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Post-fire compressive behaviour of carbon fibers woven-ply Polyphenylene Sulfide laminates for aeronautical applications
Composites Part B 119 (2017) 101e113
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Post-fire compressive behaviour of carbon fibers woven-ply Polyphenylene Sulfide laminates for aeronautical applications M.A. Maaroufi a, Y. Carpier a, *, B. Vieille a, L. Gilles a, A. Coppalle b, F. Barbe a a b
Groupe de Physique des Mat eriaux, UMR 6634 CNRS, Universit e et INSA de Rouen, 76801 Saint Etienne du Rouvray, France CORIA, UMR 6614 CNRS, Universit e et INSA de Rouen, 76801 Saint Etienne du Rouvray, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 November 2016 Received in revised form 27 January 2017 Accepted 19 March 2017 Available online 28 March 2017
The influence of fire exposure on the residual compressive behaviors of carbon fibers woven-ply Polyphenylene Sulfide has been investigated for aeronautical applications. For heat fluxes ranging from 20 to 50 kW/m2, prior fire exposure is highly detrimental to the compressive mechanical properties as the residual strength and stiffness decrease by 75% and 55% respectively. Thermogravimetric analyses have been conducted under inert and oxidative atmospheres to quantify the mass loss resulting from the thermal decomposition of the outer layer directly exposed to heat flux and oxygen-rich atmosphere and internal layers respectively. Fire exposure results in gradually increasing damages within exposed laminates: PPS matrix thermal decomposition leaves intra- and inter-laminar voids leading to more or less extensive delamination depending on fire testing conditions. In order to discuss the compressive damage mechanisms after fire, the early deformation mechanisms have been analyzed by means of 2D Digital Image Correlation. C-scan inspections have also been performed to evaluate the delaminated areas which are quite well correlated with the surface fire-degraded areas, suggesting that delamination is primarily associated with thermal degradation. As heat flux increases, the fire-induced delamination and the onset of local plastic kink-bands during compressive loading ultimately cause delamination extension and global plastic buckling. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Fire behaviour Thermoplastic High temperature Mechanical testing
1. Introduction The reduction of structural mass is one of the privileged ways to improve propulsion systems in aeronautics. High performances composites consisting of polymer matrix is a good compromise between weight and robustness. The strength and stiffness/density ratio are one of the benefits offered by composites materials, but several questions are raised by their use in more critical applications. Among them, their behaviour under fire exposure is obviously an issue because of their multiple alteration potentialities: matrix and fibres degradation, oxidization as well as delamination [1]. Fire weakens the material mechanical properties and consequently the structure resistance. When exposed to fire, polymer matrix softens for T > Tg and loses its properties (in term of stiffness and strength). Thus, in order to obtain the best post-fire behaviour, different types of composites can be considered [2]. Among polymer matrix composites, thermoset-based composites (denoted TS)
* Corresponding author. E-mail address: [email protected] (Y. Carpier). http://dx.doi.org/10.1016/j.compositesb.2017.03.046 1359-8368/© 2017 Elsevier Ltd. All rights reserved.
have been widely used for aeronautical applications over the past four decades. As a consequence, most of the studies available in the literature on the fire behaviour deal with carbon/Epoxy composites [3e5] and E-Glass/Vinylester composite [6e9]. Therefore, the post fire behaviour of TS-based composites is well established in the literature [3e14], with particular attention to the effects of heating and fire duration on the residual mechanical properties of epoxy composites [9,12e14]. The post-fire mechanical properties can rapidly decrease with the increase of the temperature of the fire exposure time due to the high flammability of the epoxy matrix, but also because of its rapid thermal decomposition [12]. Recently, thermoplastics (denoted TP) received considerable attention as matrix constituents in structural composites because they present several advantages compared to thermoset composites, such as damage tolerance or impact resistance [15]. In addition, there are still issues regarding the manufacturing process of these TS-based composites: low-temperature storage, long curing cycles, irreversible process, etc. TP-based composites can be easily be manufactured by means of specific processes such as welding [16], and automatic processes such as stamping and co-consolidation [17].
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All these advantages are related to the specific microstructure of the TP matrix consisting of an amorphous and a crystal phase. TPbased composites are increasingly used in critical applications (e.g. aircraft engine nacelles), but only few studies deal with their fire resistance. Recently, the post-fire behaviours of carbon fibers PPSand epoxy-based has been compared [18]. A smaller decrease was observed in the tensile properties of TP laminates as compared to the TS ones for thermal heat flux up to 50 kW/m2. Such observations have been correlated with the fire-induced damages and with the high char yield of PPS matrix [19]. Indeed, char formation arising from the thermal decomposition of the organic resins plays a major role on the post-fire behaviour. It notably may act as an insulation layer against the fire propagation due to its high porosity in the case of TS-based laminates. In TP-based laminates, the char composition seems to be different as the TP resins have different chemical nature and the thermal degradation does not give a similar residue [19]. As a consequence, the effects of thermal degradation on the fire behaviour are not well understood and the issue of the mechanical behaviours of thermally degraded TP-based composites is still an open question, particularly for hightemperature applications. Compression being one of the most severe mechanical loading modes [3], and due to the lack of experimental data under high temperature conditions, the purpose of the present work is to study the compressive behaviour of carbon reinforced PPS-based composites at 120 C (T > Tg) after prior fire exposure. More precisely, the question is to ultimately know whether it is relevant to replace epoxy-based composites by PPSbased composites for applications in nacelles of aircraft's engines, or not [18e21]. Finally, the main objective of the present work is to estimate how detrimental prior fire exposure is on C/PPS post-fire compressive properties [22], and to investigate the influence of fire exposure on compressive damage mechanisms. 2. Materials and methods 2.1. Materials The composite materials studied in this work are 7 plies carbon fabric-reinforced PPS prepreg laminate plates [18,19,23]. The toughened PPS resin (Fortron 0214) is supplied by the Ticona Company. The woven-ply prepreg, supplied by the SOFICAR Company, consists of 5-harness satin weave carbon fiber fabrics (T300 3 K 5HS), with a mass fraction of fibers of 58%. The prepreg plates are hot pressed according to the following lay-up [(0/90), (±45), (0/ 90), (±45), (0/90), (±45), (0/90)]. Test specimens are 100*150 mm2 plates (2.2 mm thickness) cut by water jet to reduce edge and
machining effects. The interest of using large specimens is to limit heat losses but also to reduce the thermal degradation along specimen edges. 2.2. Experimental set-up and methods 2.2.1. Fire exposure test method As used in many studies [18,24], the radiant heater of a cone calorimeter was used here to burn composites specimens. With this heating method, the heat flux and the heating conditions are controlled and repeatable. However, the exposure to a cone calorimeter represents an idealized fire condition: it is stable and involves a continuous heat flux. A heat flux gauge has been used to calibrate the cone calorimeter and to control the heat flux on the specimen surface. Four different heat fluxes have been considered: 20, 30, 40 and 50 kW/mm2. After a 2 min fire-exposure, the specimens were cooled in the air during one night. Four specimens were tested in each fire testing condition. Analyses have been performed by comparison to reference virgin specimens, which have not been exposed to prior fire. In order to monitor the temperature (see Fig. 2) and the temperature gradient DT between these two surfaces (see Fig. 3a), one thermocouple has been placed on the centre of the fire-exposed surface and another one on the unexposed surface of specimens. The accuracy of the temperature measurement depends on the contact of the thermocouple on the specimen surface which can be difficult on the fire-exposed surface because of the matrix thermal degradation. To limit this effect, the thermocouple bond with the surface is ensured by means of an iron wire wrapped around the specimen. 2.2.2. Compression test method The mechanical loading mode is a conventional compression test requiring the use of a specific anti-buckling system as buckling is the primary failure mode in compression due to specimen dimensions. This system originally designed by Boeing incorporates adjustable side plates enabling to accommodate variations in thickness and overall dimensions, as well as to prevent specimen buckling (see Fig. 1). Such method has often been reported in the literature [5,6]. Compressive tests were performed using a 100 kN capacity load cell of a MTS 810 servo-hydraulic testing. The temperature control system includes an oven and a temperature controller, and the tests were conducted at 120 C. Specimens were subjected to compressive loads at a constant displacement rate V ¼ 0.2 mm/min, complying with the standard Airbus AITM 1e0010 (compression after impact test) [25]. Because of the fire exposure
Fig. 1. Compression tests after fire exposure: experimental set-up and anti-buckling fixture [15].
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Fig. 2. Influence of fire testing conditions on the surface measured temperature: (a) on the fire-exposed surface e (b) on the unexposed surface (back face).
