Effect of confining pressure on the structural integrity of NEPE solid propellant grain

In order to investigate the effect of confining pressure on the structural integrity of Nitrate Ester Plasticized Polyether (NEPE) propellant grain, a NEPE propellant, whose maximum tensile stress increased from 0.71 MPa to 1.30 MPa and ultimate strain increased from 81% to 175% at 0.0667 s⁻¹ strain rate with the confining pressure increased from 0 to 5.4 MPa, was chosen to be analyzed and discussed. An existing three-dimensional viscoelastic damage model considering confining pressure effect developed for solid propellant was generalized for wider use in a finite element model. The structural integrity of NEPE propellant grain under ignition pressurization was evaluated. The results indicate that the effect of confining pressure within 5.4 MPa on the mechanical response of NEPE propellant grain is related to the strain damage threshold, when the equivalent strain of NEPE propellant grain is lower than the strain damage threshold of NEPE propellant without confining pressure, the confining pressure has little effect on the mechanical response of grain; otherwise, the confining pressure increases the equivalent stress and decreases equivalent strain of NEPE propellant grain under ignition pressurization. The maximum equivalent stress increased 24% and maximum equivalent strain decreased 10% under the confining pressure of 5.4 MPa. The confining pressure can reduce the “dewetting” damage, the “dewetting” damage coefficient can be reduced about 40% under the confining pressure of 5.4 MPa. When considering the confining pressure effect, the safety factor of the grain obtained by twin shear strength criterion and Von Mises stress criterion are 8.57 and 4.36, respectively. The safety factor obtained by the traditional Von Mises stress criterion is 2.38 with non-considering confining pressure effect. The confining pressure effect can improve safety factor of NEPE propellant grain significantly, and the traditional method of structural integrity assessment is conservative.