Surprising complexity in a simple particle model of composite materials

Tokyo, Japan – Researchers at Tokyo Metropolitan University used computer simulations to model how a composite material reacts to loading. They studied a particle model of a pillar of stiffer material in a soft matrix, finding a concentration of force around the pillar with a width that differed from theory. The team discovered that this was due to subtle density changes near the pillar, a new principle to consider in the design of composite materials.

When building a house of cards, it’s not enough to have cards with the right weight or stiffness: the trick is to put them together to create a stable structure. The same can be said of composite materials. When scientists think of materials made up of many different components, the properties of the final product are not simply a linear sum of its parts, but are highly dependent on its complex internal structure. Composites encompass a wide range of materials, from those that make up our bodies like bones and muscles, to bulk materials like plastic resins and reinforced concrete; the study of how physical properties such as stiffness and ductility arise from their structure is a key facet of modern materials science.

A team led by Professor Rei Kurita of Tokyo Metropolitan University used computer simulations to study how composite materials react to external forces. They constructed a “soft particle model” consisting of beads arranged in a pattern that attract or repel each other in a confined space. The force with which they do this determines the softness or hardness of the “model” material. In their most recent work, they investigated how a pillar of stiff beads surrounded by softer beads reacts to an external load, as if weight were applied to the material. Their work aimed to study how simple composite materials react to a load. Although they quickly found that the material’s reactive force was concentrated around the rigid pillar, as predicted by conventional models, it was clear that the lateral range over which the force was distributed was significantly wider than predicted.

The team explored how each of their soft particles moved under load, finding that the vertical displacement was very uniform, much like the different segments of a spring under load. However, this was not the case for displacements perpendicular to the load. Contrary to theory, the effect of the load was distributed over a significant distance from the pillar. This caused minor changes in the distance between the particles, which effectively led to fewer particles per volume or density. This incredibly subtle change has led to a significant deviation in the material’s response from “continuum” models which overlook the particulate nature of the material. The effect was not observed when the interparticle interactions were made attractive instead of repulsive since the attraction made the material much less prone to local density changes.

Despite the simplicity of the model, the results highlighted by this work show that incredibly subtle characteristics of composite materials can lead to very different physical properties. The team hopes that the generality of their findings will help inspire a wider range of explorations in the design and applications of composite materials.

This work was supported by JSPS KAKENHI Grant Numbers 20K14431, 17H02945 and 20H01874.

/Public release. This material from the original organization/authors may be ad hoc in nature, edited for clarity, style and length. The views and opinions expressed are those of the author or authors. See in full here.

Sharon D. Cole