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  1. KSshopper

    Core Crush Strength vs Compressive Strength

    Honeycomb core can be used to absorb energy by crushing under load. Crush Strength is a property of the core required to analyze an energy absorption problem. See hexcel TSB122 for the analysis method. Compressive strength is the appropriate property for use in a static strength analysis.
  2. KSshopper

    Electrical enclosure bonding.

    I don't know anything about the specifics of your box, but the case needs to be bonded to the aircraft ground plane. See SAE ARP 1870 for more guidance.
  3. KSshopper

    2024 "DD" alloy rivet heat treat... and other

    Trick Question! The FAA acceptable information for mechanics doing structural repairs will always be the Structural Repair Manual (SRM). If the SRM doesn't refer you to a more in depth fastener installation/processing spec (i.e. BAC5004), and doesn't include any information on how to heat treat...
  4. KSshopper

    Von Mises in Hand Calc?

    If I imagined this right, you have a control rod that undergoes torque and bending together while loaded. Both the peak shear from the torsion and peak bending interact at the extreme fiber which must be accounted for. Assuming elastic behavior up to ultimate, writing your margin of safety using...
  5. KSshopper

    Honeycomb Sandwich Close Outs Modeling (FEA/Nastran)

    Clarification on item 3, you represent the ramp in the model as you would in item 1, just evaluate the stresses at the boundary of the edgeband and full height core. It's simple, but you should have some test data backing it up. Strange that your hand calcs differ. That's definitely something...
  6. KSshopper

    Honeycomb Sandwich Close Outs Modeling (FEA/Nastran)

    I don't think there is any industry best practice. I've worked for three separate companies and seen it done three different ways: 1. Model the entire composite bondment with shell elements (PCOMPS). Model the core ramp area with average (1/2) height core. Use a hand analysis to calculate...
  7. KSshopper

    Bearing stress in a single shear lug loaded by moment and force

    Single shear lugs are addressed in section 9-5 of the Air Force's "Stress Analysis Manual" by Maddux. A pdf can be downloaded at: Link
  8. KSshopper

    How to quickly I.D. a part as Magnesium

    Well, the company line is you know the part is mag if the drawing tells you it's made from mag. If for some reason you don't have the drawing, an NDT technician should be able to determine the alloy. I don't think there is anything special about removing corrosion from mag, scotch brite pads...
  9. KSshopper

    Transverse shear of plates

    RPStress gave you the ratio between shear and bending deflection. Running that calculation should tell you if you're in the ballpark. Based on your problem description, I still think shear deflection would be small relative to bending deflection. I recommend you use an orthotropic material...
  10. KSshopper

    Transverse shear of plates

    Unless the thickness of the honeycomb is very large, I wouldn't think there would be much transverse shear deflection. NASA has a good reference for modeling honeycomb plates: http://femci.gsfc.nasa.gov/hcplate/honeycomb_pshell.html
  11. KSshopper

    Ultimate stress in a pressurized fuselage

    Koopas: Your method is certainly the most conservative way to repair: w = Ftu*t (where w is running load in lbs/in). The 15% "fitting factor" may be a little over the top. We normally only apply a fitting factor to single pin joints. The second method which Nigel alluded to and I...
  12. KSshopper

    Ultimate stress in a pressurized fuselage

    I've seen a fuselage fail in compression! Although it was no design condition. The mechanics raised the nose gear while the airplane was parked and it buckled the crown skin and stringers. I agree with Nigel. The least conservative thing you can do and get away with is copy a local fastener...
  13. KSshopper

    Short ED, again

    "Ligament" is simply the material left between the hole and the edge. On the parallel edge margin problem: I've heard of engineers doing a similar calculation, but I think it's overly conservative. I've found I can redistribute the shear in the shy ED fastener to the fasteners on...
  14. KSshopper

    Fatigue curves

    Mil-HDBK-5 has S-N curves for various aerospace materials: http://www.grantadesign.com/cgi-bin/stat-tree.cgi?src=MIL5-H.xml If these don't work you'll have to go to the library and look at ASTMs
  15. KSshopper

    stress stiffening effect

    yeah you're right. The modes should have changed. Apparently there's a problem in your model. Maybe try and simplify the model to a mass-spring system, that you can verify using hand calculations.
  16. KSshopper

    Trouble with Bruhn, A17

    Okay, I think I understand. It sounds like you have a couple of things going on. The first is the antenna is experiencing aeroacoustic buffeting. air flow across the antenna sheds vortexes as it passes. Simply put, these vortexes are a forcing function with a frequency that varies with...
  17. KSshopper

    Trouble with Bruhn, A17

    SparWeb: I've delt with antenna modifications as well, so I can appreciate the dilemma. The static analysis for your airplane's fuselage skin was most likely based on hoop stresses due to pressure (pr/t). Additionally, Bending stresses in the longitudinal direction due to pressure (pr/2t) and...
  18. KSshopper

    Wing Skin Buckling on FEMAP Model

    When a skin buckles, it still carries the load it carried at the onset of buckling. Any additional load is transfered to the stringers and "effective" skin. Another approach may be in order. Change your CShear elements to Plates (Shell64 in ANSYS). Change your Rods to Beams. Then...
  19. KSshopper

    Trouble with Bruhn, A17

    Is your skin a flat plate or is it a cylinder? If it is a cylinder, I'd recommend calculating hoop stress using classical methods: f=pr/t. If you're skin panel is a flat plate, I'd recommend using FEM to determine the plate stresses. Remeber to turn on stress stiffenning to capture the...
  20. KSshopper

    Fatigue Life of FAR 23 Aircraft Repair

    I like the last paragraph in your first post! However, if I'm having trouble following your methodology. I understand you are trying to substantiate a repair for fatigue, but what in the repair are you looking at? The damage which was repaired or the first row of fasteners? In your first...

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