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New designs of lattice cores based on the mathematically-deduced sheet-triply periodic minimal surface is proposed in this work. Elastic-plastic simulations are performed to evaluate the superiority of the novel designs based on their quasi-static loading compressive properties.

The industrial application of high strength steels in structural components has increased the demand on understanding the ductile failure behavior of this type of materials. In practical situations the loading experienced on components made out of these materials can be very complex, which may affect the failure behavior. The objective of this work is to study the effect of stress state on ductile failure and the mechanisms leading to rupture in high strength steels. The stress state is characterized by the stress triaxiality T and the Lode parameter L, which is a deviatoric stress state parameter that discriminates between axisymmetric or shear dominated stress states. For this purpose experiments on two different specimen configurations are performed; a double notched tube (DNT) specimen tested in combined tension and shear and a round notched bar (RNB) specimen tested in uniaxial tension. The two specimens give rise to different stress states at failure in terms of T and L. The failure loci for the DNT specimen show an abrupt change in ductility, indicating a transition between the rupture mechanisms necking of intervoid ligaments and shearing of intervoid ligaments. A clear difference in ductility between the two specimen configurations is also observed, which is closely associated with the difference in stress state at failure. A micromechanical model is developed, which assumes that ductile material failure occurs when the deformation becomes highly non-linear and localizes into a band. The model, which is applied to analyze the experiments, consists of a band with a square array of equally sized cells, with a spherical void located in the center of each cell. The model, extended with a shear criterion, captures the experimental trend rather well. The model also shows that the effect of the deviatoric stress state (L) on void growth, void shape evolution and coalescence is significant, especially at low levels of T and shear dominated stress state. QC 20100621

This licentiate thesis is generally concerned with the ductile failure and rupture mechanisms encountered under combined tension and torsion loading. In the first part entitled Paper A, an experimental investigation of the rupture mechanisms in a mid-strength and a high strength steel was conducted employing a novel test configuration. The specimen used was a double notched tube specimen loaded in combined tension and torsion at a fixed ratio. The effective plastic strain, the stress triaxiality and the Lode parameter was determined in the centre of the notch at failure. Scanning electron microscopy of the fractured surfaces revealed two distinctively different ductile rupture mechanisms depending on the stress state. At high stress triaxiality the fractured surfaces were covered with large and deep dimples, suggesting that growth and internal necking of voids being the governing rupture mechanism. At low triaxiality it was found that the fractured surfaces were covered with elongated small shear dimples, suggesting internal void shearing being the governing rupture mechanism. In the fractured surfaces of the high-strength steel, regions with quasi-cleavage were also observed. The transition from the internal necking mechanism to the internal shearing mechanism was accompanied by a significant drop in ductility. In the second part entitled Paper B, a micromechanics model based on the theoretical framework of plastic localization into a band introduced by Rice is developed. The model employed consists of a planar band with a square array of equally sized cells, with a spherical void located in the centre of each cell. The periodic arrangement of the cells allows the study of a single unit cell for which fully periodic boundary conditions are applied. The micromechanics model is applied to analyze failure by ductile rupture in experiments on double notched tube specimens subjected to combined tension and torsion carried out by the present authors. The stress state is characterized in terms of the stress triaxiality and the Lode parameter. Two rupture mechanisms can be identified, void coalescence by internal necking at high triaxiality and void coalescence by internal shearing at low triaxiality. For the internal necking mechanism, failure is assumed to occur when the deformation localizes into a planar band and is closely associated with extensive void growth. For the internal shearing mechanism, a simple criterion based on the attainment of a critical value of shear deformation is utilized. The two failure criteria capture the transition between the two rupture mechanisms successfully and are in good agreement with the experimental result. QC 20101109