This thesis explores the quality of hand-manufactured carbon-epoxy IsoTruss® grid structures for use as reinforcement in concrete piles. Large IsoTruss® grid structures were manufactured and embedded in 14.0" (35.6 cm) diameter concrete to create IsoPiles™. The IsoPiles™ were designed to have flexural characteristics similar to steel reinforced concrete piles of equal diameter. Bending stiffness was matched based on the longitudinal members. A method for comparing transverse steel reinforcement to helical IsoTruss® members was developed, along with equations to facilitate the design of IsoTruss® structures with rounded nodes. Compression tests were performed on 3.0 ft (0.91 m) long sections taken from the ends of each of the two 30 ft (9.14 m) long IsoTruss® grid structures manufactured. Fiber volume fraction, void fraction, and cross section area inspections were performed on IsoTruss® samples to determine quality. The strength, stiffness, and fiber volume fraction data obtained from these tests are compared to values obtained previously [1] for the same consolidation method. The quality of hand-manufactured large IsoTruss® grid structures was quantified by performing microscopic inspection of the members, by testing the reinforcement cage in compression, and by testing short section of IsoTruss® and steel reinforced concrete piles in compression. Compression tests were performed on short sections taken from the ends of the IsoPile™ specimens. These were compared with compression tests performed on equivalent steel-reinforced piles to evaluate the viability of the IsoTruss® as reinforcement in concrete piles. Insufficient tension on the fiber during manufacturing and insufficient radial compression during the cure resulted in an average fiber volume fraction 13% lower than previously obtained, causing the ultimate compressive strength and Young's modulus of the IsoTruss® reinforcement cages to be 51% and 22% lower, respectively, than previous data. The IsoTruss®–reinforced piles had an ultimate compressive load that was within 4% of the ultimate compressive load of the steel-reinforced piles.

The purpose of this research is to investigate the structural efficiency of 8-node IsoTruss structures subject to uniaxial compression using numerical, dimensional, and optimization methods. The structures analyzed herein are based on graphite/epoxy specimens that were designed for light-weight space applications, and are approximately 10 ft. (3 m) long and 0.3 lb. (0.14 kg). The principal failure modes considered are material failure, global buckling, local buckling at the bay level, and longitudinal strut buckling. Studies were performed with the following objectives: to correlate finite element predictions with experimental and analytical methods; to derive analytical expressions to predict bay-level buckling; to characterize interrelations between design parameters and buckling behavior; to develop efficient optimization methods; and, to compare the structural efficiency of outer longitudinal configurations with inner longitudinal configurations. Finite element models were developed in ANSYS, validated with experimental data, and verified with traditional mechanics. Data produced from the finite element models were used to identify trends between non-dimensional Pi variables, derived with Buckingham's Pi Theorem. Analytical expressions were derived to predict bay-level buckling loads, and verified with dimensional analyses. Numerical and dimensional analyses were performed on IsoTruss structures with outer longitudinal members to compare the structural performance with inner longitudinal configurations. Analytical expressions were implemented in optimization studies to determine efficient and robust optimization techniques and optimize the inner and outer longitudinal configurations with respect to mass. Results indicate that the finite element predictions of axial stiffness and global buckling loads correlate with traditional mechanics equations, but overestimate the capacity demonstrated in previously published experimental results. The buckling modes predicted by finite element predictions correlate with traditional mechanics and experimental results, except when the local and global buckling loads coincide. The analytical expressions derived from mechanics to predict local buckling underestimate the constraining influence of the helical members, and therefore underestimate the local buckling capacity. The optimization analysis indicates that, in the specified design space, the structure with outer longitudinal members demonstrates a greater strength-to-weight ratio than the corresponding structure with inner longitudinal members by sustaining the same loading criteria with 10% less mass.

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.

This thesis utilized a linear analytical approach to explore the damage tolerance or residual strength as a function of increasing damage in traditional single and hybrid-grid IsoTruss® structures. Residual strength was studied for structures subjected to axial compression, torsion and flexural bending, independently. Carbon/epoxy material properties were applied in all load cases, and fiberglass/epoxy material properties were also applied in the flexural bending case. Prior to imposing damage conditions, the IsoTruss® structures were parametrically optimized to achieve the highest strength-to-weight ratios. Typical compression strut, driveshaft, and utility pole specifications governed the design strength dimensions and boundary conditions. Damage growth was achieved by removing members from IsoTruss® structures progressively about the circumference in a symmetrical manner. The sequence of member removal, beginning with primary or secondary members, was examined, and is described as primary and secondary analyses. ABAQUS finite element analysis was employed to quantify the residual strength of each IsoTruss® configuration. Reduction in residual strength trends are compared to net section strength, which assumes a linear relationship between damage size and residual strength. Results indicate that the 6-node IsoTruss® configuration is the most damage tolerant structure in the sense that the 6-node configuration deviates the least from the net section strength. As more nodes are added, IsoTruss® structures behave more like a composite tube, exhibiting a brittle behavior characterized by an increase in strength reduction for a given damage size. Bending results reveal that carbon fiber IsoTruss® structures are more damage tolerant under primary bending conditions than fiberglass poles. On the other hand, fiberglass IsoTruss® poles prove to be more damage tolerant under secondary bending conditions than carbon fiber structures. Most importantly, however, hybrid-grid IsoTruss® poles are definitively more optimal structures than single-grid poles in terms of both strength-to-weight ratio and damage tolerance. The results and conclusions from this thesis provide benchmark capacities for mechanically manufactured IsoTruss® structures. Also included in this thesis is documentation of a special program written to analyze IsoTruss® structures.

