51st AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference<BR>18th
12 - 15 April 2010, Orlando, Florida
AIAA 2010-2768
Abaqus Fire Interface Simulator Toolkit (AFIST)
For Coupled Fire and Structural Response Prediction
Changsong Luo1, Liguo Chen2, Jim Lua3, and Philip Liu4
Global Engineering and Materials, Inc.
1 Airport Place, Suite 1, Princeton, NJ, 08540
An Abaqus Fire Interface Simulator Toolkit (AFIST) is developed to predict fire
growth, heat transfer through fluid/structure interfaces, time dependent material
softening, structural stability, and residual strength of advanced composite
structures. A real fire environment is simulated using an efficient fire dynamics
simulator (FDS) and its effect on the thermal response and failure progression is
captured via a two way coupling. Exchange of heat flux and temperature is achieved
at the coupling interface subjected to a constraint on the conservation of mass and
energy. To capture the interactions between the thermal decomposition, gas
pressure, and mechanical response, a multi-layered element approach is used in
conjunction with a 3D constituent based thermal decomposition model and a
composite damage and delamination model for composite materials and their
sandwich structures. Validation studies are performed for a compressively loaded
sandwich plate exposed to a fire. The two-way coupling scheme between FDS and
Abaqus in AFIST is also validated for both non-combustible and combustible gases.
An important phenomenon in flame spread is also explored via a demonstration
example.
Nomenclature
i ,  g
= density of solid phases and gas in decomposed composite material (kg/m3)
i
= volume fraction of composite component i
c pi , c pg
= specific heat of solid phases and gas  J/kg-K 
Asg , Ea
= pre-factor and activation energy of Arrhenius law for pyrolysis  s 1 , J/kg-mol 
 k , k
= stress/strain tensors of solid phase k  Pa,  
h _ dec
= heat of decomposition  J / kg 
ki , k g
= thermal conductivity of solid phases and gas W / m-K 
K
= composite gas permeability (m2)
mi, mg
= mass change rate of solid and gas phases (kg/m3-s)
Pg
= gas pressure (Pa)
Rg
= the constant of decomposed gas  J/kg-K 
T
= temperature (K)
1
Senior Scientist, GEM-NJ Office, Princeton, NJ, 08540, AIAA Member
Senior Scientist, GEM-NJ Office, Princeton, NJ, 08540
3
Senior Principle Scientist, GEM-NJ Office, Princeton, NJ, 08540, AIAA Member
4
Senior Scientist, GEM-MD Office, Baltimore, MD 21124
2
Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
I. Introduction
T
he key challenge to the modeling of composite and its sandwich structures in fire is a multidisciplinary
problem that involves thermal, chemical, and mechanical processes. At high temperatures, the resin
decomposes, releasing volatiles that may burn, and significantly reducing the overall strength of the
structure. Both the stiffness reduction of the polymer laminate and the growing fire via the flammable
polymer matrix material will result in a skin buckling, skin microcracking, and skin core interface
delamination. After the heat penetration through the skin, the core material can be degraded rapidly via its
charring and decomposition. Significant gas pressure may build in the structure causing the material to
delaminate and buckle. The challenge for modeling the structural response and failure progression of
composite structures in fire is to accurately represent each process and capture the coupling among these
processes. The modeling is further complicated because many of the processes are not in isolation from
each other.
Figure 1. Summary of key capabilities of AFIST.
The goal of this research is to develop an Abaqus Fire Interface Simulation Toolkit (AFIST) for better
prediction of the response of composite structure in fire. The coupled analysis toolkit is capable for
characterization of fire, material degradation, failure progression, and structural instability under thermal
mechanical loadings. The fire dynamics simulator (FDS) developed by NIST 1-2 is integrated with a
customized Abaqus via a two way coupling. A suite of user-defined subroutines in Abaqus is developed for
thermal and mechanical damage characterization. Both the full coupling module and a simplified fire curve
approach have been implemented in AFIST as summarized in Fig. 1. A customized GUI interface based on
Abaqus/CAE for AFIST is developed, so that users can easily generate both Abaqus and FDS input files.
Figure 2 shows a snapshot of the GUI interface in Abaqus/CAE.
Extensive research work has been performed in material characterization of a composite material
during a fire3-9 and fire and load induced damage progression and structural instability prediction10-12. Very
limited work has been done on the fire and structural coupling to accurately capture the thermal and mass
transfer at a fluid and solid interface. A fluid structural integration model coupled with a thermomechanical degradation model has been studied by the UB group 3,13-15. Existing computational tools and
test data were used to assess the adequacy of fluid-structure coupling algorithms for predicting the flame
spread. A cut cell approach has been developed by the UB team to characterize an arbitrary coupling
interface within a CFD domain. Given the higher computational cost associated with the CFD solver, it is
