Neutron residual stress analysis for multiphase materials with depth gradients of the strain free / independent lattice parameter D0

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The aim of the research project for the continuation period remains unchanged and covers the investigation on the influence of phase-specific micro residual stresses on the determination of the reference lattice parameter D0 for coarse multi-phase materials and to provide appropriate measuring and evaluation strategies. For non-destructive analysis of residual stress depths gradients using neutron diffraction the knowledge of this reference value is of high importance. Hereby, the focus is on the realization of neutron through surface strain scans by using very small depth increments with the particular aim to determine residual stress depth distributions non-destructively that are very close to the surface. Here, the prediction and the correction of the surface effects and the associated spurious strains are of vital importance. In the first funding period fundamental investigation on the development of phase-specific residual stresses for coarse two-phase duplex steels have been carried out. The results indicate that phase-specific textures have an immense impact on the development of phase-specific (residual) stresses and strongly affect the analyses. Moreover, it was shown that the fundamental understanding of the micromechanical processes in the materials volume at elasto-plastic materials loading is of enormous importance for the understanding of residual stress development and particularly for the correct residual stress evaluation and for the assessment of the results. Intergranular strains bring out highly non-linear relationships between the diffraction based determined {hkl} dependent lattice strains and the mechanical materials loading. Within the scope of the project continuation a self-consistent material model for the numeric prediction of the plastic anisotropy effects will be formulated and extended to the application on coarse two-phase, textured duplex steels. For the validation of the model and for the assessment of grain orientation dependent strains and stresses supplementary phase-specific strain polefigures will be determined using neutron diffraction. Furthermore, the further development of the simulation routine SIMRES for calculating spurious strains caused by the surface effects during through surface neutron strain scanning will be subject of future investigations. In the continuation project the existing model will be extended to be able to handle also phase-specific textures and texture gradients as well as chemical gradients (D0-Gradients). These enhancements of the simulation software will be realized in close collaboration with our Czech partner J. Šaroun. Finally, at the end of the project period the residual stress analyses carried out for the mechanically surface treated duplex steels and the case hardened stated will be re-evaluated using the implemented software enhancements and the final results will be compared with residual stress distributions that are determined using complementary methods.