Residual Stresses in Microcomposites and Macrocomposites

Existing models for built-in residual stresses in composite materials are reviewed and discussed. In particular, the thermal longitudinal stress present in the fiber prior to a single-fiber fragmentation experiment is studied using various model composite data. It is found that this stress is typically compressive in nature and that, quantitatively, it depends on the fiber content, the degree of undercooling, and the thermoelastic constants of the fiber and the matrix. In the case of single-fiber composites (or microcomposites), the thermal longitudinal stress present in the fiber is high enough to either induce fiber sinewave buckling (such as in E-glass/epoxy), or extensive fiber fragmentation (such as in graphite HM/polypropylene) that may then be used to measure the dependence of compressive fiber strength upon length. This has to be accounted for in quantitative models that calculate interfacial adhesion parameters using single-fiber tests, such as the fragmentation test or the microbond test. Implications for high fiber content composites (or macrocomposites) are discussed.     

Fibers from polypropylene/nano carbon fiber composites

Fibers from polypropylene and polypropylene/vapor grown nano carbon fiber composite have been spun using conventional melt spinning equipment. At 5 wt% nano carbon fiber loading, modulus and compressive strength of polypropylene increased by 50 and 100%, respectively, and the nano carbon fibers exhibited good dispersion in the polypropylene matrix as observed by scanning electron microscopy.     

Crack Initiation in Borosilicate Glass-SiC Fiber Composites

The initiation of matrix microcracking was investigated in unidirectional glass matrix composites having controlled fiber spacing. Observations were taken from composites consisting of regular arrays of TiB₂-coated SIGMA 1240 and carbon-coated SCS-6 monofilament SiC fibers in a series of borosilicate glasses. The thermal expansion mismatch between the fibers and glass matrix was varied such that the resulting radial stresses after processing ranged from tensile to compressive. The glass strongly bonds to the TiB₂-coated SIGMA 1240 fiber but weakly bonds to the carbon coating of the SCS-6 fiber, allowing the investigation of the effects of bonding at the fiber/matrix interface. The observed crack initiation stresses of the various composites are compared to predictions based on a previously developed semiempirical model and used to study the influence of the volume fraction of fibers, residual stress state and interface strength.     

Axial compressive strength of carbon fiber

Efforts were made to estimate the axial compressive strengths of carbon fibers from the fiber fragment lengths produced by subjecting to a strain greater than the fiber ultimate strain for PAN-based and pitch-based carbon fibers. The estimated compressive strength of carbon fibers decreases with increasing temperature in a temperature range from room temperature to 100°C. This decrease in compressive strength may be accounted for by a decrease in the radial compressing force. The real compressive strength, determined by extrapolating a linear relationship between the estimated compressive strength and the radial compressing force, is approximately 25–60% of tensile strength for PAN-based fibers, while it is approximately 10–35% for pitch-based fibers.     

Some observations on stress graphitization in carbon-carbon composites

The in situ stress graphitization behavior of hard carbons in unidirectionally aligned carbon-carbon (C-C) composites was studied for three carbon fibers (PAN-based T-50, pitch-based PX7 and rayon-based WCA) and two carbon precursor resins [phenol-formaldehyde (SC1008) and polyarylacetylene (PAA), a high char-yielding, low shrinkage resin]. Graphitization was followed by measurements of density, transverse thermal expansion, d-spacing by X-ray diffraction (XRD) and by scanningelectron microscopy (SEM). In conjunction with xenon-ion etching, the SEM technique was found to be particularly effective in identifying localized regions of graphitized matrix. Results reveal that the graphitization of the composite is significantly greater than graphitization of fiber or matrix alone to the same temperatures. SEM observations indicate that graphitization is confined to the matrix, usually as a sheath-like structure adjacent to the fiber and 1–3 μm thick. Such localized graphitization, usually termed stress graphitization, is believed to be the result of thermally induced tensile or compressive stresses acting at the fiber-matrix interface. Debonded regions, which are believed to either initiate at heatup or grow from pre-existing cracks in the resin-matrix composite, show less stress graphitization than well-bonded regions, presumably because the debond gaps impede stress buildup at the fibermatrix interface.Studies with three different fibers and one matrix (PAA) in matrix-rich composites showed variable degrees of localized stress graphitization, suggesting that the thermal expansion stresses responsible for stress graphitization vary with different fiber-matrix combinations. One consequence of a well-oriented stress-graphitized sheath was found to be debonding of fiber and matrix. Possible reasons for such debonding are discussed briefly.     

