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Toughening and healing of continuous fibre reinforced composites by supramolecular polymers
Composites Science and Technology 128 (2016) 84e93
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Toughening and healing of continuous fibre reinforced composites by supramolecular polymers V. Kostopoulos a, *, A. Kotrotsos a, S. Tsantzalis a, P. Tsokanas a, T. Loutas a, A.W. Bosman b a Applied Mechanics Laboratory, Department of Mechanical Engineering and Aeronautics, University of Patras, Patras University Campus, GR-26504 Rio Patras, Greece b SupraPolix BV, Horsten 1, 5612 AX Eindhoven, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 November 2015 Received in revised form 16 March 2016 Accepted 19 March 2016 Available online 21 March 2016
Interleaves comprising self-healing materials based on hydrogen bonded supramolecular polymers (SP) were successfully incorporated in the mid-plane of unidirectional (UD) carbon fibre reinforced polymers (CFRPs). The fracture toughness of these hybrid composites and their healing capability were measured under mode I loading. The fracture toughness appeared to have increased considerably since the maximum load (Pmax) of the hybrid composite had increased approximately 5 times, and the fracture energy I (GIC) displayed a dramatic increase by almost one order of magnitude when compared to the reference composite without the SP. Furthermore, the double cantilever beam (DCB) hybrid composites displayed a healing efficiency (H.E.) value for the mode I interlaminar characteristics around 60% for the Pmax and the GIC after the first healing cycle, dropping to 20e30% after the seventh cycle. During the mode I interlaminar fracture toughness tests the acoustic emission (AE) activity of the samples was also monitored. It was found that AE-activity strongly reduced due to the presence of the SP. Moreover, optical microscopy not only showed that the epoxy matrix at the interface is partly infiltrated by the SP, but it also revealed that cross-sections of both fractured surfaces were covered with the SP comprising pulled-out carbon fibres, indicating a strong interfacial adhesion. Finally it was shown that the SP fractured surfaces were partially covered with pulled-out carbon fibres emanating from the edges of the SP film in which the epoxy system exists. © 2016 Elsevier Ltd. All rights reserved.
Keywords: CFRPs Supramolecular polymers Fracture mechanics Self-healing Acoustic emission
1. Introduction During the last decades the use of carbon fiber reinforced polymers (CFRPs) has been rapidly increased. Their enhanced specific properties make them attractive for structural applications in emerging fields of industrial technology such as aerospace, automotive, rail, marine, as well as the defense industry. One of the most important mechanical characteristic of fiber-polymer composites is their resistance to delamination. The interlaminar fracture toughness plays an important role in damage formation and propagation in FRPs. The presence of delaminations may lead to a loss of stiffness which can be a very important design consideration and may result to complete failure of the composite structure. It is therefore obvious that the delamination resistance of a laminate is
* Corresponding author. E-mail addresses: [email protected] (V. Kostopoulos), bosman@ suprapolix.com (A.W. Bosman). http://dx.doi.org/10.1016/j.compscitech.2016.03.021 0266-3538/© 2016 Elsevier Ltd. All rights reserved.
critical for the in-service performance of structural composites and reliable non-destructive evaluation techniques are required for its early detection. In addition, high performance composites are sensitive to microcracking formation within the material due to inservice induced mechanical and thermal loading. Joining of microcracks under service loads may be another way for delamination. In general, both extensive microcracking and/or delaminations lead to rapid degradation of the materials performance. The composites conventional repair techniques are expensive, time consuming, and practically not applicable in the case of invisible defects. Thus, the widespread utilization of composites especially in human safety critical applications is always accompanied with damage diagnostic tools. This challenging situation acted as an inspiration for the seeking of new repair methods; cheaper, faster, and easier, applied at the early stages of damage formation. Self-healing materials [1,2] have been proposed as an emerging method of improving the performance of materials. This technology has been recently applied to composites and promises to extend the effective life-span of them, to reduce the maintenance
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needs and costs, and to improve the damage tolerance and reliability of composite structures. A constant concern of various researchers has been the enhancement of the fracture behavior of high performance structural composites. A variety of methods are proposed in the literature which includes interleaving, hybridization, stitching, shortfibres, and z-pinning [3e7]. Kostopoulos et al. [8] achieved the 100% increase in the fracture energy after the addition of 1% CNF in the matrix of CFRP laminates. Magniez et al. [9] succeed the toughening of the CFRPs by interleaving a thin layer (~20 mm) of electrospun poly(hydroxyether of bisphenol A). This type of enhancement improved the fracture toughness in mode I and mode II by up to 150% and 30% respectively. Masahiro's et al. [10] experimental results about hybrid laminates, showed an increase at the rate of 50% for the fracture toughness, in mode I fracture test compare to the base CFRP laminates. Furthermore, the results of the mode II fracture toughness test confirmed that the interlaminar fracture toughness for hybrid laminates is 2e3 times greater than the base CFRP laminates. Kostopoulos et al. [11] dealt with the use of CNFs and PZT particles as dopants for the epoxy matrix of CFRP laminates. The presence of CNFs led to a remarkable increase in the mode I fracture energy of the laminates (about 100%) whereas the addition of PZT particles demonstrated reduction in the mode I fracture energy (GIC). In mode II loading, both the CNFs and the PZTs improved the fracture properties of the CFRPs. Recently, Kostagiannakopoulou et al. [12] demonstrated the increase of the interlaminar fracture toughness of carbon fibre composites by modifying the epoxy matrix with graphene nano-species. Yasaee et al. [13,14] investigated the mode I and mode II fracture behavior of glass fibre reinforced plastics (GFRPs) with embedded strips of a thermoplastic strip, thermoplastic particles, chopped fibres, etc. From the various interleaved materials tested, those that improved the GIC relatively to the baseline value were the polyimide thermoplastic film (79% increase), the chopped aramid fibres (46% increase), the 90 E-glass/epoxy prepreg strip (46% increase), the thermoset adhesive film (43% increase) and the chopped glass fibres (16% increase). All these interleaved strips had the potential to increase the GIIC [14] with values from 75% to 123%. Self-healing composites have previously been developed by embedding healing agents (i.e. reactive chemicals and catalysts) into the matrix using microcapsules that will release the healing agent upon crack damage [15,16]. Additionally, matrices with embedded vascular network have been developed in which the network serves as reservoir for the distribution of the healing system [17]. A different approach towards self-healing composites, are matrices that comprise thermoplastic polymers. Zako and Takano [18] were the first to achieve a restoration of the fatigue performance of FRPs comprising a thermoplastic-modified matrix. In a comparable approach, Hayes et al. [19,20] blended a polybisphenol A-based thermoplastic into the epoxy matrix in order to get up to 70% healing in the resulting FRP. Whereas, Pingkarawat et al. [21] were able to get a high recovery after healing in the quasistatic mode I fracture toughness in CFRPs with poly(ethylene-comethacrylic acid) (EMAA) as thermoplastic modifier. Recently, Selver et al. [22] explored the healing potential of GFRPs of a matrix modified with glass-polypropylene hybrid yarns. The resulting composites displayed a 65% recovery from low-energy impact after a simple heating treatment. Interestingly, a synergistic combination on improved toughness and self-healing in high performance composites, has been shown by Wang et al. [23] who incorporated rectangular-shaped patches of two copolymers (EMA and EMAA) into the mid-thickness of CFRPs. A more promising approach for self-healing composites might be found by merging reversible bonds into epoxide networks, since this approach allows the healing to be unlimited as no chemicals
