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Reliability subtasks to better estimate inspection sensitivities and POD values for Nickel billet inspections. . During the quarter, work continued on four “strip coupons” cut from representative Ni alloy billets and designated • GFMA : From a high-noise site of a 10”-diameter IN718 GFM-forged billet. • VdieA : From a high-noise site of a 10”-diameter IN718 V-die-forged billet. • VdieB : From a low-noise site of the above billet. • WaspA : From a high-noise site of a 10”-diameter Waspaloy billet. Figure 1a shows the nominal geometries for each strip coupon (used for UT measurements in the axial and hoop directions) and the “baloney slices” that were later cut from the strip coupons (used for UT measurements in the radial direction). Ultrasonic data acquisition on all coupons has been completed, although analysis of the data is ongoing. Metallographic Microstructure A related goal of the subtask is to correlate the measured ultrasonic properties with the local billet microstructure. To this end, small metallographic coupons from selected billet sites have been polished and etched to reveal the grain structure, and grain size distributions have been measured from micrographs. During the Jan-Mar quarter, the majority of the research effort was spent completing the metallographic measurements and analyses. We now have deduced grain size information from 12 metallography coupons (4 sites x 3 views) for each of the four Ni-alloy strips. Labels and locations of the metallographic coupons in a typical strip are shown in Figure 1b. Two different methods, illustrated in Figure 2, were used to deduce average grain diameters from a given micrograph. The first, known as the Mean Chord Length (MCL) method is equivalent to: (1) drawing a straight line through the micrograph; (2) noting where the line crosses grain boundaries; (3) calculating the average length of the straight segments which lie within a single grain; and (4) multiplying this average chord length by 1.5 to obtain a grain diameter estimate. A computer program operating on a digital image of the grain boundaries is used to “draw” all possible lines in either the horizontal (X) or vertical (Y) direction. The conversion factor of 1.5, which translates from 2D images to 3D diameters, is believed to be appropriate for equiaxed grains of the kind seen in these billets. In an alternative approach, known as the “P(L) Method”, one determines the probability P that a line segment of length L, randomly placed on the micrograph, lies entirely within a single grain . Again a computer program is used to generate the line segments and to decide whether a given segment crosses a grain boundary. Again the method can be applied separately in the X and Y directions of Figure 2. The X or Y analysis of a micrograph yields a probability-versus-length curve, P(L). This curve is then fit to an exponential function P(L) = exp(-L/b), and 2b is used as an estimate of the mean grain diameter. Typical measured P(L) curves do not exactly follow an exponential function, leading to some uncertainty in the fitting parameter “b” and the associated grain diameter estimate. To document this uncertainty, we perform separate fits to different regions of the measured P(L) curve . This is illustrated in Figure 3, where fits to the “low L’’ and “high L” halves of the data are shown for one case. In practice we perform separate fits to 10 overlapping regions of the Quarterly Report – January 1, 2002 –March 31, 2002 print date/time: 6/6/2002 - 8:39 AM – Page 5
measured P(L) curve, and tabulate the minimum, maximum, and mean values of “b” thus obtained. We note that P(L) curves, sometimes referred to as two-point correlation functions, are of interest because they appear in a direct manner in certain models of ultrasonic attenuation and backscattered grain noise. In all of the cases examined, the grain cross-sections seen in the micrographs have roughly similar average dimensions in the X and Y directions, indicating an equiaxed grain structure. This is demonstrated in Figure 4 for the MCL method. The P(L) analysis similarly finds nearly equiaxed grain diameters. As shown in Figures 5 and 6, the two methods generally yield similar average grain diameters. When a given micrograph is analyzed, the MCL estimate usually falls in between the minimum and maximum estimates from the P(L) method. As shown in Figure 6, the average grain diameter estimate from the P(L) method tends to be about 5% below that of the MCL method. Since the grains appear to be equiaxed and the two analysis methods yield similar diameter estimates, we average over the two analysis directions [X and Y] and over the two methods [MCL and average P(L)] to arrive at an overall grain diameter estimate for each micrograph. One step in the analysis of each micrograph is the construction of a tracing showing only the true grain boundaries. When such tracings are made, so-called “twin boundaries” are intentionally excluded, since it is believed that they do not significantly contribute to attenuation and backscattered noise. Our goal is to correlate the grain diameter