基于纤维束/环氧树脂复合材料试验的单向层合板横向拉伸强度预测方法

实验研究表明,纤维束/环氧树脂复合材料试件的横向拉伸强度与工程上常用的单向层合板横向拉伸强度在趋势上具有很好的相关性,但是数值上存在一定差距。本文使用两种碳纤维和两种环氧树脂制备了三种纤维束/环氧树脂复合材料和单向层合板,并分别测量了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度,以及环氧基体的拉伸强度。在实验基础上,应用Griffith断裂强度理论建立了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度的关系模型,通过两种复合材料实验的结果拟合了该模型中的参数。利用第三种复合材料实验进行校验,发现该模型预测的单向层合板横向拉伸强度与实测强度之间达到很好的一致性,相对偏差为9%。采用本文提出的方法,可以用较为简单的纤维束/环氧树脂复合材料和环氧基体拉伸试验预测单向层合板的横向拉伸强度。     

2.5D-C/SiC复合材料的拉伸损伤研究

通过2.5D-C/SiC陶瓷基复合材料的面内拉伸试验, 研究了材料在拉伸载荷作用下的力学性能和损伤演化过程, 建立了2.5D-C/SiC复合材料的应力型和应变型拉伸损伤演化模型. 结果表明, 材料沿纵向和横向的拉伸应力－应变曲线相似, 损伤过程基本相同. 对应于拉伸应力应变曲线的三个特征切线模量, 面内拉伸的损伤演化过程可以分为三个阶段: 初始损伤阶段、损伤加速阶段和损伤减缓阶段. 由应力型损伤演化模型可以推导出三个损伤阶段的两个特征应力, 其中第一特征应力可以作为工程比例极限的参考值.     

单向复合材料弹塑性变形行为的研究

本文利用微观力学方法研究了单向连续纤维增强的金属基复合材料的弹塑性变形行为。纤维是线弹性材料,基体是弹性一粘塑性各向同性材料。在复合材料的纵向拉伸、横向拉伸和纵向剪切变形状态下,预测了复合材料的弹性模量和初始屈服应力值,并考虑了应变率对弹塑性变形行为的影响。以硼/铝复合材料为例,进行了数值分析,预测结果与实验值符合较好。      

基于单胞解析模型的单向复合材料强度预报方法

基于单胞解析模型,建立一种从复合材料细观组分到宏观单向板的强度预报方法。根据连续介质力学和均匀化方法构建细-宏观关联矩阵,通过该关联矩阵将细观组分材料的弹性和损伤性能传递到宏观单向板中。考虑复合材料细观损伤状态,当纤维和基体满足各自强度准则时失效,并通过失效因子折算成刚度的衰减。在此基础上,结合有限元分析,实现复合材料单向板纵横向拉伸模拟,从而预报单向板的拉伸强度。结果表明:该方法预报的模量和强度与实验值基本一致,验证了该方法的有效性与高效性。     

低速冲击作用下碳纤维复合材料铺层板的损伤分析

建立了一个有效计算模型, 以分析碳纤维复合材料层合板在低速冲击作用下的层内和层间失效行为。针对铺层板的层内损伤, 在基于应变描述的Hashin 失效准则的基础上, 建立了单层板的逐渐累积损伤分析模型;针对铺层板的脱层损伤, 建立了各向同性脱层损伤模型, 通过结合传统的应力失效准则和断裂力学中的能量释放率准则定义了界面损伤演化规律, 并在潜在产生脱层的区域模拟为粘结接触, 并将脱层损伤模型作为界面的接触行为。该计算模型通过商用有限元软件ABAQUS/ Explicit 的用户子程序实现。使用该计算模型对碳纤维增强环氧树脂复合材料层合板在横向低速冲击作用下的损伤和变形行为进行预测分析。数值仿真的结果与试验结果进行了比较, 取得了满意的结果, 验证了该模型的正确性。      

不规则孔隙对复合材料横向拉伸力学性能的影响

本文主要研究分析了不规则形状孔隙对复合材料单向板横向拉伸力学性能的影响。首先通过C++编写不规则孔隙随机分布算法。然后通过Python参数化生成包含随机分布纤维和不规则孔隙的重复胞元（Repeating unit cell,RUC）。最后使用有限单元法（Finite element method）分析研究了不规则孔隙对单向板横向拉伸性能（横向弹性模量和横向拉伸强度）的影响。研究结果显示,孔隙的形状会影响单向板的初始损伤、损伤扩展和最终破坏。随着孔隙率的增大,横向弹性模量和横向拉伸强度都减小。相对于横向弹性模量,孔隙率对横向拉伸强度的影响较大。      

