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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - MicroFrac (Visualization and modelling of fracture at the microscale)

Teaser

Summary

In recent years, large spills from oil pipelines and tankers, leaks from nuclear reactors and the constant need for lighter, stronger, and safer materials in the transportation industry illustrate how breaks or cracks (fracture) can have detrimental effects in terms of health and safety, the environment, and the economy. Recent reports also suggest that the costs of fracture in Europe reach 4% of Europeâ€™s gross domestic product which means about 500 billion euros. There is therefore a growing need for materials with improved fracture resistance. When materials are deforming during in-service use, there is a point at which very small voids start appearing inside the material, whose diameters are less than a hundreds of the width of a human hair. These tiny voids grow, and when they are large enough, they link with each other resulting in material failure. Knowing how fast these voids grow is therefore a key aspect to understand when materials will fail. Most work to date has been focused on rather large voids and there is very little experimental information on the mechanisms of void growth at lower scales (i.e. the microscale). This lack of information on fracture at the microscale is one of the key factors that prevent better fracture predictions and the design of damage resistant materials.
The investigations carried out under the MicroFrac project (a research project funded by the European Union under the Marie SkÅ‚odowska-Curie actions) aimed at providing a contribution towards our understanding of fracture at the microscale through a combination of state-of-the-art experiments and models.
These results allowed us to better understand the early stages of fracture in metals, when voids are still very small. The preliminary simulations results are very promising for the development of advanced simulation tools to more accurately predict when materials will break and to design fracture resistant materials.

Work performed

In order to better understand fracture at the microscale, the approach taken in this project was to create microscopic voids (microvoids) in metallic sheets in particular suitable places called grains. The voids diameters were down to less than a micrometer in diameter. The void-containing sheets were then deformed until fracture, in-situ, combined with advanced imaging techniques.
A 2-dimensional configuration was first created using the Focused Ion Beam, a technique which allows for the machinig of a void within a single grain. The sample was then deformed inside a Scanning Electron Microscope where the growth of the voids can be observed in details. 3-dimensional samples were also created by drilling voids in metallic sheets and bonding them at high temperature. The growth of the voids was visualized using x-ray tomography as shown in the figure.
Experimental void growth results were then compared to advanced simulations, where the effect of the underlying grain orientation was accounted for.
These results allowed us to better understand the early stages of fracture in metals, when voids are still very small.
The preliminary simulations results, some of them already published, are very promising for the development of advanced simulation tools and to design fracture resistant materials.

Final results

One of the main outcome of this project was the validation of models used for deformation and fracture predictions in heterogeneous materials such as titanium. As titanium is widely used in the aerospace and other industries, it is expected that the benefits of such fracture models will become important. While we did not have a chance to look into new material designs, the validation of the fracture model opens the door for the implementation of new materials with improved fracture resistance.