Epoxy Laminates Modified with Boron Nitride Nanotubes

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Enhanced Shear Performance of Hybrid Glass Fibre Epoxy Laminates Modified with Boron Nitride Nanotubes Meysam Rahmat, Behnam Ashrafi, Alex Naftel, Drazen Djokic, Yadienka Martinez Rubi, Michael Jakubinek, and Benoit Simard ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00413 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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ACS Applied Nano Materials

Enhanced Shear Performance of Hybrid Glass Fibre − Epoxy Laminates Modified with Boron Nitride Nanotubes Meysam Rahmat1*, Behnam Ashrafi2, Alex Naftel1, Drazen Djokic1, Yadienka Martinez-Rubi3, Michael B. Jakubinek3, Benoit Simard3 1

Aerospace Research Centre, National Research Council Canada, 1200 Montreal Road, Ottawa, ON K1A 0R6, Canada 2 Aerospace Research Centre, National Research Council Canada, 5145 Decelles Ave., Montreal, QC H3T 2B2, Canada 3 Security and Disruptive Technologies Research Centre, Emerging Technologies Division, National Research Council Canada, 100 Sussex Drive, Ottawa, ON K1A 0R6, Canada * Corresponding author ([email protected])

Keywords: Boron nitride nanotubes; Hybrid nanocomposites; Glass-fibre reinforced polymers; Shear test; Charpy impact

ABSTRACT Matrix-enhancement using nanotubes is one method to produce hybrid, multiscale fibre reinforced polymer (FRP) composites with improved interlaminar performance and added functional properties. Carbon nanotubes (CNTs) have been shown to be promising and recent advances in the manufacturing of boron nitride nanotubes (BNNTs), which are largely unexplored for structural reinforcement of hybrid composites with microscale fibers, offer new opportunities to employ BNNTs in reinforced hybrid composite structures. This study investigates the shear and impact properties of BNNT hybrid composites, specifically glass fibre – epoxy/BNNT composite laminates. Two manufacturing techniques were used to fabricate the specimens: wet layup, and vacuum assisted resin transfer moulding (VARTM). Shear punch, short beam shear, and modified Charpy tests were selected for their relevance to complex loading systems that involve shear, such as ballistic or other impact loading. The addition of 1 wt% BNNTs to the epoxy

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SHEAR PROPERTIES OF HYBRID GLASS FIBRE − EPOXY MODIFIED WITH BORON NITRIDE NANOTUBES

resin was found to improve the performance of the laminates: 8% increase in specific shear punch strength, 15% increase in the specific short beam shear strength and an average of 22% increase in the specific fracture energy per area in modified Charpy tests. Improvements were lower in test cases approaching pure shear, which led to the conclusion that BNNT reinforcement most effectively improves laminate performance in more complex loading situations in which an element of normal stress, such as bending, is present. As such, BNNT reinforcement, which offers different functional properties than CNTs, is also promising to improve the impact performance in multiscale, hybrid composites.

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INTRODUCTION

Fibre reinforced polymer (FRP) composites, including glass-fibre-reinforced polymers (GFRPs), are employed in a wide variety of fields from structures (aerospace, automotive, wind turbines) to electronics (circuit board substrates) to armor (impact resistant materials). Glass fibre (GF) is an effective and affordable primary reinforcement fibre in many applications [1-3] and the type of GF can be selected to the application (e.g., S-glass for tensile strength but C-glass for chemical durability). However, a limitation of such composites is their interlaminar, matrix-dominated properties including resistance to delamination and impact damage. One approach to improve matrix-dominated properties of FRP is modification of the matrix polymer, and various reports have considered the use of carbon nanotubes (CNTs) as an additional reinforcement to create multiscale hybrid composites consisting of a primary reinforcement fibre (e.g., GF or carbon fibre) and a CNT-enhanced polymer matrix [4-15].

