Numerical and experimental study on morphing bi-stable composite laminates actuated by a heating method

This paper presents a new heating actuation method to induce the snap-through phenomenon of bi-stable laminates by manipulating the residual stress in laminates. The mechanism of the heating actuation method is analyzed, and the snap-through process is simulated by the finite element method. The heating actuation experiments on two types of laminates with different stacking sequences are performed. Good agreement is obtained between the experimental data and the finite element analysis (FEA). Subsequently, the heating actuation method is applied to several different bi-stable laminates and investigated by FEA. The FEA results show that this heating actuation method is an effective way to induce the snap-through phenomenon of bi-stable laminates of different thicknesses, sizes and shapes.     

复合材料非对称正交薄层板的固化变形

利用Rayleigh-Ritz 法研究了复合材料非对称正交薄层板的固化变形。建立了考虑几何非线性的固化变形分析模型, 预报了其固化后的变形形状及变形量。利用热压釜工艺进行了实验研究。实验发现, 方板边缘发生了较大的向内卷曲变形, 板边缘附近理论预报值与实验结果差别较大, 在距板边缘一定距离内理论预报值和实验结果吻合较好, 矩形板实验结果与理论预报值吻合良好。      

On the calculation of energy release rates for cracked laminates with residual stresses

Prior methods for calculating energy release rate in cracked laminates were extended to account for heterogeneous laminates and residual stresses. The method is to partition the crack tip stresses into local bending moments and normal forces. A general equation is then given for the total energy release rate in terms of the crack-tip moments and forces and the temperature difference experienced by the laminate. The analysis method is illustrated by several example test geometries. The examples were verified by comparison to numerical calculations. The residual stress term in the total energy release rate equation was found to be essentially exact in all example calculations.     

Optimisation of blended bistable laminates for a morphing flap

Multistable composites offer significant deformations in stable shapes, and, this makes them interesting for morphing applications. Moreover, bistable laminates can be manufactured to have variable angle tows (VATs) in a ply using a tow-steering technique to ensure continuity of fibres over the planform of the laminates and, in doing so, may impart additional structural strength due to load path continuity along-with the prospect of easier integration with the major structure by blending lay-ups across components. The use of ant colony systems as an optimisation concept has been implemented, incorporating the feedback from the finite element analysis to identify blended VAT (equivalent) bistable laminate for a morphing flap application. Proof-of-concept is demonstrated by manufacture of VAT (equivalent) laminates. Presented research findings highlight the potential of blended bistable laminates, developed through optimisation based design methodology, for morphing applications.     

双稳定矩形非对称复合材料层板的跳变研究

对双稳定矩形非对称复合材料层合板两种稳定构型的跳变行为进行了实验和数值研究。通过实验测得中心加载下层合板两种稳定构型跳变的临界载荷, 建立了两种稳定构型的非线性有限元分析模型, 成功预报了层合板固化后的两种稳定构型, 并进一步预报了两种稳定构型的跳变临界载荷, 预报结果与实验结果吻合良好。实验和有限元模拟结果表明, 中心加载下矩形非对称层合板两种构型的临界载荷相差较大, 并且跳变的过程有所差异。      

Measurement of residual stresses in CFRP laminates by X-ray diffraction method

The fabrication of CFRP laminates from prepregs involves curing at elevated temperatures. Residual stresses are set up due to the difference in thermal expansion coefficient between the matrix and the fibre. In this investigation, the X-ray diffraction method is used to measure the curing stresses in CFRP laminates by incorporating a very fine layer of aluminium particles during the lay up of the laminate. A calibration procedure is followed to correlate the strain in the crystalline particles, as measured by X-rays, with the composite strain and stress. Curing stresses measured by this technique are quite close to the value calculated from the differential coefficient of thermal expansion.     

Edge effects of uniformly loaded cross-ply composite laminates

Interlaminar stresses resulting from bending of rectangular cross-ply composite laminates are determined using a layer wise laminate theory. Two types of laminates are considered. First a fully simply supported laminate subjected to bi-directional bending is analyzed. The results obtained from this theory are compared with those of the published three-dimensional elasticity solutions to verify the validity and accuracy of the present theory. Then laminates with two edges simply supported and the other two edges free are examined. The results indicate the presence of significant interlaminar stresses near the free edges.     

