how to get the slip value between concrete and rebar when using *Constrained_beam_in_solid
Wong Edward
James Kennedy
Dear Edward
From LS-DYNA User’s Manual:
*CONSTRAINED_BEAM_IN_SOLID
1. The example below shows how to define a function and use it to prescribe
The debonding process. User defined functions are supported. The function
Computes the debonding force and has two internally calculated arguments: slip
And leng. Slip is the relative axial displacement between the beam node (or
Coupling point) and the material in which the beam is embedded. Leng is the
tributary length of the beam node or coupling point. Implicit calculations
require a third argument which is output by the function: stiff. Stiff is the
debonding spring stiffness. The asterisk in front of stiff (*stiff) is required
to indicate that it is called-by-reference, meaning that its value is returned
after the function is evaluated. Please note that in the function body this
asterisk cannot be placed in the first column because the LS-DYNA keyword reader
would assume that it is the start of a new keyword.
$...|....1....|....2....|....3....|....4....|....5....|....6....|....7....|....8
* *CONSTRAINED_BEAM_IN_SOLID
$# bsid ssid bstyp sstyp ncoup cdir
2 1 1 1 2 1
$# start end axfor
0.000 0.000 -10
*DEFINE_FUNCTION
10
float force(float slip,float leng, float *stiff)
{
float force,pi,d,area,shear,pf;
pi=3.1415926;
d=0.175;
area=pi*d*leng;
pf=1.0;
if (slip<0.25) {
shear=slip*pf;
} else {
shear=0.25*pf;
}
force=shear*area;
*stiff=pf*area;
return force;
Sincerely,
James M. Kennedy
KBS2 Inc.
February 23, 2023
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ming fang
I am currently working on a numerical model of UHPFRC under uniaxial tension in LS-DYNA.
In my model, the UHPC matrix is represented by solid elements, the steel fibers are modeled by beam elements, and the fiber–matrix interaction is described using CBIS with a user-defined bond–slip law.
My main concern is that, after adding steel fibers, the overall uniaxial tensile strength of the specimen increases only slightly compared with the plain UHPC model. In other words, the fibers do not seem to provide the expected strengthening effect at the specimen level.
From the simulation results, it appears that the fibers begin to contribute only after the matrix has already reached its tensile peak and started to soften. Therefore, the fiber effect comes too late, and the global peak tensile strength is not improved much.
I would like to ask:
* In this type of LS-DYNA model, what are the most likely reasons why fibers fail to increase the overall uniaxial tensile strength significantly?
* Is this usually caused mainly by the CBIS bond–slip law, such as initial stiffness, peak bond stress, or slip at peak bond stress?
* Or is it more related to the matrix constitutive model, especially if the matrix post-peak softening is too abrupt?
* Could fiber orientation and effective bridging across the crack zone be the controlling factor, even if the fiber volume fraction is reasonable?
* In your experience, what is the most important point to check first when the simulated strengthening effect of fibers is much smaller than that observed in experiments?
Any suggestions or references would be greatly appreciated.
Thank you very much for your time and help.
Best regards,
Fang Ming
James Kennedy
Dear Ming,
See if the following is of some help.
To obtain the slip value between concrete and rebar when using *CONSTRAINED_BEAM_IN_SOLID in LS-DYNA, you must model the bond-slip behavior using the axial force function feature (AXFOR). In this formulation, the slip is defined as the relative axial displacement between the beam node (or coupling point) and the solid element material, which can be output through user-defined functions or specifically requested history variables.
Methods to Obtain/Output Slip Values
AXFOR User-Defined Function (Recommended):
By setting the AXFOR flag in *CONSTRAINED_BEAM_IN_SOLID to a negative integer, you can call a *DEFINE_FUNCTION or a user subroutine (rebar_bondslip_get_force).
The function computes the debonding force based on internal arguments, one of which is slip (relative axial displacement).
You can output this slip variable from your user-defined function to a file or monitor it in the solver output to get the precise slip value at any coupling point.
Use *CONTACT_1D:
If not using the AXFOR option, you can use *CONTACT_1D to define the bond-slip behavior. This allows you to define a specific bond stress-slip curve, and CONTACT_1D provides outputs related to bond behavior.
Post-Processing d3plot (Approximate):
If using perfect bond, you cannot extract slip directly. However, if using bond-slip, you can visualize the "debonding force" or the relative nodal displacement between the beam and the surrounding solid nodes using LS-PrePost.
Key Setup Requirements for Slip Calculation
Define *CONSTRAINED_BEAM_IN_SOLID: Set CDIR = 1 to allow for released constraints along the beam axial direction.
Activate AXFOR: Set AXFOR to a negative integer to call *DEFINE_FUNCTION, which calculates slip.
Define Bond Curve: The *DEFINE_FUNCTION must accept slip and leng (tributary length) as inputs to compute the bond force.
Note: The "slip" calculated by the solver is the relative displacement at the bond-slip interaction points defined at the rebar nodes.
-------------------------------------------------------
https://onlinelibrary.wiley.com/doi/10.1155/2021/8857491
Seung H. Yang, Kwang S. Woo, Jeong J. Kim, Jae S. Ahn, “Finite Element Analysis of RC Beams by the Discrete Model and
CBIS Model Using LS-DYNA”, Advances in Civil Engineering.
There are several techniques to simulate rebar reinforced concrete, such as smeared model, discrete model, embedded model,
CLIS (constrained Lagrange in solid) model, and CBIS (constrained beam in solid) model. In this study, however, the interaction
between the concrete elements and the reinforcement beam elements is only simulated by the discrete model and CBIS (constrained
beam in solid) model. The efficiency and accuracy comparisons are investigated with reference to the analysis results by both models
provided by LS-DYNA explicit finite element software. The geometric models are created using LS-PrePost, general purpose pre-
processing software for meshing. The meshed models are imported to LS-DYNA where the input files are then analyzed. Winfrith
and CSCM concrete material options are employed to describe the concrete damage behavior. The reinforcement material model is
capable of isotropic and kinematic hardening plasticity. The load versus midspan deflection curves of the finite element models
correlate with those of the experiment. Under the conditions of the same level of accuracy, the CBIS model is evaluated to have the
following advantages over the discrete model. First, it has the advantage of reducing the time required for FE modeling; second,
saving computer CPU time due to a reduction in total number of nodes; and third, securing a good aspect ratio of concrete elements.
Advances in Civil Engineeri
Sincerely,
James M. Kennedy
KBS2 Inc.
April 21, 2026
To view this discussion visit https://groups.google.com/d/msgid/ls-dyna2/8fedcfa1-e611-4f0d-a251-15c1566da073n%40googlegroups.com.
银星
James Kennedy
Dear Ming,
From LS-DYNA User’s Manual:
*CONSTRAINED_BEAM_IN_SOLID *CONSTRAINED
Examples:
1. Function for Modeling Debonding. The example below shows how to define
and use a function to prescribe the debonding process. User-defined functions
are supported. The function computes the debonding force and has two internally
calculated arguments: slip and leng. Slip is the relative axial displacement
between the beam node (or coupling point) and the material in which the beam
is embedded. Leng is the tributary length of the beam node or coupling point.
Implicit calculations require a third argument which is output by the function:
stiff. Stiff is the debonding spring stiffness. The asterisk in front of stiff
(*stiff) is required to indicate that it is called by reference, meaning that
its value is returned after the function is evaluated. Please note that this
asterisk cannot be placed in the first column of the function body because the
LS-DYNA keyword reader assumes asterisks start new keywords.
$...|....1....|....2....|....3....|....4....|....5....|....6....|....7....|....8
*CONSTRAINED_BEAM_IN_SOLID
$# bsid ssid bstyp sstyp ncoup cdir
2 1 1 1 2 1
$# start end axfor
0.000 0.000 -10
*DEFINE_FUNCTION
10
float force(float slip,float leng, float *stiff)
{
float force,pi,d,area,shear,pf;
pi=3.1415926;
d=0.175;
area=pi*d*leng;
pf=1.0;
if (slip<0.25) {
shear=slip*pf;
} else {
shear=0.25*pf;
}
force=shear*area;
*stiff=pf*area;
return force;
}
2. User Subroutine for Modeling Debonding. The example below shows how to
define a user subroutine and use it to prescribe the debonding process.
$...|....1....|....2....|....3....|....4....|....5....|....6....|....7....|....8
*CONSTRAINED_BEAM_IN_SOLID
$# bsid ssid bstyp sstyp ncoup cdir
2 1 1 1 2 1
$# start end axfor
0.000 0.000 1001
*CONSTRAINED_BEAM_IN_SOLID
$# bsid ssid bstyp sstyp ncoup cdir
3 1 1 1 2 1
$# start end axfor
0.000 0.000 1002
*USER_LOADING
$ parm1 parm2 parm3 parm4 parm5 parm6 parm7 parm8
1.0 6.0
$...|....1....|....2....|....3....|....4....|....5....|....6....|....7....|....8
*CONSTRAINED *CONSTRAINED_BEAM_IN_SOLID
The user debonding law subroutine:
subroutine rebar_bondslip_get_force(slip,dl,force,hsv,
. userparm,lawid)
real hsv
dimension hsv(12),cm(8),userparm(*)
c
c in this subroutine user defines debonding properties and
c call his debonding subroutine to get force
cm(1)=userparm(1)
cm(2)=userparm(2)
cm(3)=2.4*(cm(2)/5.0)**0.75
cm(8)=0.
c
pi=3.1415926
d=0.175
area=pi*0.25*d*d*dl
pf=1.0
c
if (lawid.eq.1001) then
if (slip.lt.0.25) then
shear=slip*pf
else
shear=0.25*pf
endif
force=sign(1.0,slip)*shear*area
elseif (lawid.eq.1002) then
if (slip.lt.0.125) then
shear=slip*pf
else
shear=0.125*pf
endif
endif
return
end
Sincerely,
James M. Kennedy
KBS2 Inc.
Aprl 23, 2026
Sincerely,
James M. Kennedy
KBS2 Inc.
April 21, 2026
From: ls-d...@googlegroups.com [mailto:ls-d...@googlegroups.com] On Behalf Of ming fang
Sent: Monday, April 20, 2026 10:06 PM
To: LS-DYNA2 <ls-d...@googlegroups.com>
To view this discussion visit https://groups.google.com/d/msgid/ls-dyna2/8fedcfa1-e611-4f0d-a251-15c1566da073n%40googlegroups.com.