Crack-Wake Debonding and Toughness in Fiber- or Whisker-Reinforced Brittle-Matrix Composites

Solutions are obtained for the mechanics of debonding in the crack wake in fiber- or whisker-reinforced composites for the case where a finite shear traction exists at the fiber/matrix interface in the debonded zone. These solutions are then applied to derive expressions for the steady-state toughness increases obtained in bonded composites wherein the toughness contribution is provided by crack-wake fiber/matrix debonding and crack bridging. The solutions for an unbonded composite containing a frictional fiber/matrix interface can be obtained from the derived equations in the limit of the fiber/matrix interface toughness equal to zero. In this limit, the debond crack length reduces to the slip length and the expressions for the crack opening and the predicted toughness increase reduce to previously derived expressions for unbonded composites. The steady-state toughness is found to depend sensitively on the interface toughness, the fiber fracture strength, and the shear tractions in the debonded zone including other material parameters, such as fiber radius and volume fraction and the moduli of the constituent phases. It is shown that in order to obtain finite toughness increases, the fiber/matrix interface toughness must be less than a critical value dependent on the fiber fracture strength, fiber radius and volume fraction, and fiber and matrix moduli. The predictions of the model are applied to published experimental results from a detailed and complete study of toughness increases in a bonded whisker-reinforced composite.     

Fiber Debonding in Residually Stressed Brittle Matrix Composites

The competition between initial fiber debonding versus fiber failure marks a crucial event of the microstructural failure process in fiber-reinforced brittle matrix composites. In this study, the role of a thermal residual stress field on the debonding conditions is examined theoretically and analytically. The analysis is based on two critical observations, the first being that the mechanics at the tip of a kink crack are driven only by the singularity at the main crack tip. Following from the first is the second observation that any thermal stress effects on the debonding criteria should enter only through the phase angle ψ ^T of the total stress intensity factor at the main crack tip. In general, this stress intensity factor has a thermal as well as a mechanical load contribution. It is shown that when the thermal and mechanical stress intensities, K ^R and K ^t , respectively, are in phase , i.e., ψ ^R =ψ ^t , the existing debonding conditions are universal and can be used even in the presence of thermal loads. On the contrary, when K ^R and K ^t are out of phase , i.e., ψ ^R ≠ψ ^t , events such as the delamination of thick films or debonding of inclined aligned fibers in brittle matrix composites become sensitive to the presence of the thermal stresses.     

基于微波加热的热膨胀微球/石蜡/石墨自修复功能材料的制备及其在砂浆中的应用

以热膨胀微球、石蜡和石墨为原料制备了一种快速修复混凝土裂缝的新型微波加热自修复功能材料(MHSFM)。研究了热膨胀微球掺量对MHSFM体积膨胀率的影响,测试了在微波作用下MHSFM和掺加MHSFM砂浆的升温速率,评价了MHSFM掺量对砂浆力学性能、抗渗性能和自修复性能的影响。结果表明,MHSFM的体积膨胀率随着热膨胀微球掺量增加而增大。在微波作用下,砂浆的升温速率随MHSFM掺量的增加而明显加快。MHSFM对砂浆的力学性能和抗渗性能有一定的影响,但会显著增强砂浆的自修复性能,掺加12%(质量分数)MHSFM的砂浆在微波作用下可快速愈合0.53 mm的裂缝。     

Expansion characteristics of thermally expandable microcapsules for dismantlable adhesive under hydrostatic pressure or in resin

ABSTRACT

The expansion characteristics of thermally expandable microcapsules (TEMs) under hydrostatic pressure or in resin were experimentally investigated. For the experiments, the expansion of the microcapsules was observed in high-pressure nitrogen at high temperature utilizing optical microscopy with a digital camera installed. The TEMs used for the experiments were expanded by heating under hydrostatic pressure up to 3 MPa, but the expansion degree decreased with increasing pressure. A cured bulk specimen of epoxy resin containing the microcapsules was made, and the expansion of the microcapsules was again observed with the microscope. It was found that the expansion of the microcapsules in the resin was saturated at a certain temperature. The stress distribution in the resin produced by the expansion of the microcapsules was calculated by the finite-element method. It was found that normal stress occurred, but it was mainly compressive. Tensile stress was also generated, although the maximum value was smaller than that of shear stress. It was observed that the expansion of the microcapsules was limited when there were many microcapsules in the vicinity of the interface. In other words, a complicated stress state occurred, inducing interfacial failure along the interface.     

