Introduction
The ingestion of foreign objects into aircraft jet engines can lead to severe structural damage of the fan and/or compressor airfoils. Recent reports have shown that foreign object damage (FOD) is a prime reason for maintenance and repair of military jet engines. Indeed, the damage induced by small hard objects of millimeter size in conjunction with the typical load spectra experienced by airfoils can lead to non-conservative life prediction and unexpected fatigue failures [1-5]. Typical loading spectra consist of low-cycle fatigue (LCF) cycling, due to start/flight/landing cycles, with superimposed high-cycle fatigue (HCF) cycles due to vibrations, resonant loading, etc. in the airfoils; the latter spectra typically involve large numbers (>109) of cycles at high frequencies (>1 kHz) and high load ratios (R ~ 0.8). The objective of the present investigation is to characterize the critical level of FOD by hard particles that can lead to crack extension and failure by HCF in Ti-6Al-4V. In this report, our initial studies in simulating hard particle FOD and characterizing these damage sites are described.
The specific microstructural condition of Ti-6Al-4V chosen for this
investigation is the solution-treated and over-aged (STOA) condition. Figure
1 shows the microstructure, which consists of primary ap
grains in a matrix of Widmannstätten a+b
colonies. The volume fraction of ap is
about 51 vol.-%, the size of the ap
grains approximately 15 mm. Furthermore there
is a slight elongation of the microstructure in the S-L plane in the L-direction.
Simulation of Foreign Object Damage
Foreign object damage by hard-particles mostly occurs during motion of the aircraft on the airfield, during takeoff and during landing (thrust is reversed). Typical objects ingested are stones and other debris, sizes in the millimeter regime, from the airfield. The worst case condition is experienced during takeoff, maximum engine thrust leads to maximum impact velocity. Typical impact velocities are in the regime of 100 – 350 m/s, depending on the specific engine, the impact location on the blade and other things. A further complication for simulation is the complex shape of modern fan/compressor blades (e.g. sharp leading and trailing edges, thickness varies over length).
In a very simplified approach the damage by hard-bodies is simulated by firing chrome-hardened steel spheres on a flat surface specimen (Fig. 2). The variables for the impact are the projectile’s size, shape and velocity and the impact-angle (Fig. 2b). The conditions chosen are, a 3.175 mm chrome-hardened steel sphere, impact-velocities ranging from 100 m/s to 300 m/s and an impact-angles of 30º and 90º (normal impact). The impact facility (Fig. 3) uses compressed nitrogen to accelerate the projectile. A laser/photodiode hookup combined with a 100 MHz universal counter is used to measure the velocity of the projectile just before leaving the gun barrel and impacting on the specimen surface.
Characterization of Simulated FOD-Sites
Figure 4 shows scanning electron microscope (SEM) pictures of simulated damage-sites for two impact velocities. Figure 5 and 6 shows optical microscope (OM) pictures of cross-sections of simulated damage sites at approximately the same velocities. The microstructural features observed are illustrated schematically in Fig. 7. The characterization of damage sites involves:
2. Local microstructural parameters: plastic zone size, local strain hardening, presence and appearance of cracks, appearance and dimensions of shear-bands,
3. Local residual stresses.
Characterization of the state of microstructural damage surrounding the FOD indentations is being performed using optical sectioning and will ultimately involve transmission electron microscopy. To date, no evidence of cracking has been detected near or at the damaged sites. Of additional interest is the presence of zones of local deformation in the form of shear bands (Fig. 5). Previous studies [6] have shown that such "macro bands" only form above a critical velocity (~214 m/s for a 3.2 mm steel ball in Ti-6Al-4V). Extrapolations of their fatigue stress/life data indicate that the occurrence of such "macro bands" may have a significant effect on high cycle fatigue life.
Additional characterization of the foreign object damage will involve measurement of the residual stress profiles in the vicinity of the indent. Since spatial resolutions of ±10 mm are required, conventional X-ray techniques are severely limited by spot size and photon flux. For this reason, synchronous X-ray techniques are being used with a micro-focus facility to achieve spot sizes of ~1mm with high photon fluxes.
Acknowledgements
This work is supported by the Air Force Office of Science and Research, Grant No. F49620-96-1-0478, under the Multidisciplinary University Research Initiative on "High Cycle Fatigue" to the University of California, Berkeley.
References