三维四向编织复合材料弯曲性能的实验研究

分别对编织角度为30和45度的三维四向碳环氧编织复合材料进行了不同厚度试件的弯曲实验,验证了材料的破坏模式、本构关系与试件几何尺寸的相关性.研究表明:弯曲破坏无明显的破坏裂纹,当载荷增加到了极限载荷,载荷不再增加而位移依然逐渐变大.当进行卸载时,大部分的弹性变形都可以得到恢复,仅剩余很小的塑性变形.另外改变试件的厚度对材料的载荷位移关系没有影响.     

Mechanical Behaviour of 3D Multi-layer Braided Composites: Experimental,Numerical and Theoretical Study

Mechanical properties and failure mechanisms of a newly designed 3D multi-layer braided composites are evaluated by experimental, numerical and theoretical studies. The microstructure of the composites is introduced. The unit cell technique is employed to address the periodic arrangement of the structure. The volume averaging method is used in theoretical solutions while FEM with reasonable periodic boundary conditions and meshing technique in numerical simulations. Experimental studies are also conducted to verify the feasibility of the proposed models. Predicted elastic properties agree well with the experimental data, indicating the feasibility of the proposed models. Numerical evaluation is more accurate than theoretical assessment. Deformations and stress distributions of the unit cell under tension shows displacement and traction continuity, guaranteeing the rationality of the applied periodic boundary conditions. Although compression and tension modulus are close, the compressive strength only reaches 70% of the tension strength. This indicates that the composites can be weakened in compressive loading. Additionally, by analysing the micrograph of fracture faces and strain-stress curves, a brittle failure mechanism is observed both in composites under tension and compression.     

三维四向编织复合材料压缩力学性能实验研究

通过对三维四向编织复合材料薄板试件的宏观压缩实验,研究了三维四向编织复合材料的抗压力学性能.实验结果表明:材料的编织角对其压缩力学性能的影响很大,随编织角的变化,编织复合材料的压缩破坏机制发生了变化.编织角度较小时,材料表现为脆性特征;当编织角度大于某个临界角度时,材料的应力-应变曲线趋于非线性,更多地表现为塑性破坏.     

玻纤增强聚丙烯复合材料的应变率敏感特性

采用层合热压实验法将玻璃纤维基毡与聚丙烯薄膜复合制成不同玻纤含量的玻纤增强聚丙烯(GF/PP)复合板材,在不同的加载速率下进行拉伸测试,研究GF/PP复合材料的应变率敏感特性,分析Burgers模型对该材料本构关系拟合预测的可行性。结果表明:GF/PP复合材料在低应变率范围内对应变率是敏感的,随应变率的增加,其断裂应力和抗拉强度增大;随玻璃纤维含量的增加,其所对应的应变率效应反而有所下降。同时,Burgers模型能够有效地拟合预测出该材料的拉伸应力-应变曲线,与实验曲线相比,进一步验证了GF/PP复合材料的应变率敏感特性及其变化趋势。     

Multi-Scale Progressive Damage Model for Analyzing the Failure Mechanisms of 2D Triaxially Braided Composite under Uniaxial Compression Loads

Based on continuum damage mechanics (CDM), a multi-scale progressive damage model (PDM) is developed to analyze the uniaxial compression failure mechanisms of 2D triaxially braided composite (2DTBC). The multi-scale PDM starts from the micro-scale analysis which obtains the stiffness and strength properties of fiber tows by a representative unit cell (RUC) model. Meso-scale progressive damage analysis is conducted subsequently to predict the compression failure behaviors of the composite using the results of micro-scale analysis as inputs. To research the free-edge effect on the local failure mechanisms, meso-scale models of different widths are also established. The stress-strain curves obtained by numerical analysis are verified with the experimental data. Results show that fiber and matrix compression failure inside the fiber tows are the major failure modes of the composite under axial compression. For transverse compression, the dominated failure modes are recorded for matrix compression failure inside the fiber tows. It is also presented that the free-edge effect plays an important role in the transverse mechanical response of the composite, and the failure behaviors of the internal fiber tows are strongly influenced as well.     

