The increasing incidence of military aircraft engine failures that can be traced to high-cycle fatigue (HCF) has prompted a reassessment of the design methodologies for HCF-critical components, such as turbine blades and disks. Because of the high-frequency vibratory loading involved, damagetolerant design methodologies based on a threshold for no crack growth offer a preferred approach. As impact damage from ingested debris is a prime source of HCF-related failures, the current study is focused on the role of such foreign-object damage (FOD) in influencing fatigue crack-growth thresholds and early crack growth of both large and small cracks in a fan blade alloy, Ti-6Al-4V. FOD, which was simulated by the high-velocity (200 to 300 m/s) impact of steel spheres on a flat surface, was found to reduce markedly the fatigue strength, primarily due to earlier crack initiation. This is discussed in terms of four salient factors: (1) the stress concentration associated with the FOD indentation, (2) the presence of small microcracks in the damaged zone, (3) the localized presence of tensile residual hoop stresses at the base and rim of the indent sites, and (4) microstructural damage from FOD-induced plastic deformation. It was found that no crack growth occurred from FOD impact sites in this alloy at ⌬K values below ϳ2.9 MPaΊm, i.e., over 50 pct higher than the "closure-free", worst-case threshold value of ⌬K TH ϭ 1.9 MPaΊm, defined for large cracks in bimodal Ti-6Al-4V alloys at the highest possible load ratio. It is, therefore, concluded that such worst-case, large fatigue crack thresholds can, thus, be used as a practical lower-bound to FOD-initiated cracking in this alloy.
Gas turbines for aeroengines are generally manufactured by attaching individual blades into slots in the disk. However, an alternative design consists in producing bladed disks, where disk and blades are a single piece. Previous studies have shown that linear friction welding is a technology which fits well for this application. On the other hand, bladed disks can experience in service damages and therefore affordable repair technology must be available. The present paper presents the microstructural and mechanical evaluation of double and triple welds produced to simulate replacements of damaged blades. The results show that, even though the successive welds induce a small strength decrease, both tensile and fatigue failures always initiated far away of the weld zone.
Alloy Udimet 720LI was designed for high temperature application of aircraft-engine disks up to about 730°C. Operating at such high temperatures for long times raises the question of thermodynamical stability of the alloy (bulk stability) and surface integrity. Subject of the present paper was to investigate the influence of long time exposure up to 1000 hours at high temperatures up to 845°C in air on microstructure and phase stability as well as on surface integrity. It could be shown by appropriate microscopy (LM, SEM, TEM) that coarsening of the y'-phase occurred with increasing time of exposure at temperatures of 760°C and 845°C. Additional formation of topological closed packed (TCP) phases (0) could be observed. It was found that this lack of stability exhibited an impact on mechanical properties such as tensile strength, creep resistance, and low cycle fatigue strength, especially when a dwell time was superimposed. Furthermore, the reaction of the alloy in the near-surface area of the specimen exposed to air at high temperatures with superimposed stresses was investigated by light microscopy (LM) and SEM. A quantitative analysis by EDX investigation in a SEM of the thus formed surface oxide layer and the subsurface area with a depletion of alloying elements was performed.
Superalloys 2oooEdited byT
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