Structure-Property Relations in Plane Particulate Polymeric Composites under Simple Extension and Compression

An approach is offered for evaluation of effective stress-strain curves of participate composites under plane strain conditions based on absolution of the boundary-value problem providing simultaneously the data on the corresponding structural stress and strain fields. The influence of randomness in the particle arrangements and the filler concentration on the effective behavior is established. It is demonstrated that the observed plateaus on the tensile and compressive curves are caused to a degree by the geometrical non-uniformity of the composite structures. The difference between tensile and compressive curves is examined and explained from the structural point of view.     

化学气相渗C／SiC材料结构与性能的研究

采用碳布层叠然后用化学气相渗方法制备了Ｃ／ＳｉＣ复合材料，这种材料纤维与基体间的界面是决策材料力学行为的重要因素，带有热解碳作为界面层的Ｃ／ＳｉＣ材料，在断裂进表现出大范围的脱粘，纤维与周围的基体不同发生断裂，有大量的纤维拨出，断口类似毛刷，无界央层材料表现为脆性平面断口，裂纹直接通过纤维和基体向前扩展，没有发生脱粘。     

Effects of interface bonding properties on cyclic tensile behavior of unidirectional C/Si3N4 and SiC/Si3N4 composites

In this paper, the effect of fiber/matrix interface bonding properties on the cyclic loading/unloading tensile stress?strain hysteresis loops of 2 different ceramic‐matrix composites (CMCs), ie, C/Si₃N₄ and SiC/Si₃N₄, has been investigated using micromechanical approach. The relationships between the damage mechanisms (ie, matrix multicracking saturation, fiber/matrix interface debonding and fibers failure), hysteresis dissipated energy and internal frictional damage parameter have been established. The damage evolution processes under cyclic loading/unloading tensile of C/Si₃N₄ and SiC/Si₃N₄ composites corresponding to different fiber/matrix interface bonding properties have been analyzed through damage models and interface frictional damage parameter. For the C/Si₃N₄ composite with the weakest fiber/matrix interface bonding, the composite possesses the lowest tensile strength and the highest failure strain; the hysteresis dissipated energy increases at low peak stress, and the stress?strain hysteresis loops correspond to the interface partially and completely debonding. However, for the SiC/Si₃N₄ composite with weak interface bonding, the composite possesses the highest tensile strength and intermediate failure strain; and the hysteresis dissipated energy increases faster and approaches to a higher value than that of composite with the strong interface bonding.     

双弹簧、双活塞力学模型在颗粒填充聚合物复合材料粘弹性质描述中的应用研究

推导了双弹簧、双活塞黏弹性力学模型的本构方程。在多功能试验机及轮廓摩擦磨损仪上考察了CuO、Al2O3及Cu等无机物颗粒填充聚酰胺1010复合材料的静态蠕变特性和松弛特性。实验结果表明,该力学模型能较好地反映颗粒增强聚合物复合材料的黏弹性质。     

新型轮胎加热硫化技术的研究进展

介绍3种新型硫化加热技术,即微波加热硫化技术、辐射加热预硫化技术和电磁感应加热硫化技术在轮胎硫化中的应用研究进展以及3种加热硫化技术的优缺点。指出随着金属内模的不断发展,电磁感应加热硫化技术发展优势明显,两者结合应用有望将我国轮胎硫化技术提升到一个新阶段。     

复合材料泡沫夹层结构的缺陷评定方法研究

本文以厚壁碳纤维复合材料为面板,硬质聚氨酯泡沫为芯材制造复合材料泡沫夹层结构,模拟实际生产过程中容易出现的面板与芯材之间界面的脱粘和界面胶层过厚的现象,采用人工制造试块的方法,研究了超声波探伤对夹层复合材料缺陷的评定方法,解决了实际检测过程中的疑问,为夹层复合材料结构产品的质量检验提供依据。得出了粘接良好区胶层过厚不会被判定为脱粘的结论。