Accelerated Creep Testing of High Strength Aramid Webbing

Abstract

A series of preliminary accelerated creep tests were performed on four variants of 12K and 24K lbf rated Vectran webbing to help develop an accelerated creep test methodology and analysis capability for high strength aramid webbings. The variants included pristine, aged, folded and stitched samples. This class of webbings is used in the restraint layer of habitable, inflatable space structures, for which the lifetime properties are currently not well characterized. The Stepped Isothermal Method was used to accelerate the creep life of the webbings and a novel stereo photogrammetry system was used to measure the full-field strains. A custom MATLAB code is described, and used to reduce the strain data to produce master creep curves for the test samples. Initial results show good correlation between replicates; however, it is clear that a larger number of samples are needed to build confidence in the consistency of the results. It is noted that local fiber breaks affect the creep response in a similar manner to increasing the load, thus raising the creep rate and reducing the time to creep failure. The stitched webbings produced the highest variance between replicates, due to the combination of higher local stresses and thread-on-fiber damage. Large variability in the strength of the webbings is also shown to have an impact on the range of predicted creep life.

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] .…”

Creep Burst Testing of a Woven Inflatable Module

“…The restraint layer webbings form the primary structural elements of an inflatable space module, and must sustain a design load for the operational life of the inflatable structure. An extensive test program of individual webbings has been conducted to characterize both the webbing's short-term and long-term behavior 4,7 . The results of these tests inform ongoing design work and support the testing of full-scale modules as discussed and summarized in this section.…”

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

“…nflatable space structures have been utilized in various aerospace applications [1][2][3][4][5] for over 30 years. Three examples of inflatable aerospace structures are shown in Figure 1.…”