Study of Micro-Crack on Metals During Fatigue by using Positron Beam Technique

本研究的目的之一是了解疲劳引起的缺陷的演化过程，特别是在疲劳循环的早期阶段，并评估材料的疲劳损伤。二是利用正电子束技术开展疲劳试验。为此，设计了一台常规的正电子束装置，并在疲劳试验过程中同时对304不锈钢和纯铁进行了正电子湮没寿命和多普勒展宽实验，以阐明空位团形成的基本特征。结果表明，正电子的平均寿命和S参数都随着疲劳循环次数的增加而增加(疲劳的早期阶段，即直到断裂的10%循环)。此外，在样品的某一位置，寿命迅速增加。那里可能已经形成了空位集群。此外，疲劳破坏几乎发生在正电子寿命较长的位置。这些结果表明，疲劳试验过程中微裂纹的产生与三维空位团的形成有关。研究发现，利用正电子湮没技术可以很容易地探测疲劳早期与微裂纹形成有关的应力集中区域。另一方面，我们设计的正电子束装置的计算得出正电子直径约为0.23 nm。二维光子计数测量证实了正电子束的轮廓，但其直径并不收敛。出现这个问题的原因是必须再供应一个高电压。我们有一个在未来改进这一点的实验计划。

One of the aim of this study is to understand the evolution processes of defects induced by fatigue especially in the early stage of fatigue cycles and evaluating the fatigue damages of materials. Another one is development of fatigue test by using positron beam techniques. For these purposes, a conventional positron beam apparatus was designed, and the usual positron annihilation lifetime and Doppler broadening experiments have been performed simultaneously for type 304 stainless steel and pure Fe during fatigue tests in order to clarify the fundamental feature of vacancy cluster formation. From these experiments, following results are derived.Both the average lifetime of positron and the S-parameter have increased with increasing number of fatigue cycles (very early stage of fatigue, i.e., until 10% cycles for fracture) at each position. Moreover, the lifetime has increased rapidly at a certain position of the sample. Vacancy clusters have possibly been formed there. Moreover, the fatigue failure has occurred at almost the same position as the one where the longer positron lifetime was observed. These results suggest that the generation of micro cracks during fatigue test is related to the formation of three-dimensional vacancy clusters. It has been found that the domain of stress concentration in the very early stage of fatigue, related to the formation of a micro-crack, can be probed easily by using the positron annihilation technique.On the other hand, calculation of positron beam apparatus we designed leads to the positron diameter of about 0.23 nm. Positron beam profiles were confirmed by 2-dimentional photon counting measurement, but its diameter was not converged. The reason for this problem is that another high voltage must be supplied. We have a plan to do experiment that improved this point in future.