FRACTURE AND FATIGUE-CRACK GROWTH ALONG ALUMINUM-ALUMINA INTERFACES

The mechanical failure of metal/ceramic joints subjected to monotonic and principally cyclic loading under nominal model I (Far-field) condi tions was investigated for a model Al/Al2O3 bimaterial system using a ceramic/metal/ceramic sandwich geometry in four-point bending. Crack g rowth was seen to follow a path along the interface, except at very hi gh applied driving Forces (defined in terms of the range of stress int ensity Delta K or the elastic strain energy release rate Delta G), whe re a transition to growth in the metal layer took place, often involvi ng a change in fracture mode to microvoid coalescence. The growth of f atigue cracks proceeded over a wide range of applied Delta G levels, e xtending from values well below to values well above those required to cause fracture in the adjoining ceramic. Interfacial crack-advance me chanisms under cyclic loading were found to be similar to that in duct ile metals, as evidenced by the presence of fatigue striations on the metal fracture surface. Rapid (final) failure, conversely, involved du ctile fracture in the metal or activation of defects in the ceramic su bstrate; both scenarios occurred at similar G(c) (or K-c) fracture tou ghness values. Quantification of the results focused attention on the extensive crack-tip blunting that occurs at high driving forces; this requires significant corrections to the usual small-scale yielding (SS Y) assessments of the driving force and yields fracture energies that are orders of magnitude above those reported for other metal/oxide sys tems.