Principal Research Results

Development of Grain Boundary Local Stress Analysis Methodfor High Temperature Structural Materials−Application to Void Growth Simulation under Creep Condition−
Background
Creep voids are observed in detailed microscopic inspections for high temperature structural components of thermal power
plants. The voids grow up to microcracks under a long term creep condition. In order to predict the void growth behavior under thecreep condition quantitatively, a void growth simulation program ＊ 1 was developed. The stress used for the void growth simulation iscalculated geometrically in this program. It might be possible to predict the void growth behavior with higher accuracy by using grainboundary local stress values.

Objectives
This study aims to develop an analytical method and a finite element analysis program to analyze grain boundary local stressunder creep condition quantitatively. The developed program is applied to analyze grain boundary local stress and to void growthsimulation under creep condition.

Principal Results
1. Development of a grain boundary local stress analysis method
A grain boundary local stress analysis method was developed (Fig.1). In this method, a crystal plasticity model ＊ 2 wasapplied to consider the crystal orientation dependence of material properties. By incorporating this method into a finite element analysis program, it became possible to quantitatively analyze the grain boundary local stress distribution of polycrystalline materials undercreep condition. Reduction of calculation time was achieved by introducing an automatic mesh generation procedure into the developed program.
2. Application to a void growth simulation under creep condition
A grain boundary local stress analysis and a void growth simulation were performed for a turbine rotor material (Fig.2)
under creep condition (580℃, 180MPa, and 5000 hours duration). Results were obtained as follows.
(1) It was observed that the distribution of grain boundary local stress (Fig.3) and creep strain (Fig.4) were inhomogeneous under thecreep condition. Moreover, it was revealed that the normal stress on each grain boundary changed with time respectively becauseof stress redistribution near grain boundaries (Fig.5). The change in grain boundary local stress under creep condition with time
could be analyzed by applying the developed program, while it is difficult to conduct by conventional analysis based on continuum
mechanics.
(2) By using grain boundary local stress values, a void growth simulation was performed. As a result, the change of maximum void
length with time by the void growth simulation agreed well with that observed by a microscope (Fig.6).

Future Development
This method will be improved to analyze grain boundary local stress under creep-fatigue condition. Using the modified
method, a quantitative void growth simulation of high temperature structural materials will be carried out.
Main Researchers: Takayuki Sakai, Research Scientist and Takashi Ogata, Ph.D., Group Leader,
Materials for Thermal Power Plants Sector, Materials Science Research Laboratory
Reference
T.Sakai et al, “Development of Grain Boundary Local Stress Analysis Method for High Temperature Structural Materials Application for Void Growth Simulation under Creep Condition”, Technical Report Q07011 (in Japanese)＊1：T.Ogata, 2003, “Clarification of creep void growth behavior and development of void growth simulation program”, CRIEPI Report T03007(in Japanese)＊ 2：Constitutive model to calculate inelastic deformation amount from movement of dislocation in crystal grain

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6. Fossil Fuel Power Generation(1) Mesh Generation for Crystal Grain(2) Elastic Stress Analysis(3) Crystal Plasticity Stress Analysis

Fig.2 Analysis model(result of automatic mesh generation)(1) All crystal grains are divided into finite element meshes.
(2) Elastic stress analysis is carried out.
(3) Crystal plasticity stress analysis is performed.

Tensile stress of 180MPa was applied in horizontal direction
under temperature of 580 ℃ and kept for 5000 hours.
Experimental creep rupture time under the same condition wasabout 4200 hours.

Fig.3 Stress distribution in applied loadingdirection(after 5000 hours stress duration)

Fig.4 Creep strain distribution in applied
loading direction(after 5000 hours stress duration)

Stress distribution became inhomogeneous because the crystal
grains mutually constrained creep deformation in each other.

By performing a crystal plasticity analysis, it became possible
to simulate the band of creep strain concentration.

Stress was maximum in elastic deformation.

Calculated Results by Proposed Method

Stress was maximum in creep deformation

Void Observation Results

250200150

Void Length (o m)Stress (MPa)Fig.1 Flow of grain boundary local stressanalysis100
50002000

4000

1412108642001000

2000

3000

4000

5000

Elapsed Time (Hour)

Elapsed Time (Hour)Fig.5 Time change of grain boundary normal stressFig.6 Time change of maximum void length

The normal stress of one grain boundary was the maximum atfirst but gradually reduced with the progress of creep
deformation. On the other hand, that of another grain boundary
was the minimum at first but increased oppositely.

By considering grain boundary local stress condition, it became
possible to obtain creep void simulation results that agreed wellwith the experimental observation results.

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