Process-induced deformation of composite T-stiffener structures

Composite T-stiffener structures are widely used structural elements in weight sensitive structural, aerospace and marine applications for the purpose of weight reduction. A study on the process-induced deformation of composite T-stiffener structures is presented in this paper. The deformation was calculated numerically by Finite Element Analysis (FEA), and the resulting displacements show that the deformation can be represented by a single angular displacement: the spring-in of the skin. A parametric study was conducted by Design of Experiments (DOE) and FEA to investigate the influence of design parameters on the spring-in of the skin. It is shown that the spring-in of the skin increases with the radius and the bonding length, and decreases with the fiber volume fraction.     

模具对柱面复合材料构件固化变形影响的有限元分析

针对复合材料构件固化变形问题, 分析了复合材料热压罐成型固化过程的多场耦合关系, 考虑模具的作用, 建立了柱面复合材料构件固化过程的有限元分析模型。基于此模型, 研究了模具材料、 模具厚度和模具形式对柱面复合材料构件固化变形的影响。结果表明: 模具对柱面件固化变形的影响较大, 模具材料与构件材料热膨胀系数(CTE)不匹配程度影响构件回弹角的大小; 模具的厚度不同, 导致构件的不同回弹角; 采用阴模时, 构件回弹角小于阳模的, 且回弹方向相反。     

Role of tool-part interaction in process-induced warpage of autoclave-manufactured composite structures

The role of tool-part interaction in process-induces warpage of a large composite structure was studied using a three-dimensional process model, developed by integrating sub-models that describe the evolution of cure and properties of composite as well as various physical phenomena encountered, during autoclave processing. The process model was implemented through user sub-routines interfaced with the finite element software, ABAQUS. The tool-part interaction during processing was modeled using contact elements. The predicted temperature and warpage of an aircraft part, using a frictional tool-part interface and experimentally measured cure-dependent tool-part interfacial friction coefficients, compared very well with experimental temperature and warpage, validating the 3-D process model. A comparison of predictions using various models for the tool-part interface suggests that the two components of tool-part interaction that contribute to warpage are change in shape of the tool and part, and process-induced stress caused by constrained deformation of the tool and the part.     

热固性树脂基复合材料固化变形影响因素分析

采用整体-子模块化方法建立了描述复合材料固化全过程的三维有限元模型。以L 形层合板为例,分析了固化工艺、结构设计和模具等因素对固化变形的影响方式和程度。数值模拟结果表明：升温速率和对流换热系数通过改变峰值温度影响回弹角,固化压力通过改变树脂分布和含量影响回弹角;铺层方向引起的结构力学性能的变化是回弹角差异较大的主要原因,厚度对固化变形的影响需考虑其对峰值温度和结构刚度变化两方面因素的综合影响,拐角半径的变化对固化变形的影响较小;模具形式通过改变树脂分布梯度和模具对结构的作用力位置影响回弹角,模具材料和形式的选择对于固化变形控制具有重要意义。     

Thermally induced deformation of laminated composite structures

The implications of somewhat unfamiliar thermo-elastic phenomena in laminated fibre composites are discussed in physical terms. By this approach certain simplified relationships are developed and the attendant physical interpretations are encouraged. Specifically, laminated flat plates exhibiting finite, negative thermal expansion coefficients and hoop-wound composite rings subjected to temperature changes are treated so as to emphasize the importance of the phenomena. It is also shown that the translaminar expansion coefficient is an important parameter in the hoop-wound ring which develops significant circumferential stresses when subjected to uniform changes in temperature.     

Numerical and experimental analysis of delamination in the T-stiffener integrated composite structure

In this article, two kinds of cohesive zone models (CZMs; exponential and bilinear) are used to evaluate the delamination behaviors of a composite T-stiffener integrated structure. First, based on the analysis of the bilinear CZM using maximum nominal stress damage initiation criterion and power law energy criterion, both the macroscopic mechanical response and the failure process are predicted, which analyzed the influences of the various cohesive zone parameters on the failure load and the damage patterns. Second, both the strength and the fracture characterizations about various T-stiffener integrated composite structures are studied in the experiment, which have a good agreement between the numerical result and the experimental data. Finally, the relationships among the failure load and the thickness of the skin, and the clamp distance are established; also, the energy release rates of the T-stiffeners for the failure process are predicted. These results will play an important role for designing and evaluating the strength and reliability of composite T-stiffener integrated structures.     

