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Finite element method (FEM) modeling of creep of notched nickel based single crystal superalloys


Directional coarsening of γ’ precipitates in nickel based single crystal superalloys (so called rafting) is an important process within the context of life prediction of structural parts exposed to creep conditions. Crystallographic orientations of γ’ rafts are highly dependent on the orientation of applied stress. Structural parts like turbine blades with relatively complicated shapes have many regions which are exposed to the stresses with different orientation. Therefore, investigations of microstructure degradation under the multi axial creep loading are of a great importance. Multiaxial stress state conditions can be reached in notched affected regions of creep specimens. For the purpose of examination of the microstructure around the notched region it is necessary to know the values and also directions of stresses in every point of the notched region. Finite element method (FEM) calculations can be used for obtaining these values and directions.

Two types of specimens are used:
  • bar specimen
  • plate specimen (designed in order to minimize the effect of slip stresses within the notch)
model1
Notched bar specimen with two types of structural notches (U-type and L-type) created in program Ansys.

model2
(a) Wired model of plated notched specimen created in program Abaqus, (b) Detail showing two types of structural notches.

distribucia
Typical output from FEM analysis. Left image illustrates tensors of maximal principal stress and right image presents the distribution of maximal principal stress values.

orientacia


First calculations within the frame of FEM modeling of creep of single crystalline nickel based superalloys were made in cooperation with the Institute of Production Systems and Applied Mechanics at the Slovak Technical University (doc. Ing. Bohumil Taraba, CSc.) and Institute for Materials Technology (IfW) at the Darmstadt University of Technology (Dr.-Ing. Alfred Scholz, Dr.-Ing. Andreas Simon, Dipl.-Ing. Matthias Lyschik).

Contact persons: Dr. Juraj Lapin, Ing. Marek Gebura, PhD.