Study on crack initiation in ceramic materials by ushing hybrid-observing systems

本文主要研究了以下四个方面的内容：(1)利用虚拟弯曲试件进行了考虑缺陷尺寸分布和微裂纹扩展行为的蒙特卡罗模拟。对于利用微裂纹扩展行为进行模拟的情况，估计的弯曲强度与实际弯曲强度吻合得很好。通过对散布对裂纹扩展速率的影响的评估，再次确认了断裂强度的广泛散布主要来源于缺陷尺寸的散布。(2)在静态疲劳条件下，研究了Si3N4烧结体光滑试件中疲劳裂纹的萌生和扩展。光滑试件中自然萌生的小裂纹扩展的应力强度因子远低于人工长裂纹的表观门槛值应力强度因子。在某些情况下，小裂纹在裂纹萌生后停止扩展或扩展非常缓慢。对于自然萌生的小裂纹，循环荷载对其扩展行为的影响小于人工长裂纹。此外，利用原子力显微镜，我们还可以检测到自然引发的小裂纹上的一些晶粒桥联退化的证据。(3)为了评价陶瓷材料的裂纹萌生特性，利用光子倍增管测量了光子发射。在骨折的瞬间会发射大量的光子。光子发射会持续一小段时间。在Si3N4的情况下，随着断裂的进行，光子数逐渐增加。本研究利用最新研制的原位双向弯曲机研究了玻璃表面裂纹的扩展行为。混合模式下的门槛应力强度随应力混合比R的增大而减小，提出了复合型裂纹开始扩展的判据。

In this study, following four subjects were investigated.(1) A Monte-Calro simulation taking into account the defects size distribution and the micro-crack propagation behavior was conducted by using virtual bending specimens. For the cases of the simulation using micro-crack propagation behavior, the estimated bending strength well coincided with the factual bending strength. Through the evaluation of the effects of scatter in a crack propagation rate, it is reconfirmed that the wide scatter in fracture strength mainly originated from the scatter of defect size.(2) Fatigue crack initiation and propagation in smooth specimens of a sintered Si_3N_4 have been investigated under a static fatigue conditions. Naturally initiated small cracks in smooth specimens propagate with stress-intensity factors which are far lower than the apparent threshold stress-intensity factor of artificial long cracks. In some cases, small crack stops propagation or propagates very slowly after the crack initiation. In case of a naturally initiated small crack, the effect of cyclic loading on the propagation behavior is less than that of artificial long crack. Also, using the AFM, we can detect the some evidence of grain-bridging degradation on the naturally initiated small cracks.(3) To evaluate the crack initiation properties in ceramic materials, photon emission are measured by using a photon multifier tube. A lot of photon are emitted at the instant of fracture. Photon emission continues for a little while. In case of Si_3N_4, photon count number increases gradually by the fracture. It is considered that this phenomen on caused by crack initiation.(4) In this study, crack propagation behavior on glass have been studied by using the newly developed in-situ type bi-axial bending machine. The threshold stress intensity under mixed mode tends to decrease with increase in the stress mixing ratio R. The criteria in which the crack began to grow under mixed mode condition were proposed.