三维编织SiC/SiC复合材料拉伸和弯曲损伤机制

为了研究三维编织SiC/SiC复合材料损伤机制,开展了室温条件下的单调拉伸和三点弯曲试验。实验前,利用CT扫描手段,明确了三维编织SiC/SiC复合材料试样的编织组织形态。对拉伸和三点弯曲试样的微观分析表明:原生孔洞和微裂纹导致了材料在单调拉伸过程中形成局部应力集中,随着拉伸载荷的增大,基体的横向开裂和纤维束间纵向层间裂纹逐渐演化形成纤维内部裂纹,导致材料最终的脆性断裂失效;在三点弯载荷作用下,表现为剪切、拉压共生的多耦合破坏模式,拉应力一侧首先发生失效,随后在中性面处发生剪切破坏,紧接着失效迅速向上下两侧扩展,直至截面在整个厚度方向发生失效;断口与纤维束的走向相关性很大,裂纹基本上沿着纤维束之间的界面进行扩展,导致最终失效未发生在理论失效位置处。      

混杂比对碳/芳纶纤维混杂纬编双轴向多层衬纱织物增强复合材料力学性能的影响

利用层内混杂的方式制备碳/芳纶纤维混杂纬编双轴向多层衬纱织物,通过对材料进行拉伸、三点弯曲等实验研究该织物增强复合材料的力学性能及混杂比对其力学性能的影响。结果表明,按照一定的混杂比加入芳纶纤维后复合材料的拉伸性能提高,表现出积极的混杂效应。由于延伸性好的芳纶纤维的加入,使复合材料的拉伸断裂伸长率明显提高,材料破坏模式出现了完全脆性断裂模式(C12材料破坏形式)和“扫帚”形纤维断裂模式(C8A4,C6A6材料破坏形式)。此外,按照一定的混杂比加入芳纶纤维也有效改善了碳纤维增强复合材料的破坏韧性,碳/芳纶纤维混杂MBWK织物增强复合材料的弯曲强度和弯曲模量随混杂比的提高而呈下降趋势,当复合材料中芳纶含量从42%(体积分数,下同)(C6A6)到59.2%(C4A8)的变化过程中,弯曲强度和弯曲模量的降低率较高。0°试样在混杂比为59.2%(C4A8)时,弯曲挠度最大,达到7.49 mm,远高于纯芳纶纤维或纯碳纤维增强的复合材料。所有90°混杂复合材料试样的弯曲挠度均高于纯芳纶纤维或纯碳纤维增强的复合材料,表现出积极的混杂效应。     

Framework for a generalized four-stage fracture model of cement-based materials

In cement-based materials the full range from brittle to ductile fracture can be achieved by changing the material structure, the loading conditions, the specimen size and/or the boundary conditions. Considering just the material, at one side of the spectrum hardened cement paste behaves brittle, whereas at the other side, new fibre reinforced cements may behave ductile. Structural conditions affect the brittleness/ductility as well, and by simply changing the loading (uniaxial tension, uniaxial and confined compression, etc.), the specimen/structure size or by changing the boundary conditions the full range from brittle to ductile response can be observed. Basically there is no difference in behaviour between the various loading cases and the same four-stage fracture process can always be identified. The four ‘universal’ stages are the linear elastic regime, the microcrack regime (before the maximum load is reached), the macrocrack regime (viz. the first, usually steep part of the softening curve), and the bridging stage. Microcracks are defined as cracks that can be arrested by elements in the material structure, whereas macrocracks can only be delayed/ arrested by means or structural measures at a larger scale than the material structure. In this paper it is tried to develop a unified view on fracture of materials belonging to this broad class, which may be seen as conceptual framework for an all encompassing fracture model for cementitious materials.     

瓦楞型竹束单板复合材料的制备及力学性能表征

探索了一种新型竹质工程构件--瓦楞型竹束单板复合材料(CBLC)的制备工艺, 研究了三种典型铺装类型(Ⅰ型: (0°)₆; Ⅱ型: (0°/90°)₃; Ⅲ型: (90°)₆)对其拉伸、 双向弯曲、 三维压缩性能的影响, 同时利用数字散斑相关方法(DSCM)对其弯曲应变场信息进行了表征。结果表明: 铺装类型对各项力学性能有明显影响, 对于拉伸、 纵向弯曲性能, Ⅰ型> Ⅱ型> Ⅲ型, 对于横向弯曲, Ⅱ型> Ⅲ型> Ⅰ型。不同铺装类型下CBLC的拉伸断裂机制亦各不相同: Ⅰ型为延性断裂, Ⅱ型为逐渐分层断裂, Ⅲ型为脆性断裂。 x和y方向应变场集中分布在试样底端最外层瓦楞波形连接处, 且E_xx< E_yy。多重比较分析表明: 抗压缩性能在三维方向上存在明显差异, 且y>x>z; 铺装类型对CBLC的抗压载荷、 抗压强度有很大影响, 而抗压模量差异不明显。     

