Reviewed by Lexie CornerMar 25 2024
Intricate crack patterns often emerge when objects break, but the patterns go unnoticed in everyday life. However, John Kolinski and his group at the Laboratory of Engineering Mechanics of Soft Interfaces (EMSI) in EPFL's School of Engineering are experts at capturing the formation of such patterns.
The scientists induced cracks in hydrogel samples with a standard Swiss Army knife. Image Credit: EPFL CC BY SA
To design and test safe and affordable composite materials for usage in construction, sports, and aerospace engineering, they seek to understand the propagation of cracks in brittle solids.
Conventional mechanical methods of crack analysis presume that cracks are planar, meaning they develop on a material's two-dimensional surface. However, most cracks, including those in common brittle substances like glass, propagate into three-dimensional networks of ridges and other complicated features; simple planar cracks are merely the tip of the iceberg.
Monitoring this complexity in real time is quite challenging because of the opacity of the material and the speed at which cracks occur. Now equipped with a Swiss Army knife and a confocal microscope, Kolinski and his team have achieved precisely that feat. They have also unveiled a positive correlation between crack complexity and material toughness.
The energy required to drive cracks has traditionally been considered a material property, but our work yields unique insights into the key role of geometry: namely, that by increasing the complexity of geometric features at the crack tip, a material can be made effectively tougher, because more strain energy is required to advance a complex crack than a simple one, this highlights an important gap in the current theory for 3D cracks.
John Kolinski, Institute of Mechanical Engineering, School of Engineering, Ecole Polytechnique Fédérale De Lausanne
The research was published in the journal Nature Physics.
A Fundamental Link Between Length and Strength
The technique used by the researchers entailed cutting an elastomer and four distinct hydrogels into extremely thin slices. The hydrogels, fragile and transparent, were useful as a stand-in for glass and brittle plastics to help understand how cracks originate in these materials. They were also easy to measure and deform without breaking. The elastomer also served as a stand-in for substances like silicone polymers and rubber.
The device's scissors created geometrically intricate cracks in the hydrogel samples, which were then seen with a state-of-the-art confocal microscope. The Swiss Army knife was used to induce the experimental cracks.
A series of fluorescent images were created with the confocal microscope using special equipment constructed by the EMSI team to manage sample alignment and loading. These images were then layered to create a unique, three-dimensional of each fracture surface.
People have long known that cracks can become complex by looking at fracture surfaces after the fact, but what is lost is the understanding of the loading conditions when the crack emerged or what forces the sample was exposed to, and our innovative imaging method has made it possible to characterize this relationship rigorously in-situ.
John Kolinski, Institute of Mechanical Engineering, School of Engineering, Ecole Polytechnique Fédérale De Lausanne
The investigations showed that the lengths of the fracture ends were directly correlated with the strain energy needed to drive the sample cracks. This implies that a 3D crack's greater geometric complexity creates more fracture surface as it spreads, necessitating higher strain energy to drive it.
In another experiment, the researchers demonstrated how the planar symmetry of a smoother crack was broken as it got closer to a rigid impediment buried in the sample, lengthening the crack tip and requiring more energy to move the crack ahead.
The fact that we can isolate how geometric complexity emerges with such an inhomogeneity in the material could motivate new design approaches, and our work also highlights the importance of care in carrying out materials testing, as we now know that any geometric deviation from a planar crack front may lead to a mis-measurement – and potentially dangerous over-estimation – of material toughness.
John Kolinski, Institute of Mechanical Engineering, School of Engineering, Ecole Polytechnique Fédérale De Lausanne
Journal Reference:
Wei, X., et al. (2024) Complexity of crack front geometry enhances toughness of brittle solids. Nature Physics. doi.org/10.1038/s41567-024-02435-x.