Sheet hydroforming of woven FRT composites: non-orthogonal constitutive equation considering shear stiffness and undulation of woven structure

For the simulation of sheet hydroforming for the shaping of woven fabric reinforced thermo-plastic (FRT) composites, a non-orthogonal constitutive model was developed based on a homogenization method by considering the microstructures of composites including mechanical and structural properties of the fabric reinforcement. This model is modified to capture the wrinkling behavior due to the undulation geometry of the woven structure and shear stiffness at the crossover of the warp and weft yarns of woven FRT composites. The model was implemented in an explicit dynamic finite element code to analyze the forming behavior of woven FRT during the stamp thermo-hydroforming process. Wrinkling behavior was investigated based on the application of a counteracting fluid pressure and changes to the initial blank shape.     

Computational simulation of fabric armour subjected to ballistic impacts

In computational simulations of ballistic impacts on woven polymeric fabric armour, specialized fabric models are normally used. Attempts have also been made to use commercial finite element packages for such purposes. However, such attempts normally result in either overly simplified models or prohibitively detailed finite element discretization of the fabric to capture the unique properties of woven fabric. This paper presents an FE model of woven fabric that reflects the orthotropic properties of the fabric, the viscoelastic nature of the yarns, the crimping of the yarns, the sliding contact between yarns and yarn breakage using an assembly of viscoelastic bar elements. Excellent agreement between simulation and ballistic test data is obtained in terms of the deformation of the fabric during impact, residual velocity of the projectile and the energy absorbed by the fabric. This is achieved despite the modest number of degrees of freedom employed by the model.     

A homogenization procedure for the numerical analysis of woven fabric composites

The paper presents a procedure for the numerical evaluation of the mechanical properties of woven fabric laminates. Woven fabrics usually present orthogonal interlaced yarns (warp and weft) and distribution of the fibers in the yarns and of the yarns in the composite may be considered regular. This allows us to apply the homogenization theory for periodic media both to the yarn and to the fabric. Three-dimensional finite element models are used in two steps to predict both the stiffness and the strength of woven fabric laminates. The model includes all the important parameters that influence the mechanical behavior: the lamina thickness, the yarn orientation, the fiber volume fraction and the mechanical characteristics of the components. The capabilities of the numerical model were verified studying the elastic behavior of a woven fabric laminate available in the literature and the ultimate strength of a glass fabric laminate experimentally investigated. The procedure, that can be implemented into commercial finite element codes, appears to be an efficient tool for the design of textile composites.     

Loosely woven fabric model with viscoelastic crimped fibres for ballistic impact simulations

A computational micro‐mechanical material model of loosely woven fabric for non‐linear finite element impact simulations is presented in this work. The model is a mechanism incorporating the crimping of the fibres as well as the trellizing. The equilibrium of the mechanism allows the straightening of the fibres depending on the fibre tension. The contact force at the fibre crossover point determines the rotational friction dissipating a part of the impact energy. The stress–strain relationship is viscoelastic based on a three‐element model. The failure of the fibres is strain rate dependent. The model is implemented as user defined subroutine in the transient finite element code LS‐DYNA. The ballistic impact simulations with the model are in good agreement with the experimental results. Copyright © 2004 John Wiley & Sons, Ltd.     

An inverse method for material parameters determination of titanium samples under tensile loading

Different approaches used for the simulation of woven reinforcement forming are investigated. Especially several methods based on finite element approximation are presented. Some are based on continuous modelling, while others, called discrete or mesoscopic approaches, model the components of the fabric. A semi discrete finite element made of woven unit cells under biaxial tension and in-plane shear is detailed. In continuous approaches, the difficulty lies in the necessity to take the strong specificity of the fibrous material into account. The yarn directions must be strictly followed during the large strains of the fabric. This is the main goal of the non-orthogonal model and of the hypoelastic constitutive model based on the yarn rotation presented in this paper. In the case of discrete and semi-discrete approaches the directions of the yarns are “naturally” followed because the yarns are modeled. Explicitly, however, modeling each component at the mesoscopic scale can lead to high numerical cost.     

Computational modeling of the probabilistic impact response of flexible fabrics

The impact response of flexible woven fabrics is probabilistic in nature and described through a probabilistic velocity response curve or V₀–V₁₀₀ curve. Computational impact analyses based on deterministic methods are incapable of predicting the experimentally observed probabilistic fabric impact response. To overcome this limitation we have developed a probabilistic computational framework within a finite element analysis to predict the V₀–V₁₀₀ response. The finite element model is a yarn-based representation of the fabric architecture, with a principal stress based failure criterion implemented uniformly within each yarn, but varying for each yarn within the fabric. For each impact simulation, individual yarn strengths are mapped from experimentally obtained yarn strength distributions, resulting in fabric models with spatially non-uniform failure conditions. Impact simulations are run for the case of a spherical projectile of diameter 5.556 mm impacting a single layer of 50.8 × 50.8 mm, edge-clamped, unbacked, aramid fabric. Three different yarn strength models are implemented, representing spool yarns, and yarns extracted from greige and scoured woven fabrics. Decreases in yarn strength are found to correlate to decreases in the V₁, V₅₀, and V₉₉ velocities predicted by the simulations. The relationships between yarn strength distribution and probabilistic fabric impact response are discussed.     

A continuum shell finite element model for impact simulation of woven fabrics

A new computational approach is developed to predict the impact behaviour of fabric panels based on the detailed response of the smallest repeating unit (unit cell) in the fabric. The unit cell is constructed and calibrated using measured geometrical (weave architecture, crimp, voids, etc.) and mechanical properties of the fabric. A pre-processor is developed to create a 3D finite element mesh of the unit cell using the measured fabric cross-sectional micro-images. To render an efficient method for simulation of multi-layer packs, these unit cells are replaced with orthotropic shell elements that have similar macroscopic (smeared) mechanical properties as the unit cell. The aim is to capture the essence of the response of a unit cell in a single representative shell element, which would replace the more complicated and numerically costly 3D solid model of the yarns in a crossover. The 3D finite element analysis of the unit cell is used to provide a baseline mechanical response for calibrating the constitutive model in the equivalent shell representation. This shell element takes advantage of a simple physics-based analytical relationship to predict the behaviour of the fabric's warp and weft yarns under general applied displacements in these directions. The analytical model is implemented in the commercial explicit finite element code, LS-DYNA, as a user material routine (UMAT) for shell elements. Layers of fabric constructed from these specialized elements are stacked together to create fabric targets that are then analysed under projectile impact. This approach provides an efficient numerical model for the dynamic analysis of multi-layer fabric structures while taking into account several geometrical and material attributes of the yarns and the fabric.     

Simulating in-plane fatigue damage in woven glass fibre-reinforced composites subject to fully reversed cyclic loading

The interest in using fibre‐reinforced composites in structural components is increasing. Some of these structural composites, such as wind turbine blades, aircraft components and torsion shafts are subject to fatigue loadings. It is widely accepted that fully reversed cyclic loading is the most adverse loading for fibre‐reinforced composites, but the modelling of the material behaviour under this loading condition is very difficult. In this paper, a damage model is presented for woven glass fibre‐reinforced composites subject to fully reversed cyclic loading. First fatigue experiments have been conducted in displacement‐controlled fully reversed bending and the stiffness degradation and damage patterns have been observed. Based on these experimental data, a damage model has been developed, which includes the in‐plane stress components and the degradation of the in‐plane elastic properties. The model has been implemented in a commercial finite‐element code and simulation of the successive stages in the fatigue life has been performed. The model has been validated for a plain woven glass fabric reinforced composite and simulated stiffness degradation, damage growth and damage distribution have been compared with experimental data.     

Meshfree modeling and homogenization of 3D orthogonal woven composites

In this paper, evaluation of 3D orthogonal woven fabric composite elastic moduli is achieved by applying meshfree methods on the micromechanical model of the woven composites. A new, realistic and smooth fabric unit cell model of 3D orthogonal woven composite is presented. As an alternative to finite element method, meshfree methods show a notable advantage, which is the simplicity in meshing while modeling the matrix and different yarns. Radial basis function and moving kriging interpolation are used for the shape function constructions. The Galerkin method is employed in formulating the discretized system equations. The numerical results are compared with the finite element and the experimental results.     

2.5D机织复合材料压缩性能实验与数值模拟

为了研究2.5D机织复合材料的压缩损伤和失效机制,验证双尺度渐进损伤有限元数值模拟方法的有效性,对这类复合材料分别沿经纱方向和纬纱方向进行了准静态压缩实验,获得了其相应的应力-应变曲线,并测定了材料的初始弹性模量和极限强度。在此基础上,利用双尺度渐进损伤有限元数值方法模拟分析了材料的压缩应力-应变响应和损伤演化行为,取得了与实验吻合较好的模拟结果。结果表明:2.5D机织复合材料在纬向压缩下的主要失效模式是纬纱的轴向压溃与断裂,可获得相对较高的压缩强度;但在经向压缩下,经纱因弯曲会承受附加弯矩作用,从而对周围基体造成挤压,故在经纱轴向断裂之前容易出现经纱之间基体的压溃和纱线之间的分层开裂,使强度降低,不利于发挥纤维的承载优势。     

