Load transfer from fiber to polymer matrix,studied by measuring the apparent elastic modulus of carbon fiber embedded in epoxy

Load transfer from a single carbon fiber to the surrounding epoxy matrix was studied by measuring the apparent tensile modulus of the fiber while the fiber was embedded in epoxy and comparing the apparent modulus (1650 GPa) with the real modulus (230 GPa). Thus, it was found that 87% of the tensile load applied to the fiber was transferred to the epoxy.     

Using self-assembled monolayer technology to probe the mechanical response of the fiber interphase-matrix interphase interface

In this paper, a brief review of the fiber-matrix interphase/interface region is given for carbon- and glass-fiber composites. The substructure of the interphase/interface region is discussed in terms of three interphases: (a) the fiber interphase (FI), (b) the sizing interphase (SI), and (c) the matrix interphase (MI), and two interface regions: (a) the FI-SI interface and (b) the SI-MI interface. These substructures are a synthesis of the ideas advanced by Ishida and Koenig and Drzal. The schematic model of interphase deformation behavior originally given by Bascom is reconstructed to include research results from the above researchers. To systematically probe adhesion at the SI-MI interface, functionalized self-assembled monolayers (SAMs) using bonding and non-bonding C₁₁- type trichlorosilanes are prepared using the research of Menzel and Heise, and that of Cave and Kinloch as a guide. Results from this research are compared with short chain bonding and nonbonding silanes prepared by aqueous and non-aqueous deposition processes. The data were interpreted using the mechanisms proposed by Sharpe, Ishida and Koenig, and Drzal and the mathematical equation proposed by Nardin and Ward. For the non-bonding short-chain silane deposited by aqueous deposition, 90% of the adhesion was found to be due to mechanical interlocking, with the remaining adhesion due to physicochemical interactions. For the bonding short-chain silane deposited by aqueous deposition, the interface strength relative to the non-bonding short-chain silane increased by 31%. However the interfacial shear strength (IFSS) of this system was approximately 40% lower than the comparable bonding SAM interface. This difference was interpreted in terms of the propensity of the C₃-alkylamine to form cyclic ring structures in the MI region as described by Ishida, Koenig, et al. The SAM data also indicates that 70-85% of the maximum IFSS is obtained with 25-50% of the surface covered with functional groups. This suggests that steric hindrance, due to the size of the DGEBA molecules, restricts access to the functional groups on the surface. Therefore, only 35% of the surface functional groups are accessible for bonding in the DGEBA/m-PDA epoxy resin system.     

Modelling of the transverse strength of fibre reinforced epoxy composite at low and high temperature

Process-induced thermal residual stresses and matrix failure of unidirectional carbon fibre reinforced composites (CFRP) have been investigated by finite element analysis (FEA). We used a partial discrete FEA model based on a unidirectional composite consisting of a microscopic area of fibres and matrix surrounded by a homogenised composite area. The FEA provided information about the stress state in the matrix and the fibre–matrix interface. The transverse strength of the composite was calculated regarding matrix failure and fibre matrix debonding. The influence of the temperature on the Young's modulus, the non-linear stress–strain behaviour and the strength of the matrix were investigated in detail. Following this approach it was possible to incorporate the resulting microresidual stresses on the transverse strength of the composite. Tensile tests of the neat resin and of the composite were performed in the temperature range of ?40°C to 60°C. The results of the FEA modelling are in good agreement with the experimental results of the transverse tests.     

Interphase sizing temperature effect of LaRC PETI-5 on the dynamic mechanical thermal properties of carbon fiber/BMI composites

The dynamic mechanical thermal properties of carbon fiber-reinforced bismaleimide (BMI) composites processed using polyacrylonitrile(PAN)-based carbon fibers unsized and sized with LaRC PETI-5 amic acid oligomer as interphase material at 150°C, 250°C, and 350°C were investigated by means of dynamic mechanical thermal analysis. It was found that the storage modulus, loss modulus, tan δ and the peak temperature significantly depend on the sizing temperature as well as on the presence and absence of LaRC PETI-5 sizing interphase. The result showed that the carbon fiber/BMI composite sized at 150°C had the highest storage modulus at a measuring temperature of 250°C. The storage modulus decreased with increasing sizing temperature from 150°C to 350°C, being influenced by interdiffusion and co-reaction between the LaRC PETI-5 interphase and the BMI matrix resin. The present result is quite consistent with the interfacial result reported earlier in term of interfacial shear strength and interlaminar shear strength of carbon fiber/BMI composites. It is addressed that in the present composite system the sizing temperature of LaRC PETI-5 interphase critically influences not only the interfacial properties but also the dynamic mechanical thermal properties and its control is also important.     

