Thermal Response of Sandwich Panels in Fire
P.A. Cutter1, R.A. Shenoi1, H. Phillips2 and S.S.J. Moy1
University of Southampton, Highfield, Southampton, SO17 1BJ, UK
2
Royal National Lifeboat Institution, Poole, Dorset, BH15 1HZ, UK
1
ABSTRACT
A major factor preventing further widespread use of polymeric sandwich materials is their behaviour in fire. The fire aboard
the Norwegian composite mine hunter “Orkla” in 2002 [1] did much to harbour the belief that sandwich materials are not
safe to be used in structures where there is a risk of fire. One of the main factors leading to the eventual loss of the vessel
was the collapse of the composite sandwich wheelhouse. The ability to be able to predict changes in strength of such
structures when subjected to fire in an economical manner would be a valuable asset to designers and operators in the
future.
The Royal National Lifeboat Institution (RNLI) is a registered charity that provides 24-hour lifeguard and lifeboat search
and rescue cover on inland waters and up to 100 nautical miles off the coast of the United Kingdom and the Republic of
Ireland. The majority of the all-weather lifeboat fleet are constructed from advanced composites including sandwich
materials. This research is being conducted as part of the RNLI’s general safety assessment of these composite lifeboats
and is being entirely funded by the RNLI at the University of Southampton. The International Maritime Organisation
stipulate the requirements for materials to be used in maritime structures with regards to fire safety [2]. This covers the
many aspects of material response to fire including the heat transfer through and strength of, decks and bulkheads
exposed to fire. It does not however provide a cost effective method of trialing different materials or an assessment of the
overall structural response in fire.
This paper covers the work carried out to develop a method for assessing the thermal and mechanical response of
sandwich composites to fire. The low conductivity and complex manner of thermal degradation in polymeric composite
materials means that a non-linear temperature profile develops within any given structure exposed to fire. A numerical
model to predict this profile is discussed and experimental tests are carried out in order to asses its validity. The strength
and stiffness of a given material can be related to this temperature profile.
Background
Materials for use in marine vessels have to pass specific standards set by the industry’s governing bodies [2]. In terms of
the performance in fire, it is necessary to go through what can be expensive and time-consuming testing regimes. This
has the effect of limiting designers in their choice of materials to those which have already passed the tests. There is
therefore a need to be able to predict the fire performance of materials in these tests from small scale, low cost testing and
numerical modelling.
Fire testing and modelling of composites has been evident since Bamford et al. [3] where the heat transfer through wood
subjected to fire was measured and a numerical model proposed to predict this heat flow. The most influential work in this
area came from Henderson et al. [4] with the development of a numerical model to predict the heat transfer through glass
filled phenolic composite samples. Much work has been done since the Henderson model was proposed, by Gibson et al.
[5-7] and Davies et al. [8-10], but essentially the same general equation has been used in each case as shown in Equation
1. This consists of three terms on the right hand side where the first term represents pure heat conduction of a transient
nature, the second term takes account of the cooling effect of the volatile gasses produced by pyrolysis and the last term
takes account of the energy absorbed by the composite during decomposition.
∂T
=
ρC p
∂t
∂ 2T
k 2
∂x
Pure heat conduction
−
∂T
m g C pg
∂
x
Cooling effect of volatiles
Ρ= density
Cp= specific heat capacity
T= temperature
t= time
k= thermal conductivity
hg= enthalpy of gas
−
∂ρ
( Q + h − hg )
∂t Decomposition endotherm
x= distance from fire exposed face
mg= mass gas flow rate
Cpg= specific heat capacity of gas
Q= heat of decomposition
h= enthalpy
(1)
There have been different approaches taken by each investigator in determining the fire resistance of composite materials.
The approach adopted by Henderson et al. [4] was on a small scale using a 20mm diameter cylindrical sample fully
insulated leaving only one end exposed to the heat source. This allowed simplifications of boundary conditions and that
there was minimal feedback from the combustion of the sample to the measured temperatures. The approaches adopted
by Davies et al. and Gibson et al. [5-10] have used composite panels up to 1m × 1m without insulating the unexposed face
in some cases. These methods have been shown to produce less consistent results; this is thought to be due a number of
factors including the convection and radiation taking place at the unexposed face of the panels and the burning of the
panels having a greater effect on the overall temperature on the heat exposed face. The larger scale approach however is
more representative of a material in service.
