Non linearanalysis settings

[mathias_peron]

Hi everyone,

As I'm trying to run a non linear analysis with robot, I would like to have some precisions about the settings window.

(I am sorry if some questions may seem stupid but I'm new in the world of non linear analysis)

1 - What's the difference between P-delta and large displacement?

2 - Incremental and Arc-Length methods are different way to reach a solution. How do we decide which method to use? I have always heard that the arc-length method was the most powerful but is it usable in all cases?

3 - Incremental method's parameters I don't understand:

- incremental length reduction number

- incremental length reduction factor

- maximum number of line searches

- control parameter for line-search method

- maximum number of BFGS corrections

3 - Arc-length:

What represent the maximum displacement for the selected degree of freedom? Is it a parameter to stop the analysis?

Any idea about the 'Stiff' checkbox?

Thank you in advance,

Have a nice weekend,

Mathias.

[Artur.Kosakowski · Accepted solution]

1 - What's the difference between P-delta and large displacement?

P-Delta: 2nd order effects responsible for instance for increased lateral displacements and additional moments when vertical downwards load coexists in frame with lateral load
Large displacements: 3rd order effects responsible for instance for occurrence of tension in slender simply supported horizontal beam (with at least one internal node) under vertical load.

2 - Incremental and Arc-Length methods are different way to reach a solution. How do we decide which method to use? I have always heard that the arc-length method was the most powerful but is it usable in all cases?

For the prior the analysis is governed by (increase of) load whereas for the later it is defined by deformation. You may need to use the later in the situation as below:

3 - Incremental method's parameters I don't understand:

- incremental length reduction number

When there is lack of convergence for the current increment of load (by default 20% of the total value) the program will reduce the increment 3 times trying to reach convergence before it displays lack of convergence warning

- incremental length reduction factor

The reduction of load  for the above point (by default 0.5 so 10%, 5% 2.5%)

- maximum number of line searches

It influences the speed of convergence but setting too large value may have the result in having slower convergence 

- control parameter for line-search method

Control parameter for line-search method is what is described as the alpha_k in the article below:

http://en.wikipedia.org/wiki/Broyden%E2%80%93Fletcher%E2%80%93Goldfarb%E2%80%93Shanno_algorithm

- maximum number of BFGS corrections

k index from the above article

3 - Arc-length:

What represent the maximum displacement for the selected degree of freedom? Is it a parameter to stop the analysis?

Correct.

Any idea about the 'Stiff' checkbox?

I don't know how it works.

See also:

http://help.autodesk.com/cloudhelp/2017/ENU/RSAPRO-UsersGuide/files/GUID-B240FC5F-1D69-43F5-89F7-7D7...

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