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  1. Mark172

    Stress at junction of plate and solid elements with skins not realistic. Is this normal?

    Thanks for the thoughts. I will think on it. The penetrating plate elements (2nd example from top) seem to do an ok job of showing ballpark stresses. Perhaps we can use the results of FEMs built like this to point us to locations where we need to perform classical calculations.
  2. Mark172

    Stress at junction of plate and solid elements with skins not realistic. Is this normal?

    I'm sorry, let me upload better pictures. The cantilevered beams are made of plates, the foundation cubes are solids. I had toggled "view thickness" in FEMAP, which makes the plates look like solids. Note that I have removed 3 solid elements from the second example (penetrating skins) to show...
  3. Mark172

    Stress at junction of plate and solid elements with skins not realistic. Is this normal?

    ...of the cubes that are acting as the foundations for the cantilevered plates. Why do people skin solids? That's what I'm trying to figure out. I *thought* it was a method for constraining rotations at junctions between solids and plates, but it doesn't seem to do this very well and produces...
  4. Mark172

    Stress at junction of plate and solid elements with skins not realistic. Is this normal?

    ...NOT provide realistic stress distributions nor values. Very thin skins, on the order of 1 um as I've seen commonly used, do not provide accurate *deflection* let alone stress predictions. In fact they often do not prevent pivot errors and deform well past elastic assumptions. I surmise that...
  5. Mark172

    Stress at junction of plate and solid elements with skins not realistic. Is this normal?

    I am somewhat new to using skin elements to account for moments at the junction between plates and solids, but I believe I'm doing it correctly. Yet the stress distributions I see have stark discontinuities and are nothing like what I would expect in the real world. My intuition is confirmed...
  6. Mark172

    Modal testing as the most efficient way to verify FEM accuracy?

    Hi all, thanks for your comments, I've spent time reading up on your pointers. Very useful. @SWComposites, you say that modal testing tells you nothing about strength, can you explain that a bit? My thinking is that if you've got the stiffness correct, as exhibited by matching the first 5...
  7. Mark172

    Modal testing as the most efficient way to verify FEM accuracy?

    Our current outsourced FEMs are sufficiently accurate. They vary by approx. +/- 7-10% for first modes. I'd like to produce in-house models that nail all the primary and most of the secondary modes to within just a few percent. We don't have the capital for our own vibration testing lab at this...
  8. Mark172

    Modal testing as the most efficient way to verify FEM accuracy?

    Hi All, I lead a mechanical team at a growing aerospace company building small satellites up to 500 kg. I've been tasked with bringing structural analysis in-house to rely less on outside contractors. I intend to accomplish this through a mix of training and hiring, but I am concerned about...
  9. Mark172

    Confused about thermal units in FEMAP material database

    Yeah it seems to be a bit of a nightmare! Unfortunately I've built a FEM in US common so have to stick with it to see the last step of the analysis campaign (thermal stresses) through. The conductivity checks out as BTU/in-sec-F. The specific heat only checks out as BTU/lb-F if I multiply by...
  10. Mark172

    Confused about thermal units in FEMAP material database

    ...my unit system. From the Nastran user's guide, the conductivity should be lbf/s-F, but when I convert for aluminum at ~200 W/m-k, I get 200 w/m-K * .1249 (lbf/s-F)/(W/m-k) = 24.98 lbf/s-F which is off by what looks like a factor of 10000. Similarly for specific heat: according to the...
  11. Mark172

    Best machine for cutting packaging foam for a small business?

    Great perspectives, all. Rputvin's comments made me think die-cut cardboard may be better packaging for our product all around (cheaper, less waste). For the stuff that absolutely requires foam I'll look into local waterjet outfits. Again, thank you all for your input.
  12. Mark172

    Best machine for cutting packaging foam for a small business?

    Thanks for the suggestion! Unfortunately appearance is pretty important. I'll need to deliver with this level of quality: https://www.innerspacecases.com/
  13. Mark172

    Best machine for cutting packaging foam for a small business?

    I run a small business on the side that sells motorized astrophotography mounts. Cutting our product's packaging foam by hand is a pain (and quality is inconsistent between techs), but ordering custom cut foam is too expensive. I'm a mechanical engineer by day and have broad (albeit mostly...
  14. Mark172

    Coolant Hose Material Selection for 10 yrs No Service

    Thanks tugboat for the tip on the spring clamps, I have to admit that was a blind spot. Thank you all for your responses, there is a wealth of useful information here. I should really come here more often.
  15. Mark172

    Coolant Hose Material Selection for 10 yrs No Service

    SS and copper are in the design space, but we need a flexible connection to the detector. That would require a bellows of some kind, which introduces a break in the line and additional connectors that work against the requirement to minimize leak risk. Furthermore such bellows are ~$100/each...
  16. Mark172

    Coolant Hose Material Selection for 10 yrs No Service

    Ah rb, I wish it stopped at impossible performance and budget demands. Did I mention they want it yesterday? Oh well, this is our lot. Thank you tugboat for the perspective on buna-N and the EPDM recommendation. I'll look into EPDM options. This is for a 1/4" ID.
  17. Mark172

    Coolant Hose Material Selection for 10 yrs No Service

    Happy to get into the details - 10 years, no leaks, no service are the requirements handed to me for the coolant assembly. Yes that's manager speak, in terms of probability at least 3 sigma odds of no leak across 10 years and 24 cameras (48 total lines for the cameras plus 16 more for cooling...
  18. Mark172

    Coolant Hose Material Selection for 10 yrs No Service

    Hi all, I'm a mechanical engineer at a research institution building an array of spectrographs. I'm currently designing the coolant assembly and attempting to specify the coolant lines for a minimum of 10 yrs operation with no line maintenance. The other constraints and requirements are listed...
  19. Mark172

    Applying stress concentration factors to only certain elements

    Does anyone know if it's possible to apply stress factors to only specific groups of elements in post? I have an analysis of a structure that requires different stress factors to be applied to different regions of the structure. Each analysis run has 160+ load cases that all need these factors...
  20. Mark172

    How many nodes/elements/DOFs do you work with?

    I agree, with both of you. Having the ability to do quick stress/deformation/thermal/frequency analyses of simple parts integrated into a CAD package is great to have. The problem comes when people mistake SW for a full-up analysis tool and attempt to predict the response of assemblies with...

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