Anisotropic Shear Properties of Organic Interfaces in Bio‐composite Materials

Understanding the functional role of organic interfaces in highly mineralized biogenic composites remains a challenge in biological materials research. Here, a novel experimental and analytical approach is presented to directly probe the anisotropic shear properties of sub‐micron interfaces in composite materials. Using focused ion beam (FIB) milling, specialized shear cantilevers containing a single organic interface is fabricated from the prismatic ultrastructures of two molluscan shells— Atrina vexillum and Unio pictorum . Nanoindentation‐based bending experiments at distinct locations enable decoupling of anisotropic lateral and torsional shear responses. Analytical formulations, supported by finite element simulations, allow the extraction of the elastic, lateral, and torsional shear moduli. The findings reveal isotropic shear behavior in A. vexillum , contrasted by marked anisotropy in U. pictorum , where torsional shear modulus exceeds the lateral value by a factor of two. This is attributed to differences in the microstructural architecture of the organic interfaces. Despite similar compositions, structural differences result in distinct mechanical behaviors. Crucially, the developed methodology is not limited to biocomposites and can be applied to a wide range of composite materials where thin, confined interfaces critically influence performance. This work provides a novel, versatile tool for characterizing anisotropic interface mechanics across biological and engineered composite systems.