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are consumed. Indeed, when thermo-reversible cross-links based on DielseAlder chemistry were in an epoxy network based on diglycidyl ether of bisphenol A (DGEBA), self-healing was observed after considered time (hours) of exposing the material to elevated temperatures [24]. In a more recent paper, GFRPs comprising DielseAlder based thermo-reversible cross-links, showed good self-healing behavior combined with good compatibility with the glass fibers [25]. A new technology that could be beneficial for self-healing in composites has been built on supramolecular polymers (SP) [26,27]. Especially those based on reversible hydrogen bonding arrays show great promise for self-healing materials [28e31], since these materials can typically withstand multiple healing cycles without substantial loss of performance, as a consequence of the highly directional and fully reversible non-covalent interactions present within the polymer matrix. In this study, we have employed the ureiodypyrimidone hydrogen bonding unit (UPy) as developed by Meijer and coworkers [32] because of its strong selfassociation, its synthetic accessibility, and the highly dynamic nature of low glass transition temperature (Tg)-polymers comprising the UPy [33]. Most interestingly, UPy-polymers have recently been shown to give unprecedented toughening in polybutadiene based interpenetrating networks [34]. The scope of the present work is the use of flexible SP comprising UPy-moieties as interleave additives into conventional unidirectional (UD) CFRPs in order to enhance the fracture properties of these hybrid composites and to take advantage of the SP's healing capability. This study is presenting an overview of the role of the SP in the fracture behavior and the repeatable ability to heal the cracks in CFRPs. The reference as well as the hybrid composites were subjected to mode I interlaminar fracture toughness tests and compared. After the fracture, the hybrid composites were subjected to heating under controlled loading in order to activate the SP interleave and the cracks to be healed. The healing process was repeated up to seven times. Finally acoustic emission (AE) recordings and optical microscopy examination led to better understanding of the involved failure mechanisms as well as some conclusions regarding the healing process. 2. Experimental 2.1. Materials The composite materials which used in this study were fabricated by UD carbon fibre-epoxy prepreg CE-1007 150-38. The prepreg tape was supplied by SGL Group, Germany having a tensile strength of 2.4 GPa. The SP was the SPSH01 material as provided by Suprapolix that is based on a low Tg (66 C) polymer modified with UPy-moieties, and was chosen to play the role of the toughening and healing agent interleave in the present study. Fig. 1a depicts the originally received SPSH01 piece (approximately 45 mm 30 mm 5 mm), which has been further processed into films in the present work. This polymer owes its mechanical properties to the reversible, non-covalent interactions, such as hydrogen bonding, between the macromolecules. 2.2. Preparation of the SPSH01 interleaves and composite manufacturing The preparation process of the SPSH01 interleaves is illustrated in Fig. 1. The as-received polymer piece (Fig. 1a) was converted into a thin film by a two-step heating/pressuring treatment (Fig. 1b) using a heat press machine. Firstly, the bulk SPSH01 block was pressured under 5 kN at 90 C for 30 min. Then, thermal energy was stopped to be provided to the system and the SP material was left
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Fig. 1. Illustration of the initial raw material and the preparation process of the SPSH01 interleaves.
under 25 kN overnight to cool-down and take its final form (Fig. 1c). The melting temperature (Tm) of the SPSH01 material appeared to be 77 C, using the PerkineElmer DSC 8500 differential scanning calorimeter. The DSC samples were heated from ambient temperature to 150 C at a rate of 5 C/min. Above this temperature, the UPy-UPy hydrogen bonding interactions start to diminish significantly and thereby the interactions between adjacent polymer chains of the material [28]. Consequently, a thin film of SPSH01 was obtained after the above mentioned heating/pressing treatment. The thickness of the film was measured to be approximately 120 mm with no significant thickness variations, using digital caliper. Finally, the SPSH01 film was cut into interleaves of standard sizes using a conventional film cutter (Fig. 1d, e). Two UD laminated plates made out of 22-layers were manufactured for the needs of the current study; the reference laminate and the hybrid laminate containing the SPSH01 interleafs at the mid-plane, both appropriate for mode I interlaminar fracture tests. Fig. 2 shows schematically the plate configuration. The dimensions of the plates were 300 mm 150 mm 3 mm. During the manufacturing process two 13-mm-thick sheets of polytetrafluoroethylene (PTFE) film were placed in the mid-thickness plane of both laminates as shown in Fig. 2 to act as initial pre-crack according to the request of the interlaminar fracture test. In the case of the hybrid laminate, SPSH01 strips were carefully placed at the mid-plane as shown in Fig. 2b, c. Following the lay-up, the laminates were vacuum bagged and cured in autoclave for 2 h at 130 C under 6 bars applied pressure, according to the prepreg manufacturer guidelines. C-scan inspection of the manufactured plates, using a Physical Acoustics Corporation (PAC) UT C-Scan system with a 5 MHz transducer secures the high quality of the manufactured plates. Thickness measurements were also performed. Five mode I samples were cut from both the reference and the hybrid plate. Two aluminium tabs were glued on the double cantilever beam (DCB) specimen outer surfaces (Fig. 3) using a two-component epoxy adhesive in order to bear the peel forces from the load cell.
2.3. Testing Quasi-static mode I interlaminar fracture toughness tests were performed at a 25 kN Instron Universal testing machine (Instron, High Wycombe, UK) at room temperature conditions using the DCB method according to AITM 1.0005 standard [35]. DCB specimen dimensions and experimental set-up are both illustrated in Fig. 3. The edges of the specimens were painted in white and scaled to observe the crack growth. The pre-cracked DCB test specimens were loaded in tension at a cross-head velocity of 10 mm/min until the crack was propagated to 100 mm. Five replicates were tested for each fracture toughness assessment. The GIC of the CFRPs was calculated using the areas method [35] that is:
GIC ¼
A $106 a$w
J m2
(1)
where, A is the required energy to achieve the total propagated crack length, a is the propagated crack length (final crack length minus initial crack length) and w is the specimen's width. In-situ with the mechanical testing, the AE activity was monitored. AE is an ideally suited non-destructive technique for the online monitoring of the crack propagation and was utilized in order to contribute to the extraction of useful conclusions, regarding the damage mechanisms activated during the mechanical experiments. An AE sensor was mounted on the specimens’ surface as shown in Fig. 3. The sensor type is wideband WD 100e900 kHz manufactured by PAC, USA. The transducer was attached on the specimens’ surface using a suitable glycerine-based coupling agent. AE signal acquisition was performed via a four channel 16-bit PCI/DSP-4 by PAC data acquisition system. Pre-amplification of 40 dB and bandbass filtering of 20e1200 kHz was performed using general purpose voltage pre-amplifiers with 0/20/40 dB variable gain (2/4/6AST Auto Sensor Testing Pre-amplifiers by PAC). A threshold of 40 dB was chosen and the timing parameters Peak Definition Time (PDT), Hit Definition Time (HDT) and Hit Lockout Time (HLT) were
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Fig. 2. Schematic representation of (a) the reference plate and (b) the SP containing hybrid plate. (c) Photograph of the mid-plane placement of SPSH01 film, together with the PTFE initial crack formation.
