Fractures Unveiled: Microstructures to Earthquakes

Fractures are something we normally try to avoid: nobody likes to break an arm, and material fractures in buildings or bridges can even be life-threatening. But for David Kammer they are a key to understanding the physical mechanisms of materials and natural processes. Kammer is an assistant professor for computational mechanics of materials at the Department of Civil, Environmental and Geomatic Engineering at ETH Zurich, where he investigates how fractures occur and propagate - and how they can be avoided or controlled.

From small to large

"Our research covers a wide range of sizes - from the smallest components in concrete or fibres in the human body to tectonic plates that play a role in earthquakes", Kammer explains. His research group mainly uses mathematical models to get to the bottom of the often mysterious properties of fractures. "These models are quite simple, but they include many physical processes simultaneously and thus allow us to gain deep insights and a better understanding of the phenomena."

All fractures begin with a weak point: a flaw occurs somewhere in the material, setting off a chain reaction that eventually leads to fracture. Exactly how this happens depends very much on the material. In a joint project with researchers in the USA, for example, Kammer modelled fibres in human bone. Clinical data had shown that the bones of people with type-2 diabetes were more susceptible to fractures than those of healthy people. One hypothesis is that diabetes causes collagen molecules to bind more tightly together. Kammer's simulation confirmed this hypothesis: even tiny microscopic changes lead to an increase in the brittleness of the collagen fibres, which significantly compromises the stability of the bone.

Faster than the speed limit

Once a fracture has formed in a material, the speed at which it propagates ultimately determines the fate of the material. Kammer's simulations provide explanations for seemingly mysterious or contradictory phenomena. For example, it was known that fractures in certain materials, such as those responsible for earthquakes can propagate faster than previously thought. Normally, the so-called Rayleigh wave speed - a speed limit resulting from the laws of physics and the properties of the material - limits the propagation of a fracture.

However, Kammer's research shows that small changes in the assumptions can suspend that limit. By assuming that the deformation of the material under a load is not linear but slightly non-linear - that is, that twice the force causes more than twice the deformation - the fracture can propagate faster than expected. However, the underlying physical laws remain unchanged. "Our studies show that even a small nonlinearity in the material properties is sufficient to increase the velocity of the fracture", explains Kammer. "This result opens the door to many more questions in fracture dynamics."

Placing defects prevents fractures

Another focus of Kammer's research is the study of materials with architected structures, so-called metamaterials. These materials are optimized for specific properties by using special geometric lattice structures, which are often inspired by natural shapes such as honeycombs. "Metamaterials are really a 'hot topic'", says Kammer, "but we still know too little about their fracture properties."

By simulating fractures in these materials, Kammer made a remarkable discovery: when defects are deliberately placed inside the structure, the fractures do not spread more easily - quite the contrary. The defect throws the fractures off its "racing line", and because of this forced detour, the fracture has to use more energy to propagate. In the end, this hinders its growth. This finding, which Kammer's team is now also testing in the laboratory, could lead to new approaches to designing materials that are more resistant to fracture.

Kammer also challenges basic assumptions on a macro scale, for example in the analysis of earthquakes. To better understand the onset and propagation of earthquakes, researchers often turn to laboratory experiments to test theoretical models. Kammer has taken a closer look at some of these experiments and shown that some approaches, which have, received little attention so far, describe the experimental results better than conventional models. "However, in order to understand whether our new approaches are also better at describing real earthquakes, we need a better knowledge of the conditions at the tectonic faults", says Kammer. For instance, similar to the collagen fibres in human bones, the brittleness of tectonic plates is crucial in earthquakes.

How fractures arise and how they propagate continues to puzzle scientists. A better understanding of these processes is crucial in many areas, particularly with a view to the safety of materials and structures. Kammer's research shows how small changes in the structure or in the assumptions made can have large consequences.