Premium
A MODEL FOR THE COMBINED EFFECTS OF STRESS RATIO AND GRAIN SIZE ON THE LEFM FATIGUE THRESHOLD CONDITION
Author(s) -
Lal D. N.
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.tb00056.x
Subject(s) - grain size , materials science , upper and lower bounds , aluminium , grain boundary , cleavage (geology) , fracture (geology) , metallurgy , composite material , structural engineering , microstructure , mathematics , mathematical analysis , engineering
A long‐crack, fatigue‐threshold model which explains and predicts the commonly observed effects of stress ratio, R , and grain size, d , on δ K 0 , is proposed. The inclusion of a grain‐size‐effect is an extension of a recently proposed model that examined the effect of the R ratio. The extended model is based on the hypothesis that near‐threshold, crack growth involves two micro‐mechanical processes of fracture; K max ‐controlled submicroscopic cleavage, which predominates when the defect concentration is small, and δ K ‐controlled reversed shear which predominates when the defect concentration is large, both processes occurring in a critically stressed volume, V c , ahead of the crack tip. Defect concentration in V c is reduced by a low value of R and a coarse grain size and is increased by a high value of R and a fine grain size. Good agreement is shown to exist between predicted and experimental curves of δ K 0 versus R and δ K 0 versus grain size for several steels and aluminium alloys. In particular, δ K 0 is shown to have an upper and a lower bound value for a material. The model may be used as an alternative procedure for obtaining quick, approximate but conservative estimates of δ K 0 for practical design applications.