| 講演要旨： |
Nickel-Base single crystal superalloys are widely used in many aircraft engine applications where high temperature strength materials (i.e. high creep-rupture strength, thermal fatigue resistance, and oxidation resistance) are required to meet design intents. Although adequately characterized for design purposes, persistent field failures are still observed in many single crystal Ni-base superalloy turbine blades in military as well as rocket engines. Most of these failures are classified as HCF Type Failures with cracks initiating and propagating along a single or multiple crystallographic octahedral planes. The fundamental understanding of the relation between load, crystal orientation, material anisotropy, and crack resistance forces is still lacking to prevent these types of failures.
An analytical effort was undertaken to model the crack driving forces in a single crystal nickel-base alloy under fatigue crack growth loading conditions. The testing configuration was the Brazilian Disk Specimen (A Disk with a Middle Crack under Compressive Load) commonly used in the fracture strength of rocks. Various mixed mode crack growth extension can be easily achieved by inclining the crack relative to the compressive loading line. The variation of the Mixed-Mode Stress Intensity Factors (KI, KII & KIII) with crack length is determined numerically using the Finite Element Method (FEM) for the tested crack angles and crystal orientations. The differences between the isotropic and anisotropic SIF solutions are also delineated. Furthermore, since the experimentally observed crack extension was along {111} facets, the experimentally measured FCG rates are correlated with resolve shear stress intensity factor (Krss) parameters originally proposed by Chen & Liu. This parameter is based on the projected mode II stress intensity factor along the active {111} planes in the [011] or [112] slip directions. Finally, it is shown that the DKrss parameter is able to correlate the experimentally observed FCG rates for the various tested crystal orientation.
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