单纤维与裂纹交互微力学:完整粘接纤维

使用微拉曼光谱技术研究裂纹前喙Kevlar49芳纶纤维交互微力学行为.在建立Kevlar49芳纶纤维轴向变形与拉曼频移线性关系的基础上,通过逐点测量裂尖前喙完整粘接纤维上的应力分布,探讨了裂纹与完整粘接纤维的交互作用,建立了完整粘接纤维上的应力传递模型.Ⅰ型裂纹切口前方的高应力区造成粘接完好纤维上出现应力集中,应力强度因子K1影响着纤维应力集中长度.     

横观各向同性轴对称界面端奇异性研究

套筒模型是复合材料中常用的进行纤维、基体间应力传递分析的轴对称模型.在套筒模型中,中心为纤维,纤维外包裹的"套筒"有假设为各向同性基体材料的,也有假设为横观各向同性复合材料的.不失一般性,本文将纤维和基体均视作横观各向同性材料,建立了任意楔形角的横观各向同性复合材料基体包裹横观各向同性纤维的轴对称模型,采用两次坐标变换、逐次渐近等求解方法,得到了求解该模型界面端应力奇异性指数的特征方程.考虑常见的碳纤维/环氧树脂复合材料制成的压入和拔出试件,根据得到的特征方程计算了两种试件的界面端奇异性指数随碳纤维体积百分含量的变化情况,结果发现,随纤维体积百分含量的增加,两种试件界端的奇异性均呈减弱趋势.     

原位微拉曼测试技术在碳纳米管纤维和薄膜材料力学性能研究中的应用

利用宏观应力联合原位微拉曼测试技术对双壁碳纳米管(Carbon Nanotube,CNT)纤维和薄膜材料的力学性能进行了实验分析,探讨了拉伸加载期间纤维和薄膜内CNT的载荷响应及其与宏观力学性能的关联,揭示了两种材料力学性能差异性的微观机理。实验分析表明,CNT纤维和薄膜的拉伸变形呈现弹性、强化和损伤断裂三个阶段,但其内的CNT只发生弹性变形,没有塑性形变,且没有明显的损伤或键的断裂,纤维和薄膜呈现阶段性拉伸变形的原因可归结为滑移。纤维的弹性模量显著高于薄膜,是薄膜的4.7倍,原因是弹性阶段纤维中CNT的轴向伸长对宏观应变的贡献较大。纤维和薄膜的拉伸强度相差较小,原因是强化阶段薄膜内不断有大量CNT进入承载队伍,这也使得薄膜具有比纤维更高的韧性。     

头发纤维/环氧树脂微滴脱粘测试：鳞片效应

通过纤维拉出的微滴脱粘测试实验,研究了头发纤维顺鳞和逆鳞方向拉出时的界面强度、失效模式和摩擦机制。通过分析拉伸载荷-位移曲线,给出了头发纤维/环氧树脂界面强度,发现逆鳞方向拉出的界面强度远大于顺鳞方向的强度。通过扫描电子显微镜对失效样品表面形貌分析证实,顺鳞方向拉出发生界面脱粘失效,而逆鳞方向拉出主要发生基体破坏失效模式。另外,与顺鳞方向拉出的稳定摩擦阶段不同,逆鳞方向拉出表现出振荡摩擦现象,最后发展了头发鳞片与残留树脂的相互作用微观模型来解释这种振荡摩擦现象。     

固化温度对碳纤维/环氧基体界面行为影响的分析

对界面粘结性能及热残余应力影响下的单纤维复合材料的界面行为进行了分析。采用界面的弹性-软化内聚力模型,用解析法对单纤维复合材料由固化引起的热残余应力、以及单纤维碎断过程纤维的轴向应力分布进行了模拟,得到了碳纤维/环氧树脂在常温和高温固化两种情况的界面粘结性能。结果表明：与常温固化相比,高温固化后,界面的剪切强度增幅不大,界面的断裂韧性显著增加;高温固化后形成的界面,使界面的软化提前、界面的脱粘延迟;高温固化产生的纤维轴向和界面径向热残余应力对界面的软化均有延迟作用;界面径向热残余应力还对界面的脱粘有延迟作用。     

