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RESEARCH PAPERS

Fatigue-Crack Propagation in Steels of Various Yield Strengths

[+] Author and Article Information
J. M. Barsom

Applied Research Laboratory, U. S. Steel Corporation, Monroeville, Pa.

J. Eng. Ind 93(4), 1190-1196 (Nov 01, 1971) (7 pages) doi:10.1115/1.3428061 History: Received February 08, 1971; Online July 15, 2010

Abstract

The useful life of highly constrained welded structures subjected to cyclic loads often depends on the crack-propagation behavior of the material. Thus, to predict the service life of many structures and to establish safe inspection intervals, an understanding of the rate of fatigue-crack propagation in steel is required. Accordingly, an investigation was conducted to determine the fatigue-crack-growth rates in structural steels ranging in yield strength from 36 to 191 ksi; for this study, wedge-opening-loading (WOL) specimens were used. The tests were conducted at room temperature in an air environment, and the results were compared with published fatigue-crack-growth data for steels having similar yield strengths. The results showed that the primary factor affecting fatigue-crack-growth rates in structural steels is the applied stress-intensity-factor range, ΔKI , and that conservative estimates of fatigue-crack growth per cycle of loading, da/dN, for martensitic steels are obtained from the relationship

dadN=0.66×10−8(ΔKI)2.25
where a is in inches and ΔKI is in ksi in. Similarly, the data showed that conservative estimates of da/dN for ferrite-pearlite steels are obtained from the relationship
dadN=3.6 ×10−10(ΔKI)3
As indicated in these equations, the fatigue-crack-growth rates were higher for martensitic steels than for ferrite-pearlite steels. The data also showed that the fatigue-crack growth per cycle accelerated for all the steels, and that this transition from the above relationships to increased rates occurred when the crack-opening-displacement range, Δδ, which is a measure of the strain range at the crack tip, reaches a critical value. The fatigue-rate transition in martensitic steels occurred when Δδ was about 1.6 × 10−3 in. However, the fatigue-rate transition in ferrite-pearlite steels occurred at a Δδ value slightly higher than 1.6 × 10−3 in. A model based on micro structural considerations is presented, which accounts for these differences in the fatigue-crack-growth behavior between martensitic and ferrite-pearlite steels.

Copyright © 1971 by ASME
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