最后对有待进一步研究的问题作了展望。     

A Global/Local Finite Element Approach for Predicting Interlaminar and Intralaminar Damage Evolution in Composite Stiffened Panels Under Compressive Load

This paper addresses the prediction of intralaminar and interlaminar damage onset and evolution in composite structures through the use of a finite element based procedure. This procedure joins methodologies whose credibility has been already assessed in literature such as the Virtual Crack Closure Technique (for delamination) and the ply discount approach (for matrix/fiber failures). In order to establish the reliability of the procedure developed, comparisons with literature experimental results on a stiffened panel with an embedded delamination are illustrated. The methodology proposed, implemented in ANSYS © as post-processing routines, is combined with a finite element model of the panel, built by adopting both shell and solid elements within the frame of an embedded global/local approach to connect differently modelled substructures.     

Impact Damage to Composite Laminates: Effect of Impact Location

Accidental impact loading of Composite laminates during manufacture and in-service can occur in different locations including near the edge or on the edge of a composite structure. This paper describes investigation of the effect of impact to composite laminates and compares the damage arising from central, near edge and on edge impact events. The damage tolerance of impact damaged laminates using both compression and tension tests has been measured. These results reveal the different damage mechanisms arising from different locations of impact. These different damage mechanisms have been investigated using X-Ray computed tomography. Impact on the edge of composite laminates is found to lead to smaller damage area, but more fibre failure; the severity of this damage is not revealed in standard compression after impact tests.     

The Application of Reliability Based Optimization of Tophat Stiffened Composite Panels Subject to Bi-Directional Buckling Loads

Stiffened composite panels are used within many applications, from aerospace to marine applications. Stiffened panels are utilized for their high strength to weight ratio and flexibility of layups while counteracting the low stiffness exhibited by composites. Complications arise when attempting to utilize the full variability of layups in conjunction with reliability constraints creating a complex design problem when constrained by both buckling and material strength. To aid the process of optimizing the design of composite structures and layups, while ensuring a low mass, this paper presents a bi-level optimization scheme for minimization of the weight oftophat stiffened composite panels with probabilistic deflection constraints. To improve the computational efficiency, an energy based grillage method is formulated and applied for the investigation of buckling problems under bi-directional in-plane loads The method is validated by comparing the results obtained from FE model calculations. The variables that have a large impact on the structural safety have been identified by both safety index and COV based reliability analysis. A parametric study of plate dimensions and loading ratios is conducted to investigate the coupling effects on critical buckling load. The method presented in this paper, makes it possible for engineers to improve their designs, at an early stage, with an integrated consideration between product performance and design parameters.     

温度/机械载荷作用下高强度圆环链临界损伤条件研究

以变形条件对圆环链临界损伤因子的影响为主要研究目标,确立物理试验与数值模拟仿真相互佐证寻求临界损伤因子的基本思路,完成不同温度和应变速率条件下多组试样的热物理模拟拉伸试验,利用采集到的参数完成试验的仿真再现,研究温度/机械载荷作用下刨链的强度和寿命特征.结果表明,最大损伤值总是出现在圆环链的肩部,损伤软化现象对应变速率较为敏感,临界损伤因子不是一个常数,而是在0.15～0.54范围内.     

Hybrid S2/Carbon Epoxy Composite Armours Under Blast Loads

Civil and military structures, such as helicopters, aircrafts, naval ships, tanks or buildings are susceptible to blast loads as terroristic attacks increases, therefore there is the need to design blast resistant structures. During an explosion the peak pressure produced by shock wave is much greater than the static collapse pressure. Metallic structures usually undergo large plastic deformations absorbing blast energy before reaching equilibrium. Due to their high specific properties, fibre-reinforced polymers are being considered for energy absorption applications in blast resistant armours. A deep insight into the relationship between explosion loads, composite architecture and deformation/fracture behaviour will offer the possibility to design structures with significantly enhanced energy absorption and blast resistance performance. This study presents the results of a numerical investigation aimed at understanding the performance of a hybrid composite (glass/carbon fibre) plate subjected to blast loads using commercial LS-DYNA software. In particular, the paper deals with numerical 3D simulations of damages caused by air blast waves generated by C4 charges on two fully clamped rectangular plates made of steel and hybrid (S2/Carbon) composite, respectively. A Multi Materials Arbitrary Lagrangian Eulerian (MMALE) formulation was used to simulate the shock phenomenon. For the steel plates, the Johnson-Cook material model was employed. For the composite plates both in-plane and out-of-plane failure criteria were employed. In particular, a contact tiebreak formulation with a mixed mode failure criteria was employed to simulate delamination failure. As for the steel plates the results showed that excellent correlation with the experimental data for the two blast load conditions in terms of dynamic and residual deflection for two different C4 charges. For the composite plates the numerical results showed that, as expected, a wider delamination damage was observed for the higher blast loads case. Widespread tensile matrix damage was experienced for both blast load cases, while only for 875?g blast load fiber failure damage was observed. This agrees well with the experimental data showing that the composite panel was not able to resist to the 875?g blast load.     

