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Three-dimensional multiscale model for material degradation and fracture in polycrystalline materials

Final Report Summary - 3D MULTISCALE MODEL (Three-dimensional multiscale model for material degradation and fracture in polycrystalline materials.)

AIMS. The aim of the present project is to formulate and implement a high performance three-dimensional (3D) multiscale model for the analysis of material degradation and fracture in polycrystalline materials.

WORK PERFORMED. An innovative model for intergranular degradation and failure of polycrystalline materials at the micro-scale (grain level) was developed and implemented into an in-house code. The model is based on a cohesive-frictional grain-boundary formulation, able to capture the progressive degradation and the final failure of the polycrystalline microstructure, under quasi-static loading conditions. The intergranular micro-cracking evolution has been analyzed, both for tensile and compressive loads, for two-dimensional columnar as well as fully three-dimensional microstructures. The results of the computational tests agree well with available data, confirming the high potential of the method for microstructural analysis of this class of materials and for the study of brittle polycrystalline failure.

The microstructural model wassubsequently adapted in a multiscale framework for the analysis of degradation and failure of engineering components realized in polycrystalline materials. In this framework the macro-scale is described through a classical boundary element formulation. The constitutive behavior of the material, and its progressive degradation under quasi-static loads, is however provided by the microstructural simulation, through a material homogenization procedure able to quantify the damage resulting at the macro-scale as a consequence of the progressive intergranular failure of the microstructure. The accumulation of damage at the component level is accommodated within the macro boundary element model through an initial stress approach, which is commonly used to account for plasticity in boundary element solid mechanics formulations. A non-local approach is used to avoid pathological localization of damage at the macro-scale.

Both the micro- and macro-scale formulations rely on incremental-iterative algorithms for capturing the evolution of damage. Extensive use of High Performance Computing facilities has been made, although further parallelization of the macro-structural model is still achievable and currently subjected to further investigation and refinement.

ACHIEVED RESULTS. The main outcome of the project was the formulation and the related implementation of computer codes for micro- and multi-scale analysis of degradation and failure of polycrystalline materials. The formulation was shown to allow efficient modelling of material degradation and fracture of polycrystaline materials. The project’s achievements have been disseminated in several international conferences and in several publications including:

- Benedetti, I, Aliabadi, MH, “A three-dimensional cohesive-frictional grain-boundary micro-mechanical model for intergranular degradation and failure in polycrystalline materials”, Computer Methods in Applied Mechanics and Engineering, Vol.265 pp.36-62 2013.

- Benedetti, I, Aliabadi, MH, “A three-dimensional grain boundary formulation for microstructural modelling of polycrystalline materials”, Computational Materials Science, Vol.67 pp.249-260 2013.

- I. Benedetti, M.H. Aliabadi, “Computational modeling of brittle failure in polycrystalline materials using cohesive-frictional grain-boundary elements”, Key Engineering Materials, Vol. 577-578, pp.233-236 2014.

- I. Benedetti, M.H. Aliabadi, “A grain boundary formulation for the analysis of three-dimensional polycrystalline microstructures”, Key Engineering Materials, Vol. 525-526, pp.1-4 2013.

Another work, with the tentative title “Multiscale modelling of polycrystalline materials: a grain-boundary approach to material degradation and fracture” is currently in preparation (August 2013).

POTENTIAL IMPACT. Although the current research has layed the foundation of much further investigation, the developed model has already wide scientific potential impact and societal implications:

Scientific impact. The model can be used to study and understand the mechanisms underlying the brittle failure of polycrystalline materials. This provides a more solid basis to fracture propagation criteria, that often rely on semi-empirical assumptions. The initiation and propagation of cracks is an inherently multiscale phenomenon, in which the micro-scale material features have a direct influence on the macro-continuum response and this in turn affects the micro-state evolution. The project contributes on important fundamental research and it will help Europe maintain scientific competitiveness in the field of multiscale modelling.

Socio-economic impact. Better understanding of materials degradation and damage initiation and evolution mechanisms will enhance current engineering design and maintenance practices, reducing the cost that, still today, fracture related failures have for society. The ability to model with improved accuracy and reliability the onset of damage will be highly beneficial for the European Automotive and Aerospace sectors.

Socio-economic impact. Better understanding of materials' degradation allows to design lighter and safer structures, which in turn imply a reduction of fuel emissions. This is one of the main driver of the EU support to research and the development of a low-carbon economy is a overarching target of the Europe 2020 strategy and Horizon 2020, as it was in the FP7.

Technical and commercial potential. Further refinement of the developed formulation and the implemented code could lead in the future to the inclusion in commercial FEM software packages for the analysis of progressive degradation of engineering components.

The performed research is particularly relevant for the wide engineering community in general and in particular for the Automotive, Aerospace, Civil, Petrol-chemical and Nuclear sectors, where the issues related to the impact of materials degradation and failure are of major concern.

A graphical abstract of the performed research is provided as attachment.

Relevant contact details. For further information on the project please contact Dr. I. Benedetti (i.benedetti@imperial.ac.uk; ivano.benedetti@unipa.it) or Prof. M.H. Aliabadi (m.h.aliabadi@imperial.ac.uk).

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