Internal Strain Measurement in 3D Braided Composites Using Co-braided Optical Fiber Sensors

3D braided composite technology has stimulated a great deal of interest in the world at large. But due to the three-dimensional nature of these kinds of composites, coupled with the shortcomings of currently-adopted experimental test methods, it is difficult to measure the internal parameters of this materials, hence causes it difficult to understand the material performance. A new method is introduced herein to measure the internal strain of braided composite materials using co-braided fiber optic sensors. Two kinds of fiber optic sensors are co-braided into 3D braided composites to measure internal strain. One of these is the Fabry-Parrot (F-P) fiber optic sensor; the other is the polarimetric fiber optic sensor. Experiments are conducted to measure internal strain under tension, bending and thermal environments in the 3D carbon fiber braided composite specimens, both locally and globally. Experimental results show that multiple fiber optic sensors can be braided into the 3D braided composites to meas     

三维五向编织复合材料纵向性能的实验研究

通过对具有不同编织结构参数的三维五向编织复合材料试件的纵向拉伸和压缩实验,分析了该类材料的纵向拉、压刚度和强度随编织工艺参数的变化规律以及材料的失效形式.三维五向编织复合材料在破坏前基本保持线弹性,纵向拉、压破坏具有脆性特征,拉伸模量和压缩模量比较接近,但拉伸强度远大于压缩强度.编织角和纤维体积含量对材料性能的影响显著,纱线粗细的影响不大.提高第五向纱线的比例,可提高材料的纵向性能.此外,研究中采用短标距薄板试件,以避免试件产生整体屈曲和端部纤维束开裂破坏.     

Flexural Behaviour of Unreinforced and Z-Fibre Reinforced 3D Carbon/Epoxy Composites

Applied Composite Materials - This study compares the flexural properties of unreinforced and Z-fibre reinforced 3D carbon/epoxy composites. Z-fibre reinforcement is introduced by a novel stitching...     

Experimental Investigation of Strain Behaviour of Heated Cement Paste and Concrete

The material degradation of concrete subjected to fire events has a severe influence on the load‐carrying capacity of support structures. Spalling of concrete layers, exposing the reinforcement bars and degradation of the material properties (Young's modulus, compressive strength) may lead to significant damage of the reduced cross‐section and, therefore, cause failure of the structure. In order to understand the stress build‐up at the heated surface caused by thermal expansion due to fire loading, finally leading to damage and spalling of concrete, the strain behaviour of cement paste and concrete exposed to combined thermo‐mechanical loading is the focus of this work. Hereby, the evolution of thermal strains, Young's modulus and Poisson's ratio with increasing temperature are investigated experimentally. For this purpose, the specimens are loaded uniaxially while the temperature is increased up to 800 °C. The obtained results provide the proper basis for the development of realistic material models, allowing more sophisticated simulations of structures exposed to fire.     

基于三胞模型的2.5D编织复合材料力学性能研究

为了准确预测2.5D编织复合材料的纵向力学性能,本工作以2.5D等厚度和变厚度编织复合材料为研究对象,基于细观结构表征结果,建立了一种考虑表面挤压效应和纱线错位滑移影响的2.5D等厚度和变厚度编织复合材料的全胞和三胞模型;并在所建立的全胞和三胞几何模型的基础上,基于逐渐损伤法开展了2.5D等厚度和变厚度编织复合材料的数值模拟分析,对不同结构的纵向刚度和强度进行了预测,模拟了不同结构的损伤扩展过程和损伤行为,并与实验结果进行对比验证.结果表明:考虑表面挤压效应和纱线错位滑移影响的全胞和三胞模型能更准确地预测2.5D编织复合材料的力学性能;三胞模型的数值模拟结果能更准确地反映2.5D编织复合材料的损伤扩展过程,更全面地表征2.5D编织复合材料的损伤行为.     

The Strain Rate Effect on the Compression Properties of Basalt/Carbon Fiber Reinforced Composites

Applied Composite Materials - Adding a basalt fiber layer (BF) to carbon-Fiber-reinforced plastic (CFRP) material can effectively improve its extensibility. In the work described in this study, a...     

三维编织复合材料理论研究进展

有关三维编织复合材料的理论分析研究归纳为:基于细观结构几何模型的物理性能研究和力学性能研究两部分.纤维体积分数是物理性能研究的最主要参数,力学分析以复合材料的弹性性能为主.合理的几何模型决定了力学性能分析与试验结果的一致性.建立在代表性体积单元尺度的几何模型应用最为广泛,得到了力学性能的试验验证.三维编织复合材料的力学性能的数值仿真主要以有限元方法为主,然而仅仅依赖于对其弹性性能的研究结果还远远不能满足三维编织复合材料作为关键结构部件的使用要求,建立完善的断裂准则是编织复合材料大量使用的理论依据.特殊形状的一次性编织复合材料的力学性能研究有待进一步深入.     

