The Effect of Vacuum on the Defect Size Dependence of Fatigue Crack Growth Limit (Aiming at Clarifying the Very High Cycle Fatigue Mechanism of High Strength Materials)

对于高强度材料，如高强钢和钛合金，在10^7 ~ 10^8次循环的非常高循环区，疲劳断裂发生在材料内部。通过我们到目前为止的研究，研究负责人指出，这一现象可以通过假设内部起源裂纹尖端周围的环境类似于高真空环境来解释。为了验证这一假设，在本研究中，我们使用研究负责人新开发的真空旋转弯曲机对微缺陷材料在空气和真空中进行了疲劳试验，目的是：(1)研究真空中微缺陷引起的疲劳行为，并阐明真空压力对ΔK_<th>的影响；(2)通过对比真空条件下初始断裂面与内部裂纹的断裂面，研究内部裂纹周围的环境效应。对直径为30μm、深度为30μm的高强度钢试样进行了10^<-3> ~ 10^<-4> Pa真空压力下的疲劳试验。结果表明，真空条件下的疲劳寿命比空气条件下的疲劳寿命长3 ~ 4倍。另一方面，真空中的疲劳极限比空气中的疲劳极限降低了约50MPa。当裂纹由非常小的缺陷引起时，ΔK_< >在真空中比在空气中要低。利用扫描电镜对微缺陷周围的初始断口进行了分析。已知内裂缝起源周围存在小于构造单元的凹凸图案组成的特殊表面区域；而高强度钢在10^<-3> -10^<-4> Pa的真空压力下，表面没有出现这种特殊的断口。相反，在2x10^<-5>Pa真空压力下进行的实验则明显显示出这种特殊的断口面，尤其是钛合金。由此可见，我们的假设（如上所述的下划线句）可以解释在长寿命区域低应力下发生内部断裂的现象，同时，我们假设内部起源裂纹对初始裂纹扩展断面的环境影响，特别是真空压力；似乎是不同的材料。少

With respect to fatigue of high strength materials as high strength steel and Ti-alloy, in the very high cycle region over 10^7 to 10^8 cycles, fatigue fractures occur from the interior of the materials. Through our research till now, the research leader indicated that this phenomenon can be explained by assuming that the environment around interior-originating crack tip is similar to a high vacuum environment. To verify this assumption, in this research we conducted fatigue tests of micro-defective materials both in the air and in a vacuum using a vacuum rotating bending machine newly developed by the research leader for the purpose of (1)investigating fatigue behaviors originating from micro defects in a vacuum and clarifying an effectt of vacuum pressure on ΔK_<th>, and (2)investigating environmental effects around interior cracks by comparing initial fracture surface in a vacuum with that of interior cracks.Testing involved fatigue tests under 10^<-3> -10^<-4> Pa vacuum pressure us … More ing high strength steel specimens with artificial defects of 30μm in diameter and 30μm in depth. The result showed the fatigue life in a vacuum showed approximately 3-4 times longer than those in the air. On the other hand, fatigue limit in a vacuum lowered approximately 50MPa compared with that in the air. In such case as the cracks originating from the very small defects, ΔK_<th> in a vacuum was revealed to be lowered compared with that in the air.The initial fracture surfaces around micro-defects were analyzed by SEM. It has been known that around origins of interior cracks there are special surface areas consisting of concavo-convex pattern smaller than structure unit ; nevertheless, the surface of high strength steel under vacuum pressure of 10^<-3> -10^<-4> Pa did not show such special fracture surfaces. On the contrary, the experiment under 2x10^<-5>Pa vacuum pressure conducted showed such special fracture surface clearly particularly with Ti-alloy.From the above, our assumption (underlined sentence described above) can explain the phenomena that interior fractures occur in low stress in the long life region, and at the same time, we assume environmental effects of interior originating cracks over initial crack propagation fracture surfaces, vacuum pressure in particular ; seem to be different based on materials. Less