Long Term Displacement Data of Woven Fabric Webbings under Constant Load for Inflatable Structures

Abstract

Inflatable modules for space applications offer weight and launch volume savings relative to current metallic modules. Limited data exist on the creep behavior of the restraint layer of inflatable modules. Long-term displacement and strain data of two high strength woven fabric webbings, Kevlar and Vectran, under constant load is presented. The creep behavior of webbings is required by designers to help determine service life parameters of inflatable modules. Four groups of different webbings with different loads were defined for this study. Group 1 consisted of 4K Kevlar webbings loaded to 33% ultimate tensile strength and 6K Vectran webbings loaded to 27% ultimate tensile strength, group 2 consisted of 6K Kevlar webbings loaded to 40% and 43% ultimate tensile strength, and 6K Vectran webbings loaded to 50% ultimate tensile strength, group 3 consisted of 6K Kevlar webbings loaded to 52% ultimate tensile strength and 6K Vectran webbings loaded to 60% ultimate tensile strength, and group 4 consisted of 12.5K Kevlar webbings loaded to 22% ultimate tensile strength, and 12.5K Vectran webbings loaded to 22% ultimate tensile strength. The uniquely designed test facility, hardware, displacement measuring devices, and test data are presented. Test data indicate that immediately after loading all webbings stretch an inch or more, however as time increases displacement values significantly decrease to fall within a range of several hundredth of an inch over the remainder of test period. Webbings in group 1 exhibit near constant displacements and strains over a 17-month period. Data acquisition was suspended after the 17 th month, however webbings continue to sustain load without any local webbing damage as of the 21 st month of testing. Webbings in group 2 exhibit a combination of initial constant displacement and subsequent increases in displacement rates over a 16-month period. Webbings in group 3 exhibit steady increases in displacement rates leading to webbing failure over a 3-month period. Five of six webbings experienced local damage and subsequent failure in group 3. Data from group 4 indicates increasing webbing displacements over a 7-month period. All webbings in groups 1, 2, and 4 remain suspended without any local damage as of the writing of this paper. Variations in facility temperatures over test period seem to have had limited effect on long-term webbing displacement data. Nomenclature r = radius P = pressure t = thickness in = inch σ h = hoop stress σ l = longitudinal stress FS = Factor of Safety

Review of Habitable Softgoods Inflatable Design, Analysis, Testing, and Potential Space Applications

“…Creep, the permanent deformation of a material under a sustained load, is the primary concern for long-term damage to the restraint layer, as the polymeric softgoods used demonstrate viscoelastic creep behavior ( Figure 5). Understanding this behavior was identified as one of the highest priority research areas at the end of the TransHab program, and has been a focus area for testing at NASA over the last decade [9][10][11] .…”

“…Due to the high load capacity of the webbings and cordage used in these structures, large weights are typically required to perform the tests. NASA is currently researching more efficient approaches to performing real-time creep testing on highstrength softgoods based on lessons learned from a 5-year creep test program [9][10][11] . Accelerated creep testing methodology for these materials has also been pursued, to reduce the cost and time required, but initial correlations with real time creep tests resulted in non-conservative TTF estimates.…”

“…Since the conclusion of the TransHab program in 1999, inflatables technology development and testing has continued on a smaller scale at NASA. Research has focused on investigating and characterizing areas of primary concern including: the long-duration behavior of high-strength restraint layer materials [9][10][11] , the integration of hard structure such as windows and hatches into the fabric shell, internal and secondary structures, instrumentation and measurement of strains and loads, and efficient folding and packaging of the multi-layer shell. NASA has also tested many different inflatable geometries and architectures ( Figure 1, images 8 to 11) at sub-and full-scale, fabricated both at NASA and with its industry partners [12][13][14] .…”

Inflatable structure for aerospace application: Historical perspective and future outlook

“…Therefore, only Aramid (Kevlar and Technora) and LCP (Vectran) remain as the most suitable material for inflatable wing application and the proof that these material are the best is the fact that many research group on inflatable structure are using either of this material for their research [23,27,[37][38][39][40][41][42].…”