Breaking better: How defects activate multiple toughening mechanisms in lattice materials

Toughening in heterogeneous lattice materials is commonly attributed to crack path tortuosity, but the relative importance of this and other mechanisms remains unclear because direct experimental evidence is scarce. Here, we introduce isolated, well-controlled defects into brittle lattice specimens and use mechanical testing with digital image correlation to track crack growth at the scale of individual cells. This approach allows us to experimentally evaluate how local imperfections influence fracture processes. We find that defects do not affect crack initiation or peak load, yet they can increase the work to failure. Crack path tortuosity contributes to this increase in some configurations but is not statistically significant in others, indicating that it is not the sole governing toughening mechanism. Instead, we observe ligament bridging, a mechanism that has not been reported experimentally in lattice materials, and find that it contributes to enhanced work to failure. These results show that targeted defects can activate toughening mechanisms beyond tortuosity, providing an additional route for designing fracture-resistant lattice materials.