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Static Simulation Bolt Preload Yielding

Question asked by Matthew Zola on Jul 25, 2018
Latest reply on Jan 28, 2019 by Steven McCallion

I am using a static simulation to determine whether I am using a sufficient number of fasteners for my load case. I am currently using 18-8 stainless steel socket head screws into a 6061 aluminum frame. The screws are 4-40 in size and are called out to be torqued to 5.2 in-lbs. When I make a simple static simulation as shown below, the FOS on the screw is already 0.95. If I then put 50 G's of gravity on the assembly, the FOS goes up to around 0.96. I am using the same methods on my larger assembly and am getting similar results. What I am wondering is how can I tell if these screws are sufficient given that the preload is already so close to yield and thus a low FOS?

I have read through this thread from 2011 (including the theory behind a bolted joint), and what I took from it was that I need to create two simulations - one with just the preload and one with the applied load. If the axial force on the fastener after running the simulation is greater in the applied load case, then the bolted joint has yielded. Again, I am not sure whether this logic is correct (and if it is correct, how much of a deviation from the pre-load only case is indicative of a joint failure?), or if I should be determining this in a different way.  I have also attached the simple simulation and parts that I am using to help explain what I am doing.

 

Thanks in advance for the help! Let me know if you need clarification on anything, I understand this post can be a bit hard to understand.

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