Effects of voids on shear properties and failure mode of carbon fiber/epoxy resin composites

Voids have significant influence on the shear properties of carbon fiber/epoxy resin composites. In this paper, carbon fiber/epoxy resin composite laminates with varies porosity were produced by hygroscopic saturation and stepping down the autoclave pressures. Short beam shear (SBS) tests were performed to establish the influence curve of different porosity on SBS strength. The evolution of shear damage induced by voids and the degradation mechanism of SBS strength were both studied by ultrasonic imaging and metallographic observation. The result shows that when the porosity is less than 1.0%, the SBS strength retention rate is about 88.4%-90.8%; When the porosity increases to 1.0%-1.5%, the SBS strength retention rate is about 74.9%-80.6%; When the porosity increases to 1.5%-2.0%, the SBS strength retention rate is about 66.3%-71.9%; When the porosity increases to 2.0%-3.0%, the SBS strength decreases sharply, and the SBS strength retention rate drops below 50%. The SBS shear failure mode is very sensitive to voids. Shear failure mainly occurs close to void and the surrounding stress concentration. The higher the porosity, the more obvious the promotion effect of voids on crack initiation and propagation, yielding the higher crack density, the earlier crack occurrence time, and the faster propagation speed.