Architected Interpenetrating Phase Composites Achieve Quasi‐Isotropic Ultrahigh Strength

ABSTRACT Achieving mechanical isotropy in composites with ordered reinforcement architectures remains challenging, as structural order typically induces directional dependence. This work demonstrates a paradigm‐shifting strategy that reconciles these contradictory requirements by engineering a quasi‐isotropic lattice metamaterial phase within interpenetrating phase composites (IPCs). We strategically reinforce the weak stiffness directions of the base architecture, developing skeletal frameworks that approach theoretical stiffness‐strength boundaries while maintaining isotropy. Finite element analysis and experimental characterization along crystallographic [100], [110], and [111] orientations confirm consistent quasi‐isotropic behavior, with directional variations in Young's modulus and compressive strength below 10% for both Ti6Al4V skeletons and epoxy‐infiltrated IPCs. The resulting composites exhibit exceptional mechanical performance: specific compressive strengths exceeding 100 N m kg −1 and specific energy absorption reaching 53 J g −1 for the skeletal phase—setting new benchmarks in the literature. Epoxy infiltration synergistically enhances performance, increasing Young's modulus by 70.2% and compressive strength by 105.2% compared to the bare skeleton. Fractographic analysis reveals a transition from ductile failure in pure skeletons to brittle cleavage fracture in IPCs, due to the constraining effect of the polymer matrix. This work provides a design framework for achieving isotropy in composites with ordered architectures.