High-energy absorption and light-weightiness are two critical properties for impact protection in the aerospace sector. In the past, the use of periodic honeycomb structures or random porous metallic foams were the preferred route to obtain a good specific-energy absorption performance. In recent years, the use of additive manufacturing has increased the design freedom creating a new generation of reticulated and porous materials: the metamaterials or lattice materials. The internal geometries of these lattice structures can be tuned for superior optimal properties, e.g., energyabsorption and density. However, the mechanics of these materials under impact need to be understood with the purpose of mechanical optimisation, and the computational models validated. In this work, we present the experimental compressive behaviour, at room temperature, of two Ti6Al4V lattice structures under static and dynamic conditions. The quasi-static tests were performed by using a universal testing machine while the dynamic tests were conducted at 480s-1 with a split-Hopkinson bar. In all cases, the deformation process was filmed to analyse the failure. Finally, finiteelement simulations were done, employing the Johnson-Cook model, to describe the response of the alloy. The simulations were able to reflect the failure characteristics of each metamaterial but were not able to describe the macroscopic response due to the differences between the experimental and computational volume fraction.

Additive Manufacturing technologies, such as Laser Powder Bed Fusion, have enabled the creation of complex geometrical designs which can be used for lightweighting purposes across multiple industries. One of the most common methods to reduce weight in the design stage is the use of topological optimization or lattice structures. Lattice structures consist of nodes connected with struts in different orientations in space, with the configuration of the unit cell varying depending on the final application. It is an established fact that the mechanical properties of additively manufactured components vary as a function of the size and orientation of the printed part. This inhomogeneity in properties is often neglected by material models implemented in finite element analysis, which normally just consider the mechanical properties of the bulk material. In this work, single struts of different diameters (0.5 mm and 1 mm) and orientations (0° and 45°) were tested to determine the corresponding mechanical properties and use them as an input to construct a material model to predict the mechanical properties of lattice structures. Validation against experimental behavior of two different lattice structures shows improved accuracy over bulk properties material models.

Architected metallic metamaterials fabricated by additive manufacturing are called to expand infinitely the variety of available properties observed in bulk alloys. However, the high surface-to-volume ratio of the architected metamaterials due to their intricate geometries and the surface inherited of the AM process is translated to a complex fatigue behaviour when compared with bulk conventional alloys. This is a serious concern in the use of this new class of architected AM materials in technological applications. In this work, this problematic is tackled by a systematic multiscale study of the metamaterial design -processing and defects -fatigue properties' interconnection. Commercial aluminium alloy AlSi10Mg processed by selective laser melting is used as base material. By means of combined fatigue experimentation, computational modelling and defect identification, the effect of processing conditions and design geometry on microstructural defects and surface quality is rationalised and connected with the fatigue life of metamaterials.

3D printed metamaterials are a human-designed class of material capable of providing uncommon properties unseen in nature. However the freedom power of design provided by additive manufacturing in metamaterials is useless if reliable and efficient modelling design tools are not available. The objective of this work is to evaluate, validate and study the different options for finite element simulation of bio-inspired metamaterials to provide the best solutions in terms of material properties accuracy and computational efficiency. Based on this study we have developed a new hybrid material agnostic modelling method to compute the mechanical response of beam based bioinspired metamaterials with similar precision of 3D explicit meshed models. Different variables in the latticed models were investigated, these included different element sizes and element types (volumetric and beam elements). The effects of these variables on the elastic modulus and yield strength of a lattice structure were addressed. The geometrical models were printed in Ti6Al4V using selective laser melting technique and experimentally tested for the validation of the computational results. On the basis of the volumetric results, multi-material beam models were constructed and evaluated providing with accurate results in low computational times and recreating the plastic failure phenomena observed experimentally.