Federico Bosia

Biological materials such as spider silk display hierarchical structures, from nano to macro, effectively linking nanoscale constituents to larger-scale functional material properties. Here, we develop a model that is capable of determining the strength and toughness of elastic-plastic composites from the properties, percentages, and arrangement of its constituents, and of estimating the corresponding dissipated energy during damage progression, in crack-opening control. Specifically, we adopt a fiber bundle model approach with a hierarchical multiscale self-similar procedure which enables to span various orders of magnitude in size and to explicitly take into account the hierarchical topology of natural materials. Hierarchical architectures and self-consistent energy dissipation mechanisms (including plasticity), both omitted in common fiber bundle models, are fully considered in our model. By considering one of the toughest known materials today as an example application, a synthe...

ABSTRACT In this work we study the mechanical properties and failure mechanism of nanocomposites of graphene oxide sheets embedded in polymeric systems, namely films and electro-spun nanofibers. In this last system, contrary to conventional bulk composites, the size of the nano-reinforcement (GO sheets) is comparable to the size of the nanofibers to be reinforced (approximate to 200 nm). As polymeric matrix we use gelatin. We demonstrate that the high chemical affinity of the two materials hinders the renaturation of gelatin into collagen and causes a nearly ideal mixing in the GO-gelatin composite. Adding just 1% of GO (wt of GO with respect to gelatin) we obtain an increase of Young's modulus >50% and an increase of fracture stress >60%. We use numerical simulations to study the failure mechanism of the fibers. Calculations well agree with experimental data and show that, even if cracks start at GO sheet edges due to stress concentrations, crack propagation is hindered by the nonlinear behaviour of the matrix. Moreover, the presence of the GO sheets in continuous gelatin films improves the material stability to phosphate buffer solutions from 2 days to 2 weeks, making it a better material than gelatin for applications in biological environments.

A so-called “interaction-box” formalism, which has recently been introduced to describe hysteresis in dynamical systems in the case of higher harmonic generation, is further discussed and generalized to describe the phenomenon of subharmonic generation. In this case, the increase in the periodicity of the response is reflected in the formation of multiple loops in the Effect (output) vs. Cause (input) diagrams. Conversely, we show how this type of response represents a sort of “signature” of the system, and can thus be employed to draw general conclusions about the features of the latter. A specific example of a nonlinear system is chosen to illustrate the approach, namely a vibrating cantilever beam with a breathing crack. Effect vs. Cause curves are calculated for this system in the presence of higher harmonics and subharmonics.

We present a theoretical and numerical analysis of the mechanical behavior of self-healing materials using an analytical model and numerical calculations both based on a Hierarchical Fiber Bundle Model, and applying them to graphene- or carbon-nanotube-based materials. The self-healing process can be described essentially through a single parameter, that is, the healing rate, but numerical simulations also highlight the influence of the location of the healing process on the overall strengthening and toughening of the material. The role of hierarchy is discussed, showing that full-scale hierarchical structures can in fact acquire more favorable properties than smaller, nonhierarchical ones through interaction with the self-healing process, thus inverting the common notion in fracture mechanics that specimen strength increases with decreasing size. Further, the study demonstrates that the developed analytical and numerical tools can be useful to develop strategies for the optimization of strength and toughness of synthetic bioinspired materials.