Explanation of Fish-Eye Type Crack Behavior of Austempered Ductile Iron in Very High Cycle Fatigue

本研究探讨了奥贝球铁在10^7周以上的鱼眼疲劳机理，重点研究了以下三个方面：高响应拉伸疲劳试验设备的研制、10^8周疲劳试验数据的获取、鱼眼失效断口和试件表面的观察。所开发的测试设备在加载精度方面实现了+/- 1%的性能，在响应方面实现了120- 200 Hz的性能，这使得我们能够在6-10天的短时间内获得10^8个循环的测试数据。S-N曲线在10^6-10^7次循环时趋于平稳，在长寿命区域再次下降。寿命低于10^6次的短寿命断裂起源于表面（表面断裂），而寿命超过10^7次的长寿命断裂起源于内部（鱼眼断裂）。竞争风险模型的置信度分析表明，表面和鱼眼断口的疲劳寿命分布特征完全不同。从地面观测到的 ...更多信息 通过鱼眼断裂的试样，发现表面裂纹在10^7次循环后停止扩展。从断口观察，澄清了鱼眼裂纹甚至在小于该材料的DELTAK_的DELTAK区域扩展<th>。尤其是鱼眼断口上留下了表面断口所没有的奥氏体和铁素体的细小层状组织。我们计算疲劳寿命的基础上的裂纹扩展速率使用DELTAK测量的投影面积的每个断裂起源的通常法律。表面断口的疲劳寿命几乎可以预测，而鱼眼断口的疲劳寿命则远小于实际数据。研究结果表明，表面断裂向鱼眼断裂的转变是由于表面裂纹扩展受阻所致，鱼眼裂纹在较低应力下扩展，表面断裂与鱼眼断裂存在不同的断裂机制。特别是，我们发现了一种可能性，鱼眼裂纹的扩展速度比通常的裂纹扩展速率定律所预期的要慢得多。少

This research has discussed a fish-eye fatigue mechanism of ADI at a range over 10^7 cycles, and focused on the following three items : a development of the high response tensile fatigue testing equipment, obtaining many fatigue data till 10^8 cycles, and observation of fracture surfaces and specimen surfaces which failed by fish-eye. The developed testing equipment achieved a performance of +/- 1% in a loading accuracy and 120-200Hz in responsiveness, which enabled us to obtain a test data of 10^8 cycles in a short period of 6-10 days. The S-N Curve leveled off at 10^6-10^7 cycles and at a long life area it dropped again. The fracture origins of short life below 10^6 cycles existed at surfaces (surface fracture) and those at a long life over 10^7 existed at interior (fish-eyefracture) . Confidence analysis by the competing risk model showed that characteristics of fatigue life distribution between surface and fish-eye fracture were completely different. From surface observation of the … More specimens fractured by fish-eye, surface cracks were found to stop their propagation over 10^7 cycles. From fracture surface observation, it clarified that the fish-eye cracks propagated even at the DELTAK region smaller than DELTAK_<th> of this material. Especially, fracture surfaces of fish-eye left traces of fine lammelar structures of austenite and ferrite which were not seen on surface fracture. We calculated fatigue life based on the usual law of crack propagation rate using DELTAK measured by an projection area of each fracture origin. Then the fatigue life of surface fracture is almost predictable but that of fish-eye fracture shows much smaller than the actual data. In conclusion, results clarified that transition from surface fracture to fish-eye fracture is caused by stoppage of surface crack propagation, fish-eye cracks propagate at quite a low stress, and the different mechanism is existed between surface fracture and fish-eye fracture. Especially, we found a possibility that fish-eye cracks are propagating much slowly than they are expected by usual law of crack propagation rate. Less