subjected to FOD, with a portion of the specimens having undergone a post-FOD event stress relief annealing in order to examine the effect of residual stresses. EXPERIMENTAL PROCEDURES Axial fatigue specimens were machined from a single Ti-6Al-4V forged plate that had been heat-treated to the STOA condition [3]. Two basic specimen configurations were used for this effort: a diamond cross-section tension (henceforth known as the LE, i.e. leading edge) specimen and a simple rectangular (flat) cross-section sample. The LE specimen was designed such that the edges of the gage section are tapered to radii representative of the leading edge on a fan blade. To span a range of leading edge configurations, two edge radii were examined: 0.38 mm and 0.127 mm. Details of the specimen geometries can be found in Ref [3]. Stress calculations for all samples were based on the gross area, excluding the slight loss in area due to the various FOD sizes. The LE samples were ballistically impacted using a single-stage compressed gas gun. All of the samples were shot with a 1 mm glass sphere at a velocity of approximately 305 m/s. The glass sphere was chosen as being representative, in both size and properties, of sand or runway debris that is of concern in U.S. Air Force operational engines. It also produces damage which is geometrically similar to what is often observed in the field. Details of the ballistic impact procedures are likewise presented in Ref [3]. Quasi-static impact damage was performed on the flat samples. A hardened steel indentor with a 2 mm diameter round tip was used to induce damage to a specific depth on the thin side of the rectangular sample at a 0° angle [4]. To establish the fatigue strength, samples were fatigue tested using the step-loading procedures described by Maxwell and Nicholas [5]. Steps of 107 cycles were used in this investigation, while ∆σ was taken typically at 10 percent of the initial load block. This large increment (compared to 5 percent or less in other tests) was used to minimize the number of load blocks because of the wide scatter in fatigue strengths observed under FOD conditions. Testing was conducted at stress ratios (R) of 0.1 and 0.5 at a frequency of 350 Hz using an electro-dynamic, shaker-based test machine. All testing was performed under ambient laboratory air conditions. The choice of tension-tension testing was made for several reasons. While a real blade may be subjected to combinations of bending and tension, it was felt that understanding one of the basic properties, tension fatigue, was important in characterizing FOD. Second, baseline data are available for the material tested to compare smooth [6] and notch [7,8] fatigue behavior. Finally, test apparatus is available to conduct high frequency axial fatigue testing whereas fully instrumented bend tests are more difficult to perform at high frequencies. Prior to fatigue testing, the FOD sites of each LE sample were examined with a scanning electron microscope (SEM) to document the initial FOD defect. After test, the FOD site from which the fatigue cracking failure initiated, along with the fracture surface of each sample, were again examined to correlate the microstructural features with the ensuing fatigue strength. The flat samples were examined, prior to testing, under a standard optical microscope to determine the dimensions of the damage sites. Observations of whether there appeared to be material loss (chipping out) were noted. It should be noted here that during the testing process, the fracture faces tended to be crushed when the specimen failed. This action smeared the features on the fracture faces to the point that initiation sites were compromised.
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