Problems with Fatigue Analysis

Altair Forum User

Hey guys, I’m a mechanical engineer student and I participate in a project where I analysis the structure of a modern tricycle. I need help with the fatigue analysis. I make the training of fatigue by Altair and tried make the analysis, but I’m not sure of the results. I think the problem it’s on the TABFAT.

 

About the TABFAT:

               My analysis has 1D and 2D elements. I obtained the following values in the Linear Static Analysis:

 

	Analysis 1D	Analysis 2D
σ_max	24,59 MPa	267,9 Mpa
σ_min	-19,71 MPa	0 MPa

 

               Can I represent the load cycle of my “vehicle” like a sinusoidal curve? Can you show me how I represent it?

              Should I represent these values for each of the fatigue analysis like one analysis for 1D elements and other analysis for 2D elements? Can I “mix” these values for the two analyses representing σ_max = 24,59 MPa and σ_min = -19,71 MPa?

               I think that a correct method is consider my load cycle like a variable load, but I don’t obtain experimental data.

 

About the MATFAT:

               I acquired the MATFAT data by the eFatigue website, but I don’t obtain the SE (standard error of log(N)). How I find out this? If I consider SE like “Default” (zero), how will affect my analysis?

               

Note: I’m making a SN analysis.

 

Thanks for helping!

 

Altair Forum User

Hi @Thiago Marinho

 

Sorry for a late reply,

 

1. A sinusoidal curve can be represented by a constant amplitude fully reversed loading.

 

This can be done using the TABFAT card, or can also be visualized using Utility Browser >> TABLE Create >> TABFAT,

where time points and load points can be added.

 

Both constant amplitude and a variable amplitude load history can be used (if variable amplitude, rain flow count is made internally  to convert to constant amplitude loads)

 

2. The certainty of survival is based on the scatter on the Stress-life curve, which depends or modifies the SN curve based on SE (Standard error).

When SE is non zero the reliability level can be considered using certainty of survival.

 

3. The elements can be defined in the FATDEF, using the PTYPE entry.

Nachiket Kadu_22143

Altair Forum User said:

Hey guys, I’m a mechanical engineer student and I participate in a project where I analysis the structure of a modern tricycle. I need help with the fatigue analysis. I make the training of fatigue by Altair and tried make the analysis, but I’m not sure of the results. I think the problem it’s on the TABFAT.

 

About the TABFAT:

               My analysis has 1D and 2D elements. I obtained the following values in the Linear Static Analysis:

 

	Analysis 1D	Analysis 2D
σ_max	24,59 MPa	267,9 Mpa
σ_min	-19,71 MPa	0 MPa

 

               Can I represent the load cycle of my “vehicle” like a sinusoidal curve? Can you show me how I represent it?

              Should I represent these values for each of the fatigue analysis like one analysis for 1D elements and other analysis for 2D elements? Can I “mix” these values for the two analyses representing σ_max = 24,59 MPa and σ_min = -19,71 MPa?

               I think that a correct method is consider my load cycle like a variable load, but I don’t obtain experimental data.

 

About the MATFAT:

               I acquired the MATFAT data by the eFatigue website, but I don’t obtain the SE (standard error of log(N)). How I find out this? If I consider SE like “Default” (zero), how will affect my analysis?

               

Note: I’m making a SN analysis.

 

Thanks for helping!

 

Hi 

The stresses which you have mentioned under 2d what kind of stress you are talking about?? U are checking for von-mises or signed von-mises here?? 

Altair Forum User

Nachiket Kadu .

I'm making anything wrong?

Altair Forum User

Prakash Pagadala

Thanks for helping!

Will I have a variable amplitude all right? Then if I represent in TABFAT like this (1.3395;2.679;1.3395;0) am I doing right?

 

Does rainflow happens automatically? I understood that in this analysis the software will check that have a variable amplitude and will convert this automatically? Am I rigth?

 

Nachiket Kadu_22143

Altair Forum User said:

Nachiket Kadu .

I'm making anything wrong?

Linear static is ok! But when you r viewing results in hyperview, evaluating fatigue life using signed vonmises will be a correct approach. You can view signed von-mises for static analysis which will give u correct representation of max and min stresses.

Nachiket Kadu_22143

Altair Forum User said:

Prakash Pagadala

Thanks for helping!

