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Hi All, I was working on some different FEA test on domes, and thought you guys would find the results to be of interest. For a 135 degree dome that is 580 mm ID and a consistent thickness of 20 mm, I got a FOS of 13.4289, when the factor is 155.91 PSI external hydrostatic pressure to simulate 350 fsw. For a dome that has a consistent 40 mm thickness, that is 580 mm OD, I got a FOS of 24.7079. Now of course I'll need to verify that the material properties I used are spot on, but it's a good start. Here is the Frappr link to a number of pictures, that also includes two screen captures of the acrylic material properties I used. One in metric and one in standard. http://www.frappr.com/?a=viewphoto&id=4001713&pid=9303006 As I expected, the most stressed area on a 135 degree dome was it's inner bottom corner. Acrylic likes to start to crack on corners, especially when it gets old and/or cold. The dome is deformed in a over exaggerated way, but I didn't expect the circular dent in the sides. Now that I've seen that, the type of metal head reinforcements that I see on the Euronaut make even more sense to me. Also that makes me think that for the vacuum formed domes that are thinner at the very top, that perhaps they are more balanced to resist a even hydrostatic pressure being applied to them, then I once thought. I'm planning to test some 130 degree, as well as some different half sphere domes next. "Factor of Safety COSMOSXpress uses the maximum von Mises stress criterion to calculate the factor of safety distribution. This criterion states that a ductile material starts to yield when the equivalent stress (von Mises stress) reaches the yield strength of the material. The yield strength (SIGYLD) is defined as a material property. COSMOSXpress calculates the factor of safety at a point by dividing the yield strength by the equivalent stress at that point. Interpretation of factor of safety values: A factor of safety less than 1.0 at a location indicates that the material at that location has yielded and that the design is not safe. A factor of safety of 1.0 at a location indicates that the material at that location has just started to yield. A factor of safety larger than 1.0 at a location indicates that the material at that location has not yielded. The material at a location will start to yield if you apply new loads equal to the current loads multiplied by the resulting factor of safety. COSMOSWorks offers several failure criteria for ductile and brittle materials." Regards,
Brent Hartwig |