固化度与固化收缩对非对称复合材料层合板固化变形的影响

采用三维有限元方法研究复合材料非对称层合板在热载荷和固化收缩载荷下的固化变形情况, 建立了材料力学特性、 固化体积收缩量和温度与固化度之间的函数关系, 考察了层合板变形曲率与温度和固化度之间的关系。数值计算结果表明: 非对称层合板变形曲率与固化终止时固化度有密切关系; 固化变形主要发生在降温阶段; 固化收缩对层合板变形曲率影响很小, 主要发生在第二个保温平台的前半段。      

Characterisation and modelling of the cured shapes of arbitrary layup bistable composite laminates

Asymmetric laminates can have a bistable response to loading, making them of particular interest for applications requiring a large deflection with relatively small and removable energy input. After nearly 30 years of research effort the effects of ply orientation and laminate geometry on the room-temperature shapes of such laminates are well understood and the temperature dependent deformations of laminates of arbitrary layup can be quantitatively predicted. With attention switching to the design of mechanisms incorporating bistable laminates for actuation there is an increasing importance placed on the accuracy and the sensitivity of modelling techniques to predict out-of-plane deflections. This paper presents the results of a novel experimental investigation to map the surface profiles of a series of arbitrary layup laminates. It is concluded that while existing modelling techniques are successful in accurately predicting room-temperature shapes, the sensitivity of solutions to imperfections is significant.     

Low-velocity impact and residual tensile strength analysis to carbon fiber composite laminates

In this paper, low-velocity impact characteristics and residual tensile strength of carbon fiber composite laminates are investigated by experimentally and numerically. Low-velocity impact tests and residual tensile strength tests are performed using an instrumented drop-weight machine (Instron 9250HV) and static test machine (Instron 5569), respectively. The finite element (FE) software, ABAQUS/Explicit is employed to simulate low-velocity impact characteristics and predict residual tensile strength of carbon fiber composites laminates. These numerical investigations create a user-defined material subroutine (VUMAT) to enhance the damage simulation which includes Hashin and Yeh failure criteria. The impact contact force and the tensile strength are accurately estimated using the present method. Two different tensile damage modes after different impact energies are observed. The degradation of residual tensile strengths can be divided to three stages for different impact energies, and amplitudes of degradation are affected by stacking sequences.     

Effect of in-plane boundary restraints on optimal design of skew laminates under buckling loads

Symmetrically laminated cross-ply and angle-ply skew plates subject to uniaxial buckling loads and various combinations of in-plane boundary restraints are studied using a shear deformable theory. For this purpose a finite element code is developed and applied to a couple of verification problems. The formulation of the parabolic iso-parametric plate element is briefly given and numerical results obtained for the verification problems related to stability analysis and stress diffusion are presented. The effect of in-plane restraints on the non-uniform distribution of in-plane stresses is studied by means of contour graphs. Next the buckling loads are maximized with respect to layer thicknesses in the case of cross-ply laminates and with respect to fiber orientations in the case of angle-ply laminates. The optimization results show that the exclusion of the in-plane restraints, which arise in several engineering applications, may lead to errors in the stability analysis and consequently in the design of laminated plates against buckling.     

Experimental characterization of the crack-tip-opening angle in fibre metal laminates

Although the crack-tip-opening angle (CTOA) has been shown to be well suited for modelling stable crack growth in monolithic sheet aluminium alloys, its applicability for fibre metal laminates has not been fully analyzed yet. Fracture test were performed on M(T) panels made of Glare 2-3/2-0.4, Glare 3-3/2-0.4 and laminated 2024-T3 3/2-0.4. Different fatigue pre-crack lengths were created to study the effect of bridging fibres on the CTOA measured on the external layer. The effect of bridging fibres resulted in small deviations of the CTOA vs. crack extension curve with respect to the reference panel made of metal laminate. The CTOA criterion could be successfully used for predicting the residual strength in fibre metal laminates.     

Design of deployable bistable structures for morphing skin and its structural optimization

The morphing skin is the surface of a deployable frame which can change its shape to present optimal performance in all stages of a task for an aerospace vehicle. This article proposes a deployable frame with low kinematic error and high stability. Mechanical design and optimization of the deployable mechanism are developed, and specifically the analysis model of the deployable mechanism is established by the use of extendable units. The genetic algorithm is employed to find the optimal geometric parameters of the foldable structure. Furthermore, the design method of a bistable compliant mechanism is investigated to achieve the bistability. An example of a morphing leading edge (MLE) is analysed using the proposed method. The results show that the position errors converge to the optimal solution which supplies the minimum value. Numerical analysis indicates that the mechanism can provide two bistable positions for the actual use of the MLE.     

Design of multistable composites for application in adaptive structures

During the manufacturing process and operating conditions of multilayered fibre-reinforced composites with variable fibre orientations, residual stresses build up due to the directional expansion of the unidirectionally reinforced single layers. Dependent on the laminate lay-up, these inhomogeneous residual stresses, which are primarily caused by thermal effects, moisture absorption and chemical shrinkage, can lead to large multistable out-of-plane deformations in the case of unsymmetric laminates. Instead of avoiding these laminate's curvatures, they can be advantageously used for technical applications such as novel adaptive structures. In order to adjust the laminate deformations to technical requirements, a dimensioning tool based on a modified stability analysis in combination with a novel optimisation procedure has been developed and experimentally verified. Based on the theoretical investigations, an adaptive prototype of a multistable composite with integrated smart alloys has been designed and manufactured.     

