Learning Forum

[Fayçal ABDOUNF]

Hi everybody, 

the goal of my work here it's to do a fatigue analysis on a component in mechanical APDL from a signal data. 

I have a data signal file which is the forces in two axis that the body will see in function of the time (1000seconds).

I have to import it in Mechanical APDL and recuperate the cycles by an Rainflow algorithm. 
After I will create an APDL command to applicate every loads on the body to do a fatigue analysis.

If someone has already done that, I will be very interested about its experiences.

Best regards

[Daniel Shaw]

MAPDL does not have rainflow capabilities.  Ansys nCode DesignLife does, if you have a license for it.  Also, rainflow does not produce a time vs load signal.  It produces bins of cycles (range and mean stress) the produce similar fatigue damages.

[Fayçal ABDOUNF]

Hi thank's for you reply, 

I've a rainflow algorithm that is written in APDL commands. I have to use it into mechanical APDL from my time history to extract the main/maximum stress in function of the cycles as you said above. 

[Daniel Shaw]

Ok.  MAPDL does not have any supported internal fatigue capabilities.  MADPL had a fatigue module that is no longer supported, but it did not use rainflow for cycle counting.  It calculated fatigue damage using a ASME B&PV code approach.  It is probably possible to create a script to do time-based fatigue using rainflow, but I am not aware of any existing MAPDL macros that do it.

[Fayçal ABDOUNF]

Hi everevebody, 

I'm coming back about this subject becase I understand better what I have to do.

So, I've a table of forces, I read it into my mechanical APDL session whith commands 

The first values in columns 2 representating FY (force in Y-axis on a specifical node) and the second values representating FZ. 

So, I create an apdl commands to applicate those combinations FY/FZ forces 100 times (beause I've 100 lines in my array). 

I have to retrieves the SXX stress (Von mises) and save them into an another array trough an Etable like that :

So, I get the SXX vector Array in all elements (here 1122 elements) representating the stress in whole element with 100 combinations of forces Fy,fz.

So now i want to create an array to save all of my stress (it will be an array with 1122 lines and 100 culumns).

This array will be use to calculate the damage in whole 100 calculation to make fatigue calculation in my component. 

So here what I have in my array :

Using those commands :

Can anyone help me to fill correctly my array ? 

Thank you 

[Fayçal ABDOUNF]

Hi, 

I succeeded to fill my Stress array with the following commands :

The table  : 

Now I have all stresses in whole elements for every 100 types of loads. 

Let's calculate the damage..If anyone have an idea to make it by commands ^^

 

Thank's

 

[Daniel Shaw]

There is no simple set of MAPDL commands to perform the next tasks.

11. You need to identify the stress ranges and associated mean stresses (at least the stress ranges that produce fatigue damage).  Typically, rainflow cycle counting is used in fatigue evaluations to establish the stress ranges and associated mean stress, but MAPDL does not provide any rainflow tools.  You would need to create the MAPDL script to perform the rainflow cycle counting.  In lieu of using the rainflow cycle counting method, you could use a min/max stress approach.  You would need to create a MAPDL script that looped through all possible stress events to identify the stress ranges and associated mean stresses that can occur (again at least those that produce fatigue damage).  With either cycle counting approach, you also need to develop a look-up table or an equation to define the SN curve.  You would need to use that table/equation along with the stress ranges and mean stress from the cycle counting for each event to calculate the life for that event (damage is inverse of life).  You could then sum each individual event damage (Miner’s rule) to calculate the total damage. 

2.       Even if you can accomplish task#1, the results may not be accurate because you are using Seqv to determine the fatigue damage.  Seqv is a scalar quantity.  It lacks a direction.  Using Seqv is not valid, if the direction of the principal stresses is not consistent (Non-Proportional) between the loadings.  You need to either confirm that the direction of the principal stresses is consistent, or you need to invoke the critical plane approach to assess the fatigue damage.  To use critical plane, you need to extract the component stress and perform multiple fatigue calculations using those component stresses at each node/element to determine which orientation produces the most damage (i.e., critical plane).  Again, MAPDL does not have any tools for confirming the applicablity of using Seqv or for applying the critical plane method.