Modify the control parameters to work best with Helius PFA.
It is a widely accepted notion that good convergence (or any convergence at all) is difficult to achieve in a progressive failure simulation of a composite structure when using an implicit finite element solver. In fact, many progressive failure simulations terminate early, not due to global structural failure, but simply due to the inability of the finite element code to obtain a converged solution at a particular load step. In light of this problem, Helius PFA has been optimized to improve the overall convergence rate and robustness of progressive failure simulations of composite structures. However, in order to take full advantage of its superior convergence characteristics, you must change some of the default settings that govern the nonlinear solution process used by Abaqus/Standard. These changes can be enacted using the *CONTROLS keyword statement. Note that these solution controls do not apply to Abaqus/Explicit analyses.
In Abaqus/Standard, the default settings for the nonlinear solution process are based on the fundamental assumption of the Newton-Raphson algorithm. This states that the nonlinear response of the composite structure is sufficiently smooth at both the global and local levels. However, in a progressive failure simulation of a composite structure, the nonlinear response of the composite structure is not smooth, especially at the local level where material failure results in an instantaneous reduction of material moduli. This non-smooth material response is one of the primary factors responsible for the difficulty in obtaining convergence in progressive failure simulations. Helius PFA's method of managing material nonlinearity is specifically designed to handle this localized non-smooth material response. However, the default settings of Abaqus' convergence control parameters must be changed in order to allow Helius PFA to improve the convergence characteristics of the finite element simulation. These default settings can be changed via the first data line of the *CONTROLS keyword statement. The *CONTROLS keyword statement should be placed in the input file immediately after the *STATIC keyword statement. The first data line in the *CONTROLS keyword statement contains 11 quantities. The default values of these 11 quantities are shown in the *CONTROLS keyword statement below.
*CONTROLS, PARAMETERS=TIME INCREMENTATION 4,8,9,16,10,4,12,5,5,3,50
Qualitatively speaking, the changes that should be made to these default values are intended to significantly increase the number of equilibrium iterations that Abaqus/Standard will perform before the code evaluates the need for a reduction (or cut-back) in the time increment size. If Helius PFA is used in the finite element solution, then the Abaqus input file should use the following *CONTROLS keyword statement.
*CONTROLS, PARAMETERS=TIME INCREMENTATION 1000,1000,1000,1000,1000, , , , ,10,1000
Note that the value of the quantities 1, 2, 3, 4, 5 and 11 have been set to 1000, while the value of the 10th quantity has been set to 10 . For all other quantities on the data line, the default values are acceptable. These changes force Abaqus to wait until 1000 equilibrium iterations have been completed before evaluating the need to reduce the time increment size.
The familiar *STATIC keyword statement is present in the Abaqus input file for all quasi-static analyses. The single data line used by the *STATIC statement contains four quantities that collectively specify the desired time incrementation scheme. The first quantity specifies the size of the initial time increment. The second quantity specifies the total amount of time to be analyzed in the current step. The third quantity specifies the minimum allowable size of the time increments used in the current step. The fourth quantity specifies the maximum allowable size of the time increments used in the current step. Since the use of Helius PFA significantly improves the ability of Abaqus to obtain a converged solution for any particular time increment, it is likely that the entire analysis can be performed without any time increment reductions; therefore, you may wish to experiment with the parameters that you routinely employ in the data line of the *STATIC keyword statement.
Contact and Erroneous Failure
When contact is included in a Helius PFA simulation, the above set of solution controls (*CONTROLS) may produce erroneous failure predictions when the contact is being initially established. As such, it is recommended that the initial contact is established in a separate step that does not include the above solution controls.