The Mixed Mode Bending (MMB) test is a standardized material characterization technique used to determine the strain energy release rate and fracture toughness of layered composite materials under a range of combined mode I and mode II loading conditions. The compliance of any fixture’s internal components lessens the material characterization accuracy by introducing error into the displacement measurement. Correcting for system compliance often involves complex post-processing of data. To reduce error in the test and improve material characterization quality, the authors have redesigned the MMB test fixture for minimal compliance. An analytical model of the fixture was developed to isolate the components most responsible for system compliance. The components were modified, guided by a finite element optimization algorithm, to reduce deformation. The redesigned features reinforce the fixture without creating nonlinearity in the system or changing the test’s basic mechanisms that allow for variable mixed-mode ratios. The new MMB fixture will improve the accuracy of the experiment and reduce the complexity of data post-processing, leading to a safer crack-resistant design.
The equine hoof wall has a complex, hierarchical structure that can inspire designs of impact-resistant materials. In this study, we utilized micro-computed tomography (Micro CT) and serial block-face scanning electron microscopy (SBF-SEM) to image the microstructure and nanostructure of the hoof wall. We quantified the morphology of tubular medullary cavities by measuring equivalent diameter, surface area, volume, and sphericity. High-resolution Micro CT revealed that tubules are partially or fully filled with tissue near the exterior surface and become progressively empty towards the inner part of the hoof wall. Thin bridges were detected within the medullary cavity, starting in the middle section of the hoof wall and increasing in density and thickness towards the inner part. Porosity was measured using three-dimensional (3D) Micro CT, two-dimensional (2D) Micro CT, and a helium pycnometer, with the highest porosity obtained using the helium pycnometer (8.07%), followed by 3D (3.47%) and 2D (2.98%) Micro CT. SBF-SEM captured the 3D structure of the hoof wall at the nanoscale, showing that the tubule wall is not solid, but has nano-sized pores, which explains the higher porosity obtained using the helium pycnometer. The results of this investigation provide morphological information on the hoof wall for the future development of hoof-inspired materials and offer a novel perspective on how various measurement methods can influence the quantification of porosity.
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