Fig. 3. Influence of fire testing conditions: (a) changes in the temperature gradient between exposed and unexposed faces e (b) approximate estimation of the surface degraded area after fire exposure (macroscopic observations).
method used in this study, the normalized test method for compressive properties of composites [26] has not been chosen here to characterize the post fire compressive behavior. Indeed, it would require cutting specimens in the fire-degraded and delaminated plates, leading to a more degraded material and more scattered results than with the compression after impact method. For this reason, this study is focused on structural testing rather than a pure compressive loading. As a consequence, the post-fire thermo-mechanical responses of the laminates subjected to different prior fire-exposures are compared in term of apparent stiffness and strength. 2.2.3. Digital Image Correlation technique (DIC) Fire-exposure leads to local damage within the laminates, resulting in singularities in the strain field on the outer surface. 2D Digital Image Correlation (DIC) is an appropriate tool to continuously track the effects of damage on the surface of specimens such as fire-exposed laminates subjected to compressive loads. This method is based on the measurement of the displacement field between a reference time (stress-free state) and a given time of the loading (deformed state). This method requires the painting of a black and white speckle on the specimen surface to obtain different
shades of grey. Consequently, the Green-Lagrange strain field can be derived from the 2D displacement field by means of the VIC-2D correlation software. A 3D DIC method should be used to examine the out-of-plane displacements when buckling occurs. However, as a first approximation and for lack of better means, the 2D DIC technique was used during compressive loading to investigate the early deformation mechanisms on the fire-exposed faces, but also to detect the onset of local damage events (transverse matrix cracking, breakage of 0 fibers and onset of global buckling). 2.2.4. C-scan ultrasonic inspections Even though the qualification of the fire-induced damage (type, size and severity of damage) remains a complex task, standard nondestructive evaluation methods such as ultrasonic C-scan imaging are often used, particularly to detect delamination in polymer-based composites. In the present case, the purpose of Cscan analysis is to correlate the fire-degraded area on laminates surface and the delaminated area through the thickness. The underlying idea is to show that the thermal degradation on the exposed surface leads to increasing macroscopic damages (mostly delamination) in the meso-structure of the laminates. C-scan inspections have been performed with an ultrasonic device
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ULTRAPAC II system (automated immersion system). Data acquisition, control and C-scan imaging were conducted by means of UTwintm software. 2.2.5. Dynamic mechanical analysis (DMA) Dynamic mechanical tests have been performed using a TA Instruments Q800 device. Because of the fire-induced delamination, tests were not conducted on fire-exposed specimens. Nevertheless, the aim of these tests was to characterize the loss of modulus at a service temperature during the glass transition. Specimens whose dimensions are 35 12 2.15 mm were heated from room temperature to 200 C at a constant heating rate of 2 C/min. 2.2.6. Thermogravimetric analysis The thermal degradation of C/PPS was investigated by thermogravimetric analysis (TGA) under inert (N2) and oxidative (O2) atmospheres using a TA Instruments Discovery device. Both atmospheres have been considered because the fire-exposed face is in an oxygen-rich atmosphere whereas the core layers are in an oxygen-lean atmosphere [27], therefore leading to different thermal decompositions. Rectangular samples of 10 mg (±2 mg) were cut from C/PPS virgin laminates and heated from 30 to 700 C at a constant heating rate of 10 C/min. The onset of thermal decomposition Td refers to the temperature corresponding to a 5% mass loss. 3. Results and discussion 3.1. Fire exposure tests 3.1.1. Temperature evolution and thermal conduction When samples are exposed to heat flux between 20 kW/m2 to 50 kW/m2, the maximal surface temperature ranges from 355 C to 662 C. The melting temperature (Tm ¼ 280 C) of PPS matrix is thus reached on the fire-exposed surface for all heat fluxes [28]. For the two highest heat flux, the thermal degradation onset temperature is reached. When specimens are subjected to 50 kW/m2 heat flux, the sudden increase of temperature after 110s (see Fig. 2a) is attributed to a thermocouple debonding resulting from matrix pyrolysis. Consequently, the monitored value after 110s represents the atmosphere temperature rather than the specimen surface temperature. On unexposed surfaces, the pyrolysis onset temperature is not reached for any heat fluxes (see Fig. 2b), confirming that thermal conduction and subsequent thermal decomposition is not uniform through the thickness. Some thermal inertia can indeed be observed for all heat fluxes as the maximum temperature is reached after 140e150s, whereas the samples were only exposed to fire during 120s. From the heating kinetics standpoint, the temperature changes as a function of the fire-exposure time are significantly different on both fire-exposed and unexposed surfaces. The temperature gradient between the exposed and the unexposed faces increases with increasing heat flux (86 C after 120s for the 20 kW/m2 and 230 C for 50 kW/m2) what tends to indicate that the higher the heat flux is, the less easy the thermal conduction is (see Fig. 3). It suggests that the thermal conduction within the laminates depends on both the applied heat flux and the resulting thermal degradation through the thickness and on specimen surface. Indeed, the thermal decomposition may contribute to the insulation of the outer layers of the composite (unexposed surface) through the formation of char, as it has been shown for TSbased composites [29]. The presence of melted resin into a fibrous network suppresses heat radiation and physical modification of the char [23]. Thus, the char formation in TP-based composites also protects the outer layers of the laminates against thermal degradation. However, the nature of char has only been investigated at
the meso- and micro-scales by means of a Scanning Electron Microscope (SEM) and needs to be characterized thermo-chemically in the case of TP-based composite because the PPS matrix degradation mechanisms are different from those observed in epoxybased composites [19]. 3.1.2. Fire-induced damage mechanisms Depending on fire testing conditions, different thermallyinduced damages can be observed within the laminates (see Figs. 4 and 5). On fire-exposed surface, fire exposure leads to a circular damage area whose surface linearly increases as heat flux increases (see Fig. 3b). At low heat flux (e.g. 20 kW/m2), it is observed that the fire-exposed surface is slightly thermallydegraded mainly because of the melting of the matrix (see Fig. 4), but fire-exposure does not seem to induce damages through-thethickness (see Fig. 5), and there is thus no visible degradation on the unexposed face. For higher heat fluxes (ranging from 30 to 50 kW/m2), fire-exposure leads to gradually increasing damages such as intra- and inter-laminar debonding, as well as extensive delamination which can be observed at the macroscopic scale along specimens free-edges (see Fig. 5). 3.1.3. Thermal degradation and microstructural changes In order to better understand the thermomechanical processes induced by such temperatures, the thermal degradation has been investigated by TGA. Under nitrogen, the thermal decomposition is a single-stage process beginning at around 505 C and leading to a 25% mass loss of specimens. Since carbon fibers degradation does not occur at such temperatures in nitrogen [27], the mass loss is in fact only due to the pyrolysis of PPS which loses 60% of its mass. The resulting char, described in Paragraph 3.1.1, is expected to play an insulating role. The thermal-oxidative decomposition is a twostages process. In the first decomposition step, pyrolysis starts at 515 and carbonaceous char is formed, as it happens in a nitrogen atmosphere. During the second stage beginning at 630 C, both char and carbon fibers are oxidized [19] and the material is practically fully decomposed at 700 C. For low heat fluxes (e.g. 20 and 30 kW/m2), no noteworthy thermal decomposition is expected to occur (see Fig. 6). Moreover since melting temperature is reached (see Fig. 2), the PPS matrix is redistributed within the fibers network and protect carbon fibers against oxidation [19]. The pyrolysis onset temperature is reached on surface for a 40 kW/m2 heat flux, which means that part of the matrix covering the fibers starts its decomposition and becomes char. As long as the PPS is not completely degraded, it prevents carbon fibers from oxidization. As a result, a partial cohesion of fibers and matrix can be maintained contributing to the load transfers within laminates plies, and significantly influencing the post-fire compressive mechanical properties. For the highest heat flux, the first decomposition step is complete; all the matrix on the surface has become char, which is partially oxidized on surface among with fibers (see Fig. 6). The microstructural changes are also expected to strongly influence the mechanical properties. The degree of crystallinity (DOC) decreases from 27% (virgin state) to 0 (strongly degraded state) when the heat flux increases to 50 kW/m2 [18]. More precisely, the DOC decreases slowly for low heat fluxes and rapidly for heat fluxes higher than 30 kW/m2 (see Table 1), suggesting the importance of the exposure time at temperatures higher than the material degradation temperature Td (see Fig. 2). Indeed, below Td, the only loss of crystallinity is due to the rapid cooling preventing from a total recrystallization. Above Td, the thermal degradation leads to different mechanisms like chain scission or charring responsible for the loss of crystallinity [30]. For a 40 kW/m2 heat flux, a part of the matrix on surface is “charred” (see Fig. 6). On the one hand, the
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Fig. 4. Influence of fire testing conditions on the fire-degraded of quasi-isotropic C/PPS laminates: (a) fire-exposed surface e (b) unexposed surface.
