Flex Fatigue

Flexural fatigue is a critical concern in many applications where yarns or fabrics are subject to repeated bending or creasing. Examples include ropes, sailcloth, inflatable and/or temporary structures, etc. Improving the service life of products by increasing flex fatigue resistance is an important driver for the use of Vectran^® fibers in a variety of applications.

The actual mechanism of flex fatigue has been a subject of considerable study, due to the significant variability in flexural failure resistance of fibers made from linear chain polymers. For example, typical polyesters, Vectran^® (wholly aromatic liquid crystalline polyester), and aramids (wholly aromatic liquid crystalline polyamide) all exhibit a microfibrillar structure. In addition, the ultimate compressive strength of high modulus organic fibers is generally about 1/10 of the ultimate tensile strength, and for all of the examples above, the first visual manifestation of flex damage is the appearance of kink bands in the fiber. Kink bands, often explained as dislocations (buckling or breaking) in the molecular chains, could involve the entire microfibril, or propagate through the microfibril with repeated flexing or compressive strain at the same location.

Figure 15: Folding Endurance Tester In spite of these structural commonalities, these fibers differ considerably in their resistance to flexural fatigue. Typical polyester can not provide the tensile and thermal stability of high performance fibers, but it does offer higher flex fatigue resistance when cycled at a similar percentage level of its ultimate breakload. Vectran® routinely outperforms aramids when tested for fatigue resistance and tenacity retention in yarn, rope/cable, and fabric forms. Comparative data for yarns appear in Table 12 and were collected using the Folding Endurance Tester (Figure 15). While aramid results varied considerably with type, Vectran® clearly outperforms the aramid class as well as PBO. Flexural test data should always be considered as a tool to rank various materials since controlled component testing can not mirror actual results in the fully constructed product’s environment. However, relative material rankings are consistent from test to test, as seen in Table 13. These rope testing data, generated by a high performance rope and cable company, show a range of lifetimes observed for aramids and PBO, with clearly the best results obtained from the Vectran® sample.

(Tinius Olsen/M.I.T.)

An aerospace company compared flexural fatigue resistance of Vectran^® to aramids in coated fabric form. In this study, base fabrics of aramid and Vectran^® were coated in an identical fashion with the company’s proprietary formulation. Specimens 1” (weft direction) × 60” were cut and tested to simulate hard creasing and folding in a cyclic fashion. Each cycle consisted of folding the sample in half, dragging a 10 lb. steel roller over the fold, refolding the specimen at the same point but in the opposite direction, and again dragging the roller over the fold. Strength losses were compared using a test compliant with FED-STD-191, Test Method 5102. As Table 14 illustrates, Vectran’s^® tenacity losses were minimal after 100 cycles, with the tensile failure point occurring away from the fatigued fold line. Aramid strength losses were significant, with tensile failures occurring at the fold line.

Vectran’s^® higher load bearing capability after equivalent fatigue levels is also demonstrated in Figure 16. In this comparison, 400 denier Vectran^® and aramid yarns were subjected to the indicated cycle level in a Tinius Olsen tester, after which the samples were removed and tested for strength. In this study, Vectran’s^® load bearing capability was twice that of the aramid after as few as 500 cycles, and the gap appears to widen as cycling continues. Fiber samples for each material and cycle level were examined by microscopic techniques in an effort to compare kink band formation. Vectran^® samples showed kink band formation increasing with cycle level as expected; however, the most noted observation for aramid samples was the presence of split and fibrillated fibers, even at the 500 cycle level. Possibly, kink band formation in the aramids was initiated at much lower cycle levels, but catastrophic failures later masked or interfered with microscopic examinations.

Figure 16: Tensile Strength vs Flexural Fatigue

Flexural fatigue failure and differences between the resistance of various fibers is not a simple mechanism. However, one relevant consideration might be the relative extent of crystalline order in these three fibers. For example, standard polyesters are ordered along the axis with considerable amorphous content. Vectran^® is a liquid crystalline fiber oriented along the axis with no amorphous regions and no observed three-dimensional crystallinity. Aramids are liquid crystalline fibers in which three-dimensional crystals have been observed. While each of these fibers has exhibited kink band formation in response to compressive strains, lower degrees of dimensional order may more effectively block damage propagation across microfibrils and/or fibers.