Introduction
Steel is a robust shipbuilding material with a high limit for destruction, both when
it comes to temperature and loading. Uninsulated structural steel divisions
generally start to deteriorate at 400-500°C. However, permanent deformation and fire
spread may occur to large areas when structures are heated to temperatures below
those levels, both due to deformations and due to heat conduction. An exemplified
alternative non-combustible material in SOLAS
is aluminium, despite relatively poor structural behaviour at elevated temperature.
Similarly, FRP composite could provide the same rigid and strong qualities as steel
if excessive temperature increase is avoided. Other benefits with FRP composite are
the minimization of maintenance, lack of corrosion, prolonged fatigue life, reduced
efforts for repairs and, above all, reduction in weight. However, the material is
not non-combustible according to SOLAS
definitions and this has effects on fire safety. Below follow descriptions of how
different materials can be combined to make up FRP composite as well as more details
on the different materials. Thereafter follow descriptions of their behaviour when
exposed to fire.
B.1 FRP composite compositions
1 A typical FRP composite structure in shipbuilding is the sandwich panel with a
lightweight core separating two stiff and strong FRP laminates, as illustrated in
figure 1. When the laminates are bonded on the core the composition altogether makes
up a lightweight construction material with very strong and rigid qualities. The key
to these properties is anchored in the separation of the laminates. It makes them
effective in carrying all in-plane loads and bending loads. The core, separating the
face sheets, carries local transverse loads as shear stresses, comparable with how
webs of stiffeners contribute in stiffened steel panels. The way the materials are
combined makes the construction altogether function as a "stretched out I-beam"
which may not need additional stiffeners. The FRP composite sandwich panel has a low
in-plane modulus of elasticity compared to steel. However, due to the "I-beam" type
of construction, the panel becomes very stiff with regard to bending. The FRP
composite structure is able to deform elastically under high strains and this can
reduce stress concentrations in the interface between for example a steel hull and
FRP composite deckhouse or superstructure. This reduces fatigue problems and steel
weight.
Figure 1: Illustrations of an FRP composite sandwich panel composition
2 Another FRP composite structure is the single skin panel, consisting of one single
fibre reinforced laminate. Other FRP designs are also viable, e.g. triple skin (two
cores and three laminates). The composite design could also include stiffeners.
B.2 FRP composite constituents and fire behaviour
The fire performance of FRP composite structures depends on the used materials and
their combined behaviour at elevated temperatures. Knowledge of the materials is
therefore crucial. Common core materials in FRP composite structures are for example
polymer-based foams, cellulosic or metallic honeycomb cores and balsa wood. The
laminate face sheets are generally made of carbon or glass fibre reinforced polymer.
However, there is a constant development of new FRP composite materials and the
variety of materials is large. These guidelines are not extensive when it comes to
the description of various FRP composite materials but some common materials for
marine structures, i.e. where most experience has been accumulated, are briefly
described below.
B.2.1 Polymers
1 A common processing method is hand layup with resin infusion and curing at elevated
temperature (60-80°C) or post-curing. The resins normally used are polyester,
vinylester and epoxy. Marine grades of these materials do not differ very much with
respect to behaviour in fire or at elevated temperatures; unmodified they give
comparable smoke production and heat release. Heat weakens the polymer of an FRP,
which means that structural strength is challenged in a fire event. A key property
is therefore the heat distortion temperature for the cast resin (not the laminate),
where half the stiffness is reached, comparable to glass transition temperatures for
polymers. For normal room temperature cured systems the heat distortion temperature
is usually about 70-100°C but systems may be produced with significantly improved
properties.
2 With regard to fire contribution, figure 2 shows the weight loss (left Y-axis) of a
moderately performing polyester polymer used in an FRP laminate as a function of
temperature increase and also its derivative (right Y-axis). It can be seen that the
polymer will not contribute significantly to a fire until heated to ~350°C, which is
a common range for the polymer pyrolysis temperature. It should be noted that this
temperature of significant weight loss is significantly higher than the point at
which aluminium is structurally useful. Therefore, FRP composites do not contribute
to a fire until reaching a temperature beyond which a currently acceptable
non-combustible material has ceased to either provide structural support or restrict
the spread of fire.
Figure 2: Thermo Gravimetric Analysis of a standard FRP polyester polymer
3 The resins referred to above are all combustible and with comparable smoke
production and heat release. There are also numerous modified resin systems that can
provide better fire performance in terms of fire, smoke and toxic gas formation
properties, sometimes at penalty of processing properties, mechanical properties or
increased fire smoke production.
B.2.2 Fibres and reinforcements
1 When it comes to reinforcing fibres, E-glass and carbon fibres are currently most
common. Polymeric fibres such as aramids (e.g. Kevlar and Twaron) are also used and
other fibre types may be developed in the future.
2 E-glass fibres have been common mainly due to a good strength to cost ratio.
E-glass fibres remain unaffected in fire until heated to about 830°C when viscous
flow starts. Nonetheless, mechanical properties such as strength and stiffness
decrease from around 500°C.
