# Comparison of the Tensile Behavior of Carbon Fibers Reinforced Laminates Exposed to Fire Conditions

J. Vacandare, B. Vieille, S. Patel, C. Naejus, M. Denize

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J. Vacandare, B. Vieille, S. Patel, C. Naejus, M. Denize. Comparison of the Tensile Behavior of Carbon Fibers Reinforced Laminates Exposed to Fire Conditions. Fire Technology, 2025, 10.1007/s10694-025-01765-3. hal-05243634

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# Comparison of the Tensile Behavior of Carbon Fibers Reinforced Laminates Exposed to Fire Conditions

J. Vacandare¹ · B. Vieille¹ · S. Patel² · C. Naejus² · M. Denize²

Received: 2 September 2024 / Accepted: 19 May 2025 / Published online: 27 June 2025
© The Author(s) 2025

## Abstract

This study investigates the influence of a kerosene flame aggression (characterized by a 116 kW/m² heat flux and an 1150 °C flame temperature) and a tensile loading (monotonic or creep) on the deformation and damage mechanisms of quasi-isotropic carbon fibers reinforced laminates consisting of different thermosetting matrix systems (epoxy and bismaldeide). The influence of the carbon fibers reinforcement (unidirectional and woven fibers) along matrix nature were examined on both the fire and mechanical responses of the laminates. A specific fire bench was used to conduct such tests while monitoring the changes in the axial stress and strain as well as temperature along flame exposure time. First, from monotonic testing conducted under fire conditions, the axial strength is about 50 and 30% lower than woven ply C/BMI laminates, in unidirectional (UD) and woven ply C/Epoxy laminates, respectively. These results agree with the tendency observed in virgin specimens. After a 300 s exposure, the axial strength decreases by 40 to 60% of their initial values for studied materials with respect to their virgin state. After a 900 s exposure, the drop has almost stabilized at about 30 to 40% of their initial values. Woven ply laminates being characterized by matrix-rich areas at the crimp, the thermal decomposition primarily occurs in these areas. As a result, the thermally-induced damages (porosities formation and extensive delamination) in these areas contribute the thermal transfers to be modified within the laminates. Second, creep testing under fire conditions were performed to comply with the fire certification standards. The underlying idea of these tests is to identify the axial creep stress to be applied to ensure that the loading bearing capabilities of the composite part are preserved for 900 s. With respect to C/BMI laminates, under the same fire conditions, the maximum applied stress is 30 and 20% lower in UD and woven ply C/Epoxy laminates, respectively. Based on the changes in the time-to-failure as a function of the creep tensile force, it is possible to get analytical expressions, which are the first step towards the definition of simple design rules useful to engineers willing to meet safety requirements.

**Keywords** Thermoset composites · Thermal decomposition · Tension · Creep · Kerosene flame
# 1 Introduction

Given the growing utilization of polymer matrix composites (PMCs), understanding their mechanical behavior under fire conditions becomes crucial due to their inherent flammability. In their review, Quan et al. offer guidance for the development of fire-safe PMCs by exploring the thermal decomposition and mechanical behavior in the cone calorimeter under various testing conditions (heat flux, thermal boundary conditions, specimen geometry and mechanical loading) [1]. Tests conducted at varying incident heat fluxes are relevant to estimate critical service conditions (influence of heat flux on ignition temperature), establishing correlations between thermal aggression by a cone calorimeter and time to ignition [2].

