Modal analysis and random vibrations using PSD on a PCB Navigate through samples of the tutorial by clicking on the sides of the above pictures! For YouTube results video click HERE The YouTube video is proof that applying exactly the steps and the values prescribed in our tutorial and using our 3D model - will lead you to a successful solve. Modifying the settings or the geometry used in this FEA without proper knowledge can lead to unconvergence in the solution. Please pay attention and good luck!
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Thank you for understanding our natural intentions to protect our continuous efforts to bring you only the top quality documentation! Toggle navigation Pages.Hi all, I'm Piergiorgio, an italian mechanical engineer that is starting to learn Ansys Workbench. I just registered on this forum a nice forumI have read a lot of old discussion in the past weeks and I need an help for a random vibration analysis with fatigue. Number of modes to use: all. There is something I'm missing?
ANSYS staff are not permitted to open attachments. In future posts please use the Insert Image button. I have done that for you below. I will study this problem and return with more information, but maybe someone else will comment too. Can you check the material properties displayed in Mechanical? Does the S-N curve pass in successfully?
Solution have the red flash symbol. Thanks in advance Piergiorgio. Permalink 1 0 0. Attached Files. Liked by. Order By: Standard Newest Votes. Permalink 2 0 0. Thanks a lot, Peter. I'm using Ansys Workbench Permalink 0 0 0. Admin Online. Wenlong posted this 4 weeks ago. Hi, Can you check the material properties displayed in Mechanical? Regards, Wenlong. Search in Post Topic.
ANSYS Workbench TUTORIAL 8
Popular Tags fluent ansys udf workbench cfd asc mesh fluid-dynamics error mechanical apdl cfx meshing structural-mechanics maxwell hfss static-structural general transient student This Weeks High Earners peteroznewman rwoolhou abenhadj tsiriaks 82 kkanade 64 pblarsen 49 Aniket 35 Wenlong 33 Amin 30 Kremella Hi all, I'm Piergiorgio, an italian mechanical engineer that is starting to learn Ansys Workbench.
I just registered on this forum a nice forumI have read a lot of old discussion in the past weeks and I need an help for a random vibration analysis with fatigue. Number of modes to use: all. There is something I'm missing? ANSYS staff are not permitted to open attachments. In future posts please use the Insert Image button. I have done that for you below. I will study this problem and return with more information, but maybe someone else will comment too.
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PSD is statistical in nature and as a result of the solution technique, many results that would typically be available in WB Mechanical for a classic static structural analysis, are not natively available in WB for a PSD analysis. This document discusses and presents the command language that is available for an analyst to extract force reaction results for beam connections used in WB Mechanical in a PSD analysis. There is also discussion given to obtain results at various confidence intervals 1-sigma, 2-sigma, 3-sigma.
This connectivity between the systems is first set up on the WB project page, which is illustrated in Figure 1. The modal analysis can be performed as usual. However, in the Output Controls of the Modal analysis, the option to output nodal forces must be set to YES Figure 2thereby populating the results files with the necessary data to later extract beam forces from the PSD analysis.
The other output options are at the discretion of the analyst, depending on what other results are required in post-processing.
Beam connections are created in WB Mechanical under the Connections branch. However, the beam elements are not actually created and assigned element numbers until the solve process.
Therefore, it is not possible to know upfront what number is assigned to each beam element and, as shown later, the beam element numbering scheme is pertinent to post-processing the results. To address this, it is necessary to insert a Command Snippet under each beam connection.
In the above command line, the numeral 1 increments for each beam. Therefore, the above command is intended to define a numerically unique parameter e.
See Figure 3 below to illustrate the unique sequential numbering of each beam. After a modal analysis has been setup and connected to a PSD analysis, the procedure to extract force reaction results from beam connections is fairly straightforward. The results obtained using this approach will give results in the global Cartesian coordinate system for the 1-sigma confidence interval.
An example is illustrated in Figure 4. Begin DO loop to process through each beam connection individually! Conclude the DO loop. Once solved, the results will then be printed node-by-node, in the global Cartesian coordinate system, for each beam element in the Solver Output, which is available from the Solution Information branch.
