Corrosion Fatigue Strength of High Tensile Steels and Its Fractographic Study.

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  • 高張力鋼の腐食疲労とフラクトグラフィ
  • コウチョウリョク コウ ノ フショク ヒロウ ト フラクトグラフィ

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Abstract

Considering that the corrosion fatigue is especially time-dependent, it is necessary to confirm the long-life fatigue properties of structural steels to be adoptable to the practical design curve. Fatigue tests have been performed on three kinds of high tensile steels for offshore structures in air or by dropping fresh water and sea water to the midsection of each specimen under rotating bending load at various cyclic speeds including lower frequencies. In addition, fractographic observation has been carried out on the crack initiation and fracture surface of each specimen.<br>The main results obtained on the basis of the long-time corrosion fatigue tests (maximum period; 1.45 years/specimen) are as follows:<br>(1) The fatigue strength decreases with higher cyclic numbers and lower frequencies. There exists a good linear relation between stress amplitude and cyclic frequency. The relation can be expressed by a linear logarithmic equation.<br>(2) The effect of corrosion liquid on fatigue strength is remarkable in long period tests and the fatigue strength in sea water becomes lower than that in fresh water in the frequency range between 1500 and 4rpm.<br>(3) The fatigue strength is a little dependent on tensile strength in the low cycle region and the difference in fatigue strength among high tensile steels becomes reversed in the high cycle region. This phenomenon appears not only in fresh water but also in sea water.<br>(4) The crack initiates from pits and propagates mainly by transgranular fracture. Both quasi-cleavage fracture and intergranular fracture are observed in the heat-treated steels under low cyclic frequencies in the high cycle region. These fracture patterns would be caused by corrosive effect and accelerate the crack propagation rate of the heat-treated steels.

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