Delamination is a primary failure mode for layered materials such as composites and nanocomposites. The Mixed Mode Bending (MMB) test is a standardized material characterization technique used to determine the fracture toughness of layered composite materials under combined mode I and mode II loading conditions. The MMB fixture relies on the interaction of eight main components to create any mode I to mode II ratio from 0 to approximately 5. It was proved that the compliance of any fixture’s internal components lessens the material characterization accuracy. The inherent compliance of the fixture introduces error into the load point displacement measurement and affects the fracture toughness of the specimen. The compliance of the system must therefore be accounted for to reduce these errors. Since the system compliance changes for each mode ratio, data post-processing can be relatively complex and impractical.&nbsp;&nbsp; &nbsp;The present work will improve upon the design of the mixed-mode bending fixture by reducing overall system compliance. A stiffer fixture will improve the accuracy of the experiment and reduce the complexity of data post-processing, leading to a safer crack-resistant design. The increase in the accuracy will also allow for materials with a more extensive range of elastic moduli to be tested and characterized with reliability.Finite element analysis, analytical modeling, and optimization techniques will be used to optimize the fixture design for minimal compliance while keeping the overall weight low. The relationship between each component's stiffness and the compliance of the overall system will be determined using an analytical model that approximates each component as a linear spring. The components that contribute the most to system compliance will be analyzed using finite element analysis to identify the critical members that experience the greatest deformation. Each of these components will be redesigned to reinforce the areas that experience maximum deformation. The new component designs will feature modifications to the geometry of critical members guided by a finite element optimization algorithm as well as changes in the interaction between components. Design changes must also not interfere with the existing mechanisms that allow for variable mixed mode loading conditions or prevent geometric nonlinearity to the system. Once all the critical components have been redesigned and incorporated into the final design, the compliance of the new reinforced mixed-mode bending fixture will be determined.&nbsp; The maximum deformation of the new design will be compared to that of the original fixture under identical loading conditions allowing us to quantify the reduction of system compliance.

Reinforced Mixed Mode Bending Fixture for Improved Characterization

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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