Effects of surface topography and specimen thickness on high-speed raindrop impact damage of CFRP laminates

Raindrop impact erosion has been observed since early days of aviation, and can be catastrophic for exposed materials during supersonic flight. A single impact waterjet apparatus is established for mimicking drop impacts at the velocities between 350 m/s and 620 m/s. Carbon Fiber Reinforced Polymer (CFRP) laminates with three different surface morphologies and specimen thicknesses are tested here. A central region with no visible damage has been noticed, surrounded by a “failure ring” with common damage patterns including resin removal, matrix cracking, fiber breakage and mass fiber loss. Asymmetric features are presented in the “failure ring” whose whole scope extends larger along the longitudinal direction than the transverse direction of the top layer. The mechanism of the resin removal is related to fiber–matrix debonding, and its onset and propagation can be facilitated by initial surface asperities with the shear action of the lateral jetting. In cases of multiple impact, good surface quality can slow down the evolution of resin removal and fiber exposure on the CFRP surface, reducing the erosion speed and delaying the occurrence of structural damages in the subsequent impacts. Rayleigh wave dominates the occurrence of matrix cracking on the CFRP surface, and subsequently, results in material loss and peeling of the top-layer because of lateral flow. With the increase of the specimen thickness, both the interlaminar and intralaminar failures decrease as the impact damage mechanism changes from plate bending stress to the reflection of stress waves.