Researchers at CEA-IRAMIS [1], SVI (CNRS/Saint-Gobain) [2] and LTDS [3] (CNRS/Ecole Centrale de Lyon/Ecole Nationale d’Ingénieurs de Saint-Etienne), have just demonstrated that in cases of brittle material fracture, the crack propagation rate is four times lower than theories suggested. The study is expected to have many consequences for materials science and engineering, both in terms of methods employed and results generated. An article on the study has just been published in the scientific journal PNAS.

In brittle materials such as glass, fracture is caused by the propagation of cracks in the material. In order to predict a material's fracture performance, the crack propagation rate must be known and the factors on which it depends understood. Until now, theories have been based on a maximum velocity, known as Rayleigh wave speed, which is equal to that of the surface acoustic waves in the material. Researchers working on the study published in PNAS have demonstrated that the propagation rate of microcracks on material flaws observed on a scale fine enough to distinguish individual cracks is, in fact, four times lower than expected! The high speed at which overall fracture occurs can be explained by the geometric effect produced by all these microcracks coming together.

Researchers obtained this result by breaking Plexiglas® samples and varying the force applied until the crack was opened up. They found, quite logically, that the greater the force applied to the sample, the faster it fractured. Beyond a certain fracture speed, a multitude of microcracks form around minute flaws that are always found in the material, ahead of the main fracture front, as the crack develops. Hundreds of millions of these microcracks are formed every second (meaning one microcrack every 10 nanoseconds), making it impossible to track their development in real-time during the experiment. Each microcrack, however, leaves a mark on the fracture surfaces (see figure below), which researchers can analyze at their leisure after the experiment.

A mark left by a microcrack on a fast fracture surface of a Plexiglas® sample seen under the microscope (polarized light). Image dimensions: 128 µm x 137µm.

© Image CEA-CNRS

As a result, researchers were able to reconstruct in detail the entire process leading to a fast fracture. This is because the point of origin of individual microcracks, the chronological order in which they are formed, and the speed at which each one develops, can be determined from the pattern of all the marks left.

These findings challenge the traditional view of material fracture. Surprisingly, all microcracks are propagated at the same speed of roughly 200 m/s, regardless of the force applied to open up the main crack (for a Rayleigh wave speed of around 900 m/s). Performance observed on a microscopic scale is very different from that of large-scale observations, where fracture speed increases with the force applied, and can reach much higher values of up to 500 m/s. The apparent fracture speed increases as the microcracks merge with the main crack. These results contradict general opinion in the scientific community, where it was previously thought that the generation of microcracks would slow down fracturing by dissipating extra energy.

Pattern of marks left by the nucleation, propagation and merging of microcracks during fast fracture of a Plexiglas® sample seen under the microscope. Image dimensions: 2.5mm x 3.5mm. Top right image: digital reconstruction of the pattern of fracture marks © CEA-CNRS

This study highlights the extent to which materials are affected in terms of fracture performance by the microscopic flaws they contain. Taking these effects into consideration should help to assess more effectively, and eventually improve, the breaking strength of materials. In addition to this essential aspect, the methods used to reconstruct a detailed sequence of events occurring during fracture could also be used in other applications. The analysis of marks left on fracture surfaces could, for example, provide clues as to a structure's collapse.

Reference: PNAS – Proceedings of the National Academy of Sciences

Understanding fast macroscale fracture from microcrack post mortem patterns,
C. Guerra, J. Scheibert, D. Bonamy, D. Dalmas, Proc. Natl. Acad. Sci. USA 109 (2012) 390.

http://dx.doi.org/10.1073/pnas.1113205109

[1] IRAMIS (Institut rayonnement matière de Saclay – The Saclay Institute of Matter and Radiation).

[2] SVI (Surface du verre et interfaces – Glass surface and interfaces)

[3] LTDS (Laboratoire de tribologie et dynamique des systèmes – Tribology and System Dynamics Laboratory)