二氧化硅纳米颗粒对碳纤维与环氧树脂基体粘合强度的增强

纤维与基体的粘合强度是决定纤维增强高分子复合材料性能的关键因素.本文采用横向纤维束拉伸实验的方法研究了碳纤维与经过纳米颗粒改性的环氧树脂基体间的粘合强度.平均直径为25纳米的二氧化硅纳米颗粒用特殊的溶胶-凝胶法引入环氧基体(由Hanse Chemie AG提供),可以达到很高的含量,同时保持较为理想的分散状态.实验结果表明,二氧化硅纳米颗粒对于碳纤维与改性环氧基体的粘合强度有显著的增强效应.当纳米颗粒含量为14 vol.%时,横向纤维束拉伸的断裂强度相比纯环氧基体提高了104%.通过对横向纤维束拉伸样品断裂面的扫描电镜观察,以及二氧化硅纳米颗粒改性环氧树脂基体材料的力学性质的测量,可以发现横向纤维束拉伸的断裂强度与改性环氧基体本身的断裂韧性之间存在良好的相关性.由此可推测纳米颗粒对环氧树脂基体材料的增韧是碳纤维与基体间界面增强的一个重要原因.     

压入实验界面端奇异性研究

纤维压入实验是复合材料界面剪切强度细观实验方法之一,其试件通常由复合材料中切割下来制备而成,从中选取单根纤维,进行压入试验,所以被选中的纤维可看成是被纤维和纯基本材料构成的横观各向同性复合材料所包裹。本文以此为依据,建立了横观各向同性复合材料基体包裹各向同性纤维的轴对称模型,采用逐次渐近等求解方法,得到了求解该模型界面端应力奇异性指数的特征方程,并计算了碳纤维／环氧树脂、碳纤维／铝和碳纤维／Al2O3压入试件界面端奇异性随碳纤维体积百分含量的变化情况。     

二氧化硅纳米颗粒对碳纤维与环氧树脂基体粘合强度的增强

纤维与基体的粘合强度是决定纤维增强高分子复合材料性能的关键因素。本文采用横向纤维柬拉伸实验的方法研究了碳纤维与经过纳米颗粒改性的环氧树脂基体间的粘合强度。平均直径为25纳米的二氧化硅纳米颗粒用特殊的溶胶-凝胶法引入环氧基体（由 Hanse ChemieAG提供），可以达到很高的含量，同时保持较为理想的分散状态。实验结果表明，二氧化硅纳米颗粒对于碳纤维与改性环氧基体的粘合强度有显著的增强效应。当纳米颗粒含量为14v01．％时，横向纤维柬拉伸的断裂强度相比纯环氧基体提高了104％。通过对横向纤维柬拉伸样品断裂面的扫描电镜观察，以及二氧化硅纳米颗粒改性环氧树脂基体材料的力学性质的测量，可以发现横向纤维柬拉伸的断裂强度与改性环氧基体本身的断裂韧性之间存在良好的相关性。由此可推测纳米颗粒对环氧树脂基体材料的增韧是碳纤维与基体间界面增强的一个重要原因。     

含界面相复合材料热残余应力分析

本文分析了含有界面相纤维增强复合材料热残余应力的空间分布。针对材料实际微结构几何特点,建立含有界面相的三维三相单丝模型,用均匀和梯度函数描述界面相模量随空间变化规律,由轴对称体弹性力学理论得到单丝热残余应力分布,结果表明梯度界面降低了残余应力。通过碳纤维电阻法测出T300/环氧树脂单丝体系固化后的纤维轴向应变,与梯度界面的分析结果基本一致。用叠加方法得到密排六方结构代表性体积元(RVE)中纤维间相互偶合的应力场,同时应用有限元法分析RVE中纤维间的残余应力分布,两者结果相互验证。     

单纤维细丝微力学性能实验研究

使用数字标记点识别方法分别对T300碳纤维和Kevlar29芳纶纤维单丝进行了拉伸实验测试,给出了单纤维细丝的拉伸性能参数。其中T300碳纤维单丝的直径在 7μm左右,比Kev lar29芳纶纤维的直径 (约 12μm左右 )要小;T300碳纤维的拉伸模量为 230GPa左右,远大于Kevlar29芳纶纤维的拉伸模量(约 80GPa);它们的拉伸强度相当,都在 2. 5GPa左右;而T300碳纤维的断裂伸长率在 1. 0%左右,小于Kevlar29芳纶纤维的 3. 0%。使用扫描电子显微镜(SEM)对纤维单丝断裂前后的结构进行了观察,这些不同的微观结构决定了纤维单丝拉伸性能的差异。     