Interaction between cracking, delamination and buckling in brittle elastic thin films

A discrete lattice based model for the interaction of cracking, delamination and buckling of brittle elastic coatings is presented. The model is unique in its simultaneous incorporation of the coating and of disorder in the interface and material properties, leading to realistic 3D bending (and buckling) behavior. Results are compared to the literature. In the case of cracking, the key role of a stress transfer correlation length ξ in establishing a scaling behavior for the brittle fracture of thin films is shown to extend to all geometrical and material properties involved. In the scaling regime of crack density in uniaxial tension cracking and delamination are found to occur simultaneously. In uniaxial tension of films with an internal biaxial compressive stress, the predicted initiation of buckles above delaminated areas near crack edges in the model is remarkably similar to experimental results.     

ZrB_2基超高温陶瓷复合材料的高温拉伸损伤行为

开展了SiC(20vol%)-石墨(15vol%)/ZrB2复合材料室温及高温拉伸性能实验,发现高温时复合材料的拉伸强度和弹性模量有所降低,并且具有明显的非线性特征。引入热损伤来表征弹性模量随温度的衰减规律,利用强度统计分析方法确定单向应力状态下材料的机械损伤演化方程,建立了材料在热力耦合条件下的高温拉伸损伤非线性本构模型。分析表明:随着温度的升高,SiC-石墨/ZrB2复合材料的热损伤和机械损伤不断增加,延性增强,且脆性-延性破坏转变温度范围为1 250~1 350℃。     

Fracture at V-notches with contained plasticity

Fracture at V-notches with contained plasticity was studied. The tested material was a soft annealed tool steel, AISI O1, at −50°C. The material behaviour at this temperature was brittle for a standard fracture toughness test and ductile for a tensile test. Single-edge notched tension and three-point bend specimens were tested, with notch angles ranging from 0° to 140°. For the same notch angle, both brittle and ductile fracture occurred depending on the geometry. However, transferability between different geometries prevailed if the fracture was brittle.Fracture loci derived from simple point-stress or mean-stress criteria were fitted to the experimental results. The trend of the experimental results was followed well even though the fracture loci were derived under the assumption of linear elastic stresses. However, a fracture criterion based on the fracture toughness and a tension test alone will provide a conservative fracture locus.The effect of introducing a notch radius on the used fracture criteria is discussed within the context of linear elastic analysis. As expected, a mean-stress criterion is more sensitive to changes in the notch radius than a point-stress criterion. For both criteria, the sensitivity for changes in the notch radius is less for larger notch angles.     

Fabrication and mechanical properties of oriented SiC short-fiber-reinforced SiC composite by tape casting

SiC fiber-reinforced SiC composites with nearly unidirectionally and randomly aligned SiC short fiber were prepared by tape-casting and hot-pressing (HP). Volume fractions of fibers were 10 and 20 vol.%. Three-point bending test was carried out at room temperature. The SiC short-fiber-reinforced SiC composites showed completely brittle fracture for any fiber volume fraction and orientation. The maximum strength increased with increasing sintering temperature regardless of orientation of short fiber. In the unidirectionally and randomly aligned composites sintered at 1700 °C containing 20 vol.% fiber, the maximum bending strength was about 390 and 280 MPa, respectively.     

The properties of carbon fibre/SiC composites fabricated through impregnation and pyrolysis of polycarbosilane

Unidirectional carbon fibre reinforced SiC composites were prepared from four types of carbon fibres, PAN-based HSCF, pitch-based HMCF, CF50 and CF70, through nine cycles or twelve cycles of impregnation of polycarbosilane and subsequent pyrolysis at 1200°C. The polycarbosilane-derived matrix was found to be -SiC with a crystallite size of 1.95 nm. The mechanical properties of the composites were evaluated by four-point bending tests. The fracture behavior of each composite was investigated based on load-displacement curves and scanning electron microscope (SEM) observation of fracture surfaces of the specimens after tests. It was found that CF50/SiC and CF70/SiC exhibited high strength and non-brittle fracture mode with multiple matrix cracking and extensive fibre pullout, whereas HSCF/SiC and HMCF/SiC exhibited low strength and brittle fracture mode with almost no fibre pullout. The differences in the fracture modes of these carbon fibre/SiC composites were thought to be due to differences in interfacial bonding between carbon fibres and matrix. Values of flexural strengths of CF70/SiC and CF50/SiC were 967 MPa and 624 MPa, respectively, which were approximately 75% and 38% of the predicted values. The relatively lower strength of CF50/SiC, compared with CF70/SiC, was mainly attributed to the shear failure of CF50/SiC during bending tests.     

