The general goal of an ABAQUS simulation is to determine the response of the model to the applied loads. In ABAQUS the term load refers to nonzero boundary conditions or applied displacements, as well as point loads or pressure loads. In some cases these loads are relatively simple, such as a single set of point loads on a structure. In other problems the loads applied to a structure can be very complex. For instance, different loads may be applied to different portions of the model in a particular sequence over some period of time, or the magnitude of the loads may vary as a function of time. The term load history is used to refer to such complex loading of a model.

In ABAQUS the user divides the complete load history of the simulation into a number of steps. Each step is a period of “time,” specified by the user, for which ABAQUS calculates the response of the model to a particular set of loads and boundary conditions. The user must specify the type of response, known as the analysis procedure, during each step and may change analysis procedures from step to step. For example, static dead loads, perhaps gravitational loads, could be applied to a structure in one step, and the dynamic response of the loaded structure to earthquake accelerations could be calculated in the next step.

ABAQUS divides all of its analysis procedures into two main groups: linear perturbation and general. ABAQUS makes a very clear distinction between perturbation and general analysis procedures because loading conditions and “time” are defined differently for the two cases. Furthermore, the results from each type of procedure should be interpreted differently.

The response of the model during a general analysis procedure, known as a general step, may be either nonlinear or linear. In a step that uses a perturbation procedure, which is called a perturbation step, the response can only be linear. ABAQUS treats such steps as a linear perturbation about the preloaded, predeformed state created by any previous general steps; therefore, its capability for doing linear simulations is rather more general than that of a purely linear analysis program.