The influences of grain size and stress ratio on near threshold fatigue crack growth behavior have been studied by means of fracture mechanics and fractography on annealed 60-40 brass where the grain size was varied from 15 to 33 and 92μm. The results obtained are summarized as follows.
(1) For the mid-range of crack growth rates above 5×10-9
m/cycle, the dominant fracture appearance was striation formation regardless of grain size and stress ratio. On the other hand, for the near threshold crack growth rates below 5×10-9
m/cycle, the striation-like appearance a specified crystallographic plane was mixed with the microstructure sensitive fracture appearance such as cleavage-like mode and intergranular facet. In particular, the intergranular facets were observed on only the smallest grain size.
(2) According to the changes in dominant fracture appearance, various effects of grain size and stress ratio on crack growth rate were observed. In particular, the resistant for near threshold crack propagation and the ΔKth
decreased with decreasing grain size. If the microstructure was before recrystallization, the ΔKth
remarkably depended on grain size as the stress ratio increased. On the other hand, the ΔKth
of recrystallized structure had almost the same dependence on grain size regardless of stress ratio. This fact may be well explained by the difference in fracture appearance and the concept of fracture surface roughness induced crack closure.
(3) As the stress ratio increased, the near threshold crack growth rate increased and the ΔKth
decreased regardless of grain size. The ΔKth
also depended remarkably on stress ratio with decreasing grain size. However, when the microstructure was the recrystallized one, the ΔKth
had almost the same dependence on stress ratio regardless of grain size.