基于应变能耗散的复合材料层合板面内缺口强度分析CDM模型

基于伴随能量释放的渐进损伤演化思想,建立了复合材料层合板面内失效分析的连续介质损伤力学（CDM）分析模型,该模型包含损伤表征、损伤起始判定和损伤演化法则3个方面。基于CDM模型,通过引入损伤状态变量表征损伤,建立了平面应力状态下的材料损伤本构模型。采用损伤参量 f_E改写Hashin准则,以判定损伤的起始。损伤演化由特征长度内的应变能释放密度控制,建立了损伤状态变量关于等效应变的渐进损伤演化法则。模型中还同时考虑了面内剪切非线性和网格敏感性,并进行了对比分析。对含缺口的[90/0/±45]_3s和[（±θ）₄]_s 2类典型复合材料层合板的面内拉伸失效进行了分析,结果表明,本文中的模型能有效预测复合材料层合板的面内拉伸强度。     

含材料非线性的热塑性聚乙烯自增强复合材料逐渐损伤分析

以分段线弹性方法考虑了单向复合材料纵向、横向与剪切的非线性特性,建立了静拉伸热塑性PE/PE层合板逐渐损伤模型,利用有限元技术模拟研究了UHMWPE/LDPE层合板逐渐损伤的过程及机理.研究表明,纵向非线性对层合板的拉伸力学行为非线性有显著影响;各向非线性的分段线弹性处理可简便有效地分析复合材料及其结构的非线性问题,结合逐渐损伤分析可清楚揭示基体开裂、纤一基剪切和纤维断裂等损伤模式及其进程.算例分析的理想结果验证了模型的有效性.     

针刺C/C复合材料面内拉伸强度预测

为研究针刺碳纤维增强碳基体复合材料(针刺C/C复合材料)面内拉伸强度与渐进损伤,建立了针刺C/C复合材料代表性体积单元有限元模型。模型包含无纬布层、网胎层、针刺纤维束、界面4类子区域,并考虑了孔隙的影响。采用基于应变的破坏准则及指数型损伤演化规律研究无纬布层及针刺纤维束损伤,采用弹塑性本构研究网胎层损伤,采用内聚力牵引分离定律和二次应力破坏准则分析界面损伤。通过两步法计算了孔隙对材料性能的折减效果,并得到上述4个子区域的力学性能,通过ABAQUS UMAT预测了材料的面内拉伸应力-应变曲线及各子区域损伤起始、演化与失效过程,非线性趋势及拉伸强度数值与试验数值吻合较好,验证了该模型有效性。      

三维四向编织CMCs拉伸性能及损伤演化数值预测

发展了一种能够预测三维四向编织陶瓷基复合材料(3D-B-CMCs)拉伸模量与强度以及损伤演化过程的数值计算方法.首先,利用复合圆柱(CCA)和全局载荷分担(GLS)两种模型预测了纤维束的弹性模量和拉伸强度;然后,利用微焦点CT技术建立了能够反映3D-B-CMCs真实编织几何结构的胞元模型;其次,采用Hashin纤维束失效模型以及考虑单元尺寸的各向异性损伤力学本构模型,编制了ABAQUS/UMAT子程序,对3D-B-CMCs材料宏观拉伸的整个过程进行了计算模拟,预测了宏观拉伸应力-应变曲线,并与试验结果相吻合,证明了所建立方法的合理性和UMAT程序的有效性.同时,研究和讨论了拉伸过程中材料内部不同的损伤破坏模式对复合材料整体力学性能的影响,为材料的疲劳和蠕变等力学行为的内部损伤演化提供了依据.     

Effects of interphase material properties in unidirectional fibre reinforced composites

A three-dimensional (3D) micromechanical study has been performed in order to investigate local damage in unidirectional (UD) composite materials with epoxy resin under transverse tensile loading. In particular the effect of different mechanical properties of a 3D interphase within the hexagonal array RVE have been considered and effects of thermal residual stress arising during the curing process have been accounted for in this study. To examine the effect of interphase properties and residual stress on failure, a study based on the temperature-dependent properties of matrix and interphase and a stiffness degradation technique has been used for damage analysis of the unit cell subjected to mechanical loading. Results indicate a strong dependence of damage onset and its evolution from the different interphase properties within the RVE (representative volume element). Moreover, predicted mechanical properties, damage initiation and evolution are also clearly influenced by the presence of residual stress. Numerical results and experimental data (in the literature) have also shown an interesting agreement.     

Modeling of Nonlinear Mechanical Behavior for 3D Needled C/C-SiC Composites Under Tensile Load

This paper established a macroscopic constitutive model to describe the nonlinear stress–strain behavior of 3D needled C/C-SiC composites under tensile load. Extensive on- and off-axis tensile tests were performed to investigate the macroscopic mechanical behavior and damage characteristics of the composites. The nonlinear mechanical behavior of the material was mainly induced by matrix tensile cracking and fiber/matrix debonding. Permanent deformations and secant modulus degradation were observed in cyclic loading-unloading tests. The nonlinear stress–strain relationship of the material could be described macroscopically by plasticity deformation and stiffness degradation. In the proposed model, we employed a plasticity theory with associated plastic flow rule to describe the evolution of plastic strains. A novel damage variable was also introduced to characterize the stiffness degradation of the material. The damage evolution law was derived from the statistical distribution of material strength. Parameters of the proposed model can be determined from off-axis tensile tests. Stress–strain curves predicted by this model showed reasonable agreement with experimental results.     