## 1.1 Fire Reaction of PMCs

Understanding how PMCs behave at high temperatures is crucial for ensuring passenger safety in aeronautics. When exposed to temperatures above their thermal decomposition temperature, PMCs can experience degradation in their mechanical and physical properties, affecting their ability to bear loads [3–6]. Two main types of matrix systems, thermoset (TS) and thermoplastic (TP), are used in aeronautical applications. While TS matrices have been predominant for decades, TP matrix-based composites are gaining interest due to their quicker manufacturing process and enhanced damage tolerance and impact resistance [7]. The major types of fire damage suffered by laminates are char formation, softening and degradation of the matrix and fibers, delamination and matrix cracking. The thermal decomposition behaviors that determine the fire reaction properties of fiber-reinforced polymers (TS and TP) have been investigated [8]. When exposed to fire-induced thermal loads, composite materials with different matrix systems and fiber architectures respond in distinct ways, primarily due to the unique properties of the matrix and the fiber structure. In a composite material the polymer matrix plays a key role in its behavior under thermal loads because it is more thermally sensitive than the reinforcing fibers. The matrix nature dictates how the material decomposes during pyrolysis. TS resins decompose through complex chemical reactions, typically forming volatile gases, char, and potentially carbon dioxide. They tend to have higher thermal stability compared to TPs, but they still undergo significant degradation at elevated temperatures. As they pyrolyze, they can form a protective char layer on the surface, which can act as a heat barrier, slowing the rate of degradation for the underlying material. These materials are often used in applications requiring good fire resistance, as their char layer can slow down the combustion process. In contrast, TPs soften when heated and eventually melt before decomposing. The process involves the polymer chains breaking down into smaller volatile molecules, leading to gas production. These materials are less thermally stable than thermosets and may melt away from the heat source. It is observed that TP-based composites generally have higher decomposition temperatures, yield high amounts of char, and are less susceptible to delamination cracking than thermoset laminates [8]. As a result, TP-based composites usually have higher post-fire mechanical properties (stiffness and strength). The fibers in composite materials are typically more thermally stable than the matrix but their behavior under thermal loads is influenced by how they are arranged in the composite material. In UD composites, fibers are aligned in a single direction, leading to a high strength-to-weight ratio. However, the fibers themselves are resistant to thermal damage, while the matrix undergoes
decomposition. The unidirectional arrangement can cause the material to lose its structural integrity as the matrix decomposes, even though the fibers themselves retain their form. The fibers do not undergo pyrolysis themselves but can be affected by the degradation of the matrix. If the matrix is prone to significant pyrolysis, the fiber-matrix bond can weaken, leading to potential failure of the composite. In woven or fabric-based fiber reinforced **composites**, the composite’s mechanical response to heat may differ from UD composites. Woven fabrics can provide greater stability and toughness under thermal loading. During pyrolysis, woven fabrics can offer better resistance to structural collapse due to the multi-directional fiber reinforcement, which helps maintaining the mechanical properties despite the matrix decomposition.

## 1.2 Fire Resistance of PMCs

Despite extensive research, understanding the fire resistance of PMC materials remains a complex challenge due to various thermal and mechanical factors [8]. Recent studies have focused on analyzing the post-fire mechanical behavior of PMCs to comprehend how they respond under fire conditions [9, 10]. Sorathia et al. conducted experiments comparing the mechanical behavior of various PMCs (TS vs TP laminates) under low heat fluxes [11]. They found that carbon/PolyEtherEtherKetone—PEEK—and carbon/phenolic composites exhibited the highest retention of flexural strength. Levchik associated a specific chemical structure with the ability to produce a substantial carbonaceous residue called char resulting from polymer matrix pyrolysis [12]. Other studies refer to the comparison of the fire-behavior of TS- and TP-based composites (carbon/epoxy vs carbon/PolyPhenylene-Sulfide—PPS—composites) under different heat fluxes [13–15]. They concluded that the residual tensile properties of quasi-isotropic C/PPS laminates are higher to the ones obtained in C/epoxy laminates, as a result of a higher char yield. They also observed variations in char structure between the two composites.

When it comes to the compressive behavior after exposure to various heat fluxes (ranging from 20 to 50 KW/m²), the mechanical response of C/PPS laminates is ruled by the ductility of PPS matrix at temperatures higher than its glass transition temperature (Tg) [16]. Fire-induced damage in TS-based composites leads to micro-buckling, while TP-based laminates are characterized by the initiation and propagation of plastic kink bands. More recently, Loh et al. investigated the impact of thickness on the behavior of composite structures under fire conditions and compression loads [17]. By testing glass fiber reinforced vinyl-ester laminates of different thicknesses, they showed that thicker laminates exhibit slower rates of polymer matrix softening and thermal decomposition. The thickness variations significantly affect the structural performance and failure mode under fire and compression loads, with complex failure mechanisms observed. Additionally, they developed a thermal-mechanical model allowing the time-to-failure to be predicted as a function of the compressive loading and applied heat flux. According to many Refs. [3, 8, 18–24], there is a significant reduction in mechanical performance of polymer matrix composites during exposure to flame, with a particular interest in creep-induced failure [6]. However, as a result of the inherent thermal decomposition of polymer, polymer composite parts often quickly form a layer of protective char and can then sustain a significant proportion of strength over extended periods of time [25]. This results from the onset of pyrolysis that has an endothermic nature and helps to reduce the temperature [18, 26, 27]. Preliminary studies were conducted on hybrid carbon/glass fibers reinforced PEEK thermoplastic matrix laminated composites [3, 24]. The influence of a prior kerosene flame exposure
(116 kW/m² and 1100 °C [28]), on the residual mechanical properties (in tension and in compression) was examined as a function of exposure time [24]. From this study, it is possible to conclude that the mechanical properties in tension are severely affected (– 50% in stiffness and – 70% in strength) by prolonged exposures to kerosene flame (15 min) with respect to as-received specimens. Flame exposure time (ranging from 5 to 15 min) seems to have very little influence on tensile properties and the effect is moderate on compressive properties. The barrier formed by an extensive thermally-induced delamination helps to relatively preserve the structural integrity of the plies near the back-surface. The mechanical loading is taken up by the 0° fibers in non-delaminated areas of the specimens, resulting in preserving the residual mechanical properties.

