, STABILIZE=dissipated energy fraction Abaqus/CAE module: Create Step: General: any valid step type: Basic: select Specify dissipated energy fraction from the Automatic stabilization field Considerations when the first increment is unstable or singular There are cases where the first increment is either unstable or singular (due to a rigid body mode). In such cases it is not possible to obtain a solution to the first increment without applying some damping. Therefore, some damping is already applied during the first increment. The damping factor used for the initial increment is chosen such that the average element damping matrix component, divided by the step time, is equal to the average element stiffness matrix component multiplied by the dissipated energy fraction. If the calculated strain energy change in this increment indicates that the solution without damping is stable, the damping factor is recalculated based upon the energy method described previously. However, if the strain energy change indicates that the solution is unstable or singular, the initially calculated damping factor is maintained, and a warning message is issued indicating that the amount of damping applied may not be appropriate. In many cases the amount of damping may actually be rather large, which can affect the solution in ways that are not desirable. Therefore, if the above mentioned warning message is issued, check the viscous forces (VF) and compare them with the expected nodal forces to make sure that the viscous forces do not dominate the solution. If necessary, follow the stabilized step with another step in which stabilization is not used or with a step in which a much smaller damping factor is used. Directly specifying the damping factor You can also specify the damping factor directly. Unfortunately, it is generally difficult to make a reasonable estimate for the damping factor unless a value is known from the output of previous runs; the damping factor depends not only on the amount of damping but also on mesh size and material behavior. Input File any of the following options to specify the
Unfortunately, the damping factor is problem dependent; therefore, you must rely on experience from previous runs. Compare the viscous forces (VF) with the overall forces in the analysis, and ensure that the viscous forces are relatively small compared with the overall forces in the model. Compare the viscous damping energy (ALLSD) with the total strain energy (ALLIE), and ensure that the ratio does not exceed the dissipated energy fraction or any reasonable amount. The viscous damping energy may be large if the structure undergoes a large amount of motion. The automated procedure of computing damping factors well for many applications. However, there are cases where the computed damping factor is either too small, thus not controlling the instability, or too high, thus leading to inaccurate results. These problems are more likely to occur when using a constant damping factor—the damping factor is computed in the first increment, which may not be representative of behavior in the rest of the step. For example, consider a sequentially coupled thermal-stress analysis in which a mechanical analysis reads temperatures from a previous transient thermal analysis. Typically the thermal analysis exhibits a diffusive process, where rapid changes in temperature occurs early in the analysis and minor changes in temperature occur once steady state is reached. In such a case Abaqus will compute the extrapolated strain energy based on the temperatures corresponding to the time of the first increment (in this case there may be a significant change in temperature for the first increment), thus yielding a larger then expected extrapolated strain energy. This in turn leads to a damping factor that is too large, resulting in inaccurate results. If one of the automatic stabilization methods is not working appropriately, you can try using the other automatic stabilization method; the adaptive stabilization scheme is generally preferred. Alternatively, you can try directly specifying the damping factor.