Strength and Toughness of Continuous-Alumina-Fiber-Reinforced Glass-Matrix Composites

Unidirectional, continuous-fiber composites were fabricated using polycrystalline alumina fibers and four different silicate glass matrices of differing thermal expansion. Fracture toughness measurements, strength measurements, and fractographic analysis of failed specimens are used to identify the failure mechanism. Results show that the elastic modulus mismatch between the matrix and the fibers shields the reinforcing fibers from matrix crack extension, thereby increasing composite toughness without fiber pullout. Fractographic analysis shows that fiber shielding leads to fiber failure ahead of matrix crack. Composite toughness increases linearly with increases in the residual compressive stress in the matrix phase. Ultimate composite strengths are dependent upon thermal-expansion-induced residual stresses and fiber strength.     

Performance of spalling resistance of high performance concrete with polypropylene fiber contents and lateral confinement

This paper presents an experimental study on the spalling resistance of high performance concrete with polypropylene (PP) fibers and fabric or sheet material for lateral confinement subjected to fire. According to the test results, spalling occurred on all specimens that did not contain PP fiber in the concrete mixture. However, spalling did not occur on specimens containing PP fibers above 0.05% by volume. A metal fabric showed beneficial effect on spalling resistance, but glass or carbon fiber fabrics do not show the same effect on the spalling resistance due to reduction of bond strength at high temperatures. Spalling did not occur on all specimens in which PP fibers and metal fabric were applied at the same time, and hence spalling resistance performance was significantly improved. The residual compressive strength was maintained at about 90% of its original strength, and this can be considered as an improved performance against fire damage.     

The effects of transcrystalline interphase in advanced polymer composites

Surface-induced transcrystallization in fibers has been reported in some advanced polymer composites. It is believed that transcrystalline interphase may affect stress transfer efficiency between the reinforcing fiber and the matrix. In this study, attempts were made to examine the effects of transcrystallinity on composite performance, particularly on fiber-matrix interfacial bond strength, and to investigate possible attributes of transcrystallization. Three polymer resins, poly(etherketoneketone) (PEKK), poly(etheretherketone) (PEEK), and poly(phenylenesulfide) (PPS), and four types of fiber, polyacrylonitrile (PAN)-based AU-4 (untreated AS-4) carbon, pitch-based carbon, poly (p-phenylene terephthalamide) (PPDT) aramid, and E-glass were used. It was found that PPDT aramid and pitch-based carbon fibers induce a transcrystalline interphase in all three polymers because of an epitaxial effect. Under certain conditions, transcrystallization was also observed in PAN-based carbon and E-glass fibers, which may be partially attributed to the thermal conductivity mismatch between the fiber and the matrix. Plasma treatment on fiber surface showed a negligible effect on inducing transcrystallization, whereas solution-coating of PPDT on the fiber surface showed a positive effect. The Microdebonding test, which measures the interfacial bond strength between the fiber and the matrix, consistently showed more than 40% increments for various single filament systems with transcrystalline interphase versus without. However, the effects of transcrystallinity on the interfacial bond strength appeared to decrease as the fiber content increased in composites.     

创新为世界高性能纤维带来勃勃生机

陈述了国内外近期聚丙烯腈基碳纤维、对位芳酰胺及其共聚纤维、超强聚乙烯纤维、玄武岩纤维、液晶聚芳酯纤维及沥青基碳纤维生产厂家的研发、创新和扩产情况,显示了这些高性能纤维对全球开发可再生新能源、节能减排和各种产品的更新换代有重要作用,因此在当前世界经济不景气的形势下,通过不断革新生产工艺、提高生产效率和产品质量、降低生产成本、开发新品种及发展下游复合材料等制品,仍可保持两位数的需求增长。中国的主要高性能纤维正在实现初步产业化,但有些性能档次较低、性能稳定性稍差,远未形成系列化产品,与国外同类产品相比差距较大,今后应进一步加大研发投入,通过不断创新发展自己的知识产权,才能逐步缩小差距,提高产品的国际竞争力。     

高掺量聚丙烯纤维自密实混凝土制备及其性能研究

为了有效提高高掺量聚丙烯纤维自密实混凝土的工作性能,将调整水胶比和砂率进行配制聚丙烯纤维体积掺量为0.5％的自密实混凝土并对其材料性能进行试验研究.研究表明:高掺量聚丙烯纤维的掺入对自密实混凝土的流动性有较大影响,适当调整水胶比和砂率可配制满足工作性能要求的高掺量聚丙烯纤维自密实混凝土;水胶比的增大提高了高掺量聚丙烯纤维自密实混凝土的扩展度,同时也提高了其离析的风险,降低了其抗压强度;砂率的增大对高掺量聚丙烯纤维自密实混凝土的抗压强度基本没有影响,但可提高其拌合物的粘聚性.     

