SEDL / STP / STP1184-EB / STP18120S

Fatigue Crack Growth and Crack Bridging in SCS-6/Ti-24-11

Ghosn, LJ
Research engineer, Case Western Reserve University, NASA Lewis Resident Research Associate, Cleveland, OH

Kantzos, P
Research engineer, Ohio Aerospace Institute, Cleveland, OH

Telesman, J
Material engineer, NASA Lewis Research Center, Cleveland, OH

Pages: 23    Published: Jan 1994

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Abstract

Interfacial damage induced by relative fiber/matrix sliding was found to occur in the bridged zone of unidirectional SCS-6/Ti-24Al-11Nb intermetallic matrix composite specimens subjected to fatigue crack growth conditions. The degree of interfacial damage was not uniform along the bridged crack wake. Higher damage zones were observed near the machined notch in comparison to the crack tip. The interfacial friction shear strength, τ^f, measured in the crack wake using pushout testing revealed lower values than the as-received interface. Interfacial wear also reduced the strength of the bridging fibers. The reduction in fiber strength is thought to be a function of the magnitude of relative fiber/matrix displacements and the degree of interfacial damage. Furthermore, two different fiber bridging models were used to predict the influence of bridging on the fatigue crack driving force. The shear lag model required a variable τ^f in the crack wake (reflecting the degradation of the interface) before its predictions agreed with trends exhibited by the experimental data. The fiber pressure model did an excellent job in predicting both the FCG data and the ΔCOD in the bridged zone even though it does not require a knowledge of τ^f.