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This paper presents a new framework for lightweight optimisation of functionally graded lattice structures (FGLSs) with a particular focus on enhancing and guaranteeing structural safety through three main contributions. Firstly, a design strategy of adding fillets to the joints of body-centred cubic (BCC) type lattice cells was proposed to improve the effective yield stress of the lattices. Secondly, effective properties of lattice metamaterials were experimentally characterised by conducting quasi-static uniaxial compression tests on selective laser melted specimens of both Ti6Al4V BCC and filleted BCC (BCC-F) lattices with different relative densities. Thirdly, a yield stress constraint for optimising FGLSs was developed based on surrogate models quantifying the relationships between the relative density and the effective properties of BCC and BCC-F lattices developed using experimental results assisted by numerical homogenisation. This framework was tested with two case studies. Results showed that structural safety with respect to avoiding yield failure of the optimised FGLSs can be ensured and the introduction of fillets can effectively improve the strength-to-weight ratio of the optimised FGLSs composed of BCC type lattices. The BCC-F FGLS achieved 14.5% improvement in weight reduction compared with BCC FGLS for the Messerschmitt-Bölkow-Blohm beam optimisation case study.

Artificially designed lattice based structures, enabled by additive manufacturing are promising in various engineering applications due to their high stiffness and strength with low density and attractive multifunctional properties. In this work, a robust framework has been developed for structural optimisation by generating graded lattice structures. The goal of optimisation was to achieve the minimum structural weight while satisfying the stiffness requirement. Periodic representative volume element (RVE) homogenisation method was employed to calculate the effective mechanical properties of a unit cell of the lattice structure. A metamaterial model was determined to represent the relationship between the effective elastic constants and the geometric parameter, i.e. the strut radius of quasi-isotropic BCC lattice unit cell. Mesh effect analysis was carried out to capture the optimal Finite Element (FE) mesh size for numerical simulation, taking into consideration of the trade-off between accuracy and efficiency. The optimisation process was conducted through commercial software Optistruct by applying the method of feasible directions (MFD) algorithm for optimisation, to achieve the optimal distribution of lattice strut radii. In post-processing, local maximum radius values were applied to joints of lattice unit cells to avoid sharp changes of strut radii between adjacent unit cells. Finally, a case study of 3-point bending beam was conducted to examine this framework and it was found that the proposed optimisation framework is valid for design and optimising graded lattice structures.

Herein, the compressive strength and reliability of a prime cubic (PC) lattice structure are studied. The PC lattice structure is treated as a 3D consecutive k‐out‐of‐n(F) system to study the compressive strength and the reliability of lattice structures. To do this, a reliability analysis model for compressive strength is proposed to predict the strength and dispersion of the whole structure based on the strength and dispersion of PC unit cells. Compressive strength tests on four kinds of PC lattice structures printed by selective laser melting (SLM) process have been accomplished and analyzed. The main failure modes of PC lattice structures have been analyzed afterward to simplify the strength model. Specifically, the unit cell compressive strength of lattice structures can be calculated by the finite element method (FEM) and can use this to calculate the compressive strength of the whole lattice structure. The predicted results are in good agreement with the experimental results, which shows the rationality of the model. The results indicate that as the diameter of the strut increases, the strength dispersion of the lattice structure also increases. This model offers an approach for studying the reliability of lattice structures.