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J. Japan Inst. Metals, Vol. 36, No. 12 (1972),

pp. 1180-1188

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The Fatigue Crack Propagation Law in Water of Ni-Cr-Mo Martensitic Steel

**
Keijiro Nakasa**^{1}, Hideo Takei^{1} and Tetsuhiro Asamoto^{2}

^{1}Faculty of Engineering, Hiroshima University, Hiroshima

^{2}Graduate School, Hiroshima University, Hiroshima

#### Abstract:

The effect of delayed failure on the fatigue crack propagation rate in water of Ni-Cr-Mo martensitic steel was investigated. The results obtained are as follows:

(1) The relation between the fatigue crack propagation rate da/dN in air and the stress intensity factor range Δ K is expressed as da/dN=C_{1}(Δ K)^{m1}, and m_{1} increases with increase of K_{m}/Δ K, where K_{m} is the mean stress intensity factor.

(2) The relation between the crack propagation rate da/dt in delayed failure and the static stress intensity factor K_{D} is expressed as da/dt=C_{2}(K_{D})^{m2}.

(3) The fatigue crack propagation rate (da/dN)_{w} in water is expressed as the addition of both fatigue crack propagation rate in air and crack propagation rate in delayed failure, and the following equation holds:

(da/dN)_{w}=da/dN+β(da/dt) · 1/f |

where f is the frequency (cpm); β is a coefficient of addition and depends mainly on f and K_{m}/Δ K. Under the condition in this experiment (f=55 ∼ 345 cpm, K_{m}/Δ K=0.5 ∼ 4), β is 0.2 ∼ 0.4.

(4) Physical meaning of β is considered to be the ratio of the time in which the tri-axial tensile stress state region at the crack tip meet a cluster of hydrogen atoms during one cycle.

(Received 1972/7/17)

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