Yoshiko Nagumo

Analysis of damage behaviour based on EBSD method under creep–fatigue conditions for polycrystalline nickel base superalloys

Abstract In order to detect creep–fatigue damage before the crack initiation stage, an investigation into damage behaviour based on the electron backscattered diffraction (EBSD) method for a polycrystalline nickel base superalloy has been carried out. The average misorientation within grains increases linearly up to the initiation of cracks with the increase in creep strains. Even if stress distributions exist by stress concentration, assessment in all the damage areas allowed the evaluation of creep damage regardless of geometrical influence. Furthermore, the influence of strain rates, introducing fatigue and testing temperatures are hardly observed in the misorientation analysis. Misorientation almost corresponded to inelastic strain regardless of those influences. Quite a small misorientation caused by fatigue led to an equivalent result between creep conditions and creep–fatigue conditions. It is concluded that the misorientation analysis of damaged materials based on the EBSD method allows the quantitative estimation of creep strain and the assessment of remaining creep fracture life.

Geometrical influence for creep damage evaluation of IN738LC using electron backscatter diffraction

Abstract The Ni-based superalloy IN738LC is an outstanding heat-resistant material, but it does have the disadvantage of being brittle. Therefore creep degradation such as deformation of components or initiation of microscopic creep voids only appear at the end of its available creep life. To date, electron backscatter diffraction (EBSD) in conjunction with scanning electron microscopy has been used for damage evaluation. Some results showed that the misorientation was correlated with the magnitude of macroscopic plastic or creep strains. In this study, by using the EBSD method, the characteristics of the change and distribution tendencies of crystallographic misorientation on creep life have been investigated. In addition, the geometrical influence of stress concentrated areas for misorientation analysis has been investigated, which is important to evaluate damage in practical components like blades. As a result, the grain reference orientation deviation parameter which corresponds to the spread of creep strain has been found to be useful to estimate creep life among the misorientation parameters. Also, misorientation analysis allows the evaluation of creep damage before the appearance of obvious defects without being subjected to the geometrical influence of the stress concentrated areas.

Theoretical Representations of a Characteristic Curved Surface for the Life of IN738LC Under High Temperature Creep and Fatigue Interaction Based on Non-Equilibrium Science

The Ni-base supperalloy IN738LC, developed as a gas turbine blade material, is used under the conditions of creep-fatigue multiplication. In this paper, using IN738LC, in situ observational tests under the conditions of creep-fatigue multiplication were conducted and the effects of cycle-dependent and time-dependent mechanisms on the fracture life tf were investigated. Furthermore, on the basis of the concept of non-equilibrium science, the multiple effects of creep and fatigue on the fracture life tf were clarified.© 2009 ASME

Continuum damage mechanics modeling of composite laminates including transverse cracks

In this study, the continuum damage mechanics model for predicting the stiffness reduction of composite laminates including transverse cracks is formulated as a function of crack density. To formulate the model, first the damage variable in the direction normal to the fiber of a ply including transverse cracks is derived. The damage variable is derived by the model assuming a plane strain field in the isotropic plane and using the Gudmundson–Zang model for comparison. The effective compliance based on the strain equivalent principle proposed by Murakami et al. and classical laminate theory are then used to formulate the elastic moduli of laminates of arbitrary lay-up configurations as a function of the damage variable. Finally, the results obtained from this model are compared to the finite-element analysis reported in previous studies. The model proposed in this paper can predict the stiffness of laminates containing damage due to transverse cracks (or surface crack) from just the mechanical properties of a ply and the lay-up configurations. Furthermore, this model can precisely predict the finite-element analysis results and experiment results for the elastic moduli of the laminate of arbitrary lay-up configuration, such as cross-ply, angle ply, and quasi-isotropic, including transverse cracks. This model only considers the damage of the transverse crack; it does not consider damage such as delamination. However, this model seems to be effective in the early stage of damage formation when transverse cracking mainly occurs. The model assuming plane strain field in the isotropic plane which is proposed in this paper can calculate the local stress distribution in a ply including transverse cracks as a function of crack density. The damage evolution of transverse cracks can thus be simulated by determining the fracture criterion.

Creep damage evaluation of IN738LC based on the EBSD method by using a notched specimen

The Ni-base superalloy IN738LC developed as a heat-resistant material has excellent high temperature strength, but at the same time it has a brittle property. As a result, any evidence of creep degradation such as deformation of components or initiation of creep voids has hardly made an appearance by the end of its available creep life. This paper investigates the possibility that creep damage can be evaluated by using the electron backscatter diffraction (EBSD) method to analyze misorientation which is considered to be caused by creep strain. The use of an in-situ observational testing machine with a notched specimen allowed creep-EBSD round tests which consisted of creep tests in the in-situ obser- vational testing machine and EBSD observations using scanning electron microscopy (SEM) alternately against an identical specimen. The outcome of these experiments was that crystal orientation change tendencies corresponding to the creep strain were continuously observed. It has been found that there was a good correlation between the creep strains and the misorientations referenced to the average orientation in the grain, the parameter of Grain Reference Orientation Deviation (GROD) defined by TSL EBSD system. Consequently, this study shows that the creep damage could be estimated by analyzing misorientations using the EBSD system automatically. Also, it has been found that crystal orientation changes appeared around notches in the direction of maximum shearing stress, which implies that the misorientation development was caused by the crystallographic lattice rotation.