The accuracy of predictions made using the various adaptations of the Henderson model are largely dependent on the
values of specific material properties used in the calculations. Henderson et al. [4] proposed methods for the determination
of the thermophysical and kinetic properties of the composite in its virgin and charred states to be used as input for the
heat transfer model [11-13].
In recent times with the development of fire testing devices such as the cone calorimeter which can apply controlled fire
conditions to small samples, the focus for the research has moved onto the development of methods to obtain the relevant
material properties as well as the impact of each property on the results of the numerical models [14-17].
Research into the effects of fire on sandwich composites has used the same principles as for single skin composites in
terms of the testing and modelling. Davies et al [8-9], Looyeh et al [18] and Krysl et al. [14] have proposed numerical
models based on the Henderson model to predict the heat transfer through sandwich structures with composite skins and
Vermiculux (calcium silicate) cores. The heat transfer through the core is assumed to be linear in these cases. Thus far
the testing and modelling of fire resistance of sandwich structures with lightweight foam cores has not been extensively
researched.
The purpose of this paper is to outline the novel approach developed by the authors in the fire resistance testing of
sandwich composites. The small scale approach, which has been adopted, provides an economical method to measure
fire resistance. Results from the initial testing carried out using the apparatus are given and compared with existing
predictive techniques.
Composites in Fire Phenomena
Work has been carried out to look at the effects of fire in composite materials since the early 1980s with the collected
papers of Henderson et al. [4, 11-13] providing the basis for most of the modern methods of predicting heat flow and mass
change in single skin composites. This work along with more recent publications by Davies et al and Gibson et al. [5-10]
have shown that composite skins are very good insulators of heat and when exposed to intensive heat sources and
certain phenomena which occur can add to the insulating properties.
Sandwich materials with foam cores have not been widely tested or modelled and as such little is known about their
performance both in order to insulate and in terms of their retention of structural integrity.
Composite skins react to fire in a manner similar to that of a common natural composite, wood. A thick (>10mm) piece of
polymer composite material will slowly char as it is heated to high (>300°C) temperatures. The volatile gasses given off
from a material will aid the combustion process on the surface of the material.
Initially, during heating, pure conduction will occur through a composite material. The resin then undergoes an
instantaneous charring reaction, known as pyrolysis, at around 200°C-400°C depending on the material. This reaction
produces a carbonaceous char, which is less thermally conductive than the original material. Volatile gases are also
produced by the reaction, which are initially trapped within the composite due to the low porosity. This can cause a degree
of expansion within the matrix. As the pressure of the volatiles increase and the porosity decreases they begin to flow
back through the material towards the heat source. This has a cooling effect on the composite as a whole and results in a
contraction of the composite. The layer of char material progresses through the material at a decreasing rate. This is due
to the endothermic nature of the reaction, the cooling provided by the volatile gasses and the fact that the char material is
less thermally conductive than the virgin composite. At temperatures of over 1000°C the char can react with the silica in
glass fibres to decompose further and release more volatiles. Eventually the char material will be totally consumed,
leaving just the fibres, which melt at around 1400°C for glass. The fibres aid in holding the material together but the type of
fibre used has little overall effect on the thermal performance of a composite in fire [5]. In terms of resisting the flow of heat
through the material, single skin composites around 10mm thick or more perform exceptionally well when compared to
other commonly used engineering materials. This is down to the low conductivity of the virgin material, even lower
conductivity of the char material and the endothermic pyrolysis reaction. The temperatures at which all of the above
processes occur are highly dependant on heating rate, with higher heating rates causing the reactions to occur at higher
temperatures.
The strength of single skin composites is strongly related to the temperature within the laminate. By the time the pyrolysis
reaction has occurred the strength of the composite in that region will have negligible strength and stiffness.
In sandwich structures composite skins will obviously react to high temperature in the manner described above. Foam
cores have low thermal conductivities and the large thicknesses used mean that sandwich structures tend to be very good
insulators at lower temperatures. Little work has been done on the performance of foams at elevated temperatures, but
there is a small amount of expansion at temperatures of around 100°C before decomposition occurs between 150°C and
300°C [19]. The decomposition of the foam causes it to recede and the adhesion between the faces is then lost [8,9,19].
The mechanical performance of a sandwich structure exposed to fire is very much reliant on the performance of the fireexposed face.
