MEASURMENT OF R-CURVES FOR CERAMIC COMPOSITS,AND ITS MACROMECHANIC ANALYSIS

为了弄清控制材料断裂行为的微观机制，有必要寻求精确的表征，给出材料对裂纹扩展的阻力对裂纹长度(R曲线行为)的依赖关系。在这项工作中，解决了与脆性材料中裂纹的稳定扩展有关的一些基本问题。首先，开发了一种在样品中引入尖锐(最大刻度直径)缺口的技术。然后，通过使用使用弯曲几何形状的稳定器装置来稳定裂纹扩展的问题被克服了(根据JIS)。通过这种方式，即使是在非常脆性的材料中，获得R曲线的常规程序已经被很好地建立并应用于两种类型的材料：1)由晶须或纤维状(细长)颗粒在微米尺度上增强的复合材料，2)由长纤维在更大范围内增强的复合材料。除了断裂力学表征外，还通过原位光谱测量来表征微观结构尺度上的断裂机制。结果表明，在脆性固体中，作用于裂纹尖端的机制，即弹性或摩擦桥联，在阻碍断裂扩展中起主要作用。最后，还对纤维增强材料的抗剪强度进行了精确的表征，特别强调了界面脱粘的微观机制与宏观剪切破坏过程之间的联系。对上述破裂过程的结果和最终解释进行了总结，并发表在科学论文中。

In order to make clear the micromechanisms governing the fracture behavior of materials, it is necessary to pursue precise characterizations giving the depecdence of the resistance showed by the material to crack propagation on the crack length (R-curve behavior). In this work, some basic problems related to the stable propagation of cracks in brittle materials have been solved. First, a technique to introduce a sharp (mum scale diameter) notch in the specimen has been developed. Then, the problem of stabilizing the crack propagation through the use of a stabilizer apparatus using the bending geometry (according to JIS) has been overcome. In such a way, the routine procedure to obtain R-curve in even very brittle materials has been well established and applied to two type of materials : 1) composite materials reinforced on the micron scale by whiskers or fiber-like (elongated) grains and, 2) composite materials reinforced on a larger scale by long fibers.In addition to fracture mechanics characterizations, in situ spectroscopy measurements have been performed to characterize the mechanisms of fracture on the microstructural scale. As a result of this characterizations, it has been shown that mechanisms operating in the wake of the crack tip, namely, the elastic or frictional bridging, play the major role in hampering fracture propagation in brittle solids.Finally, precise characterizations of the shear resistance of fiber-reinforced materials have also been performed with particular emphasis on relating the microscopic mechanism of interface debonding to the macroscopic failure process by shearing. The results and the final interpretation on the above fracture processes have then summarized and presented in scientific papers.