Crack Growth Behavior under Low Loading Ratio with Negligible Oxide-Induced Crack Closure

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  • TAZOE Koki
    Reliability Group, Research & Development Center, YANMAR CO., LTD
  • OKA Masanori
    Reliability Group, Research & Development Center, YANMAR CO., LTD
  • YAGAWA Genki
    Emeritus Professor, University of Tokyo and Toyo University

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Other Title
  • 酸化物誘起き裂閉口が無視できる低応力比下でのき裂進展特性

Description

<p>In order to investigate near threshold crack growth behavior without oxide-induced crack closure, ΔK decreasing tests under the stress ratio 0.1 and the loading frequency 20 Hz, 10 Hz, 5 Hz and 3 Hz conditions were carried out, respectively. In case of Cr-Mo steel, oxide films were observed at the near threshold macroscopic fracture surface at 20 Hz and 10 Hz, but not observed at 5 Hz and 3 Hz. The EDX analysis on the microscopic fracture surface at either 5 Hz or 3 Hz showed that no oxide was detected in the threshold area. It is noted that the ΔKth values were 7.0 MPa √m at 20 Hz, 5.0 MPa√m at 10 Hz, 4.6 MPa√m at 5 Hz and 3 Hz, respectively. In the range of 20 Hz to 5 Hz, the threshold showed a tendency to decrease with the loading frequency due to the decrease of the oxide-induced crack closure. On the other hand, the values became constant with the frequency smaller than 5 Hz, because there were no oxides in this range. In case of low carbon steel and cast iron, similarly to Cr-Mo steel, no clear oxides were observed in the macroscopic fracture surface tested under 5 Hz condition. It is concluded from the above results that the oxide-induced crack closure would disappear under a low frequency condition.</p>

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