# Nathan

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1. ## Damping in Linear Transient Analysis

Well, thanks @Adriano A. Koga but I had read that help page before asking, and it doesn't help as it is written "Frequency of interest in radians per unit time (PARAM, W3) for the conversion of overall structural damping into equivalent viscous damping." It doesn't say what this frequency is supposed to be, or why the 0.02 damping ratio in TABDMP1 is replaced by 0.2 with W3=300 in the tutorials I quoted.
2. ## Damping in Linear Transient Analysis

Hello all, I have a question about damping definition in linear transient analysis, either direct or modal. In OptiStruct tutorial OS-T 1315 Modal Transient Dynamic Analysis, a 2% damping ratio is defined in TABDMP1 table with 0.02% between 0Hz and upper EIGRL value 1000Hz. But in OS-T 1310 Direct Transient Dynamic Analysis, PARAM,G = 0.2 with PARAM,W3 = 300 are used. I do not understand the last case (what are these values?) and the link with 0.02 value in Modal Transient analysis. More precisely, why G is 0.2 and not 0.02, and why W3 (which if I am right is supposed to be a frequency??) is 300? In addition, I have read in OptiStruct help that TABDMP1 refered in SDAMPING subcase in a Loadstep could combined with PARAm,KDAMP,-1 to be equivalent to PARAM,G. All those different information make me a bit confused. Is it also possible to use PARAM,G in Modal Transient ? Naturally, I would have done the same thing as Modal Transient in any case : a TABDMP1 table with 0.02 CRIT value called in SDAMPING Loadstpe subcase. But if I try to use PARAM,G I get the same thing as if I did not refer SDAMPING, so without damping definition. Can someone clarify correct and possible setups for damping definition in (Modal) Transient Analysis? Thank you!
3. ## Base Motion in Modal Based Frequency Response.

@IRA_EM In addition to Adriano's answer, I think you will also need to specify Type as "DISP" in RLOAD2 card for enforced displacement
4. ## Stress output and resolved in Global system

Hi, Yes Tinh I know that equivalent stress are not system-dependent. I plotted Sxx (or Syy) stress
5. ## Fatigue analysis "Life" result

Hi, Thanks for you answer. It's been a while now since I wrote that post, and it is completely clear now. It can be considered as a "closed" topic
6. ## Stress output and resolved in Global system

Hello all, I noticed that stresses results depend on the coordinate system you use in HyperView, and that makes sense. I understood that Analysis System option gives the result as it is calculated by the solver, and according to HyperView User's Guide it depends on the type of element: "different elements in a model can have different systems, depending on the way they were defined in the results file. For example, a shell element can be in terms of its elemental system whereas a solid element could be in terms of the global system". In order to get the correct stress value if we want to work in the global coordinates system, we only have to switch "analysis system" to "Global system, (proj none)" in HyperView. However, while doing Frequency Response Analysis, I would like to plot Stress over frequency sigma=graph(freq). It seems not be possible to export these values in a "HG" format to plot it in HyperGraph, so i tried with "Builds Plot" tool directly in HyperView, and it can plot stresses over frequency, but only stresses in "Analysis System" if I'm not wrong. What I would like to do is plotting a graph with stress in Global System coordinate for 2D Shell elements. I tried to associate the global system and even a user defined coordinate as MCID in Card Edit, Elements, but it didn't change anything. Therefore, do you know if it is possible to choose an output option in OptiStruct to plot such a graph, or something that can do it directly in HyperView? Thank you very much
7. ## Missing property error

I looked at your model, 1D elements are defined as CBAR elements, but you give a PBEAML property. Try to use a PBARL property instead, or to use CBEAM elements if you really want to use PBEAML property. But here because cross-section are tube, you can use CBAR elements. To understand the difference between CBAR and CBEAM (which are both "beams" elements from a mechanical point of view, compared to CROD which is only a "bar" i.e only see tensile/compressive) please read https://altairuniversity.com/wp-content/uploads/2017/03/1D_Elements_Extract.pdf Regards, Nathan
8. ## Can we define material property in Tabled Type of Fatigue?

Hi, I had the same problem a few minutes ago. In OptiStruct help it was well explained that you can use TABLEXN to define manually one S-N curve, and that you can also use SNCM continuation line with FATMCRV card to define multi-mean SN curve. I found these options in HyperMesh 2019.1 but not in 2017.3. So, I wonder if there is a way to define one SN curve with that version other than using SRI1/b1 coefficients, in other words if there is a way to define an SN curve manually. Because I am able to create a FATMCRV Load collector, but there isn't any SNCM continuation line to reference FATMCRv or TABLEXN data.
9. ## Frequency Response values FREQi

Ok, thank you for your help!
10. ## Frequency Response values FREQi

Hi Prasanna, Thank you for your answer, I understand. However, I am not sure to get the point (2). PSD in only used at computed FRF values, isn't it? For instance, in my example of PSD input (please see file attached) if I compute FRF at 2, 4 and 6Hz, then PSD response will be computed at 2, 4 and 6Hz, but the peak at 5Hz will be missed because there is no value at 5Hz for the FRF. Am I right?
11. ## Frequency Response values FREQi

Dear Altair community, I just want to be sure about something in FRF FREQi specified values. I know that when you are doing a Frequency Response Analysis, then you are computing the structure's response at the frequency points specified in FREQi cards. But if I use the computed FRF in a Random Response Analysis which basically multiply the square of this FRF by a (PSD) input, then I know that if a computed value of the FRF (let's say for f=10) which is not a specified value in the PSD, then it will be computed either with linear or logarithmic interpolation. But on the contrary, if a PSD point is specified at f=10 whereas f=10 is not a specified value of the FRF specified in the FREQi card, then will it be just ignored? Only specified frequency values in FREQi are used, so if a point on the Input PSD does not line up with a point on the Transfer Function, that Input PSD point is ignored in the Response PSD calculation? Or is it interpolated too? I have found nothing about it in OptiStruct help. Thank you
12. ## Model analysis

Hi, As Farouk said, V1 and V2 are respectively the lower and upper frequency bound. For example, if you write V1=20 and V2=2000, then it will search for Natural Modes frequency only in this range [20,2000] (according to your unit system, generally Hz). ND is to specify the number of roots (so of modal frequencies) you want. Please find the table attached from EIGRL OptiStruct help. If you combine V2 and ND, it will only compute the minimum number of frequencies allowed by either V2 or ND. Regards, Nathan
13. ## Isolate elements/components

Oh yes it seems you're right. Thank you Tinh
14. ## Isolate elements/components

Hello, I am wondering what is the difference between "Isolate" and "Isolate Only" options. For me, it is doing the same so far. Can someone please tell me the difference? Thank you very much
15. ## Fatigue analysis "Life" result

By the way, I asked my question for a time-based input signal. So there is a "time dimension". But in the case where One work with frequency-based input such as a PSD. If the "Life" result after Fatigue analysis is 1 000 000, and because there is not "time dimension", what does that mean in term of "Life time" ? PSD is computed with FFT from a time history source, am I right? So do we need to know the time duration of this time history source to interpret the "Life" result from fatigue analysis ? EDIT : I found for this one, you have to check "Real" box in Fatseq load collector to define the time duration T
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