Fig. 3. Schematic depiction of the hybrid DCB test specimen configuration as well as the topology of the AE transducer. Dimensions in mm.
set at 50, 100 and 300 ms, respectively. 2.4. Healing procedure and healing efficiency calculations After the completion of mode I interlaminar fracture tests, the composite laminates were subjected to a healing cycle consisted of heating at 100 C for 15 min under a loading of 1 kN, using a heat press. The applied temperature was chosen to be 23 C higher than the Tm value (approx. 77 C) for 15 min in order to be sure that the SP will flow between the crack flanks and to achieve the healing effect. The loading value was chosen as the minimum necessary to ensure that the adjacent fracture flanks were kept in intimate contact during the healing procedure. Then, the samples were left to cool-down at room temperature. After the healing process, the samples were tested again using the same mode I interlaminar fracture configuration. The testing-healing process was repeated so that the DCB samples had received 8 testing cycles (the initial one
and 7 healing cycles). Reference composites were tested on their original (no-healed) conditions. The calculations of the healing efficiency (H.E.) of the estimated system were based on Eq. (2).
H:E: ¼
ahealed $100% ahybrid
(2)
where, a is the property under examination, ahealed refers to the value of the property after healing and ahybrid refers to value of the property before healing. 3. Results and discussion 3.1. Composite quality issues C-scan images confirmed good plate qualities and showed absence of induced porosity and delaminations due to the
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Fig. 4. C-scan recording photographs of the plates. (a) Reference plate. (b) SP containing hybrid plate.
manufacturing process, as can be seen in Fig. 4. Additionally, Fig. 4b shows clearly the position of the SPSH01 film inserts, which were incorporated in the mid-plane of the hybrid laminate. The fiber volume fraction of all manufactured plates was calculated to be close to 60% and was calculated using a combination of volumes, weights and densities of the components. Moreover, the incorporation of the SPSH01 film in the mid-plane of the hybrid laminate did not appear to have a significant effect on the densities (approx. 1550 kg/m3) and the thicknesses (approx. 2.94e3.16 mm) of the samples.
3.2. Mode I e cleavage fracture resistance and recovery In order to assess the fracture toughness of the laminates, loadedisplacement measurements were performed on the precracked plates using mode I interlaminar fracture tests. The resulting loadedisplacement curves for the reference as well as for the hybrid plates are depicted in Fig. 5. For both composite DCB specimens, the applied load initially increased linearly as the interlaminar pre-crack propagated, followed by a deviation from linearity, and ends with a load drop. Most interestingly, the hybrid laminate that comprises the SPSH01 film as interleaf in its midplane, displays a significantly higher Pmax and calculated GIC, when compared to the reference laminate without SPSH01 interleave (Figs. 7 and 8). Fig. 6 illustrates the bridging that occurs during crack propagation, resulting from the presence of the SPSH01 film in between the upper and lower crack surfaces of the sample. The crack appeared to propagate through the SPSH01 interleaf rather than along the SPSH01/thermoset interface. Clearly, the presence of the SPSH01 interleaf reduces the crack opening displacement at a given applied load, due to the developed bridging within the crack
Fig. 6. Illustration of the extended bridging phenomenon of the SPSH01 interleaf between the upper and the lower adjacent surfaces during the mode I testing.
Fig. 7. Comparison between the reference and the hybrid CFRP, for the Pmax in mode I experiments.
Fig. 5. Representative load-displacement curves during the mode I fracture toughness test of the reference and the hybrid CFRPs.
flanks, and thus results in a lowering of the stress at the crack tip, and consequently in an increased mode I interlaminar fracture toughness. The results are summarized in Table 1 and they clearly show that the presence of the supramolecular interleave in the plates resulted not only in a considerable increase of Pmax (545%) but even more into a calculated GIC that was increased with more than one order of magnitude (1552%). Typical interlaminar crack growth resistance curves (R-curves) under mode I loading conditions for the reference and the supramolecular hybrid composites are depicted in Fig. 9. The GIC value in composite laminates is of great importance, since it controls the initiation and the propagation of the delamination damage. As it is
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Fig. 8. Comparison between the reference and the hybrid CFRP, for the GIC in mode I experiments. Table 1 Maximum load (Pmax) and fracture energy under mode I (GIC) values for the reference as well as for the hybrid CFRPs. Material group
Pmax (N)
StDev (SSD)
Increase (%)
GIC (kJ m2)
StDev (SSD)
Increase (%)
Reference Hybrid
96.7 623.5
4.091 6.889
e 544.8
0.330 5.450
0.062 0.168
e 1551.5
Fig. 9. Representative R-curves of the reference and the hybrid CFRPs comprising the SP interleaves.
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shown in Fig. 9, the reinforcing effect of the supramolecular film is prominent. The presence of the SPSH01 interleaf promotes the development of a bridging traction zone at the interlaminar region, which suppresses the crack tip opening stresses and as a result increases the resistance of the composite to both the initiation and the propagation of the delamination damage. The R-curve for the reference composite shows the plateau value already after reaching a crack length of 30 mm, resulting in a very limited damage process zone (lower than 5 mm). On the other hand, the R-curve for the supramolecular hybrid interleaf composite shows the plateau value only after reaching a crack length of 45 mm. This results in an extended damage process zone followed by a self-similar crack propagation pattern showing the typical delamination evolution mechanism up to a crack growth length of 80 mm and eventually resulting in a significant reduction of GIC value at longer crack lengths. The significant toughness increase observed for the hybrid interleaf is attributed to the enhanced supramolecular material interface with the epoxy matrix, which is extremely strong thereby forming strong bonding between the SPSH01 polymer and the epoxy resulting in a transfer of the propagation of delamination to the supramolecular material. This enhanced interface is visible in Fig. 10b, which shows infiltration by the supramolecular material into the epoxy matrix as evidenced by the darkened region of the matrix next to supramolecular interleaf. In addition, there is also a clear difference between the failure types of the matrix. The reference material clearly shows brittle failure with intact carbon fibers as observed with optical microscopy (Fig. 11a). In contrast, the matrix of the supramolecular hybrid interleaf composite shows ductile failure, since both fracture surfaces are covered with the supramolecular interleaf comprising partially pulled-out carbon fibers emanating from the edges of the SP film that the epoxy system exists (Fig. 11b, c). This behavior reveals the strong bonding of the supramolecular interleaf with both the matrix and the carbon fibres due to the infiltration. This strong bonding is foreseen to inhibit crack propagation because of the extra energy required for interfacial failure. In this turn, interfacial failure leads to frictional sliding and/or plastic deformation at the interface and finally to the propagation of delamination through the SPSH01 material as well as carbon fiber breakage or pull-out and crack bridging. Another characteristic of the SP was the formation of agglomerates on fractured surfaces locally, after the fracture of the SPSH01 interleaf (Fig. 11d), probably originating from yielded SPSH01 bridges at the delamination interfaces. The propensity of these supramolecular interleafs for selfhealing in the CFRP composites was investigated by subjecting the samples to a healing cycle consisting of heating/compression the laminates at 100 C for 15 min under a compressive load of 1 kN.
Fig. 10. Cross-section of the (a) reference and (b) SP interleaved hybrid composite.
