Measuring Stress Distributions in Ti-6Al-4V Using Synchrotron X-Ray Diffraction

Abstract

This article presents a quantitative strain analysis (QSA) study aimed at determining the distribution of stress states within a loaded Ti-6Al-4V specimen. Synchrotron X-rays were used to test a sample that was loaded to a uniaxial stress of 540 MPa in situ in the A2 experimental station at the Cornell High Energy Synchrotron Source (CHESS). Lattice-strain pole figures (SPFs) were measured and used to construct a lattice strain distribution function (LSDF) over the fundamental region of orientation space for each phase. A high-fidelity geometric model of the experiment was used to drastically improve the signal-to-noise ratio in the data. The threedimensional stress states at every possible orientation of each a (hcp) and b (bcc) crystal within the aggregate were calculated using the LSDF and the single-crystal moduli. The stress components varied by 300 to 500 MPa over the orientation space; it was also found that, in general, the crystal stress states were not uniaxial. The maximum shear stress resolved on the basal and prismatic slip systems of all orientations within the a phase,ŝ rss ; was calculated to illustrate the utility of this approach for better identifying ''hard'' and ''soft'' orientations within the loaded aggregate. Orientations with low values ofŝ rss ; which are potential microcrack initiation sites during dwell fatigue conditions, are considered hard and were subsequently illustrated on an electron backscatter diffraction (EBSD) map.

Experimental Characterization and Crystal Plasticity Modeling of Heterogeneous Deformation in Polycrystalline α-Ti

“…[7,40] This approach was recently used to measure the stress distribution over the orientation space in a Ti6Al-4V tensile specimen, where the magnitude of variations of some stress components over the orientation space was found to be comparable to the applied tensile stress. [41] Alternatively, it is possible to adapt this approach to obtain the local stress tensor information of specific grains and then compare the result with the CPFEM simulation. Having credible knowledge of the stress distribution in a microstructure from a simulation would be very helpful to predict possible damage nucleation sites in the specimen.…”

Crystallography of Fatigue Crack Initiation and Growth in Fully Lamellar Ti-6Al-4V

“…Thus, higher values of N correspond to grains that have the required combination of forces resolved on the basal plane that have been found to promote crack formation. While it is recognized that the grain-level stress state is actually orientation dependent and does not necessarily match the applied ''uniaxial'' stress state, [44] a simplified analysis of this nature can be used as a first approximation. Imposing this criterion on the orientations presented in Figure 11 identified all of the colonies that formed cracks, namely 1, 2, 5, 7, and 8.…”

Optimal polyhedral description of 3D polycrystals: Method and application to statistical and synchrotron X-ray diffraction data

“…In the past 15 years, large efforts have been conducted to analyse experimentally the local deformation of polycrystalline materials in 3D using synchrotron X-ray diffraction. These methods provide both the grain structure of polycrystalline materials [8,9] and the local stresses and lattice rotations [10][11][12][13] that develop as they are deformed. Numerical methods are available to simulate these phenomena [14][15][16][17][18][19][20][21][22], e.g.…”