Effect of in situ material properties on fatigue damage modes in titanium matrix composites

Titanium matrix composites (TMC) and their behavior under mechanical fatigue loads was the subject of this research. The primary objective was to explain fatigue damage modes in center-notched TMC specimens. Two modes of damage have been observed in continuously reinforced, zero-degree unidirectional, SCS-6/Ti-15V-3Cr-3Al-3Sn (SCS-6/Ti-15-3) laminates. The fatigue specimens were destructively analyzed using optical microscopy to determine where cracks originated and how they grew throughout the specimen. A micromechanical model was developed to explain the fatigue crack patterns observed in the interface region surrounding the fibers of the woven and acrylic-binder TMC material systems. A two-dimensional (2-D) model of a longitudinal lamina with a center hole was used to obtain a set of displacement boundary conditions for an element near the notch, yet within the net section where the spiral crack patterns were observed. These boundary conditions were then used on a three-dimensional (3-D) unit cell model of the fiber, matrix, and interface. This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee.