Controlling fracture toughness of matrix for ductile fiber reinforced cementitious composites

An experimental study was carried out to find material parameters for making fiber reinforced cementitious composites (FRCC) more ductile. One of the dominant factors to control the ductility might be hidden in fracture property of matrix as well as the interface property between fiber and matrix. Therefore this study varied air content and water-binder ratio as the parameters to change the fracture property of matrix and experimentally examined their influence on the ductility of FRCC by three-point bend test with notched beams. As a result, it is concluded that fracture toughness of the matrix could be one of key parameters to control the ductility of FRCC. In case of a polyethylene fiber used in this study, the optimum value of the fracture toughness (critical strain energy release rate): G_IC of the matrix was obtained to be 7.5-8.0 N/m.     

Interactions at fiber/matrix interface in short fiber reinforced amorphous thermoplastic composites modified with micro- and nano-fillers

This study aims at systematically extracting fiber/matrix interfacial strength in short-glass fiber-reinforced polymer composites using an experimental micromechanics approach which employs mechanical properties and residual fiber length distributions to derive the apparent interfacial shear strength. We started from neat high-impact polystyrene matrix short-glass fiber-reinforced composites (HIPS/GF) with varying fiber loading and proceeded toward HIPS/GF hybrid composites containing micro- and nano-fillers where complex fiber/matrix interfacial interactions exist. It was found that apparent interfacial shear strength does not vary with fiber content, while the presence of fillers with different length-scales alters fiber/matrix interactions depending on their influence on physical properties of the polymer matrix, particularly in the vicinity of reinforcing fiber surfaces.     

Multiscale considerations for interface engineering to improve fracture toughness of ductile fiber/thermoset matrix composites

Mechanical interlocking at both large-scale and fine-scale as well as continuous and intermittent chemical bonding were employed to engineer the friction during fiber pullout to improve toughness. Excellent toughening results have been achieved over a large range of typical fracture feature size in the large micro-/small meso-scale of 1–100 μm. Interphase engineered features for size scales below the microscale were smoothed over or subsumed during fracture. For mechanical interlocking at the large scale, the overall fiber shape is critical and there is little interfacial adhesion. For mechanical interlocking at the fine scale, a protruding “break-away” feature performs better than a concave, cavity-like feature. For chemical bonding, intermittent coating gives superior toughness. In all of the interface or interphase designs, it is found that there is an intermediate anchoring of the fiber to best utilize fiber plasticity to improve composite toughness.     

On the fracture toughness of ceramic matrix composites

Based on the resistance curve (R-curve) behaviour of ceramic matrix composites (CMCs) determined under either quasi-static or cyclic loading, the crack-face fibre bridging stress field is determined for the compact tension (CT) test specimen geometry. Two different methods have been used for the analysis of the bridging stresses. The first considers a compliance approach. Using the difference in compliance calibration curves with and without bridging and assuming a power-law relation between bridging stress and crack opening displacement, the bridging stress field was calculated. The second approach uses the existence of an invariant stress reversal point in the CT geometry and assuming that the material exhibits linear elastic fracture behaviour, yields a recurrence relation for the bridging stresses resulting in a piece-wise constant stress function. Both models are applied to the experimentally determined fracture behaviour of a 2D carbon/carbon (C/C) composite, and the resulting bridging stress distributions are discussed.     

短切炭纤维增强沥青基C/C复合材料的组织特征

利用新型、高效的模压半炭化成型工艺，在大气环境下制备出了短切炭纤维增强沥青基C／C复合材料制品，并借助光学显做镜和扫描电镜对其微观组织和断口形貌进行了观察。通过分析，解释了短切炭纤维增强沥青基C／C复合材料中炭纤维损伤的形成机制，提出了作为增强体相的短切炭纤维和焦炭颗粒与基体炭之间独特的界而结构模型。研究还表明：复合材料中明显存在着基体相和颗粒相一基体相的显微结构不仅呈层片状，而且层片状的结构好像数层桔子皮，将颗粒相包裹起来，这种“桔皮包裹”式的结构与炭纤维表面的POG结构基本相似。     

有机黏土对碳纤维/聚醚砜-环氧复合材料层间断裂韧性的影响

采用溶剂法和热熔法制备了不同有机黏土质量分数的有机黏土/聚醚砜（PES）-环氧复合材料,通过对其微观形态和力学性能的研究,揭示了复合材料的增韧机制。在有机黏土/PES-环氧复合材料中添加T800H（12K）碳纤维,制备了T800H-有机黏土/PES-环氧复合材料预浸料单向带,采用热压罐工艺制备了复合材料单向板,对其I型、II型层间断裂韧性进行了研究。结果表明:T800H-有机黏土/PES-环氧复合材料的层间断裂韧性随有机黏土质量分数变化趋势与有机黏土/PES-环氧复合材料的断裂韧性趋势一致,证明了增韧机制的正确性。      

