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Article

Determination of the Mechanical Properties of Flax and Its Hybrid Flax/Carbon Composite Laminates with Vinyl Ester Resin for Wind Turbine Rotor Blades

by
Sriman Ram Marimuthu Rajendran
1,*,
Prem Anand Balakrishnan
2 and
Balasubramanian Visvalingam
3
1
School of Mechanical & Design Engineering, University of Portsmouth, Portsmouth PO1 3DJ, Hampshire, UK
2
Department of Manufacturing Engineering, Annamalai University, Chidambaram 608001, India
3
Department of Mechanical Engineering, Annamalai University, Chidambaram 608001, India
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(5), 229; https://doi.org/10.3390/jcs9050229
Submission received: 18 March 2025 / Revised: 22 April 2025 / Accepted: 26 April 2025 / Published: 2 May 2025
(This article belongs to the Collection Editorial Board Members’ Collection Series: Fibre Composite Materials)
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Abstract

:
In this research paper, the ±45 biaxially oriented woven flax and its hybrid flax/carbon composite laminates are manufactured by the vacuum bag technique using vinyl ester as the resin binder and the samples are characterized to evaluate their tensile, flexural and impact properties. Combining natural fibers with conventional materials typically creates a hybrid composite that shows optimal mechanical properties with partial sustainability. The flax/carbon variant exhibited superior tensile strength values of 383.88 MPa and 32.60 GPa, which are about 3.5 and 2.7 times higher than the flax composites, their flexural strengths are around 415.57 MPa and 25.02 GPa, respectively, and they have an impact resistance of 12.67 J.