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Fig. 11. Fracture surfaces of the CFRPs. (a) Reference. (b) Bottom fracture plane of the SPSH01 interleaf hybrid CFRP. (c) Top fracture plane of the SPSH01 interleaf hybrid CFRP. (d) Illustration of the agglomerates on the top fracture plane of the hybrid CFRP. In the left side is distinguished the epoxy area, while in the right side the infiltrated area by the SP material (photograph taken from the edge of the SP interleaf).
Indeed, when the fractured samples obtained during the mode I interlaminar fracture tests had been subjected to this healing cycle, a large recovery of the fracture toughness was observed in the same mode I interlaminar fracture configuration. As can be seen in the resulting loadedisplacement curves for the supramolecular hybrid composites before and after the healing (Fig. 12), recoveries around 60% of Pmax and GIC values after the first healing cycle were observed. In subsequent healing cycles on the same samples, a drop in H.E. was observed after each cycle until the recoveries of Pmax and GIC values at the level amounted to 26% and 22% respectively after the seventh healing cycle, when compared to the pristine samples (see Figs. 13 and 14 for the Pmax and GIC values respectively as well as Table 2). Although, the bridging phenomenon was
observed again during the mode I experiments for the healed samples, its efficiency appeared to decrease with increasing number of healing cycles. The ability of the healing interleaf to heal the cracks and to recover the fracture properties depends highly on the possibility of the supramolecular material's chains to reconnect themselves after mechanical rupture during the mode I experiments. This is critically depending on a sufficiently low viscosity of the SPSH01 polymer at 100 C in order to be able to flow into the crack flanks and to provide a good coverage of the debonded adhesive surfaces. Clearly, the presence of the thermo-reversible hydrogen bonds between the polymer chains facilitates this low viscosity at high temperatures and rebonding upon cooling [27]. The observed
Fig. 12. Representative load-displacement curves before and after the healing cycles during the mode I experiments.
Fig. 13. Bar diagrams for the Pmax value, before and after the healing cycles in mode I experiments.
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Fig. 15. Representative R-curves of the hybrid composite before and after the healing cycles. Fig. 14. Bar diagram for the GIC value, before and after the healing cycles in mode I experiments.
reduction in the Pmax and the GIC values with increasing number of healing cycles, can probably be attributed to an uneven distribution of the SPSH01 film over the debonded surfaces during the healing cycle at 100 C, due to inhomogeneous polymer coverage of the surface resulting from the fracturing events. Inhomogeneous polymer coverage after fracturing is supported by the observed polymer bridging phenomenon during the fracture event (Fig. 6) and the polymer agglomerates formed after fracture (Fig. 11d). It is important to notice however that the characteristic values for the Pmax and the GIC after the seventh healing cycle are still much higher compare to the values monitored for the unfractured reference material (see Figs. 13 and 14 in comparison with Figs. 7 and 8). The typical R-curves under mode I loading conditions before and after the application of the healing cycles are depicted in Fig. 15 and they are consistent with the loadedisplacement data. The general trend is that with increasing number of healing cycles, the GIC values decrease and the plateau values of the GIC are reached at a slightly later stage than the corresponding plateau value of the pristine supramolecular hybrid interleaf composite; at 50 mm instead of 45 mm. In line with the pristine SPSH01 sample, the plateau value remains almost constant until a crack growth length of about 80 mm. After that crack length, a slight decrease is observed until the end of the test, however this decrease is smoother for the healed SPSH01 samples and their fracture process zone is slightly greater when compared to the pristine samples. It is necessary to stress that after the first rupture of the interleaf modified composite, the only active material that works and keeps the upper and lower fractured surfaces together is the reversible polymer material and the epoxy matrix infiltrated by this.
Nevertheless further assessment was needed for other structural properties (i.e. knock down effect of tensile properties). According to tensile experiments that were conducted, it was shown that the knock down effect of the hybrid samples, was calculated to be approximately 8.5% in terms of the elastic modulus. In a recent investigation by Pingkarawat et al. [36], it was observed the same behavior and was shown that the elastic modulus of the mendable composites with various types of thermoplastics as healing agent was decreased compared to the reference one without thermoplastics. 3.3. Acoustic emission recordings AE was utilized as a complementary non-destructive technique towards the extraction of useful conclusions regarding the damage accumulation process during the interlaminar crack propagation. To this direction Fig. 16 shows the cumulative number of AE hits versus crack length as calculated for four specific crack lengths for both the reference composite and the supramolecular hybrid interleaf composites. Apparently, the reference composite manifests much more intense AE activity. This is a strong indication that for the same crack length propagation the reference composite undergoes more damage events than the supramolecular hybrid composite i.e. it gets more damaged and thus emits more AE. This fully alignes with the fact that the supramolecular hybrid material is tougher [12]. In addition, the AE activity versus crack length for the healed hybrid samples is summarized in Fig. 17 where the data from the first, third, sixth and seventh healing cycle are depicted. A significant decrease of AE activity is a solid indication of the fact that the H.E. as well as the ability of the material to partially remedy previously damaged sites in the vicinity of the crack path and thus produce new AE events (from the healed sites under re-loading) is finite and with a decreasing trend.
Table 2 Maximum load (Pmax) and fracture energy under mode I (GIC) values for the hybrid CFRP, before and after the healing cycles. Number of healing cycles
Pmax (N)
StDev (SSD)
H.E. (%)
GIC (kJ m2)
StDev (SSD)
H.E. (%)
0 1 2 3 4 5 6 7
623.5 397.2 337.2 294.2 300.3 237.8 208.6 162.9
6.889 1.319 7.878 16.843 10.322 3.724 6.300 12.404
e 63.7 54.1 47.2 48.2 38.1 33.5 26.1
5.450 3.231 2.562 2.355 2.081 1.990 1.689 1.208
0.168 0.381 0.321 0.169 0.103 0.283 0.217 0.042
e 59.3 47.0 43.2 38.2 36.5 31.0 22.2
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material (matrix and fibres), to the remarkable peel resistance, as well as the healing capability of the host SP. A careful optical microscopy examination and AE recordings were utilized to verify the phenomenon. Acknowledgements The present work has been funded by EU FP7 Transport (including Aeronautics) Programme within the frame of the project: Self-healing polymers for concepts on self-repaired aeronautical composites e HIPOCRATES (ACP3-GA-2013-605412). References
Fig. 16. Cumulative number of AE hits vs. crack length for the baseline and the hybrid composites.
Fig. 17. Cumulative number of AE hits vs. crack length for the hybrid composite after the first, third, sixth and seventh healing cycle.
