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- 2005 (6) (entfernen)
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- LCF (2)
- Modellierung (2)
- Superlegierungen (2)
- Axial-torsional Loading (1)
- Crack Propagation (1)
- Crack-opening displacement (1)
- Duktile Schädigung (1)
- FE modeling (1)
- Fatigue (1)
- Finite-Element-Simulation (1)
A material family to replace the current superalloys in aeronautical gas turbine engines is
considered to be that of gamma Titanium Aluminide (γ-TiAl) alloys. Structural components in
aeronautical gas turbine engines typically experience large variations in temperatures and multiaxial
states of stress under non-isothermal conditions. The uniaxial, torsional and bi-axial thermomechanical
fatigue (TMF) behaviour of this γ-TiAl alloy have been examined at 400 800°C with
strain amplitudes from 0.15% to 0.7%. The tests were conducted at both in-phase (IP) and out-ofphase
(OP). The effects of TMF on the microstructure were also investigated. For the same
equivalent mechanical strain amplitude uniaxial IP tests showed significantly longer lifetimes than
pure torsional TMF tests. The non-proportional multiaxial OP test showed the lowest lifetimes at
the same equivalent mechanical strain amplitude compared to the other types of tests.
Laird and Smith [(1962). Philosophical Magazine 8, 847857] proposed a plastic sliding-off mechanism for the stage II fatigue crack growth via striation formation. In their view, the fatigue crack extension results solely from the changing character of deformation at the crack tip during loading and unloading. In particular, the crack tip blunts during the loading stage and folds into a double notch during the unloading stage, resulting in striation formation. In order to verify Lairds plastic blunting mechanism for ductile polycrystals as well as for ductile fcc single crystals, FE calculations were performed for a rectangular plate with an initially sharp crack under plane strain conditions. The plate was subjected to a fully reversed tension-to-pressure cyclic load perpendicular to the crack plane (Mode 1). In the single crystal case the crack propagation simulations were carried out for cracks with crack plane (001) for two different crack growth orientations [110] and [100]. No initial radius for the crack tip was assumed. The actual shape of the crack tip followed from an initially sharp crack by repeated remeshing. To model the constitutive behavior typical for polycrystalline ductile metals, J2 hypo-elasto-plasticity model with ArmstrongFrederick kinematic hardening was used. To model the constitutive behavior typical for ductile fcc single crystals, a geometrically nonlinear version of Cailletauds model based on the multiplicative elasto-plastic decomposition of the deformation gradient was implemented into the FE program ABAQUS. For simplicity, only octahedral slip systems were considered. Using repeated remeshing for severely distorted elements at the advancing crack tip, deformation patterns in the sense of Lairds mechanism for fatigue crack propagation with striation formation were obtained in the case of the polycrystal simulation as well as in the case of the single crystal simulation for [110] crack growth direction. The simulation for [100] crack growth direction with the same stress level as for [110] direction also yielded crack extension by progressive large deformations but without striation formation. The dependence of the fatigue striation formation on the crack growth direction as predicted by the simulation of crack propagation in single crystals is verified by the experimental results of Neumann [(1974). Acta Metallurgica 22, 11551165] on pure copper single crystals.
In this work, the influence of the development of geometrically necessary dislocations (GNDs) at a crack tip in single crystals on the hardening and crack propagation behaviour is investigated. In particular, we are interested in examining the effect of such additional hardening on the development of glide and kink bands at the crack tip as well as on the process of crack opening. To this end, following Nye and many others, local deformation incompatibility in the material is adopted as a measure of the density of GNDs. Their development results in additional energy being stored in the material, leading to additional kinematic-like hardening. A thermodynamic formulation of the model in the context of the dissipation principle facilitates the derivation of the corresponding hardening relation. Results suggest that this additional hardening retards kink-band development, but has little or no influence on glide-band development. It also influences the crack tip opening displacement (CTOD). It turns out that the simulated CTOD correlates well with experimentally determined crack-propagation rates for different crack growth direction in the crystal.