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Article Direct Comparison of the Structural Compression Characteristics of Natural and Synthetic Fiber-Epoxy Composites: Flax, Jute, Hemp, Glass and Carbon Fibers

Mike R. Bambach

Department of Civil Engineering, The University of Sydney, Sydney, NSW 2006, Australia; [email protected]; Tel.: +61-2-9351-2193

Received: 8 September 2020; Accepted: 25 September 2020; Published: 28 September 2020

Abstract: Recent decades have seen substantial interest in the use of natural fibers in continuous fiber reinforced composites, such as flax, jute and hemp. Considering potential applications, it is of particular interest how natural fiber composites compare to synthetic fiber composites, such as glass and carbon, and if natural fibers can replace synthetic fibers in existing applications. Many studies have made direct comparisons between natural and synthetic fiber composites via material coupon testing; however, few studies have made such direct comparisons of full structural members. This study presents compression tests of geometrically identical structural channel sections fabricated from fiber-epoxy composites of flax, jute, hemp, glass and carbon. Glass fiber composites demonstrated superior tension material coupon properties to natural fiber composites. However, for the same fiber mass, structural compression properties of natural fiber composite channels were generally equivalent to, or in some cases superior to, glass fiber composite channels. This indicates there is substantial potential for natural fibers to replace glass fibers in structural compression members. Carbon fiber composites were far superior to all other composites, indicating little potential for replacement with natural fibers.

Keywords: natural ﬁber composites; ﬂax; jute; hemp; glass; carbon; channels; buckling; column

1. Introduction Natural fibers such as flax, jute and hemp have attracted substantial interest for use in continuous fiber reinforced composites in recent decades, notably for their favorable sustainability properties including, for example, resource renewability; carbon sink; short growth cycle time; low herbicide requirements; low energy production; recyclability; biodegradability; low hazard manufacturing; and composite handling and working [1–13]. As the material properties of natural fibers and their composite laminates have become better understood, attention has turned to potential applications for natural fiber composites. A major focus for applications has been the potential for natural fibers to replace synthetic fibers in existing fiber resin composite applications, such as glass fiber and carbon fiber composite. Direct comparisons between natural and synthetic fiber composite material properties are relatively straightforward, since such material coupon tests are undertaken in accordance with relevant testing standards (for example, compression, tension and flexure composite material testing in accordance with ISO 604 [14], ISO 527 [15] and ISO 14125 [16], respectively). While there exists a wide range of results due to different fiber types, fiber processing techniques, fiber layouts, matrix materials and fabrication methodologies, several review studies have been published in recent years summarizing

Fibers 2020, 8, 62; doi:10.3390/fib8100062 www.mdpi.com/journal/fibers Fibers 2020, 8, 62 2 of 14 data from a wide range of sources, which provide comparisons between different natural fibers and natural fiber composites, and their synthetic counterparts (see, for example, [1–7]). Direct comparisons between natural and synthetic fiber composite full structural members are relatively few. Such comparisons are more complex than comparisons of material coupon tests, due to the wide variety of possible geometries and testing methodologies of structural members and systems, making direct comparisons between different studies difficult. Indeed, there are relatively few studies of natural fiber composite structural members, including for primary [17–20] and secondary [21–25] structural applications. Direct comparisons of structural characteristics of flax composite and glass composite wind turbine blades were reported in [20]. Other investigations highlighting the importance of natural and renewable materials include those in [26–31]. The present study attempts to address these limitations by undertaking direct comparisons of natural and synthetic fiber composite structural compression members, where members were fabricated with identical cross-section and member geometries, fabrication mandrels, fabrication processes, epoxy matrix materials and fiber masses; and were tested under identical testing procedures in the same apparatus. The member type was the structurally generic cross-section of a plain channel section in compression. The experimental results allow direct comparison of the structural compression characteristics of natural and synthetic fiber composite structures, as described in the following sections.

2. Materials and Methods

2.1. Materials Five different fabrics were utilized for composite fabrications: flax, jute, hemp, glass and carbon. The natural fiber channel members were tested by the author previously (flax, jute and hemp) [17]. In the present study, synthetic fiber channels (glass and carbon) were fabricated using the exact same methods and materials as in the previous study [17]. Full data for the natural fiber channels are provided in [17], while relevant data were extracted for direct comparisons with the synthetic fiber channels for the present study. The flax and jute fabrics were 400 gsm [0,90] 2 2 twill weaves, × a generic pattern consisting of bi-directional yarns woven over-over-under-under, where the yarns are perpendicular (i.e., 0◦/90◦). These fabrics were commercially produced by Composites Evolution, termed Biotex Flax and Biotex Jute, and use a low-twist yarn. Nominal density, tensile strength and modulus values for the flax were 1.5 g/cm3, 500 MPa and 50 GPa, and for the jute were 1.46 g/cm3, 400 MPa and 40 GPa. The hemp fabric was a 287-gsm [0,90] plain weave pure unbleached hemp fabric with a nominal density of 1.48 g/cm3. This fabric was not produced specifically for fiber resin composite fabrication; however, it was recommended as the most appropriate fabric for composites by the manufacturer. The glass fabric was a commercial 1 1 twill weave [0,90] 300-gsm E-glass with nominal density, × tensile strength and modulus values of 2.55 g/cm3, 1950 MPa and 72 GPa. The carbon fabric was a commercial 2 2 twill weave [0,90] 320-gsm Toray T300 carbon with nominal density, tensile × strength and modulus values of 1.76 g/cm3, 3530 MPa and 230 GPa. The epoxy resin used was the bulk laminating epoxy resin Kinetix R240 with H160 hardener. This commercial epoxy resin has a density of 1.1 g/cm3 and measured a (neat) tension strength of 33.1 MPa, and it is a room temperature out-of-autoclave cure resin. Tension material specimens were water jet cut from untested laminates and tested in accordance with ISO 527 [15]. The tension coupons were 25 mm wide and 250 mm long, and bonded steel end tabs were used. Since the width of the fabric weaves varied from approximately 1 mm for natural fiber fabrics to 2.5 mm for synthetic fiber fabrics, the tension coupons consisted of many weaves across the width of the coupon. The coupons were extracted from the thickest specimens fabricated, and they thus had total fiber areal masses equal to the maximum values for each fiber type in Table1. Fibers 2020, 8, 62 3 of 14