Fig. 5. Macroscopic observations of the fire-induced damages through the thickness in quasi-isotropic C/PPS laminates subjected to different fire testing conditions.
crystalline phase disappears in this “charred” matrix (see Table 1). On the other hand, it contributes to the formation of an insulating layer, protecting the layers underneath from thermal degradation
and decrystallization. For higher heat fluxes, the thermal degradation is more significant (65% of mass loss) at the exposed surface, and oxidized char and fibers cannot act as an insulation layer
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Fig. 6. Thermal decomposition of C/PPS and mass loss rates under nitrogen and oxygen.
Table 1 Influence of fire conditions on C/PPS laminates' degree of crystallinity. Testing conditions
Virgin
20 kW/m2
30 kW/m2
40 kW/m2
50 kW/m2
Degree of crystallinity DOC (%) Decrease in DOC (%)
26.69 ± 1.76 e
24.52 ± 0.94 8
22.83 ± 1.11 15
13.19 ± 0.68 51
0 100
anymore. This phenomenon combined with longer exposure times at T > Td are responsible for the total loss of crystallinity. These physical transformations are expected to be of the utmost importance with respect to post-fire mechanical behavior, especially when compression tests are carried out at temperatures higher than Tg, as will be discussed in the next section. Indeed, the mechanical properties of the PPS matrix are degraded in a first step at T ¼ Tg due to mechanisms affecting the amorphous phase, whereas the properties of the crystalline phase decrease when temperature reaches T ¼ Tm [31]. Thus, the respective contribution of these mechanisms to the degradation of mechanical properties significantly depends on the degree of crystallinity. Such degradation is usually characterized by means of Dynamical Mechanical Analysis (DMA). The influence of prior fire-exposures has not specifically been studied here, but a DMA has been conducted on virgin specimens (see Fig. 7). As specimens were heated up to 200 C, only the loss due the transition from glassy to rubbery states can be observed around the glass transition temperature (Tg ¼ 98 C). The decrease in the storage modulus starts at 95 C and leads to a 15% loss at 120 C. In the whole glass transition interval, the storage modulus decreases by 40%. As prior fire-exposures have a significant influence on the degree of crystallinity, the higher the heat flux, the higher the modulus loss due to glass transition. This result is expected to have an influence on failure mechanisms discussed in 3.2.4.
3.2. Post-fire compressive tests 3.2.1. Macroscopic response of laminates under compression The different compressive responses can be compared for all fire-testing conditions (see Fig. 8). Compressive damage mechanisms are discussed in the next section, but the macroscopic response of C/PPS laminates appears as elastic-ductile until it reaches a maximum value which seems to be associated with the
macroscopic plastic buckling. From the obtained results, tests repeatability is better for lower heat fluxes (20 and 30 kW/m2), and it is expected that local instability (such as local micro-buckling) is promoted as heat flux and through-the-thickness damages increase. The qualitative responses can be compared for all testing conditions (see Fig. 9a), and show that the ultimate longitudinal apparent strength decreases by 80% with respect to the value of virgin specimens (see Fig. 9b). At the same time, the longitudinal apparent stiffness of the structure decreases by 60%. For comparisons purposes, under the same fire-testing conditions in tension, both longitudinal stiffness and strength decrease by virtually 35% between the virgin state and the highest heat flux (50 kW/m2), confirming that the influence of fire-exposure strongly depends on the type of loading [18]. It is worth noting that the results are here expressed in term of apparent mechanical properties based on constant cross section. Indeed, depending on the heat flux, fire damaged areas are quite different from each other, hence the cross section of effective material. These mechanical tests do not consequently accurately represent the evolution of material properties under different specific fire conditions but more the loss of properties of a specific composite structure. Fire-induced delaminated area is circular, and it is expected that compressive loading will promote further delamination. To a first approximation, it can be assumed that the degraded area within the laminates is somehow equivalent to a plate with a central hole. On the one hand, both longitudinal stiffness and compressive strength can be evaluated in term of nominal values from the constant initial cross section S ¼ t:w (with t and w thickness and width of the laminates, respectively):
F S
snom ¼ ¼
F Dsnom DF and Enom ¼ ¼ t:w S:Dε Dε
snom therefore represents the stress in a cross section far from the circular hole, and the area S at this section is called the gross
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Fig. 7. Influence of temperature on the evolution of storage modulus and tan delta of a virgin C/PPS specimen.
Fig. 8. Thermomechanical responses of quasi-isotropic C/PPS laminates subjected to a compressive loading after different prior fire testing conditions.
cross-sectional area [35]. On the other hand, the compressive strength can be obtained from the cross section at the hole, which is formed by removing the circular hole (whose diameter is d is the diameter of the delaminated circular area) from the gross cross section. The corresponding area Snet ¼ t ðw dÞ is referred to as the “undamaged” or net
cross-sectional area (see Table 2). If the stresses at this cross section are uniformly distributed and equal to snet :
snet ¼
F F snom ¼ ¼ Snet t:ðw dÞ ðw dÞ
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Fig. 9. Influence of different prior fire testing conditions on residual compressive thermomechanical behaviors: (a) Macroscopic response e (b) Changes in the longitudinal stiffness and the compressive strength.
Table 2 Influence of fire conditions on delaminated area and corresponding “undamaged” net cross-sectional area. Testing conditions 2
Delaminated area (mm ) Snet “Undamaged” net cross-sectional area (mm2) Snet S
(%)
Virgin
20 kW/m2
30 kW/m2
40 kW/m2
50 kW/m2
e 220 100
6000 141 64
8600 126 57
11000 113 52
12500 107 48
Fig. 10. Green-Lagrange longitudinal strain distribution (obtained by Digital Image Correlation) in C/PPS laminates subjected to compression after fire exposure: (a) 20 kW/m2: failure by transverse crack propagation and breakage of 0 fibers e (b) 30-40-50 kW/m2: onset of global plastic buckling is detected from the localization of positive and negative longitudinal strains.
From this definition, new values can be computed for the compressive strengths. On Fig. 9b, it appears that the effective strength snet is virtually the same for heat flux ranging from 20 to 40 kW/m2 (about 200 MPa). The net stress reflects the structural
loading and cannot be compared to the compressive strength of virgin specimens in which the stress state is homogeneous. However, the calculated values can be compared to the buckling theoretical stress sbuckling (see Fig. 9b) obtained from the Euler's
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Fig. 11. C-Scan ultrasonic inspections of quasi-isotropic C/PPS laminates subjected to compressive loading after fire exposure.
Fig. 12. Comparison between approximate estimation of the surface degraded area after fire exposure (macroscopic observations) and estimation of the delaminated area after compression (C-scan inspection).
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buckling theory [34]. These values remaining lower than sbuckling , it suggests that the degraded specimens should not experience elastic buckling along the hole as specifically addressed in section 3.2.4. In addition, when the laminates are severely degraded at 50 kW/m2, the compressive strength significantly decreases due to critical fireinduced degradation: pyrolysis of the matrix followed by the oxidation of both matrix and fibers at the exposed surface (see Fig. 6). Finally, to ease the comparison of the mechanical properties between the different fire-resting conditions, it is necessary to consider another experimental set-up to conduct compressive tests. To this aim, a cover will be designed to promote the same degraded area on the exposed surface, but also to reduce the differences in the delaminated areas. 3.2.2. Analysis of deformation mechanisms by Digital Image Correlation The early deformation mechanisms taking place in firedegraded laminates subjected to compressive loadings have been
analyzed by means of 2D DIC on the fire-degraded area. Obviously, such a 2D technique is not fully operational to investigate the deformation mechanisms as medium to high heat fluxes induce significant out-of-plane effects. However, this technique can be helpful to detect the onset of local damage events (transverse matrix cracking, breakage of 0 fibers and onset of global buckling). At low heat flux (20 kW/m2), compressive failure primarily results from transverse matrix cracking and the 0 fibers breakage (see Fig. 10). For higher heat fluxes (30-40-50 kW/m2), the onset of global plastic buckling can be detected from the localization of successive positive and negative longitudinal strains (see Fig. 10). The longitudinal strain maps also confirm that elastic buckling does not occur during compressive loading as there is no localization of simply positive or negative strains. 3.2.3. Post-compression C-scan inspections The C-scan maps of specimens subjected to compressive loading after fire-exposure provide further information on the damage
Fig. 13. Damage mechanisms in woven-ply quasi-isotropic C/PPS laminates subjected to compression after fire exposure: (a) Local micro-buckling of fiber bundles at the crimp in a 5-harness satin weave [34] e (b) Delamination and global plastic buckling induced by post-fire delamination and the formation of plastic kink bands.