Design of Experimental Apparatus
The University of Southampton, in collaboration with the RNLI, have developed a method to asses the performance of
composite sandwich materials subjected to fire. The initial stages of the project involved designing and building apparatus
with the capability of subjecting samples to a controlled fire source whilst also having the capability to subject the samples
to a structural load.
One of the aims was to develop an economical method for the testing of sandwich samples and as such a small scale
approach was adopted. The apparatus was also designed to be portable and in being portable it also became modular.
The apparatus (Vulcan), as shown in Figure 1, is designed to fit inside a standard laboratory fume cupboard so no
separate extraction system is needed.
The rig consists of a steel furnace approximately 500mm × 500mm × 500mm lined with fibrous insulation giving an active
3
volume of 0.064m . One face of the furnace is removable and houses the test sample as shown in Figure 2. The Maxon
Kinemax MVG 70 30kW propane burner is fixed on the side adjacent face of the furnace.
Figure 1. Vulcan Fire testing apparatus
Figure 2. Detachable front plate holding test sample
o
The flame fires out through a refractory block into a stainless steel tube with a 90 bend and directly onto the centre of the
test panel. The temperature inside the furnace is controlled using an Omron Digital Controller E5CK. Further details of the
apparatus can be found in Cutter et al. [14]
The sample is bolted into the front plate with 8 off M12 bolts and a square plate as shown in Figure 3. The sample is
therefore exposed to the fire on one side and ambient conditions on the other side. The samples measure 240mm ×
240mm with an area exposed to the fire of 200 mm × 200 mm. The edges of the samples are covered with a stainless
steel plate to prevent the insulation sticking to the degrading PVC foam and epoxy. The insulation around the edges of the
®
samples is 150mm thick WDS ULTRA Fibre Board. This has a thermal conductivity of less than 0.05 W/m-K even at
800°C.
Figure 3. Clamping arrangement for test samples
The loading module is fixed onto the front plate and can apply a patch load at the centre of the panel up to 50 kN through
a mechanically driven screw jack. The loads and deflections are then recorded along with temperatures within the panels
by a data acquisition unit developed at the University.
Experimental Programme
The apparatus is being used to prove theoretical models to predict the heat transfer through sandwich panels as well as
the change in mass and mechanical properties. The experimental program also serves to create a method for the different
test procedures using the apparatus.
The first stage in the experimental programme was to test the panels under fire only. This was done with the following
aims in mind:
1.
To verify the heat transfer model for degrading sandwich composites proposed by Krysl et al. [15] for a sandwich
panel at as wide a range of temperatures as possible.
2.
To obtain a level of consistency for the measured temperatures within test panels when subjected to the same
heating rate. If this can be shown then minimum temperature measurement will need to be carried out when
combining mechanical and thermal loading later in the research. It is suspected that the thermocouples laminated
into the panels reduce the overall strength.
3.
To create a link between the visual effects of the heating (thickness of char layer) to the duration and intensity of
the heating. This can then be linked to mechanical performance at a later stage.
The second stage is to test the panels under a mechanical load only in order to prove the loading module of the apparatus
is loading as expected.
The measurement of temperatures during the fire testing raised an issue rarely mentioned by other investigators in similar
experiments. In order to validate the heat transfer model it is necessary to measure the temperature of the fire exposed
surface and to use the readings as the boundary condition for that surface in the calculations. This presents a number of
problems, firstly this surface is inside the furnace and it is not possible to observe the panels from this side and
instruments attached to the surface during the experiments. The second problem is finding a method of adhesion for any
o
device to a surface which is being subjected to temperatures of around 800 C. The surface is also in a constantly
changing state as decomposition reactions occur as well as combustion of the resin matrix. The most effective method
was that used by Urbas and Parker [1993] in the surface temperature measurements of burning wood samples. In this
method two small diameter holes were drilled from the unexposed side of the sample 10mm apart through to the exposed
face. The wires of a thermocouple were inserted through the holes so the hot junction was in contact with the exposed
surface in between the holes. The thermocouple cable was then tightened so that the hot junction was kept in contact with
the surface on the exposed side and any temperature gradient was eliminated which would conduct heat toward or away
from the surface. With this method it was possible to be sure that the hot junction of the thermocouple was in good contact
with the surface, even when the surface was receding.
The sandwich panels chosen for the initial testing are representative of a typical lifeboat deck. The panels were
constructed using the lay up given in Table 1 and were vacuum consolidated with an Ampreg 22 epoxy resin system.
