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

在这项研究中，以下四个主题进行了调查。(1)采用虚拟弯曲试件进行了Monte-Calro模拟，考虑了缺陷尺寸分布和微裂纹扩展行为。对于采用微裂纹扩展行为模拟的情况，估算的抗弯强度与实际抗弯强度吻合较好。通过对裂纹扩展速率分散性影响的评价，再次证实了断裂强度的大分散性主要来源于缺陷尺寸的分散性。(2)在静态疲劳条件下研究了烧结氮化硅光滑试样的疲劳裂纹萌生和扩展。自然萌生的小裂纹在光滑试样中以远低于人工长裂纹表观门槛应力强度因子的应力强度因子扩展。在某些情况下，小裂纹在裂纹萌生后停止扩展或非常缓慢地扩展。对于自然萌生的小裂纹，循环载荷对裂纹扩展行为的影响小于人工长裂纹。此外，利用原子力显微镜，我们可以检测到一些证据的晶粒桥接退化的自然发起的小裂纹。(3)为了评价陶瓷材料的裂纹萌生性能，用光子倍增管测量了陶瓷材料的光子发射。在断裂的瞬间会发射出大量的光子。光子发射持续了一段时间。在Si_3N_4的情况下，光子计数随着断裂而逐渐增加。认为这种现象是由裂纹萌生引起的。(4)本研究利用最新开发的原位式双轴弯曲机，研究玻璃上的裂纹扩展行为。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。提出了复合型条件下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.