Concurrent multi-scale topology optimization for continuous fiber reinforced lattice structure with spatially tunable fiber morphology

Continuous fiber-reinforced polymer (CFRP) composites manufactured via additive manufacturing (AM) offer significant potential for the design of high-performance lattice structures. However, existed design methods have not yet fully tapped the potential of gradient infill lattice with varying fiber ratio. In this work, a novel concurrent multi-scale topology optimization framework is proposed for CFRP lattice structures with spatially tunable fiber morphology, leveraging an RVE-based homogenization approach. The effective mechanical properties are evaluated by incorporating micro-scale fiber characteristics (i.e., fiber volume ratio and orientation) together with meso-scale lattice unit cell geometry. The divergence constraint based on fiber decomposition is introduced to suppress abrupt curvature transitions and guide the formation of smooth fiber trajectories. The proposed optimization problem simultaneously considers macro-scale structural compliance as the objective and treats fiber parameters and unit cell dimensions as design variables across multiple scales. Furthermore, an enhanced wave projection method is developed to convert discrete design variable fields into printable fiber paths with multiple tows, enabling a continuous and realizable fiber layout. In general, the proposed concurrent multi-scale optimization scheme leverages spatially tunable fiber ratios to realize gradient infill within the lattice architecture, thereby offering superior load-bearing capacity and improved manufacturability over existing design methods. Overall, this framework provides a robust and manufacturable design strategy for CFRP lattice structures with customized topology and fiber morphology.