Life Prediction Methodologies for Aerospace Materials

Superalloys IN-100, Ren 88DT, Waspaloy, and titanium alloys were investigated. For life-limiting responses in superalloys, highly localized stress-deformation behaviors associated with both typical and unusual defects were developed. Characterizing the relaxation of residual stresses in a component due to temperature, exposure time, and mechanical loading is essential to development of a physics-based life prediction model. Current life management concepts based on damage tolerance have not incorporated the effects of surface and bulk residual stresses to retard crack initiation and crack growth. Finite element models (FEMs) of the feature specimens have been developed and results analyzed. Characterization of fretting fatigue samples revealed that fretting wear and multiaxial fatigue occur simultaneously and that multiple mechanisms contribute to the onset and propagation of cracking. No significant improvement was observed for a soft coating system. Only shear wave ultrasonics was capable of producing a signal from which a crack could be identified. A crystal-plasticity model was developed for gamma-titanium aluminide and data from tensile tests of specimens were useful for determining slip strengths and colony and grain boundary influences on material yielding. The results of elevated temperature tensile and creep tests show that off-axis tensile and creep strengths of an oxide/oxide 45 Nextel 720/AS were significantly lower than the strengths in the reinforcing fiber direction.