考虑拉剪耦合的二维编织物各向异性超弹性本构模型

基于连续介质力学理论,提出了一种考虑二维编织物拉剪耦合作用的各向异性超弹性本构模型。该模型中应变能被分解为纤维拉伸应变能和拉剪耦合作用下的剪切应变能两部分。给出了模型参数的确定方法,通过拟合实验数据,得到了本构模型参数。利用确定的本构模型对文献中不同预拉状态下的镜框剪切实验进行了预测,通过与实验结果对照,验证了所提出的本构模型正确性。该模型不仅具有明确的物理意义且参数确定简单,为更加全面精确的二维编织物成型有限元分析奠定了理论基础。     

碳纤维机织物增强热塑性树脂复合材料热冲压叠层模型

针对碳纤维增强热塑性树脂复合材料(CFRTP)在热冲压成型过程中涉及到大变形、各向异性和多场耦合的现象,为了表征CFRTP在成型中的力学特征,基于有限元方法与连续介质力学理论提出了一种热塑性树脂基体与碳纤维机织物的叠层模型。与单独采用碳纤维机织物超弹性本构模型预测CFRTP成型性能的方法相比,提出的叠层模型能够表征成型温度、压边力和纤维取向对CFRTP成型缺陷的影响,并能优化热冲压成型工艺参数。这一叠层模型具有简单实用和材料参数容易确定的优点,为碳纤维机织物增强热塑性树脂复合材料成型的数值模拟和成型工艺优化奠定了理论基础。     

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.     

T700轴向增强经编织物复合材料力学性能的模拟分析与试验研究

应用一种新型界面元模型研究了复合材料层间剪切损伤。通过引入双线性损伤准则和损伤演化,预报复合材料层间裂纹扩展。轴向增强经编织物复合材料由针织纱线引起的纤维变形(KYD)产生了富树脂区域,基于细观力学理论提出了一种新的研究轴向增强经编织物单胞模型受单向拉伸和剪切时KYD区周围应力分布的方法,得出了单胞在受载时首先在这一区域产生裂纹。对单轴向T700经编织物复合材料进行了三点弯曲性能和层间剪切性能试验测试,分析了经编织物复合材料的力学特性。分别模拟了弯曲性能和层间剪切性能试验,得出了最大预报载荷值与试验值误差小于10%,并基于有限元模型研究了复合材料面内损伤和层间裂纹扩展损伤机制。     

Glass/Epoxy复合材料叠层板面内剪切连续损伤模型

为确定S2玻璃纤维/环氧树脂(S2-Glass/Epoxy) 叠层复合材料面内剪切应力-应变关系,对S2-Glass/Epoxy 叠层复合材料面内剪切拉伸载荷下的弹、塑性连续损伤本构模型及应用进行了研究。基于平面应力状态下的连续损伤力学模型,通过典型面内剪切拉伸实验,分别建立了忽略塑性应变和考虑塑性应变的两种连续损伤力学(CDM)模型,并确定相关参数。通过ABAQUS/Explicit 用户子程序VUMAT接口,分别采用两种CDM模型对S2-Glass/Epoxy 叠层复合材料面内剪切拉伸实验进行有限元数值计算,与实验结果对比,验证模型可靠性,并分析单元类型对有限元计算结果的影响。研究结果表明: 忽略塑性应变的CDM模型可以很好地预测复合材料面内剪切失效强度,但不能较好地预测其非线性力学响应; 考虑塑性应变,将塑性硬化与损伤耦合后的CDM模型则能较好的预测复合材料非线性力学响应和面内剪切失效强度; 该平面应力状态下建立的CDM模型可用于壳单元进行复合材料有限元数值计算,横向剪切作用导致传统壳单元数值计算的载荷位移曲线略低于平面应力单元计算结果; 减缩积分算法有利于提高有限元数值计算结果的准确性。     

复合材料用机织物非正交本构模型的半球形冲压成型验证

为了描述复合材料用机织物在大变形下由于经纱和纬纱之间角度变化所引起的非线性各向异性材料行为, 前期工作中建立了一个非正交本构模型。利用半球形冲头对复合材料用平纹机织物进行冲压模拟, 并将非正交本构模型和正交本构模型的模拟结果与实验结果进行对比, 对非正交本构模型进行验证, 以充分说明该模型的有效性和正确性。结果表明: 采用非正交本构模型模拟的复合材料用平纹机织物变形后的边界轮廓与实验结果基本一致, 并且剪切角都在实验结果的误差范围内; 而采用正交本构模型, 复合材料用机织物变形后的边界轮廓和剪切角与实验结果相差较大。研究表明, 与正交本构模型相比非正交本构模型能更好地描述复合材料用机织物在大变形下的材料属性。      