An example is illustrated below in Figure 5. Because the leading command was SET,3,1, the results are for the 1-sigma confidence interval. There is discussion later for obtaining results at other confidence intervals. Similar to the previous option, the procedure again is fairly straightforward. An example is illustrated in Figure 6. Once solved, the results will then be printed element-by-element, in the Solver Output, which is available from the Solution Information branch.
An example is illustrated below in Figure 7. A PSD analysis is statistical in nature.I am performing random vibration analysis for a cubesat by applying the PSD graphs in all the three directions considering all the fixed supports. I am getting unusually high amount of stresses in the bolt holes and around the bolt hole region.
My modelling for bolt is the beam connection along with pretension given in the static structural which is then connected to modal and random vibration. Can you add some images of these regions and high stresses. Also are bolts modelled in 3D or by beams or the like? The bolts are modelled by 3D beams in ansys workbench. The stress in abnormally very high around the bolt hole and I'll add some images later. It is most likely what is called a stress singularity - look it up and you will see a lot of info on that.
But these stresses are mostly around most of the bolt holes, do you think the mesh issue for stress singularity might be occuring on all of them?
Also, when I tried to bond the surfaces instead of making the bolts, the stress diminished around the bolts holes and reduced greatly. This is a very long chapter - it depends what you are interested in really bolts, or welds, If you scoped it to the edge, that creates a higher stress than the real bolt head which makes contact to an area under the bolt head.
You can reduce this stress by imprinting a circle around the hole to create a face to scope the end of the beam to.
The area of the face distributes the load and reduces the stress. As jj77 requested above, please post some images in your reply so we can see your model and mesh. Yes, in fact I did scope the bolt hole to its edge. I will be uploading the images in a few hours since I'm not at my laptop and it would be great if you could help me out. So I tried with scoping the bolt hole to the face which reduced the stress to some extent. For a 2. Would that suffice? I have attached an image for you to see the split lines.
Also how does one realize a stress singularity at the sharp corners of the geometry? As in, how does one recognize it to ignore it? To remove the stress singularity at a sharp interior corner, edit the geometry and add a blend fillet radius to the sharp corner to remove it from the geometry. Also, how would one recognize if the given stress at a point is a stress singularity and can be ignored?
Secondly I am still getting unusually high stresses on the corner of the structure. I made a 0. You can have a look at the images. What should I do about it? It looks to me like the high stress is in an appropriate location. Changes to the design can move the natural frequency of the structure away from a high point in the PSD. Please show the PSD plots and the list of modal frequencies from the Modal analysis marked on that plot.
Which mode s are near the high values in the PSD? This Random Vibration analysis is linear, which means it has linear materials and linear connections. You would need to synthesize a transient waveform that meets the PSD specification as the input load to the model.
Random Vibration analysis
Transient Structural with nonlinear effects is a lot more work and requires a lot more computational resources and solve time than the Random Vibration analysis. Also I have some optical components inside the structure also having a problem with the stress.
They have been bonded with the optical mounts which are bolted inside since I thought I can consider them to be in place while the loads are being applied. Could there be a problem with my modelling there?I'm currently trying to run a tri-axial random vibration analysis PSD. I don't see a way of combining random vibration results from each independent axis, but I did run a single case with accelerations in all three axis. Comparing this to the results from the single axis analysis, the equivalent stress results from the multi-axis run seems to be the RSS of the three independent axis.
Is this the case? More generally, where in the help would this information be located? The web based help on the topic seemed fairly limited You can apply all three axis PSD accelerations into one analysis, as long as they are uncorrelated. Random vibration analysis is a statistical analysis so a special algorithm is used to compute the equivalent stress.
Multiple PSD excitations uncorrelated can be applied. Correlation between PSD excitations is not supported. Meaningful equivalent stress is computed using a special algorithm by Segalman-Fulcher. Note that the probability distribution for this equivalent stress is neither Gaussian nor is the mean value zero.
Thanks, Peter. Please share the file s96aif. Search in Post Topic. Popular Tags fluent ansys udf workbench cfd asc mesh fluid-dynamics error mechanical apdl cfx meshing structural-mechanics maxwell hfss static-structural general transient student This Weeks High Earners peteroznewman rwoolhou abenhadj tsiriaks 82 kkanade 64 pblarsen 49 Aniket 35 Wenlong 33 Amin 30 Kremella Recent Activity vincenzopersano is a new member in the forum.
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