二维机织复合材料弹性常数的有限元法预测

为了预测二维机织复合材料的弹性性能,建立了有限元力学分析模型。基于二维机织复合材料的几何特征,建立了参数化的单胞模型;考虑了织物纤维束呈现出的各向异性材料特征,将有限元中材料主方向转化到纤维屈曲方向,建立其力学分析有限元模型;分析了单胞边界面保持平面假设的不足,提出了对于二维机织复合材料通用的周期边界条件,获得了更为准确的二维机织复合材料的工程弹性常数。结果表明:织物衬垫单胞边界面,在单向拉伸载荷和纯剪切载荷下,呈凹凸翘曲变形,即为周期边界;应用给出的织物参数化几何建模方法与有限元求解方法,可以精确地获得工程弹性常数,数值计算结果与实验值吻合较好。      

Effect of Frictions on the Ballistic Performance of a 3D Warp Interlock Fabric: Numerical Analysis

3D interlock woven fabrics are promising materials to replace the 2D structures in the field of ballistic protection. The structural complexity of this material caused many difficulties in numerical modeling. This paper presents a new tool that permits to generate a geometry model of any woven fabric, then, mesh this model in shell or solid elements, and apply the mechanical properties of yarns to them. The tool shows many advantages over existing software. It is very handy in use with an organization of the functions in menu and using a graphic interface. It can describe correctly the geometry of all textile woven fabrics. With this tool, the orientation of the local axes of finite elements following the yarn direction facilitates defining the yarn mechanical properties in a numerical model. This tool can be largely applied because it is compatible with popular finite element codes such as Abaqus, Ansys, Radioss etc. Thanks to this tool, a finite element model was carried out to describe a ballistic impact on a 3D warp interlock Kevlar KM2? fabric. This work focuses on studying the effect of friction onto the ballistic impact behavior of this textile interlock structure. Results showed that the friction among yarns affects considerably on the impact behavior of this fabric. The effect of the friction between projectile and yarn is less important. The friction plays an important role in keeping the fabric structural stability during the impact event. This phenomenon explained why the projectile is easier to penetrate this 3D warp interlock fabric in the no-friction case. This result also indicates that the ballistic performance of the interlock woven fabrics can be improved by using fibers with great friction coefficients.     

Multiscale modeling of the impact of textile fabrics based on hybrid element analysis

In this work, a multi scale modeling approach has been developed to simulate the impact of woven fabrics using a finite element (FE) analysis. A yarn level of resolution is used in the model. This approach, referred to as the hybrid element analysis (HEA) is based on decreasing the complexity of the finite element model with distance away from the impact zone based on the multiscale nature of the fabric architecture and the physics of the impact event. Solid elements are used to discretize the yarns around the impact region, which transition to shell elements in the surrounding region. A new method for modeling the shell yarns is incorporated that more accurately represents the contours of the yarn cross section. Impedances have been matched across the solid–shell interface to prevent interfacial reflections of the longitudinal strain wave. The HEA method is validated by first applying it to the FE model of a single yarn for which an analytical solution is known. The HEA method is then applied to a woven fabric model and validated by comparing it against a baseline model consisting of yarns discretized using only solid elements.     

碳纤维织物力学性能和冲压成形实验研究

在实际成形过程中,碳纤维复合材料往往处于复杂的应力状态,开展近于真实载荷环境下的力学试验分析,能够更准确地认识实际应用中材料的成形性能和变形机理.为获得碳纤维织物的基本力学特性,设计了平纹碳纤维织物拉伸试样及成形试样,进行了单轴拉伸、双轴拉伸、镜框剪切试验和方盒冲压成形实验研究,对比了不同双拉比及纱线取向对力学性能及成形性能的影响.研究结果表明:碳纤维织物具有高度的非线性、各向异性和双拉耦合特性,即经纬向纤维的力学性能会相互影响;剪切变形是成形过程中的主要变形模式,当剪切角达到临界锁死角时,织物发生起皱现象;同种织物不同纱线取向试样表现出不同的成形性能,因此可以根据零件几何形状选择合适纤维取向的织物,从而减少缺陷,优化成形零件的力学性能.研究结果为后续建立碳纤维织物本构模型和成形仿真奠定了基础.     

Effect of statistical yarn tensile strength on the probabilistic impact response of woven fabrics

The probabilistic impact response of flexible woven fabrics can be described through the V₀–V₁₀₀ or probabilistic velocity response (PVR) curve which describes the probability of fabric penetration as a function of projectile impact velocity. One source of variability that affects the probabilistic nature of fabric impact performance is the statistical distribution of yarn tensile strengths. In this paper the effects of the statistical yarn strength distribution characteristics on the probabilistic fabric impact response are computationally studied using five different strength distributions with differing mean strengths and distribution widths. Corresponding fabric PVR curves are generated for each strength distribution using a probabilistic computational framework that involves randomly mapping yarn strengths onto the individual woven yarns of a fabric finite element model and then running a series of impact simulations for the case of a four-sided clamped fabric impacted at the center by a spherical projectile.     

