Multiaxial lifetime predictions of single-crystal superalloys: Use of reference stresses

The high temperature microstructural stability and good thermal-mechanical properties of single crystal superalloys has played a crucial role in the development of these materials for gas turbine blade application. A form of Continuum Damage Mechanics has been successfully developed over a decade or so, to model the anisotropic creep deformation of single crystal superalloys, where in a general formulation the creep rate can be obtained by the evolution of several state variables, each related to changes in the alloy microstructure during its lifetime. The model assumes that creep strain arises from the accumulation of shear strain on at least two families of active slip systems: the octahedral system G(1) = 1 1 1}<1 (1) over bar 0> and cube system G(2) = 0 0 1}<1 1 0>. Primary creep is viewed to result from a stress transfer process between the gamma-matrix and the gamma'-particles, and is described by the normalized variable S-k. The tertiary creep behavior in these materials is dominated by strain softening process, associated with an increasing mobile dislocation density in the gamma-channels. It is assumed that the rate of damage, omega(k), is proportional to the creep rate. The resulting set of constitutive relations has been incorporated into a finite element routine, in order to carry out component calculations that may be under complex states of multiaxial stress. However, the CPU times require to carry out component calculations can be considerable for complex structures such as a turbine blade. In this paper, an alternative procedure is investigated for the predictions of lifetimes under a multiaxial loading using the skeletal point concept. Lifetime calculations of the Bridgman notch specimens were done using equation (1) under the assumption that the multiaxial stress state in the notch be represented by a uniform multiaxial stress, identical to that at the skeletal point.