PREDICTING THE EFFECTS OF LOAD RATIO ON THE FATIGUE-CRACK GROWTH-RATEAND FATIGUE THRESHOLD

The effect of the load ratio, R, on fatigue crack growth behaviour is analysed on the basis of the recently proposed inelastic discrete aspe rities 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. Experiment al data have indicated that the primary crack face contacts which obst ruct closure are immediately adjacent to the crack lip, although segme nts of the crack face more distant from the crack tip are not neglecte d. However, the more distant asperities are a part of the past crack a dvance history which does not influence current behaviour. By use of t his model, it is shown that the effect of the load ratio can be adequa tely predicted once some baseline information on mechanical material p roperties and surface roughness is provided. The model also provides u seful trend information and explains many of the observed phenomena, e .g. the 'saturation' of the compressive underload effects. For a const ant applied nominal stress intensity factor range, Delta K-nom, it is shown that the effective stress intensity factor range, Delta K-eff, i nitially decreases as the positive R decreases (corresponding to the i ncreasing influence of closure), reaches a minimum around R=0, and the n starts increasing with negative R (corresponding to the plastic crus hing of the asperities which reduces closure), eventually reaching a s aturation level below Delta K-nom. Conversely, for an assumption of a constant Delta K-eff, the applied Delta K-nom increases as the positiv e load ratio decreases, reaching a maximum around R=0, and then decrea ses with more negative R values, eventually reaching again a saturatio n level (above Delta K-eff). It is also shown that the effect of mater ial hardness can be directly analysed based on this model.