Fatigue Life and Short Crack Behavior in Ti-6Al-4V Alloy; Interactions of Foreign Object Damage, Stress, and Temperature
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TRODUCTION
DURING the last decade, the use of titanium alloys in the aeronautical industry has been increased constantly, enforced by the necessity for weight reductions.[1–4] The Ti-6Al-4V alloy, due to its unique properties such as high strength and toughness and excellent corrosion resistance combined with weldability and light weight, has been known as the best alternative to substitute the heavy nickel-base superalloys in the construction of the compressor disk and blades of gas turbines of advanced jet engines.[5] Hence, the special applications of Ti-6Al-4V alloy, which are not only at ambient temperature but also at elevated temperatures, require more and more studies on the high-temperature fatigue behavior of this alloy. However, there have been a limited number of investigations on the fatigue behavior of this alloy at elevated temperatures.[6–9] It has been shown that fatigue strength at elevated temperatures is significantly less than that at ambient temperature.[9] High-cycle fatigue (HCF) has been known as one of the prime causes of turbine-engine failures in aircrafts.[10] It can result in unpredictable failures due to the propagation of fatigue cracks in blade, where the cracks initiate from small defects often associated with microstructural damage caused by fretting or foreign object impacts.[11] The FOD from bird strikes or hard body impacts such as stones can BEHZAD MAJIDI, Research Assistant, is with the Department of Materials and Mining Engineering, Amirkabir University of Technology, Tehran, Iran 15875-4413. Contact e-mail: behzadmaterial@ yahoo.com Manuscript submitted June 10, 2007. Article published onlined February 9, 2008 772—VOLUME 39A, APRIL 2008
cause immediate fan blade failures or damage from notches depending on the impact velocity.[12–14] It is believed that the residual stresses introduced from FOD cause the effective stress ratio or mean stress in the vicinity of FOD to be different from that applied to the structure.[12] These modified stress states provide the explanation for reduction in the fatigue strength of the material with FOD. Some authors[15,16] studied the residual stresses and geometric stress concentration due to FOD. Applying laboratory and synchrotron X-ray diffraction to evaluate residual stresses and comparing the obtained results with those predicted using numerical models, Duo´ et al.[16] attempted to provide more knowledge about the effects of foreign object impacts. Investigations on the role of FOD-induced microcracks on the earliest stages of FOD-induced fatigue failures in Ti-6Al-4V alloys[17] have shown that the resistance to HCF is significantly reduced due to earlier crack initiation in the damaged materials. Ritchie et al.[18] investigated the HCF and short cracks behavior in Ti6Al-4V alloy and showed that ‘‘worst-case’’ thresholds may be used as a practical lower bound to describe the onset of naturally initiated short crack growth and initiation and growth of cracks emanating from sites of simulated FOD. Oakley and Nowell[19] introduced an elastic short
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