Anisotropy of hygrothermal damage in fiber/polymer composites: Effective elasticity measures and estimates

Measured elasticity moduli of a highly (68%) glass-fiber reinforced epoxy matrix for different amounts of fiber/matrix interface weakening and debonding, due to different hygrothermal ageing stages, are compared to estimated ones. Ultrasonic measurements provide seven of the nine elasticity moduli of the orthotropic material samples, including all the moduli significantly affected by damage. Theoretical estimates combine homogenization modeling techniques and Finite Element (FE) calculations, the latter when the effect of observed partial debonding on effective moduli is to be specified. These estimates are performed under different assumptions for the composite structure, with special attention to the existence of a fiber–matrix interphase. Analytical comparisons for the undamaged composite establish that matching US measurements with estimates cannot be obtained, regardless of the chosen model, without the assumption of an interphase layer of modified resin coating the fibers. This coating resin, when in relevant concentration with regard to literature data about fiber coating thickness, typically conserves the epoxy moduli transversally to the fiber orientation, while, in the fiber direction its moduli approach those of the fibers. The comparison of the US measurements on damaged samples to FE calculations assuming progressive one-directional debonding shows that most of the composite stiffness loss can preferentially be due to an initial interphase weakening, while the fiber/matrix debonding seems more likely delayed to long H-ageing times. This is consistent with physical interpretation of damage by water pooling through silane bridges bonding epoxy to glass. The calculations also provide the effective stiffness, at different damage stages, of the “Undamaged Equivalent Inhomogeneity” for this damaged inclusion type.     

Modeling elastic properties of short flax fiber-reinforced composites by orientation averaging

Natural fibers of plant origin, used as reinforcement in polymer matrix composite materials, exhibit highly anisotropic elastic properties due to their complex internal structure. Mechanical properties can be evaluated not only by tests but also by mechanical models reflecting the principal morphological features of fibers. Such a FEM model is applied to estimate the elastic properties of a unit cell of a short-fiber-reinforced composite, an elementary flax fiber embedded in a polymer matrix. Orientation averaging approach is used for prediction of the stiffness of short flax fiber reinforced polymer matrix composite. The numerical estimates of Young’s modulus are compared to the test results of extruded flax/polypropylene composite.     

A finite element model of the fragmentation test for the case of a coated fiber

The fiber/matrix interface in composite materials is generally studied by means of the fragmentation test by using a single filament embedded in a polymeric matrix. The average shear strength, , of the interface is computed from the well-known shear-lag theory. For the case of a fiber coated with a thin layer of an interphase having a low modulus, a finite element analysis model has been developed to describe the profiles of the tensile and shear stresses along a fiber fragment. The introduction of a soft interlayer decreases the stresses at the ends of the fragment, and as the interlayer thickness increases this effect becomes more important. This model shows the limits of the shear-lag theory and could explain experimental results obtained for the case of an elastomer-coated glass fiber in an epoxy matrix.     

Effect of fiber coating on the longitudinal damping capacity of fiber-reinforced metal matrix composites

A novel model for calculating the damping capacity of continuous fiber-reinforced metal matrix composites (FMMCs) is proposed based on the viewpoint of energy loss. Finite element method (FEM) has been employed to investigate the effect of fiber coating on the longitudinal damping capacity of a composite by varying the thickness and the material properties of the coating. The results show that the damping of a composite containing the elastic coating increases with a decrease in the elastic modulus of the coating, while for the case of plastic coating, the weak coating or the high elastic modulus coating may help in improving the overall damping of composite.     

CVD-SiC界面改性涂层对气相渗硅制备C_f/SiC复合材料力学性能的影响

在气相渗硅制备C_f/SiC复合材料时,界面改性涂层非常重要。良好的界面改性涂层一方面起到保护碳纤维不受Si反应侵蚀的作用,另一方面起到调节纤维和基体界面结合状况。通过在C纤维表面制备CVD-SiC涂层来进行界面改性,研究CVD-SiC界面改性涂层对GSI C_f/SiC复合材料力学性能和断裂特征的影响,并分析其影响机制。结果表明:无CVD-SiC涂层改性的C_f/SiC复合材料力学性能较差,呈现脆性断裂特征,其强度、模量和断裂韧度分别为87.6MPa,56.9GPa,2.1MPa·m1/2。随着CVD-SiC涂层厚度的增加,C_f/SiC复合材料的弯曲强度、模量和断裂韧度呈现先升高后降低的趋势,CVD-SiC涂层厚度为1.1μm的C_f/SiC复合材料的力学性能最好,其弯曲强度、模量和断裂韧度分别为231.7MPa,87.3GPa,7.3MPa·m1/2。厚度适中的CVD-SiC界面改性涂层的作用机理主要体现在载荷传递、"阻挡"Si的侵蚀、"调节"界面结合状态3个方面。     