Tensile Properties and Failure Mechanism of a New 3D Nonorthogonal Woven Composite Material

Tensile properties and failure mechanism of a newly developed three-dimensional (3D) woven composite material named 3D nonorthogonal woven composite are investigated in this paper. The microstructure of the composite is studied and the tensile properties are obtained by quasi-static tensile tests. The failure mechanism of specimen is discussed based on observation of the fracture surfaces via electron microscope. It is found that the specimens always split along the oblique yarns and produce typical v-shaped fracture surfaces. The representative volume cell (RVC) is established based on the microstructure. A finite element analysis is conducted with periodical boundary conditions. The finite element simulation results agree well with the experimental data. By analyzing deformation and stress distribution under different loading conditions, it is demonstrated that finite element model based on RVC is valid in predicting tensile properties of 3D nonorthogonal woven composites. Stress distribution shows that the oblique yarns and warp yarns oriented along the x direction carry primary load under x tension and that warp yarns bear primary load under y tension.     

Compression modeling of plain weave textile fabric using finite elements

Textile composite are used extensively in aerospace as they offer a 3D reinforcement in a single layer providing better mechanical properties in both in‐plane and transverse directions. This paper reports on the mechanical behavior of a plain weave textile fabric under the compressive loading. Unit cell geometry of the plain weave fabric structure is identified and its model is created using TexGen geometric modeling scheme developed by the University of Nottingham (U.K.). Later on its mechanical behavior is predicted using finite element modeling (FEM) based simulation software ABAQUS^® incorporating a transversely isotropic material law. Strain energy of the developed model has been compared with that of the published results and shows very good agreement. The analysis indicated that transverse‐longitudinal shear (TLS) modulus plays an important role in characterizing the behavior of the woven fabric under compression, while the friction between the yarns and longitudinal stiffness has less significant influence on compaction behavior. In order to ascertain the effectiveness of the developed model, exhaustive parametric studies have also been conducted to investigate the effect of transverse‐longitudinal shear modulus on some of the important parameters such as artificial strain energy, external work, frictional dissipation, internal energy, kinetic energy, strain energy and total energy of the model. The developed model has the capacity to predict and simulate the behavior of variety of fabric architectures based on their constituent yarn properties under various regimes of service loads.     

Modelling inter‐yarn friction in woven fabric armour

The effects of inter‐yarn friction on the ballistic performance of woven fabric armour are investigated in this paper. Frictional sliding between yarns is implemented in a computational model of the fabric that takes the form of a network. Yarn crimp and its viscoelastic properties are taken into account. Ballistic experiments are performed to verify the predictions of the model. Parametric studies show that the ballistic response of woven fabric is very sensitive to yarn friction when the friction coefficient is low but insensitive beyond a certain level. The results also show that very high inter‐yarn friction can lead to premature yarn rupture, thus reducing the ability of the fabric to absorb impact energy. Copyright © 2005 John Wiley & Sons, Ltd.     

卵形弹斜冲击作用下平纹织物的动态响应分析

利用LS-DYNA有限元软件模拟了卵形弹倾斜冲击作用下平纹织物的动态响应,分析了织物的变形、纱线断裂以及能量吸收特性,讨论了边界约束条件对织物动态响应的影响。结果表明:在弹体贯穿织物过程中,纱线应变能和摩擦耗散能是弹体动能转变的主要形式。在给定的计算时间内,对于四边无约束、对边(经向和纬向)固定约束和四边固定约束的四种边界条件,两者之和占弹体动能损失量的比例不小于80.5%。模拟结果还表明,边界条件对织物的变形、纱线断裂以及能量吸收特性均有明显影响。边界条件不同,纱线断裂数目不同,弹体动能损失量转变成其它能量的比例也不同,导致织物的能量吸收特性也发生变化。     

基于镀银纱线的电加热织物温度场模拟与电热性能

运用ANSYS有限元模拟软件对镀银纱线在织物中加热过程进行数值模拟,并通过调整镀银纱线之间的距离和施加电压分析不同条件下加热织物内部和周围空气中热场分布情况。根据模拟结果制备镀银纱线加热织物,验证模拟结果并研究电加热织物电热性能。结果表明,随着电压的增加,镀银纱线平衡温度升高,当输出电压为7V时,镀银纱线在织物中实测温度可达109.7℃。设定镀银纱线间距为3mm,使镀银纱线在较低成本下获得较高的表面温度均匀性。加热织物的升温速度和平衡温度随着功率密度的增加而增加,模拟结果与实测结果趋势一致且结果偏差小于4.5%,说明有限元分析结果能够作为镀银纱线加热织物制备的重要参考依据。