A parametric study on the failure of bonded single-lap joints of carbon composite and aluminum

A parametric study on adhesively bonded carbon composite-to-aluminum single-lap joints was experimentally conducted. FM73m, a high strength adhesive produced by Cytec, was used for bonding. The primary objective of this study is to investigate the effects of various parameters, such as bonding pressure, overlap length, adherend thickness, and material type, on the failure load and failure mode of joints with dissimilar materials. While metal bonded joints generally fail at the adhesive, the final failure mode of all the tested bonded joints with dissimilar materials was delamination of the composite adherend. Bonding strengths of the tested joints were lower than the metal-to-metal bonded joint strength. The specimens bonded under pressure of 4 and 6 atm yielded higher failure loads than under pressure of 3 atm, which is within the range of the manufacturer-recommended bonding pressure. Failure loads of the joint increased slightly at an overlap length larger than 30 mm. Increasing adherend thickness resulted in an increase of the failure load, but was not linearly proportional to the failure load.     

Progressive damage modeling for large deformation loading of composite structures

A nonlinear constitutive model for large deformation loading at different strain rate condition was developed to represent tensile progressive damage of the nonlinear large deformation rate dependent behavior of polymer-based composite materials. The material was characterized by using off-axis composite specimens at different strain rates. A new failure criterion was proposed for the analysis of different loading directions and strain rates. Based on a method of combining the nonlinear constitutive theory and the proposed failure criterion for different strain rates, the progressive damage behavior of large deformation composites was represented. The strength of the material was also successfully represented with a single material constant.     

结构参数对复合材料V型构件固化变形影响试验及解析分析

为研究结构参数对复合材料V型构件固化变形的影响,完成了针对V型构件厚度、拐角半径、拐角角度及铺层等结构参数的变形影响研究试验。基于剪力滞后理论和弯曲理论,利用解析法建立了考虑结构参数影响的复合材料V型构件固化变形预测模型,利用模型预测了V型构件的回弹变形并分析了不同结构参数对V型构件回弹变形的影响机制。结果表明:回弹变形随着厚度的增大而减小,厚度为1~3mm之间,角度回弹变形差异最大在30%左右;回弹变形与拐角角度的补角呈约为0.014的比例;拐角半径的不同导致变形的差异不超过5%;准各向同性铺层试验件展现了最大回弹变形,0°铺层的变形减小了23.5%,90°铺层几乎不发生变形。模型分析结果表明,厚度主要通过弯曲刚度和剪切变形两方面影响回弹变形;铺层引起的力学性能和泊松效应的变化是使回弹变形有较大区别的主要原因;V型构件直边变形最大为0.20°,对回弹变形影响较大。变形预测结果与试验结果对比验证了解析法模型的准确性。     

Modeling the process-induced dimensional variations of general curved composite components and assemblies

Dimensional variations are induced during the processing of composite materials. General curved components are commonly used in composite structures. Their performance is affected by the dimensional variations associated with the manufacturing process. This paper presents a piece-wise approach for predicting the dimensional variations of general curved composite components and assemblies. For a general curved composite component, it is first divided into a number of pieces of simple geometry. For each piece, the dimensional variation, i.e. spring-in, is calculated using the effective coefficients of thermal expansion. Based on the dimensional variation of each piece, the dimensional variations of the general curved component are calculated sequentially. This approach was validated against the finite element analysis. It shows that it offers excellent accuracy while avoiding time-consuming numerical computations. Besides general curved components, this approach can also be applied to composite assemblies. It provides the foundation for the tolerance analysis/synthesis of composites.     

热固性复合材料固化过程三维有限元模拟和变形预测

分析了复合材料热固化过程中各种复杂的物理化学变化之间的相互影响,在此基础上建立了复合材料固化过程数值模拟和固化变形预测的三维有限元分析模型。采用整体-子模块方法将固化过程分为热-化学、流动-压实和应力-变形三个相对独立的子模块。热-化学模块的控制方程基于Fourier 热传导方程和树脂固化动力学方程建立,解决了温度和固化度之间的强耦合问题。流动-压实模块的控制方程基于Darcy定律和有效应力原理建立,反映了树脂流动和纤维网络紧密压实之间的流固耦合关系。应力-变形模块建立了考虑热载荷和固化收缩载荷时复合材料层合板的有限元方程。各模块之间的相互作用通过它们之间的数据交换来实现,以树脂在固化过程中的凝胶点和玻璃化转化点为判断依据确定是否运行各模块及其子程序。典型结构的计算结果与实验对比验证了本文三维有限元模型的有效性。     

A study of the integrated composite material structures under different fabrication processing

Based on the typical T-shaped integrated structures, three manufacturing schemes, such as co-cure, co-bonding and secondary bonding, are discussed in this paper. In the curing process of composite T-shaped integrated structures, the mechanism of the warpage deformation and internal stresses are analyzed, and a theoretical model is established. Some important curing mechanical properties of the T-shaped integrated structures are evaluated using the finite element method, including the internal temperature and the degree of cure, the warpage deformation and the internal stresses due to the internal exothermic chemical reaction and the volumetric shrinkage. The evolutions of material parameters are also considered while the degree of cure varied. And the relationships among the different manufacture schemes, the warpage deformation and internal stresses are studied. The results show that both warpage deformation and internal stresses are closely correlated with the fabrication process and the selection of different fabrication process can prominently reduce the warpage deformation and the internal stresses.     

