Modeling and failure analysis of composite I-beams for UAV wing spar design

Abstract

In unmanned aerial systems (UAS), the composite I-beam using carbon fiber caps and a cross-grain balsa shear web is often used for the main spar. This manufacturing method is underdeveloped in analysis causing engineers to have to use intuition and iterative test procedures until a spar is made that meets the requirements for the flight envelope. Beyond analysis, the material properties of balsa wood with the geometry and loading unique to these I-beams is unknown. This research plans to experimentally determine the material properties of balsa wood and determine its failure modes to create and validate an analysis tool for the composite I-beams. The Wing Spar Analysis Program is created in MathCad that simulates bending of composite I-beams to determine the failure modes. The material properties of balsa wood are tested for tensile failure and shear failure. These material properties are then used to create prototype I-beams, using readily available materials, and composite I-beams, using carbon fiber caps and cross-grain balsa wood shear webs. The I-beams are tested in a three-point bender and the data is used to validate the Wing Spar Analysis Program. It is found that the Wing Spar Analysis Program gives a percent difference between experimental and expected stress in the range of 2.571% - 36.256% for composite I-beams of lengths between five and ten inches. Through visual inspection, it is also found that shear failure in the composite I-beams creates a vertical crack in the balsa wood shear web that causes a failure in the epoxy that bonds the I-beam together. It is also found that balsa wood is highly dependent on its thickness, where thicker balsa wood samples have a higher probability for defects and failure.