Analysis of hygrothermal effects on mixed mode I/II interlaminar fracture toughness of carbon composites joints

Adhesive joints exhibit several advantages over conventional joints based on mechanical fasteners such as more uniform stress distribution, enhanced fatigue performance, stiffer connection, lower weight, smooth surface countor. However, the influence of environmental effects related to temperature and moisture absorption on the mechanical performance of these types of composite joints are not yet fully understood. This work investigates the hygrothermal effects on fracture toughness of composite carbon/epoxy joints under Mixed Modes I/II loading in different mode ratios (35%, 50% and 75%). Joints were produced using co-curing (CC), co-bonding (CB) and secondary bonding (SB) technologies. The specimens were submitted to an elevated temperature wet condition (ETW) at a high moisture content (90% R.H.) and high temperature (80 °C) and compared with results obtained at a room temperature ambient (RTA). The SB samples under ETW condition exhibited higher GII/GT values when compared to CC and CB specimens tested under the same aging condition. Using the scanning electronic microscopy (SEM) technique, it was possible associate the delamination process behavior of aging specimens with the fracture toughness values. The wet and hot environment affect both adhesive and adherent what results in significant changes in the failure aspects during the delamination induced failure process.