Femtosecond laser fabrication of cross-scale pre-microcracks in PBX: Toward accurate fracture toughness determination

The reliable measurement of fracture toughness ( K IC ) in polymer-bonded explosives (PBX) is critical for advancing material science, but has long been hindered by the difficulty in creating pre-cracks with precise depth and sharpness. Traditional approaches—blunt notches that overestimate K IC or irreproducible manual methods—undermine accuracy. This study introduces a femtosecond laser-based "cold processing" technique to overcome these limitations, enabling ultra-sharp pre-cracks (sub-μm tips) with minimal thermal impact. To overcome this long-standing barrier, we introduce an innovative femtosecond laser “cold processing” technique. This method enables the precise fabrication of ultra-sharp pre-cracks (micrometer-scale tips) in TATB-based PBX while preserving material integrity (Raman-verified). Crucially, addressing the depth constraint inherent in standard SEVNB methods, we developed the Single Edge Multiple V-Notched Beam (SEMVNB) approach. SEMVNB eliminates the need for error-inducing U-notches through direct multi-stage laser processing, significantly extending controllable crack depths beyond previous limits. The critical relative crack depth for stable K IC in PBX ( a / W ≥ 0.17) is substantially lower than the ASTM-specified range for metals/ceramics, revealing distinct fracture characteristics. Validation on MJ-03 substitute confirms the universal applicability of this methodology to brittle composites. This work provides a robust and reliable solution for fracture resistance assessment in PBX, addressing a safety-critical challenge in material engineering by enabling accurate prediction of crack propagation risks. Accurate measurement of the fracture toughness of polymer-bonded explosives (PBX) has long been hindered by the difficulty in fabricating pre-cracks that combine controlled depth with ultra-sharp tips. This study employs a femtosecond laser-based "cold processing" technique to successfully achieve the controllable preparation of cross-scale, ultra-sharp pre-cracks with micrometer-scale crack tip radii. The stable measurement interval for the fracture toughness of PBX was determined, and the universality of the method was validated using a surrogate material. This work provides a novel solution for the reliable performance assessment of brittle composites. 1. A femtosecond laser-based “cold processing” technique enables precise fabrication of ultra-sharp pre-cracks (tip radius ∼2.5-5 μm) in PBX with controlled depths ranging from micrometers to millimeters. 2. The developed method achieves exceptional crack-depth control and sharpness, providing highly reliable fracture toughness ( K IC ) measurements that show excellent agreement with the standard SEPB method. 3. A material-dependent critical relative crack depth ( a/W ≥ 0.17) is identified for valid KIC measurement in PBX. This threshold, significantly lower than standards for metals/ceramics, ensures the accurate determination of intrinsic fracture toughness.