Bond-slip
Osama Elshamy
James Kennedy
Dear Osama,
Some notes previously shared.
Sincerely,
James M. Kennedy
KBS2 Inc.
June 27, 2026
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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
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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 re-
quested 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.
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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 Engineering
Sincerely,
James M. Kennedy
KBS2 Inc.
April 21, 2026
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Hello all,
I am asking if anyone here is familiar with reinforced concrete models with modeling the bond-slip interaction between rebar and concrete, as I need help in a model that once I change the bond from full bond to bond-slip using functions in the BEAM_IN_SOLID card the results become worse. The .k file is attached for reference.
Best regards,
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