4. Conclusions In the current study a new type of high performance carbonepoxy composite concept is proposed having supramolecular interleafs that offer increased mode I interlaminar fracture toughness and self-healing capabilities. The SP interleaf was successfully incorporated into CFRP material with no effects to the composite architecture and its manufacturing process. Interleaves treatment and fabrication of composites were both prepared in-house and quality control of the manufactured UD laminates showed acceptable quality of the laminates and absence of defects. The laminates were tested under mode I remote loading and the presence of the supramolecular film led to a significant increase of the test peak load (Pmax) and of the mode I interlaminar fracture energy (GIC) about 540% and 1550% respectively. Furthermore, high recovery of the interlaminar properties was achieved up to 64% and 59% after the first healing cycle for the Pmax and the GIC values respectively. It was also shown that the recovery of the interlaminar properties after the seventh healing cycle is still high and concludes to interlaminar fracture toughness much higher compare to the reference one laminate. This trend is attributed to the strong bonding between the interleaved material and the constituents of the host
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Toughening and healing of continuous fibre reinforced composites by supramolecular polymers V. Kostopoulos a, *, A. Kotrotsos a, S. Tsantzalis a, P. Tsokanas a, T. Loutas a, A.W. Bosman b a Applied Mechanics Laboratory, Department of Mechanical Engineering and Aeronautics, University of Patras, Patras University Campus, GR-26504 Rio Patras, Greece b SupraPolix BV, Horsten 1, 5612 AX Eindhoven, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 November 2015 Received in revised form 16 March 2016 Accepted 19 March 2016 Available online 21 March 2016
Interleaves comprising self-healing materials based on hydrogen bonded supramolecular polymers (SP) were successfully incorporated in the mid-plane of unidirectional (UD) carbon fibre reinforced polymers (CFRPs). The fracture toughness of these hybrid composites and their healing capability were measured under mode I loading. The fracture toughness appeared to have increased considerably since the maximum load (Pmax) of the hybrid composite had increased approximately 5 times, and the fracture energy I (GIC) displayed a dramatic increase by almost one order of magnitude when compared to the reference composite without the SP. Furthermore, the double cantilever beam (DCB) hybrid composites displayed a healing efficiency (H.E.) value for the mode I interlaminar characteristics around 60% for the Pmax and the GIC after the first healing cycle, dropping to 20e30% after the seventh cycle. During the mode I interlaminar fracture toughness tests the acoustic emission (AE) activity of the samples was also monitored. It was found that AE-activity strongly reduced due to the presence of the SP. Moreover, optical microscopy not only showed that the epoxy matrix at the interface is partly infiltrated by the SP, but it also revealed that cross-sections of both fractured surfaces were covered with the SP comprising pulled-out carbon fibres, indicating a strong interfacial adhesion. Finally it was shown that the SP fractured surfaces were partially covered with pulled-out carbon fibres emanating from the edges of the SP film in which the epoxy system exists. © 2016 Elsevier Ltd. All rights reserved.
Keywords: CFRPs Supramolecular polymers Fracture mechanics Self-healing Acoustic emission
1. Introduction During the last decades the use of carbon fiber reinforced polymers (CFRPs) has been rapidly increased. Their enhanced specific properties make them attractive for structural applications in emerging fields of industrial technology such as aerospace, automotive, rail, marine, as well as the defense industry. One of the most important mechanical characteristic of fiber-polymer composites is their resistance to delamination. The interlaminar fracture toughness plays an important role in damage formation and propagation in FRPs. The presence of delaminations may lead to a loss of stiffness which can be a very important design consideration and may result to complete failure of the composite structure. It is therefore obvious that the delamination resistance of a laminate is
* Corresponding author. E-mail addresses: [email protected] (V. Kostopoulos), bosman@ suprapolix.com (A.W. Bosman). http://dx.doi.org/10.1016/j.compscitech.2016.03.021 0266-3538/© 2016 Elsevier Ltd. All rights reserved.
critical for the in-service performance of structural composites and reliable non-destructive evaluation techniques are required for its early detection. In addition, high performance composites are sensitive to microcracking formation within the material due to inservice induced mechanical and thermal loading. Joining of microcracks under service loads may be another way for delamination. In general, both extensive microcracking and/or delaminations lead to rapid degradation of the materials performance. The composites conventional repair techniques are expensive, time consuming, and practically not applicable in the case of invisible defects. Thus, the widespread utilization of composites especially in human safety critical applications is always accompanied with damage diagnostic tools. This challenging situation acted as an inspiration for the seeking of new repair methods; cheaper, faster, and easier, applied at the early stages of damage formation. Self-healing materials [1,2] have been proposed as an emerging method of improving the performance of materials. This technology has been recently applied to composites and promises to extend the effective life-span of them, to reduce the maintenance
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needs and costs, and to improve the damage tolerance and reliability of composite structures. A constant concern of various researchers has been the enhancement of the fracture behavior of high performance structural composites. A variety of methods are proposed in the literature which includes interleaving, hybridization, stitching, shortfibres, and z-pinning [3e7]. Kostopoulos et al. [8] achieved the 100% increase in the fracture energy after the addition of 1% CNF in the matrix of CFRP laminates. Magniez et al. [9] succeed the toughening of the CFRPs by interleaving a thin layer (~20 mm) of electrospun poly(hydroxyether of bisphenol A). This type of enhancement improved the fracture toughness in mode I and mode II by up to 150% and 30% respectively. Masahiro's et al. [10] experimental results about hybrid laminates, showed an increase at the rate of 50% for the fracture toughness, in mode I fracture test compare to the base CFRP laminates. Furthermore, the results of the mode II fracture toughness test confirmed that the interlaminar fracture toughness for hybrid laminates is 2e3 times greater than the base CFRP laminates. Kostopoulos et al. [11] dealt with the use of CNFs and PZT particles as dopants for the epoxy matrix of CFRP laminates. The presence of CNFs led to a remarkable increase in the mode I fracture energy of the laminates (about 100%) whereas the addition of PZT particles demonstrated reduction in the mode I fracture energy (GIC). In mode II loading, both the CNFs and the PZTs improved the fracture properties of the CFRPs. Recently, Kostagiannakopoulou et al. [12] demonstrated the increase of the interlaminar fracture toughness of carbon fibre composites by modifying the epoxy matrix with graphene nano-species. Yasaee et al. [13,14] investigated the mode I and mode II fracture behavior of glass fibre reinforced plastics (GFRPs) with embedded strips of a thermoplastic strip, thermoplastic particles, chopped fibres, etc. From the various interleaved materials tested, those that improved the GIC relatively to the baseline value were the polyimide thermoplastic film (79% increase), the chopped aramid fibres (46% increase), the 90 E-glass/epoxy prepreg strip (46% increase), the thermoset adhesive film (43% increase) and the chopped glass fibres (16% increase). All these interleaved strips had the potential to increase the GIIC [14] with values from 75% to 123%. Self-healing composites have previously been developed by embedding healing agents (i.e. reactive chemicals and catalysts) into the matrix using microcapsules that will release the healing agent upon crack damage [15,16]. Additionally, matrices with embedded vascular network have been developed in which the network serves as reservoir for the distribution of the healing system [17]. A different approach towards self-healing composites, are matrices that comprise thermoplastic polymers. Zako and Takano [18] were the first to achieve a restoration of the fatigue performance of FRPs comprising a thermoplastic-modified matrix. In a comparable approach, Hayes et al. [19,20] blended a polybisphenol A-based thermoplastic into the epoxy matrix in order to get up to 70% healing in the resulting FRP. Whereas, Pingkarawat et al. [21] were able to get a high recovery after healing in the quasistatic mode I fracture toughness in CFRPs with poly(ethylene-comethacrylic acid) (EMAA) as thermoplastic modifier. Recently, Selver et al. [22] explored the healing potential of GFRPs of a matrix modified with glass-polypropylene hybrid yarns. The resulting composites displayed a 65% recovery from low-energy impact after a simple heating treatment. Interestingly, a synergistic combination on improved toughness and self-healing in high performance composites, has been shown by Wang et al. [23] who incorporated rectangular-shaped patches of two copolymers (EMA and EMAA) into the mid-thickness of CFRPs. A more promising approach for self-healing composites might be found by merging reversible bonds into epoxide networks, since this approach allows the healing to be unlimited as no chemicals
