Microstructure-based Modelling of Ductile Fracture in Aluminium Alloys

The demand for lightweight structures increases the need for high-strength aluminium alloys while fully utilizing their valuable properties. Strength and ductility are vital factors in the design of aluminium structures. Increased strength often comes at the expense of ductility, increasing the risk of structural failure. Therefore, reliable models for ductility predictions of aluminium alloys are essential. This task is the core of the Toppforsk project FractAl. The project has developed a microstructure-based modelling framework for ductile fracture that paves the way for a new way of designing aluminium structures. The modelling framework extends our understanding of ductile fracture in aluminium alloys. It can also aid designers and engineers in selecting or tailoring alloys of ideal strength and ductility for a given structure. The research involves modelling and simulation at multiple scales and customized laboratory experiments. These innovations reduce the need for time-consuming and costly mechanical tests. FractAl started in August 2016 and ended in May 2022. The project team counts four doctoral students, one postdoctoral fellow and one researcher. A core team of scientists at the host institution, the Department of Structural Engineering at NTNU, has led the research work. International collaborators are two renowned scientists from KTH in Stockholm and ENS Paris-Saclay in Paris. The results have been published in doctoral dissertations, in international, peer-reviewed journals, and at international conferences. Major results and establishments: - A comprehensive experimental database for ductile fracture of cast and homogenized, extruded and rolled 6xxx aluminium alloys. The experiments form the basis for validating the modelling framework. Also, they contribute to a deeper understanding of the mechanisms that govern ductile fracture. The database contains material tests for different stress states and strain rates, crash tests of extruded profiles, and impact and explosion tests on plates with and without defects. Extensive experimental studies investigate how grain structure, crystallographic texture, and iron-rich particles affect plasticity and ductile fracture. - A flexible microstructure-based numerical framework for multiscale modelling of aluminium structures. The framework consists of models at the nano-, crystal-, micro- and macro-scales. The models have been validated against experimental results on different scales. The models can be linked in various ways, depending on the task and the required accuracy of the results. - Extension of the NaMo nanostructure model. NaMo calculates yield stress and work-hardening for 6xxx aluminium alloys based on chemical composition and thermomechanical processing. FractAl extended the model to include effects of strain rate and temperature, reversed loading, and pre-straining before artificial ageing. The model has been used in non-linear elemental analyses of energy dissipation in extruded aluminium profiles subjected to axial folding. This has been done without using experimental calibration data, with convincing results. - A crystal plasticity model with ductile damage for explicit and implicit finite element analysis of plasticity and fracture. The model is validated against results from the experimental database. NaMo calculations can reduce the need for experimental data in calibrating the model. The model has been used in non-linear finite element analyses to show the significance of crystallographic texture (plastic anisotropy) on plasticity and ductile fracture in 6xxx aluminium alloys. - A numerical framework to simulate the mechanical behaviour of representative volume elements for implicit finite element analyses. Here, user-defined loading states are imposed on a model of the microstructure. Thus, it is possible to perform detailed studies of the mechanisms governing the fracture. Such simulations have been used to assess the predictions of porous plasticity models relevant for aluminium alloys. They have also proven useful for examining the effects of strain rate sensitivity on the micromechanical behaviour and demonstrating how the predictions depend on the model formulation. - A numerical framework for localization analyses based on the imperfection band approach. Here, porous plasticity models describe the plasticity and damage evolution of the material. Strain localization can work as an indicator of material failure, and the numerical framework can be used to establish fracture loci for large-scale simulations of components and structures. Several studies have used localisation analyses to predict the effects of plastic anisotropy, non-proportional loading, and strain rate effects on ductile fracture.

Safer and more eco-friendly structures. Time and money saved for industry and society. Increased competitiveness. These are potential gains from the multiscale modelling framework for 6000 series aluminium alloys developed by FractAl. The new framework enables us to predict how variations in nano- and microstructure affect the metal's mechanical properties. That goes for both the unique alloy and the whole structure's performance. Reduced raw material consumption and the design of more eco-friendly structures are pivotal in shifting towards a green society. At the same time, we must maximize the structural performance. FractAl is expected to contribute to this transformation. The framework enables industrial designers to select the most suitable material and tailor the perfect strength and ductility for a given structure. As the overall methodology is generic, the framework can be modified and applied to other structural alloys.

In this project, we will develop and validate a novel microstructure-based modelling framework for ductile fracture in aluminium alloys - thus reliably introducing multi-scale simulation in design of aluminium structures against failure. By combining a nanostructure model with underpinning crystal plasticity knowledge, we will be able to predict the strength, work-hardening and plastic anisotropy of the materials, based solely on chemical composition, microstructure and temperature history. Ductile fracture can then be modelled using computational cell simulations where microstructural features (e.g., grains, precipitate-free zones and particles) are explicitly described. From this, we will scale up to develop computational plasticity and failure models for large-scale simulations of structural failure. In the project, the nanostructure-based crystal plasticity modelling and the computational cell simulations will be underpinned and validated through a programme of experimental work to characterize the microstructure of the materials and reveal the underlying physical mechanisms of ductile fracture in aluminium alloys. The computational plasticity and failure models will be validated against physical testing of structural components subjected to various loading conditions. Besides extending our fundamental understanding of ductile fracture in aluminium alloys, the microstructure-based modelling framework will make it possible for designers to select the most appropriate material for a given structure and to analyse the structural behaviour under various loading conditions without having to use time-consuming and expensive mechanical tests. The designer will not only be able to perform simulation-based design of the structure, but also to tailor the alloy-temper combination to the structural application at hand. This opens up for an entirely new way of designing aluminium structures against failure.