纤维增强韧性基体界面力学行为

分析了纤维增强韧性基体的界面力学行为及其失效机理,按剪滞理论和应变理化规律研究微复合材料的弹塑性变形和应力状态,讨论了幂硬化和线性硬化基体的弹塑性变形和界面应力分布,并给出纤维应力和位移的表达式。按最大剪应力强度理论建立了纤维／基体界面失效准则,推导出弹塑性界面失效的平均剪应力随纤维埋入长度的变化关系。     

Local displacements and load transfer in shape memory alloy composites

Although there has been a significant amount of research dedicated to characterizing and modeling the response of shape memory alloys (SMAs) alone, little experimental work has been done to understand the behavior of SMAs embedded in a host material. The interaction between SMA wires and a host polymer matrix was investigated by correlating local displacements and stress fields induced by the embedded wires with SMA/polymer adhesion. Most SMA composite applications require transfer of strain from the wire to the matrix. In these applications, maximum interfacial adhesion between the SMA wire and the polymer matrix is most desirable. The adhesion was varied by considering four different surface treatments: untreated, acid etched, hand sanded and sandblasted. The average interfacial bond strength of the SMA wires embedded in an epoxy matrix was measured by standard pull out tests. Sandblasting significantly increased the bond strength, whereas hand sanding and acid cleaning actually reduced interface strength. In situ displacements of embedded, surface-treated SMA wires were measured using heterodyne interferometry, whereas the resulting stresses induced in the polymer matrix were investigated using photoelasticity. Increased wire adhesion resulted in lower axial wire displacement and higher interfacial stresses due to the restraining effect of the matrix on the actuated wire. A simplified theoretical analysis was carried out to estimate the shear stress induced in the matrix due to wire actuation. The maximum shear stress predicted for the case of a perfect interfacial bond was about 7 percent larger than the value measured experimentally for the sand-blasted wire.     

Strain distribution within compressed circular cylinders

A theoretical solution has been obtained for the stress distribution throughout a cylindrical specimen loaded in compression, which takes into account different degrees of friction at the end surfaces. To check the solution experimentally, a cylindrical model was constructed from epoxy sheets, with electrical-resistance strain gages embedded between the sheets and also bonded to the cylindrical surface. With the specimen under axial compression, strain measurements were recorded within the elastic range of the epoxy. Reasonable agreement with the theoretical solution was observed for the two types of end conditions. In one case the cylinder was in direct contact with the platens of the testing machine, in the other case a Teflon sheet was placed between the two surfaces.     

Mechanisms of stress transfer and interface integrity in carbon/epoxy composites under compression loading: Part I: Experimental investigation

Raman spectroscopy is used to get an insight into the microstructural aspects of the compressional behavior of carbon fiber composites. This is done by a comparative assessment of the stress transfer efficiency in tension and compression in single-fiber discontinuous model geometries. It was found that axial stress is transferred in the fiber through the generation of shear stresses at the interface for both tension and compression loading. Experimental evidence is presented to verify that the values of the maximum interfacial shear stress that the system sustains is a function of the applied strain and independent of the type of loading. However, compressive failure is quite different as fiber fragments remain in contact, thus can still bear load.     

Stress relaxation in broken fibers in unidirectional composites: modeling and application to creep rupture analysis

This paper describes a model of stress relaxation in broken fibers in unidirectional metal matrix composites reinforced with long brittle fibers. A cylindrical cell with a broken fiber embedded in a power law creeping matrix is employed, and the broken fiber is assumed to have a bilinear distribution of axial stress. Then, on the basis of energy balance in the cell under constant overall strain, a relaxation equation of interfacial shear stress acting on stress recovery segments is derived in a simple form. The relaxation equation is approximated rationally and integrated to obtain an analytical solution, which is shown to agree fairly well with the numerical analysis of Du and McMeeking. (Du, Z.-Z., McMeeking, R.M., 1995. Creep models for metal matrix composites with long brittle fibres. J. Mech. Phys. Solids 43, 701–726.) Moreover, the relaxation equation is combined with Curtin's model (Curtin, W.A., 1991. Theory of mechanical properties of ceramic-matrix composites. J. Am. Ceram. Soc. 74, 2837–2845.), so that rupture time in long term creep is evaluated analytically and explicitly on the assumption of global load sharing. It is shown that the resulting relation represents well the dependence of creep rupture time on applied stress observed experimentally on a unidirectional metal matrix composite.     

