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Simple stress analysis

  1. May 20, 2010 #1

    I need help with a simple stress analysis. I have a 25 in x 25 in sheet of Lexan that's 0.125 in thick and is constrained from all sides (the Lexan is used a protective shield). I want to know if a small water jet from a pressure vessel at 7000 psi would crack./break that sheet of Lexan or not. I am assuming that the narrow water jet hits the Lexan at the same pressure. How do I do that?

    Poisson's ratio is 0.37
    Tensile strength at yield is 9500 psi
    Young's modulus is 345000 psi

    Thanks a lot in advance and I am looking forward to your feedback.
  2. jcsd
  3. May 21, 2010 #2
    A good shot would be to look in Roarks for a plate supported along all edges and a point load. Calculate what the point load would be based on the diameter of the stream and the pressure.

    Its probably best to treat it this way then to consider a load distributed over an area.

    Keep in mind that this will be a static analysis and will not consider "blast" effects when the stream is first turned on.
  4. May 22, 2010 #3
    Thank you very much for your help...I am already looking into Roarks methods :)

    I would love to have more info. though about how to consider the initial impact effect...anyone?
  5. May 23, 2010 #4
    There are lots of ways to go further.

    Is this flow "suddenly" applied? To consider this you would need to know very specific timing information.

    Are there harmonics to be considered? Again, you would need to know more about the flow.

    Best thing to do is to first do the static analysis. If the factor of safety based on yield is over, say 5? you are probably safe. But it it is closer you would need to do more work.
  6. May 23, 2010 #5
    fj, I think you need more information too. You say a "small water jet", I assume the size of the jet would be a factor. Maybe the jet diameter would be a place to start. S
  7. May 24, 2010 #6
    Thank you sardonicus. For the area of the water jet my boss says to just make an assumption like 1in^2.

    Does anybody know how to do bullet impact analysis? In other words, does anybody know how to do stress analysis to see if a material will fail under the impact of a bullet? I see the initial water jet impact very similar to a bullet's impact. If I can figure out how a bullet's impact analyzed I think I can figure out the water jet. Any thoughts?
  8. May 24, 2010 #7
    I would have thought that the temperature, both ambient and water would ahve been very imortant to a material such as lexan.

    Secondly the history of the lexan sheet makes a big difference to such materials. That is why plastic safety helmets in construction. mining etc cannot be reused after impact, even though they appear to be sound.
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