Experimental And Constitutive Model Research Of EPDM Insulation Under Impact Load

With the technology development of rocket missiles, the increase of rocket motor’s internal working pressure, the promotion and application of high-energy propellant and the development of anti-high overload rocket, security issues of the solid rocket motors have become more prominent. However, during the process of grain manufacturing, assembly, storage, transportation and work, high transmit overload, drop impact shock etc extruded the propellant and insulating coating layer, resulting in the propellant structure deformed or unable to complete the intended target, even more likely to cause catastrophic accidents. In order to investigate the characteristics of solid rocket motor insulation material, combining the experimental research and mathematical analysis method to analysis the ethylene propylene diene monomer (EPDM) insulation materials and the mechanical properties under a large range of strain rate after pyrolysis, constitutive models were established to describe those mechanical responses effectively. The main contents of this paper include the following aspects:(1)Pyrolysis experiments of EPDM material. The thermal gravimetric analysis (TGA) was used to research the thermal stability of EPDM material. The thermal gravimetric analysis curves were acquared by heating the samples in a nitrogen environment with different heating rates. The different temperatures pyrolysis experiments of EPDM were completed basing on the results of TGA curves.Energy Dispersive Spectroscopy (EDS) was used to observe the content of the different elements of EPDM material at various stages, for later research on the mechanical properties of EPDM material after after pyrolysis.(2) Experimental study of mechanical properties of EPDM material. The quasi-static compression tests of EPDM including the raw and pyrolysis material were conducted by universal testing machine at different compression rates. And the dynamic compression experiments under different strain rates of the raw materials and varying pyrolysis degrees of EPDM material were performed by the split Hopkinson pressure bar (SHPB) apparatus. The differences of mechanical behaviors of EPDM between low strain rate and high strain rate and the sensitivity to the strain rate were analyzed to study the effects on mechanical properties of the material due to different pyrolysis degrees.(3) EPDM material constitutive model at low strain rates. Because of the hyperelastic properties of raw material in quasi-static experiments, hyperelastic model combined with strain-rate-related items was used to establish EPDM raw material constitutive model at low strain rates to describe the mechanical responses.According to the quasi-static mechanical properties of EPDM material after pyrolysis, the material exhibited different transition from hyperelastic material to loose brittle material on account of different pyrolysis degrees.In order to be able to describe the mechanical properties of EPDM material in different pyrolysis degrees accurately and efficiently, different stage models were adopted. The model parameters were obtained by fitting. In addition, the theoretical predictions and experimental results were compared to verify the reliability of the models.(4) Constitutive model of EPDM material at high strain rates. To describe the raw material hyperelastic and viscoelastic properties at high strain rates, hyperelastic response items was used to replace the polynomial nonlinear elastic term in ZWT model to exhibit mechanical properties of EPDM at high strain rates. Furthermore, the temperature-related item was introduced on the basis of improved ZWT model to describe the mechanical behavior changes caused by the different degrees of pyrolysis. Adding appropriate corrections to ensure the prediction accuracy under different conditions. The complete constitutive model which can characterize mechanical properties of EPDM material at different stages of pyrolysis and different strain rates was built. The validity of the model was guaranteed by the results verification.Through the experimental research and analysis, this paper systematically established the mechanical constitutive models of EPDM insulation material in different stages of pyrolysis under high and low strain rate, which provided theoretical reference including structural integrity analysis of propellant grain, rocket motor design and numerical simulation for the use of EPDM as solid rocket motor insulation.