Shattering Classical Speed Limits – New Discovery Shows Tensile Cracks Can Approach Near-Supersonic Velocities

Snapshot of the material deformations formed by a single rapidly propagating crack moving left to right. Credit: Meng Wang, Hebrew University

Scientists from the Racah Institute of Physics at the Hebrew University of Jerusalem have unveiled findings that question traditional beliefs in fracture mechanics. Spearheaded by Dr. Meng Wang, Dr. Songlin Shi, and Prof. Jay Fineberg, the research team has experimentally shown the presence of “supershear” tensile cracks which surpass established classical speed limits and approach nearly supersonic speeds.

Classical crack moving at half of the sound speed. Credit: Meng Wang, Hebrew University

Traditionally, brittle materials have been observed to fail through the rapid propagation of cracks. Classical fracture mechanics describes the motion of tensile cracks that release elastic energy within a localized zone at their tips, limiting their speed to the Rayleigh wave speed (CR). However, the recent findings by the Hebrew University researchers indicate a paradigm shift in this understanding.

Utilizing brittle neo-Hookean materials in their experiments, the team identified the occurrence of “supershear” tensile cracks that smoothly accelerate beyond the classical speed limit of CR. Surprisingly, these cracks were observed to surpass the shear wave speed (cS) as well. In certain cases, the velocities of these supershear cracks approached dilatation wave speeds, presenting phenomena previously unobserved in classical fracture mechanics.

The shockwave from the super-sonic crack – analogous to a sonic boom. Credit: Meng Wang, Hebrew University

One of the most remarkable aspects of the discovery is the observation that supershear dynamics are governed by different principles than those guiding classical cracks. This non-classical mode of tensile fracture is not a random occurrence; rather, it is excited at critical strain levels that depend on the material properties.

“This finding represents a fundamental shift in our understanding of the fracture process in brittle materials,” commented Prof. Jay Fineberg, the corresponding author of the research. “By demonstrating the existence of supershear tensile cracks and their ability to exceed classical speed limits, we have opened up new avenues for studying fracture mechanics and its applications.”

Experimental system that generates super-sonic laboratory earthquakes at Hebrew University Laboratory. Credit: Meng Wang, Hebrew University

The implications of this research extend beyond the realm of physics. By showing that tensile cracks can surpass their classical speed limits, the researchers have paved the way for a new understanding of fracture mechanics.

Reference: “Tensile cracks can shatter classical speed limits” by Meng Wang, Songlin Shi and Jay Fineberg, 27 July 2023, Science.
DOI: 10.1126/science.adg7693