Dislocation processes at the crack tip, due to cyclic loads, were examined and this showed that the 2-parameter requirement followed naturally from the analysis. Thus, there were 2 fatigue thresholds, Kmax,th and ΔKth, instead of just ΔKth; as was normally assumed. Examination of the dislocation behaviour revealed that the Kmax threshold could be related to the stress which was necessary to nucleate the additional slip required for crack growth. Similarly, ΔKth could be related to the stress required to overcome reversible slip. The value of Kmax,th was always greater than, or equal to, ΔKth. The 2-parameter requirement could be related to the presence of the monotonic and cyclic plastic zones which were characteristic of a fatigue crack. The size of the monotonic plastic zone was always greater than that of the cyclic plastic zone. The former moved the crack tip forwards while the latter re-sharpened it. The analysis also showed that the effect of crack-wake plasticity upon the crack-tip driving-force was limited and became increasingly negligible with increasing crack length. Dislocation concepts and continuum concepts were combined in order to analyse the 2-parameter requirement of fatigue crack growth.

Dislocation Processes that Affect Kinetics of Fatigue Crack Growth. K.Sadananda, G.Glinka: Philosophical Magazine, 2005, 85[2-3], 189-203