TS-146
Corrosion Fatigue Crack Growth Behavior of Low and High Carbon Steel Wire in Seawater
Yohnosuke SUZUKI*, Shin-ichi MOTODA*, Fumio ITO** and Shigeo TSUJIKAWA***
ABSTRACT
The investigation of surface fatigue crack growth behavior in seawater was made for low carbon steel wire SWRM10, and high carbon steel wire SWRH42A by varying test frequency between 15 Hz and 0.1 Hz, and temperature between 308K and 288K. The rates of crack growth for both steels in air were nearly equal to each other in relatively low Δ K regime. A particular frequency was found at which the enviromnental acceleration factor, (dl/dN)CF / (dl/dN)air, was at a maximum for both steels. The maximum acceleration factor tended to increase with decreasing temperature, and with greater steel strength in order of SWRM10 and SWRH42A. The environmental acceleration factor decreased with increasing test frequency over the particular frequency, and decreased with testing frequency below it for both steels. When testing at lower frequencies, there appeared a threshold frequency, at or below which the rate of crack growth in seawater slowed, and finally stopped for both steels. This threshold frequency decreased with decreasing temperature, and with greater steel strength in order of 0.6 Hz for SWRM10 and 0.2 Hz for SWRH42A at 308K, 0.3 Hz for SWRM10 and 0.1 Hz for SWRH42A at 303K, and 0.15 Hz for SWRM10 at 298K. These observations can be explained reasonably by crack tip rounding caused by dissolution in seawater.
Key Words : Surface fatigue crack, Low and high carbon steel wire, Environmental acceleration factor, Threshold frequency, Crack tip rounding.
1. Introduction
In our earlier publication [1], we reported that the growth of semi-ellptical surface fatigue cracks on mild steel wire could be suppressed or even stopped by low frequency cycle around ocean wave frequency, and that the suppression brcame more efficient with decreasing cyclic frequency, We concluded that these crack grown retardation phenomena were explained by crack tip blunting arising from enhanced corrosion dissolution at the crack wall, where the rate approached the average crack tip growth rate at lower frequencies [2]. In the present study, in order to clarify the effect of steel strength on surface fatigue crack growth behavior, tests were performed on hardness-enhanced mild steel wire and on hard steel wire under a variety of seawater loading frequencies (as the representative mechanical factor).
Tablel Chemical composition of materials, wt. %.
Table2 Mechanical properties of steel.
Fig. 1 Dimensions of specimens. (dimensions are in mm)
2. Experimental
2.1 Specimens
Two commercial steel wire specimens 5 mm in diameter used in the present tests were SWRM10 and SWRH42A, and their chemical composition and mechanical properties were as shown in Table 1, and Table 2 respectively.
* Tokyo University of Mercantile Marine
2-1-6, Etchujima Koto-ku, Tokyo 135-8533, JAPAN
FAX: +81-3-5245-7436, E-mail y suzuki@ipc.tosho-u.ac.jp
** Toba National College of Maritime Technology
*** The Unversity of Tokyo
