Interlocking Geometric Interfaces for Functionally-Adapted Localized Reinforcement in Polymer Matrix Composites: Parametric FEA Design Rules and Experimental Validation

Localized reinforcement strategies offer the potential to retain most of the load-carrying capacity of integrally reinforced polymer matrix composite parts while drastically reducing the consumption of fibrous reinforcement, in line with eco-design objectives for the automotive sector. The mechanical effectiveness of such strategies is, however, controlled by the geometry of the transition between the reinforced and the unreinforced regions. This paper presents a systematic numerical and experimental investigation of four families of geometric interfaces between a rigid polyurethane matrix and a centrally placed chopped-fibre composite insert, using ASTM D638 type I tensile specimens. The four families – straight, puzzle, dovetail and T – were investigated through 75 parametric finite element simulations performed in ANSYS 2026 R1 and validated through 105 physical specimens manufactured by vacuum casting at 5 Pa in silicone moulds. For the straight interface, twenty configurations covering Dmax in the range 40–100 mm identified three behavioural regimes (stable, transition, abrupt jump) and provided a linear regression model with R 2 = 0.985 over the stable regime. For the puzzle interface, nine configurations identified two failure modes (neck failure and root failure) and design rules for Rmin (1.2–1.8 mm), Rmax/lmin (0.75–1.0) and the asymmetry ratio κ = Rmax/Rmin (2.5–3.5). For the dovetail interface, fifteen configurations organized in four blocks revealed a hyperbolic dependence of σ1,max on the real insert thickness tc and an asymmetric optimum at α ≈ 40°; a two-term regression model achieved R 2 = 0.955. For the T-interface, twenty configurations evidenced a strong Dmax × LI interaction term that dominates the linear effects. Experimental validation on seven natural and synthetic fibres confirmed mechanical efficiency ηL between 89.6 % and 93.8 % for six of the seven fibres on the straight interface; complex interfaces increased the absolute force by up to 60 N and lifted ηL to 93.8 % for the dovetail/coir combination. A Functional Performance Ratio FPR > 1 confirmed that geometric interlocking outperforms uniform reinforcement on a per-unit-fibre basis. The derived design tables provide directly usable references for hybrid matrix–composite parts. Quantitatively, the dovetail interface delivered the largest absolute force gain (+59.7 N for glass and +41.6 N for coir, i.e. +3.7% over the straight reference), the highest mechanical efficiency (ηL = 93.8% for the dovetail/coir combination) and the highest FPR (1.332, i.e. a 33.2% gain in load per unit fibre volume relative to integral reinforcement). The complex interfaces increased the absolute tensile force by 10.7–59.7 N, while six of the seven fibres reached ηL between 89.6% and 93.8% on the optimum straight-interface configuration, with a fibre-volume reduction of approximately 19%.