Interphase engineering in carbon fiber/epoxy composites: Rate sensitivity of interfacial shear strength and interfacial fracture toughness

Designing the interface in polymer composites is fundamentally a challenging task. Here, we demonstrate a strategy to engineer the interphase microstructure in carbon fiber/epoxy composites (CFRPs) using carbon nanotubes (CNTs). The incorporation of CNT modifies the interfacial mechanics and interfacial chemistry in conventional CFRPs by creating concentrated, dispersed and mixed type interphase. Therefore, a detailed study is warranted to establish the interfacial microstructure-property relationship in CNT modified CFRPs. Experimental results show that the relative improvement in interfacial shear strength (IFSS) and interfacial fracture toughness (G(ic)) depends on the microstructure of interphase. It is shown that simultaneous improvement in IFSS and G(ic) is possible with certain types of microstructural designs. Moreover, it is observed that IFSS and G(ic) are not constant material parameters but both of them show a power-law type dependence on the applied loading rate. The range of rate sensitivity parameters as a function of interphase type suggests that while concentrated and mixed interphase is more suited to maintain the interfacial integrity, dispersed interphase is beneficial for energy dissipating applications of CFRPs. In addition, IFSS and G(ic) exhibit negative rate sensitivity for certain cases. Finally, it is shown that interphase designing using CNT is an excellent tool to accurately tailor the average interfacial properties of CFRP in a broad range of 16-79 MPa and 100-453 J m(-2) for IFSS and G(ic), respectively.