双轴受压状态下的高延性纤维增强水泥基复合材料本构模型

基于Darwin和Pecknold考虑混凝土双轴力学行为的方法,建立一个同时考虑双轴受压状态下非线性力学行为和抗压强度变化的高延性纤维增强水泥基复合材料(ECC)二维正交各向异性本构模型。在因双轴加载而产生的正交各向异性的2个方向上引入等效单轴应变,建立非线性应力-等效单轴应变关系以考虑ECC的双轴非线性行为,并采用一条双轴强度包络线确定2个方向上的抗压强度。推导模型的显式数值算法,编写包含该算法的用户自定义材料子程序UMAT,并嵌于有限元计算程序ABAQUS v6.14中。通过对两组不同配合比的ECC试件在不同应力比下的双轴受压加载试验进行数值分析验证本模型的有效性。数值计算得到的主压应力方向上的应力-应变曲线及预测的抗压强度与试验结果吻合较好,表明该文提出的本构模型能够有效地预测ECC在双轴受压状态下的非线性力学行为和破坏强度。     

基于非正交本构模型的热塑性机织物预浸料宽温域赋形褶皱缺陷仿真方法

热塑性复合材料预制体的赋形质量直接影响结构件的制造质量。由于热塑性基体具有较高的熔融温度和黏度，赋形工艺温度设计不合理会导致褶皱等赋形缺陷，给热塑性复合材料结构高质量成型带来了挑战。现有的热塑性预浸料热成型研究主要基于连续介质力学、离散元、半离散元法，通过建立多机制耦合的本构模型分析热塑性预浸料的各向异性大变形行为，未充分考虑工艺调控对赋形宏观变形过程中褶皱缺陷的影响。发展了一种热塑性预浸料宽温域赋形褶皱缺陷仿真方法。通过表征热塑性机织物预浸料在不同温度、载荷下的力学性能，获取宽温域热塑性预浸料本构参数，基于非正交本构模型，提出了温度对热塑性预浸料赋形褶皱缺陷的作用规律，揭示了赋形过程中宽温域褶皱缺陷的变形机制，获得了赋形温度优化调控方案。研究结果表明：褶皱缺陷的萌生和演化过程由不同温度下的面内剪切和压缩变形行为共同影响，预浸料的褶皱缺陷变形程度随温度的增加而减弱，非正交本构模型的模拟结果与实验结果基本一致。     

定型剂对经编织物液态成型复合材料工艺性及其力学性能的影响

研究了双马来酰亚胺树脂定型剂含量对0°/90°双轴向经编织物(Non-crimp fabric,NCF)定型效果、液态成型工艺性、复合材料力学性能的影响。采用厚度回弹、C型回弹的方法表征定型效果;采用厚度压缩与偏轴拉伸实验表征带定型剂织物成型工艺性;并采用弯曲实验与层间剪切实验表征复合材料力学性能。实验结果表明定型剂的添加显著提高了NCF织物的预定型效果;经定型剂处理后NCF织物的面内剪切模量有了较大提高,抗剪切变形能力增强;添加定型剂对0°/90°双轴向NCF织物复合材料的力学性能影响不大,但在改善其工艺性的同时并不会降低其复合材料的力学性能。     

纤维增强复合材料本构模型研究进展

本文对纤维增强复合材料的本构模型最新研究成果进行简要阐述和归纳,并结合实例进行了相关数值模拟计算。文中阐述的复合材料为纤维增强树脂复合材料、纤维增强金属基复合材料和纤维编织复合材料。目前,关于纤维增强金属基复合材料的研究较少,大多数研究集中在纤维增强树脂复合材料和纤维编织复合材料。许多学者在经典弹性本构模型和连续介质力学基础上,推导出了一些弹塑性、粘弹性和考虑损伤的非线性本构模型。通过实例计算结果表明,考虑损伤导致刚度退化的复合材料弹塑性本构模型计算得到的拉伸应力应变曲线与测试结果基本一致。本文通过对复合材料本构模型的最新研究成果的归纳和数值模拟实例计算,为后续研究工作提供借鉴,推动其在实际工程中的应用。     

Numerical study on thermo-stamping of woven fabric composites based on double-dome stretch forming

Through the international corporative benchmark works, the material characterization of the woven fabric composites has been examined to better understand their mechanical properties and to provide the process design information for numerical analysis. As the second stage of the benchmark work, the double-dome geometry has been used to illustrate the effect of numerical schemes on the forming behaviors of the woven composites parts. To account for the change of fiber orientation under the large deformation, the non-orthogonal constitutive model was utilized and nonlinear friction behavior was incorporated in the simulation. The equivalent material properties based on the contact status were used for the thermo-stamping process. Furthermore, we incorporated a recently developed non-orthogonal model which captures the dependency of shear behavior of woven fabric composites on the tensions in yarns. Simulation results showed the effect of coupling on the predicted forming behavior for the double-dome parts. As numerical results, blank draw-in, punch force history and fiber orientation after forming have been compared based on various numerical models and methods.     

