Students Working on Maps to Predict Strength of Composite Structures

Mother of perl, the iridescent layer in the shells of some mollusks has inspired a Rice University study that judge the ultimate strength, stiffness and toughness of composite materials.

Researchers Rouzbeh Shahsavari and Navid Sakhavand have made universal maps that predict the properties of natural and biomimetic platelet-matrix composites, like mother of pearl and synthetic stacks of materials like graphene and boron nitride.

These computer-drawn maps are “dimensionless” and their findings will work as well for materials built with nanoscale blocks as they would for a brick wall, or bigger.

That’s the beauty of this approach: It can scale to something very large or very small.

The research which appeared this week in Nature Communications relies on four characteristics of the individual materials under consideration for a composite: their length, a ratio based on their respective stiffness, their plasticity and how they overlap.

If you know these characteristics, you can predict the stiffness, strength and toughness of the final composite. They call this a universal map because all of those input parameters are relevant to all composites and their structural properties.

This illustration compares the properties of composite structures based on their calculations. The researchers created a design map that predicts the strength, stiffness and toughness of composites regardless of size.
This illustration compares the properties of composite structures based on their calculations. The researchers created a design map that predicts the strength, stiffness and toughness of composites regardless of size.

To materials scientists and engineers, stiffness, toughness and strength are distinct, important mechanical properties. Strength is the ability of a material to stay together when stretched or compressed, stiffness is how well a material resists deformation and toughness is the ability of a material to absorb energy before failure.

The team designed maps to show how materials rate in all three categories and where they overlap. Their goal is to help engineers calculate the ultimate qualities of a material and cut down on trial and error.

The study began when Shahsavari took a close look at the architecture of mother of pearl (nacre), which maximises both strength and toughness. Under a microscope, it looks like a well-built brick wall with overlapping platelets of different lengths held together by thin layers of an elastic biopolymer.

It has a very particular structure and property: It optimises different mechanical properties at the same time.

However, engineering nacre-like composites has been difficult so far, mainly because of the lack of a design map that can reveal the various links between the structure, materials and properties of nacre-like materials. The work is an important milestone toward a better ability to decode and replicate nacre’s architecture for lightweight, high-performance composites which could benefit the aerospace, auto and construction industries in the future.

The work has spanned three years of calculation and experimentation that involved mapping the properties of natural composites like collagen and spider silk as well as synthetic stacks like hexagonal boron-nitride/graphene and silumin/alumina. They also tested their theory on macro-scale, 3-D printed composites of hard plastic and soft rubber that mimicked the properties they observed in nacre.

A map of 15 of the materials they tested shows natural ones like nacre tend to be strong and tough while synthetics lean toward strong and stiff. Shahsavari said he hopes materials scientists will use the design maps to give their composites the best possible combination of all three properties.

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