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Propagation of microcracks at stress amplitudes below the conventional fatigue limit in Ti–6Al–4V
Author(s) -
Hines J. A.,
Lütjering G.
Publication year - 1999
Publication title -
fatigue and fracture of engineering materials and structures
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.887
H-Index - 84
eISSN - 1460-2695
pISSN - 8756-758X
DOI - 10.1046/j.1460-2695.1999.t01-1-00217.x
Subject(s) - materials science , microstructure , crack closure , paris' law , fatigue limit , composite material , fracture mechanics , amplitude , growth rate , stress concentration , grain size , mathematics , geometry , optics , physics
Fatigue crack growth below the conventional fatigue limit was examined in Ti–6Al–4V in two different microstructural conditions, bi‐modal and fully lamellar. Tests conducted at R = −1, at room temperature and in air showed that there is a stress dependency in the d a /d N –Δ K behaviour in both microstructures. The increasing crack front roughness associated with increasing crack size results in a decrease in the crack growth rate relative to the crack growth rate in a single grain. The d a /d N vs. Δ K lines were drawn for each crack size and a ‘threshold’Δ K was determined using the intercept of the lines with d a /d N = 10 −10 m cycle −1 . These values were used to construct modified Takahashi–Kitagawa diagrams to predict microcrack growth below the fatigue limit for each microstructure. A comparison of the two microstructures indicated differences in behaviour in microcrack and macrocrack growth that were explained by differences in crack front roughness at a given crack size.