Laminates
QE 600
QE1200
H100
QE1200
QE600
Description
2
Quadriaxial stitched e-glass 600g/m
2
Quadriaxial stitched e-glass 1200g/m
3
100kg/m PVC foam 25mm
2
Quadriaxial stitched e-glass 1200g/m
2
Quadriaxial stitched e-glass 600g/m
Table 1. Test panel laminate details
The panels were infused with thermocouples embedded at different locations through their thickness and were subjected
to two different heating rates which were the maximum and minimum intensities currently possible with the apparatus.
Results and Discussion
Figure 4 shows the consistency of the recorded flame temperatures from the highest intensity heating rate. A good level of
o
consistency has been achieved and the burner is able to hold the temperature at a constant level of up to 900 C.
The standard fire test curves are also displayed [2] and it can be seen that at the particular heating rates recorded, the
hydrocarbon curve can be matched up to 350 seconds.
Figure 4. Flame temperatures from fire testing
Figure 5 displays the temperatures recorded during one of the fire tests on the sandwich panels described above. It can
o
be seen that the panels produce a temperature drop of up to 800 C between the flame temperature and the unexposed
o
surface and up to 600 C between the exposed and unexposed surfaces. With the particular flame temperature ramp used
o
the panel temperatures increase at a low rate, the unexposed surface rises to 200 C in 1000-1200 seconds. It can be
seen by looking at the temperatures recorded from TC-3 and TC-5 that the insulating properties of the core are retained at
high temperatures and after prolonged periods of time, which suggests that the char formed by the core decomposing has
low thermal conductivity.
The temperature recorded by TC-3, the interface between the hot face and the core is of particular relevance when looking
at how the strength of the panel will change with given heat source. Once one of the skins has lost all stiffness, which
would occur during pyrolysis, the strength of the sandwich structure as a whole would be dramatically reduced. The results
o
from the thermogravimetry using Ampreg 22 indicated that pyrolysis occurs at approximately 350 C. Given this value at
the particular heating rate used a total loss in strength would be expected to occur by 230 seconds. The effects on the
panel can be seen in Figure 6. and compared with an un-burnt sample. It is evident that there has been some contraction
in the core due to the heating.
Figure 5. Temperature profile through sandwich panel during fire test; thick lines indicate predicted temperatures
_________
TC-1
TC-2
_________
TC-3
_________
TC-4
_________
TC-5
_________
TC-6
_________
Flame temperature
Fire-exposed face temperature
Hot face/core interface temperature
Core centre temperature
Cold core/skin interface temperature
Unexposed face temperature
Figure 6. Comparison of sandwich panel subjected to the conditions shown in Figure 5. and an un-burnt panel
The recorded temperatures were compared to the results predicted by the program developed by Krysl et al. [15], based
on the Henderson equation (1). The material properties used in the model came from a number of sources including first
hand testing of resin and core samples using thermogravimetry, differential scanning calorimetry, manufacturers’ data and
data from similar materials found in literature.
The theoretical results show a reasonable correlation with the experimental data for the temperatures recorded at the hot
skin/core interface after 350 seconds. The recorded temperatures from the core centre and the cold face are also in
reasonable agreement with the predicted results. The discrepancies are thought to be due to a lack of accurate material
properties where manufacturer’s data had to be used. This highlights the importance of obtaining accurate material
property data in order to predict the performance of sandwich composite materials in fire. This does not however help in
developing a method for trialing new materials and assessing how they will perform under a range of conditions. The
testing which needs to be conducted in order to obtain the relevant material properties requires specialist bespoke
apparatus. Much of the research which has been conducted has used a single set of material data obtained by Wu et al.
[21] to validate numerical models.
This research is concerned with developing a method to predict the change in strength of sandwich materials subjected to
fire. The authors have decided that it is not essential that the heat transfer models which have been validated by other be
validated in this case again due to a lack of material properties.
Conclusions
A study of the effects of fire on foam cored composite sandwich panels has been carried out using apparatus developed at
the University of Southampton. A series of tests proved the consistency found within the apparatus when subjecting
samples to fire. The short period of time before the exposed skin in the sandwich panels reaches temperatures high
enough to cause pyrolysis and hence a loss in strength highlight the need for a thorough safety analysis to be carried out
when using such materials in structures with the potential to be exposed to fire, such as an engine room on a lifeboat.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
The fire on board the HNoMS Orkla 19th November 2002, Technical Expert Group Norwegian Defence Logistics
Organisation. (2003)
FTP Code- International Code for Application of Fire Test procedures, IMO. (1998)
Bamford, C. H., Crank, J., Malan D.H. "The Combustion of Wood. Part 1." Cambridge Philosophical Society
Proceedings 42: 166. (1946).
