Solvers Overview: Quasi-static

Description

The Quasi-static solver has attributes of both SOLVERS: Nonlinear Static Settings and SOLVERS: Nonlinear Transient Dynamic Settings. Like the Nonlinear Static solver the Quasi-static solver calculates the nonlinear static equilibrium of the system, considering nonlinear behaviour due to geometric, material and contact nonlinearity, whilst dynamic inertia effects are ignored. Unlike the Nonlinear static solver, the Quasi-static solver defines its load steps by way of Factor vs Time tables (LAYOUTS: Tables) assigned to the selected load and freedom cases, just like the Nonlinear Transient Dynamic solver.

In addition, the Quasi-static solver considers nonlinearity due to material creep (a time-dependent nonlinearity, which can also be considered by the Nonlinear Transient Dynamic solver) in addition to geometric, material and contact nonlinearity.

The solution procedure used by the Quasi-static solver is the same as the procedure used by the Nonlinear Static solver, except when creep is considered, in which case additional nonlinear material calculations are undertaken. These relate to the change in material characteristics as a function of stress, strain rate and temperature. Each result case of a quasi-static analysis refers to a time step, rather than a load step, and within each time steps, an iterative procedure is established that also needs to converge on force and displacement criteria.

A particular feature of the Quasi-static solver, when creep is considered, is the so-called Instantaneous result case. To establish the initial stress state, which defines the initial creep rate of the materials that consider creep, the Quasi-static solver will insert an extra result step at time zero, into the results file. This extra result step is basically a nonlinear static analysis based on the initial applied loads, excluding the effects of creep; it becomes the implicit initial conditions. The quasi-static analysis then proceeds from this initial stress state for the additional time steps, considering the effects of creep and other time dependent loads. Since creep is a long term phenomenon, the static stress state will have been established long before creep effects become significant, and this will be represented by the instantaneous results at time zero.

Restart can be used to continue a previously completed or terminated solution. The restart can proceed from any of the previously saved solution steps in a quasi-static analysis or a nonlinear static analysis, provided the restart file has been saved (see SOLVERS: Files and Special Topics: Solution Restart). When the restart is from a nonlinear static analysis, the stress state in the nonlinear static analysis becomes the conditions from which the initial creep rate is calculated, and therefore the additional Instantaneous result case is not executed.