Flexure and flexure-after-impact properties of carbon fibre composites interleaved with ultra-thin non-woven aramid fibre veils

Composites Part A 131 (2020) 105813

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Flexure and flexure-after-impact properties of carbon fibre composites interleaved with ultra-thin non-woven aramid fibre veils Bingyan Yuana, Mingxin Yea, Yunsen Hua, Fei Chenga,b, Xiaozhi Hua,

T

⁎

a

Department of Mechanical Engineering, University of Western Australia, Perth, WA 6009, Australia Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, PR China

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Flexural properties Flexure-after-impact Short aramid fibre interleaving Low-velocity impact

Laminar carbon fibre reinforced polymer (CFRP) composites contain inherent weak ply interfaces due to the lack of through-thickness fibre reinforcement. In this study, all ply interfaces were interleaved with ultra-thin (13 μm) non-woven short aramid fibre (SAF) veils to generate across ply fibre bridging, formed in-situ by movements of free fibre ends during the composite forming process. The toughening effect of ultra-thin SAF veils (13 μm and 29 μm in thickness) before and after impact was measured and compared. It was found that the 13 μm SAF veils increased the bulk flexural strength and modulus of CFRP by 16.9% and 19.8% prior to impact, challenging the common belief that interleaving with micro-length or short fibres is only beneficial to post-impact properties. Xray micro-computed tomography and cross-section microscopy examinations were used to explain the mechanisms for improved flexural properties before and after low-velocity impact.

1. Introduction Large thin-shell or plate-like structures of carbon fibre reinforced polymer (CFRP) can be easily fabricated using commercial carbon fibre pre-pregs. Aircraft wings and fuselage, composite wind turbine blades, performance yachts and cars are some of the familiar examples. However, the laminar CFRPs made from pre-pregs or dry fibre fabrics and epoxy always contain the inherent weakness, i.e., poor interfacial toughness due to the lack of through-thickness fibre reinforcement or zdirectional toughening [1–3]. In spite of the eminent performance along fibre direction, CFRP laminates are susceptible to out-of-plane loads such as low-velocity impact, which can cause invisible but detrimental damages including internal delamination and matrix cracking, and result in degraded structural integrity and reduced mechanical properties [4,5]. Therefore, various approaches have been proposed to restrain crack initiation and propagation in the interfaces of CFRP under out-of-plane loading conditions, including bulk resin toughening of naturally brittle resin [6,7], through-thickness toughening (e.g. stitching [8,9] and z-pinning [10,11]) and interlaminar toughening [12–16]. Through-thickness toughening involves sewing threads or anchoring pins through stacked ply layers to produce 3D fibre structure, and enables increased resistance to delamination mainly due to effect of z-directional reinforcement in arresting crack propagation [17–19]. While interlaminar toughening puts emphasis on the resin-rich ⁎

interfaces where disadvantage mainly lies, the technique is implemented by inserting a discrete interlayer (fibre membrane/veil, thermoplastic film, etc.) between adjacent carbon fibre plies to enhance the crack growth resistance, e.g. [3,14,20]. Although these toughening methods can indeed improve interfacial properties of CFRP laminates to varied extent, it is commonly assumed that other properties may be compromised. Flexural properties are typically considered since flexural failure may be caused by tensile, compressive, shear or a combination of these in-plane stresses [21]. For example, z-pinning can cause up to 25% decrease in flexural strength due to swelling and microstructure damage by pinning process [22], although it has increased the interlaminar toughness under mode I, mode II and mixed mode loads [23,24]. Electrospun polymer nanofibre interleaving can also lead to reduced flexural properties due to the increased interleaf thickness, despite of enhanced Mode I delamination fracture toughness [25]. At best, the carbon nanofibre bucky paper interleaving at failure-prone locations saw 31% and 104% improvement in interlaminar shear strength and mode II fracture toughness respectively, with flexural property almost unchanged [26]. Recently, a sandwich interleaf (CNT/polysulfone nanofiber membrane/CNT) enabled simultaneous improvement in toughness and flexural properties in a CFRP laminate composed of three layers of carbon fibre cloth and two layers of sandwich interleaves. Up to 29% increase in flexural properties was owing to the reinforcing effect of CNT, since the

Corresponding author. E-mail address: [email protected] (X. Hu).

https://doi.org/10.1016/j.compositesa.2020.105813 Received 8 November 2019; Received in revised form 31 January 2020; Accepted 3 February 2020 Available online 04 February 2020 1359-835X/ © 2020 Elsevier Ltd. All rights reserved.

