Resistance curve behavior of polycrystalline niobium failing via cleavage

The present work illustrates the crack growth resistance behavior of polycrystalline niobium failing via transgranular cleavage fracture. Bend tests were conducted on fatigue-precracked Charpy specimens at -125 and -75 deg. C under different loading rates. Crack growth was monitored in situ during the tests in order to compute the fracture initiation toughness and develop crack growth resistance curves. The cleavage fracture initiation toughness (KQ) is shown to be relatively independent of loading rate in the range of -75 to -125 deg. C, although crack growth resistance was greater at -75 deg. C and at the lower loading rates. The spatial location of the cleavage fracture nucleation sites with respect to the fatigue precrack was not significantly affected by the changes in test conditions. Stable cleavage crack growth was associated with large plastic zones ahead of the fracture nucleation sites