Because of their excellent performance with lightweight and multifunctional integration, lattice structures have been widely used in aerospace, heat exchangers, and bone tissue engineering. Triply periodic minimal surface (TPMS) lattice structures with smooth surface morphology reduce the stress concentration under load, exhibiting higher specific strength, specific stiffness, and energy absorption capacity. Therefore, TPMS has potential applications in lightweight and energy-absorbing buffer devices in the aerospace industry. Sheet and network lattices have been proposed to utilize their advantages, which require further performance improvements with an optimal design. Thus, there is an urgent need to develop a reliable simulation analysis method to reveal the mechanism of structural strengthening and determine optimization direction.
In this study, a new surface offset method was developed to design a TPMS lattice structure (Fig. 2) to improve mechanical properties and energy absorption. Diamond, Primitive, Gyroid, and I-WP TPMS lattice structures (Fig. 3) were optimized using this method and fabricated via selective laser melting (SLM). The compression tests of the lattice structures were repeated three times to reveal the mechanical properties. In comparison, finite element models with the Johnson-Cook model were established to reflect the deformation behaviors of the lattices and predict their mechanical strength, as confirmed by the experimental results. In this study, the influence of surface offset design on the mechanical properties and energy absorption capacity under quasistatic compression was investigated, which provided insight into the optimization strategies and analysis methods of lattice structures.
The experimental and simulated compression stress-strain curves show that the finite element analysis method based on the Johnson-Cook model can precisely replicate the experimental results, including similar linear growth, stress drop, and stress plateau stages. The deviations in the mechanical strength of the lattice structures obtained via the experiment and simulation are all less than 14%, particularly for sheet structures, whose ultimate strength error is within 2%. This indicates that the finite element method can accurately predict the mechanical properties and deformation behavior of lattice structures.
The mechanical properties of the four lattice types were improved significantly using the proposed design method, as can be seen from Table 4 showing the critical mechanical properties of all the samples. With the continuous increase in the surface offset, the mechanical strength of Diamond, Gyroid and I-WP lattices increase by 101.5%-244.9% owing to the increase in the second moment of area. Among them, the I-WP sheet 45-30 exhibits the most outstanding performance, demonstrating an increased mechanical strength (111.64 MPa) compared with that of the rod lattice (32.37 MPa). However, Primitive lattices significantly differ from the other three types. The surface offset helps to improve the stability of the Primitive lattices, avoiding the sudden collapse of the entire structure. The mechanical strength was increased by 47.1%, but continuous growth of the shell offset reduced the mechanical properties owing to the weakening effect of the plastic hinges.
The cumulative energy absorption (Figs. 15 and 16) reveals that the surface offset design effectively improves the energy absorption capacity of the lattice structure. Specifically, Diamond, Gyroid, and I-WP continuously improve the cumulative energy absorption by 139.8%, 279.2%, and 312.9%, respectively, compared with the corresponding rod-type lattices. Similar to strength, the most outstanding performances are contributed by the I-WP sheet 45-30, whose cumulative energy absorption increases from 11.32 to 46.72
These results highlight the optimization effect of the surface offset on the energy absorption capacity. The shear failure mode of rod-shaped lattice structure changes into the deformation behavior of layer-by-layer collapse using this method. The large-scale collective collapse of lattice structures can be prevented to obtain a smooth, continuous stress-strain curve, which increases the plateau stress of the sheet lattices.
1. In compression experiments, the rod lattice structure is prone to a 45° shear fracture. A continuous surface offset can effectively improve the deformation behavior of an abrupt collapse, enhance the mechanical strength and plateau stress, and increase the energy absorption capacity.
2. The simulation analysis method based on the Johnson-Cook plasticity and damage model can accurately predict the mechanical strength and energy absorption performance of the TPMS lattice structure, revealing the failure process and fracture behavior of the lattices. This provides essential guidance for structural optimization and performance improvement.
3. The I-WP sheet exhibits the best performance among the four typical TPMS lattice structures through surface offset. Compared with rod-shaped lattices, the mechanical strength, plateau stress, and energy absorption of sheet-shaped lattices increased by 244.9%, 335.7%, and 312.7%, respectively. This is mainly attributed to the transformation of the deformation mode contributed by the surface offset, which minimizes the 45° shear fracture behavior and improves the plateau stress of the lattice structure, accompanied by a layer-by-layer collapse for deformation optimization.
In summary, the surface offset design and Johnson-Cook simulation model were adopted for TPMS lattices in this study, which provides a reference for optimization strategies of lattice structures. Further studies on the fatigue performance of TPMS lattice structures should be conducted to facilitate the development of new lightweight structures in laser additive manufacturing.
