Multiscale viscoelastic fracture behavior analysis of highly filled polymer based on phase field model

The viscoelastic characteristics and complex multiphase microstructures of highly filled polymers render their fracture behaviors time- and scale-dependent. As a result, the prediction of their behaviors and the analysis of their multiscale fracture mechanisms are formidable engineering challenges. In this study, a multiscale fracture model for highly filled polymers was established by coupling a viscoelastic phase field model in the form of the generalized Maxwell framework with the cohesive zone model. Numerical implementation was achieved through a dual-layer mesh structure associated with cohesive elements. The simulation results showed favorable agreement with the experimental results obtained for notched and center-holed propellant specimens. The maximum load prediction error of the developed model remained within 7.0%, while the prediction error for the displacement corresponding to the maximum load remained within 7.5%. At the mesoscale, the fracture process of solid propellants was divided into three distinct stages: particle dewetting (Stage I), crack initiation (Stage II), and matrix tearing (Stage III). Dewetting of relatively large ammonium perchlorate particles induced stress concentration, thereby triggering crack initiation and propagation (perpendicular to the loading direction under uniaxial loading, and along the ±45° directions under biaxial loading). Smaller high melting explosive particles caused the crack to propagate along a curvilinear path. During the relaxation stage, viscoelastic hysteresis of the polymer induced the enlargement of dewetting-induced voids, consequently leading to stress reduction and crack propagation. The model established in this study provides a methodological reference for other highly filled polymeric materials.