Micromechanical Study on Freeze-Thaw Cycling Hygromechanical Properties of Continuous Fiber-Reinforced Thermoplastic Composite Prepreg Filaments for 3D Printing

Continuous fiber-reinforced thermoplastic composites are widely used in aviation, aerospace, and automotive applications due to the high specific strength, modulus, and design flexibility. However, their properties are susceptible to damage in extreme environments. This study investigates the microscopic property changes of 3D printing thermoplastic composites in service environments with freeze-thaw cycling. we have established the relationship between interfacial crack damage in composites and the mechanical properties of sub-mm-scale prepreg fibers through experiment and simulation. By analysis of grayscale of AFM modulus map, the interfacial crack (interfacial porosity) size evolution law at the fiber-resin matrix interface was investigated. At the micro-scale, the degree of damage at the interface is strongly correlated with the shear properties of the interface, which were derived from micro-debonding experiments. At the sub-mm-scale, the mechanical properties of flexural and axial compressive strengths are sensitive to the extent of interfacial crack damage, as numerical simulation results have also predicted. Conversely, axial tensile strength is minimally affected. Furthermore, it was found that the mechanical properties of the prepreg filaments decreased significantly after re-absorption of water after freeze-thaw cycling, especially the flexural and axial compression properties decreased by 73.2 % and 92.8 %, respectively. Therefore, this paper designed carbon fiber/polyamide-covered-polypropylene (CF/PA-PP) water-resistant prepreg filaments with core-shell structure to reduce water absorption. The core-shell CF/PA-PP filament's mechanics are comparable to those of the CF/PA filament, with strengths of 2010.2 MPa, 1081.2 MPa, and 427.0 MPa, and it has reduced water absorption from 6.8 % to 0.4 %, demonstrating its promising application in the field of 3D printing of water-resistant thermoplastic composites.