Computational models for impact mechanics and related protective materials and structures

The mechanics of impacts is not yet well understood due to the complexity of materials behaviour under extreme stress and strain conditions and is thus of challenge for fundamental research, as well as relevant in several areas of applied sciences and engineering. The involved complex contact and strain-rate dependent phenomena include geometrical and materials non-linearities, such as wave and fracture propagation, plasticity, buckling, and friction. The theoretical description of such non-linearities has reached a level of advance maturity only singularly, but when coupled -due to the severe mathematical complexity- remains limited. Moreover, related experimental tests are difficult and expensive, and usually not able to quantify and discriminate between the phenomena involved. In this scenario, computational simulation emerges as a fundamental and complementary tool for the investigation of such otherwise intractable problems. The aim of this PhD research was the development and use of computational models to investigate the behaviour of materials and structures undergoing simultaneously extreme contact stresses and strain-rates, and at different size and time scales. We focused on basic concepts not yet understood, studying both engineering and bio-inspired solutions. In particular, the developed models were applied to the analysis and optimization of macroscopic composite and of 2D-materials-based multilayer armours, to the buckling-governed behaviour of aerographite tetrapods and of the related networks, and to the crushing behaviour under compression of modified honeycomb structures. As validation of the used approaches, numerical-experimental-analytical comparisons are also proposed for each case

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We study the ballistic properties of two-dimensional (2D) materials upon the hypervelocity impacts of C₆₀ fullerene molecules combining ab initio density functional tight binding and finite element simulations. The critical penetration energy of monolayer membranes is determined using graphene and the 2D allotrope of boron nitride as case studies. Furthermore, the energy absorption scaling laws with a variable number of layers and interlayer spacing are investigated, for homogeneous or hybrid configurations (alternated stacking of graphene and boron nitride). At the nanolevel, a synergistic interaction between the layers emerges, not observed at the micro- and macro-scale for graphene armors. This size-scale transition in the impact behavior toward higher dimensional scales is rationalized in terms of scaling of the damaged volume and material strength. An optimal number of layers, between 5 and 10, emerges demonstrating that few-layered 2D material armors possess impact strength even higher than their monolayer counterparts. These results provide fundamental understanding for the design of ultralightweight multilayer armors using enhanced 2D material-based nanocomposites

Figure S4 from Multilayer stag beetle elytra perform better under external loading via non-symmetric bending properties

Comparison of bending FEM curves obtained with three different set of interface properties (τ_lim = 1/2 σ_lim) and the experimental curves. The results suggest optimal interface strength (τ_lim = 5.5 MPa) in the elytra

Figure S3 from Multilayer stag beetle elytra perform better under external loading via non-symmetric bending properties

Detail of the FEM model with identification of the different elytra structures. The top layer and trabecular structures are built with solid elements while thick shell elements are used for the middle layer and the bottom layer. The encircled region corresponds to the volume of an hemisphere of radius r equal to the trabecular width for which the adjacent nodes of different layers are tied together, thus excluded from the delamination law. This solution was employed to taken into account the fact that trabecular structures are prolongations and folds of the layers constituting the endocuticle middle layer (Figure 1 in the main text)

Finite Element Modelling details from Multilayer stag beetle elytra perform better under external loading via non-symmetric bending properties

Insect cuticle has drawn a lot of attention from engineers because of its multifunctional role in the life of insects. Some of these cuticles have an optimal combination of lightweight and good mechanical properties, and have inspired the design of composites with novel microstructures. Among these, beetle elytra have been explored extensively for their multilayered structure, multifunctional roles and mechanical properties. In this study, we investigated the bending properties of elytra by simulating their natural loading condition and comparing it with other loading configurations. Further, we examined the properties of its constitutive bulk layers to understand the contribution of each one to the overall mechanical behaviour. Our results showed that elytra are graded, multilayered composite structures that perform better in natural loading direction in terms of both flexural modulus and strength which is likely an adaptation to withstand loads encountered in the habitat. Experiments are supported by analytical calculations and finite-element method modelling, which highlighted the additional role of the relatively stiff external exocuticle and of the flexible thin bottom layer, in enhancing flexural mechanical properties. Such studies contribute to the knowledge of the mechanical behaviour of this natural composite material and to the development of novel bioinspired multifunctional composites and for optimized armours

Figure S2 from Multilayer stag beetle elytra perform better under external loading via non-symmetric bending properties

Images of the FEM model of the simulated three point bending test (a) isometric view, (b) lateral view (c) corresponding experimental sample set up

Figure S5 from Multilayer stag beetle elytra perform better under external loading via non-symmetric bending properties

FEM images showing the von-Mises stress distribution (unit of measure GPa) in the wing and the beetle body under a concentrated load of 0.5 N .A) real structure with void, B) elytra with no void