Fatigue crack path prediction in UDIMET 720 nickel-based alloy single crystals

The effect of orientation, applied stress state, environment, and temperatu re on the crack growth and crack-path behavior of single-crystal specimens of UDIMET 720 has been examined. Stage I cracking has been promoted by mixe d-mode loading, plank stress, and vacuum conditions; increasing the test te mperature to 600 degrees C does not suppress stage I:crack growth in vacuum . Consideration of the local resolved shear-stress intensity and local reso lved normal-stress intensity for each slip system as it intersects the nomi nal crack-growth plane allows-the prediction of stage I crack paths, clarif ying the importance of secondary single-crystal testing orientation (i.e., nominal crack-growth direction as well as effective tensile axis). A combin ation of both opening and shearing are found to promote stage I crack growt h, and boundary conditions,have been established within which stage I crack ing is promoted. Highly deflected stage I cracking-gives rise to significan t shielding effects, but under suitable mixed-mode loading, highly oriented , coplanar stage I crack growth can be produced. Intrinsic stage I cracking under mixed-mode loading appears to be greatly accelerated compared with m ode I-dominated stage II crack growth for comparable stress-intensity level s.