A parametric study of skin laminates used in the composite layup of large-scale wind turbine blades

To reach an optimal design solution for the composite layup of large-scale wind turbine blades, subjected to various design load conditions, while, fulfilling numerous design requirements, is a challenging task to accomplish. Since, a large-scale blade is a slender beam structure, therefore, its thin composite layup can be assumed to be under plane stress condition. Consequently, a parametric study of the skin laminates used in the blade composite layup, is conducted to explore and identify the possible design improvements. The results show that the use of off-axis fiber angles of the skin laminate lower than the conventional 45° are more favorable to achieve higher laminate stiffness, strength, bucking stability, fatigue resistance, and bend-twist coupling value, thereby, demonstrating the potential improvements to the existing composite layup design of large-scale wind turbine blade.     

Finite element analysis and parametric study of steel-concrete composite beams

A finite element model is presented to predict the stresses and deformations in steel-concrete composite beams. The model takes into account the effect of cracking and tension-stiffening in the tensed concrete, and of longitudinal slip between the steel beam and the concrete slab due to the ‘partial interaction’ of theconnectors. Some comparisons with experimental data available in literature are reported to validate the efficiency of the proposed model. Finally, a parametric study was done to investigate the effects of the geometric and mechanical variables as boundary conditions and the slip modulus of the connectors.     

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.     

Experimental study of the parametric interaction between space-charge waves in thin-film GaAs-based semiconductor structures

An experiment on the effect of low-frequency pumping on the output spectrum of a thin-film n-GaAs semiconductor structure with electron drift is reported in the 3-cm and 8-mm wavelength intervals. It is found that the presence of a low-frequency pump signal, whose power greatly exceeds that of a test signal, considerably enhances microwave transfer within two frequency bands separated by Δf=f _s−f _p, where f _s is the frequency of a weak test signal and f _p is the frequency of the pump signal (f _p<f _s). The transfer enhancement is evidence of the existence of effective parametric coupling between space-charge waves in the drift electron stream. This effect confirms the conclusions of previous theoretic studies.     

A parametric study on the design of stitched laminated DCB specimens

Through-the-thickness stitching has been shown to have the capacity of improving the poor interlaminar toughness of laminated composites for many years. Double cantilever beam (DCB) testing has been used to assess the enhanced delamination toughness of stitched laminates. To avoid the premature flexure failure of delaminated substrate beams, thick tabs are bonded to either side of the standard DCB specimens. Consequently, higher opening load can be applied to the stitched tabbed DCB (TDCB) specimens to propagate the delamination crack due to the enlarged bending stiffness and strength. This creates a significant bending of the loading pin, which needs to be accounted for in the numerical analysis. In this paper, a parametric analysis was conducted to investigate the effect of reinforcing tab thickness on the measured and predicted delamination toughness.     

基于VMD的建筑结构模态参数识别

在建筑结构的健康监测、控制和状态评估中经常遇到的一个关键性问题是如何根据实测响应信号准确估计结构阻尼比及自振频率等模态参数。基于变分模态分解(VMD)提出一种新的结构模态参数识别方法。该方法首先对实测振动信号进行VMD分解,获得单模态信号,然后采用自然环境激励技术(NEXT)得到单模态信号的自由衰减响应,最后利用直接插值法(DI)和曲线拟合获得结构的自振频率和阻尼比。通过三层框架结构的数值模拟验证了该方法的准确性及可靠性。利用该技术对台风“达维”作用下广州中信广场的实测加速度数据进行分析,并将估计的结构模态参数和其他识别方法的分析结果进行对比,进一步验证了该方法的准确性和有效性。     

A parametric cubic modelling system for general solids of composite material

A new approach to modelling solids that are anisotropic and heterogeneous is presented with applications to structures of composite material. A parametric cubic modelling system is presented for lines, surfaces, volumes, and physical data that uses construction-in-context to generate numerical data. This system automates the construction of discrete element models and can reduce input data requirements by more than an order of magnitude. A tricubic isoparametric discrete element is presented that does not require displacement derivatives to define connectivity. This element is capable of exact displacement and strain continuity over a surface while permitting strain discontinuities at heterogeneous material interfaces. The shape of an element can be any hexahedron, pentahedron, or tetrahedron and the material properties are allowed to vary over the volume. Evaluation of modelling error with respect to closed-form solutions for curved geometries indicate a single element can model up to 90-degree segments with stresses accurate to 1 per cent. Applications of the system to composite structures are presented for interlaminar edge effects and attachment stresses in a sandwich panel.