The

incorporation of CNTs also imparts multi-functionality to traditional FRP composites, including electrical enhancement (e.g., electromagnetic shielding and electrostatic discharge) [5, 14], thermal enhancement

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ACS Applied Nano Materials SHEAR PROPERTIES OF HYBRID GLASS FIBRE − EPOXY MODIFIED WITH BORON NITRIDE NANOTUBES

(e.g., thermal conductivity and thermal stability) [14, 15] and mechanical improvement (e.g., interlaminar properties) [9, 10, 13] as well as sensing [12, 16]. Boron nitride nanotubes (BNNTs), which exhibit similarly impressive mechanical properties to CNTs, have also been shown to reinforce polymers [17]. BNNTs have been reported to interact more favourably than CNTs with polymers [18-20], and also offer different multifunctional properties including electrical insulation, lack of absorption in the visible spectrum, higher thermo-oxidative stability and high neutron absorption. These features make BNNTs a desirable component for modification of FRP composites. Recent advances in larger scale BNNT synthesis [21,22], and the availability of high quality commercial BNNTs at the g-to-kg scale [23, 24], has opened new opportunities for use of BNNTs in composites. As reviewed by Meng et al. [17], studies of BNNT-polymer composites have considered a variety of matrices and shown a reinforcement effect. However, very few reports have investigated the performance of a hybrid, multiscale composite consisting of a BNNT-infused resin within a conventional fibre-matrix composite. In the first such example, BNNTs were incorporated at the interface between plies in a GFRP composite through dipping of a prepreg into solvent with dispersed BNNTs [25]. Despite a BNNT content of only ~0.01 wt%, this increased the through-thickness thermal conductivity as high as 1.2 Wm-1K-1 [25]. A preliminary study by our group also found that BNNT-modified GFRP composites showed promising mechanical property improvements, including in the elastic modulus of the epoxy resin and the impact energy absorption of the laminate [26]. In this paper, glass fibre – epoxy /BNNT hybrid composites are fabricated using two conventional composite fabrication methods (wet layup and vacuum-assisted resin transfer molding) and the structural properties of hybrid composites are investigated. Three tests were performed: short beam shear, modified

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Charpy impact, and shear punch. These tests represent various mechanisms of shear loading and the combined performance in these tests is relevant to the matrix dominated properties of composite structures under complicated loading scenarios such as impact (and ballistic) applications where improved delamination resistance is desired. Together, the three tests provide insight about how the addition of BNNTs can improve the performance of glass/epoxy composite structures under complex loading and illustrate that the reinforcement effect increases as the loading scenarios progress from pure shear to a normal stress state.

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MANUFACTURING

Raw materials: The boron nitride nanotubes used in this study were produced in-house using the hydrogen-assisted BNNT synthesis (HABS) method [21], which uses an induction thermal plasma torch system to produce highly crystalline, small diameter (~5 nm), few-walled BNNTs from an hBN feedstock as reported in detail elsewhere [23]. Both raw (r-BNNTs) and purified BNNTs (p-BNNTs) were used in this work. The r-BNNTs produced by this method are commercially available from Tekna Advanced Materials (Sherbrooke, Canada) and via Aldrich (Product No. 802824). They are estimated to be ~50% BNNTs by weight, with elemental boron and hBN-like impurities. The purification process in this case included an oxidation step to remove boron impurities, which are responsible for giving the samples containing r-BNNTs a brown colour. A 25 g batch of boron removed BNNTs was produced and used without further purification. The polymer, a toughened epoxy resin (SC-15) was provided by Applied Poleramic (Benicia, CA, USA). The glass fiber fabric was a plain weave S-glass fabric: S-2 fiberglass fabric style #4522 with the areal density of 125 g/m2, from ACP Composites (Livermore, CA, USA).

