fatigue analysis load time table problems (tabfat)

Hi,

Which version of HyperWorks are you using?

Can you update to latest and check if that helps?

Hi,

 

Which version of HyperWorks are you using?

 

Can you update to latest and check if that helps?

Hi, I am using Hyperworks 2017, which is the latest version. I don't think it is a version problem. If I import less cycles , for example, 2 cycles (18 data), it can work. It is so weird. But if I import 60 cycles (540 data), the same error pops up:

*** INTERNAL PROGRAMMING ERROR ***
   in file 'fatdmgapp.F', at location # 1808.

Can you help me with it? 

Best,

Hi @Vin566

Can  you please share the model files?

Hi @Vin566

Please kindly notice that you can not define a time-dependent force with TABFAT if your time history is complicated.

With TABFAT, you can only import Loads because TABFAT is used for static mode.

If your time history is complicated, I suggest you use the Fatigue-transient analysis. 

For the FATLOAD load collector, instead of using a Linear static for LCID field, please choose a Transient analysis loadstep.

This means you have to perform a transient analysis first.

if you don't know about Transient analysis, I recommend you to read following tutorial: OS-1310: Direct Transient Dynamic Analysis of a Bracket

only Transient Analysis allows you to create time-dependent loads with complicated time history

Hi @Vin566

 

Can  you please share the model files?

 

Sure. I attached a very simple testing file, and the corresponding .csv file. Once I run that .fem file, the error I mentioned above will pop up. I hope you can help me solve this problem. Thank you. 

Unable to find an attachment - read this blog

Hi @Vin566

I am able to run the model by adding few more data lines. I will check with developers why is this happening. 

<?xml version="1.0" encoding="UTF-8"?>

Hi @Vin566

I also have a look in your .csv file too and I have to say, your data is not really appropriate even through it true theoretically. I think you may be having some misunderstanding about Fatigue Analysis in OptiStruct.

Your data, as you said, you have 2000 cycles. This is the first problem. You should not import all cycles to TABFAT card, only one cycle is enough. the OptiStruct will do its job is predicting the life (the number of the cycles) of your structure. Eg: I am having structure and I do a fatigue analysis with 1 cycle in TABFAT card, the life result I get is 2000. It means after 2000 time of cycle, my structure will be failed. I do the fatigue analysis again, this time I import two cycles to TABFAT card, the life I get will be 1000. Nothing changes at all. the number of cycles is a result, not a data need to be imported.

The second things, please try to avoid zero value in your data. Too many zero values, sometimes, can cause some problem in programing.

Hope this help

Your data, as you said, you have 2000 cycles. This is the first problem. You should not import all cycles to TABFAT card, only one cycle is enough. the OptiStruct will do its job is predicting the life (the number of the cycles) of your structure. Eg: I am having structure and I do a fatigue analysis with 1 cycle in TABFAT card, the life result I get is 2000. It means after 2000 time of cycle, my structure will be failed. I do the fatigue analysis again, this time I import two cycles to TABFAT card, the life I get will be 1000. Nothing changes at all. the number of cycles is a result, not a data need to be imported.

Agree...

Hi @Vin566

I also have a look in your .csv file too and I have to say, your data is not really appropriate even through it true theoretically. I think you may be having some misunderstanding about Fatigue Analysis in OptiStruct.

 

Your data, as you said, you have 2000 cycles. This is the first problem. You should not import all cycles to TABFAT card, only one cycle is enough. the OptiStruct will do its job is predicting the life (the number of the cycles) of your structure. Eg: I am having structure and I do a fatigue analysis with 1 cycle in TABFAT card, the life result I get is 2000. It means after 2000 time of cycle, my structure will be failed. I do the fatigue analysis again, this time I import two cycles to TABFAT card, the life I get will be 1000. Nothing changes at all. the number of cycles is a result, not a data need to be imported.

 

The second things, please try to avoid zero value in your data. Too many zero values, sometimes, can cause some problem in programing.

 

Hope this help

 

Thank you for your information. I do misunderstand it. I thought I need to import all cycles. Now I know 1 cycle is enough.

But there is another question. From the help file, it says the fatigue analysis only cares about the highest peak and lowest valley values. For me, it seems that for an input in terms of sine function, in one cycle, I only need to specify 2 data points: peak and valley. The remaining are useless, even if I put them into the table. Do I understand it correctly?

Hi @Vin566

You really have interesting question .Yes, for an input in terms of sin function, you don't need to puts all point of a cycle to table. However, that doesn't mean you only need two data points. If you put only two data points to the table, OptiStruct will understand your data as a linear function which is not true. Your data should contain all peak, valley values and some points around them in order to maintain the form of the Load time history curve. 

for me, I hardly delete any data in my cycle because that is the data I got from another analysis and I can not sure which is necessary, which not. So I put them all and OptiStruct will decide it for me. It calls: ' Load time history Compression'.

