Abstract: In order to determine the damage tolerance of a tough turbine disk,to obtain the actual measurement of its critical crack length and to provide the ground for verification of theoretical analyses,several rotational tests on fracture were completed for a tough turbine disk with radial cracks in a pair of opposite mortise bottoms.The whole fracture process,including initiation of crack propagation,subcritical propagation,breakaway of a turbo-vane from the joint and fracture in the end,are investigated and how to determine the critical crack length is discussed too in this paper.In author's view,for the tough turbine disk in which the breakaway of a turbovane precedes its fracture,it is suggested to define the initial crack length which causes a turbovane to break away from the joint at the maximum allowable rpm as the critical crack length.