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were manufactured by hand lay-up using 11 layers of carbon fibre fabric (ZA-T300-3K-T200, ZhongAo Carbon, China, 3K, Twill-weaved, 200 gsm) impregnated with 105 Epoxy Resin and 206 Slow Hardener (West System, USA) in a mass ratio of 5:1. The 4 gsm and 8 gsm nonwoven SAF (3–4 mm in length, 12 μm in diameter, Kevlar® 49, DuPont, USA) veils were fabricated in laboratory by wet-laid method [32,40]. SAF veils infiltrated with epoxy resin (SAFE) could be combined with an impregnated carbon fibre fabric to form SAFE pre-preg, and multilayer SAF reinforcement at all carbon fibre ply interfaces were enabled by laminating SAFE pre-pregs to fabricate CF11-SAF4 and CF11-SAF8 samples. Laminates were compressed using a flat steel mould under 1.2 MPa for 15 h at the room temperature and post cured in oven at 60 ℃ for one week. Table 1 lists the constituent, average thickness of laminates and SAF veils (back-calculated from laminate thickness), impact energy and number of specimens used for flexure (without impact) and FAI tests of three sample groups. For simplicity, flexure tests without impact were denoted as impact energy = 0 J. Due to the addition of 10 interleaving layers, CF11-SAF4 and CF11-SAF8 samples were 6.9% and 15.6% thicker than CF11. The average thickness of 4 gsm and 8 gsm SAF veil after curing with epoxy, or SAFE, was 13 μm and 29 μm respectively. In Fig. 2, the cross-section optical micrographs showed the typical distribution of both SAF veils at the ply interfaces in z-direction and X-ray μCT image demonstrated the presence of SAF in the resin-rich region where warp and weft (0° and 90°) carbon fibres crossed in the x-y plane.

polysulfone nanofiber membrane alone decreased the flexural properties slightly [27]. Since the composite industries may be reluctant to change the current carbon fibre epoxy systems due to the years of experience of using the composites and huge R&D expenditure already spent on their development, a compromise still exists for increasing the delamination resistance while leaving the current composite system unchanged. As pointed out recently [3] interleaving with micro-length or short fibres can potentially provide a good compromise between enhanced delamination toughness and bulk composite properties without adding much complication to composite fabrication using commercial prepregs. The diameter of carbon fibre is typically around 4–5 μm, and the resin-rich region between two carbon fibre plies can be anywhere between 1 and 10 μm [14,28]. Since a large section of interleaving microlength or short fibres will stay roughly parallel to the ply surfaces, any chance of across ply fibre bridging will require a fibre length of at least 30 to 50 μm. On the other hand, if the interleaving fibre is too long, the chance of across ply bridging from free fibre ends may become negligible. Therefore, CNT longer than 30 μm or short aramid fibre (SAF) around 1 to 4 mm in length can be considered both long and short enough to generate sufficient free fibre end bridging from inter-ply fibre perturbation created during composite forming. Indeed, the toughening concept of using SAF tested two decades ago had already generated over 100% improvement in delamination toughness [29–31]. Recently, more efforts have been put into fabrication of ultra-thin non-woven SAF veils [14,32] so that bulk properties of CFRP are less affected while enhancing the delamination toughness. With marginal thickness increase (as low as 20 μm), multi-layer reinforcement was achieved by laminating SAF veil reinforced pre-pregs for practical applications [14]. Its effect on impact resistance and residual compressive strength was investigated, showing improvement by 50.8% and 38.6% respectively [32]. However, its effect on bulk flexural properties has not yet been revealed. The effect of the aforementioned toughening methods on flexureafter-impact (FAI) properties has not been as extensively studied as that on compression-after-impact (CAI) properties [32–37]. In terms of evaluating damage tolerance, FAI is a good alternative for CAI because it diminishes the influence of clamping conditions, is adaptive to varied sample sizes and assesses more than simply compressive properties [37]. By far, stitching was found to degrade residual flexural properties of CFRP due to damage caused by the stitching process, although it improved delamination resistance effectively [1,4]. Bulk resin toughening produced promising results as Kim et al. [38] reported that the degradation in flexural performance after impact was 25% smaller for composites with rubber modified matrix than unmodified samples. Also, addition of milled glass fibre in the epoxy matrix improved residual flexural load-bearing capacity [39]. However, limited study has been conducted to date to investigate the effect of interlaminar toughening on FAI properties of CFRP. Therefore, both flexure and FAI properties of CFRP laminates with and without multi-layer ultra-thin un-bonded non-woven SAF veil interfacial toughening were investigated in this study. Three-pointbending (3-p-b) tests prior to and post low-velocity drop-weight impact were employed, as illustrated in Fig. 1. Non-destructive X-ray microcomputed tomography (X-ray μCT) and optical microscopy (OM) were carried out to observe sample composition and inspect internal damage patterns to reveal toughening mechanisms of SAF veils.