very costly to apply a high fidelity CFD solver for the fire response prediction of the entire structure.
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To expedite the solution process for a large scale structure with a fire structure coupling, the efficient
fire dynamics simulator, FDS, can be used to perform the fire simulation. FDS has been optimized by NIST
for characterization of a low speed and thermally-driven flow such as the fire. On the other hand, the
Abaqus’ FEM solver provides its great efficiency and versatility in solving a large scale structure with both
material and geometric nonlinearity. The integration of these two commercial codes with customization
can provide an ideal computational platform for simulation of the 3D fire response and failure prediction of
a large scale structure component.
Figure 2. Customized GUI windows of AFIST as an add-on toolkit in Abaqus.
Key features in the customized GUI shown in Fig. 2 for the two-way coupling between the FDS and
Abaqus solver include:
1) Fire model definition for FDS;
2) Structure model definition for Abaqus;
3) Embedded coupling interface for co-simulation; and
4) Analysis execution and post analysis.
For the fire curve based structural certification, the customized GUI is used to accomplish the
following:
Importing/creating structure models;
Fire curve definition for a boundary surface; and
Analysis execution and post analysis.
A 3D thermal decomposition model is developed and implemented in Abaqus via user subroutines
(UMAT and UMATHT). A coupled thermal, chemical, and mechanical response is characterized at
element level via an overlay element approach in Abaqus. A two-way coupling between FDS and Abaqus
is achieved via Abaqus’ co-simulation framework. A brief summary on the technical approach along with
example applications are given below.
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II. Summary of Technical Approach
A. Thermal Diffusion and Decomposition Model
The thermal diffusion and decomposition model is mainly based on previous study in thermomechanical damage model for composite materials3,16,17. The initial material composition is assumed to
consist of fiber, resin and a small amount of gas void. Upon heating the resin heats up and is pyrolyzed
creating additional gas and char. During heating the temperature of the gas and solid are assumed to be the
same, therefore solution of a single energy equation is only required for the determination of the local
temperature field. Assuming that the effect of woven structure on the thermal response is negligible, the
solution of phase-averaged equations for resin fraction,  r , gas void fraction,  g , and energy transport is
implemented in Abaqus’ user subroutines via a multi-layered element approach, as shown in Fig. 3. There
are two layers of elements, one of which is used to solve the temperature-displacement field and the other is
used to solve the gas pressure filed18. The effect of gas pressure on the thermal field has been considered
using effective bulk thermal properties. Figure 4 shows the flow chart of the solution procedure of the
thermal decomposition model.
Figure 3. A schematic diagram for a multi-layer element approach.
Figure 4. Implementation of thermal decomposition model via Abaqus user-defined subroutines.
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B. Composite Damage and Delamination Model
In addition to the thermally induced material softening, the microcracking resulted material softening needs
to be captured using a mechanism based damage model. A three-component system is used to characterize
the damage in X-tow, Y-tow, and resin pocket for a woven fabric ply. Modified Hashin’s criteria (1980) are
used to predict the tensile, compressive, and shear failure in X-tow, Y-tow, and resin pocket. Given the
local coordinate system, the stress along the X-tow is 1 while the stress along the Y-tow is 2. Based on
this notation, the other constituent stress components defined in the constituent failure criteria are selfexplanatory. In Table 1, the constituent strength parameters are defined by
XT, YT, and ZT – Axial and Transverse Tensile Strength of a Tow
XC, YC, and ZC – Axial and Transverse Compressive Strength of a Tow
TL, and TT – Longitudinal and Transverse Shear Strength of a Tow
Trs – Shear Strength of a Resin Pocket
Note that the delamination criterion given in Table 1 is used to predict the microcracking induced interface
damage of a woven fabric unit cell (x-tow/y-tow/resin pocket). Since the micro-debonding failure occurs
in a resin-rich zone, the stress components in the resin phase are used in the debonding failure criterion.
Table 1. Summary of mechanism-driven failure criteria in each constituent.
To capture the damage induced material softening, a set of damage parameters (i, i=1, 2, …, 6) are
introduced based on the detected failure mechanism. A damage evolution algorithm for an anisotropic
material developed by Matzenmiller et al. (1995) is employed for continuum damage progression analysis.
Given the damage thresholds ri, (i=1, 2, …6) associated with the i-th failure criterion listed in Table 4.3, the
corresponding damage variable i can be determined by
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i  1  e

