SYMPOSIA PAPER Published: 01 January 1991

Fatigue Crack Growth in a Unidirectional SCS-6/Ti-15-3 Composite


An investigation was conducted to characterize and model the fatigue crack growth (FCG) behavior of a SCS-6/Ti-15-3 metal matrix composite. Part of the study was conducted using a fatigue loading stage mounted inside a scanning electron microscope (SEM). This unique facility allowed high magnification viewing of the composite fatigue processes and measurement of the near crack tip displacements. The unidirectional composite was tested in the [0]8 (that is, longitudinal) and [90]8 (that is, transverse) orientations. For comparison purposes, unreinforced matrix material produced by the identical process as the reinforced material was also tested.

The results of the study reveal that the fatigue crack growth behavior of the composite is a function of specimen geometry, fiber orientation, and the interaction of local stress fields with the highly anisotropic composite. In the case of [0]8 oriented single edge notch (SEN) specimens and [90]8 oriented compact tension (CT) specimens, the crack growth was normal to the loading direction. However, for the [0]8 CT specimens, the crack grew mostly parallel to the loading and the fiber direction.

The unusual fatigue behavior of the [0]8 CT specimens was attributed to the specimen geometry and the associated high tensile bending stresses perpendicular to the fiber direction. These stresses resulted in preferential cracking in the weak interface region perpendicular to the fiber direction. The interface region, and in particular the carbon coating surrounding the fiber, proved to be the composite's weakest link.

In the [0]8 SEN, the crack growth was confined to the matrix, leaving behind unbroken fibers that bridged the cracked surfaces. As the crack grew longer, more fibers bridged the crack resulting in a progressive decrease in the crack growth rates and eventual crack arrest. The actual near-crack-tip displacement measurements were used in a proposed formulation for a bridging-corrected effective crack driving force, ΔKeff. This parameter was able to account for most of the experienced bridging and correlated the [0]8 SEN fatigue crack growth data reasonably well.

Author Information

Kantzos, P
NASA Lewis Research Center, Cleveland, OH
Telesman, J
NASA Lewis Research Center, Cleveland, OH
Ghosn, L
Sverdrup Technology-NASA Lewis Research Center Group, Cleveland, OH
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Developed by Committee: D30
Pages: 711–731
DOI: 10.1520/STP17745S
ISBN-EB: 978-0-8031-5169-7
ISBN-13: 978-0-8031-1419-7