Can a crack be supersonic? Can the fracture that triggers an avalanche propagate faster than the limiting velocity predicted by classical fracture mechanics?
The question remains the subject of debate within the scientific community. While some numerical and experimental studies suggest that avalanche cracks may propagate at supersonic speeds, others offer a different interpretation.
The issue stems from the fact that the snowpack is often treated as a homogeneous material, for which the reference limiting velocity is well defined. In reality, however, it consists of multiple layers with different mechanical properties. What, then, are the factors that limit crack propagation velocity in such a multilayered material?
This is the question addressed by the latest research of Nicola Pugno, Full Professor of Solids and Structural Mechanics in the Department of Civil, Environmental and Mechanical Engineering at the University of Trento. His study proposes a new theoretical approach to determine the limiting velocity of avalanche cracks.
"The analytical results reveal a clear transition between a regime governed by the snow slab that detaches and another governed by the weak layer, or its interface, along which the fracture propagates. In the presence of a stiffer weak layer or interface, the crack may 'feel' a local stiffness greater than the global one, and therefore appear to propagate at a globally supersonic speed while remaining locally subsonic. This finding at least partially reconciles the different interpretations reported in the avalanche literature and is based on a characteristic energy length previously introduced to explain the two propagation regimes that had already emerged from numerical studies," explains Pugno.
The results have been published as a Matter of Opinion - a short article written by experts also on current research challenges - in Matter, the international journal of the Cell Press group, under the title Sub-Rayleigh or supershear crack propagation in snow avalanches?
"My approach represents a brief analytical contribution, building on pioneering numerical work by fellow mechanicians. I hope it will help shed further light on a complex issue that certainly requires additional theoretical, numerical and experimental investigation. In essence, the new theory should make it possible to better understand whether, and under what conditions - for example depending on the snow layering and mass - an avalanche is capable of sustaining supersonic crack propagation," comments Nicola Pugno.
Why is it important to know whether a crack propagates at supersonic speed?
"A particularly rapid release of energy can be even more catastrophic, and this should also be taken into account when designing structures that may be impacted by the snow mass, such as avalanche barriers and exposed buildings," says Pugno.
In the longer term, Pugno's contribution may also improve our ability to predict avalanches more accurately, including their propagation front and other key characteristics.
"The uncertainties in the constitutive laws of snow - which, moreover, is not a linearly elastic material - combined with the multilayered nature of the snowpack, motivated this approach and call for the development of further theoretical, numerical and experimental frameworks."