1
1 rim
me

(no sum on index i)
(1)
where m is a strain rate softening constant. Using Eq. (1), the stiffness reduction can be characterized by
Ci=(1-i)Ci0, where Ci0 is the stiffness at its virgin state. The degraded stiffness matrix for a given set of
damage parameter i is defined by
0
B
 0 (1   )G
4
ab
[C ]  
0
0

0
0
0
0
(1  5 )Gbc
0





(1  6 )Gca 
0
0
0
(2)
where

1

(1


1 ) Ea



1
 B    A    ab
Ea



  ac
Ea


1

 (1  1 ) Ea


 A    ab
Ea



  ac
Ea


ba
Eb
1
(1  2 ) Eb

bc

Eb
ba
Eb
1
(1  2 ) Eb

bc
Eb



Ec 


 cb 
Ec 

1

(1  3 ) Ec 
ca
1


Ec 


 cb 
Ec 

1

(1  3 ) Ec 

(3)
ca
(4)
Because of the failure mode interaction, a failure logic diagram has to be defined in advance to rationally
reduce the stiffness based on the observed failure mode. Table 2 summarizes the failure logic and relation
between failure mode and damage variables. As indicated in Table 2, once a tensile failure mode in X-tow
is detected, the damage variables of 1, 4, and 6 are updated based on Eq. (1) and the corresponding
stiffness components (E1, E2, G12, G31) are reduced based on Eq. (3) and (4). If the compressive crush
failure is detected in the through-the-thickness direction (r6 > 0), all the components of the stiffness matrix
are dropped accordingly.
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Table 2. Relation between failure mode and damage variables.
The softening material behavior under loading-unloading-reloading is shown in Fig. 5. The material
response is linear up to its initial failure point. Stiffness degradation is observed from the unloading when
the stress state passes the initial failure state. No additional damage is introduced during its re-loading
process. The softening branch follows an exponential decay function described by Eq. (1).
Figure 5. Demonstration of the performance of the softening model subjected to loading-unloadingreloading.
Delamination failure has been observed as a key failure mechanism in sandwich composite material.
To capture this key failure mechanism, a surface-based cohesive model22 is used in Abaqus. The surfacebased cohesive behavior is used to model the contact between the composite skin and balsa core. To
characterize the temperature dependent cohesive parameters, a scaled cohesive model at an arbitrary
temperature is used in Fig. 6 22. The cohesive strength Sn(T) at a given temperature is determined from a
curve-fit model (see Fig. 7).
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Figure 6. Illustration of temperature dependent cohesive model.
Figure 7. An example illustration of a temperature dependent cohesive model from RMIT.
C. Fluid-Structure Two-Way Coupling
The response of an advanced structure is sensitive to the local flow environment since the heat transfer
from the fire is both spatially and temporally coupled because of turbulent mixing processes. The dynamics
of a fire depends on the structure geometry, where the ventilation pathways are defined, and view factors
for radiation hear transfer. This fluid-structure coupling is furthermore complicated if the solid phase is a
composite structure due to decomposition of resin that releases additional volatiles which burn near the
surface, establishing a surface flame. The structure response and evolution of a fire are definitely coupled
as shown in Fig 8.
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Figure 8. Summary of development of fluid-structure coupling approach.
A two-way coupling scheme is implemented to capture the interaction between the fire process and
thermal decomposition in the structure. A real-time direct coupling scheme is defined by authors, as shown
