November 28, 2020 at 5:57 pm

peteroznewman

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''You can’t "bend" a single linear solid element'', what about single quadratic solid element. By rotating the nodes on the ends, the mid side node can also translate for a quadratic solid element, as like you said for the quadratic shell element.nSolid elements are based on equations that use nodal translation. There are no equations that use nodal rotation, so the element has no knowledge of the nodal rotation.nwhy do we see a difference in stress or displacement results along the thickness of shell elements in ANSYS?nShell elements have equations that use the assigned thickness to calculate the stress through the thickness. That is why you can plot the stress on the top of the shell or the bottom of the shell. You can also render the thickness in Mechanical by turning on Thick Shells and Beams.nAnd also why do shell elements take less time to solve than the solid elements even though the nodes on shell elements have 6 DOFs while for a solid element has 3 DOFs only?nShell models take less time to solve because there are fewer equations in the model. Take an example of a 10 x 100 x 1 mm plate meshed with 1 mm linear shell elements. There are 6 x 1,000 = 6,000 equations in the model. If that solid is meshed with linear solid elements, you cannot represent the bending through the 1 mm thickness with only 1 solid element. For linear elements, a minimum of 4 solid elements are required through the thickness, which means there are 5 nodes through the thickness. That means the number of equations is 3 x 5,000 = 15,000 equations. So the shell model is less than 1/2 the size of the solid model.n''which happens if there are not at least six constraints to ground'', I think you meant to say that at least six constraints to the ground IN THE CORRECT POSITION. nYes, when you turn on Weak Springs, ANSYS puts springs to ground on each body that prevent rigid body motion. If you do that yourself, you must do it correctly.nAbout the WEAK SPRINGS, how correct and dependable will be my answers?nI usually turn off Weak Springs, but if you use them and expect there to be almost no force in the weak spring, you can probe the reaction force in the weak springs to verify that the forces are very small and having an insignificant effect on your results.nAll the forces and moments applied to the model will only travel to the boundary condition I inputted and none will go to the weak springsnNo. If the forces in your model are unbalanced and some force needs to be present in a weak spring for the solver to find equilibrium, then it will do that and you plot the reaction force in the weak spring to find that out.nthe nodes on that edge of solid elements can rotate about each of its center, but cannot resist the rotation?nYes, exactly, there is no stiffness in a solid element to resist rotation of a node. There is only stiffness to resist nodal translation. If there is a mechanism, like a hinge, in the model, the solver cannot solve (without weak springs).nif I pick a an edge of a solid model and try to apply a moment there, what happens?nNothing happens, a moment on a node connected to a solid element does nothing.nCan I input constraint equations to keep the rotations of the nodes on that edge to be equal to zero?nNo. The solid elements connected to the nodes do not care about nodal rotation. Constraint equations will only help if you pick nodes off the straight edge and choose nodes on the faces adjacent to the edge so that the collection of nodes defines an area.nthere is a solid in reality and it is fixed in such a position that its only one edge is groundednIn reality, a solid that is to be fixed must have a face with some area to be fixed. There is no way to hold a sharp edge of a solid, except in a pinned type of connection to another object. Imaging a cube rotated by 45 degrees and the edge with a 90 degree angle is sitting in a Vee shaped channel with a 120 degree angle. The edge of that cube has line contact with another body. It is free to rotate about that edge. If you want to prevent rotation about that edge, you need two lines that cover an area. Like the flat face of the cube on the flat face of another body. Put a drop of glue between them and you have a fixed support.n