2.2. Channel Fabrications The dimensions selected for the composite channels were a web depth of 100 mm (D), a flange Fibers 2020, 8, x FOR PEER REVIEW 3 of 14 width of 40 mm (B) and a length of 300 mm (L). Since the webs of the channels are stiffened elements, webwhich depth are was known selected, to typically such that buckle the webs in square had multiple half-wavelengths, half‐wavelengths a channel of lengthbuckle. of This three minimized times the endweb effects. depth wasColumn selected, member such thatlateral the‐torsional webs had buckling multiple half-wavelengthswas precluded due of buckle. to the Thisshort minimized column length.end eff ects. Column member lateral-torsional buckling was precluded due to the short column length. AA hand hand layup layup technique technique was was used used to to fabricate fabricate the the composite composite channels. channels. First First the the fabric fabric was was wet wet outout with with the the epoxy epoxy resin resin using using a a standard standard brush brush and and roller, roller, then then laid laid over over a a mandrel. mandrel. The The mandrel mandrel waswas a a steel steel square square hollow hollow section section with with nominal nominal dimensions dimensions of of 100 100 mm mm (outside (outside dimension) dimension) and and a a cornercorner radius radius of of 8 8 mm. mm. This This resulted resulted in all channels havinghaving anan internal internal web web depth depth of of 100 100 mm mm (Figure (Figure1). 1).Release Release film film was was placed placed between between the the composite composite and and the mandrel,the mandrel, such such that athat chemical a chemical release release agent agentwas not was required. not required. A full A vacuum full vacuum (approximately (approximately 100 kPa)100 kPa) was setwas over set over the channel the channel and heldand held for a forminimum a minimum of 15 of h in15 a h constant in a constant temperature temperature room atroom 23 ◦ C.at The23 °C. fabricated The fabricated dimensions dimensions of the channels of the channelswere lengths were of lengths 850 mm of and850 flangemm and widths flange of widths 70 mm. of The 70 mm. channels The werechannels then were cut into then two cut specimens into two specimensand trimmed and to trimmed size following to size following curing (using curing water (using jet cutting).water jet Tablecutting).1 provides Table 1 the provides final measured the final measuredchannel geometric channel geometric properties. properties.

(a) (b)

FigureFigure 1. 1.MandrelMandrel and fiber and fabrics fiber used: fabrics (a) used: steel square (a) steel hollow square section hollow mandrel section and (b mandrel) 50 × 50 mm and fabrics.(b) 50 50 mm fabrics. × InIn the previousprevious investigation investigation [17 [17],], flax flax and juteand channels jute channels were fabricated were fabricated with diff erentwith thicknesses different thicknessesby varying by the varying number the of 400number gsm of fabric 400 layers—channelsgsm fabric layers—channels with two, three with ortwo, four three fabric or layersfour fabric were layersfabricated, were resultingfabricated, in channelsresulting within channels total fiber with areal total masses fiber of areal 800, masses 1200 and of 1600 800, gsm.1200 Hempand 1600 channels gsm. Hemphad total channels fabric had areal total masses fabric of areal 861, 1148 masses and of 1722 861, gsm, 1148 where and 1722 masses gsm, could where not masses be matched could not exactly be matcheddue to the exactly single due layer to areal the single mass oflayer 287 areal gsm. mass Similarly, of 287 in gsm. the present Similarly, study in thethe number present ofstudy layers the of numberglass (300 of gsmlayers per of layer)glass (300 and carbongsm per fabric layer) (320 and gsm carbon per layer)fabric were(320 gsm chosen per to layer) approximately were chosen equal to approximatelythe values for flaxequal and the jute, values resulting for flax in and total jute, fabric resulting areal massesin total forfabric glass areal of 900,masses 1200 for and glass 1500 of 900, gsm, 1200and and for carbon 1500 gsm, of 640, and 960 for carbon and 1600 of gsm640, 960 (the and values 1600 for gsm carbon (the values were selected for carbon such were that selected they did such not thatexceed they those did not for exceed the other those fabrics, for the since other carbon fabrics, is since substantially carbon is sti substantiallyffer and stronger). stiffer and In the stronger). present Instudy the comparisonspresent study are comparisons made between are di ffmadeerent fiberbetween types different within these fiber three types comparator within these groups, three i.e., comparatorgroups with groups, approximate i.e., groups total fiberwith arealapproximate masses of total 800, fiber 1200 areal and masses 1600 gsm, of 800, corresponding 1200 and 1600 to groups gsm, correspondingwith actual values to groups of 640–900 with gsm, actual 960–1200 values gsm of 640–900 and 1500–1722 gsm, 960–1200 gsm, respectively. gsm and 1500–1722 gsm, respectively.