2.2. Experimental methods For flexural tests after impact, or FAI, low-velocity impact was firstly performed using a drop-weight device with a hemispherical impact tip of 16 mm in diameter and an impactor of 6.52 kg in weight [32]. An intact sample (240 mm in length and 230 mm in width) was placed upon a metal support with a 60 mm diameter cavity and impacted at 3 cm, 6 cm and 9 cm height (corresponding impact energy is 1.9 J, 3.8 J and 5.7 J) in different locations to induce barely visible impact damage without penetration, with part of the sample non-impacted. Then, samples were cut to be individual specimens (90 mm in length and 15 mm in width) for both flexure and FAI tests according to ASTM standard D 7264. Although the impactor diameter is slightly larger than the width of the specimen, only the tip area of the impactor contacted the sample surface under low impact energy. Edge polishing was applied to diminish edge defects. Instron 5982 universal mechanical testing machine with a 100 kN load cell was used for 3-p-b tests, with impact face at the compression side for FAI tests. The loading speed was 2 mm/min and the span-to-thickness ratio was 32:1. Impacted specimens and fractured samples after flexural tests were scanned at 50 kV and 79 μA using X-ray μCT system (Versa 520, Zeiss, Pleasanton, CA, USA) to assess the internal failure morphology. The visualization and analysis of data generated from X-ray μCT scans were performed using Avizo (v. 8.1.1, ThermoFisher). 3. Results and discussion 3.1. Flexure properties of CFRP with and without multi-layer SAF toughening The effects of multi-layer interfacial SAF veil toughening on flexural properties of CFRP are discussed in this section. The average flexural load - displacement curves of CF11, CF11-SAF4 and CF11-SAF8 were given in Fig. 3a, where m is the slope of the secant of the loading curve. Fig. 3b gave load - displacement curves of all five specimens of CF11 and CF11-SAF4 to illustrate data dispersion. Evidently, the addition of ultra-thin SAF veils in all carbon fibre ply interfaces was beneficial in increasing load-bearing capacity of CFRP laminates. The peak load was easily detected since a sharp drop in load was clearly seen for all three samples, indicating a sudden loss of load-carrying capacity. In order to

2. Sample preparation and experimental methods 2.1. Sample preparation and interlayer thickness Configuration of unmodified (denoted as CF11, i.e., 11-ply plain carbon fibre fabrics) and modified samples were given in Fig. 1, where 10 layers of 4 or 8 gsm (grams per square meter) SAF veils were inserted into the interfaces to form CF11-SAF4 and CF11-SAF8. CF11 samples 2

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Fig. 1. Scope of this research: flexure and FAI properties of CFRP laminates with and without multi-layer ultra-thin un-bonded non-woven SAF veil toughening. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

flexural modulus Ef since the initial strain point is zero. It can be clearly seen that CF11-SAF4 exhibited higher flexural modulus than CF11SAF8, which already outperformed CF11 slightly. Equivalent stress equivalent strain curves of all five specimens of CF11 and CF11-SAF4 were given in Fig. 3d to highlight the effect of ultra-thin 13 μm SAF veil toughening in increasing the stiffness of CFRP laminates.

