Stress Concentrations At Grain Boundaries DueTo Anisotropic Elastic Material Behavior

Scanning electron microscopy (SEM) examinations on samples of the (3-titanium alloy LCB (low cost beta by TIMET) after fatigue tests revealed a strong influence of the crystallographic misorientation of adjacent grains on crack initiation and short crack growth. From that point of view the knowledge of the anisotropic elastic properties of individual grains and their influence on the stress distribution within the microstructure is of great importance to understand crack initiation and propagation mechanisms. For this purpose the microstructure of polycrystals at sites where cracks initiated was simulated by means of three dimensional finite element (FE) calculations, considering the elastic anisotropy of the grains. However, to ensure a realistic simulation, experimental data like the grain geometry and crystallographic orientation were used, which had been measured by the electron back-scattered diffraction (EBSD) technique. Beside this, the values of the elastic constants of the single-crystalline material were required. In this study a method was developed and applied to determine the three elastic constants of the anisotropic bcc structure of single-crystalline p-Titanium (solution annealed LCB) from experimental data of polycrystals. The results of local strain measurements by ISDG (inter ferometric strain-displacement gauge) and EBSD evaluation of the microstructure as well as the Young's modulus and the Poisson's ratio of the polycrystal serve as input parameters for this method. The calculated anisotropic elastic constants allow the application of the above-mentioned FE-simulation to the microstructure of the polycrystalline alloy LCB. SEM examinations of the fatigued samples show that short cracks initiate exactly at the locations where the FE-simulation yielded the highest stresses. Damage & Fracture Mechanics VI, C.A. Brebbia, A.P.S. Selvadurai, (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-812-0