Cu-bearing high-strength low-alloy steels: The influence of microstructure on the initiation and growth of small fatigue
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I.
INTRODUCTION
SMALL fatigue cracks growing in steels have been extensively studied since it was recognized that small cracks may have different growth kinetics than large cracks. There have been so many studies and they are published in such a diversity of journals and conference proceedings that no one has established a catalog of the results. A partial reading of the literature indicates that microstructure greatly influences the growth of small fatigue cracks in steels, as might be expected from the effects of microstructure on other properties. In contrast to the growth kinetics of small cracks, fatigue crack initiation has not been extensively studied in steels, probably because it was shown in the 1970s and 1980s that inclusions (ceramics, foreign matter, dirt, and intermetallics) initiated most fatigue cracks in steels. [1,2] However, in recent years, steels have become cleaner, and fatigue crack initiation may occur at sites other than inclusions,t31 The number of cycles to fatigue crack initiation and the growth of small fatigue cracks are important in assessing the lifetimes of structures that are limited by fatigue. For large structures, such as oil production offshore platforms, cracks may be assumed to exist because of the fabrication methods used and the large volume of material. For smaller and more critical structures, such as aircraft landing gear, made of clean materials, there are probably no cracks in the as-fabricated structure. In one steel, it was found that -~70 pct of lifetime was occupied by the combined initiation and growth of fatigue cracks to a surface length of 1 ram. t4~Another study found that crack initiation, which was
D.L. DAVIDSON, Institute Scientist, K.S. CHAN, Staff Scientist, and R.C. McCLUNG, Manager, are with the Materials and Structures Division, Southwest Research Institute, San Antonio, TX 78238. Manuscript submitted June 27, 1995. 2540--VOLUME 27A, SEPTEMBER 1996
not at inclusions, was 10 to 20 pct of lifetime,tSJ For the Cu-bearing steel BIS 812 EMA, fatigue crack initiation was found to be ~50 pct of lifetime during tests in sea water, t61 As examples of the effects of microstructure on fatigue initiation sites, consider the work of Kunio et al.,tT] who found that cracks initiated in prior austenite grain boundaries for the low carbon martensitic steels investigated. De los Rios et al.tS] reported about the same result for a 0.4 wt pct C steel of mixed pearlite and ferrite microstructure with ferrite in the prior austenite grain boundaries. Shang et al.[g] observed that cracks initiated within ferrite grains in steels having 26 to 67 pct martensite. Tokaji et al.[5] found that grain boundaries were implicated in a high strength ferritic steel with 10-~m-diameter grains. Daeubler et al.m found crack initiation parallel to the lamellae direction in a pearlitic steel. The nucleation sites of fatigue cracks in pure polycrystalline iron may be within grains or grain boundaries, depending on loading frequency (Guiu et aL[~O]).That effect was probably due to those tes
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