Unidirectional basalt fiber rods consolidated with an aramid sleeve were measured for compression strength after impact at various energy levels and compared to undamaged control specimens. These structural elements represent local members of open three-dimensional composite lattice structures (e.g., based on isogrid or IsoTruss® technologies) that are continuously fabricated using advanced three-dimensional braiding techniques. The unidirectional core specimens, nominally 8 mm (5/16") and 11 mm (7/16") in diameter, were manufactured using bi-directional braided sleeves or unidirectional spiral sleeves with full or partial (approximately half) coverage of the core fibers. The 51 mm (2") specimens were shorter than the critical buckling length, ensuring the formation of kink bands, typical of strength-controlled compression failure of unidirectional composites. The test results indicate an approximate decrease in the average undamaged compression strength of approximately 1/3 and 2/3 when impacted with 5 J (3.7 ft-lbs) and 10 J (7.4 ft-lbs) for the 8 mm (5/16") diameter specimens and 10 J (7.4 ft-lbs.) and 20 J (14.8 ft-lbs.) for the 11 mm (7/16") diameter specimens, respectively. The aramid sleeves improved the damage tolerance of the composite members, with the amount of coverage having the greatest effect; full coverage exhibiting up to 45% greater strength than partial coverage. Braided sleeves improved compression strength after impact by up to 23% over spiral sleeves, but generally had little effect on damage tolerance. Larger diameter specimens tend to be more resistant to damage than those specimens of a smaller diameter. The compressive material properties for undamaged basalt composites are also presented with the average compressive strength being 800 MPa (116 ksi).

Composite IsoTruss™ structures incorporate intersecting longitudinal and helical members. At the intersections, the fiber tows can be interwoven to achieve mechanical interlocking for increased joint integrity. Interlocking introduces gaps and curvilinear fiber paths similar to the crossovers in filament-wound structures, potentially facilitating local delamination within the members, thus reducing the strength and/or damage tolerance of the structure. Optimizing the interlocking pattern at the joints along with efficient consolidation minimizes these effects. Joint specimens were fabricated using a specially designed machine. Specific tow intersection patterns at the joint were: 1) Completely encapsulating the longitudinal member with the tows of the helical member; and 2) Interweaving the tows of the helical member with the tows of the longitudinal member. Consolidation was accomplished using: 1) a braided sleeve; 2) a coiled sleeve; 3) a sparse spiral Kevlar® wrap; 4) a polyester shrink tape sleeve; 5) twisting the entire bundle of longitudinal fiber tows; and 6) cinching the joints using aramid fiber. Ultimate compression strength and stiffness is directly related to the straightness of the tows in the longitudinal members at the intersections. An encapsulated joint reduces member strength by only 4.6%; whereas, an interwoven joint reduces member strength by 30.5%. The fiber paths of the longitudinal member in encapsulated joints are straighter than in interwoven joints, resulting in an average strength difference of 26.2%. Physical properties, strength, and stiffness show that consolidation quality directly affects performance. Consolidation using sleeves provides high quality consolidation, high strength, and high stiffness. Encapsulated joints consolidated using sleeves have an average ultimate strength and Young's modulus 34% and 21% higher, respectively, than encapsulated joints consolidated using other methods. Interwoven joints consolidated using sleeves have an average ultimate strength and Young's modulus 28% and 19% higher, respectively, than interwoven joints consolidated using other methods. Consolidating specimens using a braided sleeve yields the highest quality based on consistency, strength, and stiffness. Consolidating specimens by twisting the longitudinal member yields the lowest strength and stiffness. These conclusions will be applied to IsoTrussâ„¢ grid structure design and manufacturing technology.

