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H. Najmi et al.
one-piece design and eliminate the complex assemblies and joints which are the
potential failure points [1, 2].
As part of the fire safety regulations, all the manufactured airplanes must comply
with definitive standards (i.e. FAR 25.853). It states that all the materials used in
aircraft interiors should have a maximum peak heat release rate of 65 kilowatts per
square meter, a maximum total heat release of 65 kilowatt minute per square meter,
and the specific optical smoke density of 200 (i.e., the OSU 65/65/200 fire safety
standard defined by Ohio State University) [3]. The fire performance of materials
used in aircraft interiors is evaluated at an incident flux of 35 kW/m
2 in an OSU/Cone
calorimeter [3, 4]. The OSU calorimeter is difficult to access outside of accredited
testing laboratories and consequently expensive to use [5]. Therefore, in numerous
research works, a cone calorimeter is used [4, 5].
In order to comply with fire norms set by the Federal Aviation Administration
(FAA) different types of fire retardants are also used. These flame-retardants can have
significant harmful effects too such as invisible toxic gases, release of increased
amounts of carbon monoxide and hydrogen cyanide at the time of fire [6]. It is
also found that continuous exposure to some of the fire retardants can also cause
serious health problems such as cancer [7]. Therefore, in order to overcome all the
above drawbacks, the concept of multilayer materials has been brought into picture.
The regulation (i.e. FAR 25.853) in case of multilayer materials stipulates that each
element must be qualified and certified according to a standardized test, i.e. each of
the materials of the whole assembly must meet normative requirements, just like the
assembly itself [7]. In some cases, the individual materials pass the fire tests but the
whole assembly does not because the thermal interaction between the materials is
very complex to predict. This results in loss of money as well as time.
In most of the multilayer materials, Nomex honeycomb structure is used as a
core material due to its lightweight and flexibility. Typically, a honeycomb is sandwiched between fiberglass-reinforced phenolic resin skins joined with an adhesively
[8]. A study on a phenolic glass fiber suggests that a phenolic resin is completely
exhausted at 600 °C) [9]. The primary reactive agent in phenolic-based composite is
the phenolic resin itself [10]. As per the author’s knowledge, there are some studies
in which non-destructive evaluation is carried out on a honeycomb structure [11, 12]
but no dedicated study concerning thermal degradation of a honeycomb is found. In
some previous studies, the thermal decomposition of a multilayer composite in cone
and OSU calorimeter at the incident flux of 35 kW/m
2 is studied [8, 13]. It is difficult
to identify which material is taking part in the decomposition process at any given
instance only by looking at the global decomposition process. These studies are not
sufficient to have an insight into the decomposition process and determine the contribution of each layer into the fire. To understand the decomposition process of such
complex material, it is important to study the decomposition process of each material
separately prior to the multilayer assembly test. Therefore, in the present study, first,
the materials are studied separately and then with different assemblies in order to
understand how the addition of different layers affects the thermal decomposition
process.
H. Najmi et al.
one-piece design and eliminate the complex assemblies and joints which are the
potential failure points [1, 2].
As part of the fire safety regulations, all the manufactured airplanes must comply
with definitive standards (i.e. FAR 25.853). It states that all the materials used in
aircraft interiors should have a maximum peak heat release rate of 65 kilowatts per
square meter, a maximum total heat release of 65 kilowatt minute per square meter,
and the specific optical smoke density of 200 (i.e., the OSU 65/65/200 fire safety
standard defined by Ohio State University) [3]. The fire performance of materials
used in aircraft interiors is evaluated at an incident flux of 35 kW/m
2 in an OSU/Cone
calorimeter [3, 4]. The OSU calorimeter is difficult to access outside of accredited
testing laboratories and consequently expensive to use [5]. Therefore, in numerous
research works, a cone calorimeter is used [4, 5].
In order to comply with fire norms set by the Federal Aviation Administration
(FAA) different types of fire retardants are also used. These flame-retardants can have
significant harmful effects too such as invisible toxic gases, release of increased
amounts of carbon monoxide and hydrogen cyanide at the time of fire [6]. It is
also found that continuous exposure to some of the fire retardants can also cause
serious health problems such as cancer [7]. Therefore, in order to overcome all the
above drawbacks, the concept of multilayer materials has been brought into picture.
The regulation (i.e. FAR 25.853) in case of multilayer materials stipulates that each
element must be qualified and certified according to a standardized test, i.e. each of
the materials of the whole assembly must meet normative requirements, just like the
assembly itself [7]. In some cases, the individual materials pass the fire tests but the
whole assembly does not because the thermal interaction between the materials is
very complex to predict. This results in loss of money as well as time.
In most of the multilayer materials, Nomex honeycomb structure is used as a
core material due to its lightweight and flexibility. Typically, a honeycomb is sandwiched between fiberglass-reinforced phenolic resin skins joined with an adhesively
[8]. A study on a phenolic glass fiber suggests that a phenolic resin is completely
exhausted at 600 °C) [9]. The primary reactive agent in phenolic-based composite is
the phenolic resin itself [10]. As per the author’s knowledge, there are some studies
in which non-destructive evaluation is carried out on a honeycomb structure [11, 12]
but no dedicated study concerning thermal degradation of a honeycomb is found. In
some previous studies, the thermal decomposition of a multilayer composite in cone
and OSU calorimeter at the incident flux of 35 kW/m
2 is studied [8, 13]. It is difficult
to identify which material is taking part in the decomposition process at any given
instance only by looking at the global decomposition process. These studies are not
sufficient to have an insight into the decomposition process and determine the contribution of each layer into the fire. To understand the decomposition process of such
complex material, it is important to study the decomposition process of each material
separately prior to the multilayer assembly test. Therefore, in the present study, first,
the materials are studied separately and then with different assemblies in order to
understand how the addition of different layers affects the thermal decomposition
process.