Creep and Relaxation Behavior of Braided Ropes

Due to the inherent viscoelasticity of synthetic fiber materials such as aramid and polyarylate, synthetic fiber ropes exhibit significant time-dependent behaviors under load, such as creep and stress relaxation. Under the influence of complex space environmental conditions, including vacuum, alternating high and low temperatures, and ultraviolet radiation, the creep and stress relaxation behaviors of ropes become even more complex. NASA has conducted extensive experimental research to analyze the effects of fiber material, environmental conditions, load levels, and other factors on the creep and stress relaxation behaviors of braided ropes. Studies have found that the creep and stress relaxation process of aramid braided ropes can be divided into three stages: initial, steady-state, and accelerated. The steady-state creep strain exhibits a linear relationship with time; the stress relaxation rates in both the initial and steady-state stages show a linear logarithmic relationship with time, with the relaxation rate in the initial stage being approximately 1.4 times that of the second stage. At a 50% ultimate tensile strength (UTS) load level, the creep rate of aramid braided ropes is about four times that of polyarylate braided ropes. Ding Xu’s research indicated that at a 25% UTS load level, the relaxation rate of aramid braided ropes over 40 days is approximately 2.13 times that of polyarylate fiber braided ropes, suggesting that polyarylate braided ropes exhibit better resistance to creep and stress relaxation under the same load conditions.

During the accelerated creep stage, the plastic strain of braided ropes increases rapidly, ultimately leading to failure. NASA's Langley Research Center developed a novel creep testing apparatus integrated with heating and cooling systems to study the creep behavior and lifespan of polyarylate braided strap specimens under controlled environmental conditions. The results showed no significant difference in the creep patterns between two polyarylate braided strap specimens of different lengths. Under alternating temperature conditions, the creep behavior of polyarylate braided straps during the steady-state stage aligned with temperature variations, but higher load levels (>50% UTS) could eliminate the effect of alternating temperatures on creep behavior. Thus, the creep behavior of braided straps is primarily influenced by load level and temperature. Additionally, the load level significantly affects the creep life of braided straps; the creep life at a 65% UTS load level is about 12 times that at a 70% UTS load level. NASA plans to conduct future creep experiments at load levels below 50% UTS to quantify the effects of high and low alternating temperatures, vacuum, and creep load on the creep life of polyarylate braided straps, which is crucial for predicting the creep behavior and lifespan of space habitats (Transhub).

Since polar groups can associate with water molecules to form hydrates, the mechanical properties of high-performance fibers are, to some extent, influenced by moisture absorption. Yang Huijie studied the creep and stress relaxation behaviors of polyimide braided ropes under different humidity conditions. The results indicated that the steady-state creep rate of braided ropes in a 95% relative humidity environment was approximately twice that in a 60% relative humidity environment. In a vacuum environment, the relaxation rate of braided ropes was much lower than that under normal temperature and pressure or water-immersed conditions. This may be because water molecules can bind with the amide groups in polyimide molecules, disrupting the hydrogen bonds between polyimide molecular chains. Therefore, humid environments increase the stress relaxation rate of hydrophilic fibers, such as aramid.

Creep and stress relaxation testing is a very time-consuming process and requires specialized experimental setups to provide long-term constant load or strain. Theoretical models developed based on short-term experimental tests can effectively reduce testing costs: Tang et al. established a stress relaxation equation for ropes based on the Schapery nonlinear viscoelastic constitutive model and derived a creep-recovery constitutive model for ropes under step loading using Prony series, providing theoretical support for the design and optimization of mesh deployable antenna cable-net structures. However, the model did not account for the viscoplastic behavior of ropes. Huang et al. combined the Schapery model with Owen’s viscoplastic theory to develop a linear constitutive model for predicting the creep-recovery behavior of aramid and polyester fiber ropes. The model’s calculations aligned well with experimental results, but its applicability requires further validation and holds potential for extension to smaller-scale fiber geometries, such as strands and yarns. Lian et al. developed creep models incorporating viscoelasticity, viscoplasticity, and viscous damage based on thermodynamics. The model’s calculations showed good consistency with creep test data of high-modulus polyethylene (HMPE) ropes under different load levels. This model can effectively simulate the creep fracture behavior of HMPE ropes but overlooks the influence of the braided structure on creep behavior. Currently, research in this area is still in its preliminary stages, and the applicability of these models requires further testing.