Research On Creep Behavior And Life Time Prediction Of Polyimide Fibers

Polyimide(PI)fibers have excellent mechanical,thermal,dielectric and environmental resistance properties,and have great application potential in aerospace,bulletproof equipment,flame retardant and heat insulation and other fields.During long-term service,PI fibers will creep deformation or even rupture under load,so it is necessary to study the creep behavior of PI fibers.In this paper,the creep behavior of PI fibers was firstly studied through a series of constant temperature and constant load creep experiments.Then,the long-term creep behavior of the PI fibers was characterized by the time-temperature superposition principle and the stepped isothermal method,and the long-term creep behavior curve and the creep rupture prediction curve of the PI fibers were obtained.Finally,the effects of different post-thermal treatment conditions on the mechanical properties and creep behavior of PI fibers were studied.The main research contents and results of this paper are as follows:The thermal decomposition and mechanical properties of PI fibers at high temperature were studied.The results show that PI fibers have good thermal decomposition performance and mechanical properties at high temperature,and its strength and modulus can still reach 1.55 GPa and 27.74 GPa at 350 ℃.By comparing the effects of different linear densities,loading rates and pre-tension conditions on the creep behavior of PI fibers,the test method for the creep behavior of PI fibers was improved.The short-term and long-term creep properties of PI fibers were tested by the improved creep behavior test method.The results show that the creep behavior of PI fibers is a logarithmic function of time and non-linear under the experimental conditions.The long-term creep curves of PI fibers at room temperature show that the fibers enter the steadystate creep stage after about 30 h.Several commonly used creep models were used to fit the PI fibers creep data.The results show that the Findley model and the Norton-Bailey model can achieve better fitting effect.The accelerated characterization of the creep behavior of PI fibers by the time-temperature superposition principle and the stepped isothermal method was studied,and the prediction of the creep rupture time based on the creep suspension experiment and the stepped isothermal method was studied.The results show that: TTSP can obtain a relatively smooth main curve of creep at room temperature,which is in good agreement with the measured long-term creep data.Using the Arrhenius equation to calculate the creep activation energy of PI fibers,the results show that increasing the stress will increase the creep activation energy of the PI fibers and increase its resistance to creep motion.The reason for this phenomenon may be that the increase of strain further improves the orientation degree of PI fibers.In addition,with the increase of the stress level,the obtained creep activation energy is closer to the activation energy of the secondary transition,indicating that the creep behavior of PI fibers may also be related to its secondary transition.The stepped isothermal method can obtain a creep master curve that is in good agreement with the long-term creep curve at high creep stress level,but the agreement is poor at the medium creep stress level.The creep rupture time of PI fibers by the stepped isothermal method,compared with the creep rupture time of PI fibers based on the creep suspension test datas,it was found that the stepped isothermal method is more conservative in predicting the creep rupture life time of PI fibers,and its predicted rupture time is 19 years under the load level of 68.2% ABL.The mechanical properties and viscoelastic behavior of PI fibers after different post-thermal treatments were studied.The results showed that the tensile strength and modulus of PI fibers increased first and then decreased with the increase of temperature after 10 min of tension-free treatment at280 ℃～360 ℃,and the non-linear behavior of fibers was greatly improved.Tensile strength and modulus of PI fibers after tension-free treatment at 340 ℃increased with the increase of treatment time,and the linear behavior was more obvious,but the change was small after more than 40 min.After being treated with different tensions at 340 ℃ for 40 min,the modulus of PI fibers increased and the strength decreased as the tension increased,and non-linear behavior was more obvious.