Study On The Mechanical Properties Of Short Fiber Reinforced EPDM Inhibitor Film

In order to meet the needs of modern local battlefield for remote propulsion technology,we must improve the overall performance of solid rocket motors and strive to explore high specific impulse,long time operation and controllable technical approaches.To satisfy the working reliability of motors and make the powder charge burning in a certain law,the flame-retardant and combustible elastic materials are usually coated on the propellant surface.Short fiber reinforced EPDM materials have excellent ablation resistance,but researches on its mechanical properties are not deeply enough,and the winding process is in the preliminary exploration stage as well.Based on experimental mechanics and theoretical analysis,the mechanical properties and application of short fiber reinforced EPDM inhibitor film materials were studied in this paper.The mechanical properties and failure mechanisms of materials were revealed,constitutive models and tensile failure strength criterions were established,and winding EPDM coating layer had been studied.The specific research contents are as follows:(1)Experimental study of short fiber reinforced EPDM materials.First,according to GB/T228-2010 standard,uniaxial and off-axial tensile test specimens were designed.The stress-strain curves of materials in different fiber directions were obtained by different tensile rates,and the tensile failure morphology was obtained.An improved single lap specimen was used to carry out the II interfacial debonding experiment of EPDM coating/polyurethane adhesive system.Then,the interface debonding load displacement curve was obtained and the interfacial debonding in the experiment was observed.According to GB/T25048-2010 standard,annular tensile test clamp was designed.The thin-walled cylindrical specimen with winding process was measured to get the circumferential force.The influence of winding angle on its mechanical properties was analyzed.It may provide reference and evaluation criteria for theoretical research and calculation.(2)Study on strength criterion of short fiber reinforced EPDM materials.Based on the uniaxial and off-axial tensile test results,the tensile failure mechanics behavior of short fiber reinforced EPDM thin films was studied.It was found that it exhibits obvious rate dependent plane orthotropic anisotropy and complex failure mechanism.Based on Tsai-Hill strength criterion and cosine series criterion,the curves of tensile strength,elongation at break and initial modulus with different off-axis angles were predicted.And applicabilities of two strength criterions were analyzed.To more accurately describe rate related failure mechanics behavior,rate dependent function and rate intensity factor methods were introduced to extendTsai-Hill strength criterion of orthotropic single plate.Comparative analysis showed that Tsai-Hill strength criterion based on rate dependent function method had higher prediction accuracy.Meanwhile,the prediction error of rate intensity factor method had strong relations with selection of reference rate and function expression of rate intensity factor.(3)Study on constitutive model of short fiber reinforced EPDM materials.In order to more accurately describe the mechanical properties of short fiber reinforced EPDM materials,based on the theory of fiber reinforced continuum mechanics,a transversely isotropic visco-hyperelastic constitutive model considering strain rate effect was proposed to characterize the nonlinear,anisotropic and rate dependent mechanical properties of materials at low strain rates.In which,the visco-hyperelastic strain energy was decoupled into the hyperelastic strain energy that included the isotropic matrix and the anisotropic fiber tensile,and the viscoelastic strain energy,a macro phenomenal constitutive model which was used to characterize the viscous response of rubber and fiber.Prediction results were compared with the experimental results by selecting the function form that characterized each strain energy.Results showed that the proposed transversely isotropic visco-hyperelastic constitutive model had high accuracy,and the steps of obtaining material parameters were clear.The form of the model was convenient to realize the finite element development.It can provide new ideas for the design of short fiber composite materials and have reference value for the structural integrity analysis of solid rocket motor.(4)Study on mechanical properties of winding EPDM coating layer.To describe the mechanical properties of the wrapped EPDM coating layer,it was equivalent to three parts:short fiber reinforced EPDM material,polyurethane adhesive and the interface phase between them.Based on the constitutive model of short fiber reinforced EPDM thin film and polyurethane adhesive,the bilinear cohesive model parameters obtained from the experimental results of interfacial debonding of type ? coating were used to predict the circumferential force of the wrapped coating.The introduction of the bilinear traction displacement law with interface softening function at the beginning of interface strengthening can improve the trend of the theoretical curve and make the final error less than 5%.It was proved that the constitutive model of short fiber reinforced coating material was of practical value and had reference value for engineering design and application.Through this research,the mechanical properties of short fiber reinforced EPDM materials were comprehensively studied.The Tsai-Hill strength criterion,rate independent hyperelastic and rate dependent visco-hyperelastic constitutive model were constructed.Also,the wrapping method of its winding was studied.The research results show that the failure strength criterion and constitutive model established in this paper were highly accurate,which was of guiding significance for the design of short fiber reinforced composites,and can provide a theoretical reference for the structural integrity analysis,design and numerical simulation of solid rocket motor.