corrected for concentric, angular, and rotational misalignment. For the experiments we report here, alignment was performed using a cruciform shaped alignment specimen instrumented with 32 strain gages. The maximum misalignment was less than 50 με over all degrees of freedom as measured at 5 kN load and no-load conditions. The MTS controller has been verified to not move the specimen center more than 5 μm during 1 Hz out of phase cyclic loading, which represents a worst-case intended scenario. For the quasi-static loading used in the X-ray experiment, sample center position is held constant to better than 1 μm during loading. The sample is centered in the X-ray beam using a 6-axis goniometer. The top rotation stage is used to rotate the load frame 360 as each data point is recorded. For the experiments we report here, X-ray data collected during over 280 of this rotation is free from any shadowing from the load frame or grips. Furthermore, the “drift” of the sample center during the rotation was less than 5μm over the entire 360 rotation. To achieve this result, stress in the hydraulic hoses was minimized by hanging them above the load frame in a helical configuration, which expanded and contracted uniformly during rotation in a manner that did not impose a bending moment on the goniometer. The DIC assembly was mounted on an actuator with better than 1μm position repeatability that allowed it to extend into focal position to capture images and retract to prevent interference with load frame rotation. 7.2.2 Finite Element Analysis Approach Finite element analysis (FEA) using ABAQUS [26] was used to design the specimen assuming isotropic linearly elastic material behavior with both aluminum 6061-T6 and fully annealed 4140 tool steel properties as inputs. Specimen symmetry allowed for one half-thickness quadrant of the geometry to be modeled with mirror boundary conditions, as is displayed for all FEA results (Figs. 7.4, 7.5, and 7.7). Displacement boundary conditions were used at the grips. Mesh convergence study was performed to ensure adequate element size. A minimum 10-element through-thickness was used to allow for out of plane stresses. The FEA elastic behavior was compared with an isotropic linearly elastic plane-stress analytic calculation for an infinite plate. Plane-stress comparison is supported by FEA results, as in-plane stresses in the gage were at least four orders of magnitude greater than out-of-plane stresses at all loads. Fig. 7.1 Experimental setup in the 1-ID hutch of the APS at Argonne National Lab showing (1) the area detector, (2) the specimen mounted in the planar biaxial load frame, (3) the final X-ray beam collimation slits before it penetrates the sample, (4) and the DIC setup in image capturing position 7 ANewIn Situ Planar Biaxial Far-Field High Energy Diffraction Microscopy Experiment 63
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