Analysis of Fatigue-Crack Growth in a High-Strength Steel—Part II: Variable Amplitude Block Loading Effects

[+] Author and Article Information
A. M. Sullivan, T. W. Crooker

Naval Research Lab., Strength of Metals Branch, Engineering Materials Div., Washington, D.C.

J. Pressure Vessel Technol 98(3), 208-212 (Aug 01, 1976) (5 pages) doi:10.1115/1.3454402 History: Received July 30, 1975; Online October 25, 2010


To investigate load sequencing effects in fatigue-crack growth, surface flaw or part-through crack (PTC) specimens of a high-strength pressure vessel steel were subjected to variable-amplitude cyclic loading. Blocks of different load sequences and stress-ratio (R) patterns and cyclic lengths were employed. Crack-growth rate (da/dN) data were analyzed within the framework of linear elastic fracture mechanics using the stress-intensity range parameter, ΔK. The da/dN data were found to correlate well with a seauence-independent formulation of ΔK derived using nominal mean stress, σm , for the loading blocks studied. However, it proved necessary to utilize the normalizing relationship discussed in “Analysis of Fatigue-Crack Growth in a High-Strength Steel—Part I: Stress Level and Stress Ratio Effects at Constant Amplitude”, to account for stress-ratio effects observed under the variable-amplitude block loading. Only the sequential block pattern of high-low-intermediate produced a significant crackgrowth retardation. Predictions made using the sequence-independent normalizing procedure compare favorably with those using the Willenborg model. It is concluded that useful predictions for structural life under variable-amplitude cyclic loading sequences can be made for a wide variety of high-strength pressure vessel applications using sequence-independent analysis procedures.

Copyright © 1976 by ASME
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