Loading rate effect of the interfacial tensile failure behavior in carbon fiber–epoxy composites toughened with ZnO nanowires

This paper attempts to study the loading rate effect of interface tensile failure behavior of carbon fiber-epoxy composites toughened with ZnO nanowires (ZnO NWs). ZnO NWs were grown onto woven carbon fiber fabrics through a two-step hydrothermal method, the growth time was carefully controlled to vary the nanowire length. Subsequently, a series of quasi-static and dynamic transverse fiber bundle (TFB) tensile tests were carried out under different loading velocities, ranging from 1 × 10⁻⁶ m/s on the universal testing machine to 9 m/s on the electromagnetic Hopkinson bar. A 2D representative volume element (RVE) model has been established to identify the load transfer and failure behavior of the ZnO NW modified interface. The results show that under quasi-static loading, the maximum TFB tensile strength improvement owing to ZnO NWs was about 36.9% and 16.5% when compared with the pristine fibers and the fibers with surface sizing. However, due to the loading rate enhancement of the interface bonding strength, the corresponding values were decreased to 13.9% and 5.0% under dynamic loading. Besides, nanowires with medium length are recommended, because too long nanowires shall reduce the matrix impregnation space between fibers, and the microcracks from the fiber/matrix interface can easily deflect into the matrix along the nanowires. This side-effect may neutralize the enhancement mechanism of ZnO NWs when their length increases to a certain extent.