Importance of in-plane shear rigidity in finite element analyses of woven fabric composite preforming

A finite element made of woven unit cells under biaxial tension and in-plane shear is proposed for the simulation of fabric forming. The simulation is made within an explicit dynamic approach and is based on a simplified dynamic equation accounting for tension and in-plane shear strain energy. The biaxial tensile properties (given by two surfaces) and the in-plane shear properties (given by a curve) can be determined both by biaxial tensile tests and picture frame experiments or obtained by mesoscopic 3D finite element analyses of the woven unit cell. The interior load components of the proposed finite element are calculated explicitly and simply from the tensions and shear torque on four woven cells. The results obtained by the simulations of a hemispherical forming process on a very unbalanced fabric are compared to experiments. It is shown that the tension strain energy permits to describe the asymmetry of the response but that the computation of wrinkles and of the deformed states when the locking angle is exceeded needs to take the in-plane shear stiffness and its evolution with shear angle into account.     

Characterising the shear–tension coupling and wrinkling behaviour of woven engineering fabrics

Modelling the forming process for engineering fabrics and textile composites using a mechanical approach, such as the finite element method, requires characterisation of the material’s behaviour under large shear deformation. For woven engineering fabrics, a coupling between in-plane tension and both shear compliance and the onset of wrinkling is to be expected. This paper focuses on a novel testing technique, the biaxial bias extension test, as a means to investigate this shear–tension coupling and fabric wrinkling. Novel methods of determining the wrinkling behaviour are demonstrated. The main difficulty with the technique lies in extracting the material contribution to the recorded signal. To do this, an experimental method is proposed and demonstrated using a plain weave glass fabric. Biaxial bias extension test results are compared against picture frame and uniaxial bias extension results.     

Evaluation of bias-extension and picture-frame test methods for the measurement of intraply shear properties of PP/glass commingled fabrics

Efforts are currently devoted to the development of numerical tools for predicting the forming process of continuous fiber-reinforced thermoplastic composites. To ensure good predictions, reliable experimental measurements of the laminate properties must be performed at the processing temperature. This paper presents experimental results of the intraply shear properties measured on a polypropylene/glass fabric using two different methods known as the bias-extension and the picture-frame testing methods. Based on kinematic equations, both methods of measurement are discussed in regards to their respective weaknesses, mainly related to the difficulty to measure precise shear angles in the bias-extension method and the undesired fiber tension invalidating the load–displacement curve in the picture-frame test. To remove these difficulties, a modified version of the picture-frame test is proposed. This new method avoids fiber tensioning during testing and is considered more appropriate to capture the influence of the fabric architecture on the measured properties. Good correlations between the load–displacement curves and viscosities measured with the modified picture-frame and bias-extension methods were obtained compared to the standard picture-frame method.     

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。      

Numerical and Experimental Investigations on Deep Drawing of G1151 Carbon Fiber Woven Composites

This study proposes to simulate the deep drawing on carbon woven composites in order to reduce the manufacturing cost and waste of composite material during the stamping process, The multi-scale anisotropic approach of woven composite was used to develop a finite element model for simulating the orientation of fibers accurately and predicting the deformation of composite during mechanical tests and forming process. The proposed experimental investigation for bias test and hemispherical deep drawing process is investigated in the G1151 Interlock. The mechanical properties of carbon fiber have great influence on the deformation of carbon fiber composites. In this study, shear angle–displacement curves and shear load–shear angle curves were obtained from a bias extension test. Deep drawing experiments and simulation were conducted, and the shear load–displacement curves under different forming depths and shear angle–displacement curves were obtained. The results showed that the compression and shear between fibers bundles were the main deformation mechanism of carbon fiber woven composite, as well as the maximum shear angle for the composites with G1151 woven fiber was 58°. In addition, during the drawing process, it has been found that the forming depth has a significant influence on the drawing force. It increases rapidly with the increasing of forming depth. In this approach the suitable forming depth deep drawing of the sheet carbon fiber woven composite was approximately 45 mm.     

Hyperelastic modelling for mesoscopic analyses of composite reinforcements

A hyperelastic constitutive law is proposed to describe the mechanical behaviour of fibre bundles of woven composite reinforcements. The objective of this model is to compute the 3D geometry of the deformed woven unit cell. This geometry is important for permeability calculations and for the mechanical behaviour of the composite into service. The finite element models of a woven unit cell can also be used as virtual mechanical tests. The highlight of four deformation modes of the fibre bundle leads to definition of a strain energy potential from four specific invariants. The parameters of the hyperelastic constitutive law are identified in the case of a glass plain weave reinforcement thanks to uniaxial and equibiaxial tensile tests on the fibre bundle and on the whole reinforcement. This constitutive law is then validated in comparison to biaxial tension and in-plane shear tests.