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Synthesis of S-N and da/dn Life Evaluation Technologies

“Synthesis of S-N and da/dn Life Evaluation Technologies” W. J. O’Donnell, presented at the American Society of Mechanical Engineers, Pressure Vessels and Piping Conference, Pittsburgh, Pa., PVP Vol. 10, 1988.

Keywords: design criteria; crack initiation; crack propagation; fatigue failure; J-integral solutions; reactor water effects

Abstract
The S-N technology used in the ASME Code and similar design criteria includes both the crack initiation and crack propagation phases of fatigue failure. Crack growth rate technology has successfully quantified various environmental (corrosion) cyclic rate, and (to a lesser extent) on the threshold crack growth values. The relationship between safe life evaluations based on these technologies is analyzed herein, and a general method of quantifying known crack growth rate effects in S-N curves is developed.

Fatigue tests on polished specimens are characterized by the nominal stress amplitudes over yield, where linear elastic fracture mechanics methods, such as those used in the ASME Code, are not valid. The small plastic zone corrections used in the Code do not account for the plastic driving energies encountered in low-cycle fatigue testing. J-integral solutions, equivalent to COD solutions are adapted herein to evaluate the growth of cracks in these specimens. This approach is shown to correlate the growth of cracks over the entire range of loading from elastic to grossly plastic conditions, in specimens of widely different geometries and sizes, including the growth of very short cracks for materials of major interest in pressure vessels and piping. The analytical approach developed herein can be used to correct S-N fatigue life evaluation curves for known differences in crack growth rates whether they are due to corrosion-assisted fatigue or other variables. As an illustration, the crack growth rates given in Section XI of the ASME Code are used to include reactor water environmental effects on A533 reactor pressure vessel steel in the fatigue design curves of Section III.

INTRODUCTION
The fatigue design evaluation criteria in Sections III and VIII of the ASME Code are based on 30-year-old technology and do not include recent major advances in the technology of accounting for environmental and aging factors. The latter must be considered in order to evaluate the safety and reliability of operating components and systems. The analytical methods developed herein combine existing Codified S-N technology with elastic – plastic crack propagation technology to include environmental effects on crack growth rates in improved S-N fatigue life evaluation methods.

These methods can also be extended to account for imperfections and residual stresses in weldments. The results of environmental degradation testing during the past fifteen years have shown that such effects are much more deleterious than anticipated when the ASME Code adopted the current S-N fatigue design curves. Therefore, environmental and aging effects must be taken into account in evaluating the reliability and dependability of components for extended periods of operation. Since most of the available data on environmental effects focuses on measured crack growth rates, methods of developing improved fatigue life evaluation methods which include environmental effects on crack growth rates are needed. The general method developed herein is qualified and verified using data for A533B pressure vessel steel because a great deal of data is available for this material.

The method is then applied to include reactor water effects on the fatigue strength of this material. However, the method is quite general, and can be used to include any known crack growth rate effects in S-N life evaluation curves. Environmental effects on the crack initiation phase of fatigue failure can be directly incorporated into S-N life evaluation curves. Once the crack propagation effects are included as indicated herein, the resulting improved S-N curves will provide a means for plant operators to evaluate the current condition of their components and systems, taking into account the cumulative damage from operating transients and cycles which the plant has experienced.

The safe residual life can then be evaluated using the S-N curves in order to verify that Code safety margins are maintained. This plant life extension approach is applicable even where in-service inspections are not feasible. It provides a quantitative basis for making repair/replacement decisions based on the reliability and dependability of continued operation.

 
Resources
Springerlink – Introduction to Fatigue of Structures and Materials
ASTM Standards on Fatigue and Fracture
ASME Committee on Design Methods


Our company President, Bill, Sr. began his career at Westinghouse/ Bettis in the Naval Nuclear Program under Admiral Rickover. He has served as the Chairman of the ASME Subgroup on Fatigue Strength for forty years, and has published numerous papers on design, fatigue and fracture.

O’Donnell Consulting performs engineering design and analysis to ASME Code including B&PV Section VIII Division 1 and 2 – as well as the Power & Process Piping Codes.