When running a quasi-static analysis (/IMPL/QSTAT), the positive definite property of stiffness matrix will be reinforced by including the inertia matrix. The extra stiffness is a function of masses, inertia and the time step. Smaller time steps will add more stiffness. In addition, a scaling factor can be defined for the added matrix through the keyword /IMPL/QSTAT/DTSCAL (this factor is inversely proportional to this added matrix just like time step). For nonlinear analysis, this will only modify the convergence speed; whereas for linear analysis, time step (one step) should be chosen carefully (neither too small, as this will change the result significantly; nor too big, as this might lead to a non-positive definite matrix).

This option is quite suitable for a model that consists of parts connected only by contact interfaces and has failed with other analysis types. A linear quasi-static analysis can also be used for model checking of high level (even for explicit analysis). When using a time step that is not too large, results can always be obtained, whether the model is well constrained or not.

As mentioned in Activating an Implicit Analysis, when /IMPL/QSTAT/DTSCAL is used with /IMPL/LINEAR/INTER (two steps), the scaling factor is only applied in the second step. By carefully choosing this scaling factor and the termination time, the correct contact in the first step can be found and minimizes the error (due to added stiffness) on the final step.

A typical example with this methodology is the initial state simulation under gravity of a full car with dummies, in which dummies link the car only by contact. A short stop time is defined, so that a large displacement between the parts does not occur during the contact research on the first step, and a large quasi-static scaling factor is applied for the second step to get the static solution.