Multi-objective optimization of a dual-hardness thermoplastic polyurethane embossed sandwich shin guard for enhanced low-velocity impact protection

Despite the mandatory use of shin guards, lower-leg injuries remain prevalent due to limitations of existing materials and structural designs in attenuating impact. This study introduces a novel dual-hardness thermoplastic polyurethane (TPU) sandwich shin guard comprising a stiff TPU-95A outer shell, a compliant TPU-87A embossed core with solid protrusions, and an inner comfort fabric. Unlike conventional foam and lattice-based structures, the embossed core dissipates impact energy through progressive deformation and material hysteresis, thereby enhancing impact attenuation. Low-velocity impact tests were conducted using a modified NOCSAE (National Operating Committee on Standards for Athletic Equipments)-inspired protocol with a 5.13 kg impactor at 1.73 m/s with a customized curved magnesium alloy plate as a physiologically relevant surrogate interface. The effects of cell structure, cell-wall length, offset distance, and thickness of embossed geometry were systematically optimized using a hybrid Taguchi-based grey relational analysis and utility approach. Sensitivity analysis further confirmed the robustness of the optimized design, with only the offset distance changing under extreme prioritization of peak force reduction. The optimized design achieved a 75% reduction in peak force, a 6.4% increase in energy absorption without structural failure, and a sevenfold increase in contact time compared with the unguarded condition. It shows a mean peak acceleration of 44 g under repeated impacts, well below the NOCSAE guideline threshold of 200 g. The optimized guard showed better force reduction than a commercial guard under tested conditions. The proposed architecture enables tailoring of the embossed geometry to achieve the desired impact response and offers a potential alternative to conventional foam-based padding.