85
are consumed. Indeed, when thermo-reversible cross-links based on DielseAlder chemistry were in an epoxy network based on diglycidyl ether of bisphenol A (DGEBA), self-healing was observed after considered time (hours) of exposing the material to elevated temperatures [24]. In a more recent paper, GFRPs comprising DielseAlder based thermo-reversible cross-links, showed good self-healing behavior combined with good compatibility with the glass fibers [25]. A new technology that could be beneficial for self-healing in composites has been built on supramolecular polymers (SP) [26,27]. Especially those based on reversible hydrogen bonding arrays show great promise for self-healing materials [28e31], since these materials can typically withstand multiple healing cycles without substantial loss of performance, as a consequence of the highly directional and fully reversible non-covalent interactions present within the polymer matrix. In this study, we have employed the ureiodypyrimidone hydrogen bonding unit (UPy) as developed by Meijer and coworkers [32] because of its strong selfassociation, its synthetic accessibility, and the highly dynamic nature of low glass transition temperature (Tg)-polymers comprising the UPy [33]. Most interestingly, UPy-polymers have recently been shown to give unprecedented toughening in polybutadiene based interpenetrating networks [34]. The scope of the present work is the use of flexible SP comprising UPy-moieties as interleave additives into conventional unidirectional (UD) CFRPs in order to enhance the fracture properties of these hybrid composites and to take advantage of the SP's healing capability. This study is presenting an overview of the role of the SP in the fracture behavior and the repeatable ability to heal the cracks in CFRPs. The reference as well as the hybrid composites were subjected to mode I interlaminar fracture toughness tests and compared. After the fracture, the hybrid composites were subjected to heating under controlled loading in order to activate the SP interleave and the cracks to be healed. The healing process was repeated up to seven times. Finally acoustic emission (AE) recordings and optical microscopy examination led to better understanding of the involved failure mechanisms as well as some conclusions regarding the healing process. 2. Experimental 2.1. Materials The composite materials which used in this study were fabricated by UD carbon fibre-epoxy prepreg CE-1007 150-38. The prepreg tape was supplied by SGL Group, Germany having a tensile strength of 2.4 GPa. The SP was the SPSH01 material as provided by Suprapolix that is based on a low Tg (66 C) polymer modified with UPy-moieties, and was chosen to play the role of the toughening and healing agent interleave in the present study. Fig. 1a depicts the originally received SPSH01 piece (approximately 45 mm 30 mm 5 mm), which has been further processed into films in the present work. This polymer owes its mechanical properties to the reversible, non-covalent interactions, such as hydrogen bonding, between the macromolecules. 2.2. Preparation of the SPSH01 interleaves and composite manufacturing The preparation process of the SPSH01 interleaves is illustrated in Fig. 1. The as-received polymer piece (Fig. 1a) was converted into a thin film by a two-step heating/pressuring treatment (Fig. 1b) using a heat press machine. Firstly, the bulk SPSH01 block was pressured under 5 kN at 90 C for 30 min. Then, thermal energy was stopped to be provided to the system and the SP material was left
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Fig. 1. Illustration of the initial raw material and the preparation process of the SPSH01 interleaves.
under 25 kN overnight to cool-down and take its final form (Fig. 1c). The melting temperature (Tm) of the SPSH01 material appeared to be 77 C, using the PerkineElmer DSC 8500 differential scanning calorimeter. The DSC samples were heated from ambient temperature to 150 C at a rate of 5 C/min. Above this temperature, the UPy-UPy hydrogen bonding interactions start to diminish significantly and thereby the interactions between adjacent polymer chains of the material [28]. Consequently, a thin film of SPSH01 was obtained after the above mentioned heating/pressing treatment. The thickness of the film was measured to be approximately 120 mm with no significant thickness variations, using digital caliper. Finally, the SPSH01 film was cut into interleaves of standard sizes using a conventional film cutter (Fig. 1d, e). Two UD laminated plates made out of 22-layers were manufactured for the needs of the current study; the reference laminate and the hybrid laminate containing the SPSH01 interleafs at the mid-plane, both appropriate for mode I interlaminar fracture tests. Fig. 2 shows schematically the plate configuration. The dimensions of the plates were 300 mm 150 mm 3 mm. During the manufacturing process two 13-mm-thick sheets of polytetrafluoroethylene (PTFE) film were placed in the mid-thickness plane of both laminates as shown in Fig. 2 to act as initial pre-crack according to the request of the interlaminar fracture test. In the case of the hybrid laminate, SPSH01 strips were carefully placed at the mid-plane as shown in Fig. 2b, c. Following the lay-up, the laminates were vacuum bagged and cured in autoclave for 2 h at 130 C under 6 bars applied pressure, according to the prepreg manufacturer guidelines. C-scan inspection of the manufactured plates, using a Physical Acoustics Corporation (PAC) UT C-Scan system with a 5 MHz transducer secures the high quality of the manufactured plates. Thickness measurements were also performed. Five mode I samples were cut from both the reference and the hybrid plate. Two aluminium tabs were glued on the double cantilever beam (DCB) specimen outer surfaces (Fig. 3) using a two-component epoxy adhesive in order to bear the peel forces from the load cell.
2.3. Testing Quasi-static mode I interlaminar fracture toughness tests were performed at a 25 kN Instron Universal testing machine (Instron, High Wycombe, UK) at room temperature conditions using the DCB method according to AITM 1.0005 standard [35]. DCB specimen dimensions and experimental set-up are both illustrated in Fig. 3. The edges of the specimens were painted in white and scaled to observe the crack growth. The pre-cracked DCB test specimens were loaded in tension at a cross-head velocity of 10 mm/min until the crack was propagated to 100 mm. Five replicates were tested for each fracture toughness assessment. The GIC of the CFRPs was calculated using the areas method [35] that is:
GIC ¼
A $106 a$w
J m2
(1)
where, A is the required energy to achieve the total propagated crack length, a is the propagated crack length (final crack length minus initial crack length) and w is the specimen's width. In-situ with the mechanical testing, the AE activity was monitored. AE is an ideally suited non-destructive technique for the online monitoring of the crack propagation and was utilized in order to contribute to the extraction of useful conclusions, regarding the damage mechanisms activated during the mechanical experiments. An AE sensor was mounted on the specimens’ surface as shown in Fig. 3. The sensor type is wideband WD 100e900 kHz manufactured by PAC, USA. The transducer was attached on the specimens’ surface using a suitable glycerine-based coupling agent. AE signal acquisition was performed via a four channel 16-bit PCI/DSP-4 by PAC data acquisition system. Pre-amplification of 40 dB and bandbass filtering of 20e1200 kHz was performed using general purpose voltage pre-amplifiers with 0/20/40 dB variable gain (2/4/6AST Auto Sensor Testing Pre-amplifiers by PAC). A threshold of 40 dB was chosen and the timing parameters Peak Definition Time (PDT), Hit Definition Time (HDT) and Hit Lockout Time (HLT) were
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Fig. 2. Schematic representation of (a) the reference plate and (b) the SP containing hybrid plate. (c) Photograph of the mid-plane placement of SPSH01 film, together with the PTFE initial crack formation.