## 1.3 Objectives of the Study

There's a lack of research on the mechanical behavior of PMCs after kerosene flame exposure. Existing literature primarily focuses on UD carbon fibers reinforced PMCs (e.g. C/ epoxy), particularly in compression failure studies, which are crucial for long fiber-reinforced polymer composites [18, 21–23]. Defining the boundary between non-decomposed and fire-decomposed zones is challenging [29–32]. Additionally, carbon fibers undergo oxidation beyond 500 °C, a common temperature in fire scenarios, posing a significant problem [31]. Research on strain evolution during compression under fire conditions is scarce, with most studies concentrating on time-to-failure [23, 32]. Based on the literature review, comprehensive investigations are necessary to fully understand the contribution of matrix nature and carbon fibers reinforcement to the tensile behavior of quasi-isotropic laminates subjected to fire conditions depending on the loading conditions (monotonic or creep testing). The influence of flame exposure time on the tensile properties (axial stiffness and strength) has been quantified. Creep testing provides an estimate of the time-to-failure depending on the axial creep stress to determine an analytical design rule. The understanding of the thermo-mechanical coupling within the studied laminates relies on the examination of damages along temperature changes during thermal/mechanical loading.

# 2 Material and Methods

## 2.1 Materials and Specimens

Three types of carbon fibers reinforced laminates consisting of different thermosetting matrix systems (epoxy—8552—and bismaldeide—BMI) are considered in this study. The carbon fibers reinforcement is either unidirectional (denoted UD) in C/Epoxy laminates or consists of a 5-harness satin weave in C/Epoxy and 8-harness satin weave in C/BMI laminates. The stacking sequence of each material is as follows:

* C/Epoxy 8552 UD: 12 plies with a quasi-isotropic lay-up [+45, 0, – 45, 90, 45, 0]s—33% of 0° plies.

* C/Epoxy 8552 W: 8 plies with a quasi-isotropic lay-up [(+ – 45), (0/90), (+ – 45), (90/0),]s—25% of 0° plies.

* C/BMI W: 6 plies with a quasi-isotropic lay-up [(+ – 45), (0/90), (90/0)]s—33% of 0° plies.
The temperature of interests (Glass transition $T_g$ and onset of pyrolysis $T_d$) are given in Table 1. The average thickness of specimens is 2.2, 2.38, 2.38 mm in C/epoxy UD, C/ Epoxy W and C/BMI laminates, respectively. The tensile specimens (Fig. 1) were cut by water jet from large panels (500×500 mm²). The geometry of these specimens is designed to meet the requirements of the boundary conditions imposed by the fire bench.

Figure 2 shows the through-the-thickness microscopic observations of studied laminates. These observations provide valuable information on the mesostructures of studied laminates in the virgin state to assess the influence of flame exposure on internal damage and swelling (resulting from pyrolysis and thermal expansion).

## 2.2 Experimental Set-Up

In a previous study, a kerosene-flame burner has been designed and built allowing to thermally stress one side of small samples with a flame, reproducing the scenario of an engine fire [19]. The burner has been optimized to provide the same conditions encountered in standard tests [28], in terms of flame temperature (1100 °C) and heat flux (116 kW/m²). Fire resistance testing of components made of carbon fibre reinforced polymers (CFRP)

<table><caption>**Table 1** Temperatures of interest for studied composites</caption><thead><tr><th>Materials</th><th>C/Epoxy UD</th><th>C/Epoxy woven</th><th>C/BMI woven</th></tr></thead><tbody><tr><td>Glass transition temperature T<sub>g</sub>(°C)</td><td>210</td><td>215</td><td>254</td></tr><tr><td>Onset of pyrolysis T<sub>d</sub>(°C)</td><td>369</td><td>375</td><td>485 [31]</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>**Fig. 1** Tensile testing under fire conditions: geometry and dimensions of specimens [33]</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Fig. 2** Through-the-thickness microscopic observations of studied laminates with a quasi-isotropic stacking sequence showing the presence of matric-rich regions: a C/Epoxy UD—b C/Epoxy woven—c C/BMI woven</figcaption></figure>
usually demands intermediate-scale or full-scale testing. Small-scale testing is a cost-effective means of characterizing load-bearing behavior of composites in fire and a useful framework for materials development. At small scale, highly repeatable screening tests can be performed to enable better analysis of the fire degradation of composite materials through advanced metrology. On the one hand, there is also a need to reproduce calibrated thermal aggressions by a flame at small scale (hundreds of millimetres). On the other hand, the fire-induced degradation behaviour of materials can be investigated at small scale to get close to the ones encountered in standardised fire tests.

