High-cycle fatigue tests, low-cycle fatigue tests, and static tensile tests on notched specimens of a maraging steel were conducted. not only is the fatigue fracture toughness K
fc approximately constant for high-cycle and low-cycle fatigue, regardless of applied stresses and strains, but also K
fc is approximately equal to the static fracture toughness K
IC;K
fc<cong>K
IC<cong>36 MPa·m
1/2. Moreover, for its high tensile strength (yield stress σ
0.2=2440 MPa, tensile stress σ
B=2490 MPa), the maraging steel has an extremely small value of K
IC in comparison with other low-and medium-strength steels. Considering that the crack propagation period occupies only 20-30% of total life in high-cycle fatigue and also that the crack lengths at final fracture in low-cycle fatigue are very small in comparison with specimen size, the fatigue life prediction method on the basis of the integral of crack growth rate equation is inadequate and should not be applied to high-strength steels such as maraging steels. Instead, K
fc will be useful to predict the final fracture of maraging steels.
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