Compatibility of Methacrylate Based Resins Controls Interfacial Failure and Toughness in 3D‐Printed Multimaterial Composites

Thermosetting polymers are widely used in load‐bearing applications for their strength, durability, low cost, and ease of processing, yet they are limited by low toughness. This work explores how digital light processing (DLP) 3D printing can address this limitation by creating multimaterial structures with controlled resin formulations, spatial patterns, and interfacial characteristics. Two neat resins were used, a strong but brittle formulation (DA‐2) and a ductile and tough formulation (Tenacious), along with three blends (DT80, DT60, DT40). A variety of patterned architectures was printed and tested under uniaxial tension. To interpret failure behavior, a phase‐field fracture model was developed to analyze stress distributions and crack initiation, propagation, and branching. The fracture responses of pure bars (DA‐2, Tenacious, DT80) and composite grid‐filled bars (Tenacious with either DA‐2 or DT80) were examined. Experiments and simulations showed that mismatches in material properties strongly affect load transfer across interfaces and thus govern overall mechanical performance. Structures containing blended resins exhibited substantially higher toughness than those with only pure DA‐2, demonstrating that transitional interfaces between dissimilar materials improve stress transfer and enable synergistic behavior. These findings provide design guidelines for multimaterial DLP‐printed thermoset composites with enhanced interfacial performance.