NCG Reinforced MMC Fabricated by the Squeeze Casting Method

In order to increase the wettability between carbon fiber and aluminum matrix, pure nickel was applied as coating on carbon fiber. And scanning electron microscopy was used to characterize the microstructure of the coating, fiber and matrix. The tensile tests of carbon fiber and aluminum matrix were done to examine how the behavior of nickel layers with variation of applied pressure make an effect on the mechanical properties of composites. As the applied pressure increases, nickel layers were resolved into the aluminum matrix and ultimate tensile strength of the composite decreased. This was due to premature fracture of the reaction layer acting as a surface notch.     

Closed-form solution for a coated circular inclusion under uniaxial tension

A coated circular inclusion embedded in an infinite matrix is analyzed in the framework of two-dimensional isotropic linear elasticity. A closed-form solution is obtained for the case of far-field uniaxial tension using Muskelishvilis complex potential method. The solutions for the stress and strain distributions for all three regions, that is, matrix, coating, and inclusion, were obtained for various coating-to-matrix shear modulus ratios, while keeping the fiber and matrix shear moduli the same. Test cases for an inclusion without the coating and hollow inclusion were also studied. The energy release rate was evaluated using the path-independent M-integral, which is used to calculate the energy release rate for the self-similar expansion of defects surrounded by the closed contour of the integral. The results for the stress and strain concentrations along with the energy release rate due to this material inhomogeneity were analyzed to yield a better understanding of the mechanics of materials with circular inclusions. This can be helpful in designing intelligent composite structures with embedded optical fiber sensors.     

Simultaneous sensing of the strain and points of failure in composite beams with an embedded fiber optic Michelson sensor

A fiber optic Michelson sensor was embedded in composite beams to sense the internal strain and points of failure of the composite structures. The bending deformation and matrix cracking were investigated by four-point bending tests of cross-ply composite beams with the embedded fiber optic sensor. The failure points of composite beams were detected by using both a PZT sensor and a fiber optic sensor in order to investigate the fiber optic failure signals. The failure due to matrix cracks in a composite beam was confirmed by the edge replica method. The digital processing of the fiber optic signal was carried out to determine the strains and failure points of composite beams. The failure points were observed from the processed failure signal by high-pass filtering. The initial failure strain of the composite beam was measured and processed from the fiber optic strain signal after low-pass filtering.     

复合材料螺旋弹簧刚度及其影响因素分析

该研究制备了一种与中空圆截面簧丝轴线成±45°铺层的新型复合材料圆柱螺旋压缩弹簧,并对该弹簧进行准静态压缩试验。将纤维复合材料平板的剪切模量等效替代各向同性材料弹簧刚度表达式中的剪切模量,从而理论导出该类复合材料弹簧的刚度表达式。针对该表达式分析影响弹簧刚度的各种因素及其影响程度。结果表明:弹簧的刚度系数随着纤维弹性模量、纤维体积分数、簧丝外直径的增加而急剧增加,随它们的下降而显著下降;基体弹性模量与复合材料泊松比对弹簧刚度的影响十分微小;簧丝内直径在一定范围内变化时对弹簧刚度的影响几乎可以不计,但却能显著降低弹簧质量,超出该范围后,弹簧刚度随内直径增加而急剧下降;刚度系数随弹簧有效圈数和弹簧圈平均直径的增加而剧烈下降,随着它们的下降而急剧增加。     

The effect of thermoplastic coating on the mechanical properties of woven fabric carbon/epoxy composites

The effect of a polyetherimide (PEI) coating on the mechanical properties of woven fabric carbon/epoxy composites was investigated by thermal mechanical analysis, fractographical analysis and mechanical properties measurements. PEI coating enhanced the mechanical properties of carbon/epoxy composites mainly through the improvement of matrix properties. This was because most of the PEI coated on the carbon fiber diffused into the bulk of epoxy matrix due to its good miscibility with epoxy resin. As for mechanical properties of woven fabric carbon/epoxy composites, the extent of improvement by PEI coating highly depended on the applied stress state. Among the mechanical properties, mode II delamination resistance of carbon/epoxy composites showed the highest increment because matrix shear property played an important role in delamination resistance of woven fabric carbon/epoxy composite. Because of the woven geometry of carbon fiber, the improvement in impact property of carbon/epoxy composite was trivial except the large amount of PEI coated case.     

