Near-threshold fatigue crack propagation behavior was examined in four pressure vessel steels (SA508 Cl 2a, SA533 Gr A Cl 2, SA508 Cl 3a, and SA533 Gr B Cl 2) and two submerged arc weldments (SA508 Cl 2a and SA533 Gr A Cl 2). At a given temperature and load ratio, near-threshold crack growth rates in the four base materials and two submerged arc weldments were essentially identical. Increasing the load ratio from 0.20 to 0.50 increased the rate of near-threshold crack propagation at a given temperature. At a fixed load ratio, increasing the temperature from 24 to 288°C (75 to 500°F) increased the rate of near-threshold crack growth. The effects of load ratio and temperature on the near-threshold crack growth behavior of pressure vessel steels can be explained based on a crack closure concept.

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