2.3. Channel Tests Compression testing was conducted in an MTS machine with fixed‐end bearings. The channel column test specimens had bonded steel end plates of 2 mm thickness. This reduced possible end effects (stress concentrations, eccentric loading, etc.) resulting from the fabrication procedure. The end plates precluded edge rotations, resulting in the ends being fixed‐ended [17,18,32]. The testing machine contained spherical end bearings that were initially unlocked while the specimen was installed and a small load applied. The bearings were then locked providing a fixed‐end bearing Fibers 2020, 8, 62 4 of 14

2.3. Channel Tests Compression testing was conducted in an MTS machine with fixed-end bearings. The channel column test specimens had bonded steel end plates of 2 mm thickness. This reduced possible end effects (stress concentrations, eccentric loading, etc.) resulting from the fabrication procedure. The end plates precluded edge rotations, resulting in the ends being fixed-ended [17,18,32]. The testing machine contained spherical end bearings that were initially unlocked while the specimen was installed and a small load applied. The bearings were then locked providing a fixed-end bearing condition. Buckling deformations (out-of-plane) were measured with displacement transducers, which were located at 75 mm intervals longitudinally along the web, and at the mid-length of both flanges. The loading procedure was displacement control, with a speed of 0.5 mm/min.

3. Results

3.1. Laminate Thickness An important difference between the channels fabricated from the various fabrics is the laminate thickness. While comparator channels had approximately equal total fiber areal masses, they did not have equal laminate thicknesses. This results primarily from the different density of the fabrics, resulting in different nominal fabric thicknesses for the same fabric mass and different epoxy uptake during fabric wet out. Other factors may include that natural fibers contain a lumen core which may compress differently to synthetic fibers under vacuum pressure, and different fabrics and/or weaves may lead to different void contents in the matrix. The measured laminate thickness values are tabulated in Table1, where it is evident that when comparing laminates with approximately equal total fiber areal masses, the laminate thickness is generally proportional to the fiber density. Laminates in order from thinnest to thickest (for the same total fiber areal mass) were glass, carbon, flax, jute and hemp.

Table 1. Composite channel dimensions and test results.

Total Web Flange Thick- Ultimate Buckling Strength Axial Areal Depth Width ness Load Load Fiber Eﬃciency Stiﬀness Pu/Pb Mass (D) (B) (t) (Pu) (Pb) (gsm) (mm) (mm) (mm) (N) (N/gsm) (N/mm) (N) Flax 800 104.1 39.8 1.70 2683 3.35 3409 949 2.8 Flax 800 104.0 39.8 1.70 2922 3.65 3433 982 3.0 Flax 1200 105.5 41.2 2.45 6898 5.75 7249 3898 1.8 Flax 1200 105.9 41.4 2.45 6381 5.32 7270 3667 1.7 Flax 1600 107.5 41.3 3.30 11,023 6.89 9240 8933 1.2 Flax 1600 107.4 41.2 3.30 11,655 7.28 8670 9287 1.3 Jute 800 104.0 41.2 1.75 2996 3.75 4422 825 3.6 Jute 800 104.1 41.1 1.75 2967 3.71 4313 1027 2.9 Jute 1200 106.1 42.0 2.75 7678 6.40 7742 4414 1.7 Jute 1200 105.9 42.0 2.75 7611 6.34 7755 4253 1.8 Jute 1600 107.1 41.7 3.50 9555 5.97 8243 7655 1.2 Jute 1600 107.3 41.8 3.50 10,078 6.30 8705 7514 1.3 Hemp 861 104.8 41.8 2.15 1985 2.31 2849 1031 1.9 Hemp 861 104.6 42.1 2.15 2207 2.56 2850 985 2.2 Hemp 1148 106.2 41.2 2.70 4663 4.06 4949 3205 1.5 Hemp 1148 106.0 41.0 2.70 4903 4.27 6365 2748 1.8 Hemp 1722 109.0 41.5 3.90 11,879 6.90 6669 10,780 1.1 Hemp 1722 108.9 41.7 3.90 13,262 7.70 6598 11,687 1.1 Carbon 640 101.6 39.6 0.60 4231 6.61 5825 3275 1.3 Carbon 640 101.5 39.3 0.60 4122 6.44 5786 3095 1.3 Carbon 960 102.5 41.2 0.90 9089 9.47 9629 7246 1.3 Carbon 960 102.4 42.2 0.90 9340 9.73 9699 7452 1.3 Carbon 1600 103.5 42.0 1.35 25,270 15.79 14,311 15,584 1.6 Carbon 1600 103.3 40.3 1.35 24,527 15.33 14,175 14,714 1.7 Fibers 2020, 8, 62 5 of 14

Table 1. Cont.

Total Web Flange Thick- Ultimate Buckling Strength Axial Areal Depth Width ness Load Load Fiber Eﬃciency Stiﬀness Pu/Pb Mass (D) (B) (t) (Pu) (Pb) (gsm) (mm) (mm) (mm) (N) (N/gsm) (N/mm) (N) Glass 900 101.8 39.1 0.60 3041 3.38 3191 2094 1.5 Glass 900 101.8 39.3 0.60 3028 3.36 3432 2036 1.5 Glass 1200 102.4 41.7 0.80 5488 4.57 5303 3645 1.5 Glass 1200 102.5 41.9 0.80 5241 4.37 5222 3591 1.5 Glass 1500 103.3 39.3 1.15 7857 5.24 6495 4912 1.6 Glass 1500 103.2 39.5 1.15 8373 5.58 6840 5044 1.7

