Observations of subcritical crack initiation and growth processes by scanning acoustic microscope

点击翻译按钮获取中文摘要

Scanning acoustic microscope (SAM) was used to investigate the the crack initiation and propagation processes in the subcritical crack growth problems such as fatigue, stress corrosion and creep. The main interest was put on examining when and how the slip bands grow into the microcracks in fatigue. The materials selected in this study were pure copper, pure nickel and type 304 stainless steel. Fatigue tests were conducted and interrupted in a certain stage. The samples were provided for the observations by SAM and an optical microscope. The results are summarized as follows.1. The fine fatigue slip bands were able to observe when a frequency of ultrasonics was selected beyond 600MHz in the SAM observation. Subsurface informations, on the other hand, were able to obtain only when the lower frequency less than 40OMHz was used in the observations. The slip bands and cracks in the SAM image were visible best in intensity in the pure nickel samples.2. A clear difference was often observed in the SAM images in the certain two slip bands in which no differences were found in the optical microscopic images. This was found to be due to the differences in the subsurface situation and in the surface topography of the two slip bands.3. An interference fringes were observed around the initiated cracks, These fringes, however, were not observed for any slip bands. The transitional process from a slip band to a crack was unfortunately not examined in this study.4. The depth and the inclined angle of the initiated cracks were estimated quantitatively by the analyses of a V(z) curve and an interference fringe.