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Crack Growth Behavior in the Threshold Region for High Cycle Fatigue LoadingThis paper describes the results of a research program conducted to improve the understanding of fatigue crack growth rate behavior in the threshold growth rate region and to answer a question on the validity of threshold region test data. The validity question relates to the view 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-ratio data fans out from the high R-ratio data. This fanning behavior or elevation of threshold values in the low R-ratio tests is generally assumed to be caused by an increase in crack closure in the low R-ratio tests. Also, the increase in crack closure is assumed by some experimentalists to result from using the ASTM load shedding test procedure. 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 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 results from 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.
Document ID
20140002491
Acquisition Source
Johnson Space Center
Document Type
Conference Paper
Authors
Forman, R. G. (NASA Johnson Space Center Houston, TX, United States)
Zanganeh, M. (Oak Ridge Associated Universities, Inc. Houston, TX, United States)
Date Acquired
March 31, 2014
Publication Date
March 2, 2014
Subject Category
Structural Mechanics
Report/Patent Number
JSC-CN-29595
Meeting Information
Meeting: International Fatigue Congress 2014
Location: Melbourne
Country: Australia
Start Date: March 2, 2014
End Date: March 7, 2014
Sponsors: Royal Melbourne Inst. of Tech
Distribution Limits
Public
Copyright
Public Use Permitted.

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