In Table1 it is evident from the buckling loads that, for approximately equivalent fiber total areal mass, the buckling loads for the natural fiber composite channels are similar to those for the glass fiber composite channels and in some cases exceed them. This results from the thickness values discussed above. Thin laminate compression buckling is dependent on the thickness of the laminate squared and the material elastic modulus (E) (in the present study the other relevant geometric properties are the same for all channels). While the glass fiber composite elastic modulus is greater than those for the natural fiber composites (Table2), the buckling load of the glass fiber composites can be lower than the natural fiber composites in cases where the thickness is much lower. In Table1 it is evident from the axial sti ffness values that, for approximately equivalent fiber total areal mass, the axial stiffness of the glass fiber composite channels in many cases is similar to or less than those for the natural fiber composite channels. The axial stiffness is dependent on the cross-section area of the laminate and the material elastic modulus (E). While the glass fiber composite elastic modulus is greater than those for the natural fiber composites (Table2), the cross-sectional area of the glass fiber composite channels is much less than those of the natural fiber composite channels, since the thickness values are much lower, while the other relevant geometric properties are the same for all channels. In Table1 it is evident from the ultimate load values that, for approximately equivalent fiber total areal mass, the ultimate load of the glass fiber composite channels in many cases is similar to or less than those for the natural fiber composite channels. The ultimate load is dependent on the buckling load, the post-buckling ductility and the material strength. While the glass fiber composite material strength is greater than those for the natural fiber composites (Table2), the buckling loads are lower in many cases, and the glass fiber composite response is more brittle than that of the natural fiber composites with post-buckling ductility values (Pu/Pb) less than those for natural fibers in many cases (Table1). Meanwhile, since the material properties of the carbon fiber composites far exceeded those of the natural and glass fiber composites (Table2), the performance of the carbon fiber composite channels with respect to stiffness, buckling and strength were superior to natural and glass fiber composite channels in all cases, despite the carbon fiber laminates being relatively thin (Table1).

3.2. Fiber Volume Fractions Fiber mass fractions were calculated using the mass of fabric prior to fabrication and the mass of composite after fabrication, and fiber volume fractions were estimated from mass fractions and constituent densities. Natural fiber mean mass fractions for flax, jute and hemp were 0.48, 0.50 and 0.47, respectively. Synthetic fiber mean mass fractions for carbon and glass were 0.75 and 0.77, respectively. These data indicate that natural and synthetic fibers took up epoxy resin at significantly different rates, due to their different densities (and weaves). The fiber volume fraction mean values for flax, jute, hemp, carbon and glass were 0.41, 0.44, 0.41, 0.66 and 0.60, respectively. It should be noted that since natural fibers contain a lumen core, these fibers may compress slightly under a vacuum pressure. Fibers 2020, 8, 62 6 of 14

This may slightly increase the natural ﬁber density resulting in the volume fractions being an upper bound estimate for these ﬁbers.

3.3. Material Tension Test Results Material tension strength results are tabulated for all laminates in Table2, where the mean values for flax, jute, hemp, carbon and glass were 45.4, 52.1, 35.3, 771 and 484 MPa, respectively. It is noted that these are uni-axial stresses on bi-directional fabric [0,90] composites, and thus the material values are nominally one half of those for uni-directional fabric composites. As expected, the synthetic fiber compositesFibers 2020, 8, x had FOR substantially PEER REVIEW higher tension strengths than the natural fiber composites, and carbon6 of 14 was the strongest composite. It should be noted the natural fiber composite tension strengths are relativelyfrom the multilayer low and may bi‐ resultdirectional from thetwill following: weave; the voids epoxy in the resin matrix may may not resultcompletely from thepenetrate multilayer the bi-directionallumens; the natural twill weave; fiber is the a epoxylow‐twist resin yarn, may however, not completely if the penetratetwist is not the zero lumens; the twist the natural angle fibermay ischange a low-twist during yarn, tension however, loading; if the and twist the is natural not zero fiber the yarn twist contains angle may elementary change during fibers tension that can loading; move and(relative the natural to each fiberother) yarn during contains tension elementary loading. fibers that can move (relative to each other) during tension loading. Table 2. Measured mean uni‐axial tension coupon material properties of the fiber composite Tablelaminates 2. Measured (ET is the meanelastic uni-axial modulus tension in tension, coupon εuT is material the failure properties strain in of tension, the fiber and composite fuT is the laminates ultimate

(EtensionT is the stress). elastic modulus in tension, εuT is the failure strain in tension, and fuT is the ultimate tension stress). ET εuT fuT

ET (GPa) (%)ε uT(MPa) fuT Flax(GPa) 6.03 3.1 (%) 45.4 (MPa) FlaxJute 6.035.18 1.6 3.152.1 45.4 JuteHemp 5.18 3.83 1.8 1.635.3 52.1 HempCarbon 3.83 72.9 1.1 1.8771 35.3 CarbonGlass 72.9 22.9 2.3 1.1484 771 Glass 22.9 2.3 484 Exemplar tension material stress–strain responses are plotted in Figure 2, and mean elastic moduliExemplar and fracture tension strains material are stress–strain tabulated responsesin Table 2. are These plotted data in Figureindicate2, and that mean the synthetic elastic moduli fiber andcomposites fracture had strains highly are tabulatedlinear stress–strain in Table2. responses, These data while indicate natural that the fiber synthetic composites fiber had composites highly non had‐ highlylinear responses. linear stress–strain The carbon responses, fiber composite while natural had the fiber lowest composites fracture had strain highly (1.1%), non-linear while the responses. fracture Thestrain carbon of the fiber glass composite fiber composite had the (2.3%) lowest was fracture comparable strain (1.1%), with those while of the the fracture natural strain fiber ofcomposites the glass fiber(1.6–3.1%). composite (2.3%) was comparable with those of the natural fiber composites (1.6–3.1%).

