The critical length of the hydride cluster in delayed hydride cracking of Zr-2.5wt% Nb

In delayed hydride cracking (DHC) of Zr-2.5wt% Nb alloy, the hydride cluste r at the crack tip has a critical length, which is a function of the stress intensity factor K-I and other parameters. When K-I > KIH, the threshold s tress intensity factor, the hydride cluster must grow to this critical leng th before it will fracture. On the other hand, when K-I < KIH, there is a m aximum length to which the hydride cluster can grow, and this length is ins ufficient for fracture i.e. less than the critical length. In this work, th e lengths of the hydride cluster were experimentally studied for K-I < KIH and K-I > KIH near KIH. A modified experimental method was used, that permi tted the hydride clusters to be formed and fractured individually. The hydr ide clusters were observed to be wedge-shaped, in agreement with the predic tions by Metzger and Sauve (PVP, vol. 326, ASME, 1996). The lengths of hydr ide cluster measured in this work are compared with existing theoretical pr edictions. A good general agreement was obtained, but some differences are discussed.