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mechanisms, and more particularly they can be used to calculate the delaminated area (see Fig. 11). As it was observed in section 3.1.2, prior fire-exposure leads to a circular damage area whose surface linearly increases as heat flux increases. Fire-induced delaminated area is circular, and it is expected that a compressive loading will promote further delamination. C-scan inspections performed after compressive tests clearly show that delaminated area gradually increases under the degraded surface as heat flux increases. The comparison between an approximate estimation of the fire degraded area after fire exposure and the delaminated area after compression (respectively based on macroscopic observations and C-scan analysis) show that they are rather well correlated (see Fig. 12), suggesting that delamination is primarily associated with thermal-degradation. 3.2.4. Investigations on compressive damage mechanisms after fireexposure In the case of compressive loadings, damage mechanisms are complex [28,29], but they are even more difficult to investigate as both matrix and fiber/matrix bonding are severely thermallydegraded. As it was shown in section 3.1.2, fire-exposure causes two major damages: intra-and inter-laminar cracking and delamination. As it is well documented in the literature, carbon fibers under compressive loadings are prone to instabilities such as
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buckling or fiber crushing, and the initial alignment of fibers in reinforced composites has a large influence on their compressive response [32,33]. Thus, the crimp region plays a significant role in controlling the onset of failure in woven-ply laminates, mostly because of the undulating structure of carbon fabrics (see Fig. 13), and the presence of matrix-rich areas at the crimps. More specifically, both matrix toughness and ductility contribute to specific failure mechanisms within a fiber network in PPS-based laminates subjected to compressive loadings at T > Tg [34]. Indeed, localized bending at the crimp is found to cause micro-buckling in woven-ply misaligned structures (see Fig. 13a). In addition, micro-buckling comes along with a plastic deformation of the matrix in highly ductile matrix systems, resulting in the formation of deformed inclined kink-bands also called plastic buckling (see Fig. 13b). Such bending is also amplified by the formation of voids resulting from the degradation of the PPS matrix in matrix-rich areas and the fireinduced delamination. As it was shown by the C-scan analysis (cf. section 3.2.3), delamination is the primary failure mechanism ruling the high-temperature compressive behavior of C/PPS laminates when the material is thermally-degraded by medium to high heat fluxes. At low heat flux (20 kW/m2), fire-induced delamination is less significant, and compressive failure results from transverse matrix cracking and the breakage of 0 fibers (see Fig. 14 and cf. section 3.2.2). As the heat flux increases, the fire-induced
Fig. 14. Macroscopic observations of fire-exposed specimens after compressive tests: transverse matrix cracking and formation of global plastic buckling bands.
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delamination and the onset of local plastic kink-bands during compressive loading ultimately cause delamination extension and global plastic buckling following the maximum load in the loaddisplacement response (see Fig. 14). As a result, the compressive response of fire-damaged laminates is elastic/ductile with a gradual failure. 4. Conclusions From the fire-performance standpoint, the basic idea of the present work was to investigate how detrimental different prior fire-testing conditions are on the residual compressive behavior of Polyphenylene Sulfide laminates to be used under hightemperature service conditions (e.g. 120 C) for aeronautical applications (e.g. aircraft engine nacelles). Depending on the firetesting conditions, different conclusions can be drawn from this study: With respect to the values of virgin specimens (unexposed to fire), a severe fire-exposure (typically 50 kW/m2 for 2 min) contributes to a significant decrease in laminates longitudinal stiffness (55%) and strength (75%). Prior fire-exposure results in gradually increasing damages within tested laminates, as PPS matrix thermal decomposition leaves intra- and inter-laminar voids leading to more or less extensive delamination. Thermogravimetric analyses have been conducted under inert (N2) and oxidative (O2) atmospheres to quantify the mass loss resulting from the thermal decomposition of the material in the laminates exposed and core layers respectively. In order to investigate the compressive damage mechanisms after fire, the early deformation mechanisms in compression have been analyzed by means of a 2 dimensional Digital Image Correlation technique. Thus, the onset of global plastic buckling can be detected from the localization of positive and negative longitudinal strains. The delaminated areas have been evaluated by means of C-scan inspections, and these areas are rather well correlated with the surface fire-degraded areas, suggesting that delamination is primarily associated with thermal-degradation. As the heat flux increases, the fire-induced delamination and the onset of local plastic kink-bands during compressive loading ultimately cause delamination extension and global plastic buckling. Finally, compression behavior being the Achilles' heel of composite behavior under load in fire, this study is a preliminary work dealing with the design of an experimental platform whose purpose is to simultaneously apply fire-exposure and mechanical loading (critical service conditions). To ease the comparison between the mechanical properties of the laminates subjected to different fire conditions, a specific cover will be designed to promote the same degraded area on the exposed surface, but also to reduce the differences in the delaminated areas. Acknowledgement This work is a part of the DECOLLE project funded by the Institute Carnot ESP (Energy and Propulsion Systems). The authors also would like to acknowledge the Aircelle Company for supplying the composite materials. References [1] Burns LA, Feih S, Mouritz AP. Compression failure of carbon fiber-epoxy
laminates in fire. J Aircr 2010;47:528e33. http://dx.doi.org/10.2514/1.45065. [2] Bourbigot S, Delobel R, Dusquene S. Comportement au feu des composites. Doss Tech Ing 2006:1e10. [3] Feih S, Mathys Z, Gibson AG, Mouritz AP. Modelling the compression strength of polymer laminates in fire. Compos Part Appl Sci Manuf 2007;38:2354e65. http://dx.doi.org/10.1016/j.compositesa.2007.04.013. [4] Feih S, Mathys Z, Gibson AG, Mouritz AP. Modelling the tension and compression strengths of polymer laminates in fire. Compos Sci Technol 2007;67:551e64. http://dx.doi.org/10.1016/j.compscitech.2006.07.038. [5] Mouritz AP, Feih S, Kandare E, Mathys Z, Gibson AG, Jardin PED, et al. Review of fire structural modelling of polymer composites. Compos Part Appl Sci Manuf 2009;40:1800e14. http://dx.doi.org/10.1016/ j.compositesa.2009.09.001. [6] Gibson AG, Torres MEO, Browne TNA, Feih S, Mouritz AP. High temperature and fire behaviour of continuous glass fibre/polypropylene laminates. Compos Part Appl Sci Manuf 2010;41:1219e31. http://dx.doi.org/10.1016/ j.compositesa.2010.05.004. [7] Boyd SE, Bausano JV, Case SW, Lesko JJ. Mechanistic approach to structural fire modeling of composites. Fire Technol 2009;47:941e83. http://dx.doi.org/ 10.1007/s10694-009-0122-8. [8] Boyd SE, Lesko JJ, Case SW. Compression creep rupture behavior of a glass/ vinyl ester composite laminate subject to fire loading conditions. Compos Sci Technol 2007;67:3187e95. http://dx.doi.org/10.1016/ j.compscitech.2007.04.009. [9] Boyd SE, Case SW, Lesko JJ. Compression creep rupture behavior of a glass/ vinyl ester composite subject to isothermal and one-sided heat flux conditions. Compos Part Appl Sci Manuf 2007;38:1462e72. http://dx.doi.org/ 10.1016/j.compositesa.2007.01.006. [10] Bhat T, Chevali V, Liu X, Feih S, Mouritz AP. Fire structural resistance of basalt fibre composite. Compos Part Appl Sci Manuf 2015;71:107e15. http:// dx.doi.org/10.1016/j.compositesa.2015.01.006. [11] Bhat T, Kandare E, Gibson AG, Di Modica P, Mouritz AP. Compressive softening and failure of basalt fibre composites in fire: modelling and experimentation. Compos Struct 2017;165:15e24. http://dx.doi.org/10.1016/ j.compstruct.2017.01.003. [12] Bausano JV, Lesko JJ, Case