Mechanical characteristics of interpenetrating phase composites (IPCs) made of 3D-printed polylactic-acid scaffolds and epoxy resin matrix, the effect of infill pattern and density

• IPCs show 92% higher tensile strength (25 vs. 13 MPa) and 53% greater elastic energy absorption than 3DP samples. • Concentric patterns achieve maximum strength (26 MPa) while ZigZag patterns provide optimal balance between strength and energy absorption. • Tensile strength triples (13→27 MPa) and energy absorption quadruples when infill density increases from 25% to 100% in IPCs. • IPCs demonstrate gradual failure behavior whereas 3DP samples fail abruptly through delamination or localized cracking at pattern intersections. • At 75% infill, IPCs maintain full strength while 3DP samples show 12-44% strength reduction compared to 100% infill. • IPCs excel in stiffness and initial energy absorption while 3DP samples perform better in post-yield plastic energy dissipation. This study investigates the mechanical performance of 3D-printed (3DP) and interpenetrating phase composite (IPC) samples, analyzing the effects of ten infill patterns and densities (25%, 50%, 75%, 100%) on tensile strength, tensile energy, and failure mechanisms. Failure patterns varied for 3DPs, Concentrics exhibited delamination, Grid45°/-45° showed diagonal cracks, and other patterns had uniform fracture pattern. IPCs were more brittle, but enhanced strength. IPC samples outperformed 3DP in tensile strength (25 MPa vs. 13 MPa) and elastic energy absorption (15.18 vs. 9.92 kJ/m², +53%), attributed to their composite structure. 3DPs showed superior plastic ( G II ) and post-ultimate ( G III ) energy absorption due to greater deformation capacity. Tensile strength increased from 13 MPa (25% infill) to 27 MPa (100% infill), and total energy ( G A ) rose 297% (17.51 to 52.06 kJ/m²). Concentric patterns achieved the highest strength (26 MPa) and energy (60 kJ/m²), while Cross patterns lagged (10 MPa, 10 kJ/m²). ZigZag and Line45°/-45° patterns balanced strength and ductility (60 kJ/m²). Normalized strength at 75% infill for IPC (0.11–1.16) surpassed 3DP (-12% to -44%), highlighting IPC’s strength retention. These results suggest IPC’s suitability for structural applications and 3DP’s advantage in energy dissipation. Concentric or ZigZag patterns are recommended for optimal performance.