Discussion Forum

Hello all,

I am building a model to predict the deflection of an actuated beam; both for a single segment uniform beam and a segmented beam with indepedent actuation in each segment. The model is actuated by inputting actuation stress into the equation for second Piola-Kirchoff membrane stress. The beam is clamped at the beam and has a prescribed zero displacement in the z with free rotation at the tip. I am looking to find the reaction forces at the tip that occur as a result of the prescribed zero displacement boundary condition. I am using shell elements for thin beams and am using Comsol v. 5.2a or 5.3.

I find the reaction forces in one of two ways. The first way I use a line integration under the derived values of the results. I select the tip edge and select the active layer (.RFz) for the expression. I then select the summation over nodes method in the integration settings.

The second method I use involves exporting the reaction force in the z in a text file. I am looking to export the nodal data along the tip edge. In so doing under Output I select text, evaluate the points in a grid and format the data in a spreadsheet. I then select the beam length in the X, and select a range of data points in the Y which I believe correspond to the initial locations of the nodes, and the origin in the Z due to the prescribed zero displacement.

Both of these methods results in my finding odd numbers for the reaction forces. As far as exporting what I suspect to be the nodal data (but may not be) the nodes flip signs and change magnitudes unpredictably. When I sum up the nodes under the derived values, the same occurs as I change the inputted actuation.

I am looking to reliably and accurately find the reaction forces at the tip for a uniform or segmented beam (where each segment can be actuated indepedently) and any help is greatly appreciated.