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THE EFFECTS OF FREQUENCY AND R ‐RATIO ON FATIGUE CRACK GROWTH BEHAVIOUR IN Al‐Li‐Cu‐Mg ALLOY (8090) PLATE
Author(s) -
Kemp R. M. J.,
Wilson R. N.,
Gregson P. J.
Publication year - 1992
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 - 10.1111/j.1460-2695.1992.tb01271.x
Subject(s) - alloy , materials science , paris' law , metallurgy , crack closure , composite material , structural engineering , fracture mechanics , engineering
— The effects of frequency and R ‐ratio on the fatigue crack growth rate of the Al‐Li‐Cu‐Mg alloy 8090 have been assessed in air. Compact tension test pieces ( B = 25 mm) were tested at constant P max using a triangular waveform (CA) and the crack length was monitored by a four‐wire pulsed potential drop technique. Crack closure measurements were made during the test by a front face compliance technique. The crack growth rate was found to be dominated by the high level of crack closure and the fatigue fracture morphology. Two fracture types were identified; a highly deviated “shear” fracture mode which was in competition with a macroscopically flat “tensile” fracture mode. The tensile fracture mode was predominant at low test frequency, high R ‐ratio and short crack lengths whereas the shear mode was predominant at high frequency, low R ‐ratio and long crack lengths. The transition from tensile to shear mode was primarily controlled by the effective stress intensity factor range, Δ K eff . The effect of loading variables on the transition was explained in terms of the diffusion distance of hydrogen relative to the effective cyclic plastic zone diameter. Test piece thickness had a secondary effect on the transition and this was explained in terms of gas transport considerations.