缝纫泡沫夹芯复合材料的刚度预测与试验验证

基于材料细观结构,建立了缝纫泡沫夹芯复合材料的刚度预测模型,并进行了刚度性能的相关试验验证。其中,对缝纫复合材料层合面板部分,考虑了缝纫角对单胞尺寸和富脂区大小的影响,以及缝纫前后层合面板厚度的变化对复合材料面板纤维体积含量的影响,采用改进的纤维弯曲模型计算了缝纫复合材料层合面板的刚度;对缝纫增强的泡沫夹芯部分,把缝线树脂柱看作是泡沫基体中的增强相,将其简化为特殊的单向增强复合材料,提出了用串并联组合模型来预测其刚度。试验测试了缝纫泡沫夹芯复合材料板试件的刚度。应用本文模型对缝纫层合面板和缝纫泡沫夹芯复合材料板的刚度进行预测,结果均与试验结果吻合较好。采用理论模型系统研究了缝纫参数和结构参数对缝纫泡沫夹芯复合材料刚度的影响。     

Mathematical investigation of interfacial property in fiber reinforced model composites

In the current paper, we have investigated the dependence of the effective elastic properties of a composite material on the fiber/matrix interface elastic property. The model composite consists of a single cylindrical fiber embedded in a concentric cylindrical matrix material. A three dimensional mathematical method has been developed connecting the interface properties with the effective axial Young’s modulus of the composite structure. Special effort has been devoted to decode information about the quality of the interface by exploiting the information provided by the elastic effective parameters. In particular, the effective modulus vs. stiffness coefficient curves have been generated for Ti/SiC composites. The aforementioned curves reveal the characteristics of the transition from the regime of perfect interface to the realm of complete debonding.     

The mechanical characteristics of smart composite structures with embedded optical fiber sensors

In this study, the mechanical characteristics of composite laminates with embedded optical fiber sensors were evaluated to investigate the effect of embedded optical fiber on the mechanical properties of composite laminates under the static tensile and the low cycle fatigue load. Testing specimens were fabricated with glass fiber/epoxy composites with embedded optical fiber sensors to observe initiation and growth of damage in the specimens and laser signal behavior transmitted through the optical fiber visually and directly. By using this transparency of glass fiber/epoxy composites, the damage of sensors and associated laser signal behavior was observed. Under the static load, the embedded optical fibers do not have significant effect on the stiffness and the strength, while the embedded optical fibers show significant effect on the fatigue life of composite specimens. Especially, the embedded optical fiber sensors show the very low resistance to the fatigue load.     

Effects of interface coating and nitride enhancing additive on properties of Hi-Nicalon SiC Fiber reinforced reaction-bonded silicon nitride composites

Strong and tough Hi-Nicalon SiC fiber reinforced reaction-bonded silicon nitride matrix composites (SiC/RBSN) have been fabricated by the fiber lay-up approach. Commercially available uncoated and PBN, PBN/Si-rich PBN, and BN/SiC coated SiC Hi-Nicalon fiber tows were used as reinforcement. The composites contained 24 vol% of aligned 14 m diameter SiC fibers in a porous RBSN matrix. Both one- and two-dimensional composites were characterized. The effects of interface coating composition, and the nitridation enhancing additive, NiO, on the room temperature physical, tensile, and interfacial shear strength properties of SiC/RBSN matrix composites were evaluated. Results indicate that for all three coated fibers, the thickness of the coatings decreased from the outer periphery to the interior of the tows, and that from 10 to 30 percent of the fibers were not covered with the interface coating. In the uncoated regions, chemical reaction between the NiO additive and the SiC fiber occurs causing degradation of tensile properties of the composites. Among the three interface coating combinations investigated, the BN/SiC coated Hi-Nicalon SiC fiber reinforced RBSN matrix composite showed the least amount of uncoated regions and reasonably uniform interface coating thickness. The matrix cracking stress in SiC/RBSN composites was predicted using a fracture mechanics based crack bridging model.     