(a) (b)

Figure 2.2. ExemplarExemplar uni-axial uni‐axial tension tension coupon coupon material material tests tests of the of fiber the compositefiber composite laminates: laminates: (a) natural (a) fibernatural composite fiber composite laminates laminates [17] and [17] (b) syntheticand (b) synthetic fiber composite fiber composite laminates. laminates.

3.4. Channel Test Results Full results for the ultimate compression strength of all channel sections are tabulated in Table 1, and exemplar axial compression–axial displacement test data are shown in Figures 3 and 4 for carbon and glass channels, respectively, including photos at key points (a to g). Test data for the flax, jute and hemp channels are provided in [17]. The compression results indicate typical behavior for thin‐walled structural sections, with reference to Figures 3 and 4: an initial linear elastic response (a,b); followed by compression buckling of one or more web or flange elements (typically in multiple half‐wavelengths) with corresponding bi‐furcation from linear elasticity (c); followed by a post‐ buckling response characterized by increasing out‐of‐plane buckling deformations and concomitant softening, resulting from the re‐distribution of axial stresses away from these buckled regions (d); the attainment of an ultimate limit state resulting from material fracture in the channel corners (e,f); Fibers 2020, 8, 62 7 of 14

3.4. Channel Test Results Full results for the ultimate compression strength of all channel sections are tabulated in Table1, and exemplar axial compression–axial displacement test data are shown in Figures3 and4 for carbon and glass channels, respectively, including photos at key points (a to g). Test data for the ﬂax, jute and hemp channels are provided in [17]. The compression results indicate typical behavior for thin-walled structural sections, with reference to Figures3 and4: an initial linear elastic response (a,b); followed by compression buckling of one or more web or ﬂange elements (typically in multiple half-wavelengths) with corresponding bi-furcation from linear elasticity (c); followed by a post-buckling response characterized by increasing out-of-plane buckling deformations and concomitant softening,

Fibersresulting 2020, from8, x FOR the PEER re-distribution REVIEW of axial stresses away from these buckled regions (d); the attainment7 of 14 of an ultimate limit state resulting from material fracture in the channel corners (e,f); followed by followedpost-ultimate by post load‐ultimate shedding, load typically shedding, also typically involving also the involving development the development of additional of fractures additional (g). fracturesMaterial failure(g). Material is discussed failure further is discussed in the further following in the section. following section.

Figure 3. ExemplarExemplar carbon carbon fiber fiber composite composite channel channel (960 (960 gsm) gsm) compression compression test (dashed line indicates thethe channel compression stiffness; stiffness; circles indicate fractures; allall columncolumn heightsheights areare 300300 mm).mm). Exemplar axial compression—axial displacement test data for each fabric material and each of Exemplar axial compression—axial displacement test data for each fabric material and each of the three total fiber areal mass comparator groups are shown in Figures5–7, including photos at the three total fiber areal mass comparator groups are shown in Figures 5–7, including photos at the the ultimate limit state, for direct comparison between different fabrics. These data indicate that the ultimate limit state, for direct comparison between different fabrics. These data indicate that the natural fiber channels demonstrated a much more ductile response, partly due to lower elastic moduli, natural fiber channels demonstrated a much more ductile response, partly due to lower elastic but predominantly due to their much more ductile post-buckling and post-ultimate responses. As moduli, but predominantly due to their much more ductile post‐buckling and post‐ultimate demonstrated in Figure2, the synthetic fiber laminates demonstrated highly linear material behavior, responses. As demonstrated in Figure 2, the synthetic fiber laminates demonstrated highly linear while the natural fiber laminates demonstrated highly non-linear material behavior. These material material behavior, while the natural fiber laminates demonstrated highly non‐linear material characteristics allowed the natural fiber channels to develop large out-of-plane buckling displacements behavior. These material characteristics allowed the natural fiber channels to develop large out‐of‐ prior to reaching the ultimate limit state, during which the axial stresses redistributed and allowed plane buckling displacements prior to reaching the ultimate limit state, during which the axial the channel to continue to resist load. This produced a pronounced post-buckling strength, gradual stresses redistributed and allowed the channel to continue to resist load. This produced a pronounced attainment of the ultimate load and gradual unloading post-ultimate. Conversely, the synthetic fiber post‐buckling strength, gradual attainment of the ultimate load and gradual unloading post‐ultimate. channels reached the ultimate state in an abrupt manner and failed suddenly in a brittle manner, Conversely, the synthetic fiber channels reached the ultimate state in an abrupt manner and failed shedding load rapidly post-ultimate. This was also qualitatively evident during the experiments, where suddenly in a brittle manner, shedding load rapidly post‐ultimate. This was also qualitatively evident during the experiments, where the synthetic fiber composite channels failed suddenly with an audibly loud noise, corresponding to the sudden release of the stored compression energy associated with material fracture. Fibers 2020, 8, 62 8 of 14 the synthetic fiber composite channels failed suddenly with an audibly loud noise, corresponding to Fibers 2020, 8, x FOR PEER REVIEW 8 of 14 theFibers sudden 2020, 8, x release FOR PEER of theREVIEW stored compression energy associated with material fracture. 8 of 14

FigureFigure 4. Exemplar 4. Exemplar glass glass ﬁber fiber composite composite channel channel (1200 (1200 gsm) gsm) compression compression test (dashedtest (dashed line indicatesline indicates Figure 4. Exemplar glass fiber composite channel (1200 gsm) compression test (dashed line indicates the channelthe channel compression compression stiﬀness; stiffness; circles circles indicate indicate fractures; fractures; all column all column heights heights are 300 are mm). 300 mm). the channel compression stiffness; circles indicate fractures; all column heights are 300 mm).

