Mechanical Characterization of a Graphite Epoxy IsoTruss

Graphite epoxy IsoTruss specimens were filament wound and experimentally tested to failure under simple compression, tension, and torsion and compared with simple analytical predictions. Failed specimens were subsequently retested to gain further insight into the mechanical interaction of the various components of the IsoTruss. Simple analytical techniques were used to predict the approximate strength of the IsoTruss. A total of 15 five inch (12.7 cm) diameter, five-bay IsoTruss specimens were filament wound on a reusable, silicone inner mandrel. Nine 3-tow specimens with a nominal member cross-sectional area of 0.27 in.2 (1.7 cm2) and six 5-tow specimens with a corresponding area of 0.45 in.2 (2.9 cm2) were fabricated and tested. Axial and torsion loads, axial displacements, and rotations were synchronized with axial strain gauge data from the three main regions of the IsoTruss (axials, tetrahedrons, and the cross-zone). Additionally, four 3-tow IsoTruss specimens with various lengths tested in simple compression showed that global buckling does not affect a five inch (12.7 cm) IsoTruss six bays in length or less, and that the failure of a single bay has little or no effect on the capacity of the remaining IsoTruss to resist compressive loads. The results indicate the relative contributions of primary and secondary load members. The influence of the secondary load members increases after failure of a primary load member. The redundancy of the structure coupled with the influence of the secondary load members causes a ductile type failure to occur for all load types, but it is most pronounced in torsion. In all load types, failure was initially isolated to a single bay. Generally, reloaded specimens were only minimally affected by the prior damage. Finally, increased fiber interconnectivity at the nodes of the IsoTruss yields higher ultimate stresses and greater toughness in IsoTruss structures.