Direct Observation of the Mechanisms of Fatigue Crack Growth

A.研究了频率、温度和材料对疲劳裂纹扩展尺寸效应的影响。三个研究组都尝试了相同的材料(今年是镍基高温合金)。(1)在谢菲尔德大学，在室温和800℃下进行了大试件疲劳小裂纹萌生和扩展的自动观察。微结构对疲劳小裂纹萌生和扩展的影响正在被阐明。(2)在九州大学，在与伺服液压疲劳试验机相连的Instron型扫描电子显微镜上，对小试件的疲劳裂纹扩展进行了5万倍的自动观察。裂纹尖端变形(裂纹张开-闭合行为)之间的关系。在大阪大学，对连接在伺服液压疲劳试验机上的大阪型扫描电子显微镜进行了变幅载荷下疲劳裂纹的动态观察。通过三维表面裂纹扩展分析，研究了裂纹形状与试件尺寸之间的关系。(4)在九州大学，发展了基于边界单元法的三维表面裂纹应力强度因子分析方法。(5)将(4)的结果与谢菲尔德大学的有限元方法的结果进行了比较，证明有限元方法的精度适合于三维裂纹扩展的预测。

A.Investigation of the effect of frequency, temperature and material on size effect of fatigue crack propagation.The same material (Ni-base superalloy in this year) was tasted in the three research groups.(1) At University of Sheffield, the automated observation of the initiation and propagation of small fatigue cracks in large specimens is conducted at both room temperature and 800*C.The effect of micro-structure on the initiation and propagation of small fatigue cracks is being made clear.(2) At Kyushu University, the automated observation of the fatigue crack propagation of small specimens in the Instron-type scanning electron microscopes connected to servohydraulic fatigue testing machine is conducted at 50 thousand magnifications. The relationship between the deformation of the crack tip (crack opening-closing behavior). micro-structure and fatigue crack growth ratio is being made clear.(3) At Osaka University, the dynamic observation of fatigue crack in the Osaka-type scanning electron microscopes connected to servohydraulic fatigue testing machine is conducted under variable amplitude loads. The effect of variable amplitude loads on short fatigue crack growth is being made clear.B.Investigation of the relationship between the crack shape and specimen size by 3-D surface crack propagation analyzes.(4) At Kyushu University, the analytical method based on Boundary ElementMethod, which is extremely precise, for stress intensity factors of 3-D surface crack is developed.(5) Comparing the result of (4) and that by Finite ElementMethod at University of Sheffield, the precision of FEM is confirmed to be suit for the prediction of the 3-D crack growth.