三维五向编织复合材料低速冲击及冲击后压缩性能实验研究

通过实验研究三维五向碳纤维/环氧树脂编织复合材料低速冲击及其冲击后压缩(CAI)性能。测试试件虽然有不同的编织角度,但承受相同的冲击能力。采用冲击后压缩测试表征不同编织结构的冲击后剩余力学性能。结果表明:编织角较大的试件由于其更紧密的空间结构,能承受更高的冲击载荷且冲击损伤区域更小。CAI强度和损伤机理主要取决于编织纤维束的轴向支撑。随着编织角的增加,CAI强度降低,材料的破坏模式也由横向断裂转变为剪切破坏。     

Preliminary Investigation of the Wear Behaviour of Self-lubricating Carbon Fibre Reinforced Glass Matrix Composites in Vacuum

The self-lubricating wear behaviour of a C-fibre reinforced borosilicate glass matrix composite was investigated using a vane-stator configuration resembling a rotating vacuum pump. The vanes were made of the composite material and the stator was of cast iron. Tests were carried out under varying loads in the range 3.8 to 5.8 N. Rotational velocities varied between 5.5 and 7.8 m/s. Fibre fracture and glass matrix particle detachment were the damage mechanisms observed by scanning electron microscopy. Composite wear was accompanied by material transfer onto the stator surface. The formation of isle-type and continuous graphitic films on the counter-body surface was observed. These films provide adequate lubrication during friction, leading to a relatively low wear of the composite under the conditions investigated.     

三维编织复合材料力学行为研究进展

随着三维编织复合材料力学行为研究工作的开展,这种材料在很多领域得到越来越广泛的应用。本文从细观结构几何模型、力学行为的理论研究、实验研究三个方面综述了三维编织复合材料力学行为的研究现状,并提出了当前工作存在的问题,对今后研究的趋势进行了展望。     

变截面三维编织复合材料的净形制备及弯曲性能

介绍变截面三维编织预型件的减纱净形制备工艺,通过实验观察与理论建模相结合的方法,分析减纱对预型件细观结构的影响,对比整列减纱、行单元减纱与切削复合材料在变截面区域的弯曲性能。结果表明表面与内部减纱单元是减纱时的基本单位,保证编织规律不变是减纱工艺的基本原则;表面或内部单元减掉后分别会形成两组特殊的纱线轨迹,其中一组纱线的长度与编织角大小较不减纱时增大,而另一组纱线只改变编织角的方向;行单元减纱与整列减纱复合材料的弯曲性能均明显优于切削试件,且行单元减纱试件的弯曲性能比整列减纱试件略高。     

2.5维编织复合材料力学性能的有限元分析

在已有研究的基础上,提出了一个新的2.5维编织复合材料有限元模型,该模型较为真实地模拟了织物内纤维束的轮廓结构和走向,可用于2.5维编织复合材料力学性能的有限元数值分析.利用该模型采用有限元法对2.5维编织复合材料进行了有效弹性性能的数值预报,并合理确定了复合材料内部应力场分布;同时提出经向纤维束横截面宽厚比是影响2.5维编织复合材料有效弹性性能的主要因素,分析了其影响情况.采用VARTM工艺制备了试验件,并对其进行力学性能测试.通过实验值和理论值的对比,结果表明:有限元计算得到的结果与实验值吻合较好,从而验证了该模型的有效性.     

Damping Behaviour and Mechanism of Graphite ParticulateReinforced Metal Matrix Composites

The effect of graphite particulate reinforcement on the resultant damping behaviour of ZA27metal matrix composites (MMCs) has been investigated in an effort to develop a new functional material. The MMCs were processed by a spray atomization and deposition technique. The damping capacity, as well as the relative dynamic modulus, was measured at frequencies of 1and 4 Hz in the 30 to 200℃ temperature range. There exists a transformation point at 55℃in the internal friction and temperature spectrum of the MMCs. It is shown from microstructural analysis that the damping capacity of ZA27 can be significantly improved by the addition of graphites particulates through spray deposition processing. Finally, the operative damping mechanism is discussed in light of the data obtained from characterization of microstructure and damping capacity.     

Finite Element Analysis of Thermo-Mechanical Properties of 3D Braided Composites

This paper presents a modified finite element model (FEM) to investigate the thermo-mechanical properties of three-dimensional (3D) braided composite. The effective coefficients of thermal expansion (CTE) and the meso-scale mechanical response of 3D braided composites are predicted. The effects of the braiding angle and fiber volume fraction on the effective CTE are evaluated. The results are compared to the experimental data available in the literature to demonstrate the accuracy and reliability of the present method. The tensile stress distributions of the representative volume element (RVE) are also outlined. It is found that the stress of the braiding yarn has a significant increase with temperature rise; on the other hand, the temperature change has an insignificant effect on the stress of the matrix. In addition, a rapid decrease in the tensile strength of 3D braided composites is observed with the increase in temperature. It is revealed that the thermal conditions have a significant effect on the strength of 3D braided composites. The present method provides an effective tool to predict the stresses of 3D braided composites under thermo-mechanical loading.