3 Carbon fibres are more heat resistant than glass fibres and are also
common. They are unaffected by temperatures up to about 350°C and oxidize at a
temperature of 650°C to 700°C (i.e. far above the temperature at which typical
resins decompose). In addition, carbon fibre mats exhibit better heat distribution
properties than glass fibres, which can avoid the occurrence of "hot spots".
4 While the polymer may contribute to the fire and increase its severity, the
reinforcing fibres do not normally add to the fire intensity. On the contrary, as
they often are quite inert, they serve as a temperature barrier and thermal
insulator. However, a hazard is the possibility of fibres being spread to the
environment from a fire event. Such fibres are known to cause skin/throat/eye
irritation in the vicinity of a fire.
B.2.3 Core materials
1 Polymer-based foams and balsa cores are often used in shipbuilding. Figure 3 shows
a similar analysis as in figure 2 but for a PVC (polyvinyl chloride) foam core
material. It shows no weight loss, and thereby no fire contribution from the
material, until reaching ~250°C. The high smoke and toxicity generation potential of
PVC has led to an increased use of other polymer-based foams.
Figure 3: Thermo Gravimetric Analysis of PVC core foam
2 Different core materials have varying responses to fire exposure. Typical behaviour
of polymer-based foams at high temperature is melting, softening and shrinking,
whereas end-grain balsa wood chars (generally at temperatures exceeding 200°C to
250°C). Balsa wood does not have a softening temperature nor does it shrink in the
same way as a polymer, and the smoke generation potential is generally more limited.
Note that in this context PVC and balsa cores have been provided as examples but
other cores exist and may be developed. In each case a clear understanding of the
fire performance of the core material is necessary.
B.3 Fire performance of FRP composite, key issues and means for
improvement
1 The performance of an FRP composite structure when exposed to fire varies with the
composition of core and laminates but mainly depends on the following five
conditions:
-
.1 type of polymer and thickness of laminate;
-
.2 type and density of core;
-
.3 type and amount of fire protection (e.g. insulation); and
-
.4 structural support, e.g. stiffeners.
2 Some typical critical temperatures for an FRP composite sandwich panel using
standard polyester-based FRP laminates and a PVC foam core are summarized in figure
4. Spontaneous ignition of the laminate could typically occur at 350°C to 400°C and
the core material will lose structural integrity at certain temperatures due to
phase transitions (melting, vaporizing). However, the composite sandwich
construction will generally lose its structural strength at temperatures well below
such temperatures (discussed above for the individual materials). For a load-bearing
structure it is thus more critical to manage structural integrity than ignition and
fire involvement. Loss of the mechanical properties of a sandwich panel may be
claimed to be associated with delamination, i.e. when a significant part of the
laminate is detached from the core. In fire testing sandwich panels under load, it
has been found, e.g. for the above-mentioned sandwich systems, that overall
structural failure of the panel often occurs when the bond of the laminate skin to
the core reaches a critical temperature. It is important to note that this will
generally occur much sooner than ignition in a fire situation. Softening of the skin
to core bond then results in the structure ceasing to act as a sandwich panel and
failing by buckling of the resulting thin skin structure. However, it should be
noted that the thermal insulating quality of the composite allows for local hot
spots without compromising an entire structure. It is in other words required that a
sufficient percentage of a load bearing element is heated before a collapse occurs.
There are also remedies to lower the risk of structural collapse, e.g. supporting
stiffeners or pillars.
Figure 4: Typical critical temperatures for an FRP composite sandwich (PVC core,
polyester FRP)
B.3.1 Structural fire performance of FRP composite structures
1 FRP composite can never fulfil "A" class requirements as defined by SOLAS,
since "A" implies "non-combustible" according to SOLAS regulation
II-2/3.2. It further implies 60 minutes fire resistance, represented by a
temperature rise in a large furnace according to the standard temperature-time
curve, as defined by the ISO. FRP composite and metallic construction materials
differ conceptually from a fire safety point of view. Not only from a reaction to
fire perspective (ignitability, smoke and heat production) but also from a
resistance to fire perspective (structural integrity and heat transfer). In the
SOLAS requirements for fire resistance, metallic materials are expected
to keep the temperature increase at the unexposed side of the bulkhead or deck in
the standardized fire test below ~200˚C for 0, 30 minutes or 60 minutes, depending
on the requirements for the particular space. The motive is to control the risk for
fire spread to compartments adjacent to the fire compartment. A steel construction
could still be load carrying for a long time after such temperatures are reached,
whereas, e.g. an aluminium construction would start to lose its structural strength
at about 200˚C. A steel construction is therefore allowed with insulation on one
side of the division, whereas aluminium constructions must be insulated on both
sides. The same would be true also for FRP composites.
