Fatigue crack growth in unidirectional and cross-ply SCS-6/Timetal(RTM)21S titanium matrix composite
Abstract
Fatigue crack growth in unidirectional and cross-ply SCS-6/Timetal$\sp\circler$21S titanium matrix composite was investigated. Fatigue crack growth tests were performed on (0) $\sb4,$ (90) $\sb4,$ and (0/90) $\sb{\rm s}$ center notch specimens. The (0) $\sb4$ and (0/90) $\sb{\rm s}$ fatigue crack growth rates decreased initially. Specimens removed prior to failure were polished to the first row of fibers and intact fibers in the wake of the matrix crack were observed. These bridging fibers reduced the stress intensity range that the matrix material was subjected to, thus reducing the crack growth rate. The crack growth rate eventually increased as fibers failed in the crack wake but the fatigue crack growth rate was still much slower than that of unreinforced Timetal$\sp\circler$21S. A model was developed to study the mechanics of a cracked unidirectional composite with any combination of intact and broken fibers in the wake of a matrix crack. The model was correlated to fatigue crack growth rate tests. The model was verified by comparing predicted displacements near the crack surface with Elber gage (1.5 mm gage length extensometer) measurements. The fatigue crack growth rate for the (90) $\sb4$ specimens was faster than that of unreinforced Timetal$\sp\circler$21S. Elber gage displacement measurements were in agreement with linear elastic fracture mechanics predictions, suggesting that linear elastic fracture mechanics may be applicable to transversely loaded titanium matrix composites.
Degree
Ph.D.
Advisors
Hillberry, Purdue University.
Subject Area
Mechanical engineering|Aerospace materials|Materials science
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