A new method for quantifying anisotropic martensitic transformation strains accumulated during constrained cooling

Martensitic phase transformations during welding can play a major role in determining the final residual stresses and they can be anisotropic if the transformation occurs under stress. Traditionally, the Satoh test has been used to quantify the response, but it suffers from the fact that the temperature is not uniform along the specimen length, making it difficult to interpret the data. This shortcoming is overcome in our new experimental method using digital image correlation (DIC) to quantify the temperature dependent evolution of the transformation strain locally both parallel and perpendicular to an applied load, in this case for a high-strength low alloy (HSLA) steel and a tough, low transformation temperature weld consumable designed to mitigate tensile weld residual stresses. The method is able to separate the volumetric component of the transformation strain from the deviatoric transformation plasticity component. The volumetric component is shown to be independent of applied load, while the deviatoric component varies approximately linearly with applied load. For the HSLA steel studied here the method also reveals that the transformation start temperature rises under both tensile and compressive loading, confirming previous work. From a weld modelling viewpoint our method provides sufficient information to include the stress dependency of the anisotropic transformation strain in numerical finite element models of the weld process

Back-stresses and geometrical hardening as competing mechanisms enhancing ductility in HCP metals

By recourse of computational mechanics, back-stresses are unveiled as a major source for the increase in work hardening during forming of hexagonal close-packaged (hcp) metals. Polycrystalline visco-plastic self-consistent (VPSC) and crystal plasticity finite element modelling (CPFEM) simulations of tensile uniaxial experiments were used along with experimental texture information. Simulations took into account the analogous variation in the critical resolved shear stress (CRSS) values of each slip family that could result from an increase in the test temperature. As the CRSS ratio between secondary and primary slip families increased, two different contributions to the variation of the work hardening rate were observed depending on the simulation framework: (i) a decrease in the work hardening rate in VPSC simulations attributed to texture evolution or geometrical hardening and (ii) an increase in the work hardening rate in CPFEM simulations due to back-stresses. While geometrical hardening is present in both simulation frameworks, only CPFEM is able to capture the influence of back-stresses on the increase of the work hardening rate with temperature. The results provided here contribute to a better understanding of the deformation mechanisms present in warm forming of hcp metals, showing also that CPFEM is a better simulation framework to study warm forming of hcp metals.publishe

Back-stresses and geometrical hardening as competing mechanisms enhancing ductility in HCP metals

Inside the folder "Textures" researchers can find the sets of orientations obtained through EBSD for the three HCP materials studied in the publication "Back-stresses and geometrical hardening as competing mechanisms enhancing ductility in HCP metals" The Origin file data_for_curves.opj contains the data coming from VPSC and CPFEM simulations of uniaxial tensile tests on the aforementioned HCP metals