Interview with Professor Pablo Zavattieri, Director of the Multi-Scale Mechanics and Materials by Design Lab at Purdue University and SIMULIA Champion
By @SJ
Biological materials show tremendous strength and toughness. Simulation is revealing the mechanisms that nature uses and inspiring new kinds of engineering materials with superior properties to conventional ones.
Humans have made huge advances in materials science, but evolution has given nature a billion-year advantage. Many artificial materials have high degrees (maybe levels) of strength or stiffness: they can withstand strong forces without deforming or breaking. Many have high toughness: a lot of energy is needed to form a crack. In general, there is a trade-off between these two – to increase strength, the engineer must sacrifice toughness and vice versa, according to the so-called “banana curve”.
The “banana curve” for stiffness (X-axis) versus toughness (Y-axis) for engineering materials (on lower and left axes) and biological materials (on upper and right axes)
Biological materials – materials found in nature produced by living organisms – can be both strong and tough at the same time, following an “inverted banana curve”. Their secret is in their architecture. Biology can work at extremely small scales, producing complex composites. For instance, many hard biological materials such as bone and shell consist of small pieces of strong but brittle minerals in a precise arrangement bound with a tough organic “glue”.
Being able to mimic biological structures, opens up the possibility of creating artificial materials with properties that are currently impossible. As a result, there is a huge amount of research into understanding precisely how biological materials work.
One group at the forefront of this study is the Multi-Scale Mechanics and Materials by Design Lab at Purdue University, under its director, Professor Pablo Zavattieri. Their work includes examining biological materials at the microscopic level, simulating their behavior to understand how they work, and constructing artificial material prototypes that mimic this behavior.
Zavattieri started thinking about biological materials 15 years ago when he was working for the automotive industry. “My former advisor told me that the ceramic models that I developed during my Ph.D. work could be extended and used to explain the toughness of nacre, which has a toughness that is 3 orders of magnitude higher than that of the main constituent, aragonite: CaCO3 – or calcium carbonate, which is known to be a very brittle material.([1])” This discovery inspired him to think more deeply about biological materials and how to study their mechanisms.
There are three steps to understanding biological materials. The first is to characterize the structure. That means discovering the geometrical aspects of the architecture, the hierarchy of components and their distribution. They also need to characterize the fundamental mechanical properties of the individual constituent materials and how the individual building blocks behave.
Once the components of the structure are identified, the team then determines their function. This involves collaborating with biologists to work out the loading conditions, constraints, and environment that the material operates in, and the evolutionary forces that produced it.
Finally, the researchers identify and understand the damage mitigation mechanisms that are active. This requires “breaking” the material under the microscope and making observations. Most often, the group finds that the mechanisms of failure are counterintuitive or unexpected, and that is ultimately the inspiration for hypotheses and further questions.
To investigate these hypotheses, Multi-Scale Mechanics and Materials by Design Lab researchers use physical experiments and computer models. One important tool is structural simulation using Abaqus. The simulation can be quickly re-run with different parameters – for example, different geometries or layer thicknesses – in order to find the critical values that determine the strength and toughness of the material. The simulation can also visualize the exact distribution of forces and strains within the structure for a better understanding of how it works.
For example, the mantis shrimp has club-like front limbs that it can accelerate at over 100,000 ms-2 (10,000 g) to club or impale prey and attackers. Its limbs, therefore, need a very strong material in order to withstand these incredible forces. It achieves this with a “Bouligand” structure, also known as a helicoidal structure: the impact layer is constructed of layers of fibers, each layer being slightly twisted relative to the one above. Each fiber consists of many fibrils in a mineral matrix, and each fibril is in turn made of many proteins and polysaccharides.
Multiscale simulation captures the behavior of the materials at each level, allowing the lab to study why this structure is so strong and how cracks propagate through the structure. “These biological materials are so complex that you can actually see that in microscopes. It's what we call hierarchical structures. At different scales, you see different geometries and patterns, and they actually have different roles of course. So understanding that at different scales is just one step.”
They can then use 3D printing to construct synthetic composites with the same structure – experiments can verify the failure mode identified by the simulation and the best selection of materials to substitute for the biological minerals and proteins.
“If you understand the concept, then you realize it's not the geometry that I want to replicate, what you want to replicate are the underlying mechanisms and functionality. I want to see how it breaks, how it deforms. Finite element analysis is the key. We put all these mechanisms into this model, we build the geometry, and then we start looking into questions. What if I replace mineral with ceramic?”
The helicoidal architecture found in the mantis shrimp is now being applied to carbon-fiber and glass-fiber reinforced composites, and there is interest from industries ranging from aerospace to sport.
Photo (left) and computer model (right) of the helicoidal structure in a mantis shrimp
“We are working on 3D printing of cement, using the kind of architectures that we learned from the mantis shrimp or lobsters or crabs, and we are seeing an increase in toughness,” Zavattieri continues. “In the future, we are going to actually be able to add more details into these materials. Adding more nanomaterials into the filament or the ink that we are printing, trying to replicate nature. And we are going to be able to combine different materials as we develop better additive manufacturing techniques.”
Another example comes from the diabolical ironclad beetle, a beetle with an exceptionally hard shell that can withstand even the weight of a car. The wing cases (elytra) of the diabolical ironclad beetle are made of layers of chitin in a protein matrix, similar to other insects. However, these are fused together, connected by a complex interface that resembles a jigsaw. This “puzzle architecture” gives the materials incredible toughness without significantly compromising on strength.
A particular advantage of the puzzle architecture is it potentially offers a very strong way to join two different materials without welds or chemical bonds.
Different degrees of curvature with the expected failure mode of each.
Delamination in a puzzle blade, simulated in Abaqus
Not all biological materials are static. Cellular materials can move between two states with snapping mechanisms. One such example of a bistable mechanism in nature is a Venus flytrap, which can move from open to close under a second. Periodic architected materials with a series of bistable mechanisms that show this snapping phase transformation leads to interesting applications, from energy absorption to actuation. These can be used to build components for applications like crash protection in vehicles or to create medical devices such as stents that can be activated within the body.
“In critical applications where we are trying to protect something like a human being in a car, or on an airplane, you want the structure to fail, but then to fail gracefully, and to dissipate energy,” Zavattieri. “The diabolical ironclad beetle actually shows that. It's a very nice example in nature, where once it reaches the maximum load, then it still takes more time or more energy to fully break it. So the beetle can actually survive a catastrophic event like a rock falling or a predator actually trying to bite it. The shell will eventually break and the beetle will need time to recover, but at least it can actually escape from that situation.”
Although the lab has already investigated numerous biological materials, they have still only scratched the surface according to Zavattieri. “There is still a long list of species we haven’t studied yet. There could be 15 million species out there, and only 2 million are known to science – not to count those that existed in the past such as trilobites, ammonites, dinosaur bones. So imagine the number of possibilities that are there. This is the tip of the iceberg. We can ensure that we will be learning new mechanisms and principles from nature for a long time.”
References
[1] F. Barthelat, H. Tang, P.D. Zavattieri, C-M. Li and H.D. Espinosa, ”On the mechanics of mother-of-pearl: A key feature in the material hierarchical structure”, Journal of the Mechanics and Physics of Solids, 55(2), pp. 306-337, 2007
Abaqus Material Modeling Academics SIMULIA Champions Biological Structures
Interview with Professor Zavattieri
Learn how Professor Pablo Zavattieri uses simulation to reveal how the club-like front limbs of the mantis shrimp can punch through the tough outer shell of its prey. Its limbs need a very strong material in order to withstand incredibly high impact forces. Learn how simulating the structures in the Mantis Shrimp claw is providing valuable insights and some surprises into new ways of thinking about design and composites.