FigureFigure 5. Exemplar 5. Exemplar composite composite channel channel compression compression tests tests for thefor approximatelythe approximately 800 gsm800 gsm total total ﬁber fiber Figure 5. Exemplar composite channel compression tests for the approximately 800 gsm total fiber arealareal mass mass comparator comparator group group (640–900 (640–900 gsm) gsm) (all column (all column heights heights are 300 are mm). 300 mm). areal mass comparator group (640–900 gsm) (all column heights are 300 mm). Fibers 2020, 8, 62 9 of 14 Fibers 2020, 8, x FOR PEER REVIEW 9 of 14 Fibers 2020, 8, x FOR PEER REVIEW 9 of 14

Figure 6. Exemplar composite channel compression tests for the approximately 1200 gsm total fiber FigureFigure 6. Exemplar 6. Exemplar composite composite channel channel compression compression tests tests for for the the approximately approximately 1200 1200 gsm gsm total total ﬁber fiber areal mass comparator group (960–1200 gsm) (all column heights are 300 mm). arealareal mass mass comparator comparator group group (960–1200 (960–1200 gsm) gsm) (all (all column column heights heights are 300are 300 mm). mm).

FigureFigure 7. Exemplar 7. Exemplar composite composite channel channel compression compression tests tests for for the the approximately approximately 1600 1600 gsm gsm total total ﬁber fiber Figure 7. Exemplar composite channel compression tests for the approximately 1600 gsm total fiber arealareal mass mass comparator comparator group group (1500–1722 (1500–1722 gsm) gsm) (all (all column column heights heights are 300are 300 mm). mm). areal mass comparator group (1500–1722 gsm) (all column heights are 300 mm).

The post‐buckling strength was quantified using the ratio of the ultimate load to the section The post‐buckling strength was quantified using the ratio of the ultimate load to the section buckling load (Table 1). The change in column stiffness methodology was used to quantify the buckling load (Table 1). The change in column stiffness methodology was used to quantify the Fibers 2020, 8, 62 10 of 14

FibersThe 2020, post-buckling 8, x FOR PEER REVIEW strength was quantified using the ratio of the ultimate load to the section10 of 14 buckling load (Table1). The change in column sti ffness methodology was used to quantify the buckling loads.buckling The loads. linear The elastic linear axial elastic stiff nessaxial bi-furcation stiffness bi‐ pointfurcation was point identified, was identified, and the column and the buckling column loadbuckling was takenload aswas the taken compression as the compression load corresponding load corresponding to this bi-furcation to this point bi‐ (thisfurcation methodology point (this is discussedmethodology further is discussed in [17,18, 33further]). It is in noted [17,18,33]). that this It providesis noted that an upper this provides bound to an the upper column bound buckling to the load,column as the buckling reduction load, in as column the reduction stiffness in occurs column immediately stiffness occurs following immediately buckling. Thesefollowing data buckling. indicate theThese post-buckling data indicate strength the post of‐buckling the synthetic strength fiber of channels the synthetic (ratios fiber of 1.3 channels to 1.7) were (ratios generally of 1.3 to less1.7) thanwere thosegenerally of the less natural than fiberthose channels of the natural (ratios fiber of 1.1 channels to 3.6); however,(ratios of it1.1 is to noted 3.6); that however, these ratiosit is noted changed that withthese the ratios laminate changed thickness. with the laminate thickness.

3.5.3.5. FailureFailure ModesModes InIn accordanceaccordance withwith fundamentalfundamental thinthin plateplate mechanicsmechanics theories,theories, inin thethe post-buckledpost‐buckled statestate thethe stressesstresses redistributeredistribute away from from the the buckled buckled regions regions into into the the non non-buckled‐buckled regions: regions: in the in thepresent present case caseinto intothe thecorners corners of the of the channel channel sections. sections. It Itis isvia via this this mechanism mechanism that that a post-buckingpost‐bucking strengthstrength develops,develops, duringduring whichwhich thethe stressesstresses continuecontinue toto increaseincrease inin thesethese cornercorner regionsregions untiluntil thethe ultimate ultimate limitlimit state,state, characterizedcharacterized byby thethe attainmentattainment ofof aa stressstress limitlimit correspondingcorresponding toto failurefailure ofof thethe material.material. InIn accordance accordance with with the the material material behavior behavior (Section (Section 3.3 3.3),), the the channel channel material material failure failure was was brittle brittle for for the the syntheticsynthetic fiber fiber composites, composites, characterizedcharacterized byby concentratedconcentrated matrixmatrix fracturesfractures (including(including fiberfiber fracture),fracture), andand ductileductile forfor thethe naturalnatural fiberfiber composites,composites, characterizedcharacterized byby didiffuseffuse matrixmatrix failurefailure (without(without fiberfiber fracture).fracture). Exemplar Exemplar matrix matrix failures failures are are shown shown in Figurein Figure8 for 8 typicalfor typical carbon carbon and glassand glass composites, composites, and forand flax for composites flax composites which which were typicalwere typical to the naturalto the natural fiber composites fiber composites (natural (natural fiber composite fiber composite matrix failuresmatrix arefailures elaborated are elaborated further in further [17]). The in [17]). post-buckling The post‐ mechanicsbuckling mechanics model for compositemodel for structuralcomposite channelstructural sections channel is describedsections is further described in [ 17further–19]. in [17–19].

FigureFigure 8. 8.Exemplar Exemplar matrix matrix failures failures in the in cornersthe corners of the of composite the composite channels channels associated associated with the ultimatewith the limitultimate state: limit (a) carbon state: (a (inside) carbon view), (inside (b) view), carbon (b (outside) carbon view), (outside (c) view), glass (inside (c) glass view), (inside (d) view), glass (outside(d) glass view),(outside (e) view), ﬂax (inside (e) flax view) (inside and view) (f) ﬂax and (outside (f) flax view). (outside view).

