Effects of cyclic hygrothermal environments on interfacial behavior and properties of CF/EP composites

Composites are normally exposed to complex environments in service, such as high temperature and humidity. This work focuses on the moisture absorption and cyclic absorption-desorption behavior of carbon fiber reinforced epoxy resin composites under hygrothermal environments. The effects of hygrothermal aging on the matrix and interfacial properties are investigated through short beam shear, scanning electron microscope, dynamic thermomechanical analysis, and Fourier transform infrared spectroscopy tests. Experimental results indicate that the moisture absorption behavior of pure resin shows non-Fick diffusion swelling characteristics. But upon the introduction of carbon fiber, the moisture absorption behavior of the composites is transformed to conform with Fick diffusion. With the progression of hygrothermal treatment, both the interlaminar shear strength and the glass transition temperature of the composites decrease rapidly initially, and the rate of degradation decelerates after 14 days. The impairment to the properties caused by hygrothermal treatment can be classified into reversible damage and irreversible damage. Among these, the reversible damage stems from the swelling of the resin matrix as a result of the ingress of water molecules into the material interior, and the irreversible damage originates from the hydrolysis of the matrix and the formation of microcracks at the interface. After the dehumidification treatment of the samples that absorbed moisture for 56 days, the retention rate of the interlaminar shear strength recuperated from 77.40 % to 91.92 %, and the proportion of reversible damage is 63.98 %. After the cyclic moisture absorption and desorption treatment, the properties of the dry samples recover compared with the wet samples.