Fatigue-crack propagation behavior of ductile/brittle laminated composites

摘要

A study has been made of the fatigue-crack propagation properties of a series oflaminated Nbreinforced Nb3Al intermetallic-matrixcomposites with varying microstructural scale butnominally identical reinforcement volume fraction(20 pct Nb). It was found that resistance tofatigue-crack identical reinforcement volumefraction (20 pct Nb). It was found that resistanceto fatigue-crack growth improved with increasingmetallic layer thickness (in the range 50 to 250μm) both in the crack-divider and crack-arresterorientations. For a given layer thickness, however,the properties in the crack-arrester orientationwere superior to the crack-divider orientation.Indeed, the fatigue resistance of the crackarrester laminates was better than the fatigueproperties of unreinforced Nb3Al and pure Nb; bothlaminate orientations had significantly betterfatigue properties than Nb-particulate reinforcedNb3Al composites. Such enhanced fatigueperformance was found to result from extrinsictoughening in the form of bridging metal ligamentsin the crack wake, which shielded the crack tipfrom the applied (far-field) driving force. Unlikeparticulate-reinforced composites, such bridgingwas quite resilient under cyclic loadingconditions. The superior crack-growth resistanceof the crack-arrester laminates was found toresult from additional intrinsic toughening,specifically involving trapping of the entirecrack front by the Nb layer, which necessitatedcrack renucleation across the layer.