SW. Composite life under sustained compression and one sided simulated fire exposure: characterization and prediction. Compos Part Appl Sci Manuf 2006;37:1092e100. http://dx.doi.org/10.1016/j. compositesa.2005.06.013. [13] Pering GA, Farrell PV, Springer GS. Degradation of tensile and shear properties of composites exposed to fire or high temperature. J Compos Mater Jan 1980;14:54e68. [14] Seggewiss PGB. Methods to evaluate the fire resistance of carbon fiber reinforced plastic. Long Beach: Proc. SAMPE Tech. Conf.; 2004. [15] Vieille B, Casado VM, Bouvet C. About the impact behavior of woven-ply carbon fiber-reinforced thermoplastic- and thermosetting-composites: a comparative study. Compos Struct 2013;101:9e21. http://dx.doi.org/10.1016/ j.compstruct.2013.01.025. [16] Costa AP da, Botelho EC, Costa ML, Narita NE, Tarpani JR. A review of welding technologies for thermoplastic composites in aerospace applications. J Aerosp Technol Manag 2012;4:255e66. http://dx.doi.org/10.5028/jatm.v4i3.166. [17] Vieille B, Albouy W, Chevalier L, Taleb L. About the influence of stamping on thermoplastic-based composites for aeronautical applications. Compos Part B Eng 2013;45:821e34. http://dx.doi.org/10.1016/j.compositesb.2012.07.047. [18] Vieille B, Lefebvre C, Coppalle A. Post fire behavior of carbon fibers Polyphenylene Sulfide- and epoxy-based laminates for aeronautical applications: a comparative study. Mater Des 2014;63:56e68. http://dx.doi.org/10.1016/ j.matdes.2014.05.046. [19] Vieille B, Coppalle A, Keller C, Garda M-R, Viel Q, Dargent E. Correlation between post fire behavior and microstructure degradation of aeronautical polymer composites. Mater Des 2015;74:76e85. http://dx.doi.org/10.1016/ j.matdes.2015.03.002. [20] Petit A, Vieille B, Coppalle A, Barbe F, Maaroufi M-A. High temperature behaviour of PPS-based composites for aeronautical applications: Influence of fire exposure on tensile and compressive behaviors. 20th Int. Conf. Compos. Mater., 2015. [21] Petit A, Vieille B, Coppalle A, Barbe F, Lacour B, Maaroufi M-A. Thermoplasticbased composites under fire exposure: A critical issue for aeronautical applications. 1st Fr.-Chin. Symp. Damage Fract. Compos. Struct. Assess. Monit., 2015. [22] Mouritz AP, Gibson AG. Fire properties of polymer composite materials. Springer Science & Business Media; 2007. [23] Zhang H. Fire-safe polymers and polymer composites. Washingtion DC: US Department of Transport; 2004. [24] Mouritz AP. Post-fire flexural properties of fibre-reinforced polyester, epoxy and phenolic composites. J Mater Sci 2002;37:1377e86. http://dx.doi.org/ 10.1023/A:1014520628915. [25] Test standard AITM 1.0010. Determination of compression strength after impact. Fiber reinforced plastics. AITM Airbus Ind Test Method; 1994. p. 2. [26] ASTM D695-15. Standard test method for compressive properties of rigid plastics. ASTM Int; 2015. [27] Feih S, Mouritz AP. Tensile properties of carbon fibres and carbon fibreepolymer composites in fire. Compos Part Appl Sci Manuf 2012;43: 765e72. http://dx.doi.org/10.1016/j.compositesa.2011.06.016. [28] Vieille B, Taleb L. About the influence of temperature and matrix ductility on
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mechanisms of a woven carbon/polyimide laminate under compression. Mech Mater 1998;30:165e80. http://dx.doi.org/10.1016/S0167-6636(98)00053-2. [33] Osada T, Nakai A, Hamada H. Initial fracture behavior of satin woven fabric composites. Compos Struct 2003;61:333e9. http://dx.doi.org/10.1016/S02638223(03)00058-8. [34] Vieille B, Casado VM, Bouvet C. Influence of matrix toughness and ductility on the compression-after-impact behavior of woven-ply thermoplastic- and thermosetting-composites: a comparative study. Compos Struct 2014;110: 207e18. http://dx.doi.org/10.1016/j.compstruct.2013.12.008. [35] Pilkey WD, Pilkey DF. Peterson's stress concentration factors. third ed. John Wiley & Sons editors; 2008p6.
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Post-fire compressive behaviour of carbon fibers woven-ply Polyphenylene Sulfide laminates for aeronautical applications M.A. Maaroufi a, Y. Carpier a, *, B. Vieille a, L. Gilles a, A. Coppalle b, F. Barbe a a b
Groupe de Physique des Mat eriaux, UMR 6634 CNRS, Universit e et INSA de Rouen, 76801 Saint Etienne du Rouvray, France CORIA, UMR 6614 CNRS, Universit e et INSA de Rouen, 76801 Saint Etienne du Rouvray, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 November 2016 Received in revised form 27 January 2017 Accepted 19 March 2017 Available online 28 March 2017
The influence of fire exposure on the residual compressive behaviors of carbon fibers woven-ply Polyphenylene Sulfide has been investigated for aeronautical applications. For heat fluxes ranging from 20 to 50 kW/m2, prior fire exposure is highly detrimental to the compressive mechanical properties as the residual strength and stiffness decrease by 75% and 55% respectively. Thermogravimetric analyses have been conducted under inert and oxidative atmospheres to quantify the mass loss resulting from the thermal decomposition of the outer layer directly exposed to heat flux and oxygen-rich atmosphere and internal layers respectively. Fire exposure results in gradually increasing damages within exposed laminates: PPS matrix thermal decomposition leaves intra- and inter-laminar voids leading to more or less extensive delamination depending on fire testing conditions. In order to discuss the compressive damage mechanisms after fire, the early deformation mechanisms have been analyzed by means of 2D Digital Image Correlation. C-scan inspections have also been performed to evaluate the delaminated areas which are quite well correlated with the surface fire-degraded areas, suggesting that delamination is primarily associated with thermal degradation. As heat flux increases, the fire-induced delamination and the onset of local plastic kink-bands during compressive loading ultimately cause delamination extension and global plastic buckling. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Fire behaviour Thermoplastic High temperature Mechanical testing
1. Introduction The reduction of structural mass is one of the privileged ways to improve propulsion systems in aeronautics. High performances composites consisting of polymer matrix is a good compromise between weight and robustness. The strength and stiffness/density ratio are one of the benefits offered by composites materials, but several questions are raised by their use in more critical applications. Among them, their behaviour under fire exposure is obviously an issue because of their multiple alteration potentialities: matrix and fibres degradation, oxidization as well as delamination [1]. Fire weakens the material mechanical properties and consequently the structure resistance. When exposed to fire, polymer matrix softens for T > Tg and loses its properties (in term of stiffness and strength). Thus, in order to obtain the best post-fire behaviour, different types of composites can be considered [2]. Among polymer matrix composites, thermoset-based composites (denoted TS)
* Corresponding author. E-mail address: [email protected] (Y. Carpier). http://dx.doi.org/10.1016/j.compositesb.2017.03.046 1359-8368/© 2017 Elsevier Ltd. All rights reserved.
have been widely used for aeronautical applications over the past four decades. As a consequence, most of the studies available in the literature on the fire behaviour deal with carbon/Epoxy composites [3e5] and E-Glass/Vinylester composite [6e9]. Therefore, the post fire behaviour of TS-based composites is well established in the literature [3e14], with particular attention to the effects of heating and fire duration on the residual mechanical properties of epoxy composites [9,12e14]. The post-fire mechanical properties can rapidly decrease with the increase of the temperature of the fire exposure time due to the high flammability of the epoxy matrix, but also because of its rapid thermal decomposition [12]. Recently, thermoplastics (denoted TP) received considerable attention as matrix constituents in structural composites because they present several advantages compared to thermoset composites, such as damage tolerance or impact resistance [15]. In addition, there are still issues regarding the manufacturing process of these TS-based composites: low-temperature storage, long curing cycles, irreversible process, etc. TP-based composites can be easily be manufactured by means of specific processes such as welding [16], and automatic processes such as stamping and co-consolidation [17].