### 2.2.1 Thermal Degradation/Decomposition Under Fire Conditions

The kerosene burner shown in Fig. 3a is a domestic device (Cuenod manufacturer). The kerosene is injected in a nozzle generating a hollow cone spray with an angle equal to 80° and a maximum flow rate of 0.3 g/s [18, 20, 24]. This flow rate is controlled with a mass flow meter (MINI CORI-FLOW™—Bronkhorst), and it was adjusted. Airflow is also controlled with a mass flow meter (EL-FLOW® Prestige—Bronkhorst). The fuel to air ratio was selected at 0.85 of the stoichiometric value, in order to obtain heat flux and temperatures values close to the standard values (116 kW/m² and 1100 °C) at the sample location. The flame at the exit of the turbulator is a wide and turbulent jet. Therefore, a 50 mm diameter steel tube is installed after the turbulator to channel the hot combustion gases on the exposed area of the sample (Fig. 3a). The temperature was measured as a function of the exposure time by means of type-K thermocouples stuck on the back surface of specimens by means of a high temperature ceramic polymer adhesive.

While surface temperature measurements using thermocouples are useful and convenient, they have limitations in accurately representing the internal temperature distribution of a material. The primary limitations stem from the inability to directly measure internal temperatures, the time lag in response, variations in thermal conductivity, material heterogeneity, and external factors such as convective heat losses and surface conditions. In materials with significant thermal gradients—which is typically the case in laminates exposed to a one-sided kerosene flame—surface measurements may not accurately reflect internal temperatures. Pyrolyzed laminates often have heterogeneous thermal properties, meaning that their internal structure, composition, or phase can vary across the material. Variations in porosity, density, or composition can affect how heat is conducted through the

<figure><img src="image_4.png" /><figcaption>**Fig. 3** - In situ tensile behaviour of C/PPS laminates subjected to a kerosene flame during mechanical loading: a Kerosene burner—b description of the experimental bench—c tensile testing during fire exposure</figcaption></figure>
material, creating uneven thermal gradients. Thermocouples located at the surface cannot detect these internal variations and might provide misleading conclusions about the overall thermal behavior of the material. Finally, the accuracy of surface temperature measurement depends on the condition of the material’s surface. As the unexposed surface may experience thermal decomposition under extended exposure times, it may affect the thermocouple’s ability to measure the true surface temperature accurately.

### 2.2.2 Ex Situ Tensile Testing

After thermal aggression under fire conditions, the residual tensile mechanical behaviour is investigated using a uniaxial servo-hydraulic machine MTS, equipped with hydraulic grips and a load cell with a 100 kN capacity. The tensile tests are conducted in displacement-controlled mode (1 mm/min) at Room Temperature (RT). The tensile mechanical properties were determined according to the European standards EN 6035 [34]. The axial modulus ($E_x$) and ultimate strength ($\sigma^u$) were calculated from the following definitions:

$$E_x = \frac{\Delta F}{S \cdot \Delta \varepsilon_x} \quad and \quad \sigma^u = \frac{F^u}{S} \qquad (1)$$

where $\Delta F$ is the difference in the tensile loads at $(\varepsilon_x)_2 = 0.25\%$ and $(\varepsilon_x)_1 = 0.05\%$, S is the specimen cross section, $\Delta \varepsilon_x = (\varepsilon_x)_2 - (\varepsilon_x)_1$ is the difference in the axial strains obtained from a blade extensometer, $F^u$ is the maximum force borne by the specimen at failure.

### 2.2.3 In Situ Tensile Testing Under Fire Conditions

Considering the fire bench introduced in Fig. 3b, tensile specimens were subjected to a monotonic tensile loading in force-controlled mode (at a loading rate of 500 N/s) during a kerosene flame exposure with a protective shield (Fig. 3c). The axial displacement was measured using a Linear Variable Differential Transformer (LVDT) sensor. A protective shield associated with a ceramic thermal insulation layer was designed to limit the thermal decomposition of the specimens with an exposure area of 40*20 mm² during kerosene flame exposure (Fig. 3c). Three samples were tested in the following configurations:

* Monotonic tensile loading without flame exposure (referred to as virgin state),

* Monotonic tensile loading after a 300 s (5') exposure to a kerosene flame,

* Monotonic tensile loading after a 600 s (10') exposure to a kerosene flame,

* Monotonic tensile loading after a 900 s (15') exposure to a kerosene flame.

The axial strain was computed from the LVDT displacement measurements. To a first approximation, it provides an estimate of the axial strain, therefore allowing the axial stiffness to be calculated.