The Occurrence Mechanism of Periodicity of Creep Crack Path for P92

The morphology of creep crack growth in metals is usually dominated by inter-granular cracking. However, for developed heat-resistant steels with a martensitic lath strengthening structure such as ASME Grade P92, inter-granular cracking, which is a typical fracture mechanism of creep crack growth, was restrained, and the creep crack growth path was found to be composed of a fracture unit area (FA) beyond the scale of the grain size. In spite of the fact that FA is closely related to resistance against crack growth, the occurrence mechanism of FA has not yet been clarified. In this study, by conducting creep crack growth tests for P92, two-dimensional finite element analyses, and metallurgical characterization of the microstructure, the occurrence mechanism of FA for P92 was clarified. As a result, the creep crack growth path for P92 was found to be composed of an FA beyond the scale of the grain size due to the sub-critical crack growth, and the periodic sub-critical crack growth was found to be dominated by the preceding initiation of voids along a slip line. On the basis of the proposed finite element method analysis, this crack growth mechanism was found to cause dispersion of creep damage, which causes unstable crack growth and results in periodic convexo-concave crack growth, that is, FA-cracking.

Prediction for progression of transverse cracking in CFRP cross-ply laminates using Monte Carlo method

This study predicted transverse cracking progression in laminates including 90° plies. The refined stress field (RSF) model, which takes into account thermal residual strain for plies including transverse cracks is formulated, and the energy release rate associated with transverse cracking is calculated using this model. For comparison, the energy release rate based on the continuum damage mechanics (CDM) model is formulated. Next, transverse cracking progression in CFRP cross-ply laminates including 90° plies is predicted based on both stress and energy criteria using the Monte Carlo method. The results indicated that the RSF model and the CDM model proposed in this study can predict the experiment results for the relationship between transverse crack density and ply strain in 90° ply. The models presented in this paper can be applied to an arbitrary laminate including 90° plies.

The characterization of dominating region of fracture (process region) around a crack tip based on the concept of mechanical similarity and atomic mechanics

Brittle fracture at a crack tip is considered to be caused within a specified local region, that is, process region. However, the determination of this scale has not yet been theoretically clarified. The difficulty of this determination will be due to the wide range scale analysis from the range of nano scale (atomic scale) to macro scale (crack size scale). In this paper, on the basis of the atomic mechanics using super atom and hybrid method of the fractal concept which concerns self-similarity with the proposed analysis of scale projection from macro to nano scales, the disturbed region of atom arrangements around a crack tip were clarified under the local stress field by crack and dislocation. This disturbed region was related to fracture dominating region (process region) which is in good agreement with the scale obtained from experimental consideration by R. Ritchie.

Effect of Material Microstructure on Creep Damage Formation Behavior for Ni-Base Directionally Solidified Superalloy

Ni-base directionally solidified superalloy strengthened by γ′ precipitates have been developed as a gas turbine blade. However, it is difficult to detect creep damage such as creep voids by conventional observation techniques. It is important to clarify the creep damage behavior for Ni-base superalloy.In this study, creep crack growth tests for Ni-base directionally solidified superalloy CM247LC were conducted using the in-situ observational system. Additionally, the metallographical investigation was conducted on crept specimen using EBSD analysis and relationship between creep crack growth path and material microstructure were clarified. And, in order to clarify the difference of creep crack growth behavior, the designed two-dimensional elastic-plastic creep finite element analyses were conducted for the model with grain distribution obtained by EBSD analysis.Copyright

Scale effect of material structure on the similarity of subcritical crack growth under sustained loading

The irregularity of shapes of fractured units with various lengths in a creep crack path on creep crack growth re- sistance were characterized based on the concept of the fractal and scaling effect. Furthermore, the general validity of this characterization for subcritical crack growth was proposed by comparing the obtained results with those in rock fracture. As a result, both the fracture unit length denoted as FA of creep cracking in metals and the characteristic scale of rock fracture were found to have fractal dimensions that show both have the same scale effect. These results will promote that the possibility of a unified scientific description of the effect of geometrical morphology of various subcritical crack growth patterns will be possible on the basis of the concept of the fractal and characteristic scale effect.