Epoxy resins are utilized in many industrial areas as matrix of composite materials and adhesives because of high strength and stiffness. Fatigue properties are important in terms of the long time reliability. To enhance the fatigue life of the epoxy adhesives, the fracture mechanism of the resin between rigid substrates under cyclic loading should be clarified. In this study, the fatigue crack propagation behavior of toughened epoxy adhesive was investigated and was compared with that of bulk resin. The fatigue crack propagation behaviors of adhesives are similar to those of the bulk resins. As the results of fractographic observation, the fracture surface for the adhesive layer are very flat and was different from that of bulk resin where the plastic deformation was popular. The deformation around the crack tip was restrained by the rigid metal substrates and the tri-axial stress was kept in spite of the rubber particle cavitation.
