Abstract
Small diameter double braids were fabricated using poly (p-phenylene terephthalamide) (PPTA) as the core material and nylon 66 and polyester (PET) as the braided sheath materials. The degree of deterioration from abrasion was characterized by residual strength measurements, while the number of cycles to failure was used to determine fatigue resistance. Fracture zones from both, along with the failed specimens from static strength tests, were investigated using SEM. Tensile testing of both the sheathed braid and unsheathed core revealed that the sheath improves load-bearing capabilities, but pre-conditioning the braids before testing decreases strength. A modified model was applied to predict the mean failure stress of the unsheathed core, and a good fit was found, with the rope failure strain being more difficult to predict when the model was applied in a similar fashion. Abrasion testing of the sheathed samples did not result in a strength loss within the number of cycles tested, but when the unsheathed core was tested at a moderate number of cycles, a 60% loss of strength resulted. The sheath also improved the lifetime when the braids were subjected to cycling around a capstan. Lifetimes were short due to the sensitivity of the PPTA core to lateral compressive forces and the high loading force (∼50% UTS). SEM investigation of the failure zones for both the abrasion and fatigue samples resulted in similar topographies. Surface abrasion of the PPTA core resulted in fibrillation. Splitting was evident in the morphologies of both nylon and polyester sheaths. There was little evidence of self-abrasion of the PPTA in the braided structure, indicating low relative strand movement.
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