1. Introduction

The current society depends on a diverse range of natural sources like wood, coal, oil, natural gas and nuclear materials for structural applications and energy generation. This has been the trend for the past century, which has resulted in the overconsumption of fossil fuels to compensate for the increasing population and civilization. The processing of coal and petroleum-based products releases greenhouse gases that pollute the environment. Thus, an ecological imbalance is created due to the rapid depletion of nonrenewable sources, which in turn puts us behind in terms of achieving a self-sustaining economy. To counterbalance this consumption, researchers started to look for potential sources of replenishable energy as well as sources that, when processed, aid in achieving a zero-carbon footprint. This in turn has led to a focus on renewable energy sources for energy generation [1]. Wind energy is one of the potential sources that can be relied upon for electricity production without causing any emission of greenhouse gases. Wind turbines harness wind energy by using three rotor blades, each of which are aerodynamically designed to capture wind energy and convert it to mechanical energy. However, its efficiency is dependent on several crucial factors, such as the blade dimensions, load parameters, wind velocity and the mechanical properties of the blades [2].
The materials used for manufacturing wind turbine blades are steel, fiberglass, iron and some metallic alloys incorporating cobalt, chromium and rhenium. This process involves the utilization of directional solidification technology, which involves making the whole blade as a single crystal to further enhance the mechanical properties. However, this advanced manufacturing technique is quite expensive and requires high levels of precision. Thus, the trend shifted to composite materials due to their enhanced mechanical properties of the synthetic carbon and glass fibers. However, after their service life, they are dumped in landfills or dumping areas, which creates immense waste generation. Besides this, they are non-biodegradable and their recycling process demands more energy [3]. The trend then begins to shift to a sustainable economy by encouraging researchers to utilize sustainable materials like natural fibers. These natural fibers are usually processed by physical and chemical means to increase their interfacial bonding and mechanical performance. The integration of conventional fibers with natural fibers promotes partial sustainability without compromising the mechanical performance. Their availability, biodegradability and easy processing make them stand out for structural applications. This renders hybrid composite laminates more suitable for diverse applications in the building, automotive and energy-generating sectors. Several numerical and experimental studies have been undertaken to understand their mechanical behavior and their potential benefits for structural applications like wind turbine blades [4].
This hybridization technique of mixing natural fibers with conventional fibers has created a knowledge vacuum with potential areas for future research. This, in turn, has pushed researchers to look for the optimal combination of materials concerning the stacking arrangement, number of plies, thickness of plies, fiber dimensions, matrix material, fiber orientations and so on. The mechanical properties of the composites directly depend on the orientation of the fibers due to their anisotropic behavior. Beyond this, the mechanical properties are also significantly influenced by the parameters mentioned above. Therefore, a good understanding of the composite materials and a thorough examination are needed to evaluate the mechanical performance before incorporating it effectively in structural applications [5]. Experimental investigations regarding the bidirectional woven flax fiber and carbon fiber epoxy composite laminates have been separately undertaken [6]. The research indicates that the flexural properties regarding strength and modulus values are around 8 and 12 times greater, respectively, than those of the flax fiber-reinforced epoxy composite. They have also revealed why it is necessary to incorporate hybrid composites for impact-related applications. A study on flax/carbon fabric fiber-reinforced epoxy composites demonstrated that the tensile strength rose to 344.14 MPa from 226.36 MPa when utilizing the vacuum bag technique as opposed to the hand layup method [7]. However, the micro hardness value remains close to around 200 Hv for both production methods.
The unidirectional hybrid bio-composite made of carbon and flax fibers with poly-butyl-succinate as resin binder showed that, substituting one layer of carbon with a flax resulted in superior mechanical performance [8]. The resulting tensile strength value is 500 MPa, with flexural strength and modulus values of 62 MPa and 4.5 GPa, respectively. Investigations have revealed that replacing one layer of flax ply with a carbon ply resulted in superior tensile strength and modulus values of about 266.4% and 262%, though the strain decreased to its failure rate. A comparable situation is noted for the flexural strength, which increases from 76.42 MPa to about 160.42 MPa, though the bending characteristics are impacted by the stacking arrangement of the hybrid composites [9]. One researcher used vinyl ester resin for conventional fiber-reinforced composites, which displayed increased tensile, flexural and hardness properties and showed improvements in interlaminate shear strength [10]. The influence of fiber orientations on the mechanical performance of banana, jute, hemp, kenaf and pineapple leaf fibers reinforced with epoxy demonstrated enhanced tensile, flexural and hardness characteristics at 450 orientations [11].
A comparative experimental study on natural and hybrid fiber-reinforced composites included flax/sisal, E-glass/flax/sisal and jute/coir epoxy composite laminates [12]. The findings indicate that natural fiber composites could potentially replace traditional materials due to their enhanced mechanical properties and ability to biodegrade. Their attributes encompass tensile strength, Young’s modulus, flexural strength, flexural modulus and impact energy values ranging from 37–57 MPa, 1.5–3.3 GPa, 57–89 MPa, 2.8–5.6 GPa and 20–52 KJ/m2, respectively. The researcher demonstrated that woven flax/carbon cross-ply composite laminates had superior impact performance regarding absorbed energy, penetration energy, fracture propagation and indentation regions when compared with their unidirectional counterparts [13]. Nonetheless, both variants exhibited improved impact characteristics compared with the flax fiber epoxy variant. This research study seeks to assess the mechanical performance of biaxially orientated woven flax and its hybrid variant flax/carbon composite laminates with vinyl ester resin as the matrix material. They are biaxially orientated and progressively arranged in a ±45-degree direction, one above the other. The mechanical characteristics to be assessed include tensile, flexural and low-velocity impact properties of the composite laminates by using appropriate test equipment.
This research contributes to the development of hybrid composites by combining sustainability for structural applications with the mechanical characterization of flax fiber-reinforced vinyl ester composites. Additionally, we seek to provide novel information about which composites may be best suited for the rotor blade of a wind turbine, in turn helping design engineers to choose appropriate materials for blades.

2. Methodology

2.1. Properties of Wind Turbine Blades

The blade is the essential component of a wind turbine; therefore, the material utilized for wind turbine blades must possess all requisite attributes to operate efficiently and effectively in its service environment. The required properties of the wind turbine blade are as follows:
-
High strength: To support the blades from aerodynamic wind loads, pressure loads, inertial loads and gravitational loads.
-
High modulus: To ensure the blade has sufficient rigidity to prevent any kind of deformation during loading, thereby maintaining dimensional stability and structural integrity.
-
Impact resistance: To ensure the blade has optimal crack resistance during collision of debris or birds.
-
Lightweight: To reduce the effect of the weights of blades acting on the tower in the form of gravitational load.
-
Fatigue resistance: To withstand the cyclic loading conditions provided by the combination of all of the loads for around 108 cycles i.e., for more than 20 years].
-
Corrosion resistance: To prevent degradation of the blade due to any oxidation reaction or photochemical reaction when exposed to the atmospheric ambiance.

