Question

FEA ( finite element analysis )

c) Describe the boundary conditions needed to impose a symmetry plane when using beam elements. d) How do you know when to turn on "Large Deflection? e) Describe the difference between mesh convergence and force convergence. force converge

Answer 1

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51MMET 111111 一个个了 SYMMETRY 24

C) This basically means that the part of the model on one side of the symmetry supports the part of the model on the other side of the symmetry. Imagine that our beam from above is in tension. This means that in the middle the “left part” of the beam wants to go left, while the “right part” of the model wants to go right. If it is symmetrical the “left part" want to go left equally strong as the "right part" want to go right. This means that the center is in perfect balance. Now imagine we cut our model in half and we only think about the “left part”. In such a case the “left part” wants to go left, and there is no “right part" (we cut it away!) to balance this movement. So what we will do is we will support the "x" direction in the middle (on entire area) and we are set. The reaction force in the support will be equally strong as the “left side action" but in reverse direction - a perfect representation of the "right side" of the model! Please notice that "x" is the “normal direction" to the symmetry plane (it's perpendicular to symmetry plane). Now let's wonder what about the "z" direction. In the middle, the beam goes downward, and nothing is preventing that. Furthermore, since the model is “symmetric" both left and right side of the model want to go downward "equally bad". This means we don't have to support the "z" direction - you need to allow for such deformations! This means that our support in 2D in the middle of the beam (where symmetry is) should be: TX = 0, TZ = free, RY = 0.

d) what are large deflection effects? In simple terms the inclusion of large deflection means that for changes in stiffness due to changes in shape of the parts you are simulating. The classic case to consider is the loaded fishing rod. In its undeflected state, the fishing rod is very flexible at the tip. With a heavy fish on the end of the line, the rod deflects downward and it is then easy to observe that the stiffness of the rod has increased. In other words, when the rod is lightly loaded, a small amount of force will cause a certain downward deflection at the top. When the rod is heavily loaded however, a much larger amount of force will be needed to cause the tip to deflect downward by the same amount.This change in the force amount required to achieve the same change in displacement implies that we do not have a linear relationship between force and displacement. Consider Hooke's law, also known as the spring equation: F = kx Where F is the force applied, K is the stiffness of the structure, and x is the deflection. In a linear system, doubling the force results in double the displacement. In our fishing rod case, though, we have a nonlinear system. We might need to triple the force to double the displacement, depending on how much the rod is loaded relative to its size and other properties, and then to double the displacement again we might need to apply four times that force This recalculation of the stiffness as the structure deflects is activated by turning on large deflection effects. Without large deflection turned on, we are constrained to using the linear equation, and no matter how much the structure deflects we are still using the original stiffness.