From microdefects to macro-fracture: A unified strength-toughness model for PBX via femtosecond laser-engineered multiscale cracks

Polymer bonded explosives (PBX) suffer from complex defect-mediated fracture due to inherent microstructural heterogeneities, where existing models for homogeneous materials prove inadequate. To resolve the critical gap in cross-scale fracture prediction, we develop a unified framework integrating microscopic defect effects with macroscopic failure criteria. The proposed model introduces a microstructure-correlated inherent equivalent defect parameter (a*) that mathematically reconciles strength- and toughness-governed fracture through defect-depth coupling. Using femtosecond laser-engineered cracks spanning three orders of magnitude (1 μm-3.5 mm), we validate the model's predictive accuracy via systematic three-point bending tests. The derived parameter a* demonstrates correlation with TATB crystal size characteristics, while the developed methodology enables fracture toughness (K_IC) determination using solely defect-free strength measurements and shallow-crack response data, significantly reducing testing complexity compared to conventional multi-specimen approaches. This work provides both a practical tool for PBX quality assessment and fundamental insights into defect-mediated fracture mechanisms in particulate composites.