Additively manufactured porous metamaterials: an investigation on compressive performance, failure characterisation and the effects of geometry variations of uniform and stochastic lattices

Abstract

Natural porous materials have evolved over millennia to optimise lightweight
mechanical performance, including properties such as stiffness, strength and energy absorption at low density. As a result, these natural materials occupy a large range of material property space, enabling properties to be efficiently matched to performance requirements. As manufacturing techniques have developed, the property space occupied by synthetic materials has expanded accordingly, but often without the combined performance and efficiency of natural materials. Advances in additive manufacturing (AM) have enabled the synthetic material property space to be expanded further by enabling porous materials with variations in relative density, pore shapes and spatial distribution to achieve tailored mechanical properties.

Synthetic structures consist of assemblies of repeated unit cells. Analytical models based on axial (stretching-dominated) or bend (bending-dominated) modes of loading for different unit cells can predict the mechanical response of uniform (regular) lattice structures, but these models break down with increasing relative density as they are based on slender beam assumptions that become invalid, and are also not applicable to non-uniform (spatially varying) lattices. A large region of the accessible material property space is therefore not described by these analytical models. Additionally, an often under-reported feature of additively manufactured porous materials is the presence of distortion due to residual stress, which can alter their boundary conditions and consequently their apparent properties. This project aimed to further our understanding of how relative density, geometry, and distortion affect the behaviour of additively manufactured lattice structures by characterising the geometries, and mechanical behaviour of both uniform and non-uniform bending and
stretching-dominated lattices over a range of relative densities.

Lattice specimens were manufactured using stereolithography AM of a commercial glass-reinforced thermosetting resin with a high base elastic modulus but low strain to failure. Height distortions were observed in all types of lattices but decreased with relative density and for non-uniform structures. An adapted version ofWinkler’s elastic foundation model determined that a typical distortion of 100 μm reduced the initial apparent elastic modulus by approximately 50%, with greater distortions reducing this further. The as-built density was greater than as-designed for all lattice geometries, and analytical models from literature were empirically adjusted to account for the increase in strut diameters observed. Other analytical models from literature that described the relationships between relative density and mechanical properties for uniform lattice structures were validated over a wide relative density range (15 to 70% depending on geometry type), with improvements suggested using empirical fits. These revealed that the apparent elastic modulus relationship was similar to natural materials such as wood and bone. For all uniform lattices, relative density increased the apparent elastic modulus, maximum stress and energy absorption. Further increases for non-uniform geometries were observed for the apparent elastic modulus and maximum stress, thought to be due to the accumulation of excess material at the joints between dissimilar adjacent cells. The failure strains for all geometries were at least double that of the base material with trends dependent upon the geometry of the unit cells. They increased with relative density for uniform stretching-dominated lattices, but decreased for uniform bending-dominated geometries. Non-uniform structures had similar or reduced levels of energy absorption and failure strains to uniform structures, with the lower-density unit cells often resulting in catastrophic failure at lower strain.

Both the uniform and non-uniform lattices produced in this project added to the
material property space in regions that overlap with cancellous bone, an example of an evolutionary optimised natural porous material. The methodologies developed in this project provide a good basis for designing and characterising further non-uniform lattice geometries to continue expanding this space.

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Identifiers

ORCID for Maria Stagno Navarra: orcid.org/0009-0000-0816-7356

ORCID for Martin Browne: orcid.org/0000-0001-5184-050X

ORCID for Andrew Hamilton: orcid.org/0000-0003-4627-849X

ORCID for Charles Burson-Thomas: orcid.org/0000-0001-9308-4669

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