PC鋼線の低サイクル腐食疲労に及ぼす亜鉛メッキの効果

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  • Effect of Galvanizing on Corrosion Fatigue Crack Growth Rates in Prestressed Concrete Wires
  • PC コウセン ノ テイ サイクル フショク ヒロウ ニ オヨボス アエン メ

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Many studies conducted on smooth specimens have revealed that zinc-coated steels tested in sea water environments show a marked increase in fatigue life in comparison with that of uncoated specimens. The object of this study is to consider possible hydrogen damage in galvanized wires having preexisting cracks in steel under such high mean stresses as usually imposed on PC wires. Using a cantilever type low cycle fatigue tester made in our laboratory, rates of crack growth have been measured at the frequency of 1cpm. Through a small area of galvanized layer left undissolved on the specimen, precracks could be introduced by rotary bending in the air. The specimens in the final form of bare PC wires with precracks of 50μ in depth were immersed in 3% NaCl solution at 25°C and potentiostated at -1.10V (SCE) during fatigue cycling to examine whether permeated hydrogen accelerates the crack growth or not. By comparing the rates with those in room air, it is concluded that the effect of hydrogen becomes dominant in accelerating crack growth above KImax=80kg·mm-3/2, which coincides with the value obtained from the observation of fracture surfaces by a scanning electron microscope. This value of KImax is considered to be equal to KISCC. The threshold stress intensity range below which no detectable air-fatigue crack growth is observed, ΔKth, was also evaluated. Using the values of KISCC and ΔKth, crack growth characteristics of galvanized PC wires at a maximum stress level of 100kg/mm2 is discussed. As the threshold crack depth of 410μ, corresponding to KISCC, is considered to be larger than that of preexisting cracks, so galvanized wires could be used in sea water, as far as the coating is present, on the basis of fatigue characteristics determined in the air.

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