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About Rex

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  1. Rex


    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


    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
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