### 2.2.4 Fractographic Analysis

Microscopic observations were carried out by means of a numerical optical microscope Keyence VHX-5000 to build 3D pictures of damaged areas. Fractographic analyses were conducted on samples cut in half by waterjet after thermal degradation/decomposition.
<figure><img src="image_5.png" /><figcaption>**Fig. 4** Tensile behaviour at room temperature of studied laminates (as-received state): a stress–strain curves—b Comparison of tensile properties (axial stiffness and strength) of studied laminates (as-received state) tested at room temperature</figcaption></figure>

<table><caption>**Table 2** Tensile properties of studied laminates tested at room temperature</caption><thead><tr><th>Materials</th><th>C/Epoxy UD</th><th>C/Epoxy woven</th><th>C/BMI woven</th></tr></thead><tbody><tr><td><i>E</i><sub>x</sub>(GPa)</td><td>38.5 ± 0.4</td><td>47.4 ± 0.8</td><td>60 ± 0.5</td></tr><tr><td><i>σ</i><sub>x</sub><sup>u</sup>(MPa)</td><td>483 ± 31</td><td>566 ± 38</td><td>681 ± 19</td></tr></tbody></table>

# 3 Results and Discussion

## 3.1 Tensile Behaviour in the Virgin State

To understand the influence of fire conditions on the tensile behaviour of studied materials, the first step is to conduct monotonic tensile tests at room temperature on virgin specimens (without prior flame exposure). Figure 4a shows that all the studied laminates have elastic-brittle macroscopic responses, in agreement with their quasi-isotropic lay-up (about ¾ of the tensile load is borne by the 0° plies). The tensile properties (axial stiffness $E_x$ and ultimate strength $\sigma_x^u$) were computed from Eq. (1) and were reported in Table 2. The portion of 0° plies within the laminates slightly differs from one case to another: 33% in woven ply C/Epoxy and C/BMI on the one hand; and 25% in UD ply C/Epoxy. As a result, the tensile response and properties (axial stiffness and strength) are different depending on the materials configuration. From the stiffness standpoint, C/BMI laminates are about 60 and 30% stiffer with respect to UD and woven C/Epoxy (Fig. 4b), respectively. From the strength point of view, C/BMI laminates are about 40 and 20% more resistant with respect to UD and woven ply C/Epoxy, respectively. These differences will be discussed along the influence of flame exposure time.

## 3.2 Temperature Changes on the Opposed Surfaces During Flame Exposure

The second step—towards the understanding of the influence of flame exposure on the in situ tensile behaviour of studied materials—is to conduct temperature measurements on the opposed surfaces of specimens as a function of the flame exposure time. The exposure of composite laminates to one sided kerosene flame leads to heterogeneous temperature
distributions within the laminates. The most straightforward technique to monitor temper-
ature changes at different locations is to stick thermocouples on the opposed surface of
specimens (Fig. 5). The temperature gradient refers to the differences in the temperature’s
measurement on the back surface in the center of the flame-exposed area (thermocouple
represented by a black dot) with respect to the values measured at growing distance (ther-
mocouple represented by a grey dot) as was shown in Fig. 5. Depending on the temperature
pyrolysis of each tested material, this is important as it reflects the damage gradients within
the laminates volume as will be discussed in the sequel of the paper. To a first approxima-
tion, these measurements provide a good estimate of temperature distribution depending on
exposure time. Figure 5 shows that the maximum temperature (at the center of the flame
exposure area—black curve) is reached after 150 to 200 s depending on the composite sys-
tem. The temperature values are compared to characteristic temperatures (Table 1):

* In UD ply C/Epoxy, the pyrolysis temperature (369 °C) is reached after about 120 s on the opposed surface, whereas the temperature measured by the other thermocouples (away from the center) remains lower after 600 s. The glass transition temperature is reached for all thermocouples locations after 600 s. The temperature gradient is about 280 °C at the end of the flame exposure. These temperature measurements suggest that the epoxy matrix has virtually lost most of its mechanical properties in all the plies of the laminates.

* In woven ply C/Epoxy, and contrary to what was observed in UD ply C/Epoxy, the thermal decomposition (375 °C) is barely reached in the center of the specimen after 600 s. The glass transition temperature is also barely reached for thermocouples 2 and 3; and not even reached for thermocouple 4. The temperature gradient is about 230 °C at the end of the flame exposure. These changes therefore suggest that the fabric reinforcement along the presence of matrix-rich areas may at the crimps may modify the thermal transfers within the plies of the laminates. It also means that woven-ply C/Epoxy laminates should retain higher load bearing capabilities as the Epoxy matrix is less thermally decomposed.

<figure><img src="image_6.png" /><figcaption>**Fig. 5** Evolution of temperature on the opposed surface (measured by four thermocouples stuck at different positions) as a function of kerosene flame exposure time: a C/Epoxy UD—b C/Epoxy woven—c C/BMI woven</figcaption></figure>
* In woven ply C/BMI, whose pyrolysis temperature is 485 °C, the pyrolysis temperature is reached after only 120 s, whereas the glass transition is barely reached for other thermocouples. The temperature gradient is about 300 °C at the end of the flame exposure. With respect to C/Epoxy laminates, the maximum temperature is significantly higher in C/BMI laminates (about 550 °C vs 400–480 °C), therefore suggesting that (i) the thermal conductivity of BMI is higher—(ii) the thermal decomposition of BMI is instrumental in modifying the thermal transfers within the laminates at distance from the flame exposed area.

