Hierarchical Fractal Interfaces in 3D-Printed Stiff-Soft Polymer Composites: Fracture Resistance and Toughening...

Hierarchical hard-soft interfaces offer a route to fracture-resistant polymer composites without relying solely on intrinsically tough constituents. Here, Koch-curve-derived interfacial networks with fractal orders n = 0-3 were embedded as a compliant photopolymer phase within a rigid matrix and fabricated by multimaterial PolyJet printing. Pre-notched tensile tests, an eccentrically loaded single-edge-notch tension framework, digital image correlation, laser confocal microscopy, scanning electron microscopy, and crack-path box-counting analysis were used to determine how fractal order and pattern orientation govern fracture. At 0°, the n = 2 design increased peak nominal stress, nominal strain at fracture, tensile toughness, and crack-path length by approximately 2.2-, 1.7-, 3.6-, and 2.1-fold, respectively, relative to n = 0. Hierarchy also raised the conditional stress-intensity factor K_C from 9.78 to 13.3-13.6 MPa·m^1/2 for n = 2-3. The representative n = 2 specimen exhibited a broader strain-redistribution zone than the representative n = 0 and n = 3 specimens, while the n = 2 architecture showed the most tortuous fracture morphology, whereas added hierarchy at n = 3 yielded diminishing benefits and coincided with reduced printing fidelity. Pattern orientation altered the balance between load transfer, deformation, and crack guiding. Toughening arose primarily from interfacial crack deflection and branching, assisted by crack-tip blunting, strain delocalization, and uncracked-ligament bridging. Overall, the second-order fractal architecture printed at 0° provides the best balance of load-bearing capacity, deformability, and fracture resistance, establishing fractal order and pattern orientation as practical design variables for tough multimaterial printed composites.