3 Replies Latest reply on Aug 30, 2016 12:06 PM by Mike Pogue

    Stress analysis beams vs solids

    Marian Micik

      Hi everyone,

      I'm trying to calculate the maximum load capacity of two trolleys. See attached pics of the frames. Since I'm new to SW I first googled some articles and watched video tutorials on youtube but nothing helped me much so far. I can calculate a simple part. But I'm helpless when it comes to assemblies. I think the problem is that I don't understand how the simulation works. Every time I solve an error or a problem, I get a new one.
      These two frames are differently designed. One is made of structural members and the other one is made of solids.

       

      1. My first question is. Beams or solids? When is it better to use solids and when beams?

      - If I'm correct it doesn't really matter. If I make everything right, I should get the same results. Somehow I managed to calculate one of the frames. But then I realized it's made of solids. So I changed it to beams to see what will happen and I got different results. I came across this topic where the guy had a similar problem. But it didn't really helped me to solve my problem.

      Like I said. I can calculate one part, but with assemblies it's much more difficult. With one part I still get different results but the difference is small.

       

      2. Can I use both solids and beams in one simulation?

      Example: I have trouble to fix the beams. I don't want to fix it in the joints. I want to fix it on the bottom. Like it's laying on the ground. Maybe stupid idea but I want to make it a little more simple (and avoid troubleshooting messages). So I make the lower parts as solids and everything else will be beams. Can I do that???

       

      3. It seems that it is easier to use beams because when I use solids, I get a lot of "meshing failed" errors. And if I mesh the failed parts again the calculation times are too big.

       

      4. When I apply force on solids I can pick between "total" or "per item"  load. As you can see on the pic Frame1.png I need to calculate the maximum load of each beam and the max. load of all 3 of them in each row. How do I do that? Do I use the option "per unit lenght" or how do I do that?

       

      I know it's a lot of questions but I'm working on it for several days now and I kinda didn't move forward. And even if I do get a result, I'm not sure if I can rely on it.
      If you think it's too much to explain atleast share some links to some tutorials.

       

      Thanks in advance.

        • Re: Stress analysis beams vs solids
          James Riddell

          1. You won't get the same results. The 'exact' equations are based on beam theory - if you use beams in FEA and have the exact same loading/BCs then the results should be identical.  Real-world isn't like that of course.  In my limited experience, FEA results are conservative.

           

          Over constraint will always add stiffness to the structure and changes the results.  But then, in the linear realm, we are usually trying to prevent any damage so if we design with a SF the plan is to never reach stresses that might approach the YS.

           

          2. Good question.  A better question might be: If you can use beams & solids, how realistic is the connection between the two.  If it is no better than the way bolted joints react then it isn't too much use.

           

          3.  Try meshing with standard instead of curvature elements, sometimes that helps.  You can have twice as many std elements and get roughly the same calculation times.

           

          4.  Don't forget you can apply a split surface 'on' a surface if you know where the force(s) are actually applied.  Otherwise it is a fair assumption to evenly distribute the loading.  Usually I try to pick what might be the worst-case scenario and if it passes that it should pass all "easier" cases.

          • Re: Stress analysis beams vs solids
            Mike Pogue

            Use beams for this--that's what they are for. Solids will be computationally prohibitive unless you are doing some kind of dissertation on these trolleys.

            You should get similar results if you use enough elements and if the elements are not "draft". But they will probably be pretty different at/near the joints.

            You should extract the joint loads from the FEA and do hand calcs, rather than worrying about precisely how the FEA model is lying to you at those points--because it is lying to you.