“Through careful examination of computer models of compressed graphitic atomic layers and experiments on layered solids we showed deformation consistent with ripplocation. The fact that one can learn something about an earthquake from studying the deformation of an individual graphite layer, or vice versa, is quite remarkable indeed.”Michel Barsoum
Many layered materials, from decks of cards, to steel composites, and even the middle of tectonic plates, are known to ripple under pressure. Now researchers at Drexel University in the US have been examining the behavior underlying this buckling phenomenon in research that could help areas such as structural and mechanical engineering, geology and seismology better understand deformation under pressure.
As described in Physical Review Materials [Barsoum et al. Phys. Rev. (2019) DOI: 10.1103/PhysRevMaterials.3.013602], the elastic, rippling behavior occurring inside layered materials when compressed from the sides, described as “ripplocations”, was explored. In 2003 the team had shown that certain layered materials behave differently as they could load and unload them in compression and they would return back to their original shape – that is, they deformed elastically. While elastic deformation is fairly common, they found it surprising that during the loading and unloading process a significant portion of the mechanical energy was dissipated in the form of heat, and if pushed to a certain stress the solid fractured and formed kink bands.
A typical example is how playing cards bend when squeezed from the edges without allowing the cards to separate, or how a ripple can form in a carpet when pushed from the edge. In such pressurized environments, they looked at these internal waves – interior layers that buckle in wave-like formations – showing that they exist at the macro level and can also be modelled at the atomic level. This helped them show that the response was basically the same, why they are reversible and the origin of the energy dissipated, viz. friction between the sheets.
In atomic simulations they demonstrated that in graphite ripplocations nucleate before the material reaches its failure point, and that if the pressure is removed, the ripples dissipate and the layers return to their original shape. In addition, the team observed that ripple bands form simultaneously, with the waves emerging at the same time as the load is applied, and that the height of the ripples, their amplitude, increases with the load.
Ripplocations were previously thought to only occur in very weakly bonded layered solids, but group leader Michel Barsoum realized they are a much more fundamental micromechanism that applied to the vast majority of layered solids. As he told Materials Today, “Through careful examination of computer models of compressed graphitic atomic layers and experiments on layered solids we showed deformation consistent with ripplocation. The fact that one can learn something about an earthquake from studying the deformation of an individual graphite layer, or vice versa, is quite remarkable indeed.”
The work could help in the development of stronger layered materials, making them more damage tolerant or impact resistant, and ripplocations may also play a role in making materials more tolerant to radiation for the nuclear industry, or help predict earthquakes. They now hope to further develop their understanding at the macro-level by varying the effects of layer thickness, constraints, friction coefficient between layers, on the nucleation and propagation of ripplocations, and at the microlevel by looking for evidence of ripplocations in transmission electron micrographs in a variety of solids.