This thesis quantifies the flexural behavior (strength, stiffness and failure) of IsoTruss®-reinforced concrete beam-columns for use in deep foundation pile applications. Four-point bending tests were performed in the laboratory on two instrumented carbon/epoxy IsoTruss® reinforced concrete piles (IRC piles) and two instrumented steel reinforced concrete piles (SRC piles). The piles were approximately 14 ft (4.3 m) in length and 14 in (36 cm) in diameter and were loaded to failure while monitoring load, deflection, and strain data. The steel and IsoTruss®® reinforcement cages were designed to have equal flexural stiffness to permit a relative strength comparison. Moment curvature diagrams reveal that the stiffness values were indeed close, verifying the design objective. At failure, the IsoTruss®-reinforced concrete beams held nearly twice the bending moment as the steel-reinforced concrete beams [1,719 kip-in vs. 895 kip-in (194 kN-m vs.101 kN-m)], although the failure modes were quite different. The SRC piles exhibited the traditional ductile failure behavior, as expected, while the IRC piles lacked ductility. The IRC pile deflections remained linear to failure, while the SRC piles yielded significantly. At 35 kips (165 kN), the maximum load on the SRC piles, the ductility of the SRC piles was twice that of the IRC piles (0.0084 and 0.0042, respectively). Toughness measurements reveal that due to the lack of ductility in the IRC piles, the SRC piles absorbed approximately twice as much energy as the IRC piles. Further investigations are required to explain the absence of ductility in the IRC piles, since ductility has been observed in other IsoTruss®-reinforced concrete structures in flexure. Even with this low level of ductility, the IRC piles are substantially stronger than the SRC piles and provide an alternative for use in deep foundation environments. Not only is the IRC pile strong enough for the job, but the IsoTruss® reinforcement is approximately 62% lighter, more rigid, and more corrosion resistant than the steel reinforcement.

Unidirectional carbon fiber and fiberglass epoxy composite elements consolidated with aramid sleeves were radially impacted at 5 J (3.7 ft-lbs) and 10 J (7.4 ft-lbs), tested under compression, and compared to undamaged control specimens. These structural elements represent local members of open three-dimensional composite lattice structures (e.g., based on isogrid or IsoTruss® technologies). Advanced three-dimensional braiding techniques were used to continuously fabricate these specimens. The unidirectional core specimens, 8 mm (5/16 in) in diameter, were manufactured with various sleeve patterns. Bi-directional braided sleeves and unidirectional spiral sleeves ranged from a nominal full to half coverage. These specimens were tested for compression strength after impact. This research used an unsupported length of 50.8 mm (2.0 in) specimens to ensure a strength-controlled compression failure. Compression strength of undamaged unidirectional carbon fiber and fiberglass epoxy composites is virtually unaffected by sleeve type and sleeve coverage. Fiberglass/epoxy configurations exhibited approximately 1/2 and 2/3 reduction in compression strength relative to undamaged configurations after impact with 5 J (3.7 ft-lbs) and 10 J (7.4 ft-lbs), respectively. Increasing aramid sleeve coverage and/or increasing the interweaving of an aramid sleeve (i.e., braid vs. spiral) increases the damage tolerance of fiberglass/epoxy composite elements. Damaged carbon/epoxy composites exhibited an approximate decrease in strength of 70% and 75% after 5 J and 10 J of impact, respectively, relative to undamaged configurations. The results verify that an aramid sleeve, regardless of type (braid or spiral), facilitates consolidation of the carbon fiber and fiberglass epoxy core. Not surprisingly, full coverage configurations exhibit greater compression strength after impact than half coverage configurations.

Compression tests were performed on damaged and undamaged rods and tubes made from unidirectional carbon/epoxy composite and lightweight core materials. Tested samples represent local members in an open, three-dimensional, composite lattice structure. Testing was performed in order to establish effective core materials to use in order to increase the buckling length of local IsoTruss® members while maintaining low weight. Members were formed from T700SC-12K-50C carbon fiber with UF6639-100 resin. Core materials consisted of 3/8-inch (0.953 cm) outside diameter Teflon® rods, Teflon® tubes, nylon rods, nylon tubes, Ertalyte® rods, and Duratron® rods. All 3/8-inch (0.953-cm) cores were each surrounded by 50 tows of carbon/epoxy prepreg. Control samples were also created with 50 carbon/epoxy prepreg tows. Half-inch (1.27 cm) outside diameter copper tubes were used as core materials for tubes consisting of 100 carbon/epoxy prepreg tows. Control samples to compare against samples with copper cores were also created with 100 tows of carbon/epoxy prepreg. Impact damage was inflicted using a cylindrical tup with 20 ft-lb impact energy.In undamaged specimens, nylon tube showed the highest structural efficiency. Nylon showed structural efficiencies much higher than other materials when comparing undamaged samples. In damaged specimens Ertalyte® rods showed the highest structural efficiency. Core stiffness appeared to control the level of absorbed impact energy with stiffer cores absorbing and dissipating more energy than softer equivalents during impact.