in Figs. 9 and 10. During the simulation process, the information will be exchanged through the interface at
every coupling step. Using the surface temperature of solids predicted by the thermal decomposition model
in our user-defined Abaqus toolkit, FDS can accurately determine the heat flux that is imposed on the
solids from its coupled thermal diffusion and chemical decomposition model. FDS will get the mass flux
of decomposed gases, which include combustible and non-combustible gases. The combustion of released
gas is very important for modeling the flame spread phenomena.
Figure 9. Implementation of thermal coupling between FDS and Abaqus in AFIST.
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Figure 10. Implementation of mass coupling between FDS and Abaqus in AFIST.
III.
Results and Discussion
A. Time-to-failure Prediction of Composite Sandwich Structure
To validate the AFIST toolkit, a coupon level sandwich composite is studied and the results are
compared with the experimental data from by Feih et al.5. The sandwich panel consists of a thick core of
balsa wood and two skins of glass/vinyl ester composite laminate, as shown in Fig. 11. The thermal
properties of glass/vinyl ester and balsa are given in Tables 3 and 4, respectively.
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Figure 11. A composite sandwich panel under a 50 kW / m heat flux on left surface and constant
compression loads in vertical direction.
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Table 3. Thermal and transport properties of Eglass/vinyl ester5,20,20.
Properties
Asg
s 
-1
Values
Properties
5.6×1013
Ea  J/kg-mol 
n
h _ dec  J / kg 
f
Table 4. Thermal and transport properties of
balsa wood7,9.
 kg / m 
3
f
2.12705×10
1
C p  J/kg-K 
k W / m-K 
Ea  J/kg-mol 
116488
h _ dec  J / kg 
1500
b,ini  kg / m3 
0.55
 r ,  c  kg / m3 
6.7×107
n
3.788×105
b, final  kg / m3 
1140
960 (T<410 K)
1210 (T<550)
1360 (T>550 K)
1
556000
150
22
C pb  J/kg-K 
1420 + 0.68*T
C pb,char  J/kg-K 
3194 + 1.33*T
k W / m-K 
0.43
Values
s 
Asg
5
-1
0.2
In the experimental study, both a force control with constant compressive loading and a given heat flux are
applied to measure the time-to-failure of the sandwich structure. As shown in Fig. 11, the sandwich consisting of a
core of balsa wood and two skins of glass vinyl ester composite laminate. Both skin and core are 150mm long and
80 mm wide. The skin has a thickness of 5 mm and the core has a thickness of 30 mm. A uniform heat flux is
imposed at a 100mm (in-length) area on one side of the sandwich, and a constant compressive force is applied on the
top surface. Since the clamps used to restraint the specimen on both ends were not fixed to the compression plates,
the boundary condition allowed free rotation (pinned)5.
The mechanical properties of E-glass/vinyl ester laminate at room temperature are given in Table 5. The
temperature dependency of mechanical properties of woven E-glass/vinylester laminate is critical to the prediction
of the time-to-failure of the structure. According to the experimental studies by Kim et al19, the longitudinal
modulus and the shear modulus degrade differently as temperature increases, as shown in Fig. 12. The coefficient of
thermal expansion (CTE) of composite in this case is orthotropic. Both in-plane and out-of-plane CTEs are
temperature dependent, as shown in Fig. 13.
Table 5. Mechanical properties of E-glass/vinyl ester laminate at room temperature20, 20.
Properties
E1 (Pa)
E2 (Pa)
E3 (Pa)
v12
v13
v23
G12 (Pa)
G13 (Pa)
G23 (Pa)
Values
2.68×1010
2.68×1010
1.15×1010
0.15
0.4
0.4
5.04×109
3.64×109
3.64×109
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(a)
(b)
Figure 12. Temperature dependency of (a) Longitudinal modulus and (b) shear modulus of the E-glass/vinyl
