Thermomechanical Deformation Of Polyimide Braided Rope And Its Influencing Factors

In this paper,the telescopic cable antenna using polyimide rope is used as the application background,polyimide rope braided from polyimide fiber was used as the research object.The thermal cycling behavior,strain relaxation deformation under cyclic loading,stress relaxation and creep behavior of braided rope under different stress and environment were studied by self-made vacuum thermal cycling equipment.The mechanism of polyimide rope performance degradation was discussed by analyzing the structure characteristics of the polyimide braided rop e before and after the thermomechanical loading using Fourier transform infrared spectroscopy(FT-IR),thermogravimetric analysis(TG)and differential thermal analysis(DSC).The cyclic loading strain relaxation test results show that the amount of strain relaxation at the initial stage of cyclic loading is large,and then the slowdown tends to be stable.The amount of strain relaxation in the 50% humidity air medium is significantly reduced compared to that in the aqueous medium within its internal friction is reduced under room temperature and standard atmosphere.Under standard atmosphere air conditions,the strain relaxation is increased and the internal friction increases as the temperature increases in the temperature range of-150°C to 150°C.The thermal activation energy carry out from strain relaxation deformation is 1.6kJ/mol.The thermal expansion test results show that the polyimide braided rope exhibits a negative expansion in the stretching direction in the-150°C to 150°C temperature range.The heat distortion curves during the heating and cooling phases do not coincide and appear annular.The thermal cycling under low stress causes the strain loop to move down and the sample is shortened.With the increasing of stress,the sample is elongated and the strain loop upwards under thermal cycle,the coefficient of thermal expansion increases slightly.The creep and stress relaxation test results show that the steady-state creep rate of the polyimide braided rope at 25°C 50 percent humidity is 2.15×10-4/h,the steadystate creep rate of the polyimide braided rope at 25°C 95 percent humidity is 4.86×10-4/h,and the steady-state creep rate at 120 °C is 1.46×10-3/h.The amount of stress relaxation is the smallest in the vacuum environment compare t o the largest in the water medium under 25°C.The thermal activation energy of stress relaxation is 1.9kJ/mol in the temperature range of-150°C to 150°C under atmospheric air pressure.In the temperature range of-150 °C to +150 °C,the thermal activation energy of the strain relaxation under stress cycling,creep and stress relaxation is close to the activation energy between molecules,indicating that the mechanism of thermomechanical deformation is dominated by van der Waals force in the molecular chain of polyimide.