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Technology Reviews

Proposed New Fatigue Design Curves for Carbon and Low-Alloy Steels in High Temperature Water

[+] Author and Article Information
William J. O’Donnell

 O’Donnell Consulting Engineers, Inc., 2940 South Park Road, Bethel Park, PA 15102wjo@odonnellconsulting.com

William John O’Donnell, Thomas P. O’Donnell

 O’Donnell Consulting Engineers, Inc., 2940 South Park Road, Bethel Park, PA 15102

B. F. Langer, W. E. Cooper, W. J. O’Donnell, and James Farr were the original developers of these factors in the late 1950s and early 1960s.

Except that corrections to the air curves for austenitic stainless steels, Alloys 600 and 800 are also being proposed.

Of course much work needs to be done on bolting fatigue including consideration of rolled versus cut threads, and recognition of potentially high strain concentrations in heat treated bolts with high yield to tensile strength ratios.

The requirement to use a factor of 4 for fillet welds and threads are examples of useful guidance as well as the factors provided for piping components.

Crack Initiation is itself a process of microcrack development due to shear generated intrusions and extrusions at the surface and shear crack propagation through microstructural barriers.

The factor of 2 for environmental effects was used by B. F. Langer and W. J. O’Donnell in their development of the original fatigue design curves in the Code.

Such a dependence would also be undesirable for the plant operator since losing control of the oxygen level could raise future design life regulatory issues.

The strain rate that governs environmental effects is the average strain rate during increasing tensile straining during the cycle.

J. Pressure Vessel Technol 131(2), 024003 (Jan 27, 2009) (10 pages) doi:10.1115/1.3006896 History: Received April 28, 2006; Revised September 15, 2008; Published January 27, 2009

High temperature (>300°F, 149°C) water has been found to greatly accelerate fatigue crack growth rates in carbon and low-alloy steels. Current ASME Code fatigue design curves are based entirely on data obtained in air. While a factor of 2 on life was applied to the air data to account for environmental effects, the actual effects have been found to be an order of magnitude greater in the low-cycle regime. A great deal of work has been carried out on these environmental effects by talented investigators worldwide. The ASME Code Subgroup on Fatigue Strength has been working for 20years on the development of new fatigue design methods and curves to account for high temperature water environmental effects. This paper presents an overview of the data and analyses used to formulate proposed new environmental fatigue design curves, which maintain the same safety margins as existing Code fatigue design curves for air environments, and a historical summary of ASME Code Work in this field.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic illustration of (a) growth of short cracks in smooth specimens as a function of fatigue life fraction and (b) crack velocity as a function of crack length: LEFM is the linear elastic fracture mechanics and EPFM is the elastic-plastic fracture mechanics

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Figure 2

Typical reference fatigue crack growth curves for low-alloy ferritic material in water environments (16) for a rise time of 10min with R=−1

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Figure 3

Reactor water environmental fatigue data derived from air data for pressure vessel steels by correcting for crack growth rate differences using ΔJ from Ref. 13

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Figure 4

Dissolved oxygen effects at 290°C(554°F) at a strain rate of 0.001%/s.

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Figure 5

PVRC data for carbon steels obtained under simulated PWR conditions from WRC Bulletin 487

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Figure 6

PVRC laboratory data for carbon steel obtained under simulated BWR reactor water environments from WRC Bulletin 487

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Figure 7

PVRC data for low-alloy steel obtained under simulated BWR conditions from WRC Bulletin 487

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Figure 8

Compilation of environmental fatigue data for carbon steels

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Figure 9

Compilation of environmental fatigue data for low-alloy steels

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Figure 11

Relative fatigue life of several heats of carbon and low-alloy steels at different levels of dissolved oxygen and strain rate

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Figure 12

Comparison of the fen models with the PVRC strain rate thresholds for carbon and low-alloy steels from WRC bulletin 487

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Figure 15

Proposed environmental fatigue design curves for N>106cycles

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