Because of the expense of conducting ballistic tests, only a limited number of tests were carried out. In many cases, duplicate tests were not used. Because of this, as well as the scatter involved in this type of testing, only general trends are expected from the results. A portion of the impacted samples, both from the ballistically impacted LE samples, and the quasi-statically impacted flat samples were stress relief annealed before fatigue testing. This process was performed to allow a comparison of fatigue limit stress of specimens impacted under identical conditions with and without residual stresses. DISCUSSION OF RESULTS Damage Characterization The photos in Figures 1a and 1b show examples of ballistic impact damage on the LE samples. Although the conditions were the same for the ballistic impacting for these samples, the results were greatly different. Some of the impact sites exhibited a simple “dented” appearance with little or no loss of material evident (Fig. 1a), whereas other impact sites exhibited more damage with a much larger loss of material (Fig. 1b). The photos in Figures 2a and 2b show flat sample indents where there was no evidence of material loss (Fig. 2a) and where material loss occurred (Fig. 2b). The arrow in Figure 2b indicates an area where a crack occurred during the indenting process. A piece of material eventually chipped out during the handling of the sample. This behavior was typical in the samples with the large damage depths. LE samples were rotated in the SEM to provide the appropriate angle to measure the maximum depth of damage. These measurements (depth and width of impact notch) were used later to compute approximate values of the stress concentration factor, Kt, as well as a notch depth ratio, d/a. While the flat samples had a radius of the indent that was close to that of the quasi-static indentor, the LE specimens had notch root radii which had a larger amount of scatter. The scatter in these dimensions can be attributed to the scatter in the exact location of the impact on the thin LE samples and very small size of the projectile causing the impact. Figure 1a: Head-on view of a 30° ballistic impact FOD site for a 0.38 mm leading edge radius sample exhibiting little or no loss of material. 200 µm 200 µm Figure 1b: Head-on view of a 30° ballistic impact FOD site for a 0.38 mm leading edge radius sample exhibiting a larger loss of material.
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