Dec 5 2017
How a material breaks is one of the vital properties scientists consider when designing layered composites that mimic those discovered in nature. Rice University engineers have developed a method to decode the interactions between materials and the structures they form and this can help increase their stiffness, toughness, strength, and fracture strain.
An illustration shows a model platelet-matrix composite in the foreground and nacre, one of nature’s toughest materials, in the background. Courtesy of the Multiscale Materials Laboratory
In a study, Rouzbeh Shahsavari, a materials scientist from Rice, and Shafee Farzanian, a visiting scholar, ran more than 400 computer simulations of platelet-matrix composite materials like mother-of-pearl in order to develop a design map to assist in synthesizing staggered composites for applications at any scale, including microelectronics, spacecraft and cars where multifunctional, lightweight structural composites are important.
The new model incorporates the geometries and properties of a range of platelet and matrix components to compute the stiffness, toughness, strength, and fracture strain of the composite. Changing any compositional or architectural parameter adjusts the entire model as the user looks for the optimal psi, a quantification of its ability in order to avoid catastrophic failure.
The research is published in the Journal of Mechanics and Physics of Solids.
Natural composites are widespread. Examples include bamboo, tooth enamel, nacre (mother-of-pearl) and the dactyl clubs of mantis shrimp, all are nanoscale arrangements of hard platelets attached by soft matrix materials and arranged in bouligand, overlapping brick-and-mortar, or other architectures.
They work because hard parts are flexible enough (because of the soft matrix) to distribute stress throughout the material and strong enough to take a beating. They can often distribute or limit the damage without failing completely when they fracture.
Lightweight natural materials are abundant,in these types of materials, two kinds of toughening happen. One comes before crack propagation, when the platelets slide against each other to relieve stress. The other is part of the beauty of these materials: the way they toughen after crack propagation. Even when there is a crack, it does not mean a failure, the crack may be arrested or deflected several times between the layers. Instead of going straight through the material to the surface, which is a catastrophic failure, the crack bumps into another layer and zigzags or forms another complex pattern that delays or entirely prevents the failure. This is because a long and complex crack trajectory requires much more energy to drive it, compared with a straight crack.
Rouzbeh Shahsavari, materials scientist from Rice University
For years, researchers and engineers have worked to mimic the stiff, strong, tough, and light properties of natural materials, either with soft and hard components or combinations different types of platelet.
To engineers, strength, toughness and stiffness are distinct characteristics. Strength is the ability of a material to remain together when compressed or stretched. Toughness is the ability of a material to attract energy before failure. Stiffness is how a material resists deformation properly. In an earlier paper, the Rice lab developed a map to predict the properties of composites depending on those parameters before crack propagation.
The addition of crack-induced toughening in biomimetic and natural materials is another interesting and potent source of toughening that offers extra lines of defense against failure, Shahsavari said. “The models uncovered nonintuitive synergies between the before- and after-crack toughening phenomena,” he said. “They showed us what architectures and componensts would allow us to combine the best properties of each.”
Using the baseline model, the researchers were able to modify four values for each simulation: the platelet overlap offset, the platelet dissimilarity ratio (when over one type of platelet is involved), plasticity of the matrix, and characteristic platelet length, all of which are vital to the properties of the composite.
Shahsavari said that over the course of 400 simulations, the model demonstrated the greatest factor in psi was possibly platelet length. The model revealed that short platelets mostly yield fracture control to the plasticity of the soft matrix, whereas long platelets take it back. By distributing the fracture evenly and allowing maximum crack growth, platelet lengths can attain the optimal psi and make material better able to prevent catastrophic failure.
In addition, the model will help researchers design whether a material will fail with an unexpected facture, like ceramics, or slowly, like ductile metals, by changing components, using contrasting platelets or adjusting the architecture.
Shahsavari is an assistant professor of civil and environmental engineering and of materials science and nanoengineering.
The research was supported by Rice’s Department of Civil and Environmental Engineering and the National Science Foundation. Supercomputing resources were provided by the National Institutes of Health and an IBM Shared University Research Award in collaboration with Adaptive Computing, Qlogic, and Cisco, as well as Rice’s National Science Foundation-supported DAVinCI supercomputer administered by the Center for Research Computing and procured in collaboration with Rice’s Ken Kennedy Institute for Information Technology.