Microstructure and fracture toughness of nickel particle toughened alumina matrix composites

Al₂O₃-Ni composite materials have been made by a hot pressing technique. Two composite microstructures, i.e. a dispersive distribution of nickel particles and a network distribution of nickel particles in an alumina matrix, have been produced. The fracture toughness of the composite materials has been measured by a double cantilever beam method. Both composites are tougher than the virgin alumina matrix. The fracture toughness of the composite with a network microstructure is much higher and has a more desirable R-curve behaviour than the composite with a microstructure of dispersed particles. For the particulate dispersion microstructure, the main limitation to toughening is the lack of plastic deformation of the ductile nickel due to the pull out of nickel particles, indicating weak bonding at the Al₂O₃/Ni interface. For the network microstructure composite, the gauge length of the ductile phase is much larger, allowing the ductile nickel to stretch to failure between the crack faces. A large extent of nickel plastic deformation has been observed, and the weak bonding at the Al₂O₃/Ni interface can promote partial debonding and contribute further to toughening.     

微米级短碳纤维/铜基复合材料组织和摩擦性能研究

以电解铜粉和酚醛树脂包覆的毫米级短碳纤维为原料,通过球磨-冷压-加压烧结制备了微米级短碳纤维/铜复合材料,研究了短碳纤维长度和分散程度对材料力学和摩擦性能的影响.结果 表明:球磨工艺可有效缩短碳纤维长度,制备均匀分散且长度均匀(20 ～40 μm)的微米级碳纤维,进而保证了材料在摩擦过程中可连续产生均匀细小的碳颗粒以阻碍材料的黏着,改善材料的摩擦稳定性和耐磨性.球磨时间不足时,短碳纤维长度差异大且局部存在纤维缠结,摩擦过程中富铜区黏着加剧,易产生片状脱落,磨损较大(2.32×10-4mm3/(m· N));球磨时间过长时,短碳纤维损伤严重,产生大量碳颗粒与短碳纤维共存,过多的碳铜界面和缺陷促进了材料摩擦过程中疲劳损伤导致的大量剥层磨损,耐磨性降低;球磨3h制备的复合材料综合性能较好,弯曲强度和体积磨损率达到332.9 MPa和1.00×10-4mm3/(m·N),摩擦系数波动范围宽度约为0.0834.     

Effect of composite layering on the fracture toughness of brittle matrix/particulate composites

Glass matrix/Ni particulate composites, ranging from 0 to 25% particulate phase, were prepared both as single-volume-fraction composites and as multi-volume-fraction layered composites. Fracture toughness (K_c) measurements were made on all composites using the applied moment double cantilever beam technique. The measured toughness values for the layered composites were found to be equivalent to those of the single-volume-fraction composites. The fracture toughness measured for the layered composites was found to be dependent on the volume of composite phase tested and ultimately on the number of crack-particle interactions which occurred. R-curve like behaviour was observed in the layered composites.     

Role of controlled debonding along fiber/matrix interfaces in the strength and toughness of metal matrix composites

In metal matrix composites toughness is derived primarily from the plastic work of rupture of ductile matrix ligaments between the fractured fibers and from the plastic work of simple shear separation along steps connecting major fracture terraces. In the optimization of tensile strength in the longitudinal and transverse directions together with the respective works of fracture the most important factor is the control of the extent of debonding along interfaces between the fibers and the matrix, which develops locally in the course of deformation in a continuously changing mix of modes. In Al alloy matrix composites reinforced with Al₂O₃ fibers an effective means of controlling the key interface fracture toughness is through coarsening of Al₂Cu intermetallic interface precipitates which prescribe a ductile fracture separation layer. A combined experimental approach and micromechanical modeling, utilizing a specially tailored novel tension/shear: traction/separation law provides the means for further optimization of overall behavior. This revised version was published online in July 2006 with corrections to the Cover Date.     

Delamination toughness computation for curved thermoplastic composites

The curved double cantilever beam (CDCB) test is a method used to characterize mode I delamination toughness of curved composite materials, fabricated by filament winding technology. A finite element model for this geometry is presented, that takes into consideration nonlinear deformational effects, orthotropic material properties and local mixed mode load at crack-tip. It can be used as the theoretical basis for analysis of experimental results. Especially for long cracks, compliant materials and thin samples large deflection should not become neclected in the modelling. In spite of its highly unsymmetrical geometry the crack tip loading state proved to be nearly pure mode I.     