3.6. Comparisons of Structural Compression Characteristics—Compressive Strength and Stiffness Since the total fiber areal masses of the channels had small variations within the three comparator groups, the strength is compared using the value of the channel ultimate compression Fibers 2020, 8, 62 11 of 14

3.6. Comparisons of Structural Compression Characteristics—Compressive Strength and Stiﬀness

Fibers Since2020, 8, the x FOR total PEER fiber REVIEW areal masses of the channels had small variations within the three comparator11 of 14 groups, the strength is compared using the value of the channel ultimate compression strength (N) perstrength unit fiber(N) per areal unit mass fiber (gsm), areal i.e., mass the (gsm), strength i.e., e thefficiency strength (N/ gsm).efficiency While (N/gsm). the channels While hadthe channels different relativehad different quantities relative of epoxy quantities material, of epoxy this strength material, effi thisciency strength value wasefficiency selected value to directly was selected compare to betweendirectly compare different between types of fiber different reinforcements. types of fiber The reinforcements. strength efficiency The values strength are efficiency tabulated values in Table are1, andtabulated all data in areTable plotted 1, and in all Figure data 9are, along plotted with in aFigure linear 9, trend along for with each a fiberlinear type. trend These for each data fiber indicate type. thatThese for data each indicate fiber type, that strength for each e fiberfficiency type, increased strength with efficiency composite increased thickness. with Thiscomposite is a well-known thickness. resultThis is for a well thin-walled‐known result structural for thin sections‐walled in compression,structural sections where in thinner compression, sections where are less thinner efficient sections since theyare less are efficient susceptible since to they compression are susceptible buckling to of compression the thin plate buckling elements of ofthe the thin section. plate elements These data of also the indicatesection. thatThese carbon data fiberalso indicate composites that have carbon superior fiber strengthcomposites effi ciencieshave superior to all other strength fiber efficiencies types, despite to beingall other comparatively fiber types, thin, despite while glassbeing fiber comparatively composite strength thin, while efficiencies glass are fiber comparable composite with strength natural fiberefficiencies composites. are comparable with natural fiber composites.

Figure 9. Composite channel compression strength efficiency (strength per fiber mass, N/gsm). Figure 9. Composite channel compression strength efficiency (strength per fiber mass, N/gsm). While strength efficiencies varied with thickness, the average value for all thicknesses provides While strength efficiencies varied with thickness, the average value for all thicknesses provides a comparative measure for the strength efficiency for each fiber type, and these values were then a comparative measure for the strength efficiency for each fiber type, and these values were then normalized to the average value for flax and plotted in Figure 10a. Similarly, each channel section normalized to the average value for flax and plotted in Figure 10a. Similarly, each channel section compression stiffness (i.e., the slope of the linear elastic portion of the axial load—axial displacement compression stiffness (i.e., the slope of the linear elastic portion of the axial load—axial displacement plot) was normalized to its total fiber areal mass, then averaged for each fiber type, then normalized to plot) was normalized to its total fiber areal mass, then averaged for each fiber type, then normalized the average value for flax, and plotted in Figure 10b. It is noted that the channel section compression to the average value for flax, and plotted in Figure 10b. It is noted that the channel section stiffness thus calculated corresponds to the section EA/L value (where E is the elastic modulus, compression stiffness thus calculated corresponds to the section EA/L value (where E is the elastic A is the cross-section area, and L is the channel length). These data indicate that the carbon fiber modulus, A is the cross‐section area, and L is the channel length). These data indicate that the carbon composite channels were more efficient than all other composites, with respect to both strength fiber composite channels were more efficient than all other composites, with respect to both strength (1.9 times flax) and stiffness (1.8 times flax). Meanwhile, glass fiber composite channels were less (1.9 times flax) and stiffness (1.8 times flax). Meanwhile, glass fiber composite channels were less efficient than flax fiber composites channels, with respect to both strength (0.8 times flax) and stiffness efficient than flax fiber composites channels, with respect to both strength (0.8 times flax) and stiffness (0.8 times flax). Glass fiber composite channels were less efficient than jute fiber composite channels, (0.8 times flax). Glass fiber composite channels were less efficient than jute fiber composite channels, and approximately equally efficient with hemp fiber composite channels. Flax and jute fiber composite and approximately equally efficient with hemp fiber composite channels. Flax and jute fiber channels were approximately equally efficient, and they were slightly more efficient than hemp fiber composite channels were approximately equally efficient, and they were slightly more efficient than composite channels. hemp fiber composite channels. Fibers 2020, 8, 62 12 of 14 Fibers 2020, 8, x FOR PEER REVIEW 12 of 14

(a) (b)

FigureFigure 10. 10. MeanMean compression compression strength andand stistiffnessffness e ffiefficiencyciency of of structural structural columns columns from from channel channel tests: tests:(a) strength (a) strength and (andb) compression (b) compression stiffness stiffness (EA/ L,(EA/L, where where E is the E is elastic the elastic modulus, modulus, A is the A is cross-section the cross‐ sectionarea, and area, L and is the L channelis the channel length), length), compared compared with the with values the values for flax. for flax.