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Nanocomposite preparation: BNNTs were added to an epoxy resin (SC-15 epoxy, Part A) by solventfree planetary mixing (Thinky ARE-310) using 3 rounds of 2 minutes mixing at 2000 rpm. The resin was allowed to cool between mixing intervals. The resin was degassed under vacuum for 15 mins. The BNNT-modified SC-15 Part A component was then mixed with the hardener (SC-15 Part B) using a low velocity (10 rpm) propeller while under continuous vacuum to avoid the introduction of air into the mixture. A BNNT content of 1 wt% was selected due to the priority of keeping the viscosity of the nanocomposite epoxy, which increases with BNNT content (see supplementary information, Figure S7), in a suitable range for both laminate production methods. Laminate production: The SC-15 epoxy and BNNT-modified SC15 composite were used with the Sglass plain weave plies to create glass fibre – epoxy/BNNT hybrid composite panels using two different methods: wet-layup and vacuum assisted resin transfer moulding (VARTM). In the wet-layup process, a thin layer of resin (baseline or BNNT-modified) was applied between each ply of glass fibre. Conversely, in the VARTM manufacturing process a dry fibre preform (stack of glass fibre plies) was infused by using vacuum to pull the resin through the laminate. Each laminate consisted of 21 layers of glass fibre fabric (17 cm × 34 cm). The panels manufactured by these two techniques had thicknesses of 2.4 ± 0.2 mm, which resulted in glass fibre volume fraction of 40 ± 5 %. The VARTM-produced specimens were 22% thicker and 5% lower in density than those made using wet layup, which is due to a larger amount of resin. The addition of nanotubes did not cause noticeable changes in the thickness or fibre/resin volume fractions. All laminates were cured at room temperature under vacuum bag compaction, and the individual specimens were cut from the cured panels. Figure 1 shows that the addition of r-BNNTs to the epoxy caused the panel to turn brown in colour, which is consistent with the beige-brown colour of r-BNNTs due

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to the presence of an amorphous boron impurity (brown in colour). Panels containing p-BNNTs appeared milky white, close to the clean white colour of the panels without BNNTs. Panels containing r-BNNTs and manufactured by VARTM also showed colour differences between the top and bottom sides of the panel and along the panel length. Spots of light brown colour visible at the bottom side of the panel were attributed to through-thickness filtration [26]. Scanning electron micrographs verified the presence of BNNTs within the laminate (see: Microscopy); therefore, it was concluded that the brown-colour (boron) impurity was filtered, although partial BNNTs filtration is also possible. A flow chart summarizing the nanocomposite and laminate fabrication is included as supplementary information (Figure S1)

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TEST METHODS

Shear Punch: Five specimens for each material/manufacturing combination were tested under shear punch conditions as described in ASTM D732 [27]. The shear punch test simulates shear plugging, a common failure mechanism seen in thick composite armour plates [28]. Short Beam Shear: The short beam shear test determines how the material reacts at the close vicinity of a concentrated bending load. These tests were conducted in accordance with ASTM D 2344 [29]. For each combination of material/manufacturing process, a set of at least five specimens were tested. Modified Charpy: The modified Charpy impact test has a longer test specimen. It captures a combination of pure shear and bending at the far field, and simulates the influence of a concentrated load on the overall performance of a structure. In addition, it also directly measures the amount of energy the material absorbs. These tests were performed in accordance with the ISO 179-1 [30] and ISO 179-2 [31] standards

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using a drop weight tower testing machine. Each test was repeated for five Type 1 specimens as depicted in the standard. The details of the test methods including Photographs of the three test configurations (shear punch, short beam shear, and modified Charpy) are included as supplementary information (Figure S2 in supporting information).

4 4.1

RESULTS AND DISCUSSION

Shear Punch

Images of the shear punch specimens after testing are shown in Figure 2. The centre pieces of the specimens were completely removed. Representative load-displacement curves are presented in Figure 3a and illustrate the trend seen for all shear punch specimens. Figure 3b shows the specific shear strength, * σ shear =

Fmax , t ×c×m

(1)

where t is the thickness, c is the circumference of the punched area, and m is the mass of the specimen before testing, for each sample type. The specific shear strength was significantly higher for wet layup specimens compared to their VARTM counterparts. Approximately the same load was required to break both the VARTM and wet layup specimens, which indicates that the additional resin in the VARTM specimens had a negligible effect on the failure of the shear punch specimens. The main load-bearing elements under pure shear are the fibres; hence the addition of extra resin does not effectively enhance the shear properties and the dominant effect is added weight, which decreases the specific shear stress.

