Elastic shielding during fatigue-crack growth of titanium matrix composites

The role of elastic shielding in reducing the local stress intensity factor (SIF) range during fatigue crack growth (FCG) has been investigated using several single-ply composites with significantly different interfacial characteristics. The specimen geometry necessitated the fatigue crack to initially grow through a monolithic matrix region before impinging on a set of longitudinally oriented fibers. This facilitated the assessment of the crack shielding phenomenon from two regions: the region where the crack interacted with the first fiber, and at high stress levels when nonbridging conditions prevailed in the fibrous region. The extent of shielding was nearly identical in the two measurements for a given composite system. However, the shielding contribution was found to depend on the interface bond strength; the interface with the highest bond strength provided the largest degree of crack retardation in both cases. A preliminary assessment of this dependency has been provided. The implications of using the correct shielding factor on both fiber strength and life prediction are also discussed. Material: Ti-6Al-4V reinforced with SiC fibers.