Tooth enamel comprises parallel microscale and nanoscale ceramic columns or prisms interlaced with a soft protein matrix. This structural motif is unusually consistent across all species from all geological eras. Such invariability-especially when juxtaposed with the diversity of other tissues-suggests the existence of a functional basis. Here we performed ex vivo replication of enamel-inspired columnar nanocomposites by sequential growth of zinc oxide nanowire carpets followed by layer-by-layer deposition of a polymeric matrix around these. We show that the mechanical properties of these nanocomposites, including hardness, are comparable to those of enamel despite the nanocomposites having a smaller hard-phase content. Our abiotic enamels have viscoelastic figures of merit (VFOM) and weight-adjusted VFOM that are similar to, or higher than, those of natural tooth enamels-we achieve values that exceed the traditional materials limits of 0.6 and 0.8, respectively. VFOM values describe resistance to vibrational damage, and our columnar composites demonstrate that light-weight materials of unusually high resistance to structural damage from shocks, environmental vibrations and oscillatory stress can be made using biomimetic design. The previously inaccessible combinations of high stiffness, damping and light weight that we achieve in these layer-by-layer composites are attributed to efficient energy dissipation in the interfacial portion of the organic phase. The in vivo contribution of this interfacial portion to macroscale deformations along the tooth's normal is maximized when the architecture is columnar, suggesting an evolutionary advantage of the columnar motif in the enamel of living species. We expect our findings to apply to all columnar composites and to lead to the development of high-performance load-bearing materials.

Materials combining high stiffness and mechanical energy dissipation are needed in automotive, aviation, construction, and other technologies where structural elements are exposed to dynamic loads. In this paper we demonstrate that a judicious combination of carbon nanotube engineered trusses held in a dissipative polymer can lead to a composite material that simultaneously exhibits both high stiffness and damping. Indeed, the combination of stiffness and damping that is reported is quite high in any single monolithic material. Carbon nanotube (CNT) microstructures grown in a novel 3D truss topology form the backbone of these nanocomposites. The CNT trusses are coated by ceramics and by a nanostructured polymer film assembled using the layer-by-layer technique. The crevices of the trusses are then filled with soft polyurethane. Each constituent of the composite is accurately modeled, and these models are used to guide the manufacturing process, in particular the choice of the backbone topology and the optimization of the mechanical properties of the constituent materials. The resulting composite exhibits much higher stiffness (80 times) and similar damping (specific damping capacity of 0.8) compared to the polymer. Our work is a step forward in implementing the concept of materials by design across multiple length scales.

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Please cite this article as: Sain, T., Meaud, J., Yeom, B., Waas, A.M., Arruda, E.M., Rate dependent finite strain constitutive modeling of polyurethane and polyurethane-clay nanocomposites, International Journal of Solids and Structures (2014), doi: http://dx.Abstract A finite strain nonlinear viscoplastic constitutive model for polyurethane (PU)-Montmorillonite clay (MTM) nanocomposites is developed with the goal of characterizing the mechanical response under different strain rates and strain amplitudes. In this model, both the elastic and viscous responses are considered to be nonlinear. It is shown that a simple mathematical extension of the model used to characterize the PU determines the nonlinear material constants for the PU-MTM. A finite deformation nonlinear viscoelastic model is used to represent the mechanical behavior of PU. The rate dependent viscous behavior and multiple relaxation times present in the PU response are determined using the frequency dependent tan δ measurements from Dynamic Mechanical Analysis (DMA). The model is capable of accurately capturing both the rate dependent behavior and frequency dependent damping of PU. The entire rate dependent hysteresis behavior (loading-unloading) is predicted accurately through the constitutive model for strains up to 10%. For the PU-MTM nanocomposite, the constitutive stress update is implemented in a finite element (ABAQUS/Explicit) framework and validated using a range of experimental results. The model predictions show excellent agreement with experimental results in capturing rate dependent loading/unloading responses for both PU and PU-MTM nanocomposites. The proposed model can easily be extended to characterize other polyurethane based nanocomposites.