From Fig. 5, it also appears that C/BMI laminates exhibit a plateau in temperature after 5 min while the temperature of C/Epoxy laminates continue to gradually increase. This difference depends on the pyrolysis temperature of the polymer, which starts at about 370 °C in epoxy-based laminates, whereas it starts at about 485 °C in C/BMI composites. The saturation of temperature is related to the occurrence of delamination through-the-thickness, resulting in slowing down the thermal transfers from the exposed to the back surface. In woven-ply specimens, the presence of matrix-rich areas (Fig. 2) also leads to the formation of large porosities (by means of the polymer matrix pyrolysis) which may contribute to absorb the thermal energy brought by the flame. Ultimately, these temperatures distribution are expected to reflect on the deformation and damage mechanisms taking place in laminates subjected to a combined thermal aggression and tensile loading. To achieve a more accurate measurement of internal temperature gradients, additional techniques such as infrared thermography, embedded temperature sensors, or numerical simulations based on heat conduction models are often required to complement surface measurements.

## 3.3 Tensile Behaviour Under Flame Exposure

After they were exposed to a kerosene flame for 300–600–900 s, specimens were subjected to a monotonic tensile loading under flame exposure up to failure. Though the tensile load is primarily borne by the 0° plies, the macroscopic tensile response is elastic–plastic (Fig. 6), suggesting that the ±45° plies gradually deform when they take up the tensile load transferred from the thermally-decomposed 0° plies exposed to flame. Figure 6 also shows that the tensile behaviour is the same regardless the flame exposure time. The main difference is the ultimate load borne by the laminates, which gradually decreases as a function of exposure time (Fig. 7). With respect to the most resistant material (C/BMI), the axial strength is about 50% and 30% lower in C/Epoxy UD and woven laminates, respectively (Fig. 7b). These results agree with the tendency observed in virgin specimens (Fig. 4). After a 300 s exposure, the axial strength decreases by 40–60% of their initial values for studied materials with respect to their virgin state. After a 900 s exposure, the drop has almost stabilized at about 30–40% of their initial values.

However, Fig. 7 also shows that C/Epoxy UD laminates reach a plateau whereas woven ply laminates (C/Epoxy and C/BMI) follow the same decreasing trend. One might speculate that woven ply laminates being characterized by matrix-rich areas at the crimp (where warp fiber bundles undulate over weft fiber bundles), the thermal decomposition primarily occurs in these areas. As a result, the thermally-induced damages (porosities formation and extensive delamination) in these areas contribute the thermal transfers to be modified within the laminates (Fig. 8). In UD-ply laminates, as there are no matrix-rich areas, the temperature distribution within the laminates is more homogeneous, hence resulting in a homogenous distribution of thermally-induced damages.
<figure><img src="image_7.png" /><figcaption>**Fig. 6** In situ tensile testing of studied materials subjected to different kerosene flame exposure times (300–600–900 s): **a** C/Epoxy UD—**b** C/Epoxy woven—**c** C/BMI woven</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 7** In situ tensile testing of studied materials after different kerosene flame exposure times (300–600–900 s): **a** Evolution of the normalized tensile strength of studied materials as a function of the exposure time—**b** Influence of flame exposure time on axial strength</figcaption></figure>

Figure 8a clearly shows that all the plies are severely delaminated. It might explain why the mechanical properties rapidly decrease after a 300 s exposure and then remain virtually constant for longer exposure times. In woven-ply laminates, Fig. 8b, c show that there is a damage gradient through-the-thickness, with a rather well preserved are near the opposed surface. These different thermally induced damages may also explain the differences in temperature distribution pointed out in Fig. 5.
<figure><img src="image_9.png" /><figcaption>**Fig. 8** Through-the-thickness microscopic observations of laminates subjected to a kerosene flame for 600 s (without mechanical loading): a C/Epoxy UD—b C/Epoxy woven—c C/BMI woven</figcaption></figure>

These results suggest that, after a while, flame exposure no longer significantly affects the bearing capabilities of the laminates. These conclusions are supported by the fracto- graphic analyses of the thermally-induced damages depending on the flame exposure time (Fig. 8). The 600 s exposure case was chosen to illustrate the effect of one-sided flame exposure on the damage state within each composite material. From these through-the- thickness observations, two well defined areas are clearly observed: (i) an area near the flame exposed surface that is characterized by a damage gradient consisting of large poros- ities and extensive delamination, and (ii) an area that is rather well preserved near the opposed surface. From the mechanical standpoint, both areas are expected to play different roles in the tensile load bearing [35].

