Influence of pre-bond moisture in the adherents on the fracture toughness of bonded joints for composite repairs

The mechanical performance of composite components repaired with bonded patches depends on the moisture content of the adherents. There is a need to define drying procedures which guarantee the quality of the bonded joint. An experimental investigation has been carried out on the effect of pre-bond moisture on the fracture toughness (GIC) and failure behavior of repair joints bonded with adhesive films. Substrates were conditioned by immersion in distilled water at 70 °C for 336 h and subsequently dried for 1 h or 24 h to achieve different moisture contents before bonding. Specimens of bonded joints were tested under mode I loading at room temperature. The fracture toughness (GIC) decreased with increasing pre-bond moisture level in the substrates. Fractographic inspection revealed an enhancement of the cohesive mode of failure as the pre-bond moisture decreased. Extending the duration of the drying operation of the substrate with pre-bond moisture, caused an improvement in the fracture toughness of the joint, although not a full recovery of the reference values obtained in absence of pre-bond moisture.