Postbuckling behavior and imperfection sensitivity of elastic-plastic periodic plate-lattice materials

Advances in additive manufacturing have enabled a new generation of materials with advantageous properties inherent to their architecture. Recently, architected materials with periodic arrangements of plates, called plate-lattice materials, have been developed to reach theoretical least upper bounds for stiffness and strength of isotropic materials. This work investigates the buckling behavior of several plate-lattice architectures with relative densities between 0.5% to 25% when subjected to uniaxial compression. Finite element unit cell models with shell elements and periodic boundary conditions are used to simulate the buckling and post-buckling behavior of plate-lattices made from an elastic-perfectly plastic parent material. Five plate-lattice architectures with cubic symmetry are investigated - three anisotropic architectures (simple cubic, body-centered cubic, face-centered cubic) and two isotropic architectures (simple cubic and body-centered cubic combination, simple cubic and face-centered cubic combination). Geometric imperfections of varying amplitude are applied to determine the imperfection sensitivity of these plate-lattice materials, and to calculate their buckling knockdown factors. Consistent with the behavior of the elastic-plastic plate building blocks that comprise the lattice, this investigation reveals that plate-lattice materials are generally imperfection insensitive when their constituent plates become thinner and first bifurcation occurs in the elastic range away from the yield stress. When this occurs, the initial post-buckling behavior is stable and the ultimate strength can be many times higher than the initial buckling stress. (C) 2021 Published by Elsevier Ltd.

Mechanical properties of replicated cellular Zn and Zn1.5Mg in uniaxial compression

Karel Lietaert, Andreas Mortensen, Ludger Weber

We present here a study of the mechanical behavior of cellular Zn and Zn1.5Mg with densities between 11.9 and 24.5%, produced by replication processing using NaCl as a spaceholder. Cellular Zn and Zn1.5Mg deform homogeneously during quasi-static compression. The plateau stress scales with the relative density according to a power-law of exponent of 2.8 for both materials; recorded values are 6.0 +/- 0.6 MPa for cellular Zn of relative density 24% and 10.0 +/- 0.5 MPa for cellular Zn1.5Mg of relative density 23%. In ambient-temperature creep tests with loads up to 50% of the plateau stress, after more than 70 h deformation remains negligible (in the range of experimental error). This observation, coupled with other measured mechanical properties, indicates that replicated cellular Zn and Zn1.5Mg are viable materials in the context of their application as a low load-bearing implant material.

ELSEVIER SCIENCE INC2019

Precipitation in the high strength AA7449 aluminium alloy

Patrick Schloth

Al-Zn-Mg(-Cu) (AA7xxx) alloys are widely used structural materials owing to their excellent mechanical properties and low corrosion susceptibility. The high strength of these materials is obtained by the formation of secondary phases, e.g. precipitation. This present thesis aims at contributing to the understanding of the influence of precipitation in the high strength AA7449 alloy on the internal stress formation at different length scales. Two different cases are studied in this thesis. In the first case, the influence of precipitation during quench on macroscopic residual stresses (RS) is studied in an industrial 75 mm AA7449 thick plate. For some investigations the behaviour of the AA7449 alloy is compared with the medium strength AA7040 alloy. In the second case, the influence of different precipitation states on the internal strain formation is investigated at the microstructural length scale. For the first case, in situ small angle X-ray scattering evidenced that during quenching the heterogeneous eta phase forms at high temperatures in contrast to homogeneous GP(I) zones, which form at low temperatures. The eta formation is influenced by the cooling conditions but also by macrosegregation in the thick plate. Yet, the volume fraction of eta is very low and has a negligible effect on the macroscopic RS in thick plates. The homogeneous GP(I) zones form during quench with radii in the subnanometre range. Their formation is strongly influenced by excess vacancies. Fast cooling rates can lead to higher radius and volume fraction of GP(I) zones compared to slower cooling. The GP(I) zone formation increases the yield strength during quench and is responsible for the high RS in industrial thick plates. Therefore, the GP(I) zone formation during quench needs to be taken into account in macroscopic finite element (FE) residual stress simulations by using precipitation modelling. The GP(I) zone formation during quench is simulated by using a thermodynamic based Eulerian multi-class model. Therefore, a thermodynamic description for GP(I) zones is derived from reversion heat treatments by using the solubility product. The influence of excess vacancies is taken into account by adapted effective diffusion coefficients. This simplified approach allows reproducing reasonably well the measured GP(I) zone formation during rapid coolings. The coupling of the precipitation model with FE residual stress calculations (performed by the other PhD studentt) allows predicting very well the RS in two AA7449 thick plates. For the second case, in situ mechanical testing during tensile deformation using X-rays and neutrons evidence that the presence of different precipitate types change the internal strain formation in the aluminium matrix at the microstructural length scale. The presence of small GP(I) zones leads to an internal strain formation that is dominated by the intergranular strains of the aluminium matrix. When large eta precipitates are embedded in the aluminium matrix the interphase strains clearly dominate the internal strain formation. Further, it is shown that the eta phase has a higher elastic modulus than the aluminium matrix and shows an anisotropic behaviour during elastic and plastic deformation. In addition, it is pointed out that the internal strain formation at the microstructural length scale can have a significant effect on macroscopic RS measurements performed by XRD when the plastically deformed sample contains eta' and eta.

