Mechanical and forming properties of AA6xxx sheet from room to warm temperatures

The influence of temperature on the mechanical behaviour of the heat treatable Aluminium alloy EN AW-6061 has been investigated with a series of tensile tests. It is found that temperature has an effect on both the storage of dislocations and dynamic recovery. The results have been used to fit the dislocation based Nes work-hardening model. Simulations show that the model captures properly the dependence of yield stress and work-hardening rate with temperature and temper. The work-hardening model has been implemented into the Dieka FEM to simulate the warm deep drawing of cylindrical cups. Comparison of the simulated and experimental punch force and cup thickness reveals a good correspondence and validates the proposed modelling approach

Thermo-mechanical forming of Al-Mg-Si Sheet

In warm forming of aluminum sheet, the temperature and strain rates vary considerably. In simulations, the material\ud model must be capable to predict stresses within this wide range. Here, the physically based Nes model is used to describe\ud the behavior of AA6061-T4 sheet material under warm forming conditions. A significant change of earing behavior is\ud found between room temperature and 250 ºC. Crystal plasticity calculations showed a reasonable correspondence of\ud changing r-values if extra slip systems are considered at high temperatures. Satisfactory results are obtained for simulation\ud of tensile tests and cylindrical deep drawing

Constitutive characterization of a rare-earth magnesium alloy sheet (ZEK100-O) in shear loading: Studies of anisotropy and rate sensitivity

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.ijmecsci.2017.04.013 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Rare-earth magnesium alloys such as ZEK100-O offer improved ductility over other conventional magnesium alloys at room temperature; however, they exhibit significant anisotropy and complex yield behaviour. In this work, a systematic investigation of anisotropy of ZEK100-O rolled sheet in shear loading was conducted at room temperature under quasi-static conditions, as the shear deformation of these alloys is not well understood. Furthermore, uniaxial tensile and compressive characterization of the material was performed to provide context for its behaviour under shear loading. It was revealed that ZEK100-O exhibits strong anisotropy in shear which is markedly different than tensile anisotropy with unique trends that suggest the activation of different deformation mechanisms. To characterize shear anisotropy in HCP materials such as ZEK100-O, the shear response of the material should be investigated in three orientations of 0° (or 90°), 45°, and 135° with respect to the rolling direction. The selection and analysis of these directions is discussed in terms of the principal stress directions and activation of different deformations mechanisms. In order to further investigate this behaviour, the microstructure of the deformed specimens was studied using Electron Backscattered Diffraction (EBSD) analysis to quantify the active twinning systems in different test orientations. Moreover, the CPB06 yield criterion with two linear transformations was calibrated with experimental data to describe the complex anisotropic behaviour of ZEK100-O. It was established that the material exhibits asymmetry not only in tension-compression regions represented by the 1st and 3rd quadrants of yield locus but also in shear regions represented by the 2nd and 4th quadrants. Finally, the strain rate sensitivity of ZEK100-O was studied in shear tests at elevated strain rates of 10s−1 and 100s−1, at which positive rate sensitivity was observed.Cosma International/Automotive Partnership Canada/Natural Sciences and Engineering Research Council of Canada [APCPJ 417811-11]Ontario Research Fund [RE01-054]Canada Research Chairs Secretariat [950-220425]Canada Foundation for Innovation (30337

