Meaning of Ke in Design-by-Analysis Fatigue Evaluation

[+] Author and Article Information
Gerry C. Slagis

 G C Slagis Associates, 258 Hillcrest Place, Pleasant Hill, California 94523slagisg@asme.org

The quoted Ref. 6 is Ref. 5 in this document.

From unpublished committee documents.

J. Pressure Vessel Technol 128(1), 8-16 (Oct 26, 2005) (9 pages) doi:10.1115/1.2140798 History: Received September 09, 2005; Revised October 26, 2005

The ASME Section III Design-by-Analysis rules for pressure-retaining components include a detailed fatigue evaluation based on elastically predicted primary, secondary, and peak stresses. A prerequisite for the fatigue analysis is that the primary-plus-secondary stress range does not exceed 3Sm. If this limit is exceeded, the code provides “Simplified Elastic-Plastic Analysis” rules for the fatigue evaluation. A Ke penalty factor is applied to the elastically predicted alternating stress. The maximum value of Ke (3.3 or 5) is a severe design limitation. Test data indicate that the code specified maximum value of Ke is very conservative. The simplified elastic-plastic rules were originally developed for piping and published in B31.7. When the piping rules were incorporated into Section III in 1971, the B31.7 procedure was replaced by a less complex procedure. The development of the simplified elastic-plastic analysis approach is reviewed to establish the technical basis for the present code rules. The concepts of fatigue, shakedown to elastic action, thermal bending, elastic follow-up, notch factor, and strain redistribution are discussed. Recommendations for changes to the plastic strain correction factor are provided.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

NB-3200 stress limits (2004)

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Figure 2

Shakedown to elastic action

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Figure 3

Plastic redistribution (Figs. 1 and 2 from Ref. 7)

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Figure 4

B31.7 Appendix F correction factor for plastic strain redistribution

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Figure 5

Langer’s configurations for plastic strain analysis (from Ref. 5)

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Figure 6

Selection of Ke as 1∕n (from Ref. 5)

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Figure 7

Gerber test specimens (from Ref. 9)

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Figure 8

Gerber fatigue test data (from Ref. 9)

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Figure 9

Ke Data (from Ref. 10)

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Figure 10

Krempl test specimen

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Figure 11

Carbon steel Ke data (from Ref. 7)

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Figure 12

Piping design examples




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