For a given material, Figs. 9, 10, 11 show that the failure surfaces are similar on the exposed and opposed surfaces regardless the flame exposure time. On the exposed sur- faces, ±45° oriented plies are characterized by longitudinal splitting of carbon fibers. The orientation of fibers in these plies virtually remains unchanged, suggesting that they do not significantly bear the tensile load. On the opposed surface, ±45° oriented plies are charac- terized by an extensive debonding of the fibers/matrix interface between carbon fibers bunc- dles in woven-ply laminates and an extensive longitudinal splitting in UD-ply laminates. The main difference between UD and woven reinforcement is that the orientation of ±45° ply is unchanged in UD C/Epoxy laminates, whereas ±45° fiber bundles clearly rotate to align with the tensile load in woven ply C/Epoxy and C/BMI laminates. These observa- tions confirm that all the plies of the laminates do not contribute the same way to bear the tensile load, as was suggested by the two well defined areas observed in the through-the- thickness microscopic views (Fig. 8).

Finally, both epoxy and BMI composites experience specific failure mechanisms under high temperatures and fire exposure, which are influenced by their matrix systems and fiber-matrix interactions. Epoxy laminates are more susceptible to matrix softening, cracking, and delamination at lower temperatures compared to BMI laminates, which have better thermal stability. BMI laminates offer superior performance in high-temperature
<figure><img src="image_10.png" /><figcaption>**Fig. 9** Fractographic observations of exposed and opposed surfaces of laminates subjected to a kerosene flame for 300 s and after a monotonic tensile loading to failure: a C/Epoxy UD—b C/Epoxy woven—c C/BMI woven</figcaption></figure>

environments but can still experience issues such as delamination and thermal degradation at extreme conditions (above 250 °C). In designing with these materials, engineers must account for the temperature thresholds of each resin system (e.g., their characteristic temperatures Tg and Td—see Table 1), ensuring that the composite maintains its mechanical properties in the event of fire exposure or prolonged heat. Quantitative analysis of these failure modes typically would involve methods like ultrasonic testing, and advanced imaging techniques to evaluate damage areas such as delamination, crack density, and fiber failure. Fracture toughness tests would be relevant to further investigate the severity of delamination in composite laminates exposed to a kerosene flame for different durations, depending on the nature of the polymer matrix. Additionally, 3 points bending tests would provide valuable information on the changes in both the stiffness and strength of severely delaminated specimens.

## 3.4 Creep Behaviour Under Flame Exposure

To mimic critical service conditions (fire event under mechanical loading), a creep tensile load was applied to the laminates subjected to a one-sided kerosene flame. Figure 12 shows the changes in the axial displacement as a function of the flame exposure time. Having in mind that the safety requirements—to be met in accordance with the certification standards [3]—is to ensure that the loading bearing capabilities of the composite part are preserved
<figure><img src="image_11.png" /><figcaption>Fig. 10 Fractographic observations of exposed and opposed surfaces of laminates subjected to a kerosene flame for 600 s and after a monotonic tensile loading to failure: a C/Epoxy UD—b C/Epoxy woven—c C/BMI woven</figcaption></figure>

for 900 s, the underlying idea of these tests is to identify the axial creep stress to be applied to avoid failure after a 900 s kerosene flame exposure (Blue dotted line in Fig. 13). One may deduce from these tests the admissible creep strength to meet the 900 s certification requirements for each material: 190, 222, 274 MPa for C/Epoxy UD, C/Epoxy woven and C/BMI woven laminates, respectively. In other words, with respect to C/BMI laminates, it means that, under the same fire conditions, the maximum applied stress is 30 and 20% lower in UD and woven ply C/Epoxy laminates, respectively.

The key parameter to be also investigated here is the correlation between the applied creep load F and the time-to-failure $t_{failure}$. Figure 13 shows that the studied materials do not behave the same way in terms of creep tensile force dependence. Typically, the transition from high (900 s) to low (about 60 s) values of $t_{failure}$ is rather slow in C/BMI laminates, whereas it dramatically decreases in UD ply C/Epoxy laminates.

Based on the changes in the time-to-failure as a function of the creep tensile force, it is possible to determine the analytical expression of design rules depending on the material:

* In UD ply C/Epoxy laminates, the time-to-failure follows a sigmoid law:

$$t_{failure} = 1.3883 - \frac{1.0188}{\left[1 + e^{\left(\frac{0.66479 - F}{1.3709}\right)}\right]}$$

* In woven ply C/Epoxy laminates, the time-to-failure follows a sigmoid law:
<figure><img src="image_12.png" /><figcaption>Fig. 11 Fractographic observations of exposed and opposed surfaces of laminates subjected to a kerosene flame for 900 s and after a monotonic tensile loading to failure: a C/Epoxy UD—b C/Epoxy woven—c C/BMI woven</figcaption></figure>

$$t_{\text{failure}} = 41.558 + \frac{957.67}{\left[1 + e^{\left(\frac{F-11.045}{49.571}\right)}\right]}$$

* In woven ply C/BMI laminates, the time-to-failure follows an exponential law:

$$t_{\text{failure}} = -707.51 + 15869.\text{e}^{-0.14919\text{F}}$$

As was observed in the case of monotonic tensile tests, the changes in the time-to-failure $t_{\text{failure}}$ as a function of the applied creep load are correlated with the thermally-induced damages along the deformation mechanisms occurring under mechanical loading.