Strain rate sensitivities of deformation mechanisms in magnesium alloys

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.ijplas.2018.04.005 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/Strain rate sensitivity (SRS) is an important material property that governs the rate dependent mechanical behaviors associated with deformation rate changes, creep, stress relaxation, formability, etc. The variety of activated deformation mechanisms of magnesium alloys under different loading paths, e.g. tension vs. compression, implies that SRS of magnesium alloys obviously depends on loading paths, and each deformation mechanism has its own SRS. However, a single SRS scheme is commonly employed in numerical modeling to describe the rate dependent behaviors of magnesium alloys, which disregards the distinction of SRSs among different deformation mechanisms. The implementation of the constitutive model that works for a wide range of values of SRSs has been a challenge to crystal plasticity modeling for metals with multiple deformation mechanisms like magnesium. Especially, very small values of SRS, corresponding to low rate-sensitivity, generally lead to high nonlinearity involved in the governing equations, and then computational failure. In this paper, the elasto-viscoplastic self-consistent (EVPSC) crystal plasticity model is improved to enhance its numerical robustness for very small SRS values. Taking advantage of this improvement, different SRSs for various deformation mechanisms are employed to investigate the strain rate dependent behaviors of magnesium alloys at room temperature. First, the SRSs for various deformation mechanisms are determined based on the compressive stress relaxation tests on an AZ31 alloy plate; secondly, the obtained SRSs are applied to interpret internal elastic strain evolution of the same magnesium alloy under in-plane compression; finally, the determined SRSs are applied to investigate the deformation of another AZ31 alloy under various deformation paths and strain rates. The present work is the first effort on studying effects of strain rate-sensitivity on mechanical behavior of Mg alloys under wide range of applied strain rates by using an improved self-consistent polycrystal plasticity model. Good agreement between the experiments and simulations reveals the importance and necessity of using different SRSs for the deformation mechanisms involved. The rate dependent behaviors of magnesium alloys can be better described by using multiple SRSs associated to each operative deformation mechanism.Natural Sciences and Engineering Research Council of CanadaMinistry of Research, Innovation and ScienceNational Natural Science Foundation of China [51575346, 51675331]Shanghai Jiao Tong UniversityAutomotive Partnerships CanadaCanada Research Chairs Secretaria

Homogenization of Damaged Concrete Mesostructures using Representative Volume Elements - Implementation and Application to SLang

This master thesis explores an important and under-researched topic on the so-called bridging of length scales (from >mesomacrobridge< the representations of events that occur at two different scales. The underlying objective here is to efficiently incorporate material length scales in the classical continuum plasticity/damage theories through the concept of homogenization theory. The present thesis is devoted to computational modeling of heterogeneous materials, primarily to matrix-inclusion type of materials. Considerations are focused predominantly on the elastic and damage behavior as a response to quasistatic mechanical loading. Mainly this thesis focuses to elaborate a sound numerical homogenization model which accounts for the prediction of overall properties with the application of different types of boundary conditions namely: periodic, homogeneous and mixed type of boundary conditions over two-dimensional periodic and non-periodic RVEs and three-dimensional non-periodic RVEs. Identification of the governing mechanisms and assessing their effect on the material behavior leads one step further. Bringing together this knowledge with service requirements allows for functional oriented materials design. First, this thesis gives attention on providing the theoretical basic mechanisms involved in homogenization techniques and a survey will be made on existing analytical methods available in literature. Second, the proposed frameworks are implemented in the well known finite element software programs ANSYS and SLang. Simple and efficient algorithms in FORTRAN are developed for automated microstructure generation using RSA algorithm in order to perform a systematic numerical testing of microstructures of composites. Algorithms are developed to generate constraint equations in periodic boundary conditions and different displacements applied spatially over the boundaries of the RVE in homogeneous boundary conditions. Finally, nonlinear simulations are performed at mesolevel, by considering continuum scalar damage behavior of matrix material with the linear elastic behavior of aggregates with the assumption of rigid bond between constituents

Simulation of stretch forming with intermediate heat treatments of aircraft skins - a physically based modeling approach

In the aerospace industry stretch forming is often used to produce skin parts. During stretch forming a sheet is clamped at two sides and stretched over a die, such that the sheet gets the shape of the die. However for complex shapes it is necessary to use expensive intermediate heat-treatments in between, in order to avoid Lüders lines and still achieve large deformations. To optimize this process FEM simulations are performed. The accuracy of finite element analysis depends largely on the material models that describe the work hardening during stretching and residual stresses and work hardening reduction during heat treatments due to recovery and particle coarsening. In this paper, a physically based material modeling approach used to simulate the stretch forming with intermediate heat treatments and its predictive capabilities is verified. The work hardening effect during stretching is calculated using the dislocation density based Nes model and the particle coarsening and static recovery effects are modeled with simple expressions based on physical observations. For comparison the simulations are also performed with a phenomenological approach of work hardening using a power law. The Vegter yield function is used to account for the anisotropic and biaxial behavior of the aluminum sheet. A leading edge skin part, made of AA 2024 has been chosen for the study. The strains in the part have been measured and are used for validation of the simulations. From the used FEM model and the experimental results, satisfactory results are obtained for the simulation of stretching of aircraft skins with intermediate heat treatments and it is concluded that the physics based material modeling gives better results