When will it break?

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 Europe’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 (larger than 10 micrometres (1 micrometre is one millionth of a metre, or one hundreds of the diameter of a human hair) 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.

The investigations carried out under the MicroFrac project (a research project funded by the European Union under Marie Skłodowska-Curie action number 659575) aimed at providing a contribution towards our understanding of fracture at the microscale through a combination of state-of-the-art experiments and models. Microscopic voids (Microvoids) down to less than a micrometer in diameter were artificially created in metallic sheets in particular suitable places called grains and were then tested in-situ via advanced imaging techniques [1].

A 2-dimensional configuration was first created using the Focused Ion Beam technique which allows for the machinig of a void within a single grain. The sample is then deformed inside a Scanning Electron Microscope where the growht of the voids can be observed in detail. 3-dimensional samples were also created by drilling voids in metallic sheets and bonding them at high temperature. The growth of the voids is crearly detected using x-ray tomography, and experimental void growth results are then compared to advanced simulations, where the effect of the underlying grain orientation can be accounted for. These results allowed a better understanding of the early stages of fracture in metals, when voids are still very small. The preliminary simulations results so far performed are very promising for the development of advanced simulation tools to more accurately answer the question: “When Will it Break?” and to design fracture resistant materials.

[1] M. Pushkareva, J. Adrien, E. Maire, J. Segurado, J. Llorca, A.Weck. Three-dimensional investigation of void growth leading to fracture in commercially-pure titaniumMaterials Science and Engineering A. (2016) In press. DOI: dx.doi.org/10.1016/j.msea.2016.06.053

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