3.3.2 Time Stepper parameters

1. The dtmin and dtmax values are used to constrain the range of possible time step values. The opt_iter defines the estimated optimal number of quasi-Newton iterations. If the actual number of iterations is less than or equal to this value the time step size is increased, otherwise it is decreased.
2. When a time step fails (e.g. due to a negative jacobian), FEBio will retry the time step with a smaller time step size. The max_retries parameter determines the maximum number of times a timestep may be retried before FEBio error terminates. The new time step size is determined by the ratio of the previous time step size and max_retries+1. For example, if the last time step size is 0.1 and max_retries is set to 4, then the time step size is adjusted by 0.02: The first retry will have a step size of 0.08; the next will be 0.06, and so on.
3. The user can specify a load curve for the dtmax parameter. This load curve is referred to as the must-point curve and serves two purposes. Firstly, it defines the value of the dtmax parameter as a function of time. This can be useful, for instance, to enforce smaller time steps during rapid loading or larger time steps when approaching steady-state in a transient analysis. Secondly, the time points of the dtmax loadcurve define so-called must-points. A must-point is a time point where FEBio must pass through. This is useful for synchronizing the auto-time stepper with the loading scenario. For instance, when loading starts at time 0.5, adding a must-point at this time will guarantee that the timestepper evaluates the model at that time. In conjunction with the PLOT_MUST_POINT value of the plot_level parameter, this option can also be used to only write results to the plotfile at the specified time points. Consider the following example.

   <dtmax lc="1">0.1</dtmax>
   ...
   <loadcontroller id="1" type="loadcurve">
     <interpolate>STEP</interpolate>
       <points>
         <point>0,0</point>
         <point>0.5, 0.1</point>
         <point>1.0, 0.2</point>
       </points>
   </loadcontroller>
   

   This example defines load curve 1 as the must-point curve. This curve defines three points where FEBio will pass through (namely 0, 0.5 and 1.0). The values of each time point is the value of the maximum time-step size (dtmax). Since the curve is defined as a step-function, each value is valid up to the corresponding time-point. Thus, between time 0 and time 0.5, the maximum time step value is 0.1. Between 0.5 and 1.0 the maximum time step value is 0.2. If the plot_level parameter is set to PLOT_MUST_POINTS, then only the three defined time points will be stored to the plotfile.