Study reveals hidden slow creep stage before explosive

A new study reveals that material fracture begins with a hidden, slow process long before the explosive break we see. Researchers from the Hebrew University of Jerusalem found cracks nucleate as two-dimensional patches that creep at speeds ranging from microns to millimeters per second.

Yuval Paz and Jay Fineberg of the Racah Institute of Physics led the work, collaborating with Meng Wang of the Beijing Institute of Technology and Mokhtar Adda-Bedia of CNRS, ENS de Lyon and Université de Lyon. Their findings, published in the journal Physical Review Letters, extend the classical theory of how materials fail.

The missing beginning of fracture

Classical linear elastic fracture mechanics (LEFM) successfully describes fast-moving cracks but leaves a fundamental question unanswered. It does not explain how a crack gets started. The theory states a crack smaller than a critical size, called the Griffith length, remains stable. Only beyond that size does it become unstable and accelerate toward the speed of sound.

The team's experiments aimed to investigate this missing initiation story. They used a specially designed system and high-speed imaging to track fractures from their first appearance.

From 2D patch to explosive crack

The experiments showed fracture begins when a material reaches a critical stress. A tiny broken region nucleates not as a line but as a two-dimensional patch. This patch then expands extremely slowly in a process termed creep.

A remarkably consistent sequence emerged. A tiny 2D crack patch appears and creeps. When it grows to span the thickness of the material, everything changes. This geometric, or topological, transition transforms the patch into a one-dimensional, through-going crack. Only then does the explosive acceleration described by classical theory begin.

The slow creep stage is substantial, occupying at least 75% of the total fracture process. Once the transition occurs, the final rupture happens on microsecond-to-millisecond timescales.

Extending the classical framework

The new theory does not replace LEFM but extends it. It connects three previously separate stages within a single mechanical description: crack initiation, extremely slow creep, and explosive fracture. The framework accounts for the interplay between a crack's two-dimensional geometry and the material's thickness.

Once the patch becomes a through-crack, the behavior naturally reverts to the rapid fracture predicted by classical LEFM. The experiments also challenge the idea that a crack must reach a particular length before failure. They support a critical stress for initiation that depends on geometry.

In the experiments, the classical Griffith length was about 1 millimeter. Cracks were observed nucleating at scales roughly 10 times smaller, around 0.1 millimeters.

Implications for earthquakes and detection

Understanding this hidden slow stage could change how researchers detect impending material failure. Instead of looking only for large, dangerous cracks, monitoring for tiny, slowly expanding patches may become important.

The findings also connect to larger-scale phenomena like earthquakes. The framework builds on related research into frictional ruptures, where two surfaces begin sliding. The researchers show that tensile fracture and frictional rupture can share the same underlying geometrical mechanism.

This suggests the physics of a slowly growing lab crack may offer insight into how ruptures initiate and evolve along frictional interfaces, including those in faults. A catastrophic fracture appears instantaneous, but beneath it lies a long, nearly invisible preparation. A tiny patch forms, creeps slowly outward, and changes geometry until it spans the material's thickness. Then the crack takes off.