Cycle Skipping with ABAQUS

A special mode of operation for Z-mat with ABAQUS has been developed in order to accelerate cyclic computations in low cycle fatigue applications. Typically in low cycle and thermo mechanical fatigue analysis there is significant stress relaxation around notches due to plastic accomadation during the early loading of a component. It is of interest to predict what the stabalized stress-strain cycle is at these critical locations in order to best apply LCF, creep, and other damage computations to predict life. This cycle skipping technique can therefore greatly improve the CPU utilization for these costly fatigue analysis for real components.

Input structure

The Z-mat cycle skip allows a number of user configurable parameters concerning accuracy controls. The interface is quite general allowing the full use of ABAQUS features in the loading steps.





The material file used follows. 
***material
  *integration theta_method_a 1.0 1.e-9 1500
  *initialize_variable
     epcum  0.43
     X11   7.529e-03
     X22  -1.065e-02
     X33   3.118e-03
***automatic_time
   *limit eto22 2.0e-3
   *security   1.3
   *divergence 2.0
***behavior gen_evp
 **elasticity isotropic
   young 2.1e5
   poisson 0.3
 **potential gen_evp ep
  *flow plasticity
  *criterion mises
  *isotropic nonlinear
    R0 200.
    Q  2000.0
    b  0.26
  *kinematic nonlinear X
    C  25500.0
    D  81.
***return



Results

Some example results for a component are shown below. The red shows a computation with all cycles computed, while the green utilizes the cycle skip.

Two dimensional benchmark accuracy results.



Three dimensional benchmark accuracy results.



Post computation of fatigue

The following is a benchmark problem which was used in the testing and validation of the cycle skip method. This calculation was done using ABAQUS with Z-mat running in cycle skip mode. The model has more than 44k DOFs, and was run for 30 loading cycles. more than 40% of the cycles were skipped, with essentially no loss in accuracy. The results were loaded into Zmaster to perform post computations of the fatigue life using a general creep-fatigue interaction LCF law. The contour plot was thus created using Zmaster from the post generated results. Note that blue areas indicate low fatigue life.

NW Numerics Home | About this Site | Site Map | Recent Changes

Northwest Numerics and Modeling, Inc.
150 Nickerson Street, Suite 102
Seattle WA 98109
 Phone (206) 352-8030
Fax (206) 352-8035
info@nwnumerics.com