Fatigue-Crack Propagation Behavior of Ductile/Brittle Laminated Composites

A study has been made of the fatigue-crack propagation properties of a series of laminated Nbreinforced Nb3Al intermetallic-matrix composites with varying microstructural scale but nominally identical reinforcement volume fraction (20 pct Nb). It was found that resistance to fatigue-crack growth improved with increasing metallic layer thickness (in the range 50 to 250 mm) both in the crack-divider and crack-arrester orientations. For a given layer thickness, however, the properties in the crack-arrester orientation were superior to the crack-divider orientation. Indeed, the fatigue resistance of the crack arrester laminates was better than the fatigue properties of unreinforced Nb3Al and pure Nb; both laminate orientations had significantly better fatigue properties than Nb-particulate reinforced Nb3Al composites. Such enhanced fatigue performance was found to result from extrinsic toughening in the form of bridging metal ligaments in the crack wake, which shielded the crack tip from the applied (far-field) driving force. Unlike particulate-reinforced composites, such bridging was quite resilient under cyclic loading conditions. The superior crack-growth resistance of the crackarrester laminates was found to result from additional intrinsic toughening, specifically involving trapping of the entire crack front by the Nb layer, which necessitated crack renucleation across the layer.