A Unit‐Cell Approach of Finite Element Analysis for Transverse Impact Damage of 3‐D Biaxial Spacer Weft‐Knitted Composite

Abstract: This paper evaluates the transverse impact damage of a 3‐D biaxial spacer weft‐knitted composite using experimental results and complimentary finite element analysis. The load–displacement curves and damage morphologies during impact loading were obtained to analyse energy absorption and impact damage mechanisms of the knitted composite. A unit‐cell model based on the microstructure of the 3‐D knitted composite was established to calculate the deformation and damage evolution when the composite is impacted by a hemisphere‐ended steel rod. An elastoplastic constitutive equation is incorporated into the unit‐cell model and the critical damage area failure theory developed by Hahn and Tsai has been implemented as a user‐defined material law (VUMAT) for commercial available finite element code ABAQUS/Explicit. The load–displacement curves, impact damages and impact energy absorption obtained from ABAQUS/Explicit are compared with those FROM experiments. The good agreement of the comparisons supports the validity of the unit‐cell model and user‐defined subroutine VUMAT. The unit‐cell model can also be extended to evaluate the impact crashworthiness of engineering structures made out of the 3‐D knitted composites.     

Fatigue behaviour of aluminium matrix composites reinforced with continuous alumina fibres

Abstract

Alumina (N610) reinforced pure Al (A9) or Al–2 wt-%Cu alloy (AU2) unidirectional (UD) composites which combine a highly ductile matrix with a strong interface bonding, present high static and dynamic mechanical performances. In the present paper, the fatigue behaviour of quasi­UD N610+A9 and N610+AU2 composites is investigated. Notwithstanding the presence of the transverse bundles, the longitudinal behaviour in tension and fatigue of these quasi-UD composites is nearly equivalent to that of the pure UD. Only in the case of the N610+AU2 quasi-UD composite, the fatigue limit is about 30% lower than that of the pure UD. Acoustic emission (AE) monitoring correlated with microfractography and microstuctural examinations has enabled identification of different stages in the evolution of damage and the association of these with damage and failure mechanisms. In fatigue, three main damage mechanisms are activated in a sequential and/or superposed mode during a three stage evolution.     

Prediction for debonding damage process and effective elastic properties of glass-bead-filled modified polyphenylene oxide

A three-dimensional three-phase finite element unit cell model has been applied to predict the debonding damage process of particulate polymer composites (PPC) during tensile deformation. The model consists of a particle, an interface and a polymer matrix. The particle-matrix debonding process has been simulated by using a particle-matrix debonding criterion and a vanishing finite element technique. For verification of the predicted results, the results obtained in the in situ SEM experiments of our previous study on glass beads reinforced modified polyphenylene oxide (GB/PPO) were used The predicted results are in good agreement with the experimental micro-scale tensile debonding-damage process. Anisotropic damage in materials like PPC is very difficult to measure by conventional experimental methods. In order to provide anisotropic damage information of the damaged composites, this was amongst the first time to predict the effective elastic properties of PPC in different principal directions by performing a virtual load–unload test on the partially debonded cell model. Some predicted results would be compared with those of experimental load–unload test.     

Hierarchical modelling of a polymer matrix composite

A hierarchical modelling scheme to predict the properties of a polymer matrix composite is introduced. The stress–strain curves of amine-cured tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM) cured have been predicted using group interaction modelling (GIM). The GIM method, originally applied primarily to linear polymers, has been significantly extended to give accurate, consistent results for TGDDM, a highly crosslinked two-component matrix. The model predicts a complete range of temperature-dependent properties, from fundamental energy contributions, through engineering moduli to full stress–strain curves through yield. The predicted properties compare very well with experiment. Using the GIM-predicted TGDDM stress–strain curve, a 3D finite element model is used to obtain strain concentration factors (SCF) of fibres adjacent to a fibre break in a unidirectional (UD) composite. The strain distribution among the intact neighbouring fibres is clearly affected by the yielding mechanism in the resin matrix. A Monte Carlo simulation is carried out to predict the tensile failure strain of a single composite layer with the thickness equal to the fibre ineffective length. The effect of matrix shear yielding is introduced to the model through the SCF of surviving fibres adjacent to the fibre-break. The tensile failure strain of the composite is then predicted using a statistical model of a chain of composite layers.     

Effects of interphase properties in unidirectional fiber reinforced composite materials

A micromechanical study has been performed to investigate the mechanical properties of unidirectional fiber reinforced composite materials under transverse tensile loading. In particular, the effects of different properties of interphase within the representative volume element (RVE) on both the transverse effective properties and damage behavior of the composites have been studied. In order to evaluate the effects of interphase properties on the mechanical behaviors of unidirectional fiber reinforced composites considering random distribution of fibers, the interphase is represented by pre-inserted cohesive element layer between matrix and fiber with tension and shear softening constitutive laws. Results indicate a strong dependence of the RVE transverse effective properties on the interphase properties. Furthermore, both the damage initiation and its evolution are also clearly influenced by the interphase properties.