The effects of volume percent and aspect ratio of carbon fiber on fracture toughness of reinforced aluminum matrix composites

Carbon fiber reinforced aluminum matrix composites are used as advanced materials in aerospace and electronic industries. In order to investigate role of aspect ratio of carbon fiber on fracture toughness of aluminum matrix composite, the composite was produced using stir casting. Al–8.5%Si–5%Mg selected as a matrix. The samples were prepared with three volume fractions (1, 2 and 3) and three aspect ratios (300, 500 and 800). Three-point bending test was performed on the specimens to evaluate the fracture toughness of the materials. The results showed that the fracture toughness of composites depends on both fiber volume fraction and aspect ratio. Scanning electron microscopy (SEM) was employed to elucidate the fracture behavior and crack deflection of composites. The study also, showed that the toughening mechanism depends strongly on fiber volume fraction, aspect ratio and the degree of wetting between fiber and matrix.     

Plane strain fracture toughness test procedures for particulate metal matrix composites

Abstract

Procedures for plane strain fracture toughness tests on a number of particulate reinforced aluminium alloy metal matrix composites (MMCs) have been examined. Measurements of toughness are reported on a range of particulate aluminium alloy MMCs and the results are compared with validity criteria in standards for metallic materials. In particular, the effect of testpiece thickness was studied in a 6000 and a 2000 series aluminium alloy containing, respectively, 25 and 30% silicon carbide. The results are compared with other published work on the toughness of particulate reinforced MMCs.

MST/1225     

Prediction of Mode I interlaminar fracture toughness of stitched flax fiber composites

This paper aims to propose a simulation procedure to predict the interlaminar fracture toughness of stitched flax fiber composites through a virtual double cantilever beam test. The proposed procedure is constituted of two steps. First, the interlaminar failure of unstitched flax fiber laminate, as the parent laminate, is modeled using cohesive elements with a nonlinear softening law in order to model the large-scale fiber bridging occurred during delamination. The experimental results are used to calibrate the parameters of the cohesive law. Second, two-node beam elements are superposed onto the cohesive interface of the parent laminate at a prescribed stitch density and distribution to model the bridging stitches present in the validation samples. The stitch material behavior and properties are obtained from the tensile test of impregnated stitch fibers. The out-of-plane flax yarn stitching was found to generate a twofold increase in the delamination resistance of the composite laminate at a medium stitch density. The FE analysis results agreed well with the experimental results, where a good fit between the predicted and experimental R-curves was achieved.     

The effect of fibre length on fracture toughness and notched strength of short carbon fibre/epoxy composites

The effect of radius of curvature on the tensile notched strength of random short carbon fibre/epoxy composites containing 1, 5 and 15 mm length fibres is studied. The strength of all laminates showed a sensitivity to the radius of curvature, with the tensile strength decreasing at smaller radii of curvature. A model is developed to predict notched strength based on assumed evolution and propagation of damage from the tip of the notch. The predictions of the model depend principally on two material properties: the unnotched tensile strength and fracture toughness. Reasonable agreement is achieved between the predicted notched strength and experimental data.     

Fracture toughness of short carbon fibre reinforced thermoplastic polyimide

The fracture toughness testing of short fibre reinforced thermoplastic materials were performed. Materials tested were the polyimide resin and also that reinforced with 20 wt% or 30 wt% short carbon fibre. For introducing the initial crack, the tapping method, the sliding method and the bridge indentation method were examined. Among them, the sliding method was found to be effective for every case. The fracture tests were conducted by the three-point bending test with several loading rates. Stable crack growth was observed for the neat material while unstable fracture occurred for the reinforced materials. The critical values of the stress intensity factor at crack initiation were greater for the reinforced materials than for the neat resin. The fracture toughness of the 30 wt% reinforced material was independent of loading rate while that of 20 wt% reinforced material increased with loading rate. In order to investigate the fracture mechanisms, fractographic observations were also performed.     

Fracture toughness and fracture of WC-Co composites

Critical stress intensity factor, and related parameters have been measured in three-point bending for 18 different combinations of different volume fractions of cobalt (5 to 37%) and grain size of tungsten carbide (0.7, 1.1 and 2.2 m). In particular, a study was made of the correlations between the strength and mechanical and microstructural parameters, such as ¯L _Co,C _WC, ¯L _Co/¯D _WC, ¯L _Co ² /¯D _WC,H _V and wear resistance. A hypothesis for the mechanism of fracture has been proposed following an analysis of these results and a study of the mode of fracture.