Unbreakable materials on the horizon

Fracture, a central problem of engineering

As a leading expert in statistical and theoretical physics from the National Research Council in Italy (CNR), Dr Zapperi is well placed to examine the dynamics of complex materials and the size effects in fracture and plasticity.

“I have always been fascinated by the way things break differently at different scales; this has been a central problem of engineering”, says Dr Zapperi. “The one atom-thick sheets of graphene that you use in laboratories and large pieces of architecture such as bridges.do not fail in the same way. Size effects are immensely complex phenomena and our goal is to understand how collective effects – i.e. the motion of atomic defects and cracks in materials– can impact their physical behaviour when the scale changes”.

An area where pragmatism is still king

There already exists a lot of empirical knowledge about fracture and plasticity. “Engineers for instance know well how to avoid that elevators fall, by making them much stronger than required. These issues are already part of their everyday work but a complete theory about fracture and plasticity of materials is not yet in sight”, notes Dr Zapperi.

In the ERC project, he plans to establish a universal law for failure statistics that could be applied to a large array of different materials, including metals, glass, crystalline materials, amorphous materials such as gels, etc.

Taking the example of water which boils under a certain quantity of heat that is proportional to its volume, Dr Zapperi explains that the theories of fracture and plasticity don’t follow the same proportionality principle. There is no simple way to estimate the load at which a large sample will break just by knowing the fracture load of a smaller sample. Referring to Leonardo da Vinci’s wires that are exhibited in the Milano Science Museum, he explains that on average, longer wires are more likely to possess weak parts and to fail at smaller loads. For instance a chain will break at its weakest link. To sum up, he says “the larger, the weaker - the smaller, the stronger”.

In contrast, the stress required to deform plastic materials (the so-called yield strength) does not depend very much on the size of the object. Micron-sized samples, however, are much stronger than bulk ones and are also less predictable in the way they deform.

By using a standard method called the “renormalization group” (RG), which allows the study of how the system behaves depending on its scale, the research team plans to better understand size effects in fracture and plasticity.

Dr Zapperi is convinced that his research is particularly relevant at times where we push devices to smaller and smaller scales. “Understanding size effects in micron-scale ductile materials like metals could be helpful to better control fluctuations in their deformation, which is important in manufacturing microelectronic components, for instance”.

Freedom of research

Regarding the ERC funding, Dr Zapperi comments: “A lot of our work deals with computational work. The ERC grant was a real career boost as it helped me to hire five team members and to buy a dedicated computer cluster to make our simulations, instead of using standard desktop computers which by definition are much slower.” In terms of collaboration, he concludes: “The ERC funding allows us to make cutting-edge research and for us to cooperate with peers in other countries like the US or Finland. I find it quite exceptional to benefit from such freedom without being obliged to squeeze my research by pre-set priorities”.