How, Why and When to Relax the Convergence Tolerance for a Mechanical Event Simulation

When developing a model for a Mechanical Event Simulation (MES), one approach that can be helpful is to perform a trial analysis with the convergence tolerance relaxed. This type of trial analysis will likely reach completion and run faster than an analysis with a tighter convergence tolerance. A quick trial run can enable you to spot any modeling problems and identify areas of greatest stress concern where the finite element mesh should be refined.

In MES, the displacement tolerance is a global measure of how strictly mechanical equilibrium should be enforced at each time step. The dynamic nature of MES requires that Newton's Second Law be applied throughout the event. Equilibrium iterations are required to enforce observance of this law. Convergence is achieved when the equilibrium residual is below the displacement tolerance value. A displacement tolerance is used because the system of equations solved in MES is in terms of displacements.

Because MES is a numerical method, there is some discretion regarding how strictly equilibrium should be enforced. Very strict adherence requires that the displacement tolerance be set to the computational accuracy of the computer: the default setting of 1e-15 is representative of such a value. Under certain circumstances, the displacement tolerance can be relaxed to values ranging from 1e-3 to 1e-6. Such tolerance values are recommended when the model contains engineering elements such as shells and beams or when performing an initial trial analysis to check overall model behavior.

Here's how you can relax the convergence tolerance:

1. Access the "Analysis Parameters" screen.
2. Click on the "Advanced" button.
3. On the "Equilibrium" tab, change the value of the "Displacement tolerance" field. For example, you might change from the default value of 1e-15 to 1e-4 as shown in Figure 1.

Figure 1: Specifying the "Displacement tolerance" value on the "Equilibrium" tab of the "Analysis Parameters" screen.

Relaxing the "Displacement tolerance" value should enable a fast, convergent solution. With the MES capability to display motion and stresses combined all in one process including contact and impact, you can get an idea of the overall behavior of the model and determine whether you've made any mistakes in setting up the model. This can help to ensure that your model setup is correct for a subsequent analysis with a tighter convergence tolerance, which will give more accurate results.