Epsilon in .f06 file - big value

[Gaetan82]

Hi, i have a problem when trying to run a model in Nastran ( Sol 101) and hope that someone can help me: the epsilon value that i read in the .f06 file is about 1.70E-4; now, i know that this value is big and that, normally, a good value for epsilon is 10E-9 or smaller. I know also that this value is a measure of numerical accuracy and round off error of a run (101) and that a bigger value evidences a ill-conditioning problem (small perturbations in the system can lead to large changes in the solution);

What are possible cause of this problem ? in a blog i read:
1) high difference in stiffness between adjacent elements in the model.
For istance a mistake in the application of properties ( composites) on a panel done manually ? is it possible ?

Can someone suggest me other possible cause for this type of problem ?

Thank you

 

[BlasMolero]

Hello!,
EPSILON is an error measure of the "normalized value of the residual loading". Epsilon values that are greater than 0.001 are flagged for a possible loss of accuracy due to numeric conditioning.
EPSILON is generated for each loading condition. An acceptable value of EPSILON depends on the model complexity and the machine that it runs on. An epsilon value in the neighborhood of less than 10-9 is generally considered acceptable.

Some general causes for singularity can include:

• Degrees of freedom without stiffness because of missing elements.
• A 2-dimensional plate problem with the normal rotation unconstrained.
• A solid model with rotational DOFs at the corners unconstrained.
• Incorrect modeling of offset beams.
• Incorrect multipoint constraints.
• Mechanisms and free bodies, such as sloped plates, beam to plate connections, beam to solid connections, and plate-to-solid connections.
• Low stiffness in rotation.
• A stiff element adjacent to a very flexible element.

If PARAM,AUTOSPC,YES is specified (this is the default in the Structured Solution Sequences, except SOLs 106 and 129), the potential singularities are automatically constrained if possible.
Also if you have error when solving the model you can override this fatal message by inserting “PARAM,BAILOUT,–1" in your input file, but this should be use only to DEBUG the problem, not for final results.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

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