Abstract

This research demonstrated the fabrication, flexural testing, and analysis of nominally 5 ft (1.5 m) 6-bay and 10 ft (3 m) 12-bay carbon/epoxy IsoBeam™ structures. The rectangular cross-section was 5 in (12.7 cm) wide by 10 in (25.4 cm) high. The IsoBeam structure is a composite lattice structure that is a geometric derivative of the IsoTruss® structure. Modifying the geometry to yield a rectangular cross-section provides additional applications for these beams as structural elements in buildings, aircraft, vehicles, and other structures. The diameters of the constituent members of the IsoBeam, namely the longitudinal and diagonal members, were sized such that the IsoBeam could hold the design load of a 10K1 steel joist 550 plf (818.5 kg/m). Three IsoBeam structures were manufactured: two 5 ft (1.5 m) long and one 10 ft (3 m) long. The IsoBeam structures were manufactured with carbon/epoxy composite tows comprised of T700SC-12K-50C carbon fibers and UF3369-100 pre-impregnated (pre-preg) epoxy resin. The pre-preg tows were positioned on a modified pin-mandrel under tension using a combination of hand and machine filament winding in an interwoven pattern to create the complex geometry of the IsoBeam structure. Each member was circumferentially wrapped with 1 in (2.5 cm) wide strips of Dunstone Hi-Shrink Tape (polyester) to consolidate the tows during the manufacturer’s recommended curing process. Microscopic measurements after testing established that these careful manufacturing techniques produced high-quality specimens with an average void ratio of 0.72% and an average fiber volume fraction of 69.5%. The average compression stiffness and strength were 18.7 ksi (129 GPa) and 115.1 ksi (793 MPa), respectively.Each IsoBeam was loaded in four-point bending to failure, with other tests performed in the linear-elastic range to study load path behavior of the IsoBeam. Strain, deflection, and load data were collected to provide a detailed understanding of the behavior of individual members under load and their corresponding stresses. The 6-bay IsoBeam structures experienced failure at 8055 lbs (35.8 kN) and 11224 lbs (49.9 kN), in compression initiated by buckling. The longer 12-bay IsoBeam structure failed in a similar manner at 8035 lbs (35.7 kN) but also exhibited delamination, due to insufficient interweaving.Experimental results were compared to the predicted strength of the IsoBeam based on a linear finite element model (created using SAP 2000) and hand calculations. Validation of the design through the comparison of experimental and predicted values gave insight on design techniques and overall understanding of the performance of the IsoBeam in bending, with excellent correlation in the linear range. The assumption that longitunals are primarily responsible for bending strength and diagonals primarily carry shear was validated, indicating a strong correlation between manufacturing quality and performance of IsoBeam structures.

Degree

College and Department

Ira A. Fulton College of Engineering and Technology; Civil and Environmental Engineering

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