You are viewing an old version of this page. View the current version.

Compare with Current View Page History

« Previous Version 76 Next »

Error formatting macro: include: java.lang.IllegalArgumentException: No link could be created for 'SIMULATION:ANSYS 12 - Tensile Bar - Panel'.

Numerical Results

Before we explore the ANSYS results, let's take a peek at the mesh.

Mesh

Click on Mesh (above Solution) in the tree outline. This shows the mesh used to generate the ANSYS solution. The domain is a rectangle. This domain is discretized into a number of small "elements". For each element, ANSYS approximates how the structure responds to the forces acting on the element. A finer mesh is used in areas of greater stress concentration. We have checked that the solution presented  to you is reasonably independent of the mesh.



Units

Set the units for the results display by selecting Units > Metric (mm, kg, N, s, mV, mA). The displacements will be reported in mm and the stresses in N/mm2 which is equivalent to MPa.

Displacement

To view the deformed structure, click on Solution > Displacement in the tree outline. The black rectangle shows the undeformed structure. The deformed structure is colored by the magnitude of the displacement. Red areas have deformed more and blue areas less. You can see that the left end has not moved as specified in the problem statement. This means this boundary condition has been applied correctly. The displacement increases from left to right as we intuitively expect. There is also not much variation in the y-direction. So we can conclude that the model has been constrained properly.

Note the extremely high deformation near the point load. This extremum is unrealistic and should be ignored (there are no point loads in reality).

To view the Poisson effect (shrinking in the y direction), zoom into the top-rightright corner by drawing a rectangle around the region with the right mouse button.

You can do this multiple times to zoom in more. You do indeed see the shrinking in the y-direction as expected but it is small for this model.

You can restore the front view of the entire model by right-clicking in the background and choosing View > Front.

If you zoom into the top-left corner, you will see that the model cannot shrink in the y-direction at the left boundary where it is fixed. In other words, it "wants to" shrink in the y-direction at the top-left corner but cannot due to the displacement constraint we impose. So can expect a stress concentration near this corner.

Note that you can zoom in and out using the middle mouse wheel. You can translate the model by clicking on the Pan button and dragging the model with the left mouse button. There are also a bunch of zoom options next to the Pan button.

sigma_x

Next, let's take a look at the stress components starting with sigma_x. Click on Solution > sigma_x in the tree outline. The stress is uniform away from the ends. To check what the value is in the uniform region, click on Probe in the toolbar (see snapshot below) at the top and move the cursor on the structure; Probe values in the middle as well as at the ends. You may need to translate the model to the right to see the probe values near the left end.



The value of sigma_x away from the ends is nearly 200 MPa (the unit is indicated above the plot). This matches with the P/A value expected from the [Pre-Analysis step].



In the sigma_x plot, we see that there is deviation from the analytical value in two regions:

  • Around the point load (again the extremely high values very close to the point load are unrealistic).
  • At the fixed end.

The analytical solution is inaccurate in these regions since the 1D assumption breaks down. In fact, as the mesh is refined further, the stress at the point load will approach infinity.

sigma_y

Next, let's take a look at sigma_y. Click on Solution > sigma_y in the tree outline. Again, probe values in the middle as well as at the ends. The value in the middle is close to zero as expected from the analytical solution. There is significant deviation from the analytical solution at both ends. Note that there are areas where sigma_y is negative i.e. compressive.



tau_xy

We expect tau_xy to be zero away from the ends. Near the ends, since sigma_x and sigma_y are non-zero, we expect

Unknown macro: {latex}

[
\tau_

Unknown macro: {xy}

= \tau_

(x,y)
]

Plot tau_xy, look at the range of values and use Probe to check actual values. Are the above statements valid?



Equivalent Stress (Von Mises):

The Equivalent or Von Mises stress is used to predict yielding of the material. We can consider the maximum and minimum equivalent stresses as the critical design points. We can see that the analytical solution under-predicts the maximum equivalent stress. Thus, one would need to use a large factor of safety if using the analytical result while designing such a structure. One would use a factor of safety with the FEA result also but it does not have to be as large.



[Go To Homework Exercise] 

Go to all ANSYS Learning Modules

  • No labels