Fiber bias in nanoindentation of polymer matrix composites

Nanoindentations were performed in the near fiber region of the matrix of a polymer matrix composite to attempt to determine the nanomechanical properties of the interphase. A possible fiber bias effect was observed and this effect was confirmed by performing nanoindentations in both the presence and in the absence of fiber. Changes in experimentation by using lower loads (40 μN and 80 μN) reduced the fiber bias effect. Cutting the samples at an angle to the fiber axis was performed to try to further reduce the fiber bias effect. The experimental variations considerably reduced the fiber bias effect. Specifically, the combination of angled cutting and use of reduced loads eliminated the fiber bias effect at distances greater than 150 nm from the fiber.     

Interfacial structure analysis of polymer laminate using SPring-8 X-ray microbeam

Interfacial structure of laminated polyethylene (PE)/polypropylene (PP) films was investigated by synchrotron X-ray microbeam. The X-ray microbeam (0.9 μm (vertical) × 1.7 μm (horizontal)) formed using a phase zone plate was irradiated on the cross-section of the laminated films. In order to irradiate X-ray microbeam in the direction perpendicular to the cross-section of the film sample, adjustment of the sample setting was performed by Thomson scattering method. The Thomson scattering intensity is proportional to the number of the irradiated electrons, so the irradiated position of the X-ray microbeam could be determined from the intensity profile with high spatial resolution. By changing the sample position, diffraction patterns could be obtained from the laminated films across the PE/PP interfacial region. The thickness of the interfacial region of the annealed laminate was estimated as 5 μm judging from the changes of the diffraction intensities from the PE crystallites to the PP ones. The interfacial thickness depended on the thermal treatment of the film. It was found that the adhesion strength of the PE/PP laminate increased with increasing the interfacial thickness. Both of PE and PP chains entangled each other during laminate processing. The entangled molecular chains play important role as anchoring effect at the PE/PP interdiffusion region. However, the phase separation progressed with further crystallization by annealing. Thus, the adhesion strength of the PE/PP laminate was considered to be influenced by the interfacial thickness.     

Relationship between the flexural properties and specimen aspect ratio in unidirectional composites

It is well known that the bending test provides a simple and convenient way of measuring the strength of unidirectional composite materials and gives very repeatable results. The aim of this research work was to study and analyze the flexural properties of unidirectional reinforced carbon fiber/epoxy (UD) specimens subjected to three-point loading. The effect of span-to-thickness ratio (L/h) and width-to-thickness ratio (b/h) on the three-point bending of UD composites has been investigated. Results have shown that unidirectional composites exhibit a transition in the failure mode from shear delamination to fiber yield with the span-tothickness ratio (L/h) is increased. The observed experimental data are confirmed by theoretical considerations presented here. Using the classical beam theory the conclusions of the tests could be extended by applying some reasonable requirements and simple rational fractional functions identified. This made it possible to express the asymptotic values of the flexural strength, the flexural modulus and apparent shear stress in a form that is independent from the values of the span-to-thickness ratio applied, and characterize the bending behavior of the composite materials at a more exact level.     

The effect of surface treatment of carbon fiber on the flexural property of thermoplastic polyimide composite

Rare earth solution (RES) surface modification and air-oxidation methods were used to improve the interfacial adhesion of the carbon fiber reinforced polyimide (CF/PI) composite. The flexural property of the PI composites reinforced by the carbon fibers treated with different surface modification methods was comparatively investigated. Results showed that the flexural strength of CF/PI composite was improved after RES treatment. The improvement of impact and flexural property of the CF/PI composite was mainly due to the improvement in interfacial adhesion after RES treatment. X-ray photoelectron spectroscopy (XPS) study of carbon fiber surface showed that the oxygen concentration was obviously increased after RES treatment. The increase in the amount of organic functional groups increased the interfacial adhesion between CF and PI matrix.     