Table 1 Constituent, thickness of laminate and SAF veil, impact energy and number of specimens used for flexure and FAI tests. Specimen Constituent

Thickness SAF veil Thickness

CF11

CF11-SAF4

CF11-SAF8

11 layers of CF a

11 layers of SAFE prepreg b (4 gsm SAF) 1.99 mm 13 μm

11 layers of SAFE prepreg (8 gsm SAF) 2.15 mm 29 μm

1.86 mm /

Ef (MPa) = Efsecant (MPa) =

(1)

EquivalentStress (MPa) = Impact energy (J) Number

0 5

1.9 5

3.8 5

5.7 5

0 5

1.9 5

3.8 5

5.7 5

0 5

1.9 5

3.8 5

S3 ∗ m S 3 ∗ Flexural Load = 3 4∗b∗h 4 ∗ b ∗ h3 ∗ Displacement

5.7 5

Equivalent Strain =

a

CF: Carbon Fibre fabric. SAFE pre-preg: Un-bonded non-woven SAFE veil reinforced Carbon Fibre fabric [32].

4∗

b

σf (MPa) =

eliminate the influence of thickness increase by SAF veils, Fig. 3c gave the average equivalent stress - equivalent strain curves of CF11, CF11SAF4 and CF11-SAF8. Flexural secant modulus of elasticity Ef secant was calculated as Eq. (1), where S is span length, b is width and h is thickness of sample. Equivalent stress and equivalent strain were obtained according to Eq. (2) and (3). Herein, Ef secant was regarded as

3∗P∗S 2 ∗ b ∗ h2

Flexural Load b∗h h2

∗ Displacement S3

(2)

(3)

(4)

The flexural strength σf (calculated by Eq. (4), where P is the maximum applied force) and flexural modulus with standard deviation of CF11, CF11-SAF4 and CF11-SAF8 were given in Fig. 4a. The 4 gsm SAF veil increased the flexural strength and modulus by 16.9% and 19.8% respectively. The 8 gsm one saw 10% improvement in modulus, although it decreased strength slightly when compared to control 3

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Fig. 2. SAF distribution in z-direction and x-y plane by OM and X-ray μCT. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

compressive fibre fracture such as fibre crushing and buckle delamination were observed in the uppermost layer. And interfacial delamination propagation was detected in interfaces of upper layers, i.e., under compressive loading condition. The bottom layers under tensile loading condition exhibited limited damage. In contrast, CF11-SAF4 sample saw obvious fibre breakage as tensile fracture in bottom layers, indicating an improved resistance to compression of upper layers due to SAF veils. Also, more trans-laminar crack propagation occurred in upper layers than delamination, indicating restrained slippage between carbon fibre plies during bending [44] and higher interlaminar friction [43] through SAF veils. This might contribute to the improved loadbearing capacity of CFRP as the force needed to break carbon fibre by tensile loading is higher than that for delamination growth. The failure pattern of CF11-SAF8 was somewhere in the middle between CF11 and CF11-SAF4, consistent with the results from Fig. 4a. During the visualization of internal damage of the randomly magnified segment of CF11-SAF4, through-interlayer-thickness reinforcement by SAF was observed, as illustrated in Fig. 6a. Due to the bending stress, the thickness of the interface was enlarged to around 40 μm. One free fibre end of SAF was linked to the adjacent upper carbon fibre cloth, denoted as Z1. And its trace in the interface was revealed in x-y plane Z2 and Z3. The other free fibre end was pushed to the adjacent under layer Z4. Therefore, a strong interfacial bonding by fibre bridging was formed when bearing flexural load to reinforce the interface. This

sample. The observation challenged the common assumption that interfacial interleaving would cause reduction in flexural performance of CFRP due to the added weight proportion of “weak layer” with lower stiffness. Typically, the tensile modulus of common T300 carbon fibre is around 230 GPa [41] while that of interleaving aramid fibre Kevlar® 49 is around 112.4 GPa [42]. Result from [25] was in accord with the assumption since the continuous PEK-C nanofibre interleaving resulted in reduced flexural strength. We compared the normalised flexural strength of SAF veil toughened samples to the data from [25] in terms of the interlayer thickness, as shown in Fig. 4b. It was found that increased interlayer thickness had negative effect on the flexural strength, no matter the type (SAF or PEK-C), diameter (12 μm or 950 nm), scale (micro or nano) and length (short or continuous) of the interleaving fibre. We reached a preliminary conclusion that when the interleaf is thin enough (< 20 μm) or simply close to interleaving fibre diameter, the flexural performance would be enhanced rather than compromised. Under bending, CFRP can fail due to fibre failure (e.g. compressive kink, or tensile break), fibre-matrix interface failure (adhesive fracture like debonding and delamination) and matrix failure (cohesive fracture such as shear crack) [43]. The flexural failure morphology was then analysed to reveal the toughening mechanism of SAF veils. Samples were randomly selected for X-ray μCT scanning, and magnified lens were used for internal failure pattern inspection. Fig. 5 gave the typical bending failure modes of CF11 and CF11-SAF4. For control sample, 4