Energy Absorption Characteristics of Web-Core Sandwich Composite Panels Subjected to Drop-Weight Impact

The objective of the study was to characterise the energy absorption of composite panels with tied cores, subjected to a drop weight impact test. Numerical simulations based on explicit finite element analysis have successfully modelled low velocity impact tests carried out on sandwich panels with web-core structure and plastic foam. The numerical model has been validated in terms of the failure behaviour of the panel and the variation of the contact force after the initial peak load corresponding to flexural failure. The numerical model is used for a better interpretation of the test results and of the failure mechanisms within the structure. The contribution to the overall energy absorption of the different parts composing the panels has been studied, with the aim of evaluating the feasibility of using low density foam in combination with web-core reinforcement in structural applications.     

Analytical Solutions to Predict Impact Behaviour of Stringer Stiffened Composite Aircraft Panels

Applied Composite Materials - This paper aims to develop an analytical method to predict the low-velocity impact response of simply supported stringer stiffened panels. Since the combination of...     

Nonlinear Viscoelastic Response of Unidirectional Polymeric Laminated Composite Plates Under Bending Loads

Nonlinear bending analysis of polymeric laminated composite plate is examined considering material nonlinearity for viscoelastic matrix material through a Micro–macro approach. The micromechanical Simplified Unit Cell Method (SUCM) in three-dimensional closed-form solution is used for the overall behavior of the unidirectional composite in any combination of loading conditions. The elastic fibers are transversely isotropic where Schapery single integral equation in multiaxial stress state describes the matrix material by recursive-iterative formulation. The finite difference Dynamic Relaxation (DR) method is utilized to study the bending behavior of Mindlin annular sector plate including geometric nonlinearity under uniform lateral pressure with clamped and hinged edge constraints. The unsymmetrical laminated plate deflection is predicted for different thicknesses and also various pressures in different time steps and they are compared with elastic finite element results. As a main objective, the deflection results of viscoelastic laminated sector plate are obtained for various fiber volume fractions in the composite system.     

Damage Mechanisms in Composite/Composite Bonded Joints Under Static Tensile Loading

This paper presents an experimental study of the initiation and development of damage in composite/composite joints. The materials studied are glass fibre reinforced polyester laminates and these are bonded together using an epoxy adhesive. Two types of joint interface are examined, assemblies between woven roving layers and between mat layers. The experimental techniques employed are dye impregnation, extensometry, visual observation and photoelasticimetry. It is shown that the nature of the surface layer does not influence the damage initiation load but does affect subsequent crack propagation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Modeling Lightning Impact Thermo-Mechanical Damage on Composite Materials

Carbon fiber-reinforced polymers, used in primary structures for aircraft due to an excellent strength-to-weight ratio when compared with conventional aluminium alloy counterparts, may nowadays be considered as mature structural materials. Their use has been extended in recent decades, with several aircraft manufacturers delivering fuselages entirely manufactured with carbon composites and using advanced processing technologies. However, one of the main drawbacks of using such composites entails their poor electrical conductivity when compared with aluminium alloy competitors that leads to lightning strikes being considered a significant threat during the service life of the aircraft. Traditionally, this problem was overcome with the use of a protective copper/bronze mesh that added additional weight and reduced the effectiveness of use of the material. Moreover, this traditional sizing method is based on vast experimental campaigns carried out by subjecting composite panels to simulated lightning strike events. While this method has proven its validity, and is necessary for certification of the structure, it may be optimized with the aid provided by physically based numerical models. This paper presents a model based on the finite element method that includes the sources of damage observed in a lightning strike, such as thermal damage caused by Joule overheating and electromagnetic/acoustic pressures induced by the arc around the attachment points. The results of the model are compared with lightning strike experiments carried out in a carbon woven composite.     