The Effect of ZrO2 Interphase on Interfacial Frictional Stresses in SiC/ZrO2/SiCf Composites

Two types of SiC fiber tows (Hi-Nicalon? and Hi-Nicalon S?) were coated with stabilized ZrO₂ and composited using preceramic polymer impregnation pyrolysis to form SiC/SiC_f minicomposites. Properties of the fiber/matrix interface in composites were investigated using the indentation method in which a pyramidal indenter was used to push on an individual fiber and cause sliding at the interface. The interfacial frictional stresses were determined from the force–displacement relation. The composites reinforced by the ZrO₂-coated fibers have smaller interfacial frictional stresses than composites reinforced by the initial fibers and show fibers sliding relatively more easily with respect to the SiC matrix.     

Finite element prediction of debonding onset in short fiber reinforced polymers incorporating a hybrid interphase region

A robust finite element procedure for investigating damage evolution in short fiber reinforced polymeric composites under external loads is developed. This procedure is based on an axisymmetric unit cell composed of a fiber, surrounding interphase and bulk matrix. The hybrid interphase concept involves a degraded material phase, the extent of which is material and property dependent. One of the most significant features of the model relies on establishment of variable adhesion conditions between the primary material phases. The unit cell is discretized into linearly elastic elements for the fiber and the matrix and interface elements which allow debonding in the fiber–matrix interface. The interface elements fail according to critical stress and critical energy release rate criteria. The tension and shear aspects of failure are uncoupled, although the resulting nonlinear problem is solved implicitly utilizing quasi-static incremental loading conditions. Final failure resulting from saturation and breakage is modeled by the vanishing interface element technique. Details of the propagation of interface cracks and the initiation of debonds are also observed and discussed for various shapes of fiber end. Numerical results reveal an intense effect of the fiber-end geometry on the initial fiber–matrix de-cohesion. The present finite element procedures can generate meaningful results in the analysis of fiber-reinforced composites.     

On the relevance of the interface in the modeling of damage and plasticity in composite laminates: Peek/APC2

The publisher notifies the reader that this article has been withdrawn due to copyright reasons that came to light after publication. The publisher apologizes to the reader for any inconvenience.     

Improvement of dispersion tolerance using wavelength-interleaving and forward error correction

In this paper, we first derived the BER expression of an optical channel with chromatic dispersion impairment. The derived BER expression takes into account the influence of pulse spreading, transmitter rise/fall times, receiver Q-factor and noises. It is simpler and hence easier to use than many existing channel BER models. Then we proposed a wavelength-interleaved FEC scheme to mitigate the chromatic dispersion impairment in optical transmission system and analyzed its decoding performance. The minimum wavelength separation required for such a wavelength-interleaving FEC system to obtain the highest coding gain is also determined. Based on this design, the chromatic dispersion tolerance of the proposed system with 2-wavelength and 4-wavelength-interleaving are shown to improve by about 13% and 22%, respectively, over a non-interleaved system. In essence, wavelength-interleaving averages out the performances of a set of channels experiencing higher and lower dispersion, hence the interleaved channels have better performance than the non-interleaved channels experiencing higher dispersion, and worse performance than the non-interleaved channels experiencing lower dispersion. However, since WDM systems are typically designed based on the worst-case channel, the overall system performance is improved by wavelength-interleaving.     

Low-velocity impact damage characterization of carbon fiber reinforced polymer (CFRP) using infrared thermography

Carbon fiber reinforced polymer (CFRP) after low-velocity impact is detected using infrared thermography, and different damages in the impacted composites are analyzed in the thermal maps.The thermal conductivity under pulse stimulation, frictional heating and thermal conductivity under ultrasonic stimulation of CFRP containing low-velocity impact damage are simulated using numerical simulation method. Then, the specimens successively exposed to the low-velocity impact are respectively detected using the pulse infrared thermography and ultrasonic infrared thermography. Through the numerical simulation and experimental investigation, the results obtained show that the combination of the above two detection methods can greatly improve the capability for detecting and evaluating the impact damage in CFRP. Different damages correspond to different infrared thermal images. The delamination damage, matrix cracking and fiber breakage are characterized as the block-shape hot spot, line-shape hot spot, and “

” shape hot spot respectively.     

Improvement in a-plane GaN crystalline quality using wet etching method

Nonpolar （1120） GaN films are grown on the etched a-plane GaN substrates via metalorganic vapor phase epitaxy. High-resolution X-ray diffraction analysis shows great decreases in the full width at half maximum of the samples grown on etched substrates compared with those of the sample without etching, both on-axis and off-axis, indicating the reduced dislocation densities and improved crystalline quality of these samples. The spatial mapping of the E2 （high） phonon mode demonstrates the smaller line width with a black background in the wing region, which testifies the reduced dislocation densities and enhanced crystalline quality of the epitaxial lateral overgrowth areas. Raman scattering spectra of the E2 （high） peaks exhibit in-plane compressive stress for all the overgrowth samples, and the E2 （high） peaks of samples grown on etched substrates shift toward the lower frequency range, indicating the relaxations of in-plane stress in these GaN films. Furthermore, room temperature photoluminescence measurement demonstrates a significant decrease in the yellow-band emission intensity of a-plane GaN grown on etched templates, which also illustrates the better optical properties of these samples.     