创新推动高性能纤维及其复合材料制品的发展

主要介绍了碳纤维及其复合材料的创新与发展动向,包括主要企业最新动态,以及中间相与通用级沥青基碳纤维、其他原丝的低成本碳纤维、对位和间位芳酰胺纤维、碳化硅和氧化铝陶瓷纤维的最新创新动向和扩大应用情况。指出高性能纤维及其复合材料的工艺技术创新,体现在生产工艺的高效化、产品的高性能化和低成本化以及应用领域的不断扩大,其中各种助剂起着重要作用。     

玄武岩-粗聚丙烯纤维干硬性混凝土拉压性能试验研究

混杂纤维增强干硬性混凝土在国内外已有广泛的应用,纤维配比是影响其拉压性能的主要因素之一。为研究玄武岩纤维与粗聚丙烯纤维配比对干硬性混凝土拉压性能的影响,将玄武岩纤维与粗聚丙烯纤维单掺或按不同比例混合掺入干硬性混凝土中,开展不同养护龄期下纤维混凝土的抗压、劈裂抗拉试验,分析纤维混杂增强效应,并基于成熟度理论修正养护龄期,优化玄武岩-粗聚丙烯纤维干硬性混凝土的劈裂抗拉强度预测模型。结果表明:玄武岩纤维与粗聚丙烯纤维的掺入不仅提升了干硬性混凝土抗压、劈裂抗拉性能,而且纤维的桥接作用能明显改善混凝土的脆性破坏特征,其中玄武岩纤维与粗聚丙烯纤维混掺配比为1 ∶2(质量比)时最为明显,表现出了最优的纤维混杂正效应。根据等效龄期-抗压强度关系式计算得到的混凝土抗压强度与劈裂抗拉强度具有更好的幂函数关系,该模型便于计算及预测不同养护温度条件下玄武岩-粗聚丙烯纤维干硬性混凝土的拉压性能。     

Surface treatments of subdenier monofilament polypropylene fibers to optimize their reinforcing efficiency in cementitious composites

The surface properties of polypropylene (PP) fibers have an important effect on their reinforcing efficiency in cementitious composites. Two new methods of modifying the surface of subdenier monofilament polypropylene fibers were introduced, as well as the performances of the fiber‐reinforced mortar. The results show that the surface modification improved the mechanical performance of the fiber‐reinforced mortars, such as compressive strength and flexural strength, and the reinforcing efficiency depends on the adopted method. The enhanced interfacial bonding between treated fibers and the cementitious matrix, compared with that of unmodified fibers, was investigated using scanning electronic microscopy. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2637–2641, 2004     

混杂纤维自密实混凝土孔结构对抗压强度影响的试验研究

采用压汞法对混杂纤维自密实混凝土进行微观孔结构试验,并进行抗压强度试验,分析了混杂纤维自密实混凝土孔结构的孔隙率、孔径尺寸与级配、孔分布特征与强度的关系,得出混杂纤维自密实混凝土孔结构与抗压强度的关系趋势.研究结果表明,在自密实混凝土中混杂掺人玄武岩纤维和聚丙烯纤维,其微观孔结构的改善对抗压强度的提高有着直接的影响.     

Evaluation of Oxide–Oxide Composites in a Novel Combustor Wall Application

Oxide–oxide composites were evaluated in a novel combustor design requiring higher wall temperatures than the conventional combustors. The evaluation was based on a combination of numerical modeling and experimental rig testing. The modeling included computational fluid dynamics (CFD) calculations whose results were used in a thermo-mechanical analysis using finite element modeling (FEM). The composites tested experimentally were obtained from a commercial vendor; they were reinforced using Nextel^™ 720 fibers. The rig tests showed that aluminosilicate matrix composites with higher room temperature strengths suffered cracking while the weaker alumina matrix composites performed satisfactorily. The results were consistent with numerical models that predicted residual stresses from creep during service. The models showed that in-plane gradients and their effects were more severe than those of through-thickness gradients and suggest that tailoring fiber architecture is important in transitioning these composites to applications.     