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Fig. 3. (a) Average flexural load – displacement and (c) Average equivalent stress - equivalent strain curves of CF11, CF11-SAF4 and CF11-SAF8; (b) Five flexural load - displacement and (d) five equivalent stress - equivalent strain curves of CF11 and CF11-SAF4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

ray μCT scanning, as shown in Fig. 7a. In the middle layers, hardly any impact damage was revealed by μCT. Damage area was measured according to the crack length and fibre breakage area in warp and weft directions, as marked by yellow dash line in Fig. 7a [43,44]. The measured data of CF11, CF11-SAF4 and CF11-SAF8 under 3.8 J impact was given in Fig. 7b as representative. SAF veils effectively reduced total damage area, which was also seen in 1.9 J and 5.7 J impact conditions. Fig. 8a gave the average FAI load – displacement curves of CF11, CF11-SAF4 and CF11- SAF8 under 5.7 J impact. Compared to Fig. 3a, the maximum flexural load values were evidently reduced due to impact damage. Moreover, CF11 sample exhibited a nearly plateau period after reaching peak load, while SAF toughened samples still presented clear inflection point in load-carrying capacity. This could be explained by the scan slice of worst internal failure induced by 5.7 J impact in longitudinal section of three sample groups, as shown in Fig. 8b. Through-thickness delamination was seen for CF11 sample, while fibre breakage in bottom layers and reduced delamination distribution were observed for SAF toughened samples. Therefore, without SAF interleaving, the FAI loading of control sample showed gradual loss in loadbearing capacity due to the pre-existing severe delamination distribution. While SAF reinforced sample retained better structural integrity than control sample, thus exhibiting FAI loading mode similar to counterparts in Fig. 3a. Residual flexural strength and residual flexural modulus with standard deviation were given in Fig. 9a and Fig. 9b. Herein, test results of 0 J (flexural tests without impact) were provided as the reference for

phenomenon was not noticed in randomly selected and magnified CF11-SAF8 sample. Therefore, it is presumed that through-interlayerthickness fibre bridging is more likely to be formed in-situ during composite fabrication with 4 gsm SAF interleaving than 8 gsm one. The main reason might be the thickness, as 4gsm SAF (13 μm thick) is close to fibre diameter (12 μm) since equivalently single SAF layer existed in the interface (see Fig. 2) and it is comparable to the original resin-rich layer (1–10 μm). This also explained why CF11-SAF4 saw increased instead of degraded global flexural performance as the addition of “weak layer” was compensated by the sufficient through-interlayerthickness fibre bridging by ultra-thin SAF to adjacent carbon fibre cloths. While for CF11-SAF8, the interfacial layer was around two to three times of the aramid fibre diameter (see Fig. 2), thus not all free fibre ends could reach to the neighbouring carbon fibre plies as illustrated in Fig. 6b. Therefore, the effect of short fibre interleaving can be maximized in both the delamination toughness and in-plane properties when the possibility of fibre bridging from those free ends is maximized. This can be achieved by reducing the interleaf thickness so that more interleaving fibres could be linked to adjacent carbon fibre plies without reducing the carbon-fibre/epoxy volume ratio.