Henderson, J. B., Wiebelt, J. A. and Tant, M.R. "A Model for The Thermal Response of Polymer Composite
Materials with
Experimental Verification." Journal of Composite Materials 19: 579-595. (1985).
Gibson, A. G., Wu, Y.-S., Chandler, H.W., Wilcox, J.A.D. "A Model for the Thermal Performance of Thick
Composite Laminates in Hydrocarbon Fires." Revue De L'Institut Francais Du Petrole 50(1): 69-74. (1995).
Gibson, A. G., P. N. H. Wright, P.N.H., Wu, Y.-S., Mouritz, A.P., Mathys, Z., Gardiner, C.P. "The Integrity of
Polymer Composites During and After Fire." Journal of Composite Materials 38(15): 1283-1307. (2004).
Looyeh, M. R. E., Bettess, P., Gibson, A.G. "A one dimensional finite element simulation for the fire-performance
of GRP panels for offshore structures." International Journal of Numerical methods for Heat and Fluid Flow 7(6):
609-625. (1997).
Davies, J. M. Performance of Sandwich Panels in Fire 1. Onshore Applications. Sandwich Construction 3,
Southampton, UK, Emas. (1995).
Davies, J. M., Dewhurst, D., McNicolas, J., Performance of Sandwich Panels in Fire 2. Offshore Applications.
Sandwich Construction 3, Southampton, UK, Emas. (1995).
Davies, J. M. and H. B. Wang A Numerical and Experimental heat Transfer study of GRP Panels Subject
to
Standard Cellulosic and Hydrocarbon Fire Tests. Interflam '96, Cambridge. (1996).
Henderson, J. B., Tant, M. R., Moore, G.R., Wiebelt, J.A. "Determination of kinetc parameters for the thermal
decomposition of phenolic ablative materials by a multiple heating rate method." Thermochimica Acta 44: 253264. (1981).
Henderson, J. B., Tant, M. R., Moore, G.R., Wiebelt, J.A "A method for the determination of the specific heat and
heat of decomposition of composite materials." Thermochimica Acta 57: 161-171. (1982).
Henderson, J. B., Tant, M. R., Moore, G.R., Verma, Y. P. "Measurement of the Thermal Conductivity of
Polymer Composites to High Temperatures Using the Line Source Technique” Polymer Composites 4(4): 219
224. (1983).
Cutter, P. A., Gillitt, A. W., Hilmarson, G., Mitsotakis, N. “Development of a Method for Assessing the Strength of
Composite Materials in Fire”, MEng Group Design Project Report, School of Engineering Sciences, Ship Science,
University of Southampton. (2004).
Krysl, P., Ramroth, W. T., Stewart, L.K., Asaro, R,J. "Finite element modelling of fibre reinforced polymer
sandwich panels exposed to heat." International Journal for Numerical Methods in Engineering 61: 49-68. (2004).
Lattimer, B. Y. and Ouellette, J. "Properties of composite materials for thermal analysis involving fires."
Composites Part A: Applied Science and Manufacturing 37(7): 1068-1081(2006).
Lua, J., O'Brien, J., Key, C.T., Wu, Y., Lattimer, B.Y. "A temperature and mass dependent thermal model for fire
response prediction of marine composites." Composites Part A: Applied Science and Manufacturing 37(7): 10241039. (2006).
Ramroth, W. T., Krysl, P., Asaro, R.J. "Sensitivity and uncertainty analyses for FE thermal model of FRP panel
exposed to fire." Composites Part A: Applied Science and Manufacturing 37(7): 1082-1091. (2006).
Looyeh, M. R. E., Rados, K. , Bettess, P. "Thermochemical responses of sandwich panels to fire." Finite
Elements in Analysis and Design 37(11): 913-927. (2001).
Grenier, A. T., Dembsey, N. A., Barnett, J.R. "Fire Characteristics of Cored Composite Materials for Marine Use."
Fire Safety Journal 30: 137-159. (1998).
Wu, Y.-S., Wilcox, J. A. D. Gibson, A.G., Bettess, P. “Design for Composite Panels for Mechanical and Fire
Performance”, Final report prepared for British gas, University of Newcastle. (1993).