Correlation of tensile and flexural responses of strain softening and strain hardening cement composites

Closed form equations for generating moment–curvature response of a rectangular beam of fiber reinforced concrete are presented. These equations can be used in conjunction with crack localization rules to predict flexural response of a beam under four point bending test. Parametric studies simulated the behavior of two classes of fiber reinforced concrete: strain softening and strain hardening materials. The simulation revealed that the direct use of uniaxial tension and compression responses under-predicted the flexural response for strain softening material while a good prediction for strain hardening material was obtained. The importance of strain softening range on the flexural response is discussed using non-dimensional post-peak parameters. Results imply that the brittleness and size effect are more pronounced in the flexural response of brittle materials, while more accurate predictions are obtained with ductile materials. It is also demonstrated that correlations of tensile and flexural results can be established using normalized uniaxial tension and compression models with a single scaling factor.     

Numerical simulation of low-velocity impact loading of a ductile polymer material

Mechanical impact loading of injection-moulded components was simulated. The material was a talc-filled and elastomer-modified polypropylene used in automotive exterior parts. The material model was the linear-elastic–viscoplastic SAMP-1 model, which features pressure-dependent yield stress, plastic dilatation and a simple damage model. The model was calibrated with data from tests in uniaxial tension, shear and uniaxial compression, utilising 3D digital image correlation for full-field displacement measurements. With the calibrated model, two load cases were simulated; centrally loaded clamped plates and three-point bending of bars. The predictions of force vs. deflection were good to fair. The results are discussed in terms of deficiencies of the calibration data, heterogeneity and anisotropy of injection-moulded components, and shortcomings of the model. In particular, the hardening curves at high strain rates are uncertain, and tests in biaxial tension would be useful.     

缝合式三维编织C/SiC复合材料拉伸加卸载力学行为

本文通过开展缝合式三维编织C/SiC复合材料的准静态单轴拉伸及循环加卸载试验,研究加卸载行为对复合材料的损伤及材料内部能量耗散的影响。通过对材料的断口分析,探究加卸载对材料破坏强度的影响规律。结果表明,加卸载行为会消耗材料内部的能量,对纤维束与基体之间的界面造成损伤,进而降低材料的承载能力;材料滞回曲线的面积随着卸载点应力的增大而增大;材料的整体失效属于脆性破坏,复合材料断口表现出明显的分层现象,且单轴拉伸时断口相对于循环加卸载更整齐。      

Effects of microstructure, specimen and loading geometries on KIC of brittle honeycombs

Fracture toughness of brittle honeycombs depends on cell microstructure, specimen and geometries. A microstructure coefficient in the K_IC expression of brittle honeycombs can not be found analytically. In this paper, a finite element program is utilized to numerically determine the coefficient. In practice, fracture toughness can be measured from conducting a three-point bend or uniaxial tension test. Specimen geometry restrictions of three-point bend test for honeycombs are examined and proposed here. Meanwhile, fracture toughness of honeycombs under the two loading geometries is compared; results show that K_IC measured from uniaxial tension test is smaller than that from three-point bend test if the K_IC formulations for solid materials are employed. As a result of that, the K_IC formulation of three-point bend test is modified for honeycomb-like materials.     

基于改进型近场动力学方法的多裂纹扩展分析

在非局部键型近场动力学理论基础上,提出了能够反映混凝土、岩石类材料力学特性和非局部长程力尺寸效应的改进型近场动力学微极模型,弥补常规微观弹脆性（Prototype Microelastic Brittle,PMB）键型近场动力学本构模型的应用范围限制和定量计算误差大等缺陷,并构建了相应的适合于模拟脆性多裂纹扩展问题的近场动力学算法体系。通过对不同核函数修正项对应的近场动力学定量计算结果进行比较,验证了改进型近场动力学模型和数值算法的计算精度并确定了精度最高的核函数修正项;模拟双裂纹脆性板受压和随机多裂纹脆性板受拉的裂纹扩展全过程并与已有结果对比,进一步验证了模型和算法在模拟多裂纹扩展问题时的可靠性。分析了含多裂纹三点弯梁的起裂和裂纹失稳扩展过程,并研究了裂纹初始倾角、初始长度等因素对构件破坏形式和破坏荷载的影响规律。     

Effect of fiber modulus of elasticity on the long term properties of micro-fiber reinforced cementitious composites

