Three-point bending fatigue behaviour of DIW-printed microporous titanium filaments for orthopaedic lattices

• First systematic fatigue study of single direct-ink-written titanium filaments. • Porous filaments show earlier crack initiation than compact filaments. • Deflection and branching create complex crack paths and slow long-crack growth. • Porous filaments show higher low-cycle fatigue strength; high-cycle values converge. This study reports fatigue data on microporous titanium (Ti) filaments fabricated by direct ink writing (DIW) for orthopaedic applications. Compact (∼6% closed-pore-dominated) and porous (∼15% open-pore-dominated) variants were tested under three-point bending fatigue. Fractography and elastoplastic finite element analysis (FEA) were used to relate surface roughness and microporosity to crack path and local surface stress–strain fields. FEA showed equivalent plastic strain concentrated at surface valleys, with maximum values up to ∼50% higher in porous than in compact filaments, consistent with earlier fatigue crack initiation. Fractography revealed pronounced crack deflection and branching in the porous filaments, induced by an interconnected micropore network, increasing crack-path tortuosity and thereby slowing long-crack (Stage II) propagation. Consequently, porous filaments tended to show higher low-cycle fatigue resistance, whereas high-cycle fatigue lives were comparable between the two filament variants. These filament-scale findings complement lattice-scale observations in which porous lattices exhibit superior overall fatigue resistance, reflecting the dominance of long-crack propagation at that scale. The results highlight the promise of DIW Ti with tailored open microporosity for load-bearing implants.