Fracture toughness of random fibrous materials with ultrahigh porosity at elevated temperatures

The fracture toughness of three‐dimensional random fibrous (3D RF) material was investigated from room temperature to 1273 K by virtue of experimental method, theoretical model and Finite Element Method (FEM) in the through‐the‐thickness (TTT) and in‐plane (IP) directions. The experiments showed that the fracture toughness in the TTT and IP directions increases (from 0.0617 to 0.0924 Mpa·m^1/2 and from 0.2958 to 0.3982 Mpa·m^1/2 for the TTT and IP directions, respectively) as the temperature until reaching a transition temperature (1123 K and 1223 K for the TTT and IP directions, respectively), then the fracture toughness decreases from 0.0924 to 0.0393 Mpa·m^1/2 and from 0.3982 to 0.3106 Mpa·m^1/2 for the TTT and IP directions, respectively. The fracture behavior was related to the bulk microstructures, the mechanical properties of fibers and the blunting of crack tip. The crack tip blunting affected the fracture toughness at elevated temperatures which was verified using the theoretical model. A FEM model with a single edge crack where special attention was drawn to the influence of the morphological characteristic was developed to simulate the fracture behavior of 3D RF material. Numerical results from the FEM modeling along with a theoretical model with crack tip blunting mechanism incorporated agreed well with the experimental results.     

High‐temperature short‐range compressive responses and contact effect of ultrahigh porosity 3D random fibrous materials

Through experiments and finite element modeling (FEM) of contacting fibers, we study the compressive responses of a 3‐dimensional (3D) random fibrous (RF) material of ultrahigh porosity (89%) in the through‐the‐thickness (TTT) and in‐plane (IP) directions from 299 (room temperature) to 1273 K. The experimental results indicate that localized failure and overall compressive deformation dominate the deformation process of RF materials loaded in the TTT direction at low and high temperatures, respectively. On the other hand, only localized failure is observed in the IP direction upon loading. Based on its morphological characteristics, a FE model that considers contact between the fibers is developed to simulate the compressive responses of the tested 3D RF material. In this model, the contact mechanism between the fibers is simulated based on a user‐defined nonlinear spring element. The simulated strength and elastic modulus agree well with the observations from the compressive experiments.     

Temperature effect on dynamic compression performance of random fibrous composites in the TTT and IP directions

This study examines the effect of temperature on the dynamic compressive performance of random fibrous (RF) composites at temperatures up to 1273 K in the through-the-thickness (TTT) and the in-plane (IP) directions, using an improved high-temperature split Hopkinson pressure bar (SHPB) system. The results revealed that in the IP direction, the RF composite presented a shear fracture mode below 1073 K and initiated multiple major cracks in the specimens at 1273 K. However, the composite showed a layered fracture mode in the TTT direction from 288 to 1273 K. The dynamic strength in both directions showed a consistent trend when observed under static loading below the critical temperature. The change in the strain-rate sensitivity (SRS) of the dynamic strength was insignificant for temperatures below the transition temperature of viscous-flow and brittle deformation of the RF composite. However, above the transition temperature, the SRS of the dynamic strength became significant.     

Dynamic compressive response of zirconium diboride-silicon carbide composites at high-strain rates

The quasi-static, dynamic compression experiments and micromechanical model were employed to declare the dynamic compressive response of ZrB₂–20%SiC composite at high-strain rates. The quasi-static compressive strengths were measured to determine the range of initial microcrack length in ZrB₂–20%SiC composite. The effects of the strain rate on dynamic compressive strength, critical stain, as well as fracture mechanisms were discussed based on experimental results. Dynamic mechanical properties of ZrB₂-based composites display obvious strain rate dependence. The dynamic increase factor in the compressive strength shows a rapid increase above a transition strain rate of 1228 s⁻¹. Moreover, a micromechanical model considering initial microcrack lengths is used to predict dynamic compressive strengths, which agree with the experimental results. Additionally, the critical strain has a linear increase tendency with the increase of strain rate. The dynamic compressive fracture mechanism of ZrB₂–20%SiC composite is relative to the combination effect between strain rate and microstructure. The size of flaw distribution is critical below the transition strain rate resulting in bigger fragments, whereas the flaw density is primary with more and smaller fragments above the transition strain rate.     