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It can also be concluded that the addition of r-BNNTs, on average (considering both wet layup and VARTM), increases the specific shear strength of the composite by ~8% (Figure 3b). Our previous studies of a BNNT-reinforced epoxy as an adhesive reported improvement in tensile properties and single lap shear strength with BNNT addition [32], but insignificant improvement in the double lap shear properties [33]. The double lap shear method also limits non-shear components, which are encountered in the single lap configuration. Shear punch testing is similar in that both methods are pure shear tests with negligible tension or bending components. A smaller effect from BNNT reinforcement under pure shear situations can be understood as due to the fact that nanotube bridging and pull-out phenomena, which are the main mechanisms of nano-reinforcement [34,35], are not as effective under shear conditions. Similar conclusions can be drawn from work on CNT-enhanced adhesive joints [36]. In that case CNTreinforcement led to a large increase in peel strength but essentially no change in slotted lap shear strength [36], which is also a test method that restricts bending components. Unexpectedly p-BNNTs, which were selected both to eliminate the colour of the r-BNNT panels and to be a more efficient filler (higher proportion of nanotubes), did not improve the test performance. The most likely explanation is the experimental nature of the purification process. Purification effectively reduces the content of boron impurities, but other hBN-like impurities were not removed. These impurities and the potential for damage (e.g., shortening) to BNNTs during purification ultimately were not favourable in this case. However, removal of the boron impurity did address the effect of adding BNNTs on the colour of the GFRP composite.

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4.2

Short Beam Shear

The load-displacement curves for all sample types, shown for VARTM-produced laminates containing 1 wt% r-BNNT/epoxy as an example (Figure S3, supplementary information), have an initial peak in the load that indicates the point at which the first crack, delamination, or plastic deformation occurred. The secondary peaks indicate secondary damage events that occurred in order to allow for further deformation. The test standard outlines three conditions that indicate the end of the test: 30% drop in the observed load, specimen failure, or displacement equal to the nominal thickness of the specimen. However, in this configuration where the loading span was short, after 1.5 mm of displacement the load readouts were not representative of the specimen properties due to the interaction between the loading nose and supports [29]. The maximum load is then used to calculate the short beam strength, Fsbs,, F sbs = 0.75

Pm b×h

(2)

where Pm is the maximum load, b is the specimen width, and h is the specimen thickness. As the two manufacturing methods used in this study produce specimens with different fibre volume fractions and densities, the short beam shear strengths were divided by the mass of each specimen resulting in specific short beam strength values. Figure 4a shows that the specific short beam strength is not dependent on the manufacturing method. The VARTM specimens had slightly higher strength due to their increased thickness, which could be due to the mixed nature of loading in short beam shear tests. The specimen is dominantly under shear, but there is an element of bending involved in the loading conditions. As mentioned in the previous section, the extra resin does not improve pure shear properties; however, it makes the specimen thicker putting extra distance between the fibres (i.e. the load bearing elements) and the neutral axis of the specimen. This

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effectively increases the flexural rigidity of the specimen, which enhances the overall short beam shear results. However, the VARTM specimens were also heavier than the wet layup specimens and the specific stress calculation shows that the increased strength of the VARTM specimens was offset by the additional mass. In this case the addition of BNNTs led to larger increases in performance of the composite than observed in the shear punch test. Again the addition of r-BNNTs was found to be most effective, and on average the specific short-beam-shear strength by 15% due to r-BNNT addition. Figure 4 also illustrates a short beam specimen after the test. In thermography analysis (refer to supporting information, Figure S6) the failed specimens did not show indications of deep cracks or delamination, indicating that the specimens deformed due to the plastic deformation of the resin. Under short beam test conditions the specimens experience both shear and normal (due to bending) loads and nanotubes have proven to be more effective under normal loading conditions [33]. Hence the effectiveness of adding BNNTs to the resin is more pronounced in short beam shear results than the shear punch tests (i.e., 15% enhancement for short beam shear compared to 8% for shear punch). Hybrid laminates based on CNT composites have shown similar improvements (~8 to 20%) in short beam shear testing [9]. 4.3

