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

Titanium matrix composites (TMC) and their behavior under mechanical fatigu e loads was the subject of this research. The primary objective was to expl ain fatigue damage modes in center-notched TMC specimens. Two modes of dama ge have been observed in continuously reinforced, zero-degree unidirectiona l, SCS-6/Ti-15V-3Cr-3Al-3Sn (SCS-6/Ti-15-3) laminates. The fatigue specimen s were destructively analyzed using optical microscopy to determine where c racks originated and how they grew throughout the specimen. A micromechanic al model was developed to explain the fatigue crack patterns observed in th e interface region surrounding the fibers of the woven and acrylic-binder T MC 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 condi tions for an element near the notch, yet within the net section where the s piral crack patterns were observed. These boundary conditions were then use d on a three-dimensional (3-D) unit cell model of the fiber, matrix, and in terface.