13.7. G. User-Defined Model Initialization

Multiple methods exist to define the initial state of an analysis model. When initializing the state of a new Sierra/SManalysis from a previous analysis, restart is the recommended method (see Section 9.9). However, in some cases a restart initialization is not possible; for instance, if the initial stress/strain state of the model is coming from an external source such as another code, experimental data, or an analytic solution. Another example is when new blocks are added to an existing analysis and need initial stress/strain/material state that is defined in a secondary analysis.

The methodology of manually initializing a model is sometimes called “poor man’s restart.” Generally many node and element fields need to be set to give a fully consistent initial state. The required field set is different for different element types as shown in Table 13.3.

Table 13.3 Required Variables For Consistent Model Initialization

Element	Nodes	Formulation	Element Variables	Nodal Variables
Beam	2	Rod	beam_tshear_rs, beam_tshear_rt, beam_t_axis, cross_sectional_area, left_stretch, unrotated_stress	acceleration, displacement, velocity, rotational_acceleration, rotational_displacement, rotational_velocity
		Cylindrical Joint	cylj_rst_theta1, cylj_rst_theta2, hg_resistance, left_stretch, unrotated_stress	displacement, velocity
Shell	3	Orig_Tri_Shell, C0_Tri_Shell	element_thickness, unrotated_stress	acceleration, displacement, velocity, rotational_acceleration, rotational_displacement, rotational_velocity
	4	BL_Shell (Integration rule = Trapezoid)	membranehourglass, shearhourglass, unrotated_stress	acceleration, displacement, velocity, rotational_acceleration, rotational_displacement, rotational_velocity
		BT_Shell (Integration rule = Trapezoid)	hg_resistance_q4, unrotated_stress	acceleration, displacement, velocity, rotational_acceleration, rotational_displacement, rotational_velocity
Hex	8	Uniform Gradient	rotation, left_stretch, hg_resistance, unrotated_stress	displacement, acceleration, velocity
		Fully Integrated	unrotated_stress	displacement, velocity
Tet	4	Uniform Gradient	left_stretch, unrotated_stress	displacement, acceleration, velocity
		Fully Integrated	left_stretch, unrotated_stress	displacement, acceleration, velocity
	10	Composite_Tet	left_stretch, rotation, unrotated_stress	displacement, velocity
Particle	1	SPH	calc_dens, element_density, element_mass, left_stretch, rotation, total_stress, unrotated_stress	coordinates, acceleration, displacement, velocity, rotational_acceleration, rotational_displacement, rotational_velocity, force_internal

Material state needs to be initialized in a manner that is consistent with other element kinematic quantities, as well. Several elastic materials have no state data, but most materials have a state data array at each element. The name of the material state data field is usually defined by lame_state_[model_name], where model_name is the name of the material model. For example the name of the state field for the viscoplastic material is lame_state_viscoplastic and the name of the state field for the soil foam model is lame_state_soil_foam.

Material state variables also often have aliases, which causes some confusion when using them for restart and initialization purposes. When writing out material state in a preload analysis for initialization in a later analysis output, the following syntax is strongly recommended:

BEGIN RESULTS OUTPUT
  ...
  ELEMENT VARIABLES = lame_state_viscoplastic(:) as \#
    state_viscoplastic
  ...
END

The “(:)” operator will prevent the use of material state alias names; the material state will be output as a single contiguous field which can then be read back in a single operation. An initialization block like the following should then be used to read back in the material state:

BEGIN INITIAL CONDITION
  INITIALIZE VARIABLE NAME = lame_state_viscoplastic(:)
  VARIABLE TYPE = ELEMENT
  READ VARIABLE = state_viscoplastic
END

In addition to element- and material-specific field data, some boundary-specific conditions may have additional initialization requirements.