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All these advantages are related to the specific microstructure of the TP matrix consisting of an amorphous and a crystal phase. TPbased composites are increasingly used in critical applications (e.g. aircraft engine nacelles), but only few studies deal with their fire resistance. Recently, the post-fire behaviours of carbon fibers PPSand epoxy-based has been compared [18]. A smaller decrease was observed in the tensile properties of TP laminates as compared to the TS ones for thermal heat flux up to 50 kW/m2. Such observations have been correlated with the fire-induced damages and with the high char yield of PPS matrix [19]. Indeed, char formation arising from the thermal decomposition of the organic resins plays a major role on the post-fire behaviour. It notably may act as an insulation layer against the fire propagation due to its high porosity in the case of TS-based laminates. In TP-based laminates, the char composition seems to be different as the TP resins have different chemical nature and the thermal degradation does not give a similar residue [19]. As a consequence, the effects of thermal degradation on the fire behaviour are not well understood and the issue of the mechanical behaviours of thermally degraded TP-based composites is still an open question, particularly for hightemperature applications. Compression being one of the most severe mechanical loading modes [3], and due to the lack of experimental data under high temperature conditions, the purpose of the present work is to study the compressive behaviour of carbon reinforced PPS-based composites at 120 C (T > Tg) after prior fire exposure. More precisely, the question is to ultimately know whether it is relevant to replace epoxy-based composites by PPSbased composites for applications in nacelles of aircraft's engines, or not [18e21]. Finally, the main objective of the present work is to estimate how detrimental prior fire exposure is on C/PPS post-fire compressive properties [22], and to investigate the influence of fire exposure on compressive damage mechanisms. 2. Materials and methods 2.1. Materials The composite materials studied in this work are 7 plies carbon fabric-reinforced PPS prepreg laminate plates [18,19,23]. The toughened PPS resin (Fortron 0214) is supplied by the Ticona Company. The woven-ply prepreg, supplied by the SOFICAR Company, consists of 5-harness satin weave carbon fiber fabrics (T300 3 K 5HS), with a mass fraction of fibers of 58%. The prepreg plates are hot pressed according to the following lay-up [(0/90), (±45), (0/ 90), (±45), (0/90), (±45), (0/90)]. Test specimens are 100*150 mm2 plates (2.2 mm thickness) cut by water jet to reduce edge and
machining effects. The interest of using large specimens is to limit heat losses but also to reduce the thermal degradation along specimen edges. 2.2. Experimental set-up and methods 2.2.1. Fire exposure test method As used in many studies [18,24], the radiant heater of a cone calorimeter was used here to burn composites specimens. With this heating method, the heat flux and the heating conditions are controlled and repeatable. However, the exposure to a cone calorimeter represents an idealized fire condition: it is stable and involves a continuous heat flux. A heat flux gauge has been used to calibrate the cone calorimeter and to control the heat flux on the specimen surface. Four different heat fluxes have been considered: 20, 30, 40 and 50 kW/mm2. After a 2 min fire-exposure, the specimens were cooled in the air during one night. Four specimens were tested in each fire testing condition. Analyses have been performed by comparison to reference virgin specimens, which have not been exposed to prior fire. In order to monitor the temperature (see Fig. 2) and the temperature gradient DT between these two surfaces (see Fig. 3a), one thermocouple has been placed on the centre of the fire-exposed surface and another one on the unexposed surface of specimens. The accuracy of the temperature measurement depends on the contact of the thermocouple on the specimen surface which can be difficult on the fire-exposed surface because of the matrix thermal degradation. To limit this effect, the thermocouple bond with the surface is ensured by means of an iron wire wrapped around the specimen. 2.2.2. Compression test method The mechanical loading mode is a conventional compression test requiring the use of a specific anti-buckling system as buckling is the primary failure mode in compression due to specimen dimensions. This system originally designed by Boeing incorporates adjustable side plates enabling to accommodate variations in thickness and overall dimensions, as well as to prevent specimen buckling (see Fig. 1). Such method has often been reported in the literature [5,6]. Compressive tests were performed using a 100 kN capacity load cell of a MTS 810 servo-hydraulic testing. The temperature control system includes an oven and a temperature controller, and the tests were conducted at 120 C. Specimens were subjected to compressive loads at a constant displacement rate V ¼ 0.2 mm/min, complying with the standard Airbus AITM 1e0010 (compression after impact test) [25]. Because of the fire exposure
Fig. 1. Compression tests after fire exposure: experimental set-up and anti-buckling fixture [15].
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Fig. 2. Influence of fire testing conditions on the surface measured temperature: (a) on the fire-exposed surface e (b) on the unexposed surface (back face).
Fig. 3. Influence of fire testing conditions: (a) changes in the temperature gradient between exposed and unexposed faces e (b) approximate estimation of the surface degraded area after fire exposure (macroscopic observations).
method used in this study, the normalized test method for compressive properties of composites [26] has not been chosen here to characterize the post fire compressive behavior. Indeed, it would require cutting specimens in the fire-degraded and delaminated plates, leading to a more degraded material and more scattered results than with the compression after impact method. For this reason, this study is focused on structural testing rather than a pure compressive loading. As a consequence, the post-fire thermo-mechanical responses of the laminates subjected to different prior fire-exposures are compared in term of apparent stiffness and strength. 2.2.3. Digital Image Correlation technique (DIC) Fire-exposure leads to local damage within the laminates, resulting in singularities in the strain field on the outer surface. 2D Digital Image Correlation (DIC) is an appropriate tool to continuously track the effects of damage on the surface of specimens such as fire-exposed laminates subjected to compressive loads. This method is based on the measurement of the displacement field between a reference time (stress-free state) and a given time of the loading (deformed state). This method requires the painting of a black and white speckle on the specimen surface to obtain different
shades of grey. Consequently, the Green-Lagrange strain field can be derived from the 2D displacement field by means of the VIC-2D correlation software. A 3D DIC method should be used to examine the out-of-plane displacements when buckling occurs. However, as a first approximation and for lack of better means, the 2D DIC technique was used during compressive loading to investigate the early deformation mechanisms on the fire-exposed faces, but also to detect the onset of local damage events (transverse matrix cracking, breakage of 0 fibers and onset of global buckling). 2.2.4. C-scan ultrasonic inspections Even though the qualification of the fire-induced damage (type, size and severity of damage) remains a complex task, standard nondestructive evaluation methods such as ultrasonic C-scan imaging are often used, particularly to detect delamination in polymer-based composites. In the present case, the purpose of Cscan analysis is to correlate the fire-degraded area on laminates surface and the delaminated area through the thickness. The underlying idea is to show that the thermal degradation on the exposed surface leads to increasing macroscopic damages (mostly delamination) in the meso-structure of the laminates. C-scan inspections have been performed with an ultrasonic device
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ULTRAPAC II system (automated immersion system). Data acquisition, control and C-scan imaging were conducted by means of UTwintm software. 2.2.5. Dynamic mechanical analysis (DMA) Dynamic mechanical tests have been performed using a TA Instruments Q800 device. Because of the fire-induced delamination, tests were not conducted on fire-exposed specimens. Nevertheless, the aim of these tests was to characterize the loss of modulus at a service temperature during the glass transition. Specimens whose dimensions are 35 12 2.15 mm were heated from room temperature to 200 C at a constant heating rate of 2 C/min. 2.2.6. Thermogravimetric analysis The thermal degradation of C/PPS was investigated by thermogravimetric analysis (TGA) under inert (N2) and oxidative (O2) atmospheres using a TA Instruments Discovery device. Both atmospheres have been considered because the fire-exposed face is in an oxygen-rich atmosphere whereas the core layers are in an oxygen-lean atmosphere [27], therefore leading to different thermal decompositions. Rectangular samples of 10 mg (±2 mg) were cut from C/PPS virgin laminates and heated from 30 to 700 C at a constant heating rate of 10 C/min. The onset of thermal decomposition Td refers to the temperature corresponding to a 5% mass loss. 3. Results and discussion 3.1. Fire exposure tests 3.1.1. Temperature evolution and thermal conduction When samples are exposed to heat flux between 20 kW/m2 to 50 kW/m2, the maximal surface temperature ranges from 355 C to 662 C. The melting temperature (Tm ¼ 280 C) of PPS matrix is thus reached on the fire-exposed surface for all heat fluxes [28]. For the two highest heat flux, the thermal degradation onset temperature is reached. When specimens are subjected to 50 kW/m2 heat flux, the sudden increase of temperature after 110s (see Fig. 2a) is attributed to a thermocouple debonding resulting from matrix pyrolysis. Consequently, the monitored value after 110s represents the atmosphere temperature rather than the specimen surface temperature. On unexposed surfaces, the pyrolysis onset temperature is not reached for any heat fluxes (see Fig. 2b), confirming that thermal conduction and subsequent thermal decomposition is not uniform through the thickness. Some thermal inertia can indeed be observed for all heat fluxes as the maximum temperature is reached after 140e150s, whereas the samples were only exposed to fire during 120s. From the heating kinetics standpoint, the temperature changes as a function of the fire-exposure time are significantly different on both fire-exposed and unexposed surfaces. The temperature gradient between the exposed and the unexposed faces increases with increasing heat flux (86 C after 120s for the 20 kW/m2 and 230 C for 50 kW/m2) what tends to indicate that the higher the heat flux is, the less easy the thermal conduction is (see Fig. 3). It suggests that the thermal conduction within the laminates depends on both the applied heat flux and the resulting thermal degradation through the thickness and on specimen surface. Indeed, the thermal decomposition may contribute to the insulation of the outer layers of the composite (unexposed surface) through the formation of char, as it has been shown for TSbased composites [29]. The presence of melted resin into a fibrous network suppresses heat radiation and physical modification of the char [23]. Thus, the char formation in TP-based composites also protects the outer layers of the laminates against thermal degradation. However, the nature of char has only been investigated at
