Characterization of Mesoscale Damage Evolution in NEPE Propellant Via Digital Image Processing and Material Region Tracking

Nitrate ester plasticized polyether (NEPE) propellants are widely used as the primary energy source in solid rocket motors (SRMs), whose macroscopic mechanical degradation under loading fundamentally originates from mesoscale damage evolution. Characterization and analysis of mesoscale damage evolution are therefore essential for understanding the damage mechanisms and mechanical response of solid propellants. However, continuous quantitative characterization of damage evolution within the same material region under large deformation remains challenging due to deformation-induced variation of the tracked material region. To address this challenge, a material region tracking method based on four-fiducial-point calibration was proposed and integrated with digital image processing (DIP), enabling continuous tracking of the same material region on the propellant surface and quantitative characterization of damage evolution within it throughout large deformation. In-situ SEM tensile tests were then conducted under quasi-static uniaxial tension to investigate the mesoscale damage evolution of NEPE propellants over a strain range of 0–167%. To address the uncertainty arising from empirically tuned DIP parameters, the influence of different parameter combinations on characterization results was systematically evaluated. Quantitative characterization proved highly sensitive to parameter selection, with substantial variations among parameter combinations; however, after Min–Max normalization, a subset of the normalized evolution curves exhibited a clustered distribution. On this basis, an equivalence-screening framework integrating four metrics (Pearson r , RMSE, ICC, and MaxDev) was established, which reliably identified equivalent parameter combinations, yielded highly consistent results, and improved the robustness and reproducibility of quantitative characterization. The proposed methodology enabled the following findings regarding mesoscale damage evolution within the tracked material region: Compared with porosity, pore area provides a more direct representation of mesoscale damage evolution and is therefore more suitable for quantitative characterization under material-region tracking conditions. The evolution of pore area within the tracked material region exhibits a multi-stage sigmoidal growth pattern, which reflects the progressive and heterogeneous characteristics of mesoscale damage and correlates closely with the macroscopic mechanical response. The stage with the largest increase in pore area corresponds closely to the nonlinear damage stage of the macroscopic stress–strain response. During this stage, rapid interfacial debonding develops in initially defect-free AP particles, thereby revealing the mesoscale origin of the macroscopic nonlinear mechanical response of NEPE propellants.