Using the local stress tensor from CPFE, the large Schmid factor gradients in most grains directly connect SEM contrast variations to heterogeneous stress and strain.

Quantifying Local Shears in Pure Ti grains with SEM and Crystal Plasticity Finite Element (CPFE) Simulation to Assess Damage

Thomas R. Bieler, Michigan State University, DMR 1108211

CPFE provides a way to estimate the local stresses that are difficult to measure experimentally. The simulation shows semi-quantitative agreement with shears measured in prior NSF MWN sponsored work, and the simulation provides an estimate of the local stress tensor.

Changes in contrast with strain have been quantified by taking fast Fourier transforms (FFTs) of SEM images of poly-crystalline patches from a tensile specimen deformed sequentially in-situ. The resulting peak width at half max has been calibrated to these images, i.e., FWHM = f().

Basal slipPrism slip

Quantifying Local Shear in Pure Ti grains with SEM and Crystal Plasticity Finite Element (CPFE) Simulation to Assess Damage

Thomas R. Bieler, Michigan State University, DMR 1108211

BSE images show changes in channeling contrast with strain

30 30 mm

Evolution of mesoscale plastic strain ahead of a fatigue crack in pure Ti was quantified using the FFTs of backscattered electron contrast in a 15x15 array of images collected in the crack tip region. This characterization of the strain field was compared to the zone of plastic work measured using thermal-elastic stress analysis (TSA) using a sensitive thermal camera. Comparison with analytical solutions shows no agreement with the Von Mises analysis, but good agreement with Dugdale and Irwin analyses.

Fatigue Crack tip

These results indicate that both TSA and the approach developed here are reasonably robust and can be applied to a broader range of loading geometries, providing a diagnostic measure of damage that can impact the design of complex industrial loading and forming operations where accurate measurements of strain fields and damage is required.

Before Fatigue