溶融亜鉛めっきした中炭素鋼材の湿度制御下における疲労特性

抄録

We investigated fatigue properties and crack initiation behavior in galvanized steel when fatigue tests were conducted under conditions with controlled humidity（RH＝30% and 80%）. The fatigue strength in the low cycle（N_f ≤ 10⁵cycles）did not increase in response to increments in humidity. However, the fatigue strength in the high cycle（N_f > 10⁵ cycles）at RH＝80% was higher than that of RH＝30%, which was attributed to the humidity dependence of crack initiation behavior. That finding indicates that fatigue cracking is inhibited at RH＝80%. When fatigue strength was independent of humidity, several crack initiation sites were formed in the surface during the low cycle. However, when differences in fatigue strength were between RH＝80% and RH＝30%, only one crack initiation site occurred at RH＝80%, with numerous such sites at RH＝30%. These findings suggest that cracking is prone to occur at RH＝30%. Several subcracks were found in the test piece at RH＝80%. Furthermore, the subcrack number and length decreased as the number of cycles to failure increased. Some subcracks on test pieces that reached fatigue limits were closed by corrosion products consisting of ZnO and Zn（OH）₂. Results show that increased fatigue strength of galvanized steel resulted from corrosion products ZnO or Zn（OH）₂, which caused the cracks to close under high humidity, thereby delaying crack propagation.