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Metastable austenitic stainless steel (304L) samples with a rectangular cross-section were plastically deformed in torsion during which they experienced multiaxial stresses that led to a complex martensitic phase distribution owing to the transformation induced plasticity effect. A three-dimensional characterization of the phase distributions in these cm-sized samples was carried out by wavelength-selective neutron tomography. It was found that quantitatively correct results are obtained as long as the samples do not exhibit any considerable preferential grain orientation. Optical microscopy, electron backscatter diffraction, and finite element modeling were used to verify and explain the results obtained by neutron tomography. Altogether, neutron tomography was shown to extend the range of microstructure characterization methods towards the meso- and macroscale.
Resistance spot welds were prepared on 3 mm thick sheets of continuous cast and rolled AZ31 magnesium alloy. The microstructure and composition analysis of weld nugget, heat affected zone (HAZ) and base metal were examined using optical and scanning electron microscopy (HR-SEM and EDS/X). The resistance spot welded magnesium alloy joints consist mainly of weld nugget and HAZ. The nugget contains two different structures, i.e. the cellular-dendritic structure at the edge of the nugget and the equiaxed dendritic structure in the centre of the nugget. The structure transition is attributed to the changes of solidification conditions. In the HAZ, grain boundary melting occurred and grain boundaries became coarse. It has been shown that hardness reduction in the weld nugget and HAZ compared with base metal is evident due to dendritic microstructure and grain growth, respectively. The results showed that spot welded joints have failed in interfacial mode under torsion and tensile–shear loading conditions. Digital image correlation during tensile–shear testing showed that low surface strains occur in the interfacial failure mode, because fracture and deformation happened primarily in the nugget area.
The torsional buckling of a cruciform column under compressive load with a vertex plasticity model
(2011)
The torsional buckling of a plastically deforming cruciform column under compressive load is investigated. The problem is solved analytically based on the von Kármán shallow shell theory and the virtual work principle. Solutions found in the literature are extended for path-dependent incremental behaviour as typically found in the presence of the vertex effect that is present in metallic polycrystals.
At the critical load for buckling the direction of straining changes by an additional shear component. It is shown that the incremental elastic–plastic moduli are spatially nonuniform for such situations, contrary to the classical J2 flow and deformation theories. The critical shear modulus that governs the buckling equation is obtained as a weighted average of the incremental elastic–plastic moduli over the cross-section of the cruciform.
Using a plasticity model proposed by the authors, that includes the vertex effect, the buckling-critical load is computed for a aluminium column both with the analytical model and a FEM-based eigenvalue buckling analysis. The stable post-buckling path is determined by the energy criterion of path-stability. A comparison with the experimentally obtained classical results by Gerard and Becker (1957) shows good agreement without relying on artificial imperfections as necessary in the classical J2 flow theory.