This research seeks to understand how the internal structure of tough heterogeneous materials enhances fracture resistance and to use this knowledge to create novel tough materials. It is often very difficult to transcribe, let alone improve or optimize, clever designs in natural highly-tough materials into synthetic materials, partly because their complex structures occur over many length scales. A systematic experimental study will be conducted on several naturally-occurring materials that show the highest resistance to failure by fracturing, such as woods, bone and nacre. The study will measure all the key properties of these materials needed to describe them in a theoretical framework that is more sophisticated and powerful than the one commonly employed to describe material behavior. Not only will this deepen the knowledge of existing highly fracture-resistant materials, the more powerful theoretical framework will then be employed to explore, optimize and create new highly-fracture-resistant materials. Such materials will be useful in making stronger, lighter and safer components and structures in a wide variety of civilian, manufacturing and aerospace applications.
The approach of this project is understanding and modeling high toughness microstructures, and creation of novel ultra-tough materials: model microscale material response by a generalized continuum theory (Cosserat elasticity) that captures the effects of microstructure in a richer, yet still continuum, way. A systematic experimental investigation will determine, for the first time, all Cosserat moduli for several naturally toughest materials: woods, bone and nacre. This will reveal the key material microstructural characteristics, as captured by Cosserat theory, that produce high toughness. A closely allied theoretical investigation will: (i) analytically determine Cosserat moduli for several of these naturally tough microstructures; (ii) employ Cosserat material to represent near-crack-tip (fracture process zone) material response to confirm these characteristics lead to high toughness, and to permit Cosserat moduli optimization to produce the highest possible toughness. This approach has the fascinating prospect to escape and improve upon biomimetics (Nature has not discovered all nor perhaps even the best tough microstructures) by taking the key Cosserat features learned from testing naturally tough materials, and from the theoretical toughness optimization, and employing these to create novel fracture-tough microstructures exhibiting these key Cosserat features. Such new microstructures could differ substantially from those of the natural materials. Fabrication and testing of novel materials created in this way will confirm their high toughness. An integrated outreach an education plan consisting of demonstration materials, course enhancement, web resource development, and interdisciplinary research and training interactions will be pursued.