estimates with ultrasonic properties. Absolute errors in grain diameter estimates are believed to mainly arise from two factors: (1) the process of identifying “non-twin” grain boundaries is somewhat subjective; and (2) a given micrograph containing a few hundred grains represents only a very minute fraction of the specimen volume that is insonified in a given ultrasonic measurement. During the quarter, efforts were made to correlate the measured grain diameters at various billet sites with the ultrasonic attenuation values measured earlier. Figure 7 displays the attenuation values at 7.5 MHz for all measurement sites and inspection directions. Absolute attenuation values are believed to be accurate to within about +/-0.0015 N/cm or +/-0.03 dB/inch. Given this level of accuracy, one sees that at a given site in a given specimen, the ultrasonic attenuation is quite isotropic. For three of the specimens (strip coupons) the attenuation drops significantly as one approaches the OD from the billet center. For the fourth specimen (V-die-A), the attenuation increases as the OD is approached. The correlation between measured attenuation values and measured grain diameters is shown in Figure 8. Significant scatter is seen about the general trend. This is believed to be primarily due to the fact that the grain size determinations were made using only a very small fraction of the grains contained within the ensonified volume. Nonetheless, a general trend is seen of the same shape as that expected for equiaxed, untextured, pure Nickel grains. In the upcoming quarter, the ongoing analysis of all backscattered grain noise data will be completed, and figures similar to Figures 7-8 will be generated showing the dependence of noise FOM on position and its correlation with average grain diameter. Quarterly Report – January 1, 2002 –March 31, 2002 print date/time: 6/6/2002 - 8:39 AM – Page 6
Page 1 and 2: Engine Titanium Consortium Quarterl
Page 3 and 4: Project 1: Task 1.1: Subtask 1.1.1:
Page 5: findings for the first billet studi
Page 9 and 10: 1.2 1.0 0.8 V-DIE-A coupon "D" (axi
Page 11 and 12: Attenuation (dB/in) . Waspalloy Att
Page 13 and 14: Deliverables: Fundamental propertie
Page 15 and 16: Small Diameter Billet Assessment: A
Page 17 and 18: Under normal circumstances one woul
Page 19 and 20: Plans (April 1, 2002 - June 30, 200
Page 23 and 24: system will be at GE, and evaluatio
Page 25 and 26: fixed-focus transducers that were n
Page 27 and 28: Back-Wall Scan FBH scan 20% 57% 49%
Page 29 and 30: 63% Back-Wall Scan 74% FBH scan 58%
Page 31 and 32: Figure 6. Noise and target amplitud
Page 33 and 34: Original Date Revised Date Descript
Page 37 and 38: Objective/Approach Amendments: Obje
Page 39 and 40: (a) Transducer is: 15 MHz 0.5”-Di
Page 41 and 42: FOM (std. units) 0.20 0.18 0.16 0.1
Page 43 and 44: Peak> Range of Ave. Range of Peak G
Page 45 and 46: The average noise characteristics,
Page 47 and 48: Noise C-scans through the “Flat S
Page 49 and 50: 4.0” Convex Block; Flat Side Insp
Page 51 and 52: % FSH Noise Measurements from flat
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possible candidates. Development wo
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Forging Calibration Standard #1 1.7
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Surface Finish Studies The plans fo
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Figure 4. Beam diameter determinati
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size was adequate to support a 0.04
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phased array instruments. The desig
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Original Date Revised Date Descript
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Task 2: Task 2.1: Subtask 2.1.1: In
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Objective/Approach Amendments: Obje
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Figure 2. A preliminary design of t
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Publications and Presentations: Dat
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Survey of common practices (industr
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include air drying, oven drying, an
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aluminum oxide blasting is conceali
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Project 3: Task 3.1: Subtask 3.1.1:
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the methodology. These may be thoug
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model parts across 17 micrograph pl
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Z Y X Figure 4. Angle View of Void
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A report documenting the relative e
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naturally occurring flaws in forgin
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Milestones: Revised dates of TBD ar
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7a-7l 6 8 9 10 Key additional ingre
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incorporating the results of the pr
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