Base Element class

This class is the base class for all FE elements. It is also the abstract class, declaring some services, which have to be implemented by the derived classes. The main purpose of this class is to serve the basic common services, which are common to all finite elements (like storing the references to element's material model, nodes and applied loadings, assembling location array, storing from/to context file, etc.). The Element class does neither declare nor implement any method related to specific analysis. These services are to be declared and possibly implemented by derived classes.³ These derived classes (direct children of Element class) are therefore assumed to be base classes for particular analysis or problem type, represented by corresponding engng model. They typically declare the general services required for a specific analysis purpose - like evaluation of stiffness or mass matrices for structural analysis, or evaluation of capacity and conductivity matrices for heat transfer analysis. Usually they also provide general implementation of these services.

The Element is derived from a FEMComponent. It inherits the FEMComponent's ability to keep its number and reference to the domain, it belongs to, its error and warning reporting services, methods for field extraction functions from object record (used when element reads its description from input database). Also the FEMComponent declares several abstract services. The most important are initializeFrom for object initialization from a given record, saveContext and restoreContext methods for storing and restoring object state to/from a stream, and giveInterface service for requesting an object interface.

The attributes declared at the Element class include the variables used to keep its list of nodes and sides, its material and cross section number, lists of applied body and boundary loads, list of integration rules and array storing its code numbers (see OOFEM library reference manualfor details).

The following important services are declared at the element level:

• services for component management - include services for requesting element's geometrical characteristics (giveDofManager, giveNode, giveNumberOfDofManagers, giveNumberOfNodes), material (giveMaterial), cross section (giveCrossSection).

• services related to code numbers management:
  giveLocationArray - returns the element location array. This location array is obtained by appending code-numbers of element nodes (according to the node numbering), followed by code numbers of element sides. The ordering of DOFs for the particular node/side is specified using a node/side DOF mask, which is obtained using giveNodeDofIDMask or giveSideDofIDMask services. Please note, that this local DOF ordering must be taken into account when assembling various local characteristic vectors and matrices. Once the element location array is assembled, it is cached and reused to avoid time consuming assembly. Some engineering models may support dynamic changes of the static system (generally, of boundary conditions) during analysis, then these models use invalidateLocationArray function to invalidate location array after finishing time step, to force new equation numbering.
  computeNumberOfDofs - computes or simply returns total number of element's local DOFs. Must be implemented by particular element. The giveDofManDofIDMask service return DOF mask for corresponding dof manager (node or side). This mask defines the DOFs which are used by element at the given node/side. The mask influences the code number ordering for the particular node. Code numbers are ordered according to the node order and DOFs belonging to the particular node are ordered according to this mask. If element requests DOFs using a node mask which are not in the node then error is generated. This masking allows node to be shared by different elements with different DOFs in the same node/side. Element's local code numbers are extracted from the node/side using this mask. These services must be implemented (overloaded) by particular element implementations.
• services for requesting the so-called characteristic components. (giveCharacteristicMatrix and giveCharacteristicVector). The component requested is identified by parameter of type ``CharType'' (see cltypes.h). These are general methods for obtaining various element contributions to the global problem. These member functions have to be overloaded by derived analysis-specific classes in order to invoke the proper method according to the type of requested component.
• services related to the solution step update and termination.
  These services are used to update the internal variables at element's integration points prior to reached state (updateYourself and updateInternalState). Similar service for the internal state initialization is also declared (see initializeYourself) and re-initialization to previous equilibrium state (see initForNewStep).
• services for accessing local element's unknowns from corresponding DOFs. These include methods for requesting local element vector of unknowns (computeVectorOf) and local element vector of prescribed unknowns (computeVectorOfPrescribed).
• Services for handling transformations between element local coordinate system and coordinate system used in nodes (possibly different from global coordinate system).
• miscellaneous services. Their detailed description can be found in OOFEM library reference manual(see also ``element.h'' (declaration) and ``element.C'' (implementation) files).