ester composite19.
Figure 13. Illustration of curve-fit models for the CTEs from RMIT.
A series of compressive loads (50%, 37.5%, 22% and 15% of the compressive strength) are applied on the
composite sandwich structure. Figure 14 and 15 show the delamination failure and the displacement curves for 50%
and 15% strength of the sandwich composite. In both cases, delamination (debonding between the front skin and
balsa core) can be observed. The in-plane displacement drops rapidly when the composite sandwich fails. At the
same time, the out-of plane displacement increases quite quickly too. In this study, therefore, the time-to-failure is
defined as the time when the in-plane displacement suddenly drops.
(a)
(b)
Figure 14. Delamination and displacement histories of sandwich panel at 50% strength load.
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(a)
(b)
Figure 15. Delamination and displacement histories of sandwich panel at 15% strength load.
The time-to-failure predictions are compared with experimental data, shown in Fig. 16. There are two sets of
experimental data, for front and back skins. According to Feih et al.5, the front skin failure is resulted from the stable
plastic kinking. This failure mechanism is not included in the current study. The current model predictions agree
reasonably well with experimental failure time based on the back skin failure.
Figure 16. Time-to-failure predictions of sandwich panel at a heat flux of 50 kW/m2 5.
B. Verification of Thermal and Mass Coupling between FDS and Abaqus
Heat and mass transfer coupling is successfully implemented in AFIST. This section is focused on verification of
AFIST by comparison with stand alone FDS results.
In stand-alone FDS, a one-dimensional thermal decomposition model has been implemented within its 3D CFD
solver. FDS assumes that solid obstructions consist of multiple layers, with each layer composed of multiple
materials components that can undergo multiple thermal degradation reactions. Each reaction forms a combination
of solid residue, water vapor, non-combustible gas, and/or fuel vapor. Heat conduction is assumed only in the
direction normal to the surface. Also, additional assumptions used in stand-alone FDS solid solver are:
 Instantaneous release of decomposition volatiles from solid to the gas phase
 Local thermal equilibrium between the solid and the volatiles
 No condensation of gaseous products
 No porosity effects
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In AFIST, solid part is solved by Abaqus, which is a full 3D thermal mechanical model. In order to verify our heat
transfer and mass transfer coupling schemes, Abaqus model has to be casted as a 1D case to exactly match FDS
internal 1D solid solver.
1. Mass Coupling with Non-combustible Gases
Consider a plastic plate in a room with its front and top faces open to the air and inert walls on all the other sides.
The room has the dimension of 1.2m long, 0.6m wide and 4m tall (see Fig. 17). The plate is of 1m tall, 0.6m wide
and 0.025m thick. A hot brick at 1000 oC is used as a radiation heat source in front of the plate. For the verification
purpose, the problem is solved by both AFIST and the stand-alone FDS and their results are compared. We assume
that the decomposition gas is non-combustible. The plate will absorb the radiation energy from the hot brick and
undergo thermal decomposition. The decomposed gas from the absorbed heat at the front surface is released and
dispersed in the room. The reaction rates are functions of local mass concentration and temperatures, and calculated
as a combination of Arrhenius and power functions:
n

