Yan Li

Few existing studies focus on damping effects on second harmonic generation in nonlinear phononic crystals, especially lacking in exploring the potential of damping for nonlinear wave control. To fill the gap, we first theoretically study damping effects on second harmonic generation in nonlinear diatomic chains for both phase-matching (PM) and non-PM cases, which are validated by full-scale numerical simulations. We then extend the investigation to damped nonlinear locally resonant chains. Interestingly, we identify a special PM case in which the fundamental and second harmonics can be tuned selectively by changing the location of damping in the chain. Moreover, an unconventional phenomenon is observed that the amplitude of the damped second harmonics can be even larger than that of the damped fundamental harmonic in certain parts of the chain. This study offers new insights for wave control by exploiting damping in nonlinear phononic crystals.

Compared with traditional continuous plant fiber-reinforced thermoplastic composites, their 3D-printed counterparts offer distinct advantages in the rapid fabrication of complex geometries with integrated forming capabilities. However, the impregnation process of continuous plant fiber yarn with thermoplastic resin presents greater technical challenges compared to conventional synthetic fibers (e.g., carbon or glass fibers) typically employed in continuous fiber composites, owing to the yarn’s unique twisted structure. In addition, low molding pressure during 3D printing makes resin impregnation more difficult. To address the impregnation difficulty within plant fiber yarn during 3D printing, we employed two low-viscosity resins, liquid thermoplastic resin (specifically, reactive methyl methacrylate) and thermosetting epoxy resin, to pre-impregnate flax yarns, respectively. A dual-resin prepreg filament is developed for 3D printing of flax fiber-reinforced composites, involving re-coating pre-impregnated flax yarns with polylactic acid. The experimental results indicate that liquid thermoplastic resin-impregnated composites exhibit enhanced mechanical properties, surpassing the epoxy system by 39% in tensile strength and 29% in modulus, attributed to improved impregnation and better interfacial compatibility. This preparation method demonstrates the feasibility of utilizing liquid thermoplastic resin in 3D-printed continuous plant fiber composites, offering a novel approach for producing highly impregnated continuous fiber filaments.

3D‐printed polymer‐based composites are promising for various engineering applications due to high strength‐to‐weight ratios and design flexibility. However, conventional matrix materials, such as polylactic acid and epoxy resin, often exhibit brittleness and limited impact resistance (< 10 kJ m ^{− 2} ). Herein, a universal strategy is reported for enhancing the ductility and impact energy absorption of 3D‐printed composites by leveraging the dynamic crosslinking of B─O dative bonds. To validate its effectiveness, a smart composite (PLA/SSG) comprising shear‐stiffening gel fillers embedded in a polylactic acid matrix is designed and its rate‐dependent mechanical adjustability along with 3D printability is evaluated. The resulting composite shows significant improvements in impact resistance, ductility, and strength‐ductility balance. Specifically, the multiple crack and localized plastic yielding of polylactic acid matrix induced by shear‐stiffening gel fillers enables PLA/SSG with a 40‐times increase in ductility; the “soft‐hard” phase transition of shear‐stiffening gel induced by B─O bonds endows PLA/SSG with a 330% improvement in impact energy absorption. This B─O bonds‐inspired strategy provides a universal approach for printing smart impact‐resistant composites and structures.