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

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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.