Property of Fatigue Crack Initiation and Propagation Behavior of a Coarse-Grain Ni Alloy

Low cycle fatigue tests at elevated temperature were conducted on a Ni-based directionally solidified superalloy subjected to transverse loading. To investigate the effect of the arrangement and the orientation of crystal grains on the crack initiation, the electron back-scatter diffraction (EBSD) method was applied on the surface of the tested specimens. In addition, finite element analysis that considered the plastic behavior of crystal grains and the crystal grain shape on the specimen surfaces was performed to evaluate the relationship between the crack initiation and the local stress that develops near the grain boundaries. The results are summarized below. As for the effect of crystallographic properties in the specimen surface, many cracks initiated near the grain boundaries that neighbored the grains whose secondary axis was inclined to the normal direction of specimen surface by more than 35 degrees because the longitudinal stress larger than the nominal stress developed near those boundaries. The crack location was not confirmed to correlate with the grain boundary misorientation between the neighboring grains on the surface. However, most cracks initiated near the grain boundaries that neighbored the grain whose secondary axis was more inclined than the grain located in the opposite surface. The results of FEM analysis revealed that high stress developed near those grain boundaries. In addition, the location where high resolved shear stress developed generally corresponded to the crack location and that it correlated with the number of crack initiation cycles.

Effect of crystallographic properties on low cycle fatigue crack initiation at elevated temperature for Ni-based directionally solidified superalloy

Yokoyama¹,

Sekihara²

2016

Effect of crystallographic properties on low cycle fatigue crack initiation at elevated temperature for Ni-based directionally solidified superalloy with small holes

Yokoyama

Sekihara

2016

Low cycle fatigue tests at elevated temperature were conducted on a Ni-based directionally solidified superalloy with small holes subjected to transverse loading. To investigate the effect of the arrangement and the orientation of crystal grains on the crack initiation, the electron back-scatter diffraction (EBSD) method was applied on the surface of the tested specimens. In addition, finite element analysis that considered the plastic behavior of crystal grains was performed to evaluate the relationship between the crack initiation and the local stress that developed along the hole edges. The results are summarized below. It was shown that the number of cycles to crack initiation tended to be small along the hole edges located on the grain boundaries by classifying the hole edges according to the position of the hole edge and the grain boundary. In addition, it was small along the hole edges located in the grain whose secondary axis was more inclined than the grain located in the opposite surface. The results of FEM analysis revealed that high stress developed along those hole edges. The location where high stress developed correlated with the number of crack initiation cycles not only among the hole edges in one specimen but also among the specimens subjected to different nominal strain range.

Evaluation of pulsating tension fatigue strength and influence of microstructure on small crack initiation and growth behaviors of Ni based castalloy 246

Kuramitsu¹,

Fujii

Hasunuma

et al. 2017

The purpose of this study is to investigate low cycle fatigue (LCF) and high cycle fatigue (HCF) strengths, crack initiation and growth behaviors of Ni based castalloy 246. LCF and HCF tests are conducted under load controlled conditions. As the result of the observation of etched specimen, the grain size is from 3 mm to 5 mm. And the microstructure is mainly composed of dendritic structure. Growth orientation of dendritic structure varies in every grain. The result of elemental analysis, Si and Ti are detected at dendritic boundary. The results of LCF test show that small cracks initiate at the first cycle, followed by the crack growth along slip direction. Observation of fracture surface reveals that small cracks originate at casting defect or dendritic boundary. The results of HCF, on the other hand, show that small cracks initiate from subsurface casting defect under  ≦ 600 MPa, while fracture origin becomes surface under  > 600 MPa. The result of HCF test after introducing small crack on the surface shows that small crack on the surface does not affect on the fatigue life and the location of crack initiation. As the result of EBSD, the crack initiation site is near grain boundary and the crack grows along slip plane.