Fig. 3. Schematic depiction of the hybrid DCB test specimen configuration as well as the topology of the AE transducer. Dimensions in mm.
set at 50, 100 and 300 ms, respectively. 2.4. Healing procedure and healing efficiency calculations After the completion of mode I interlaminar fracture tests, the composite laminates were subjected to a healing cycle consisted of heating at 100 C for 15 min under a loading of 1 kN, using a heat press. The applied temperature was chosen to be 23 C higher than the Tm value (approx. 77 C) for 15 min in order to be sure that the SP will flow between the crack flanks and to achieve the healing effect. The loading value was chosen as the minimum necessary to ensure that the adjacent fracture flanks were kept in intimate contact during the healing procedure. Then, the samples were left to cool-down at room temperature. After the healing process, the samples were tested again using the same mode I interlaminar fracture configuration. The testing-healing process was repeated so that the DCB samples had received 8 testing cycles (the initial one
and 7 healing cycles). Reference composites were tested on their original (no-healed) conditions. The calculations of the healing efficiency (H.E.) of the estimated system were based on Eq. (2).
H:E: ¼
ahealed $100% ahybrid
(2)
where, a is the property under examination, ahealed refers to the value of the property after healing and ahybrid refers to value of the property before healing. 3. Results and discussion 3.1. Composite quality issues C-scan images confirmed good plate qualities and showed absence of induced porosity and delaminations due to the
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Fig. 4. C-scan recording photographs of the plates. (a) Reference plate. (b) SP containing hybrid plate.
manufacturing process, as can be seen in Fig. 4. Additionally, Fig. 4b shows clearly the position of the SPSH01 film inserts, which were incorporated in the mid-plane of the hybrid laminate. The fiber volume fraction of all manufactured plates was calculated to be close to 60% and was calculated using a combination of volumes, weights and densities of the components. Moreover, the incorporation of the SPSH01 film in the mid-plane of the hybrid laminate did not appear to have a significant effect on the densities (approx. 1550 kg/m3) and the thicknesses (approx. 2.94e3.16 mm) of the samples.
3.2. Mode I e cleavage fracture resistance and recovery In order to assess the fracture toughness of the laminates, loadedisplacement measurements were performed on the precracked plates using mode I interlaminar fracture tests. The resulting loadedisplacement curves for the reference as well as for the hybrid plates are depicted in Fig. 5. For both composite DCB specimens, the applied load initially increased linearly as the interlaminar pre-crack propagated, followed by a deviation from linearity, and ends with a load drop. Most interestingly, the hybrid laminate that comprises the SPSH01 film as interleaf in its midplane, displays a significantly higher Pmax and calculated GIC, when compared to the reference laminate without SPSH01 interleave (Figs. 7 and 8). Fig. 6 illustrates the bridging that occurs during crack propagation, resulting from the presence of the SPSH01 film in between the upper and lower crack surfaces of the sample. The crack appeared to propagate through the SPSH01 interleaf rather than along the SPSH01/thermoset interface. Clearly, the presence of the SPSH01 interleaf reduces the crack opening displacement at a given applied load, due to the developed bridging within the crack
Fig. 6. Illustration of the extended bridging phenomenon of the SPSH01 interleaf between the upper and the lower adjacent surfaces during the mode I testing.
Fig. 7. Comparison between the reference and the hybrid CFRP, for the Pmax in mode I experiments.
Fig. 5. Representative load-displacement curves during the mode I fracture toughness test of the reference and the hybrid CFRPs.
flanks, and thus results in a lowering of the stress at the crack tip, and consequently in an increased mode I interlaminar fracture toughness. The results are summarized in Table 1 and they clearly show that the presence of the supramolecular interleave in the plates resulted not only in a considerable increase of Pmax (545%) but even more into a calculated GIC that was increased with more than one order of magnitude (1552%). Typical interlaminar crack growth resistance curves (R-curves) under mode I loading conditions for the reference and the supramolecular hybrid composites are depicted in Fig. 9. The GIC value in composite laminates is of great importance, since it controls the initiation and the propagation of the delamination damage. As it is
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Fig. 8. Comparison between the reference and the hybrid CFRP, for the GIC in mode I experiments. Table 1 Maximum load (Pmax) and fracture energy under mode I (GIC) values for the reference as well as for the hybrid CFRPs. Material group
Pmax (N)
StDev (SSD)
Increase (%)
GIC (kJ m2)
StDev (SSD)
Increase (%)
Reference Hybrid
96.7 623.5
4.091 6.889
e 544.8
0.330 5.450
0.062 0.168
e 1551.5
Fig. 9. Representative R-curves of the reference and the hybrid CFRPs comprising the SP interleaves.
89
shown in Fig. 9, the reinforcing effect of the supramolecular film is prominent. The presence of the SPSH01 interleaf promotes the development of a bridging traction zone at the interlaminar region, which suppresses the crack tip opening stresses and as a result increases the resistance of the composite to both the initiation and the propagation of the delamination damage. The R-curve for the reference composite shows the plateau value already after reaching a crack length of 30 mm, resulting in a very limited damage process zone (lower than 5 mm). On the other hand, the R-curve for the supramolecular hybrid interleaf composite shows the plateau value only after reaching a crack length of 45 mm. This results in an extended damage process zone followed by a self-similar crack propagation pattern showing the typical delamination evolution mechanism up to a crack growth length of 80 mm and eventually resulting in a significant reduction of GIC value at longer crack lengths. The significant toughness increase observed for the hybrid interleaf is attributed to the enhanced supramolecular material interface with the epoxy matrix, which is extremely strong thereby forming strong bonding between the SPSH01 polymer and the epoxy resulting in a transfer of the propagation of delamination to the supramolecular material. This enhanced interface is visible in Fig. 10b, which shows infiltration by the supramolecular material into the epoxy matrix as evidenced by the darkened region of the matrix next to supramolecular interleaf. In addition, there is also a clear difference between the failure types of the matrix. The reference material clearly shows brittle failure with intact carbon fibers as observed with optical microscopy (Fig. 11a). In contrast, the matrix of the supramolecular hybrid interleaf composite shows ductile failure, since both fracture surfaces are covered with the supramolecular interleaf comprising partially pulled-out carbon fibers emanating from the edges of the SP film that the epoxy system exists (Fig. 11b, c). This behavior reveals the strong bonding of the supramolecular interleaf with both the matrix and the carbon fibres due to the infiltration. This strong bonding is foreseen to inhibit crack propagation because of the extra energy required for interfacial failure. In this turn, interfacial failure leads to frictional sliding and/or plastic deformation at the interface and finally to the propagation of delamination through the SPSH01 material as well as carbon fiber breakage or pull-out and crack bridging. Another characteristic of the SP was the formation of agglomerates on fractured surfaces locally, after the fracture of the SPSH01 interleaf (Fig. 11d), probably originating from yielded SPSH01 bridges at the delamination interfaces. The propensity of these supramolecular interleafs for selfhealing in the CFRP composites was investigated by subjecting the samples to a healing cycle consisting of heating/compression the laminates at 100 C for 15 min under a compressive load of 1 kN.
Fig. 10. Cross-section of the (a) reference and (b) SP interleaved hybrid composite.
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Fig. 11. Fracture surfaces of the CFRPs. (a) Reference. (b) Bottom fracture plane of the SPSH01 interleaf hybrid CFRP. (c) Top fracture plane of the SPSH01 interleaf hybrid CFRP. (d) Illustration of the agglomerates on the top fracture plane of the hybrid CFRP. In the left side is distinguished the epoxy area, while in the right side the infiltrated area by the SP material (photograph taken from the edge of the SP interleaf).
