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Rex

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Everything posted by Rex

  1. Rex

    MFLUID

    Hi, Thanks for your feedback, I checked all these to simulate the fluid effect during modal analysis, but I just wanted to know whether the MFLUID supports the triad 6 element in the element set, can you tell me?
  2. Rex

    MFLUID

    In MFLUID, can I use Tria6 in the WSURF 1? Or I need use the tria3?
  3. you need to learn the theory of the fatigue simulation first...... How do you think the damage be calculated? The software need to find how many cycles at different stress range/amplitude, then use the material S-N curve to find the allowable cycles at different stress range/amplitude...... The damage will be calculated using the test cycles(even you input is in time, but the software will find the cycles) and the allowable cycles..... Damage=test cycles/allowable cycles
  4. Hello Mani, The cycle result is not time, it tells how many cycles the part can resist..... In the Optistruct User Guide, page like 307 shows you how to calculate the cycles on the elements, but the way how Optistruct works is not clearly explain here, I asked in the past, but still no feedback. In the fatigue simulation, due to the material S-N curve, the vibration cycles is needed instead of time, even your input is in time, but finally software will calculate the cycles anyway. Try to read the USER GUIDE, it should be helpful!
  5. You did not attach the model here, so I can not tell you the order of the stress tensors in this GSTRESS pch file, but if you want you can find my answer in another topic:
  6. you can find the free e-books, in there you can download below book. Meanwhile, the Optistruct User Guide and the reference guide helps too. Fatigue_eBook.pdf
  7. To one of your question, why use frequency domain instead of time domain: Traditionally fatigue analyses have been dealt with by defining the time variation of the loading. Sometimes these time histories become very complex, long, and even random in nature, making conventional transient FE analysis and even pseudo-static fatigue analyses very time consuming, cumbersome, and inconvenient. Dynamicists have long used frequency domain techniques to understand responses to structures subject to these types of loading, mainly out of necessity to speed up computation times and reduce disk storage needs, but also to understand dynamic behavior. I was trying to give you a good answer, but I think it's better to find something from the famous book or article, the above answer is from Embedded Vibration Fatigue User’s Guide of Nastran, this is only part of the answer, in this document, you can find almost all your answer, they also compared the stress and damage by using time domain and frequency domain in random fatigue simulation. Hope it helps you! Meanwhile, I hope this forum can answer the difficult questions too....
  8. Dear, I am trying to know how the Optistruct calculate the damage factor in a sine sweep vibration, but currently I was confused by the way to calculate the cycles at specific frequency by using the stress results output from the FR simulation, the way to calculate the cycles is shown in Optistruct User Guide, page 307, which is attached below. Inhere the cycles at specific frequency can be calculated by using f/v, but it's not easy for me to understand. By using this theory, if we adding more frequency points in the Freqi cards, then the total cycles in a frequency range will be increased, which will induce a higher damage factor in this end. Meanwhile, I did this calculation by using Excel, and attached below: Cycles_calculation.xlsx In this sheet, the first two columns are outputted from Optistruct, the 3rd is the cycles calculated by using the formula suggested in the User Guide, the 4th is the method based on my understanding.... I see the deviation of the total cycles in this frequency range is much big, can someone tell me why? Thanks a lot!
  9. Hello Prasannak, Are you sure about this? why the software doesn't use the closed two points of the FRF result to interpolate, then use the interpolate FRF result to do the random analysis?
  10. Rex

    PSD analysis

    @Tommy Hello, I try to answer you based on my knowledge, if I am wrong, please correct me! 1. I did not use the HyperGraph to do this, so I do not know, but I know there is a free software can do this convert, usually I will use Scilab or Smath to do this work, but you can use the vibrationdata made by Tom Irvine 2. Usually, the PSD used in the vibration test is log-log format, then we should use the same in the simulation, the log-log makes the PSD looks like linear, but they are not a straight line in the real, if you use linear-linear, the vibration energy will be reduced. Hope it helps, keep in touch!
  11. Rex

    PSD analysis

    oops, it's really difficult to download the presentations, I mean the download speed, is there a way to download all the Optistruct presentations in a more efficiency way?
  12. Rex

