Directional stiffening and stress localisation in protective beams inspired by natural hierarchical structure of fish scales

This paper reports the comprehensive flexural behaviour of biomimetic protective beams inspired by the hierarchical architecture of fish scales. The models measuring 150 mm × 20 mm × 10 mm, comprised of a 5 mm thick scale-tissue hybrid layer of overlapping Acrylonitrile Butadiene Styrene (ABS) scales embedded in a Thermoplastic Polyurethane (TPU) matrix, supported by a 5 mm soft tissue sub-layer. Scale geometries were derived from four fish species (i.e., Salmon, Bass, Tuna, and Corydoras), each exhibiting unique morphology and scale volume fractions ranging from ∼21% to ∼46%. A constant scale volume baseline (∼28% scale volume fraction) was used for comparative evaluation. The results showed that increasing scale volume fraction significantly improved energy absorption in the rigid scale phase while also moderately enhancing energy dissipation in the compliant tissue analogue. Reverse loading revealed that directional anchoring and curvature of the scales further contributed to stiffness gains, underscoring the importance of asymmetric morphology in load transfer. Among the tested configurations, designs with larger scale geometry (such as in Salmon) demonstrated superior mechanical performance due to improved continuous scale-tissue interaction and stress localisation within the stiff phase. At constant scale volume fractions, mechanical behaviour was also governed by geometric attributes such as scale length, curvature, and interlocking. Overall, the study highlights the characteristics of bio-inspired design for achieving flexible and mechanically efficient protective systems.