Learning Forum

[AbelBerhanemeskel]

Hello Everyone,

I was simulating a simple beam under a cyclic displacement and axial load with plasticity using shell elements the following note appeared in the solver output

Predictor is ON by default for structural elements with rotational     

 degrees of freedom. Use the PRED,OFF command to turn the predictor    

 OFF if it adversely affects the convergence.

what does this mean for the model and how do I turn off the predictor in Ansys mechanical if it is necessary? My model end moments doesn't equalize even though the beam is fixed at both ends.

[Govindan Nagappan]

@AbelBerhanemeskel
For details on predictor, check theory reference: 14.10. Newton-Raphson Procedure (ansys.com)


Predictor-corrector algorithm is used in Newton-Raphson Procedure. Usually displacement at current step U(n) is calculated based on the previous step U(n-1). However, in predictor-corrector algorithm the predictor option extrapolates the DOF solution by adding a displacement increment, so U(n) is calculated based on U(n-1)+beta * deltaUn (see equation 14-162 in doc). Predictor is used to help convergence, but it is possible for predictor may worsen convergence in some cases. Predictor won't affect results accuracy and it is just to accelerate convergence.
To turn off predictor, right click on analysis branch (Static Structural) and Insert "Command object"
Type the command pred,off
Example: