PSI - Issue 14

Aman Arora et al. / Procedia Structural Integrity 14 (2019) 790–797 Aman Arora/ Structural Integrity Procedia 00 (2018) 000–000

792

3

corresponding strain 1%, to minimum displacement 10 µm, corresponding strain 0.1%, which turns out to be positive strain ratio of 0.1. Strain hardening was observed for consecutive cycles as can be seen in Fig. 2. Fatigue test was interrupted after every 25 cycles to see whether cracks have nucleated or not. Tests were stopped on detection of the first cracks site in both cases. Initiation sites were then correlated to the EBSD measurements.

Fig.1. Detail drawing of shallow notch nickel specimen (All dimensions are in mm)

200

150

100

50

0

-50 Stress (MPa)

-100

-150

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Strain (%)

Fig. 2. Cyclic stress-strain curve of first 20 cycles

3. Computational analysis Analysis of the microstructure of fatigue specimen after fatigue crack initiation was performed using EBSD maps. EBSD data was post analysed by Bruker’s Esprit 2.1 for IPF, misorientation and grain boundary analysis. MTEX algorithm was used for schmid factor and elastic modulus calculations. Classification of grains was done with 5 0 tolerance with grain orientation averaged in Hough space with pixel size of 76 nm. Effective elastic modulus was calculated parallel to loading direction using compliance matrix. Maximum schmid factor for {111} <110> slip system with respect to loading direction was calculated for each grain according to their average orientation angles. Grain boundaries were characterized according to misorientation angle between two neighbouring grains with an allowable angular deviation from the Brandon criterion used to characterize special coincident site lattice (CSL) grain boundaries. To visualize plastic deformation with relatively high resolution, Kernel Average Misorientation (KAM) maps extracted from EBSD data allow correlation of local lattice rotations to the deformation in crystalline materials. Local change in orientation within the grain is due to generation of GNDs during plastic deformation (Deng and Barnoush 2017).