T. Lorentzen

Self-consistent modelling of the plastic deformation of F.C.C. polycrystals and its implications for diffraction measurements of internal stresses

Using a self consistent scheme we model the development of elastic lattice strains during uniaxial loading for selected families of grains with specific orientations. These lattice strains vary dramatically for the different grain orientations, and most families of grains show a high degree of non-linearity at the start of the plastic regime. The 311 reflection does, however, respond almost linearly to loading, and therefore it constitutes a suitable reflection for characterization of macroscopic stresses and strains by diffraction for the given conditions. As a consequence of the high degree of non-linearity in the lattice strain response during loading highly anisotropic intergranular residual lattice strains develop during unloading. The evaluation of the model predictions by neutron diffraction is exemplified by selected results from in-situ loading experiments performed on austenitic stainless steel specimens. As a necessary condition for the proper understanding of the results we have included a description of the slip pattern resulting from the model applied and its relation to the slip patterns derived from the upper-bound Taylor model and the lower-bound Sachs model.

Lattice strain evolution during uniaxial tensile loading of stainless steel

Applied and residual lattice strains were determined by neutron diffraction during a tensile test of a weakly textured austenitic stainless steel and were compared to the predictions of a self-consistent polycrystal deformation model. Parallel to the tensile axis the model predictions are generally within the resolution of the diffraction measurements, but perpendicular to the tensile axis discrepancies are noted. Discrepancies between model and measurements were greater for the residual lattice strains than during loading. It is postulated that this is because the model does not predict reverse plasticity during unload.

Characterization of residual stresses generated during inhomogeneous plastic deformation

Abstract Residual stresses generated by macroscopic inhomogeneous plastic deformation are predicted by an explicit finite element (FE) technique. The numerical predictions are evaluated by characterizing the residual elastic strains by neutron diffraction using two different (hkl) reflections. Intergranular residual elastic strains between subsets of grains are predicted numerically and verified by neutron diffraction. Subsequently, the measured residual strain profiles in the test samples are modified by the intergranular strains and compared to the engineering predictions of the FE technique. Results compare well and verify the capability of the numerical technique as well as the possibilities of experimental validation using neutron diffraction. The presented experimental and numerical approach will subsequently be utilized for the evaluation of more complicated plastic deformation processes resembling forming operations.

Self thermal-plastic response of NiTi shape memory alloy fiber-actuated metal matrix composites

The present work develops a quantitative theory of the self thermal-plastic response of NiTi shape memory alloy actuated metal matrix composite materials. Model calculations are compared with existing experimental data obtained from a testing procedure consisting of an initial room temperature, 5% tensile elongation process, and a subsequent room temperature to 120 degree(s)C unconstrained (external stress free) heating process. During the unconstrained heating process the composite fiber actuators attempt to recover pseudo-plastic strain imparted during the room temperature tensile prestrain process. As the temperature increases, the fiber stress-temperature state enters increasing phase transformation intensity, resulting in strong increases in fiber longitudinal tensile stress, matrix longitudinal compressive stress and composite compressive longitudinal external strain. Sufficient temperature brings the matrix stress state to the point of plastic yield. The composite then exhibits a very unusual, self thermal-plastic compression response, recovering approximately 2.2% strain.

Thermal deformation behavior of a NiTi actuated aluminum metal-matrix composite

The present work reports macroscopic thermal mechanical and in-situ neutron diffraction measurements from a 22.9 volume percent, 50.7 at percent Ni-Ti fiber actuated 6082-T6 aluminum matrix composite and 6082-T6 homogeneous aluminum control material subjected to an initial room temperature 4 percent tensile elongation and unloading process followed by a subsequent room temperature to 120 degrees C unconstrained heating process. During the unconstrained room temperature to 120 degrees C heating process, the composite exhibited a pronounced, nonlinear thermal contraction, while the homogeneous control exhibited the expected linear thermal expansion. The composite thermal compression was clearly the result of a powerful shape memory response in the NiTi fiber actuators.