# INISTRS

Bulk Data Entry Defines initial stress values and settings for elements in nonlinear analysis.

## Format

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
INISTRS ID ETYPE CIDA
The following continuation line(s) are optional when ETYPE = SHELL. They can be used to define the sections where initial stress values are applied and they need to be given before the continuation lines starting with EORS.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
SECT NSEC SEC1 SEC2 SEC3 etc.
The following continuation lines are used to define initial stress values for an element or a set of elements. They are mandatory and can be repeated as required.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
EORS EID/ESETID CIDB
VALUE STRS1 STRS2 STRS3 etc.

When ETYPE = SHELL and the SECT line is given, each VALUE line defines initial stress for a shell section; and so, for any shell element (with EID) or shell element set (with ESETID), the number of VALUE lines must be NSEC.

## Alternate Format (Initial Stress from H3D file)

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
INISTRS ID ASSIGN SUBID
EORS EID1/ ESETID1 EID2/ ESETID2 EID3/ ESETID3 EID4/ ESETID4 EID5/ ESETID5 EID6/ ESETID6 EID7/ ESETID7
EID8/ ESETID etc. etc.

## Example 1

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
INISTRS 7
ELEM 1001
VALUE 3.5e4 -1.5e3 0.0 3.0e3 0.0 2.0e3
ESET 200
VALUE 3.0e4 -1.5e3 0.0 3.0e3 0.0 2.0e3

## Example 2

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
INISTRS 8 SHELL -1
SECT 2
ELEM 101
VALUE 3.5e4 0.0 0.0
VALUE -3.5e4 0.0 0.0
ELEM 102
VALUE 3.0e4 0.0 0.0
VALUE -3.0e4 0.0 0.0

## Example 3 (Alternate Format – Stress from H3D file)

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
INISTRS 7 19 3
ELEM 1001 2002 3003

## Example 4 (Alternate Format – Stress from H3D file)

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
INISTRS 8 29 6
ESET 2 5 9
ESET 9 19 29

## Definitions

Field Contents SI Unit Example
ID Unique identification number.

No default (Integer > 0)

ETYPE Element type which initial stress is applied.
Blank (Default)
The initial stress is applied for a collection of solid and/or shell elements.
SHELL
Applying initial stress for a collection of shell elements specifically.

CIDA Coordinate system identification number, or stress components flag.
Blank (Default)
The initial stress components are defined in material coordinate system for solids and in element coordinate system for shells.
-2
Material coordinate system
-1
Element coordinate system
0
Basic coordinate system

Integer > 0: user-defined (prescribed) coordinate system.

SECT Keyword indicating the start of continuation line(s) for the shell sections.
NSEC Number of shell sections where initial stress is applied for shell elements.

No default (1 ≤ Integer ≤ 6)

SEC(i) Through-thickness position for the shell sections. SEC(i) must be either all blank or all non-blank, i.e., the number of SEC(i) must be either NSEC or zero; and SEC(i) must be given in ascending order if all non-blank.

Blank implies that the sections are located uniformly through the shell thickness, starting at the bottom surface and ending at the top surface.

A Real value is the multiplier to shell thickness to get the location. -0.5 means the bottom surface of the shell element and 0.5 means the top surface.

The through-thickness positive direction, pointing from the bottom surface to the top surface, is determined by the right-hand rule and the order in which the grid points are listed on the connection entry. It is same as the convention for Z1/Z2 of PSHELL.

<Blank, or -0.5 ≤ Real ≤ 0.5>

Default = Blank

EORS Flag indicating an element, or element SET is selected in the following field.
ELEM (Default)
Element is selected
ESET
Element SET is selected

EID/ESETID Identification number of the element or element SET to which the stress values apply.

No default (Integer > 0)

CIDB Coordinate system identification number or stress components flag.
Blank (Default)
Implies that CIDA is used; -2, -1 or Integer ≥ 0 suppresses CIDA.
-2
Initial stress components are defined in material coordinate system
-1
Element coordinate system
0
Basic coordinate system

Integer > 0: user-defined (prescribed) coordinate system.

VALUE Keyword indicating the start of a continuation line for the stress components.
STRS(i) Stress components 4

No default (Real)

Alternate Format Definitions (Stress from H3D file): 5 - 7
ASSIGN Identification number of ASSIGN, H3DRES. When ASSIGN is specified, SUBID should be specified as well, and “VALUE” continuation line is not allowed.

This field is mandatory for Alternate Format (Stress from H3D file).

No default (Integer > 0)

SUBID Subcase identification number where initial stress is applied from a H3D file.

This field is mandatory for Alternate Format (Stress from H3D file).

No default (Integer > 0)

EORS Flag indicating an element, or element SET is selected in the following field.
ELEM (Default)
Element is selected
ESET
Element SET is selected

EID#/ESETID# Identification number of the element or the element SET to which the stress values apply.

No default (Integer > 0)

1. INISTRS and ISTSADD should not have the same ID.
2. The INISTRS Bulk Data Entry is selected by the Subcase Information command INISTRS.
3. Initial stress is supported for solid elements (CHEXA, CPENTA, CPYRA and CTETRA) and shell elements (CQUAD4, CTRIA3, CQUAD8 and CTRIA6), only.
Initial stress is currently not supported for:
• solid elements with hyperelastic material in LGDISP analysis
• solid elements with user-defined material in LGDISP analysis
4. For initial stress of solid elements, 6 stress components are needed for each VALUE line and they are:
STRS1
$\sigma$ xx
STRS2
$\sigma$ yy
STRS3
$\sigma$ zz
STRS4
$\sigma$ xy
STRS5
$\sigma$ yz
STRS6
$\sigma$ zx
Where, x, y and z are the axes in the coordinate system selected via CIDA and CIDB.
For initial stress in element or material coordinate system of shell elements, 3 stress components are needed for each VALUE line and they are:
STRS1
${\sigma }_{11}$
STRS2
${\sigma }_{22}$
STRS3
${\sigma }_{12}$
Where, 1 and 2 are the first two local directions in the element or material coordinate system.
For initial stress in the basic or a prescribed coordinate system of shell elements, 6 stress components are needed for each VALUE line and they are:
STRS1
$\sigma$ xx
STRS2
$\sigma$ yy
STRS3
$\sigma$ zz
STRS4
$\sigma$ xy
STRS5
$\sigma$ yz
STRS6
$\sigma$ zx
Where, x, y and z are the axes in the basic or prescribed coordinate system.

Alternate Format (Stress from H3D file):

5. VALUE continuation line is not allowed for the Alternate Format when Stress is sourced from a H3D file.
6. For Shell elements, currently only the equivalent of NSEC=2 is supported (Z1 and Z2 layers) to apply the initial stress, as the number of layers where stress data is available can be identified from the H3D file.
7. Currently only H3D files output from OptiStruct and HyperForm are supported.