Effects of plastic constraint on the cyclic and static fatigue behavior of metal/ceramic layered structures

The role of metal layer thickness and resultant plastic constraint in the metal layer during the failure of metal/ceramic layered structures is examined under cyclic and static loading conditions. Crack-growth experiments were conducted on sandwich specimens consisting of 99.999% pure aluminum layers bonded between 99.5% pure polycrystalline alumina with the metal layer thickness varying from 5 to 100 μm. Under cyclic loading, crack growth occurred primarily at the interface separating the two materials; additionally, stable fatigue cracks deviated into the alumina for thin-layered samples at high driving forces. Under monotonically increasing loads, the fracture toughness increased with Al layer thickness, whereas under cyclic loads the threshold driving force for crack growth conversely decreased with increasing layer thickness. Under static loading in a moist environment, interfacial crack growth was never observed at measurable rates (⩾10−9 m/s) for driving forces up to 200 J/m2; however, for thin-layered samples, subcritical cracks did deviate off the interface and grow, sometimes stably, into the alumina. Trends in crack-growth rates and crack trajectories are examined in terms of the level of constraint, loading conditions and environmental influences.