EPFL2015

Investigations and numerical modeling of mechanical properties of tempered martensitic steel Eurofer97 at various loading rates, temperatures and after spallation irradiation

Serafin Knitel

The reduced activation tempered martensitic steel Eurofer97 is a reference steel for a structural applica-tions in fusion reactors. The first-wall and blanket materials will be subjected to high thermal and neutron fluxes and will experience very complex, time-dependent mechanical loading. Owing to impinging neutrons in materials, points defects and gasesous impurities (hydrogen and helium) are created and accummulated in the form of small defect clusters in the microstructure altering the mechanical properties. Notably, fracture toughness decreases as strength increases. Eurofer97, being a body centered cubic alloy, presents a ductile-to-brittle transition temperature T0 that shifts to higher temperatures after irradiation. The main objective was to study the loading rate and irradiation effects on plastic flow and fracture properties of Eurofer97. Tensile and fracture tests were performed at temperatures in the transition region, at low and high loading rates on unirradiated material. Tensile properties of irradiated material were assessed after irradiation in the Swiss neutron spallation source SINQ to a dose of about 11 dpa and helium content of 500 appm at about 250 Â°C. The information gathered from the mechanical tests was used either as input in finite element models of fracture specimens, or as benchmark data to calibrate and verify the predictions of the models. Fractographic observations were also conducted. The tensile tests on unirradiated Eurofer97 showed a strong temperature dependence and strain rate sensitivity of the stress at low temperature, consistent with a mechanism of nucleation and propagation of double kink on screw dislocation. At high loading rate, the strain rate hardening was partially compensated by thermal softening due to quasi-adiabatic heating. The tensile properties after irradiation showed strong irradiation hardening (increase of the yield stress) accompanied by a drastic decrease of uniform elongation. The constitutive behavior at large strains, representative of those at crack tip, was obtained with an inverse method, based on finite element (FE) models of tensile specimens, developed to derive the true stress-strain curves from the load-displacement ones. These curves were used as input for the FE models of fracture specimens. Comparative fractographic observations on unirradiated and irradiated specimens did not reveal any strong effect of irradiation or helium on fracture mechanism, which remains transgranular cleavage. This observation indicates that higher helium concentration than 500 appm is necessary to initiate intergranular cleavage by helium bubble precipitation on grain boundaries. Dynamic fracture tests were performed on unirradiated Eurofer97. A shift of about 10 Â°C of T0 by one order of magnitude in loading rate was determined, which was in good agreement with results obtained on different ferritic steels. Static fracture toughness tests were also performed on disk compact tension specimens that can be accommodated in irradiation tubes of SINQ. Different FE models of fracture specimens were developed to include strain-rate sensitivity of flow stress, heating resulting from the plastic work in the fracture process zone and loading rate. The structure of the stress field at the crack tip was studied and the temperature dependence of a lower bound to toughness data in the transition was reconstructed using a calibrated criterion of local approach to brittle fracture.