Environmental effects on the Mode I fracture and fatigue of bonded composites

Bonded composite materials are being specified for an increasing number of aircraft structures and also for repairs to damaged metallic components. Thus, ensuring the long-term operation of new airframes and extending the lives of today’s aging aircraft depends, in part, on the durability of the adhesive bond line. The study described in this report used fracture mechanics concepts to assess the environmental durability of two bonded composite systems. Total strain energy release rates were determined using closed-form solutions and finite element analyses. Results showed that exposure to hot/dry or hot/wet conditions could cause significant reductions in fracture toughness and fatigue threshold. The fracture path depended upon the orientation of plies at the bond line interface. In both systems examined, threshold strain energy release rates were low compared to fracture toughness and crack growth rate sensitivities were extremely high. These results illustrate the importance of considering environmental effects in bonded composite design.