4.4. Discussion Discussion TheThe present present study study has has demonstrated demonstrated that, that, for for equivalent equivalent fiber fiber mass, mass, natural natural fiber fiber composite composite structuralstructural compression membersmembers are are generally generally structurally structurally equivalent equivalent to their glassto their fiber counterparts.glass fiber counterparts.For the specific For purpose the specific of ascertaining purpose of if naturalascertaining fibers (flax,if natural jute and fibers hemp) (flax, may jute replace and hemp) glass fiber may in replacestructural glass composites, fiber in structural it is concluded composites, that this it is would concluded be achievable that this onwould a fiber be mass achievable replacement on a fiber basis mass(subject replacement to the geometric basis (subject limitations to the of geometric the present limitations test series of thinthe present structural test laminates series of inthin compression). structural laminatesIt should bein noted,compression). however, It this should conclusion be noted, does not however, infer similar this equivalencyconclusion ondoes a volume not infer replacement similar equivalencybasis (i.e., comparing on a volume structural replacement characteristics basis (i.e., for comparing channels with structural identical characteristics thickness). for channels with identicalDirect comparisons thickness). between structural laminates of flax and glass fiber polyester composites wereDirect made comparisons in [20], comparing between nominally structural identical laminates small of wind flax turbineand glass blades. fiber Inpolyester that study composites the blades werewere made nominally in [20], volume comparing constrained, nominally whereby identical approximately small wind turbine the same blades. volume In that of flax study and the glass blades fiber werereinforcements nominally volume were used. constrained, Flax fiber whereby density approximately was 60% of glass the fibersame density, volume resultingof flax and in glass a total fiber flax reinforcementsfiber mass that were was 55%used. of Flax the totalfiber glassdensity fiber was mass. 60% However, of glass fiber the flax density, composite resulting took in up a moretotal flax resin fibersuch mass that that the resin was 55% used of in the the total glass glass composite fiber mass. was 90%However, of that the used flax in composite the flax composite took up more (the authors resin suchattributed that the this resin to used a small in the diff glasserence composite in the fiber was volume 90% of andthat aused resin in cavity the flax in composite the flax mold). (the authors The net attributedeffect was this that to the a small flax blade difference was 10% in the lighter fiber than volume the glass and a blade. resin Thecavity fiber in massthe flax fractions mold). of The the net flax effectand glasswas that composites the flax blade were was 28.2% 10% and lighter 44.3%, than respectively, the glass blade. which The are fiber somewhat mass fractions lower than of the those flax in andthe glass present composites study. The were structural 28.2% and testing 44.3%, of therespectively, blades indicated which are that somewhat the glass lower blade than had athose flexural in thesti ffpresentness 1.76 study. times The greater structural than testing that of of the the flax blades blade. indicated This is inthat contrast the glass to the blade present had a study flexural and stiffnesshighlights 1.76 the times note greater above than regarding that of the the di flaxfference blade. between This is making in contrast comparisons to the present using study equivalent and highlightsmass or equivalent the note above volume. regarding The authors the difference also noted between the flax bladesmaking had comparisons a highly non-linear using equivalent response, masswhile or the equivalent glass blade volume. was highly The authors linear, also which noted reflects the flax somewhat blades had the dia ffhighlyerent ductilitynon‐linear between response, flax whileand glassthe glass noted blade in the was present highly study. linear, The which failure reflects load ofsomewhat the glass the blade different was not ductility disclosed, between so ultimate flax andstrength glass noted comparisons in the present could notstudy. be made.The failure The authors load of the also glass noted blade the importancewas not disclosed, of investigating so ultimate the strengthcomposites comparisons at the application could not/ structuralbe made. The scale, authors rather also than noted limiting the importance analyzes to of data investigating extracted fromthe compositesmaterial coupon at the testing.application/structural scale, rather than limiting analyzes to data extracted from materialWith coupon regards testing. to potential structural applications for natural fiber composites, it is noted that in primaryWith structuralregards to applicationspotential structural in civil applications infrastructure, for both natural appropriate fiber composites, strength it and is noted ultimate that limit in primarystate ductility structural are required.applications Ultimate in civil limit infrastructure, state ductility both is especially appropriate important strength to provideand ultimate redundancy limit statein cases ductility of extreme are required. loading orUltimate overloading, limit whilestate largeductility deflections is especially prior to important the ultimate to limitprovide state redundancyprovide visible in cases indications of extreme of impending loading or overloading, failures. Thus while while large ultimate deflections strength prior is critical,to the ultimate ductility limit state provide visible indications of impending failures. Thus while ultimate strength is critical, Fibers 2020, 8, 62 13 of 14 of that state is also important. These considerations indicate that high ductility of the natural fiber composites in the present study lend themselves to primary structural applications, in such cases as when strength requirements are satisfied. It has been shown by the author in [18,19], that geometrically optimized structural channel sections of flax and jute have sufficient strength for light primary structural applications, such as in residential buildings. Conversely, in other structural applications, said ductility may make natural fiber composites inappropriate and may lend themselves to synthetic composites. In the wind turbine blade study [20], the authors noted the flax blade tip deflection was up to 2.7 times greater than that of the glass blade for the same load. While the flax blade passed the strength requirements, the higher tip deflection would need to be accommodated in the tower design.

5. Conclusions Direct comparisons between structural characteristics of synthetic fiber (glass and carbon) and natural fiber (flax, jute and hemp) composite structural members have been made experimentally for thin-walled channel sections in compression. It has been shown that natural fiber structural laminates are generally structurally equivalent to their glass fiber counterparts in equivalent fiber mass applications, with regards to compression strength and stiffness. Carbon fiber was superior to all other fiber types. The ductility of the structural members was notably different between synthetic and natural fiber composites, where the former was much less ductile than the latter, which may lend different composites to different types of structural applications. With regards to the specific issue of replacing glass fiber with natural fiber in structural compression applications, it has been demonstrated that, subject to the geometric limitations of the present experiments on thin-walled channels, structural equivalency may be generally achieved based on direct substitution with the same fiber mass.

Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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