August 8, 2023 — Researchers at the Hebrew University of Jerusalem’s (HU) Racah Institute of Physics made a discovery that challenges the conventional understanding of fracture mechanics. The team, led by Prof. Jay Fineberg, along with postdoctoral associates Dr. Meng Wang and Dr. Songlin Shi, experimentally demonstrated the existence of “supershear” tensile cracks that exceed classical speed limits and transition to near-supersonic velocities.  

Classical crack moving at half of the sound speed credit Meng Wang Hebrew University
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. 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. Surprisingly, these cracks were observed to surpass the shear wave speed as well. In certain cases, the velocities of these supershear cracks approached dilatation wave speeds, presenting phenomena previously unobserved in classical fracture mechanics. 

Shockwave from the super-sonic crack credit Meng Wang Hebrew University
Shockwave from the super-sonic crack. 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. 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.” 

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

The article was published in Science.