Fibers for reinforcing cementitious composites are typically short and randomly dispersed in the matrix. Consequently, most of the fibers are inclined to the cracks that develop in the cement matrix and suffer from bending stress as these cracks open. For brittle fibers, such as carbon fibers, the bending stress may lead to flexural fiber rupture before the fiber attains its full capacity in direct tension. As a result, the efficiency of these fibers may be reduced. This phenomenon is not expected to occur in ductile fibers, which can yield locally rather than rupture. Predictions of a theoretical model show that the bending stress increases as the matrix becomes denser and suffer (an event which occurs as the matrix ages or due to the addition of silica-fume) and decreases for fibers of lower modulus of elasticity. Therefore, a reduction in strength with time in composites with dense matrices is expected for very brittle fibers of high modulus, moderate or no reduction for low modulus brittle fibers, and no reduction in strength is expected for ductile fibers. The long term properties of cementitious composites reinforced with various microfibers was studied to validate the model; PAN and Pitch type carbon fibers represented brittle fibers of high and low modulus, respectively; polypropylene and polyacrylonitrile fibers represented ductile fibers. The results showed good agreement with the theoretical model.     

Failure behaviour of particulate-reinforced aluminium alloy composites under uniaxial tension

Tensile tests were carried out at room temperature on 6061-aluminium alloy reinforced with SiC and Al₂O₃ particulates. Although a significant increase in strength could be achieved by introducing ceramic reinforcements into the aluminium alloy matrix, it is associated with a substantial decrease in fracture strain. In order to understand the reason for the inferior ductility of such composites, analytical solutions were obtained using a simple composite model. SEM studies were carried out on the side surfaces of the fractured specimens to verify the proposed failure behaviour. Failure modes observed to operate in such composites under uniaxial tension are described.     

Microstructural failure modes in three-phase glass syntactic foams

Samples of syntactic foam containing hollow glass microspheres of 0.108 and 0.253 g/cm³ tap densities, some with a silane surface treatment, were subjected to different stress states and examined for failure modes. All foams contained the same volume fraction of APO-BMI, a bismaleimide resin binder. The samples were tested in compression and in three-point bend, and mechanical properties were compared between the various foams. Microsphere strength had a strong effect on overall uniaxial compressive strength with interface strength playing a secondary, yet significant role. In three-point bending, the role of the interface was much more critical. Cross sections of the compression test samples were examined by optical microscopy, and fracture surfaces were investigated by scanning electron microscopy.     

Thermal properties of diamond/SiC/Al composites with high volume fractions

The diamond/SiC/Al composites with high volume fractions and a large ratio of diamond to SiC particle size (7.8:1) were fabricated by gas pressure infiltration. The results show that the fine SiC particles occupy efficiently the interstitial positions around coarse diamond particles; the main fracture mechanism of the composite is matrix ductile fracture, and diamond brittle fracture was observed which confirms a high interfacial bonding strength; the diamond/SiC/Al composites with 80% and 66.7% volume fraction of diamond in the reinforcement have the higher volume fraction in the reinforcement and lower coefficient of thermal expansion compared to the diamond/Al composite. Turner and Kerner models are not in good agreement with the experimental data for the composites based on reinforcement with two phases different in shape and component. When the effect of the coating layer considered, differential effective medium (DEM) model is confirmed a reliable model in designing a composite with a given thermal conductivity based on reinforcement with two phases different in size.     

Ultimate Bending Strength Evaluation of U-Notched Ductile Steel Samples Under Large-Scale Yielding Conditions

The load-bearing capacity of several U-notched specimens, reported in literature, made of very ductile steel and loaded under symmetric three-point bending were theoretically estimated by using the Equivalent Material Concept (EMC), proposed originally by the author, combined with two well-known brittle fracture models namely the mean stress (MS) and the point stress (PS) criteria. The results revealed that the MS-EMC model with an accuracy of more than 90% was more efficient than the PS-EMC model with about 72% accuracy. By using the MS-EMC model, one can predict well the onset of mode I cracking in U-notched ductile components under large-scale yielding conditions without requiring conducting elastic-plastic analyzes or using ductile fracture criteria.     

Double torsion fracture testing of high-density polyethylene

The fracture of polyethylene has been studied extensively using conventional testing geometries such as three-point bending (TPB) and single-edge notch tension (SENT). These geometries are of limited utility for studying crack growth, because the crack speed is constantly changing and the crack front is in the centre of the specimen. Double torsion (DT) is a fracture geometry that suffers neither of these disadvantages, yet has only received limited attention in the literature. Its use has been limited to highly brittle materials such as glass, ceramics, thermosetting plastics and PMMA. In contrast to these materials, high-density polyethylene (HDPE) is an inherently ductile polymer. Before the advantages of DT can be exploited for testing HDPE, it is first necessary to demonstrate the validity of DT fracture measurements performed on such a ductile material. In this paper it is shown that at moderate rates of loading and at temperatures below 0C, valid double torsion fracture results can be obtained for an ethylene 1-butene copolymer. A novel technique for specimen preparation and a simple method for accurately monitoring crack growth are also described.