Figure 2a: Side view of a 0° quasi-static impact FOD site for a flat sample exhibiting no loss of material. 1 mm 1 mm Figure 2b: Side view of a 0° quasi-static impact FOD site for a flat sample exhibiting a large crack (eventual loss of material). Leading Edge (LE) Samples The results for strength of ballistically impacted LE specimens are presented in Figure 3 where fatigue limit stress is normalized with respect to the smooth bar fatigue strength at the appropriate stress ratio. For this material, the maximum stresses for smooth bars are σ=568 MPa for R=0.1 and σ=660 MPa for R=0.5 [3,5]. It should be noted that in the figure, AR refers to specimens that were tested in the as-received (as-impacted) condition and SR refers to specimens that were tested in the stress-relieved condition. Symbols with upward arrows indicate notch geometries where failure occurred away from the notch at the indicated stress. From the data, it can be seen that the SR samples tend to have higher fatigue strengths than the samples tested in the as-impacted condition. Also shown in the figure are points that utilize a fatigue notch factor, Kf, which was calculated from the measured dimensions for radius and notch depth [4]. It can be seen that the SR samples lie above the predictions of large notch fatigue analysis, and many of the AR samples also lie above those points. It can only be concluded that the small size of the notches being dealt with here are not as severe as geometrically similar notches, a finding supported by the observations of Haritos, et al. [8] in notch fatigue studies on the same material. They observed that the detrimental effect of a notch decreased as the root radius became smaller for the same value of Kt, and attributed that finding to a size effect as formulated by other notch fatigue theories (eg. [9]). 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0.5 0.6 All LE data 0.38 mm LE AR 0.38 mm LE SR 0.13 mm LE AR 0.13 mm LE SR Calculated 1/Kf Normalized fatigue limit stress Notch depth ratio, (d/a) Figure 3: Effect of the notch depth ratio on the normalized fatigue limit stress for ballistically impacted 0.38 mm leading edge radius samples.
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