Experimental and numerical investigation of bio-inspired CFRP structures with layer thickness gradients

Throughout evolutionary history, mineralized tissues have developed remarkable skeletal microstructures that combine exceptional damage tolerance with adaptation to challenging environmental conditions. These tissues typically feature composite architectures with spatially varying fibre orientations and layer thicknesses. This study utilizes the microstructure of deep-sea sponge spicules and the cuticle of the lobster Homarus americanus as bioinspiration. Sponge spicules, found at depths between 1100 m and 2100 m, consist of radially arranged layers of hydrated silicon dioxide. Their curvature adapts to ocean currents, and analyses reveal graded layer thickness from 0.6 µm to 10 µm. Thinner layers in tensile regions enhance tensile strength due to scaling relationship with layer thickness h − 1 / 2 , while thicker layers in compressive zones improve stability and reduce buckling. Unlike this microstructure, the cuticle of Homarus americanus comprises epicuticle, exocuticle, and endocuticle, with the latter two forming helicoidal fibre architecture. In the claws, exocuticular layers are thinner, facilitating energy absorption under impact, whereas thicker endocuticular layers provide structural stabilization through increased stiffness. Inspired by these biological systems, thin-ply carbon fibre reinforced polymer laminates were designed for out-of-plane loading conditions. A quasi-isotropic layup 45° , 90° , -45° , 0° was chosen to reflect amorphous nature of hydrated silica. To mimic natural gradients, layers of varying thin-ply thickness were employed within thermally balanced laminate sequence. Layer configurations were initially optimized using finite element three-point bending simulations and subsequently validated experimentally. The graded design approach resulted in improved flexural performance and reduced damage propagation under impact loading, demonstrating potential of bio-inspired layer thickness gradation for development of advanced composite structures. • A general design approach for graded laminates was developed. • FE simulations accurately predict graded laminate behaviour. • Graded design structure increases flexural strength. • Reduces impact induced damage through graded design.