Structural Integrity of Co-Cured Composite Panels

Skin and stiffeners with co-cured joints are widely used in aircraft structures. Co-cured parts, where stiffeners are integrally cured with the skin in one cure cycle, will gradually replace fastener structures. This paper investigates the structural strengths of co-cured joints in skin and J-shaped stiffeners. The experimental results correlated well with the predicted results for compressive panels. The compressive strength of J-stiffened panels significantly increased as the length of the web decreased and the thickness of the web increased. In addition, it was found that film adhesive that was hand rolled and placed at the intersection of the web and skin could effectively improve the structural strength.     

Impact Damage of 3D Orthogonal Woven Composite Circular Plates

The damages of 3D orthogonal woven composite circular plate under quasi-static indentation and transverse impact were tested with Materials Test System (MTS) and modified split Hopkinson bar (SHPB) apparatus. The load vs. displacement curves during quasi-static penetration and impact were obtained to study the energy absorption of the composite plate. The fluctuation of the impact stress waves has been unveiled. Differences of the load-displacement curves between the quasi-static and impact loading are discussed. This work also aims at establishing a unit-cell model to analyze the damage of composites. A user material subroutine which named VUMAT for characterizing the constitutive relationship of the 3-D orthogonal woven composite and the damage evolution is incorporated with a finite element code ABAQUS/Explicit to simulate the impact damage process of the composite plates. From the comparison of the load-displacement curves and energy absorption curves of the composite plate between experimental and FEM simulation, it is shown that the unit-cell model of the 3D woven composite and the VUMAT combined with the ABAQUS/Explicit can calculate the impact responses of the circular plate precisely. Furthermore, the model can also be extended to simulate the impact behavior of the 3D woven composite structures.     

Damage Behaviors of Foam Sandwiched Composite Materials Under Quasi-Static Three-point Bending

This paper reports the quasi-static three-point bending damage behaviors of foam sandwiched composites in finite element analyses (FEA) and experimental. Finite element calculations were performed to characterize the static response of foam sandwich composites with different ply angle face sheets. Quasi-static three-point bending tests were conducted with a MTS materials testing system to obtain the load–displacement curves and energy absorption under quasi-static bending. A crushable foam model was used in order to explore the mechanical behaviors of core materials, while the Hashin criterion was employed to predict the failure of the face sheets. The load–displacement curves show a satisfactory agreement between the experimental and numerical results. The finite element calculations can also be used to obtain the failure mode included the core damage, face sheet damage and face-core interface damage. It can be observed that the damage at the core material can be classified as either core cracking or core crushing. The damage of the face sheet was through matrix cracking and delamination, with fiber breakage. The significant indentation occurs as a result of the fiber breakage. The face-core interface crack was typically induced by the cracks initiated from the tensile side and propagated to the compressive side.     

T700/6421复合材料层板低速冲击后损伤特性

对未增韧及增韧后的T700/6421复合材料层板进行了低速冲击实验,讨论了表面凹坑深度(D)、表面凹坑直径(R)、冲击后压缩强度(CAI)及冲击能量(E)的关系,并通过记录冲击过程中的接触力与时间的变化分析了冲击时的损伤过程。实验结果表明,增韧后的复合材料其更容易出现深凹坑及更大的表面凹坑直径,更容易实现目视明显可见损伤(VID)的效果,并通过分析冲击历程响应发现:低速冲击过程中复合材料层板起始产生分层损伤时的时间与冲击能量并没有必然的联系。     

Micromechanical Modeling of Impact Damage Mechanisms in Unidirectional Composite Laminates

Composite laminates are susceptible to the transverse impact loads resulting in significant damage such as matrix cracking, fiber breakage and delamination. In this paper, a micromechanical model is developed to predict the impact damage of composite laminates based on microstructure and various failure models of laminates. The fiber and matrix are represented by the isotropic and elastic-plastic solid, and their impact failure behaviors are modeled based on shear damage model. The delaminaton failure is modeling by the interface element controlled by cohesive damage model. Impact damage mechanisms of laminate are analyzed by using the micromechanical model proposed. In addition, the effects of impact energy and laminated type on impact damage behavior of laminates are investigated. Due to the damage of the surrounding matrix near the impact point caused by the fiber deformation, the surface damage area of laminate is larger than the area of ??impact projectile. The shape of the damage area is roughly rectangle or elliptical with the major axis extending parallel to the fiber direction in the surface layer of laminate. The alternating laminated type with two fiber directions is more propitious to improve the impact resistance of laminates.