## 4 Conclusions

The tensile behavior was compared in thermosetting (Epoxy and Bismaldeide)—based laminates exposed to one-sided thermal aggression resulting from a kerosene flame exposure (116 kW/m² heat flux and an 1150 °C flame temperature). The obtained results emphasize the challenge of characterizing the thermo-mechanical coupling within
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 12** In situ creep testing of studied materials subjected to different creep loads during kerosene flame exposure: a C/Epoxy UD—b C/Epoxy woven—c C/BMI woven</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Fig. 13** In situ creep testing of studied materials subjected to different creep loads during kerosene flame exposure—Evolution of time to failure as a function of the applied creep force</figcaption></figure>

PMCs experiencing thermal decomposition of its matrix (pyrolysis) through non-stand-
ardized tests while addressing structural effects (localization of both the thermal deg-
radation and damage mechanisms). On the one hand, by means of a specific fire bench
allowing critical service conditions (mechanical loading under fire) to be reproduced at
small scale, the influence of flame exposure time on the tensile properties (axial stiff-
ness and strength) was addressed. On the other hand, by considering creep testing con-
ditions, the time-to-failure was determined to evaluate the fire-resistance of different
composite systems, which significantly depends on both their pyrolysis temperature and
reinforcement architecture.

The mechanical response of the laminates' plies under tensile loading differs from
one ply to another due to a temperature gradient, depending on the fibers orientation as
well as the state of the polymer matrix (melted or pyrolyzed). Once the thermal equi-
librium is reached (after about 100 to 200 s), the temperature is about 450 to 550 °C on
the back surface. Depending on the matrix nature, it means that all the plies of C/Epoxy
laminates have reached the onset of thermal decomposition (350 °C), whereas the pyrol-
ysis of the BMI matrix has just started in C/BMI laminates (480 °C).

The changes in polymer state with temperature result in significantly modifying the
mechanical properties in tension. After a 300 s exposure, the axial strength decreases by
40 to 60% of their initial values for studied materials with respect to their virgin state.
After a 900 s exposure, the drop has almost stabilized at about 30 to 40% of their initial
values. The decrease in strength appears to be more gradual in woven-ply laminates
due to more extensive delamination and porosities formation in matrix-rich areas. Ulti-
mately, these thermally-induced damages contribute the thermal transfers to be modified
within the laminates, and to maintain the load bearing capabilities of the opposed plies.
From the microscopic observations done on damaged specimens, two areas are clearly
defined: (i) an area near the flame exposed surface that is characterized by a damage
gradient consisting of large porosities and extensive delamination, and (ii) an area that
is rather well preserved near the opposed surface. From the mechanical standpoint, both
areas do not contribute the same way to bear the tensile load.

The creep tests conducted on laminates under fire conditions provide the admissible
creep strength to meet the 900 s certification requirements for each material: 190, 222
and 274 MPa for UD ply C/Epoxy, woven ply C/Epoxy and C/BMI laminates, respec-
tively. With respect to C/BMI laminates, the maximum applied stresses are 30 and 20%
lower in UD and woven ply C/Epoxy laminates, respectively.
Based on the changes in the time-to-failure as a function of the creep tensile force, this study provides (i) analytical expressions, which are the first step towards the definition of simple design rules useful to engineers willing to meet safety requirements for each type of composite material—(ii) valuable information on both the temperature and damage gradients within the laminates depending on the pyrolysis temperature and the reinforcement architecture, allowing a better understanding of the correlation between flame-exposure time and the mechanical response under creep loading (similar to what is undergone by the composite materials under critical service conditions).

**Funding** Open access funding provided by INSA Rouen Normandie.

**Data Availability** The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Authors and Affiliations

J. Vacandare<fnref n="1" /> · B. Vieille<fnref n="1" /> · S. Patel<fnref n="2" /> · C. Naejus<fnref n="2" /> · M. Denize<fnref n="2" />

<figure><img src="image_17.png" /></figure>

B. Vieille
benoit.vieille@insa-rouen.fr

J. Vacandare
julie.vacandare@insa-rouen.fr

S. Patel
s.patel@daher.com

C. Naejus
c.naejus@daher.com

M. Denize
m.denize@daher.com

<figure><img src="image_18.png" /></figure>

<footnote n="1">¹ INSA Rouen, Normandie Univ, CNRS, GPM, Avenue de l'Universite,
76800 Saint Etienne du Rouvray, France</footnote>

<footnote n="2">² Daher Aerospace—Shap’in, 1 Rue Claire Roman, 44860 St Aignan de Grandlieu, France</footnote>