Dispersion of micelle-encapsulated fluorophores in a polymer matrix for control of color of light emitted by light-emitting diodes

In the present study, we demonstrated that fluorescent dyes could be nanoscopically dispersed in a polymer matrix that was immiscible with the dyes; the dyes were encapsulated in micelles. Using a model polymer composite, we also showed that the color of light emitted by light-emitting diodes (LEDs) could be controlled by coating fluorescent polymer composites onto the LEDs. For this purpose, fluorophores that were insoluble in toluene were solubilized into a solution of block copolymer micelles in toluene by the selective incorporation of fluorescent dyes into micellar cores. Because the micelles could be dispersed well in the polymer matrix without the formation of aggregates, fluorescent dyes encapsulated in the micelles were also effectively dispersed in the polymer matrix without macroscopic separation. The polymer composite can be evenly coated onto most substrates, regardless of their surface characteristics. Thus, light-emitting devices with well-controlled emission wavelengths and emission intensities can be fabricated by coating the polymer composite onto the surface of the device.     

Effect of BN Coating on the Strength of a Mullite Type Fiber

Nextel 480 is a polycrystalline essentially mullite fiber (70 wt.-% Al₂O₃+28 wt.-% SiO₂+2 wt.-% B₂O₃). Different thicknesses of BN were applied as coatings on this fiber. Optical, scanning electron, and transmission electron microscopy were used to characterize the microstructure of the coatings and fibers. The effects of coating and high temperature exposure on the fiber strength were investigated using two-parameter Weibull distribution. TEM examination showed that the BN coating has a turbostratic structure, with the basal planes lying predominantly parallel to the fiber surface. Such an orientation of coating is desirable for easy crack deflection and subsequent fiber pullout in a composite. The BN coated Nextel 480 fiber showed that Weibull mean strength increased first and then decreased with increasing coating thickness. This was due to the surface flaw healing effect of the coating (up to 0.3 μm) while in the case of thick BN coating (1 μm), the soft nature of the coating material had a more dominant effect and resulted in a decrease of the fiber strength. High temperature exposure of Nextel 480 resulted in grain growth, which led to a strength loss.     

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.     

Interaction between matrix cracking and edge delamination in composite laminates

Matrix cracking and edge delamination are two main damage modes in continuous-fibre composite laminates. They are often investigated separately, and so the interaction between two damage modes has not yet been revealed. In this paper, a simple parallel-spring model is introduced to model the longitudinal stiffness reduction due to matrix cracking and edge delamination together. The energy release rate of edge delamination eliminating the matrix crack effect and the energy release rate of matrix cracking in the presence of edge delamination are then obtained. Experimental materials include carbon- and glass-fibre-reinforced bismaleimide composite laminates under static tension. The growth of matrix cracks and edge delaminations was recorded by means of NDT techniques. Results show that matrix cracks may initiate before or after edge lamination. This depends on the laminate layup, and especially on the thickness of the 90° plies. Edge delamination may also induce matrix cracking. Matrix cracking has a significant effect on the stiffness reduction in GRP laminates. The present model can predict the stiffness reduction in a laminate containing both matrix cracks and edge delaminations. The mixed-mode delamination fracture toughness obtained from the present model shows up to 50% differences compared with O'Brien's model for GRP laminates. However, matrix cracking has a small effect on the mixed-mode interlaminar fracture toughness of the CFRP laminates.     

Post-curing effect on magnetoelectric performance of single PZT rod/continuous Terfenol-D fiber/epoxy 1-1-3 composites

A magnetoelectric (ME) composite consisting of a single PZT rod embedded in a matrix of continuous Terfenol-D fiber and epoxy medium has been fabricated and characterized. With an optimized aspect ratio of the composite rods, a large ME effect has been observed. The magnetostrictive effect of the continuous Terfenol-D fiber/epoxy medium can be enhanced by imposing an optimal pre-loading stress on the material and this pre-loading stress can be induced by suitable heat treatment. Experimental results show that the ME effect of the single PZT rod/continuous Terfenol-D fiber/epoxy composites can be enhanced significantly by a post-curing process. A thermal stress-mediated continuous fiber composite model has been used to explain the ME enhancement of the post-cured composites.     

气体通过高分子复合膜的机理及其应用

本文对气体通过高分子复合膜的机理进行了研究,考虑到支撑层阻力、涂层与致密层的结合程度,提出了扩展的Henis模拟电路。从理论上和实践中,证明了由于支撑层阻力的存在,使气体由中空纤维高分子复合膜丝外向丝内渗透比由丝内向丝外渗透的渗透系数小;指出了涂层嵌入致密层深度比涂层厚度对膜的分离系数影响大;第一次定量给出了少量断丝对大型中空纤维高分子膜分离器分离系数的严重影响;提出用O_2、N_2评价大型N_2/H_2高分子膜分离器的新的计算方法,并在实验中得到验证。