Modelling of laminates using a layerwise element with enhanced strains

A layerwise finite element with enhanced strains is developed for the analysis of laminates with special emphasis on determination of interlaminar forces and study of delaminations. An interface model using the penalty function method is developed to calculate strain energy release rates. Since the interface model provides the facility for the closure of delamination by a small amount, strain energy release rates were evaluated by actual crack closure and by virtual crack closure methods for a comparative study. Numerical examples of delamination are presented to illustrate the accuracy of the computational approaches developed herein. © 1998 John Wiley & Sons, Ltd.     

Predicting fatigue crack initiation in fibre metal laminates based on metal fatigue test data

A methodology is presented to predict the cycles to crack initiation in a notched fibre metal laminate subjected to cyclic loading. The methodology contains four steps. First, the far-field metal layer stress cycle is obtained using classical laminate theory. Second, the peak stress cycle is estimated from a combination of a handbook solution for the stress concentration factor in a finite isotropic plate, and analytical solutions for the stress concentration for equal situations in infinitely large plates. The third step is to adapt the amplitude of the peak stress cycle to the characteristics of S–N data for monolithic material from the literature to allow for the cycles to initiation to be read from the S–N curve for each metal layer.In contrast to what can be found hitherto in the literature about predicting the cycles to fatigue crack initiation in fibre metal laminates, the authors of this paper leave no obscurities but rather attempt to bring understanding of the complete path from situation to prediction.Test results from the literature for Glare 4B-3/2-0.3 have been replicated using the aforementioned methodology. It is shown that it can accurately predict the number of cycles to crack initiation, although the S–N data that is used for the predictions dictates the obtained accuracy. The closer the stress cycle value of the S–N data is to the value of the case analysed, the higher the accuracy obtained. Such a trend was not observed for the stress concentration factor of the S–N curves used, although a choice for S–N data with a different stress concentration factor can cause a significant change in precision. The method is also shown to work for several other fibre metal laminates.     

基于超弹性特性的SMA复合材料层合板低速冲击损伤数值模拟方法

基于ABAQUS有限元软件结合VC++6.0程序设计,建立了含不同铺层角度、不同排列密度形状记忆合金(SMA)纤维的复合材料层合板有限元模型。将基于Brinson本构模型的SMA分段线性超弹性模型以及判断复合材料层内失效的三维HASHIN失效准则编译至ABAQUS/VUMAT子程序,使用界面单元模拟复合材料层间区域,建立了SMA复合材料层合板的低速冲击损伤及冲击后剩余强度数值模拟方法。对比了不含SMA纤维层合板、含SMA纤维层合板、含普通金属丝层合板在不同冲击能量下的损伤响应。进一步分析了SMA纤维体积分数和直径变化对冲击响应的影响。冲击后剩余压缩强度模拟结果表明:冲击能量为16J时,含体积分数25%、直径0.5mm的SMA纤维层合板的冲击后剩余压缩强度相比不含SMA纤维层合板提高5.78%、相比含普通金属丝层合板提高4.69%。随着SMA纤维体积分数提高,层合板的抗低速冲击能力增强,当体积分数一定时,较细的(0.3mm)SMA纤维比粗的(0.6mm)SMA纤维对层合板的抗低速冲击能力增强效果更好。     

基于RFID的SMA增强复合材料层合板无线监测方法及实验

将无线射频识别技术(RFID)引入结构健康监测领域, 提出了利用传感标签和读写器实现结构健康无线监测的新方法。以埋入环氧树脂复合材料结构中具有一定规格和布局的NiTi形状记忆合金(SMA)超弹丝作为电容器的电极, 分布的RFID传感标签实时获取形状记忆合金丝间的微小电容信号, 并将其与时间、 位置编码信息打包后无线传输给读写器和监控站点。对两种SMA增强复合材料层合板, 分别在万能材料试验机和冲击试验机上进行了拉伸、 弯曲和冲击实验, 结果表明, 系统能实现对埋入结构中NiTi超弹性传感元件的高精度监测, 并具有功能扩展方便、 路由选择与定位简单的特点。     

Effect of transverse cracking on stiffness reduction of cross-ply laminates

An analytical model based on the principle of minimum potential energy is developed and applied to study the effect of multiple cracks in cross-plies on the stiffness of a laminated composite. The transverse cracks are assumed to span the thickness of the cross-ply group only partially unlike in the previous studies in which they were assumed to span the entire thickness of the cross-ply group. This arrangement facilitates the study of competition between the self-similar extension of an inherent flaw within the cross-plies and the multiple parallel cracking. The numerical results for the axial stiffness as a function of both the crack density and the crack length are presented for three different composite material systems (glass/epoxy, graphite/epoxy and ceramic/ceramic) or which experimental results are available so as to validate the model.