 Hope this help!

@Vin566

Never tried but I think yes, the determined peaks and valleys of the stress/strain during cycling are reordered to start from absolute maximum.

At the end you need Sa, Mean stress, etc... which can be obtained from peaks and valleys, I guess. 

Hi @Vin566

Please kindly notice that you can not define a time-dependent force with TABFAT if your time history is complicated.

With TABFAT, you can only import Loads because TABFAT is used for static mode.

If your time history is complicated, I suggest you use the Fatigue-transient analysis. 

For the FATLOAD load collector, instead of using a Linear static for LCID field, please choose a Transient analysis loadstep.

This means you have to perform a transient analysis first.

if you don't know about Transient analysis, I recommend you to read following tutorial: OS-1310: Direct Transient Dynamic Analysis of a Bracket

only Transient Analysis allows you to create time-dependent loads with complicated time history

 

Hi, thank you for your answer. I'm doing the fatigue analysis. The results are based on the static analysis. Do you think by using transient analysis the accuracy of the results could be improved? I think the fatigue life depends on how the periodic loading is applied. As long as it is one period, the time may not be important. Please correct me if my understanding is wrong. 

Hi @Vin566

 

I am able to run the model by adding few more data lines. I will check with developers why is this happening. 

<?xml version="1.0" encoding="UTF-8"?>

 

Hi, have the developers sent you any feedback about this strange bug? 

Hi,

They are looking into it... Will update as soon as I get some feedback. 

Hi @Vin566

Please kindly notice that you can not define a time-dependent force with TABFAT if your time history is complicated.

With TABFAT, you can only import Loads because TABFAT is used for static mode.

If your time history is complicated, I suggest you use the Fatigue-transient analysis. 

For the FATLOAD load collector, instead of using a Linear static for LCID field, please choose a Transient analysis loadstep.

This means you have to perform a transient analysis first.

if you don't know about Transient analysis, I recommend you to read following tutorial: OS-1310: Direct Transient Dynamic Analysis of a Bracket

only Transient Analysis allows you to create time-dependent loads with complicated time history

Hi,

I am doing a fatigue run and I have set up transient run as first loadstep for input to the fatigue analysis. In FATLOAD collector I am leaving the TID blank and reference of transient loadstep is provided in LCID.

I am getting an error that TID is not referenced. Kindly let me know about it. As per the help, it shows TID needs to be left blank if transient subcase is used.

How to perform fatigue analysis for the result I get in explicit radioss solver and what material property is required for the same?

How to perform fatigue analysis for the result I get in explicit radioss solver and what material property is required for the same?

Hi,

What results you have from RADIOSS?

von_mises stress in with time varying load.

Hi @gopal_rathore, @Prakash Pagadala, @Toan Nguyen

Any updates, how to resolve the error ?

 

Hi,

I am doing a fatigue run and I have set up transient run as first loadstep for input to the fatigue analysis. In FATLOAD collector I am leaving the TID blank and reference of transient loadstep is provided in LCID.

I am getting an error that TID is not referenced. Kindly let me know about it. As per the help, it shows TID needs to be left blank if transient subcase is used.

  Hi @Karthic Can you please share details about the problem?

Please update to 2017.2.2 and check if you are able to run the fatigue analysis with transient subcase referenced in FATLOAD

Thank you @Prakash Pagadala, Issue solved /emoticons/default_smile.png' srcset='/emoticons/smile@2x.png 2x' title=':)' width='20' />

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Hi @Vin566

 

You really have interesting question .Yes, for an input in terms of sin function, you don't need to puts all point of a cycle to table. However, that doesn't mean you only need two data points. If you put only two data points to the table, OptiStruct will understand your data as a linear function which is not true. Your data should contain all peak, valley values and some points around them in order to maintain the form of the Load time history curve. 

for me, I hardly delete any data in my cycle because that is the data I got from another analysis and I can not sure which is necessary, which not. So I put them all and OptiStruct will decide it for me. It calls: ' Load time history Compression'.

 Hope this help!

Hi Toan Nguyen, 

I am a new member on forum. Coud you kindly tell me what analysis type can export the data of load time history as you mentioned above ? . Kindly reply me soon, i am very need to know how to define load time history table data ? 

Thank you ! 

Hi,

In my case, I used to do a lot of MBD simulation. MBD simulation could give you a time-dependent load which you could apply to OptiStruct to perform Transient Analysis and then Fatigue analysis.

To define load time history in OptiStruct, kindly refer to optiStruct tutorials about Transient Analysis and Fatigue analysis