Discrete‐Continuum Duality of Architected Materials: Failure, Flaws, and Fracture
3D nano‐ and micro‐architected materials are resilient under compression; their susceptibility to flaws and fracture remain unexplored. This work reports the fabrication and tensile‐to‐failure response of hollow alumina nanolattices arranged into 5 µm octet‐truss unit cells. Some specimens contained through‐thickness center notches oriented at different angles to the loading direction, with a length‐over‐sample‐width ratio of 0.45. In situ tensile experiments reveal that for all orientations, failure initiates at the notch root, followed by instantaneous crack propagation along lattice planes orthogonal to extension. A tensile strength of 27.4 ± 0.7 MPa is highest for unnotched samples and decreases as notch orientation varies from 0° to 90° to its minimum, 7.2 ± 0.4 MPa; their specific tensile strength is ≈4 × higher than that for all other low‐density materials. Finite element simulations reproduce observed strengths and failure mechanisms: initial cracks always initiate at the nodal junctions with highest stress concentrations by tearing of alumina walls at the nodes. Subsequent crack propagation shifts maximum stress concentration to the nodes along lattice plane orthogonal to the loading direction. A modified analytical fracture model based on the effective notch length predicts tensile strengths consistent with experiments. These findings imply that continuum fracture mechanics can predict failure in nano‐architected materials, which helps develop advanced materials through informed architectural design.
© 2018 WILEY‐VCH. Manuscript received: 25 September 2018. Manuscript revised: 17 November 2018. Version of Record online: 13 December 2018. A.J.M. and W.H. contributed equally to this work. The authors gratefully acknowledge the financial support of DoD through JRG's Vannevar‐Bush Fellowship and of the Army Research Office through Institute for Collaborative Biotechnologies (ICB). The authors thank the Kavli Nanoscience Institute at Caltech for the availability of cleanroom facilities. Part of this work was carried out in the Lewis Group facilities at Caltech. The authors thank A. Rosakis, K. Faber, K. Bhattacharya, and D. Yee for helpful discussions and D. Kochmann, C. Portela, and G. Phlipot for insight on reagents/analytic tools. Y.W.Z. gratefully acknowledges the financial support from the A*STAR, Singapore. W.H. gratefully acknowledges the financial support from the Northwestern Polytechnical University, China. A.J.M., W.H., Y.W.Z., and J.R.G. designed research; A.J.M. performed experimental research; W.H. contributed reagents/numerical simulations; A.J.M. and W.H. analyzed data; A.J.M., W.H., Y.W.Z., and J.R.G. wrote the paper. The authors declare no conflict of interest.
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