Database_cross_section
Yolanda Diego
I am trying to understand the bolt_initial_stress_section.4not1.k example and I have a question.
When I take a look at the cross section plane, a message with this text appears:
N and L vectors for *DATABASE_CROSS_SECTION are parallel - cannot show plane.
I draw a line to see the N vector, and what I understand is:
1. Prestress is defined along this 1 unit long across the bolts
2. SECFORC results are the resultant between a 1 unit long across the bolts. Is this correct?
Because in the papers, the pictures shown a perpendicular plane across the bolts. Why is not like in the pictures?
James M. Kennedy
Dear Yolando,
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*DATABASE_CROSS_SECTION_OPTION1_{OPTION2}
OPTION1 includes: PLANE, SET
To define an ID and heading for the database cross section, set OPTION2 to: ID
Purpose: Define a cross section for resultant forces written to ASCII file secforc.
1. For the PLANE option, a set of two cards defines a cutting plane, see Figure
16-2. Based on this cutting plane, a node set and element set(s) which comprise
the cross section are internally generated. Using the variable ICRFILE in
*CONTROL_OUTPUT, those sets may be output for the purpose of displaying
the nodes and elements of the cross section using LS-PrePost.
2. If the SET option is used, just one card is needed which identifies a node set
and at least one element set. In this case the node set(s) defines the cross section,
and the forces from the elements belonging to the element set(s) are
summed up to calculate the section forces. Thus, the element set(s) should
include elements on only one side (not both sides) of the cross section.
The cross-section should cut through deformable elements only, not rigid bodies.
Cutting through master segments for deformable solid element spot welds can lead to
incorrect section forces since the constraint forces are not accounted for in the force and
moment summations. Beam element modeling of welds do not require any special
precautions.
------------------------------------------------
Perhaps the following can help:
http://www.dynasupport.com/howtos/general/cross-section/
LS-DYNA has a facility to calculate and output section forces if the user provides
*database_cross_section/*database_secforc entries or through LS-PrePost as given in
the following comments.
The SPlane interface allows sections to be defined interactively and LS-PrePost computes
the section quantities if shell resultants are available in d3plot files. Generally this data is
available in d3plot files unless it is turned off by the use of *DATABASE_EXTENT_
BINARY options.
Jacob Krebs recently provided this very nice, concise explanation for SPlane to the Google
LS-PrePost User's Group:
FixS (fixed in space) means that the section plane will not move once defined.
FixM (fixed to model) means that the section plane can move if the nodes used to define
it move.
Lagr (lagrangian) means that a group of elements is cut initially, and then those same
elements are tracked throughout the animation (and are not confined to a flat plane as the
model deforms). This option is most similar to *database_cross_section.
------------------------------------------------
A note taken from the User's Manual for *database_cross_section:
If the SET option is used, just one card is needed which identifies a node set and at least
one element set. In this latter case the node set defines the cross section and the forces
from the elements belonging to the element set(s) are summed up to form the section
forces. Thus the element set(s) should include elements to only one side (not both sides)
of the cross section. The cross section should cut through deformable elements only, not
rigid bodies.
One final comment:
LS-PrePost uses element stress resultants to calculate section forces. LS-DYNA, on the
other hand, uses nodal forces (which are not available in the d3plot). *database_secforc
is the most accurate section force output you can obtain.
------------------------------------------------
Some additional/similar comments shared by Jacob Krebs :
1. The stress is multiplied by the area of intersection for each element to calculate a force.
The forces are then summed across all intersected elements. The calculation is less accurate
if the section plane does not cut cleanly through the center of the element, by the way.
2. NODFOR reports a force of zero for internal nodes unless an external load is applied
directly to the node.
-----------------------------------------------
Sincerely,
James M. Kennedy
KBS2 Inc.
March 18, 2021
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