d
E
  i   A exp

dt
 RT 
 i 
(5)
where ρ is the solid density changing with time, ρi is the initial density, E is the activation energy, T is local
temperature, R is universal gas constant, and A is pre-exponent factor. The thermal decomposition properties of the
material are listed in the following table (Table 6).
Figure 17. Problem set up, a plastic plate undergoes thermal decomposition.
Table 6. Thermal decompositions properties of plastic plate.
Properties
-1
Values
Asg (s )
3015730
Ea (J/Kg-mol)
23900
n
1
h (J/kg)
2 ×108
ρi(kg/m3)
1180
3
ρf (kg/m )
118
C p(J/kg-K)
1900
k (W/m-K)
0.17
A comparison of temperature and mass distribution at t= 150 sec is shown in Fig, 18, and 19, respectively, based on
the prediction from AFIST and the stand alone FDS. A comparison of time history curves of a point at the bottom
of the plate is plotted in Figs. 18 and 19, for the temperature and mass flux. Again both the temperature and mass
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flux curves are in very good agreement between the stand alone FDS and AFIST prediction. Due to the use of the
finite difference in FDS and finite element in AFIST, a small discrepancy can be observed from the numerical
simulation. The heat and mass transfer coupling capabilities in AFIST has been verified based on the use of the stand
alone FDS via its 1D solver in solids.
Figure 18. Comparisons of temperature contour at simulation time 150 seconds. The left is from stand-alone
FDS and the right is from AFIST
Figure 19. Comparisons of mass flux contour at simulation time 150 seconds. The left is from stand-alone FDS
and the right is from AFIST.
Figure 20. comparisons of temperature time
history curves at point A.
Figure 21, comparisons of mass flux time history
curves at point A.
2. Mass Coupling with Combustible Gases
This example is designed to explore the phenomena of flame spread using AFIST. The flame spread is an important
factor that cannot be ignored during the design since the released combustible gases can contribute the self burning
on the fire exposure surface resulting in a quick rise of the temperature. The problem set up shown in Fig. 22 is
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similar to the previous validation case except that the plate is 3 meters tall and the decomposition gas is combustible.
The material properties are the same as described in Table 1 except that additional combustion properties for the
decomposition gas are needed as shown in Table 7.
Figure 22. A 3-meter tall plate under heat flux.
Table 7. Combustion properties of decomposition gas.
Properties
Values
Chemical Formula
C5H8O2
Heat of Combustion
23900 kJ/kg
Soot Yield
0.022
Snap shots of temperature distribution on the plate surface at three different times are shown in Fig. 23 and the snap
shots of burning rate, heat release rate and smoke are shown in Fig. 24. Clearly we can see flame is spreading
upward. At first, only the bottom of the plate is undergoing decomposition due to the radiation heat from the hot
brick. Then the decomposed gas is released from the bottom and burned in the CFD domain. The burned gas which
is at higher temperature and lower density will flow upward due to buoyancy forces, heating the upper part of the
plate and causing the plate to decompose further. Wall temperature and burning rate at three device points
associated with three different heights (denoted as red dots in Figs. 23 and 24) are recorded and displayed in Fig. 25
and 26. It clearly can be seen that the higher the point location is, the later the material begin to heat it up and
decompose, since the flame is spreading upward from the bottom.
Figure 23, flame spread, plate surface
temperature contours at three different times.
Figure 24, flame spread, burning rate and heat
release rate contours at three different times.
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Figure 25, Temperature history curves for three
points at different heights.
Figure 26, Burning rate time history curves for
three points at different heights.
3. Demonstration: Sandwich Panel in Real Fire Environments
After the coupling verification study, AFIST is applied to perform fire simulation and response prediction of a
loaded composite sandwich plate subjected to a pool fire. The problem statement along with the geometric
parameters is given in Fig. 27. A pressure load of 167.5 MPa is applied on the top edge of the plate.
Figure 27. Problem set up two-way coupling via FDS/Abaqus.
The thermal/decomposition properties of the sandwich plate and the concrete walls are given in Table 3, 4 and 8,
respectively. For the pool fire shown in Fig. 27, the heat release rate is 1000 kW/m2 on a (0.4m, 0.4m) square area
and the fuel source is propane. The room is made of concrete walls with one side exposed to fire and the other side