Will I have a variable amplitude all right? Then if I represent in TABFAT like this (1.3395;2.679;1.3395;0) am I doing right?

 

Does rainflow happens automatically? I understood that in this analysis the software will check that have a variable amplitude and will convert this automatically? Am I rigth?

 

Yes Rainflow approach calculates the min and max load automatically in optistruct when there is variable loading.

Altair Forum User

Nachiket Kadu

Yes, I'm using signed von mises when I'm viewing fatigue.

About the Rainflow, the cycle that I represented in the last post is it correct?

 

Thanks for helping bro!!

Nachiket Kadu_22143

Altair Forum User said:

Nachiket Kadu

Yes, I'm using signed von mises when I'm viewing fatigue.

About the Rainflow, the cycle that I represented in the last post is it correct?

 

Thanks for helping bro!!

Rainflow will be applicable for random loading curve where crest and trough for the loading cycle is very difficult to determine, in such circumstances max and min is calculated using cycle counting technique which you can refer for better understanding. In your case if you are using a simple sine function, there max and min is already determined.

Altair Forum User

Nachiket Kadu

I'm sorry, I don't understood very well about the sine function. If I use a simple sine function, how I represent it ?

Altair Forum User

hi guys

I'm having a similar issue with my Fatigue Analysis

Request your assistant to check my setup

 

I have 2 problem

1. Is my Loading Information is correct?

Basically, I wanted to test 1 million cycles of alternating 823N force

Have i defined it correctly

 

2. I try to run it and end up error #6971 as in the picture. I don't have any clue where my mistakes are.

 

Attached is the snapshots and my HM file

 

Kindly assist

Many thanks in advance

 

 

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

 

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

 

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

 

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

 

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

 

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

 

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

 

 

Unable to find an attachment - read this blog

Nachiket Kadu_22143

Altair Forum User said:

Nachiket Kadu

I'm sorry, I don't understood very well about the sine function. If I use a simple sine function, how I represent it ?

Just use the rainflow method. Its not an issue. For more detailed explanation for rainflow you may refer the theory behind it.

Altair Forum User

Hi @yuri6996

 

Looks like PFAT is missing in your model. 

 

Could you please check for the same?

Altair Forum User

@Prakash Pagadala

Thanks for your feedback

 

May i know what is PFAT?

 

I have been trained to use the Fatigue Process Manager to perform the fatigue analysis

Just fill in the neccessary information and run

 

Is the PFAT is a new feature

I cant recall PFAT or FATDEF in HW14

 

Thanks in advance

Altair Forum User

@yuri6996

 

Please see the image for fatigue process in OptiStruct 14.0

 

 

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

Altair Forum User

Dear @Prakash Pagadala

Thanks for your feedback

 

Will try to understand that parameter

 

FYI

I undergo training somewhere in 2013/2014

Cant remember HW version

Maybe 14 or much lower

But I do remember that I didn't train using this parameter

Its noted in my notebook to just use the fatigue Process Manager (FPM)

 

Anyway many thanks for your kind assistant

Altair Forum User

Below is the FATDEF card

May i know which parameter should i enter

 

Thanks in advance

 

Altair Forum User

Hi,

 

Check PTYPE and select the element type (PSHELL or PSOLID). For PID(#) select the relevant property and PFAT for PFATID(#)

Altair Forum User

These 2 mouse clicks is the answer i am looking for this whole week

Analysis completed and result superb

 

Thanks @Prakash Pagadala

Altair Forum User

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Md Ansari

Altair Forum User said:

Hi @Thiago Marinho

 

Sorry for a late reply,

 

1. A sinusoidal curve can be represented by a constant amplitude fully reversed loading.

 

This can be done using the TABFAT card, or can also be visualized using Utility Browser >> TABLE Create >> TABFAT,

where time points and load points can be added.

 

Both constant amplitude and a variable amplitude load history can be used (if variable amplitude, rain flow count is made internally  to convert to constant amplitude loads)

 

2. The certainty of survival is based on the scatter on the Stress-life curve, which depends or modifies the SN curve based on SE (Standard error).

When SE is non zero the reliability level can be considered using certainty of survival.

 

3. The elements can be defined in the FATDEF, using the PTYPE entry.

Hi,

 

I am trying to visualize cyclic fatigue of entire no of cycle in hyperview but  only life and damage i can see. help is appreciated.

 

 

Thanks