3.2. FAI properties of CFRP with and without multi-layer SAF toughening The effects of multi-layer interfacial SAF veil toughening on FAI properties of CFRP are discussed in this section. Crack formation and propagation were seen in upper layers that face impact directly and fibre breakage due to tensile stress was observed in bottom layers by X5

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impact damage was limited. When impact energy was increased to 3.8 J or 5.7 J, the sample with 8 gsm veil had a smaller damage area than that of the sample with 4 gsm veil (shown in Fig. 7b) due to the larger amount of SAF available to absorb the impact energy. Therefore, the larger impact damage area in the CF11-SAF4 sample led to the lowered FAI load-bearing capacity in comparison to that of the CF11-SAF8 sample. However, if the impact induced damage area is the same, CF11SAF4 might potentially still exhibit higher residual flexural strength than CF11-SAF8, as shown in Fig. 10. 4. Conclusion Laminar CFRP made from pre-pregs contain naturally ultra-thin resin-rich layers between carbon fibre plies [14]. These resin-rich interfacial layers acting like “interleaving” resin layers can be anywhere between 0 to around 10 μm in thickness. When sparsely distributed SAF are used to toughen the resin-rich interfacial region, a new toughened interleaving layer consisting of SAF and epoxy resin (SAFE) is formed. If the interleaving SAFE thickness is controlled to be thin enough so that free fibre ends of SAF can reach adjacent carbon fibre plies, toughening effects will be observed, as shown in Fig. 4b. As pointed out recently [3], the interleaving method using microlength or short fibres (SAF in this study) has the potential to enhance the delamination resistance with minimum impact on the composite processing/forming process and their properties, and furthermore a solid progress has been made from pure research (mid-layer toughening) to practical applications (multiple layer reinforcements using ultra-thin SAF veils or fabrication of SAF-toughened pre-pregs) [29,32]. Compared to our previous studies [29], the length of SAF was reduced to around 3 mm or less so that 2 to 4 times more free fibre ends were created [32]. Moreover, the after-curing thickness of SAFE interleaving layer is further reduced to around 13 μm for 4 gsm SAF since the toughening effects of ultra-thin SAFE interleaving layers on flexural strength (before impact) are clearly thickness dependent as shown in Fig. 4b [14,32]. The flexural tests (without impact) conducted in this study show that 4 gsm SAF has yielded more favourable results, increase in the flexural strength as in Fig. 4b and increase in the bulk composite modulus as in Fig. 4a before impact. The ultra-thin SAFE interleaving layer around 13 μm in thickness is fairly close to the diameter of aramid fibre as can be seen in Fig. 2 for CF11-SAF4. And 4 gsm of SAF is enough to retain the residual flexural strength after impact as shown in Fig. 9a. Combining the “high strength” and “high modulus” before impact in Fig. 4a, and “high toughness” after impact in Fig. 8 and Fig. 9 and results recently reported in [32], it seems that “high strength, high toughness and high stiffness” CFRP can be fabricated through appropriate composite microstructure designs. The SAF toughening method used in this study is basically to regain the inevitable property loss from individual carbon fibres to bulk composites. Nevertheless, simultaneous improvement in toughness, strength and stiffness still challenges the common belief that multi-layer interleaving would degrade global flexural performance of CFRPs as in Fig. 4b for interleaf thickness > 20 or 40 μm. However, this study shows that using 4 gsm SAF with the controlled thickness of 13 μm, both the flexural strength and modulus have been increased by 16.9% and 19.8%. The across ply fibre bridging mechanism in CFRP with 4 gsm SAF has been revealed by X-ray μCT images shown in Fig. 6a, i.e., the free fibre ends of SAF have been pushed into the adjacent carbon fibre plies. Such quasi-z-directional toughening is only possible if the interleaving layer is sufficiently thin, and many free fibre ends are available. Ultrathin un-bonded non-woven SAF veils with micro-length fibres not too short (long enough to span the interleaf thickness) and not too long (to have more free fibre ends) may be idea for in-situ forming of the quasiz-directional reinforcement during composite fabrication. The composite forming pressure, interfacial layer thickness, the diameter, length and density of micro-length or short fibres need to be optimized to