Modified Charpy

The initial energy (5 J) was sufficient to break every specimen; however, only a few specimens broke fully into two separate pieces. Impacted samples are presented in Figure 5 and both the neat and BNNTmodified specimens were seen to fail in tension. There is also a discolouration zone (associated with plastic deformation of the resin) around the fracture surface of all specimens, reflecting the deformation of the resin in that area. This discolouration was observed consistently across all specimens, regardless of the

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manufacturing method or r-BNNT/p-BNNT addition, implying that the specimens all failed in a similar manner. When comparing the raw data from the tests with the example load-displacement curve from the ISO 179-2 standard, it can be concluded that fracture in the specimens tested in this study took the form of brittle breaks. Brittle breakage is characterized by yielding followed by unstable cracking, which is consistent with the cracking seen in Figure 5. The differences between the load-displacement curves of the various specimen types were in the slope of the linear section, where the BNNT-containing specimens exhibited higher slopes (e.g., Figure S4). This difference indicates that the addition of BNNTs increased the matrix flexural stiffness, which is consistent with effect of these BNNTs on the Young’s modulus of epoxy in tensile tests [26,32,33] and 3-point bending (refer to supporting information and Figure S5), leading to the improved Charpy performance. The load-displacement data were used to calculate the specific maximum stress, σ ∗ max =

Fmax A× m

,

(3)

,

(4)

and specific fracture energy per area,  Fi +1 + Fi    i +1 − d i )  i =1 k

G∗ =

∑  2(d

A× m

respectively. In equation (3), Fmax is the peak load, A is the cross-sectional area of the specimen, and m is the mass of the specimen. In equation (4), Fi is the measured load at time step i, and di is the measured displacement at time step i. Again the specific properties were calculated due to the variations in fibre volume fraction and mass between different manufacturing techniques..

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where σ*max is the specific maximum stress, Fmax is the peak load, A is the cross-sectional area of the specimen, and m is the mass of the specimen. where G* is the specific fracture energy per area, Fi is the measured load at time step i, and di is the measured displacement at time step i. The specific maximum stress and specific fracture energy per square meter are shown in Figure 6. The effect of the manufacturing technique on the specific maximum stress, shown in Figure 6a, mirrors the observations from the previous test methods. The modified Charpy test is a bending fracture dominant mode and the thicker specimens manufactured by VARTM have the advantage of distancing the load bearing fibres from the neutral axis. This effect is more pronounced in this case compared with the short beam shear tests, as the modified Charpy is a more bending dominant test. Hence, even after considering the extra mass of resin, the thicker VARTM specimens show higher specific maximum stress values. The exception to this is the purified BNNT specimens, which have larger error bars. The specific fracture energy per area (Figure 6b) does not follow the same trend, as the energy absorption mechanism is through both normal (bending) and shear (inter-laminar matrix) stresses and the role of normal stresses are subsequently diminished. Therefore, benefitting from lower mass, the wet layup specimens show higher specific fracture energy per area. According to Figure 6a the addition of BNNTs increased the maximum specific stress of the material. On average the addition of 1 wt% BNNTs caused a 19% increase in the maximum specific stress. For the case of wet layup with p-BNNTs, an increase of up to 34% was recorded. Additionally BNNT-containing specimens absorbed more energy than the neat specimens, leading to an average increase of 22% (with a maximum increase of 37% for r-BNNT VARTM specimen) in specific fracture energy pre area (Figure

