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.

Composite lattice structures known as the IsoBeam™ made with unidirectional carbon/epoxy were manufactured and tested in shear-dominated bending. The manufacturing process consisted of placing tows of carbon fiber pre-impregnated with epoxy resin onto a pin-type mandrel to create members with interwoven joints. The members were consolidated with a half spiral aramid sleeve. The IsoBeam structure consists of two main types of members: longitudinal and diagonal members measuring nominally 0.4 in. (10.2 mm) and 0.2 in. (5.1 mm) in diameter, respectively. The hand-manufactured specimens measured nominally 6 in. (152.4 mm) high by 3 in. (76.2 mm) wide by 2 ft (0.61 m) long with 4 bays, each 6 in. (152.4 mm) long. The beams weighed between 1.82-1.86 lbs (8.09-8.27 N). A finite element analysis of the IsoBeam was compared to the experimental results. The IsoBeam specimens were tested in four-point or three-point bending but were dominated by shear due to short-beam bending because of the low length/height aspect ratio. After testing to failure, individual members that were lightly loaded and appeared to be undamaged were removed and tested in axial compression. The void percentage and fiber volume fraction were also measured. The average maximum strength of the IsoBeam structure was 4.11 kips (18.3 kN), yielding an equivalent shear of 2.06 kips (9.15 kN) and bending moment of 20.2 kip-in (2.29 kN-m). This strength was lower than expected and is attributed primarily to low material quality, insufficient consolidation of members, and inadequate tension on the tows during manufacturing. The structure exhibited ductile behavior absorbing considerable energy after initial failure, as well as exhibiting damage tolerance due to the inherent structural redundancy. The inner diagonal members which are inherently stiffer exhibited higher strains than the side outer diagonal members after initial failure. The members removed and tested exhibited an average compression strength of 86.9 ksi (599 MPa) and compression modulus of 17.8 Msi (122 GPa) which are both lower than observed in members tested in past research. The diagonal members had a higher strength of 111 ksi (767 MPa) than the longitudinal member's compression strength of 62.5 ksi (431 MPa). Most members were seen to have a high percentage of voids with an average of 4.3% for diagonal members and 6.4% for longitudinal members. The average fiber volume fraction content of members was very low at 38%. The linear finite element analysis of the IsoBeam structure predicted failure at a load of 34 kips (151 kN). Without considering buckling, the first member predicted to fail was a vertical outer diagonal. This research demonstrates that increasing the manufacturing quality should yield an IsoBeam structure that is strong, ductile and damage tolerant.