Fig. 4. (a) Flexural strength and modulus with standard deviation of CF11, CF11-SAF4 and CF11-SAF8; (b) Effect of interleaf thickness on the flexural strength (normalised to control sample strength value respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

comparison and the purple dashed ellipse was marked to emphasize again the significance of SAF veils in improving flexural performance before impact. By vertical comparison, it is noteworthy that after SAF veil toughening, better maintenance of original flexural properties was enabled after impact loading. For example, CF11 retained only 63.9%, 39.3% and 34.7% of its original strength after 1.9 J, 3.8 J and 5.7 J impact while CF11-SAF8 still retained 83.9%, 68.1% and 53.8% respectively. This should be mainly attributed to the effect of SAF in restraining damage formation after impact as illustrated in Fig. 7b (see mechanisms in [32]). The residual flexural strength was found to be more susceptible to impact than residual flexural modulus, which is consistent with the observations from [45,46]. By horizontal comparison, significantly improved residual flexural strength was observed by SAF veil toughening when compared to control samples under impact conditions. For example, the 4 gsm SAF veil and 8 gsm one improved the residual strength of CFRP laminates by 50.9% and 22.8% respectively under 1.9 J impact. The improvement for 3.8 J impact was even higher, at 56.8% and 62.4% respectively. This should be owing to the effect of SAF in not only diminishing impact damage but also reinforcing interfaces during bending through fibre bridging as discussed in Section 3.1. Both 4 gsm and 8 gsm SAF veils increased FAI properties, and the 8 gsm one started to outperform 4 gsm one when impact energy was increased to 3.8 J. When impact energy was 1.9 J, the effect of sufficient fibre bridging provided by 4 gsm SAF veil was still dominant since 6

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Fig. 5. Typical bending failure morphology of CF11 and CF11-SAF4 (X-ray μCT). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

fibre pre-preg manufacturing process [3,14,31,32]. In this way, the extra step of interleaving in the composite forming process can be removed. Furthermore, issues related to the SAF length, density and distributions and quality control and finally delamination susceptibility in CFRP structures can all be dealt with in the pre-preg fabrication process so that end-users can simply use SAF toughened pre-pregs like those current plain carbon fibre pre-pregs.

achieve desired quasi-z-directional toughening from the interleaving techniques. As expected, the residual flexural properties of CFRP have also been improved by up to 56.8% and 62.4% through the corporation of both SAF interleaving densities, due to the reduced impact damage. With the increasing impact energy, the dominate toughening mechanism of SAF interleaving was changed from fibre bridging to fibre pull-out and fracture, but all contributed to the enhanced impact resistance and thus resulted in smaller damage areas. Through-thickness 3D weaving may degrade the flexural modulus prior and post impact, although it can retain higher flexural strength after impact than 2D composites due to the Z-tows [37]. A compromising solution between quasi-z-directional toughening and un-affected bulk CFRP properties can be provided by ultra-thin SAF interleaving. Finally, it should be mentioned that interfacial reinforcement using SAF can be incorporated into the carbon

CRediT authorship contribution statement Bingyan Yuan: Conceptualization, Investigation, Writing - original draft, Writing - review & editing. Mingxin Ye: Investigation. Yunsen Hu: Investigation. Fei Cheng: Investigation. Xiaozhi Hu: Conceptualization, Writing - review & editing, Supervision. 7

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Fig. 6. (a) Through-interlayer-thickness reinforcement by SAF with both free fibre ends pushed into adjacent carbon fibre cloth (X-ray μCT); (b) Schematic diagram of SAF fibre bridging in thickness direction of CF11-SAF4 and CF11-SAF8. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. (a) Impact damage patterns in upper and bottom plies (X-ray μCT); (b) Damage area of CF11, CF11-SAF4 and CF11-SAF8 under 3.8 J impact. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8. (a) Average FAI load – displacement curves and (b) Scan slice of worst internal failure of CF11, CF11-SAF4 and CF11- SAF8 under 5.7 J impact (X-ray μCT). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 10. Relationship between damage area after impact and residual flexural strength. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement B. Yuan thanks the financial support from Australian Government through an “Australian Government Research Training Program Scholarship”. The authors acknowledge the facilities, and the scientific and technical assistance of Microscopy Australia at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments. References Fig. 9. Residual flexural (a) strength and (b) modulus of CF11, CF11-SAF4 and CF11-SAF8. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

[1] Francesconi L, Aymerich F. Effect of stitching on the flexure after impact behavior of thin laminated composites. Proc Inst Mech Eng Pt C J Mechan 2018;232(8):1374–88. [2] Nash NH, Young TM, McGrail PT, Stanley WF. Inclusion of a thermoplastic phase to improve impact and post-impact performances of carbon fibre reinforced

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