Fatigue Crack Growth Behavior in the Threshold Region

Abstract:

This paper describes the results of a research study conducted to improve the understanding of fatigue crack growth rate behavior in the threshold growth rate region and to answer a question of the validity of threshold region test data. The validity question relates to the position held by some experimentalists that using the ASTM load shedding test method does not produce valid threshold test results and material properties. The question involves the fanning behavior observed in threshold region of da/dN plots for some materials in which the low R data fans out from the high R data. This fanning behavior or elevation of threshold values in the low R tests is generally assumed to be caused by an increase in crack closure in the low R tests. Also, the increase in crack closure is assumed by some experimentalists to result from using the ASTM load shedding test procedure [1-3]. The belief is that this procedure induces load history effects which cause remote closure from plasticity and/or roughness changes in the surface morphology. However, experimental studies performed by the authors have shown that the increase in crack closure is more a result of extensive crack tip bifurcations that can occur in some materials, particularly in aluminum alloys, when the crack tip cyclic yield zone size becomes less than the grain size of the alloy. This behavior is related to the high stacking fault energy (SFE) property of aluminum alloys which results in easier slip characteristics. Therefore, the fanning behavior which occurs in aluminum alloys is a function of intrinsic dislocation property of the alloy, and therefore, the fanned data does represent the true threshold properties of the material. However, for the corrosion sensitive steel alloys tested in laboratory air, the occurrence of fanning is caused by fretting corrosion at the crack tips, and these results should not be considered to be representative of valid threshold properties because the fanning is eliminated when testing is performed in dry air.

• Periodical: Advanced Materials Research (Volumes 891-892)
• Pages: 327-332
• DOI: https://doi.org/10.4028/www.scientific.net/AMR.891-892.327
• Online since: March 2014
• Authors: Royce G. Forman*, Mohammad Zanganeh
• Keywords: Fatigue Crack Bifurcations, Fatigue Crack Growth Testing, Fatigue Crack Growth Thresholds, Fatigue Crack Roughness

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