. Clear identification of the tree regions just described is possible.
. The presence of a threshold K1 is not clearly defined because of the difficulty of measuring crack velocities that are much smaller than 10*7 m/s
. The dramatic effect of increasing the temperature of water on the crack velocity is shown in Fig. 12.9c The velocity increases by about 2 orders of magnitude over a temperature range of =100c, typical of thermally activated processes.