Microcrack propagation in fatigued F.c.c. monocrystals I: Crack-depth distribution and propagation rate

Abstract

Nickel monocrystals oriented for single slip were fatigued at room temperature in air at constant plastic strain amplitude ε ap = 8×10 −4 (fatigue life N f about 12 000 cycles). The specimens were cycled up to different fatigue stages ( 1 6 × N f…N f ) and cut perpendicular to the surface trace of persistent slip bands (PSBs). Microcracks, running from the PSB surface profile into the interior along primary slip planes, could be observed in the cut faces by scanning electron microscopy. The frequency distributions of microcrack depths c > 1 μm were estimated for crack populations of about 1000 cracks per specimen. The curves have their centres at low crack depths and long tail-like Weibull distributions. From the frequency distributions at different cycle numbers N, mean propagation rates v depending on c and N could be calculated by a special theoretical approach for describing the development of the population. The v (c, N) dependences can be approximated by three propagation laws: for shorter cracks (1) v ∝ c| N 1 2 and for deeper cracks (2) v = constant (about 1 nm cycle −1) and later, above a critical value c ∗ = 80 μ m , (3) v ∝ c 2 for accelerating propagation of cracks. It has been shown that the calculated propagation rates are sufficient to estimate fatigue lives and that crack depths distributions in earlier fatigue stages already contain the later “fate” of fatigued monocrystals. The results of repolishing the fatigued surface layer and further fatiguing show that microcrack propagation in f.c.c. monocrystals depends not only on the PSB surface relief but also on the age of the internal structure.