faces to the outside atmosphere environment. The door is open to outside atmosphere too. For the sandwich plate,
only the face near the fuel source is applied with the coupling boundary conditions and all the other faces are
assumed perfectly insulated. The mesh size for the sandwich plate is 1mm in the through-thickness direction and
5cm in other two directions. The mesh size for the FDS domain is 5cm in all three directions. The FDS domain
(room) has the size of [5m x 3m x 3m] and its mesh density is by [50 x 30 x 30].
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Table 8. Thermal properties of concrete walls.
Property
Quantity
Density
1440 (kg/m3)
Specific
Heat
0.84(kJ/kg-K)
Thermal
Conductivity
0.48(W/m-K)
After 40 seconds of heating, the thermal-mechanical response contours are shown in Figs. 28 - 35 using
Abaqus/CAE. The distribution of the heat flux and the associated temperature is shown in Figs. 28 and 29,
respectively for the exposed structure surface (hot surface) at time of 40 sec. As we can see, the heat flux
distribution is non-symmetric, and the surface temperature has the similar distribution pattern as the heat flux
distribution. After 40 sec, the fire induced heat flux is over 30 kW / m 2 at the hot surface where the temperature
reaches 570K. This temperature is high enough for the composite to be decomposed as shown in Fig. 30. The
composite starts to pylosize at the hottest spot, where the distribution of the decomposition rate is displayed in Fig.
30. The resulting distribution of the volume fraction of the char is given in Fig. 31 for the time instant of 40 sec.
Figure 28. Heat flux contour for the hot surface at
40s.
Figure 29. Temperature contour for the hot
surface at 40s.
Figure 30. Decomposition rate contour for the hot
surface at 40s.
Figure 31. Volume fraction of char contour for the
hot surface at 40s.
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The stress distribution, damage distribution, and stiffness degradation is in Figs. 32 – 35. The delamination initiates
and propagates at the laminate-balsa interface as shown in Fig. 32. Figure 32 also displays the distribution of the
Von Mises stress of the hot surface at 40 seconds. Both the skin wrinkling and delamination occurs due to the
softening induced material instability and strength degradation. Figures 33 to 35 present the damage index of the
fiber tow, axial stiffness degradation, and the damage index for the matrix cracking, respectively, at the hot surface
for the time instant of 40 sec.
Figure 32. Stress distribution contour for the hot
surface at 40s.
Figure 34. Contour of stiffness damage index for
the hot surface at 40s.
IV.
Figure 33. Contour of X-dir fiber damage for the
hot surface at 40s.
Figure 35. Contour of matrix cracking for the hot
surface at 40s.
Summary of Conclusions
The overall goal of this study is to develop a coupled CFD and FEM toolkit, linked to an experimental protocol
for establishing material properties for performing structural fire integrity assessment of FRP composite and its
sandwich structures. An Abaqus fire interface simulator toolkit (AFIST) has been developed by packaging and
integrating our solution modules in fire simulation, thermal decomposition, non-linear damage state evolution,
thermal-mechanical response prediction, and hybrid damage and failure prediction. To enhance its commercial
viability, a customized FDS has been developed as our fire simulator and integrated with Abaqus via an in-house
coupling framework. In addition, a comprehensive user-defined library has been developed to capture the
thermal/chemical/mechanical induced material softening and the delamination induced structural buckling. The
failure sequence, failure mechanism, and time to failure in a sandwich composite have been captured via AFIST’s
response and failure prediction modules.
Given the limitation from the sequential (or one-way) coupling between a fire simulator and a structural
analyzer, a two-way coupling has been developed and implemented in AFIST. A verification and demonstration
example has been used to explore the validity via the non-combustible gas and flame spread via the combustible gas.
An example application of the two-way coupling has been performed via a loaded sandwich plate subjected to a
room fire.
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Acknowledgments
The authors gratefully acknowledge the support from ONR 331 under contract N0001408C0591 with Dr. Luise
Couchman as the program monitor. The authors would like to thank the CET Lab at University at Buffalo and
Extreme Lab at Virginia Tech to provide technical guidance during this study, and thank SUMULIA for the
technical support and guidance.
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