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

本研究对以下四名受试者进行了调查。(1)采用虚拟弯曲试样进行了考虑缺陷尺寸分布和微裂纹扩展行为的Monte-Calro模拟。对于采用微裂纹扩展行为进行模拟的情况，估计的抗弯强度与实际抗弯强度吻合较好。通过对散射对裂纹扩展速率影响的评价，再次确认了断裂强度的大散射主要来源于缺陷尺寸的散射。(2)研究了Si_3N_4光滑烧结试样在静态疲劳条件下的疲劳裂纹萌生和扩展。光滑试样中自然萌生小裂纹的扩展应力强度因子远低于人工长裂纹的表观阈值应力强度因子。在某些情况下，小裂纹在裂纹起裂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，我们可以检测到自然产生的小裂纹上的一些颗粒桥接退化的证据。(3)利用光子倍增管测量了陶瓷材料的光子发射，以评价陶瓷材料的裂纹起裂特性。在断裂的瞬间发射出大量的光子。光子发射持续一段时间。对于Si_3N_4，光子计数随着断裂逐渐增加。认为这种现象是由裂纹萌生引起的。(4)采用新研制的原位双轴折弯机对玻璃的裂纹扩展行为进行了研究。混合模式下的阈值应力强度随应力混合比r的增大而减小，提出了裂纹在混合模式下开始扩展的判据。

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.