the meso- and micro-scales by means of a Scanning Electron Microscope (SEM) and needs to be characterized thermo-chemically in the case of TP-based composite because the PPS matrix degradation mechanisms are different from those observed in epoxybased composites [19]. 3.1.2. Fire-induced damage mechanisms Depending on fire testing conditions, different thermallyinduced damages can be observed within the laminates (see Figs. 4 and 5). On fire-exposed surface, fire exposure leads to a circular damage area whose surface linearly increases as heat flux increases (see Fig. 3b). At low heat flux (e.g. 20 kW/m2), it is observed that the fire-exposed surface is slightly thermallydegraded mainly because of the melting of the matrix (see Fig. 4), but fire-exposure does not seem to induce damages through-thethickness (see Fig. 5), and there is thus no visible degradation on the unexposed face. For higher heat fluxes (ranging from 30 to 50 kW/m2), fire-exposure leads to gradually increasing damages such as intra- and inter-laminar debonding, as well as extensive delamination which can be observed at the macroscopic scale along specimens free-edges (see Fig. 5). 3.1.3. Thermal degradation and microstructural changes In order to better understand the thermomechanical processes induced by such temperatures, the thermal degradation has been investigated by TGA. Under nitrogen, the thermal decomposition is a single-stage process beginning at around 505 C and leading to a 25% mass loss of specimens. Since carbon fibers degradation does not occur at such temperatures in nitrogen [27], the mass loss is in fact only due to the pyrolysis of PPS which loses 60% of its mass. The resulting char, described in Paragraph 3.1.1, is expected to play an insulating role. The thermal-oxidative decomposition is a twostages process. In the first decomposition step, pyrolysis starts at 515 and carbonaceous char is formed, as it happens in a nitrogen atmosphere. During the second stage beginning at 630 C, both char and carbon fibers are oxidized [19] and the material is practically fully decomposed at 700 C. For low heat fluxes (e.g. 20 and 30 kW/m2), no noteworthy thermal decomposition is expected to occur (see Fig. 6). Moreover since melting temperature is reached (see Fig. 2), the PPS matrix is redistributed within the fibers network and protect carbon fibers against oxidation [19]. The pyrolysis onset temperature is reached on surface for a 40 kW/m2 heat flux, which means that part of the matrix covering the fibers starts its decomposition and becomes char. As long as the PPS is not completely degraded, it prevents carbon fibers from oxidization. As a result, a partial cohesion of fibers and matrix can be maintained contributing to the load transfers within laminates plies, and significantly influencing the post-fire compressive mechanical properties. For the highest heat flux, the first decomposition step is complete; all the matrix on the surface has become char, which is partially oxidized on surface among with fibers (see Fig. 6). The microstructural changes are also expected to strongly influence the mechanical properties. The degree of crystallinity (DOC) decreases from 27% (virgin state) to 0 (strongly degraded state) when the heat flux increases to 50 kW/m2 [18]. More precisely, the DOC decreases slowly for low heat fluxes and rapidly for heat fluxes higher than 30 kW/m2 (see Table 1), suggesting the importance of the exposure time at temperatures higher than the material degradation temperature Td (see Fig. 2). Indeed, below Td, the only loss of crystallinity is due to the rapid cooling preventing from a total recrystallization. Above Td, the thermal degradation leads to different mechanisms like chain scission or charring responsible for the loss of crystallinity [30]. For a 40 kW/m2 heat flux, a part of the matrix on surface is “charred” (see Fig. 6). On the one hand, the
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Fig. 4. Influence of fire testing conditions on the fire-degraded of quasi-isotropic C/PPS laminates: (a) fire-exposed surface e (b) unexposed surface.
Fig. 5. Macroscopic observations of the fire-induced damages through the thickness in quasi-isotropic C/PPS laminates subjected to different fire testing conditions.
crystalline phase disappears in this “charred” matrix (see Table 1). On the other hand, it contributes to the formation of an insulating layer, protecting the layers underneath from thermal degradation
and decrystallization. For higher heat fluxes, the thermal degradation is more significant (65% of mass loss) at the exposed surface, and oxidized char and fibers cannot act as an insulation layer
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Fig. 6. Thermal decomposition of C/PPS and mass loss rates under nitrogen and oxygen.
Table 1 Influence of fire conditions on C/PPS laminates' degree of crystallinity. Testing conditions
Virgin
20 kW/m2
30 kW/m2
40 kW/m2
50 kW/m2
Degree of crystallinity DOC (%) Decrease in DOC (%)
26.69 ± 1.76 e
24.52 ± 0.94 8
22.83 ± 1.11 15
13.19 ± 0.68 51
0 100
anymore. This phenomenon combined with longer exposure times at T > Td are responsible for the total loss of crystallinity. These physical transformations are expected to be of the utmost importance with respect to post-fire mechanical behavior, especially when compression tests are carried out at temperatures higher than Tg, as will be discussed in the next section. Indeed, the mechanical properties of the PPS matrix are degraded in a first step at T ¼ Tg due to mechanisms affecting the amorphous phase, whereas the properties of the crystalline phase decrease when temperature reaches T ¼ Tm [31]. Thus, the respective contribution of these mechanisms to the degradation of mechanical properties significantly depends on the degree of crystallinity. Such degradation is usually characterized by means of Dynamical Mechanical Analysis (DMA). The influence of prior fire-exposures has not specifically been studied here, but a DMA has been conducted on virgin specimens (see Fig. 7). As specimens were heated up to 200 C, only the loss due the transition from glassy to rubbery states can be observed around the glass transition temperature (Tg ¼ 98 C). The decrease in the storage modulus starts at 95 C and leads to a 15% loss at 120 C. In the whole glass transition interval, the storage modulus decreases by 40%. As prior fire-exposures have a significant influence on the degree of crystallinity, the higher the heat flux, the higher the modulus loss due to glass transition. This result is expected to have an influence on failure mechanisms discussed in 3.2.4.
3.2. Post-fire compressive tests 3.2.1. Macroscopic response of laminates under compression The different compressive responses can be compared for all fire-testing conditions (see Fig. 8). Compressive damage mechanisms are discussed in the next section, but the macroscopic response of C/PPS laminates appears as elastic-ductile until it reaches a maximum value which seems to be associated with the
macroscopic plastic buckling. From the obtained results, tests repeatability is better for lower heat fluxes (20 and 30 kW/m2), and it is expected that local instability (such as local micro-buckling) is promoted as heat flux and through-the-thickness damages increase. The qualitative responses can be compared for all testing conditions (see Fig. 9a), and show that the ultimate longitudinal apparent strength decreases by 80% with respect to the value of virgin specimens (see Fig. 9b). At the same time, the longitudinal apparent stiffness of the structure decreases by 60%. For comparisons purposes, under the same fire-testing conditions in tension, both longitudinal stiffness and strength decrease by virtually 35% between the virgin state and the highest heat flux (50 kW/m2), confirming that the influence of fire-exposure strongly depends on the type of loading [18]. It is worth noting that the results are here expressed in term of apparent mechanical properties based on constant cross section. Indeed, depending on the heat flux, fire damaged areas are quite different from each other, hence the cross section of effective material. These mechanical tests do not consequently accurately represent the evolution of material properties under different specific fire conditions but more the loss of properties of a specific composite structure. Fire-induced delaminated area is circular, and it is expected that compressive loading will promote further delamination. To a first approximation, it can be assumed that the degraded area within the laminates is somehow equivalent to a plate with a central hole. On the one hand, both longitudinal stiffness and compressive strength can be evaluated in term of nominal values from the constant initial cross section S ¼ t:w (with t and w thickness and width of the laminates, respectively):
F S
snom ¼ ¼
F Dsnom DF and Enom ¼ ¼ t:w S:Dε Dε
snom therefore represents the stress in a cross section far from the circular hole, and the area S at this section is called the gross
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Fig. 7. Influence of temperature on the evolution of storage modulus and tan delta of a virgin C/PPS specimen.
Fig. 8. Thermomechanical responses of quasi-isotropic C/PPS laminates subjected to a compressive loading after different prior fire testing conditions.
cross-sectional area [35]. On the other hand, the compressive strength can be obtained from the cross section at the hole, which is formed by removing the circular hole (whose diameter is d is the diameter of the delaminated circular area) from the gross cross section. The corresponding area Snet ¼ t ðw dÞ is referred to as the “undamaged” or net
cross-sectional area (see Table 2). If the stresses at this cross section are uniformly distributed and equal to snet :
snet ¼
F F snom ¼ ¼ Snet t:ðw dÞ ðw dÞ
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Fig. 9. Influence of different prior fire testing conditions on residual compressive thermomechanical behaviors: (a) Macroscopic response e (b) Changes in the longitudinal stiffness and the compressive strength.
