CompositesWorld

JUN 2017

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NEWS 29 CompositesWorld.com BBA vs. VA Testing companies like Lockheed already use," Assaker adds, "making the allowables process and associated testing more efficient." Cost vs. time Assaker's Fig. 3 is a rebuttal to the quantitative comparison in Rousseau's CAMX 2015 presentation which tried to compare BBA to VA in more concrete terms. Assaker concedes it is based on a number of arguable assumptions — e.g., dollar values can easily vary because different organizations do stress analysis differently. Assaker contends that Rousseau's conclusions are premised on at least one incorrect assumption. "His cost conclusions are true if you assume that the VA test coupons are built in the FEA code by hand, from scratch, every time," says Assaker. "But this is exactly what our solution was developed to streamline." To give a quick background, Assaker is talking about the Digimat software, which he says comprises FEA technology, composites expertise and failure models. Rousseau says he assumed the FE model would be built once per test configuration (compression, tensile, CAI, etc.), "but each time the model is run, you have to change the MAT and PSHELL cards for material properties, lay-up and/or environmental condition." Assaker responds, "We don't build the model once per test configuration for each material because it is already done; we built each test configuration's model into the software. ese are stan- dardized tests. By definition, they cannot change. Modifying the MAT and PSHELL cards is also automated." He explains the issue is not complexity — the coupons are very simple, with elemen- tary loading conditions — "but you have to do a lot of them and there is complexity in the damage analysis underneath." Assaker says e-Xstream worked with allowables engineers to program the mechanics into the software so that the user simply enters the unique parameters per case. Despite all the talk thus far, Assaker contends that the impetus for development of his company's virtual testing capability was not cost, but time. "Engineers came to us frustrated at the long time periods required for testing before they could begin the design process," he explains. "Our objective was to allow an engineer to quickly compare material 1, 2 and 3, for example. We wanted to shorten coupon testing to six months or less." But Rousseau responds, "In my experience, no one waits on testing to begin the design process. I can readily estimate properties required to begin design. Since all of the other design development tasks take longer than coupon testing, I can always have the final allowables done prior to drawing release." Assaker counters that he is suggesting use of VA instead of esti- mated properties. It is important to note here that definitions matter. Because the ways BBA and VA are inherently set up, this is a somewhat apples- to-oranges comparison. Physical testing is used as the input data for VA, and thus, is mainly performed for lamina (ply level) — not lami- nates (layups) — at 0°, 90° and 45° (shearing tests). A small number of physical tests on laminates are performed for verification (Fig. 4, above). Using the lamina data, the software automatically builds laminates, sets up test coupons and conditions and then runs the test matrices (e.g., compression, tension, OHC, notched, CAI, filled hole, etc.) for various environments (room temperature ambient, hot/ wet, etc.). Assaker adds, "We calculate all of these laminate proper- ties by simulation and test only at the red dots (Fig. 4) to validate our results." e software then solves for allowables. Assaker contends this doesn't require the two months projected in Rousseau's CAMX paper, but a matter of weeks after lamina test data are received. And because lamina data are simplified, this testing reportedly can be achieved more quickly than Rousseau's projected 18 months. Rousseau clarifies that his 18-month testing period includes the time for panel fabrication, machining, wet-conditioning and static testing at cold-temperature ambient, room-temperature ambient and elevated-temperature wet conditions, plus report-writing. "Wet conditioning alone takes two months, regardless of how many or few specimens you test," he points out. "In my opinion, all of this physical laminate testing will take a minimum of 9 months, with 18 being a conservative estimate." Assaker, however, explains that VA uses a formula to calcu- late wet laminate properties from the properties for dry laminates already developed from lamina testing. is formula is based on the FIG. 4 VA testing with physical validations VA tools use physical lamina test data at 0°, 90° and 45° to build laminate test coupons using finite elements and virtual testing to calculate laminate properties. Physical laminate testing is only performed at the red dots to validate results. Source | e-Xstream engineering 3000 2500 2000 1500 1000 500 0 0 10 20 30 40 50 60 70 80 90 100 IM7/8552 Unnotched tension carpet plot Test data IM7/8552 Percentage of 45/-45° piles Laminte strenghth (MPa) 0% 90° 80% 0° 20% 90° 40% 90° 60% 90° 80% 90° 60% 0° 40% 0° 20% 0°

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