纤维特性对砂加气混凝土力学性能和微观结构的影响

砂加气混凝土因其韧性较差,在生产过程中易出现缺棱掉角的现象.通过添加不同长径比和弹性模量的纤维,研究纤维掺量对砂加气混凝土抗压和抗折强度的影响,并借助X射线衍射和扫描电镜分析其微观结构,研究了纤维对砂加气混凝土增韧机理.研究结果表明:聚丙烯纤维和玻璃纤维的最佳掺量均为0.3%,砂加气混凝土的抗压强度分别提高了22.0%和27.8%,抗折强度分别提高了20.0%和26.0%.不同纤维对砂加气混凝土的水化产物含量有一定的影响,但不会生成新相.纤维对砂加气混凝土的增强增韧机理主要在于其能在基体中形成三维网络骨架,通过减缓裂纹尖端的应力集中,减缓或阻止裂纹的扩展.添加两种纤维的砂加气混凝土砌块受力时主要对纤维拔出做功,因此纤维本身的弹性模量对砂加气混凝土强度的影响较小.     

处于平稳增长期的沥青基碳纤维

概述了沥青基碳纤维的分类、特性、产品系列化情况,国内外各公司的产能发展预测,各公司所采用的工艺技术路线,通用级及中间相沥青基碳纤维的用途以及各公司所开发的主要市场。     

超级导热型沥青基碳纤维

比金属银、铜热导率(TC)还要高的中间相沥青基碳纤维倍受世人关注。这种超级热导率中间相沥青基碳纤维可广泛用于宇航工业和电子产业，如卫星的太阳能电池基板、集成电路(IC)基板、印刷线路的基板、高能密度电子仪器的散热片等。这种轻质、柔软、高导热的复合材料将随着宇航工业、信息产业和电子工业的需求而得到迅速发展。     

Self-Consistent Method to Calculate Fiber Interactions in a SiC/Al2O3 Ceramic Composite

The sintering of Al₂O₃/SiC ceramic composite leads to a state of high internal stresses in the composite material at room temperature because of the difference in thermal contraction between matrix and particles. It has been shown in a previous work¹ that the interaction among fibers must be accounted for in order to predict correctly the residual stresses. In the present paper we develop a numerical scheme that permits taking into account such interaction for an arbitrary distribution of fiber directions (DFD) and for completely anisotropic properties of the phases. We apply the formulation to calculate the average strain in the matrix (due to the interaction among fibers) and the effective thermal coefficients of the composite. We find that the average strain in the matrix depends strongly on the DFD and that the predictions agree with measurements done by Majumdar and Kupperman.³ We prove that the effective thermal coefficients of the composite are not sensitive to the DFD when the matrix and the fibers exhibit isotropic thermal and elastic properties. Finally, we analyze the effect of the DFD and of the fiber interaction on the internal stresses inside the SiC fibers and compare with experimental values.     

Residual stress distribution modification caused by weathering in polypropylene and polystyrene

A study has been made of changes in residual stress distributions caused by weathering in polypropylene and polystyrene. Chemi-crystallization has a major effect in polypropylene and an analysis based on the volumetric changes that occur on crystallization has been developed. Close to the surface, fractional crystallinity changes up to 4% are caused by photodegradation (X-ray measurements by Rabello and White (18)). It is estimated that this would cause a tensile residual stress of ～2MNm^?2 to form at the surface if there were no preexisting residual stresses; in the case examined here, the effect of this shrinkage stress was to reduce the compressive residual stress to a small value (<1MNm^?2). Additional changes caused by stress relaxation prior to completion of chemi-crystallization resulted in net tensile stresses near to the surface of the photo-degraded polypropylene. The changes occur almost symmetrically in polypropylene even if the molding is exposed on one surface only. A similar analysis has been made for thermoplastics in which the changes occur only at the exposed surface, comparing the results with measurements made on a glass fiber reinforced grade of polypropylene. In this case the analysis predicts that the stress changes by 2–3MNm^?2 near the surface, enough to develop tensile stresses up to 2MNm^?2 there if the compressive residual stress at the beginning is fairly small, in fairly good agreement with observed changes. Glass fiber reinforced polypropylene does not relax as readily as unfilled polypropylene and better agreement is to be expected without any allowance for stress relaxation. The analysis for one-sided chemi-crystallization allows calculation of the resulting distortion in terms of the curvature: this was estimated at 0.33m^?1, compared to the measured value of 0.44 m^?1. Volumetric changes also occur in noncrystalline polymers and a similar analysis based on data obtained with polystyrene (17) confirms that these changes can explain the observed development of tensile residual stresses on weathering. In the example studied here surface stress changes of 2–3MNm^?2 are predicted and this accounts for a large proportion of the change in residual stress obtained by direct measurement.