拉脱法测量液体表面张力系数实验的改进

从约利弹簧秤的结构出发,指出拉脱法测量液体表面张力系数的不合理性以及由此带来的系列问题.通过实验研究,提出合理的操作方法——降拉脱法,并给出相应的操作流程图.     

Improvement of the magnetic properties of barium hexaferrite nanopowders using modified co-precipitation method

Barium hexaferrite BaFe₁₂O₁₉ powders have been synthesized using the modified co-precipitation method. Modification was performed via the ultrasonication of the precipitated precursors at room temperature for 1 h and the additions of the 2% KNO₃, surface active agents and oxalic acid. The results revealed that single phase magnetic barium hexaferrite was formed at a low annealing temperature of 800 °C for 2 h with the Fe³⁺/Ba²⁺ molar ratio 8. The microstructure of the powders appeared as a homogeneous hexagonal platelet-like structure using 2% KNO₃ as the crystal modifier. A saturation magnetization (60.4 emu/g) was achieved for the BaFe₁₂O₁₉ phase formed at 1000 °C for 2 h with Fe³⁺/Ba²⁺ molar ratio 8 using 5 M NaOH solution at pH 10 in the presence of 2% KNO₃. Moreover, the saturation magnetization was 52.2 emu/g for the precipitated precursor at Fe³⁺/Ba²⁺ molar ratio 12 in was achieved for the precipitated precursor ultrasonicated for 1 h and then annealed at 1200 °C for 2 h. Coercivities from 956.9 to 4558 Oe were obtained at different synthesis conditions.     

Improvement of characterization accuracy of the nonlinear photonic crystals using finite elements-iterative method

We investigate nonlinear one- and two-dimensional photonic crystals by applying a finite element-iterative method. Numerical results show the essential influence of nonlinear elements embedded into a quarter-wave stack and the sharp photonic crystal waveguide bend on the spectral characteristics of these structures. We compare our results with those obtained in [21] from the discrete equation method for the case of a sharp waveguide bend. The comparison shows that neglecting the nonuniform field distribution inside the embedded nonlinear elements leads to overestimation of the waveguide bend transmissivity. PACS 42.65.-k; 42.70.Qs     

铁磁纳米阵列膜温度稳定性的损伤扩散研究

利用损伤扩散方法研究了三角点阵量子磁盘的温度稳定性问题.为建立实际体系的伊辛模型,我们作了三方面考虑：1)计入自旋间的长程相互作用;2)考虑纳米线阵列中纳米线长度的不一致性,3)推导出矫顽力与自旋交换常数间的关系并利用矫顽力定量估算出自旋交换常数的数值.模拟结果表明,自旋间的作用范围越长,损伤扩散越难,即稳定性越好;而纳米线的长度混乱度越大,温度稳定性越差. 关键词： 量子磁盘 温度稳定性 损伤扩散方法     

电光双折射扫描滤波法提高啁啾脉冲对比度

提出了一种通过扫描滤波来提高啁啾脉冲对比度的方法,并分析了该方法的原理。与传统的非线性滤波技术不同,扫描滤波方法消除预脉冲时,选通原理是基于光的频率而不是强度。以电光双折射扫描滤波器为例,说明了扫描滤波法的具体应用。通过数值模拟及实验研究,发现电光双折射扫描滤波法可以有效地消除主脉冲以前几百ps的预脉冲,并且主脉冲同时还能保证较高的透过率。     

电光双折射扫描滤波法提高啁啾脉冲对比度

提出了一种通过扫描滤波来提高啁啾脉冲对比度的方法,并分析了该方法的原理。与传统的非线性滤波技术不同,扫描滤波方法消除预脉冲时,选通原理是基于光的频率而不是强度。以电光双折射扫描滤波器为例,说明了扫描滤波法的具体应用。通过数值模拟及实验研究,发现电光双折射扫描滤波法可以有效地消除主脉冲以前几百ps的预脉冲,并且主脉冲同时还能保证较高的透过率。