Strain-rate Dependency And Constitutive Model Of Mechanical Behaviours Of CMDB And HTPB Propellant

Technologies of gun-launched missiles,extended range missiles,and terminal correction mortar represent the development direction of a new generation of weapons.This type of weapons tends to support an axial overload of up to 10⁴ g during launching.The overload is far higher than that of 10² g supported by conventional missiles,which constitutes a great challenge for structural integrity of solid propellant grain.Taking composite modified double base?CMDB?and hydroxyl-terminated polybutadiene?HTPB?propellant as investigative objects,this dissertation has carried out experiments of mechanical properties,and studied strain-rate dependency,and developed constitutive model.The specific work is as follows:?1?Experiments of mechanical propertiesA universal testing machine,a high-speed hydraulic testing machine,and a split Hopkinson pressure bar?SHPB?system were employed.Experimental method of CMDB and HTPB propellant were designed to carry out the compressive experiments that cover the whole range of low,intermediate,and high strain rates,and the tensile experiments that cover a partial range of strain rates.These experiments fill the blank of intermediate strain-rate experimental technique in field of experimental study.Compressive mechanical curves of CMDB propellant over strain rates ranging from 1.7×10^-4 to 4000 s^-1,tensile mechanical curves of CMDB propellant over strain rates ranging from 1.7×10^-3 to 1.7×10^-1 s^-1,compressive mechanical curves of HTPB propellant over strain rates ranging from1.7×10^-4 to 2500 s^-1,and tensile mechanical curves of HTPB propellant over strain rates ranging from 1.2×10^-4 to 80 s^-1 were achieved.The repeatability and the constancy of strain rate were checked to substantiate the validity of experimental data.?2?Strain-rate dependencyCompressive mechanical behaviours of CMDB propellant over strain rates ranging from 1.7×10^-4 to 4000 s^-1 were studied.The yield stress was observed to increase bilinearly with the logarithm of strain rate.The transition strain rate was experimentally determined to be 50 s^-1.Comparison of the transition strain rate and the DSR model prediction proposed by Mulliken et al.indicates that the bilinear strain-rate dependency is attributed to the transformation of the rate-dependent mechanism,from the restriction of only the?process to the cooperative restriction of the?and?processes.A mathematical model of Ree-Eyring was proposed to predict the yield behaviour of CMDB propellant over a wide range of strain rates on the basis of experimental data.Tensile mechanical behaviours of CMDB propellant over strain rates ranging from 1.7×10^-3 to 1.7×10^-1 s^-1 were studied.The stress at a given strain level was observed to increase linearly with the logarithm of strain rate.The strain-rate dependency is the same as that of compressive mechanical behaviours of CMDB propellant at low strain rates.Compressive mechanical behaviours of HTPB propellant over strain rates ranging from 1.7×10^-4 to 2500 s^-1 were studied.The stress at a given strain level was observed to firstly increase with the logarithm of strain rate in a linear manner,then followed by an exponential manner.The transition strain rate was experimentally determined to be 1 s^-1.The qualitative analysis in terms of molecular-level motions indicates that the strain-rate dependency is attributed to the restriction of molecular mobility with a certain length scale.Tensile mechanical behaviours of HTPB propellant over strain rates ranging from 1.2×10^-4 to 80 s^-1 were studied.The stress at a given strain level was observed to increase bilinearly with the logarithm of strain rate.The transition strain rate was experimentally determined to be 1 s^-1.The strain-rate sensitivity linearly enhances with decreasing the strain level.The tensile mechanical behaviours of HTPB propellant are the same as the compressive mechanical behaviours.Due to a lack of tensile experimental data at higher strain rates,the trend of exponential increase in compressive mechanical behaviours of HTPB propellant was not observed.The strain-rate dependency of tensile mechanical behaviours of HTPB propellant is also attributed to the restriction of molecular mobility with a certain length scale.?3?Constitutive modelA modified DSGZ constitutive model was proposed to describe the compressive mechanical properties of CMDB propellant over strain rates ranging from 1.7×10^-4 to4000 s^-1,and the tensile mechanical behaviours of CMDB propellant over strain rates ranging from 1.7×10^-3 to 1.7×10^-1 s^-1.The model is a phenomenological one associated with strain rate,and can describe the total phases of initial linear elasticity,non-linear rise to yield,followed by strain softening and then significant strain hardening.The predictions are in agreement with the experimental data.A phenomenological constitutive model associated with strain rate was proposed to describe the compressive mechanical properties of HTPB propellant over strain rates ranging from 1.7×10^-4 to 2500 s^-1.The model combines the linear dependence of stress on strain and the strain-rate dependency,which is a linearly elastic component as a base model,then multiplied by a rate-dependent component.The predictions are in agreement with the experimental data.A phenomenological constitutive model associated with strain rate was proposed to describe the tensile mechanical properties of HTPB propellant over strain rates ranging from 1.2×10^-4 to 80 s^-1.The model combines the bilinear strain-rate dependence and the difference in strain-rate sensitivity at different levels of strain,which is a hyperelastic component as a base model,then multiplied by a rate-dependent component.The predictions are in agreement with the experimental data.