三维机织复合材料单胞模型各向异性的有限元分析

使用三维绘图软件Pro/E绘制出三维浅交弯联机织复合材料数字化结构模型,借助大型有限元分析软件ANSYS模拟单胞模型承受不同方向压缩载荷作用下的力学性能。探究在不同方向的压缩载荷作用下复合材料单胞模型的应力分布情况,并借此分析复合材料单胞模型的各向性能;以承受X方向压缩载荷的单胞模型为例,分析复合材料中纤维与树脂的受力情况。结果表明:三维机织复合材料受到压缩载荷时,表现出明显的各向异性,表现为X方向压缩性能最好,Z方向压缩性能最差;纤维作为主要承载体,承担较多载荷作用,树脂作为次要承载体,承担较少载荷作用。     

3D surface image analysis for fracture modeling of cement-based materials

The texture and exposed phases of a fracture surface are direct evidence of the mechanical behavior of a cement-based material. Deflection, microcracking and bridging are toughening mechanisms involved in fracture of brittle matrices that affect surface roughness. This study measures crack deflection and branching using 3D surface measurement techniques with confocal laser microscopy of mechanically fractured mortar prisms and 3D stereo pair microscopy of mechanically fractured plain concrete prisms. Image analysis techniques were used to identify phase composition and out-of-surface crack branching from profiles of cracks intruded with a low melting-point alloy. The resulting data was the basis for a micromechanical model to relate surface and phase data and the measured fracture energy to the increase in energy with respect to fracture of the matrix independent from the composite behavior.     

Combined experimental and numerical study of fracture behaviour of cement paste at the microlevel

The aim of this paper is to investigate the fracture performance of cement paste at the microlevel. An experimental procedure for micromechanical testing of consecutive layers in cement paste is developed. Every layer is tested by nanoindentation and subsequently imaged in ESEM to link the phase distribution with the measured micromechanical properties. The obtained micromechanical properties were used directly as input for numerical simulation of fracture behaviour of cement paste. A comparison is made between simulated 2D fracture response from individual slices and simulated fracture response of 3D structure, which is reconstructed from the 2D slices. In addition, the influence of heterogeneity on fracture response was studied. 3D microstructure enables more stable crack propagation, branching and crack trapping, which is attributed to more restrain and phase connectivity. Neglecting heterogeneity and original phase distribution in numerical simulations of cement paste leads to overestimation of both tensile strength and fracture energy.     

Voronoi cell finite element modelling of the intergranular fracture mechanism in polycrystalline alumina

The mechanisms of fracture in polycrystalline alumina were investigated at the grain level using both the micromechanical tests and finite element (FE) model. First, the bending experiments were performed on the alumina microcantilever beams with a controlled displacement rate of 10 nm s^–1 at the free end; it was observed that the intergranular fracture dominates the failure process. The full scale 3D Voronoi cell FE model of the microcantilever bending tests was then developed and experimentally validated to provide the insight into the cracking mechanisms in the intergranular fracture. It was found that the crystalline morphology and orientation of grains have a significant impact on the localised stress in polycrystalline alumina. The interaction of adjacent grains as well as their different orientations determines the localised tensile and shear stress state in grain boundaries. In the intergranular fracture process, the crack formation and propagation are predominantly governed by tensile opening (mode I) and shear sliding (mode II) along grain boundaries. Additionally, the parametric FE predictions reveal that the bulk failure load of the alumina microcantilever increases with the cohesive strength and total fracture energy of grain boundaries.     

New observations of the fiber orientations effect on machinability in grinding of C/SiC ceramic matrix composite

In this paper, the effect of machining parameters on cutting force, force ratio, 3D surface roughness was studied, and the surface formation mechanism was deeply analyzed in view of the position relation between machining directions and fiber orientations. New observations of the fiber orientation effect on machinability are attempted to obtain in grinding of 2D C/SiC ceramic matrix composite with electroplated diamond grinding tool. Two machining directions (A and B) on one surface are taken into account to study the effect of fiber orientation on the grinding process. The results indicate that the cutting forces obtained in machining direction of A are greater than that in machining direction of B under all experimental conditions. However, the tangential force is greater than the normal force, which is different from grinding ordinary material. Whether in the machining direction of A or direction of B in grinding C/SiC composite, on the whole the surface roughness values (Sa and Sq) decrease as the feed rate increases. As depth of cut increasing, the surface roughness values in the machining direction of A and B come out inconsistency. At different feed rates, the surface roughness values in the machining direction of A and B also represent inconsistency with the change of cutting speed. The theoretical model of undeformed cutting thickness is unfit for evaluating its effect on the surface roughness. After analyzing of the surface formation, except for some fibers forming extruding fault and fracture, being pulled out, and fracture or broken, a new phenomenon that some fibers forming extruding fault and fracture is observed.     

短碳纤维的分散性与CFRC复合材料的力学性能

碳纤维增强水泥基复合材料(CFRC)是新发展起来的一种功能材料,制备CFRC复合材料过程中,碳纤维在水泥基体中的分散性直接影响CFRC复合材料的力学性能。借助超声波和分散剂羟乙基纤维素(HEC),实现了短碳纤维在水泥基体中的均匀分散。对所制备的CFRC复合材料的断口形貌,作了SEM观察和能谱分析;测试了试件的抗压强度和抗折强度。结果发现,水灰质量比为0.44,碳纤维均匀分散,其质量掺量为0.6%时,复合材料的抗压强度可提高20%,抗折强度提高129%。