Development of Test Methods for Evaluation of Bending Stiffness and Compressive Modulus of Braided Composite Lattice Structures
Type of Degreethesis
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Open Architecture Composite Structures (O-ACS) have been recently developed and are known for their high modulus per unit weight. They are truss structures preformed on a mandrel shaped as a cylindrical, elliptical or polygonal cross section tube. It is very critical to accurately measure the mechanical properties of these structures in order to evaluate and optimize them. This work focuses on development of test methods suitable and appropriate for testing O-ACS to accurately and precisely measure the bending stiffness and axial stiffness under compression. The current research includes testing the O-ACS with standard three point bending test and further improving the test method to be more accurate for O-ACS specific testing, by using collars to reduce the localized deflection. A novel test method for measuring bending stiffness of O-ACS has then been introduced with an intention to provide more accurate and consistent results. In this test method the specimen is potted in epoxy and steel extensions are attached on both ends. Forces are then applied on these extensions to apply a pure moment to the specimen, thus eliminating deflection due to shear as well as contact stresses. A test method for accurately measuring the compressive stiffness and compressive modulus is developed in this research. Different configurations of O-ACS (Varying the number of axial yarns and braid angle) are tested for a comparative study of their compressive stiffness and moduli. Custom test fixtures built for performing these test methods have also been described in detail as a part of this study. The effect of molding Expanded Polypropylene (EPP) inside the O-ACS on the bending stiffness of O-ACS has also been studied.