Programmable nonlinear response in spatially varying continuous-fibre lattices: Experimental and numerical investigations

Continuous fibre-reinforced lattice structures have emerged as a promising class of architected materials for lightweight applications, offering exceptional stiffness-to-weight ratios and energy absorption capacity through the strategic placement of fibres within cellular geometries. While extensive research has focused on their linear elastic behaviour and stiffness optimisation, the nonlinear deformation mechanisms and failure responses of spatially graded continuous fibre lattices remain insufficiently understood. This study presents a comprehensive experimental and numerical investigation of the flexural behaviour of additively manufactured graded lattice beams across the full deformation range, from initial elasticity to post-peak collapse. The lattice architectures are generated through an optimised graded lattice procedure that distributes five geometrically compatible star-hourglass re-entrant unit cells across a simply supported beam domain, with spatial variation enabling distinct mechanical roles within a single monolithic structure. Beams fabricated from pure PLA and continuous glass-fibre-reinforced PLA using fused deposition modelling are tested under three-point bending and analysed through nonlinear finite element simulations in Abaqus. The results demonstrate that structural-scale grading activates deformation mechanisms inaccessible to uniform architectures. Notably, graded Configs exhibit geometric stiffening, characterised by an increase in tangent stiffness with progressive deformation due to the sequential engagement and compaction of successive cell layers, thereby counteracting conventional softening behaviour. Furthermore, by varying unit-cell geometry and fibre reinforcement distribution, the global failure mode can be deliberately shifted between brittle localised fracture and quasi-ductile progressive collapse without altering the base material. Systematic comparison of six optimised Configs reveals a quantifiable stiffness-strength trade-off, with architectural variation enabling movement along this design frontier. The key novelties of this work include: (i) experimental demonstration of geometric stiffening in graded continuous fibre lattices, (ii) architecture-driven programmability of failure behaviour through spatial cell distribution, and (iii) validated nonlinear FE analysis capturing the observed deformation sequencing and collapse mechanisms. These findings establish spatially varying continuous fibre lattices as programmable mechanical systems in which stiffness distribution, nonlinear engagement, and failure progression can be tailored through coordinated multiscale design, expanding the attainable design space beyond conventional lightweight structures.