Characterization of Deformation and Failure of Fomes fomentarius Hymenium by Synchrotron Phase-Contrast Enhanced Micro-Computed Tomography, In situ Compression Testing and Three-Dimensional Optical Flow

Efficient design strategies found in nature have led to innovative materials and technologies. One promising blueprint for lightweight applications is the hymenium of Fomes fomentarius, a cellular, hierarchically structured, light, strong, and failure-resistant biological tissue. Phase-contrast enhanced micro-computed tomography is used to investigate deformation and failure during in situ compression testing of the tubular honeycomb structure. Specimens are loaded parallel or transverse to the long axis of the tubes , in wet or dry condition. Displacements are evaluated between defined loading steps with three-dimensional optical flow and morphological image analysis is used to correlate the micro- and mesostructure with local strains and structural damage. Key findings include that displacements are about five times larger in transverse than parallel loading, with greater displacements in regions of lower density. Higher hyphae density is found in struts compared to vertices, which leads to uniform displacements without significant local strain concentration. While cracks are observed in both struts and vertices, surprisingly, fracture is more catastrophic in the struts. Parallel loading results in plastic buckling and delamination in dry samples, while their wet counterparts show telescopic shortening from the first loading step and fewer fatal cracks, probably due to greater hyphae elasticity.