Measurement of the cyclic bridging law in a titanium matrix composite and its application to simulating crack growth

The cyclic bridging traction law for a fiber reinforced titanium matrix composite (TMC) has been measured using a multiple fiber pullout specimen geometry. The test specimens were cut from fatigued specimens containing a single fully bridged matrix crack. The two key parameters obtained from such measurements are the interface sliding stress and the fiber bundle strength. The sliding stress was found to be considerably lower than the pristine values reported for other TMCs (by a factor of similar to 5), a result of wear of the fiber coatings. The fiber strength is also reduced following fatigue, by similar to 25%. The strength reduction is associated with the formation of new surface flaws, also due to cyclic sliding. Additionally, simulations of Fatigue crack growth have been performed using the measured sliding stress and compared with experimental measurements over a range of applied stresses. Though excellent correlations are obtained between the experiments and the predictions, it is demonstrated that similar correlations can be obtained using sliding stress values that differ from the measured one by a factor of similar to 2 and by suitable adjustment of the Paris law parameter that characterizes the intrinsic fatigue resistance of the composite. (C) 1997 Acta Metallurgica Inc.