Indeed, when the fractured samples obtained during the mode I interlaminar fracture tests had been subjected to this healing cycle, a large recovery of the fracture toughness was observed in the same mode I interlaminar fracture configuration. As can be seen in the resulting loadedisplacement curves for the supramolecular hybrid composites before and after the healing (Fig. 12), recoveries around 60% of Pmax and GIC values after the first healing cycle were observed. In subsequent healing cycles on the same samples, a drop in H.E. was observed after each cycle until the recoveries of Pmax and GIC values at the level amounted to 26% and 22% respectively after the seventh healing cycle, when compared to the pristine samples (see Figs. 13 and 14 for the Pmax and GIC values respectively as well as Table 2). Although, the bridging phenomenon was
observed again during the mode I experiments for the healed samples, its efficiency appeared to decrease with increasing number of healing cycles. The ability of the healing interleaf to heal the cracks and to recover the fracture properties depends highly on the possibility of the supramolecular material's chains to reconnect themselves after mechanical rupture during the mode I experiments. This is critically depending on a sufficiently low viscosity of the SPSH01 polymer at 100 C in order to be able to flow into the crack flanks and to provide a good coverage of the debonded adhesive surfaces. Clearly, the presence of the thermo-reversible hydrogen bonds between the polymer chains facilitates this low viscosity at high temperatures and rebonding upon cooling [27]. The observed
Fig. 12. Representative load-displacement curves before and after the healing cycles during the mode I experiments.
Fig. 13. Bar diagrams for the Pmax value, before and after the healing cycles in mode I experiments.
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Fig. 15. Representative R-curves of the hybrid composite before and after the healing cycles. Fig. 14. Bar diagram for the GIC value, before and after the healing cycles in mode I experiments.
reduction in the Pmax and the GIC values with increasing number of healing cycles, can probably be attributed to an uneven distribution of the SPSH01 film over the debonded surfaces during the healing cycle at 100 C, due to inhomogeneous polymer coverage of the surface resulting from the fracturing events. Inhomogeneous polymer coverage after fracturing is supported by the observed polymer bridging phenomenon during the fracture event (Fig. 6) and the polymer agglomerates formed after fracture (Fig. 11d). It is important to notice however that the characteristic values for the Pmax and the GIC after the seventh healing cycle are still much higher compare to the values monitored for the unfractured reference material (see Figs. 13 and 14 in comparison with Figs. 7 and 8). The typical R-curves under mode I loading conditions before and after the application of the healing cycles are depicted in Fig. 15 and they are consistent with the loadedisplacement data. The general trend is that with increasing number of healing cycles, the GIC values decrease and the plateau values of the GIC are reached at a slightly later stage than the corresponding plateau value of the pristine supramolecular hybrid interleaf composite; at 50 mm instead of 45 mm. In line with the pristine SPSH01 sample, the plateau value remains almost constant until a crack growth length of about 80 mm. After that crack length, a slight decrease is observed until the end of the test, however this decrease is smoother for the healed SPSH01 samples and their fracture process zone is slightly greater when compared to the pristine samples. It is necessary to stress that after the first rupture of the interleaf modified composite, the only active material that works and keeps the upper and lower fractured surfaces together is the reversible polymer material and the epoxy matrix infiltrated by this.
Nevertheless further assessment was needed for other structural properties (i.e. knock down effect of tensile properties). According to tensile experiments that were conducted, it was shown that the knock down effect of the hybrid samples, was calculated to be approximately 8.5% in terms of the elastic modulus. In a recent investigation by Pingkarawat et al. [36], it was observed the same behavior and was shown that the elastic modulus of the mendable composites with various types of thermoplastics as healing agent was decreased compared to the reference one without thermoplastics. 3.3. Acoustic emission recordings AE was utilized as a complementary non-destructive technique towards the extraction of useful conclusions regarding the damage accumulation process during the interlaminar crack propagation. To this direction Fig. 16 shows the cumulative number of AE hits versus crack length as calculated for four specific crack lengths for both the reference composite and the supramolecular hybrid interleaf composites. Apparently, the reference composite manifests much more intense AE activity. This is a strong indication that for the same crack length propagation the reference composite undergoes more damage events than the supramolecular hybrid composite i.e. it gets more damaged and thus emits more AE. This fully alignes with the fact that the supramolecular hybrid material is tougher [12]. In addition, the AE activity versus crack length for the healed hybrid samples is summarized in Fig. 17 where the data from the first, third, sixth and seventh healing cycle are depicted. A significant decrease of AE activity is a solid indication of the fact that the H.E. as well as the ability of the material to partially remedy previously damaged sites in the vicinity of the crack path and thus produce new AE events (from the healed sites under re-loading) is finite and with a decreasing trend.
Table 2 Maximum load (Pmax) and fracture energy under mode I (GIC) values for the hybrid CFRP, before and after the healing cycles. Number of healing cycles
Pmax (N)
StDev (SSD)
H.E. (%)
GIC (kJ m2)
StDev (SSD)
H.E. (%)
0 1 2 3 4 5 6 7
623.5 397.2 337.2 294.2 300.3 237.8 208.6 162.9
6.889 1.319 7.878 16.843 10.322 3.724 6.300 12.404
e 63.7 54.1 47.2 48.2 38.1 33.5 26.1
5.450 3.231 2.562 2.355 2.081 1.990 1.689 1.208
0.168 0.381 0.321 0.169 0.103 0.283 0.217 0.042
e 59.3 47.0 43.2 38.2 36.5 31.0 22.2
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material (matrix and fibres), to the remarkable peel resistance, as well as the healing capability of the host SP. A careful optical microscopy examination and AE recordings were utilized to verify the phenomenon. Acknowledgements The present work has been funded by EU FP7 Transport (including Aeronautics) Programme within the frame of the project: Self-healing polymers for concepts on self-repaired aeronautical composites e HIPOCRATES (ACP3-GA-2013-605412). References
Fig. 16. Cumulative number of AE hits vs. crack length for the baseline and the hybrid composites.
Fig. 17. Cumulative number of AE hits vs. crack length for the hybrid composite after the first, third, sixth and seventh healing cycle.
4. Conclusions In the current study a new type of high performance carbonepoxy composite concept is proposed having supramolecular interleafs that offer increased mode I interlaminar fracture toughness and self-healing capabilities. The SP interleaf was successfully incorporated into CFRP material with no effects to the composite architecture and its manufacturing process. Interleaves treatment and fabrication of composites were both prepared in-house and quality control of the manufactured UD laminates showed acceptable quality of the laminates and absence of defects. The laminates were tested under mode I remote loading and the presence of the supramolecular film led to a significant increase of the test peak load (Pmax) and of the mode I interlaminar fracture energy (GIC) about 540% and 1550% respectively. Furthermore, high recovery of the interlaminar properties was achieved up to 64% and 59% after the first healing cycle for the Pmax and the GIC values respectively. It was also shown that the recovery of the interlaminar properties after the seventh healing cycle is still high and concludes to interlaminar fracture toughness much higher compare to the reference one laminate. This trend is attributed to the strong bonding between the interleaved material and the constituents of the host
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