Space environments are particularly harsh for the high-strength fibers NASA relies on for soft structures. Kevlar, Nomex, Nylon, and other synthetic fibers are broken down by exposure to the combination of vacuum, atomic oxygen, and ultraviolet radiation. Glass fiber offers unique advantages for structural applications, but is hampered by a sensitivity to bend radius and large sewn areas in order to distribute loads appropriately. The goal of this project was to determine the geometric limits of a potted end termination, and to assess weight and volume savings over a traditional sewn pin and clevis termination.
During the course of the investigation into potted end terminations for glass fiber webbing, a new and innovative connection was developed that has lower weight, reduced fabrication time, and superior thermal tolerance over the metallic end terminations that were to be optimized in the original proposal. This end termination essentially transitions the flexible glass fiber webbing into a rigid fiberglass termination, which can be bolted/fastened with traditional methods.
The initial termination using Hysol 9394 and aluminum termination blocks was investigated with various lengths of webbing potted and pulled to failure. Through this testing, it was determined that a 1/2” -long bonded length was required to develop full strength in 1” wide webbing that was 0.03” thick. During the course of testing, it was observed that the aluminum termination blocks had several disadvantages:
1) The end terminations must be significantly larger than the webbing itself.
2) The potting process used a “heat cleaning” step of several minutes at ~800 °F to burn off a Teflon coating. This was assumed to be needed to get a high-strength bond directly to the glass fiber. Failure was often observed at the edge of this heat-affected zone.
3) The termination block and the fiber/epoxy composite will always have differential thermal expansion, and this would have to be investigated for any reduction in strength and thermal limits to preclude de-bonding.
A new idea was to change the end termination material to FR-4 fiberglass sheet, and to bond the glass webbing/Hysol epoxy composite to the FR-4 fiberglass sheet directly. This eliminated disadvantages 1 and 3 from the aforementioned list. A final step, which started out as a control to see how much strength was gained by the heat cleaning process, demonstrated that sufficient (full) strength is obtained without heat cleaning at all.
The resulting end termination is only as wide as the webbing itself, is composed primarily of glass fiber with virtually no differential thermal expansion sensitivity, and is extremely lightweight. Attachment to other structural elements may be through a pinned or bolted arrangement, as was used in the testing, or in the case of a composite structure (carbon/epoxy or glass fiber/epoxy), this new end termination can be bonded directly to the composite structure.
In summary, an extremely compact, lightweight, and thermally insensitive end termination for glass fiber webbing was developed and tested, sustaining the full rated strength of the webbing in tensile tests. The end termination consists of fiberglass sheet bonded to the webbing using epoxy or similar structural adhesive. The fiberglass sheet has well-known properties, and can be drilled and finished through conventional methods. This work used FR-4 fiberglass sheeting, 0.09” thick. The end termination is a sandwich of fiberglass/webbing/fiberglass. The overlap of fiberglass and webbing used for the test was selected based on the prior work with potted aluminum end terminations, and was kept constant at 0.75”.