Stress characteristics and failure criterion of composite solid propellant under biaxial tensile loading

Biaxial tension is a critical load throughout the entire life cycle of solid rocket motors, substantially influencing the structural integrity of composite solid propellant grains. The failure criterion of a material is highly dependent on its mechanical response characteristics. Thus, stress calculation methods using a constant load transfer coefficient (LTC) fail to accurately capture the actual stress state. This approach tends to introduce substantial computational errors. Therefore, investigating the mechanical behavior characterization and failure criteria of solid propellants under biaxial tensile loads has theoretical importance and engineering value. This study conducted biaxial tensile tests on composite solid propellant at three strain rates (0.001, 0.01, and 0.1 s⁻¹) using digital image correlation. A stress field solution method based on the nonlinear viscoelastic constitutive model was developed, revealing the evolution pattern of the LTC during tensile deformation. A biaxial tensile failure criterion parameterized by three stress invariants (I₁, J₂, and J₃) was also established. Results demonstrate that under non-equal biaxial tension, the crack initiation locations and propagation directions show remarkable differences compared to equal biaxial conditions, with cracks propagating bidirectionally perpendicular to the principal strain directions. The LTC exhibits a characteristic three-stage evolution pattern during biaxial tension: “constant phase–rapid decline–gradual decline.” Stress calculation methods that use a constant LTC may introduce relative errors up to 25.3 %. Compared to existing typical failure criteria, the proposed criterion effectively captures the strength enhancement effect of composite solid propellants under biaxial tension and demonstrates smaller fitting errors across different strain rates.