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6b). Incorporating BNNTs into modified Charpy specimens resulted in the largest improvements in this study and this is explained by the effectiveness of nanotubes in a normal (bending) dominant type of loading. This range of Charpy results reported here with BNNTs is similar to literature examples showing 20-50% increases in energy absorption for CNT-based hybrid composites [37-39]. Figure 6 also shows that the specific fracture energy per area is significantly increased by adding BNNTs, which results from the increase in specific maximum stress and also by the increase in the Young’s modulus of the resin due to addition of BNNTs (Figure S5). To summarize, the addition of 1 wt% BNNTs increases the stiffness and strength of the matrix leading to an increase of the fracture energy of the composite. The effect of adding BNNTs to the epoxy matrix on the mechanical properties of the hybrid composites is summarized in Table 1. As mentioned above, the trend of enhancing BNNT effectiveness from shear punch to modified Charpy is clear. Three-point bending tests [40] (see supplementary information) also demonstrate the strongest influence of BNNTs occurs when the specimens are under normal stress conditions. The maximum stress in pure shear state (shear punch test) was improved by 11%, whereas for short beam shear and modified Charpy, the improvements in maximum stress were 16% and 23%, respectively. Finally, the three-point bending test with dominant normal stress state showed 35% improvement in maximum stress. The best results were almost always achieved by r-BNNT specimens manufactured by VARTM.

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5

MICROSCOPY

Scanning electron microscopy (SEM) was conducted on all specimen types, confirming that BNNTs were observed in the bulk epoxy resin as well as on the failure surfaces of the laminates. Figure 7 shows the fracture surface of neat and BNNT specimens (Figure 7a and b) as well as the matrix-glass fibre interface for specimens with and without BNNTs (Figure 7c and d). The fracture surface in the presence of BNNTs was rougher compared to the neat epoxy specimen. A rougher surface indicates a larger fracture surface area, which requires higher fracture energy. Moreover, the fracture surface of the specimen with BNNTs is consistent with a nanotube pull-out mechanism. The pull-out mechanism requires the interface between the nanotubes and the matrix to break such that the entire surface area of the BNNTs that protrude out from the surface act as fracture surface area and increase the required fracture energy to break the specimen. The matrix fracture surface in the vicinity of the glass fibre reinforcements appears rougher for the case of BNNT-modified specimens, which supports the higher fracture energy for the case of BNNTmodified specimens. There is no significant difference in the topography of the glass fibre-matrix interface and BNNTs do not appear to interfere with this fibre-matrix interface. A set of SEM images ranging from 2k to 20k magnification (Figure 8) illustrates the failure surface of a glass fiber composite. The broken surface of the resin around the glass fibres indicates a strong resin/fibre interface. This series of micrographs demonstrates the presence of BNNTs infiltrated within the bundles of glass fibre. The load transfer mechanism within a bundle of fibres is through shear stress, where the resin transfers the load from fibre to fibre. As a result, the BNNTs in this series of micrographs are generally parallel to the direction of the fibres. In contrast, in the micrograph of the BNNT/epoxy surface shown in Figure 7 the BNNTs are more likely to be perpendicular to the fracture surface.

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CONCLUSION

Hybrid BNNT-reinforced SC-15 Epoxy/glass fibre composites were produced by two composites manufacturing methods: hand layup and VARTM. The performance of the composite panels was investigated in a series of structural tests selected to cover a range of shear loading conditions from nearly pure shear to a mixed shear/normal stress state, which is relevant to complex loading scenarios including impact. It was observed that the VARTM samples were thicker, heavier, and had lower fibre volume fraction. The r-BNNT panels manufactured by VARTM also showed colour difference between the top and bottom of the laminate; where lighter brown spots were visible on the bottom side. SEM images revealed the presence of BNNTs within the light areas, and the colour difference is attributed to filtration of boron impurities through laminate thickness. The addition of 1 wt% BNNTs (both raw and purified) increased the specific shear punch strength (+8% on average), specific short beam shear strength (+15% on average) and specific fracture energy per area (+22% on average). The effect of BNNTs increased across this set of tests, from shear punch to short beam shear and then to modified Charpy, as the contribution of bending loads (normal stress) is larger in the latter scenarios. Overall, BNNTs have a small effect on the pure shear properties of hybrid composite materials; however, they are more effective in improving the performance under complex loading cases that involve bending, including the short beam shear and Charpy impact cases. This observation motivates further impact characterization, an application of which could be to the use of GFRP in ballistic protection. Optimization of the BNNT component and manufacturing methods offer significant opportunities to further improve the performance. In particular, the p-BNNTs employed in this study eliminated the effect of r-BNNTs on the colour of the panels but

