Difficulties can arise, especially in simulations involving contact, complicated material models and geometrically unstable behavior. Using , the structure's configuration is updated to. We offer regularly scheduled public seminars as well as training courses at customer sites. Who should attend This course is recommended for users with experience in Abaqus who wish to become more proficient with reaching converged solutions. Alternatively, you can specify the dissipated energy fraction for automatic stabilization directly. In most applications the first increment of the step is stable without the need to apply damping. You will learn how to develop models that will converge.
This process is continued until a solution is found. The solution is accepted without any check on the size of the displacement correction. If the instability manifests itself in a global load-displacement response with a negative stiffness, the problem can be treated as a buckling or collapse problem as described in. If is less than the current tolerance value, P and are considered to be in equilibrium and is a valid equilibrium configuration for the structure under the applied load. Automatic stabilization of static problems Nonlinear static problems can be unstable. The mechanism is triggered by including automatic stabilization in any nonlinear quasi-static procedure. Instead, the solution is found by applying the specified loads gradually and incrementally working toward the final solution.
The damping factor is then determined in such a way that the extrapolated dissipated energy for the step is a small fraction of the extrapolated strain energy. Thus, the amount of output data available from a nonlinear simulation is many times that available from a linear analysis of the same geometry. However, if the instability is localized, there will be a local transfer of strain energy from one part of the model to neighboring parts, and global solution methods may not work. In nonlinear analyses each step is broken into increments so that the nonlinear solution path can be followed. Many years of practical experience in understanding and resolving convergence issues have been condensed into this course. Using the dissipated energy fraction It is assumed that the problem is stable at the beginning of the step and that instabilities may develop in the course of the step. Generally it is to your advantage to provide a reasonable initial increment size see , for an example ; only in very mildly nonlinear problems can all of the loads in a step be applied in a single increment.
Again, the largest force residual at any degree of freedom, , is compared against the force residual tolerance, and the displacement correction for the second iteration, , is compared to the increment of displacement,. Details of the automatic load incrementation scheme are given in the Job Diagnostics dialog box, as shown in more detail in. You define the steps, which generally consist of an analysis procedure, loading, and output requests. If the increment converges in fewer than 5 iterations, this indicates that the solution is being found fairly easily. Many years of practical experience in understanding and resolving convergence issues have been condensed into this course. Using the damping factor There are cases where the first increment is either unstable or singular due to a rigid body mode. The explicit central-difference operator satisfies the dynamic equilibrium equations at the beginning of the increment, ; the accelerations calculated at time are used to advance the velocity solution to time and the displacement solution to time.
On-site courses can be customized to focus on topics of particular interest to the customer, based on the customer's prior specification. Unfortunately, it is quite difficult to make a reasonable estimate for the damping factor, unless a value is known from the output of previous runs; the damping factor includes information not only about the amount of damping but also about mesh size and material behavior. The stable time increment is discussed in. 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. Difficulties can arise, especially in simulations involving contact, complicated material models and geometrically unstable behavior.
Many years of practical experience in understanding and resolving convergence issues have been condensed into this course. This class of problems has to be solved either dynamically or with the aid of artificial damping; for example, by using dashpots. With the default incrementation control, the procedure works as follows. Alternatively, you will find an unsubscribe link in each newsletter to revoke your subscription at any time. You can change this, and all other such tolerances, by specifying solution controls see. After this training you will be able to recognize when a problem is too difficult or too ill-posed to be solved effectively.
Or call us at 044 43443000. Registration: For more details and registration write to us at simulia. Because the explicit method is conditionally stable, there is a stability limit for the time increment. Different loads, boundary conditions, analysis procedures, and output requests can be used in each step. 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. The course is divided into lectures, demonstrations and workshops.
In nonlinear analyses the total load applied in a step is broken into smaller increments so that the nonlinear solution path can be followed. Register a Free 1 month Trial Account. 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. Course Objective Obtaining converged solutions for highly nonlinear simulations can sometimes be challenging. Instead, the solution is found by specifying the loading as a function of time and incrementing time to obtain the nonlinear response.
Any case that passes such a stringent comparison of the largest force residual with the time-averaged force is considered linear and does not require further iteration. By default, this tolerance value is set to 0. At the end of each increment the structure is in approximate equilibrium and results are available for writing to the restart, data, results, or output database files. Again, the largest force residual at any degree of freedom, , is compared against the force residual tolerance, and the displacement correction for the second iteration, , is compared to the increment of displacement,. For example: Step 1: Hold a plate between rigid jaws. In a nonlinear analysis the solution cannot be calculated by solving a single system of equations, as would be done in a linear problem.