2 An FRP composite is a good thermal barrier. The fundamental condition for the FRP
composite to achieve structural integrity "equivalent" to an A-class division is
therefore not the temperature requirement at the unexposed side but that structural
resistance is maintained for 60 min. As discussed above, an FRP composite structure
will generally start to lose structural strength below 200˚C, and an FRP composite
deck or bulkhead would therefore start to lose its structural integrity long before
the temperature at the unexposed side approaches 200˚C. Thus, an FRP composite
construction generally achieves SOLAS regulation
II-2/9 "Containment of fire" much better than metallic materials due to
its insulating capacity but has problems to fulfil SOLAS regulation
II-2/11 "Structural integrity". Therefore, if structural collapse due to
heat in an FRP composite construction can be avoided, the FRP composite design has a
major advantage to metallic materials since fire spread due to heat transfer is a
much lower risk in FRP composite than in metallic materials.
3 To achieve structural resistance in FRP, it is important to keep temperatures down,
which is achievable through insulation or cooling. Structural fire performance may
also be achieved by structurally redundant design, e.g. by using pillars, stiffeners
or sandwich panels with over-capacity (e.g. triple skin panels designed so that half
of the structure is sufficient to carry the design load). If redundancy of the
constructions' load-bearing capacity is incorporated in the design, a fire could be
well contained within the fire enclosure for a long time before spreading to other
areas through the structure.
4 While structural performance is maintained, a fire will actually be better
contained than in a prescriptive steel design since the insulating capacity of the
composite will add significantly to the total insulating capacity of the
construction. Since the heat is well kept within the fire enclosure, the overall
temperature may also be higher compared to in a steel enclosure. Thus, a more
intense fire with higher temperatures is possible using an FRP composite
construction but the fire is more localized and less likely to spread due to heat
transfer than in a metallic construction. The high temperatures motivate water-based
extinguishing systems since inertion by evaporated water is well facilitated.
5 If active and passive risk control measures fail and the fire falls out of control,
a heat induced structural collapse could occur. The FRP composite could then also
take part in the developing fire.
B.3.2 Firefighting of FRP composite structures
1 The high insulation capacity also affects the way of fighting fires in the
construction material. In general when a fire appears on a vessel, water cooling of
boundary surfaces of the fire enclosure is a basic strategy in maritime
firefighting. When using FRP composites instead of metallic materials, such cooling
is more or less meaningless since the outer surfaces of the fire room will have very
low temperatures for a long time and also, the insulating capacity of the material
will make such construction cooling ineffective. Instead, firefighting must take
place inside the fire enclosure. Suitable firefighting equipment is already in use,
such as the Cutting Extinguisher or small prefabricated inlets for nozzles which
allow firefighting without entering the room. This is further discussed in 3.10
(regulation 10 – Firefighting).
2 The combustion of FRP composites is dependent on a thermal breakdown of organic
molecules in the material. The insulating quality of the material will initially
create a very steep temperature gradient in the material when subjected to a fire.
If the material is cooled down, the production of combustible gases is hindered and
the fire is stopped. This cooling should be applied to the hot surface. Empirical
testing has shown that early application of water (which also requires fast
detection) on a burning surface will quench the pyrolysis reactions in the FRP
composite quite quickly.
3 If the fire has given sufficient heat exposure for the FRP composite to reach
pyrolysis temperatures also deeply within the construction, fire tests have shown
that continuous cooling may be necessary to prevent reignition. In particular, the
core works as a thermal barrier, both in heat exposure and during cooling. Thus, for
efficient firefighting it is beneficial if surfaces within a fire enclosure are
cooled down as soon as possible. Active systems with quick response could therefore
be useful.
4 A gaseous extinguishing system should be avoided since it will not provide the
necessary cooling of the material at the surface. See also the discussion in section
B.3.1 concerning evaporation advantages in well-insulated enclosures.
B.3.3 Exterior surfaces in FRP composite
Exchanging traditional external steel surfaces for combustible FRP composite will
give a fire the ability to propagate vertically if a window breaks or if an external
door is left open. The fire can then potentially spread between decks and fire
zones. This issue has been given much attention and full scale tests have been
carried out in order to find suitable mitigating measures. Producing FRP face sheets
with low flame-spread characteristics or installing a drencher system for external
surfaces are alternatives to avoid fire spread. Fire rated windows and doors are
other fire safety measures that could be relevant. It may also prove necessary to
provide some kind of structural redundancy, as described above, addressing external
fire exposure.
B.3.4 Steel-FRP joints
1 An important area for an assessment of fire safety of FRP elements is the steel-FRP
joints. A hazard associated with steel-FRP joints is the possibility of conduction
of fire induced heat in the steel structure to an adhesive joint. If the adhesive
reaches a critical temperature, the joint will fail. Furthermore, there are combined
effects of differences in thermal expansion and other properties (e.g. heat
conductivity, elastic modulus, combustibility) which could cause loss of structural
and fire integrity.
2 Steel-FRP joints must be properly assessed to ensure that they are sufficiently
protected from fire and heat deterioration. The assessment of steel-FRP joints
should be part of the SOLAS regulation II-2/17 assessment and can include performance of
fire tests.
3 Furthermore, it must be ensured that the structural fire integrity of the steel-FRP
joint is maintained throughout its service life.