Table 2 Influence of fire conditions on delaminated area and corresponding “undamaged” net cross-sectional area. Testing conditions 2
Delaminated area (mm ) Snet “Undamaged” net cross-sectional area (mm2) Snet S
(%)
Virgin
20 kW/m2
30 kW/m2
40 kW/m2
50 kW/m2
e 220 100
6000 141 64
8600 126 57
11000 113 52
12500 107 48
Fig. 10. Green-Lagrange longitudinal strain distribution (obtained by Digital Image Correlation) in C/PPS laminates subjected to compression after fire exposure: (a) 20 kW/m2: failure by transverse crack propagation and breakage of 0 fibers e (b) 30-40-50 kW/m2: onset of global plastic buckling is detected from the localization of positive and negative longitudinal strains.
From this definition, new values can be computed for the compressive strengths. On Fig. 9b, it appears that the effective strength snet is virtually the same for heat flux ranging from 20 to 40 kW/m2 (about 200 MPa). The net stress reflects the structural
loading and cannot be compared to the compressive strength of virgin specimens in which the stress state is homogeneous. However, the calculated values can be compared to the buckling theoretical stress sbuckling (see Fig. 9b) obtained from the Euler's
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Fig. 11. C-Scan ultrasonic inspections of quasi-isotropic C/PPS laminates subjected to compressive loading after fire exposure.
Fig. 12. Comparison between approximate estimation of the surface degraded area after fire exposure (macroscopic observations) and estimation of the delaminated area after compression (C-scan inspection).
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buckling theory [34]. These values remaining lower than sbuckling , it suggests that the degraded specimens should not experience elastic buckling along the hole as specifically addressed in section 3.2.4. In addition, when the laminates are severely degraded at 50 kW/m2, the compressive strength significantly decreases due to critical fireinduced degradation: pyrolysis of the matrix followed by the oxidation of both matrix and fibers at the exposed surface (see Fig. 6). Finally, to ease the comparison of the mechanical properties between the different fire-resting conditions, it is necessary to consider another experimental set-up to conduct compressive tests. To this aim, a cover will be designed to promote the same degraded area on the exposed surface, but also to reduce the differences in the delaminated areas. 3.2.2. Analysis of deformation mechanisms by Digital Image Correlation The early deformation mechanisms taking place in firedegraded laminates subjected to compressive loadings have been
analyzed by means of 2D DIC on the fire-degraded area. Obviously, such a 2D technique is not fully operational to investigate the deformation mechanisms as medium to high heat fluxes induce significant out-of-plane effects. However, this technique can be helpful to detect the onset of local damage events (transverse matrix cracking, breakage of 0 fibers and onset of global buckling). At low heat flux (20 kW/m2), compressive failure primarily results from transverse matrix cracking and the 0 fibers breakage (see Fig. 10). For higher heat fluxes (30-40-50 kW/m2), the onset of global plastic buckling can be detected from the localization of successive positive and negative longitudinal strains (see Fig. 10). The longitudinal strain maps also confirm that elastic buckling does not occur during compressive loading as there is no localization of simply positive or negative strains. 3.2.3. Post-compression C-scan inspections The C-scan maps of specimens subjected to compressive loading after fire-exposure provide further information on the damage
Fig. 13. Damage mechanisms in woven-ply quasi-isotropic C/PPS laminates subjected to compression after fire exposure: (a) Local micro-buckling of fiber bundles at the crimp in a 5-harness satin weave [34] e (b) Delamination and global plastic buckling induced by post-fire delamination and the formation of plastic kink bands.
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mechanisms, and more particularly they can be used to calculate the delaminated area (see Fig. 11). As it was observed in section 3.1.2, prior fire-exposure leads to a circular damage area whose surface linearly increases as heat flux increases. Fire-induced delaminated area is circular, and it is expected that a compressive loading will promote further delamination. C-scan inspections performed after compressive tests clearly show that delaminated area gradually increases under the degraded surface as heat flux increases. The comparison between an approximate estimation of the fire degraded area after fire exposure and the delaminated area after compression (respectively based on macroscopic observations and C-scan analysis) show that they are rather well correlated (see Fig. 12), suggesting that delamination is primarily associated with thermal-degradation. 3.2.4. Investigations on compressive damage mechanisms after fireexposure In the case of compressive loadings, damage mechanisms are complex [28,29], but they are even more difficult to investigate as both matrix and fiber/matrix bonding are severely thermallydegraded. As it was shown in section 3.1.2, fire-exposure causes two major damages: intra-and inter-laminar cracking and delamination. As it is well documented in the literature, carbon fibers under compressive loadings are prone to instabilities such as
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buckling or fiber crushing, and the initial alignment of fibers in reinforced composites has a large influence on their compressive response [32,33]. Thus, the crimp region plays a significant role in controlling the onset of failure in woven-ply laminates, mostly because of the undulating structure of carbon fabrics (see Fig. 13), and the presence of matrix-rich areas at the crimps. More specifically, both matrix toughness and ductility contribute to specific failure mechanisms within a fiber network in PPS-based laminates subjected to compressive loadings at T > Tg [34]. Indeed, localized bending at the crimp is found to cause micro-buckling in woven-ply misaligned structures (see Fig. 13a). In addition, micro-buckling comes along with a plastic deformation of the matrix in highly ductile matrix systems, resulting in the formation of deformed inclined kink-bands also called plastic buckling (see Fig. 13b). Such bending is also amplified by the formation of voids resulting from the degradation of the PPS matrix in matrix-rich areas and the fireinduced delamination. As it was shown by the C-scan analysis (cf. section 3.2.3), delamination is the primary failure mechanism ruling the high-temperature compressive behavior of C/PPS laminates when the material is thermally-degraded by medium to high heat fluxes. At low heat flux (20 kW/m2), fire-induced delamination is less significant, and compressive failure results from transverse matrix cracking and the breakage of 0 fibers (see Fig. 14 and cf. section 3.2.2). As the heat flux increases, the fire-induced
Fig. 14. Macroscopic observations of fire-exposed specimens after compressive tests: transverse matrix cracking and formation of global plastic buckling bands.
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delamination and the onset of local plastic kink-bands during compressive loading ultimately cause delamination extension and global plastic buckling following the maximum load in the loaddisplacement response (see Fig. 14). As a result, the compressive response of fire-damaged laminates is elastic/ductile with a gradual failure. 4. Conclusions From the fire-performance standpoint, the basic idea of the present work was to investigate how detrimental different prior fire-testing conditions are on the residual compressive behavior of Polyphenylene Sulfide laminates to be used under hightemperature service conditions (e.g. 120 C) for aeronautical applications (e.g. aircraft engine nacelles). Depending on the firetesting conditions, different conclusions can be drawn from this study: With respect to the values of virgin specimens (unexposed to fire), a severe fire-exposure (typically 50 kW/m2 for 2 min) contributes to a significant decrease in laminates longitudinal stiffness (55%) and strength (75%). Prior fire-exposure results in gradually increasing damages within tested laminates, as PPS matrix thermal decomposition leaves intra- and inter-laminar voids leading to more or less extensive delamination. Thermogravimetric analyses have been conducted under inert (N2) and oxidative (O2) atmospheres to quantify the mass loss resulting from the thermal decomposition of the material in the laminates exposed and core layers respectively. In order to investigate the compressive damage mechanisms after fire, the early deformation mechanisms in compression have been analyzed by means of a 2 dimensional Digital Image Correlation technique. Thus, the onset of global plastic buckling can be detected from the localization of positive and negative longitudinal strains. The delaminated areas have been evaluated by means of C-scan inspections, and these areas are rather well correlated with the surface fire-degraded areas, suggesting that delamination is primarily associated with thermal-degradation. As the heat flux increases, the fire-induced delamination and the onset of local plastic kink-bands during compressive loading ultimately cause delamination extension and global plastic buckling. Finally, compression behavior being the Achilles' heel of composite behavior under load in fire, this study is a preliminary work dealing with the design of an experimental platform whose purpose is to simultaneously apply fire-exposure and mechanical loading (critical service conditions). To ease the comparison between the mechanical properties of the laminates subjected to different fire conditions, a specific cover will be designed to promote the same degraded area on the exposed surface, but also to reduce the differences in the delaminated areas. Acknowledgement This work is a part of the DECOLLE project funded by the Institute Carnot ESP (Energy and Propulsion Systems). The authors also would like to acknowledge the Aircelle Company for supplying the composite materials. References [1] Burns LA, Feih S, Mouritz AP. Compression failure of carbon fiber-epoxy
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