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were not favourable for structural performance. Improved purified BNNTs, as well as functionalized BNNTs, should allow for more efficient reinforcement of hybrid multiscale composites.

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ACKNOWLEDGEMENTS

The authors acknowledge NRC’s Nanotube Manufacturing Facility (K.S. Kim, J. Guan, M. Plunkett, D. Ruth, S. Walker, C.T. Kingston) for the production and purification of BNNTs, as well as R. Desnoyers and D. Klishch for the contributions to the testing, M. Harrison and R. Campbell for specimen preparation, and J. Margeson for SEM imaging. Financial support was provided by NRC through its Security Materials Technology Program.

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Supporting Information

Supporting figures and explanatory detailed text as referenced in the text including specimen preparation, test methods (i.e. shear punch, short beam shear, modified Charpy, and three-point bending), thermography analysis, and rheology.

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Figures

Figure 1: Glass fibre – epoxy laminates incorporating r-BNNT (top left), p-BNNT purified (top right), and neat resin (bottom).

(a)

(b)

Figure 2: Failure surfaces of shear punch p-BNNT specimens made by (a) wet layup, and (b) VARTM after test

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Figure 3: (a) Load-displacement curves for shear punch p-BNNT specimens made by wet layup (repeated test for specimens 1 to 5), (b) shear punch specific shear strength

(a)

(b)

Figure 4: (a) Average of the specific short beam shear strengths, (b) optical microscopy image of a short beam shear specimen after the test.

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(a)

(b)

Figure 5: (a) Profile view of the modified Charpy fracture surface of a r-BNNT specimen made by wet layup, (b) bottom view of the fracture surface of a neat specimen made by VARTM

Figure 6: (a) Specific maximum stress, and (b) specific fracture energy per area obtained from modified Charpy tests.

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(a)

(b)

(c)

(d)

Figure 7: Fracture surface of (a) neat wet layup, and (b) 1 wt% r-BNNT specimens. Glass fibrematrix interface for (c) neat wet layup, and (d) 1 wt% r-BNNT specimens.

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Figure 8: SEM images showing glass fibres and BNNTs in a tested r-BNNT Charpy specimen manufactured by wet-layup. The arrows red rectangle shows the section which is zoomed on and the arrow show the next picture in the sequence.

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Tables Table 1: Summary of the maximum property improvement for different tests. The percentage of improvement is based on the neat resin composite. Short Shear Test

Beam

Modified Charpy

Three-Point Bending

Punch Shear Specific

Specific

Specific

Specific Property

Shear

Maximum

Baseline (neat resin composites)

50

Maximum

Modulus

Stress

Fracture Energy

Stress Strength

Elastic

Stress

per Area

3.7 (GPa/kg)

26 (MJ/m2kg)

18.8 (GPa)

436 (MPa)

4.6 (GPa/kg)

36 (MJ/m2kg)

20.7 (GPa)

589 (MPa)

87

(GPa/kg)

(GPa/kg)

55

101

Best Results (GPa/kg)

(GPa/kg)

Sample with Best

VARTM

VARTM

Wet layup r-

VARTM r-

VARTM r-

VARTM r-

Results

r-BNNT

r-BNNT

BNNT

BNNT

BNNT

BNNT

11

16

23

38

10

35

Maximum Improvement (%)

27




Table of Contents graphic

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Epoxy Laminates Modified with Boron Nitride Nanotubes

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