    PSD analysis

    Hi Rahul, Where I can find all these interesting presentations you attached above? I am really interested with these! Let me know, thanks!
  13. I also faced similar problem, but I just fixed, I know what is the problem you want to solve! In Optistruct help document, the format for the stress outputs is not given, so we will confuse with all these stress tensors, it also took me long time to understand the stress tensor during FRF simulation, below is my solution. 1) First of all, you need have the .pch file for desired element, all the stress tensors is shown in it, I already saw it in you picture 2) Then you can load the .h3d file in Hyperview, and hide all the element except the desired element. 3) Now you can plot the stress of this element (please keep the averaging method as none), then compare this contour plot result with your .pch file, by using this way you can understand the format of the stress tensors Hope this can help u!
  14. oops, the attachment is failed to load in previous..... pch_calculation.xlsx
  15. I already found the way to calculate the Von Mises Stress by using the Real & Imaginary stress tensor, if anyone is interested on this topic, you can find the attachment below. I am still working on the cycles counting in Optistruct!
  16. Hi Rahul, Thanks for your answer, but I did not get your point! you only show me how to calculate the magnitude displacement in time domain, but how the Hyperview uses the 12 stress tensors to find the response Von Mises stress in time domain? To your 2nd answer, I know what you mean, you need calculate the time to increased to the top frequency, but how the software deal with the bottom frequency? and how will the software use the time to calculate the cycles? I mean the software will count all the time to the top frequency or 50% on the bottom frequency & 50% on the top frequency? For example: sweep rate 5Hz/min, bottom frequency is 5Hz, top frequency is 10Hz, so it takes 60 seconds to reach 10Hz. Then how will the software use this time to count the cycles? 10Hz*60s at 10Hz? or how? Thanks again!
  17. Dear, I did a sweep calculation for a Degas Bottle, the frequency range is from 15Hz to 100Hz, acceleration level is 3g, when I got the response stress after the model frequency simulation, I outputted the stress tensor of the most critical element, but I found there are 12 stress tensor in the pch file, so how the hyperview use these tensor to get the plot stress? I could not find the way. Meanwhile, how the Optistruct calculate the damage based on the response stress, I mean how to calculate the cycles at interested frequency, I know the help document, but it doesn't tell too much. Thanks a lot! 20191202_GWM_B30_Degas_Bottle_LT_V5_Opt.pch
  18. Hi Tommy, thanks for your answer if this is to my question. I will try on Monday, the MFLUID only supports for shell element, if I model the tank in solid, then it will be a problem apply the MFLUID on the mesh, I do not care about the eigenvalue of fluid, I just want to know the fluid influence on the tank. Thanks again
  19. Hi Rahul, Always finding good feedback from you, one question from my side, may I use tet10 instead of 2D mesh then use MFUILD in it? or I must use shell element for the container? If the MFUID can only use on the shell element, may I put a very thin layer on the solid elements, then use MFUID on this thin layer? Thanks in advance!
  20. I made a PSD fatigue model based on my understanding to Optistruct, but I do not know whether this is the right setting, I see the stress PSD and RMS stress is similar with Patran, but I am not sure about the fatigue damage results, is there someone who can help me to check? Thanks a lot! Notes: In my model, a simple beam is built, on the tip of this beam, a 0.2kg mass is added. Then the other end of this beam is fixed, and the PSD loads in here is 0.475G^2/Hz from 20Hz to 200Hz, this random test keeps for 4 hours. 20191028_Beam_PSD_FATIGUE.fem
  21. 1) ASD is Acceleration spectrum density, it always with the unit (G^2/Hz) in X direction and Hz in X direction, and it tells how much the acceleration will be in 1sigma level. 2) If you want to know why I want to run the random fatigue in frequency domain, you can search on google, they explain it better than me. For me, I would say do the random fatigue in time history is a very time-consuming work, and usually if you want to know the random input in a better way, you need enough acceleration data in time domain. Meanwhile, if you see the acceleration input on time history, sometime it does not help you too much, but in the frequency you could find which frequency is more important.... oh, by the way,by using Miner rules, we can even get the total damage considering different acceleration inputs(like the acceleration on the air intake manifold can be caused by engine and road together, it's difficult for us to consider the fatigue in time history...) Please learn more on this topic by using google or here, all these answers are based on my understanding, if I am wrong, just let me know! Thanks!
  22. Hi Rahul, Thanks for your model, now I know Optistruct can deal with fatigue in frequency domain, it will take me sometime to check the cards, meanwhile the PSD model you attached in here is not a fatigue model. I will try to find out how to do random fatigue by myself! Have nice day!
  23. Sorry Rahul, I did not find something about random fatigue simulation in frequency domain. I know that Optistrut can run random response simulation, then we can get the Stress PSD in frequency and RMS, but how can we use this information to calculate the damage factor in optistruct? The theory will be use the 1 sigma stress(RMS), 2 sigma stress & 3 sigma stress and the cycles at different stress level to calculate the damage, then sum the sub-damage at these levels to get the total damage in all the elements. Hope you can give me more information, thanks!
  24. Hi Farouk, Sorry for late replying, just came back from a business trip. To you question: I want to do the fatigue simulation in frequency domain because that usually the OEM will ask you to use the ASD&test time in different direction to check whether the parts will be survived, in this case, it makes no sense to change the input to time history. In other side, fatigue simulation in time domain takes much long time than frequency domain. If I am wrong, let me know. thanks!
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