Debonding and fiber pull-out in reinforced composites

In order to evaluate the strength of fiber-reinforced composites, there is first the need to investigate the interfacial debonding and the pull-out of fibers in a fractured composite with intact fibers. This type of problem in crack bridging has been investigated by several authors based on different models and assumptions [1–7]. In this study, we will consider a three-dimensional model of a single fiber of finite length bonded by a finite cylindrical matrix with an initial crack existing in a portion of the interface. In the model, one end of the cylinder is so constrained that the axial component of displacement vanishes. A tensile stress is applied to the fiber at the other end. The aim is to determine the pull-out of the fiber and the critical condition for interfacial debonding. Both the fiber and the matrix are treated as elastic materials. Analysis is made based on a method using Papkovich-Neuber displacement potential functions for the problem of an elastic solid subjected to axisymmetrical boundary conditions. Solutions are found by means of the technique of trigonometrical series. Effects of initial misfit strains and frictional sliding between the fiber and the matrix over the interfacial crack are also included in the study.     

Residual Strains using Integrated Continuous Fiber Optic Sensing in Thermoplastic Composites and Structural Health Monitoring

The evolution of spatially resolved internal strain/stress during the manufacturing of thermoplastic composites and subsequent relaxation from water intake are evaluated using an in-situ fiber optic sensor corresponding to a coated optical glass fiber with a nominal diameter of 160 μm. Unidirectional carbon fiber-polyamide 6 composites are produced using compression molding with an embedded fiber optic for strain measurement. The distributed fiber optic based strain sensor is placed in an arrangement to capture 0, 45, and 90° strains in the composite to resolve in-plane strain tensor. Strains are monitored in the direction of fiber optic sensor along its length at high resolution during the various stages of compression molding process. Results indicate considerable internal strains leading to residual stress at the end of processing step along the off-axis (45°) and transverse (90°) directions, and small strains in the carbon fiber pre-preg (0°) direction. At the end of compression molding process, an average of 7000 and 10,000 compressive micro-strains are obtained for residual state of strain in the off-axis and transverse direction. Since water/moisture infusion affects the mechanical properties of polyamide-6 matrix resin, these composite panels with embedded sensors targeted for marine applications are monitored in a water bath at 40 °C simulating accelerated testing conditions. Using the same fiber optic sensor based technique, the strain relaxation was observed during water uptake demonstrating in-situ strain monitoring during both manufacturing and subsequent composite implementation/application environment. The technique presented in this paper shows the potential of optimizing time-temperature-pressure protocols typically utilized in thermoplastic manufacturing, and continuous life-cycle monitoring of composite materials using a small diameter and inexpensive distributed fiber optic sensing.     

碳纤维增强树脂基复合材料压缩失效机理研究

为分析基体性质和纤维铺设方式对碳纤维树脂基复合材料(CFRP)压缩性能的影响,设计两种基体（耐高温环氧树脂基体和PA6 基体） 和两种碳纤维铺设方式[[45/0/-45/90]s和[45/-45]2s],共4 类盒型碳纤维试件．采用轴向压缩实验,研究低应变速率下碳纤维复合材料的极限载荷、压缩量和刚度等力学性能．研究表明试件的失效形式主要由基体性质决定,损伤区域及裂纹方向主要由碳纤维铺设方式决定．利用X射线显微镜(XRM)对试件典型的损伤区域进行三维扫描,并对扫描图像进行重构与渲染,获得破坏区域的内部损伤细节．根据损伤扫描结果,得到材料内部的损伤类型及破坏程度．归纳所获得的损伤测试特征,分析不同类型试件压缩时的损伤规律与失效机理．