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PREDICTING THE EFFECTS OF LOAD RATIO ON THE FATIGUE CRACK GROWTH RATE AND FATIGUE THRESHOLD
Author(s) -
Kardomateas,
Carlson
Publication year - 1998
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.1460-2695.1998.00539.x
Subject(s) - materials science , crack closure , asperity (geotechnical engineering) , paris' law , stress intensity factor , structural engineering , range (aeronautics) , crack tip opening displacement , crack growth resistance curve , composite material , mechanics , fracture mechanics , engineering , physics
The effect of the load ratio, R, on fatigue crack growth behaviour is analysed on the basis of the recently proposed inelastic discrete asperities model. A wide range of load ratios, both positive and negative, are examined. Particular emphasis is placed on compressive excursions, i.e. negative R loadings. The inelastic discrete asperities model is a micro‐mechanical analysis based on the plastic crushing of a single asperity (or multiple asperities) located on the crack face close to the crack tip and under dominantly plane strain conditions. Experimental data have indicated that the primary crack face contacts which obstruct closure are immediately adjacent to the crack tip, although segments of the crack face more distant from the crack tip are not neglected. However, the more distant asperities are a part of the past crack advance history which does not influence current behaviour. By use of this model, it is shown that the effect of the load ratio can be adequately predicted once some baseline information on mechanical material properties and surface roughness is provided. The model also provides useful trend information and explains many of the observed phenomena, e.g. the ‘saturation’ of the compressive underload effects. For a constant applied nominal stress intensity factor range, ΔK nom , it is shown that the effective stress intensity factor range, ΔK eff , initially decreases as the positive R decreases (corresponding to the increasing influence of closure), reaches a minimum around R = 0, and then starts increasing with negative R (corresponding to the plastic crushing of the asperities which reduces closure), eventually reaching a saturation level below ΔK nom . Conversely, for an assumption of a constant ΔK eff , the applied ΔK nom increases as the positive load ratio decreases, reaching a maximum around R = 0, and then decreases with more negative R values, eventually reaching again a saturation level (above ΔK eff ). It is also shown that the effect of material hardness can be directly analysed based on this model.