Life Prediction Methodologies for Aerospace Materials

Damage tolerance investigations of gamma-TiAl alloys have involved assessments of lamellar lath orientations of colonies and defects in large-scale production ingots. An investigation of elastic properties of colonies has continued with conjoint efforts in the experimental evaluation of test specimens containing large colonies and in finite element analysis of modeling of the contiguous colony locations and orientations within the test specimens. Experimental behavior of Nextel TM720/AS ceramic matrix composite was determined under tensile, creep, and thermal exposure conditions. Fracture and creep rupture behavior of MI SiC/SiC and high-cycle fatigue behavior of CVI Cf/SiC have been conducted. Analyses of CMC were conducted to predict strains at notch tips and heat transfer and stresses in a conceptual combustor design and to design rings that were tested on a combustor rig. Fretting fatigue investigations of Ti-64 have focused on the effects of coating fitting pads: the use of dissimilar materials for the fitting pads: the fatigue crack nucleation in fitting pads: the application of an ultrasonic shear wave technique to detect crack initiation: and the effects of specimen thickness and ratios of nominal normal stress to shear stress. High-cycle fatigue tests have continued to evaluate the influence of foreign object damage, mean stress, and previous loading history. A fractographic assessment of load sequences has revealed some surprising results for fatigue crack growth and load interaction in two aluminum alloys. For the engine rotor life extension program. A broad study of the behavior of residual stresses in nickel-base superalloys has begun. These efforts currently involve the determination of the compressive behavior of materials of interest, the correlation between predicted and measured inelastic strains in bend specimens. and the relaxation of residual stresses due to thermal exposure.