2.2. Composite Material Specification

The matrix and fiber materials used in this research for fabricating the composite laminate are vinyl ester resin and woven fiber layers of flax and carbon and are unidirectional. The biaxial woven fiber fabric layers are procured from Net Composites Ltd. Vinyl ester is used as the resin binder due to its superior properties to polyester and epoxies, which are the two most used matrix materials for producing composite laminates. The vinyl ester resin exhibits greater strength compared with polyester and offers enhanced resistance relative to epoxy, positioning it as a hybrid-like resin binder. The fiber layers are ±45 biaxially oriented for all of the composite types, with a total of 7 lamina. The flax (0.25 mm thickness) and carbon fiber (0.2 mm thickness) layers are 3 and 4, respectively. The resin content ratio is 0.45 and the density of carbon and flax fiber is 1.78 g/cm2 and 1.51 g/cm2.
The hybrid composite flax/carbon is sequentially stacked with alternating flax and carbon layers, with the carbon layer positioned as the outermost layer on both sides to enhance resistance to deformation under mechanical loads. Figure 1 illustrates the graphical representation of the stacking sequence for both flax and hybrid flax/carbon composites. For flax material samples 1, 2 and 3, the volume fractions of reinforcement are 29, 31 and 39, the volume fractions of resin are 2, 4 and 6, and the volume fractions of resins are 1.68, 1.72 and 1.81, respectively. For flax/carbon samples 1, 2 and 3, the volume fractions of reinforcement are 28, 32 and 27, the volume fractions of resin are 14, 15 and 13, and the volume fractions of resins are 2.93, 2.6 and 2.75, respectively. Table 1 indicates the mechanical properties of fibers.
Flax laminate ([0°/90°]) and the symmetric lay-up of the plain weave flax fabric (e.g., [0°/90° flax/0° carbon/90° flax]) are two examples of architecture types. Depending on the required flexural performance, the carbon layer or layers may be positioned centrally or close to the tensile/compressive faces.
The UD carbon fibers offer the necessary mechanical strength and stiffness for structural loads, while the flax fibers’ inherent damping properties and low weight are utilized in this hybrid design.

2.3. Resin System

A vinyl ester resin system (e.g., Derakane 411–350) was used because of its superiority in mechanical properties, corrosion-resistance and its appropriateness for use with wind blades. The resin contains a cobalt-based accelerator (such as cobalt naphthenate) and a catalyst (usually methyl ethyl ketone peroxide, or MEKP). The vinyl ester resin (100 parts by weight) and MEKP catalyst (curing agent) are mixed in a ratio of 1.5 to 2.0 parts by weight. Pre-curing took place at about 25 °C for 24 h, while post-curing took place at 80 °C for 2–3 h.

2.4. Manufacturing and Sample Preparation

The vacuum bag technique is used for manufacturing the flax and flax/carbon composite laminates. This technique involves forcing the resin into the fiber layers by the use of vacuum pressure. The manufacturing setup typically consists of a resin inlet, vacuum outlet, mold, releasing agent, sealant tape and a resin trap. The mechanical performance of the composite laminates depends on the type of manufacturing technique used. This technique is capable of fabricating composite laminates for high-performance applications. Some of the benefits of using the vacuum bag technique are a better fiber-to-resin ratio, a lower void content, good resin utilization, a better consistency and repeatability [14,15]. The composite laminates produced by using the vacuum bag technique are shown in Figure 2. The main process parameters used are a vacuum pressure of less than 20 Mbar, a mold temperature of 100–120 °C, a resin injection temperature of around 60–80 °C, and a post-curing temperature of about 180 °C. The process was undertaken for 3 h to achieve a low void content along with improved matrix–fiber interfacial bonding properties [16,17].
The fabricated composite laminates after curing are shown in Figure 3. An industrial infrared thermometer integrated with circular laser technology is used to monitor the temperature during the whole process. Then, the composite laminates are cut into samples by using a water jet cutting machine, as they cannot be processed by conventional cutting machines due to their delicate surface texture. The main reason for this is the formation of microcracks acting as local stress raisers during loading, which can cause the composite material to fail before reaching its true strength. Thus, water jet cutting is used for processing the composite laminates due to its high precision and high dimensional accuracy. The samples are then prepared according to their respective ASTM standard dimensions and are made ready for mechanical testing.The material specifications are represented in Table 2.

2.5. Experimental Procedure

The mechanical characterization is performed to ascertain the mechanical behavior and specific properties of the composite material when subjected to load in an ambient environment, such as a temperature of 23 ± 2 °C and a relative humidity of 50 ± 5% RH. The strength, modulus and impact properties of the flax and its hybrid flax/carbon composites are assessed using tensile, flexural and low-velocity impact testing.

2.5.1. Tensile Test

The tension testing is typically conducted on a universal testing machine (UTM) with a tensile clamping system. The UTM Zwick/Roell Z030, ZwickRoell GmbH & Co, Ulm, Germany, with a load cell capacity of 1 kN, is used for performing the required tensile test. The samples for the tensile test are prepared to adhere to the ASTM standard D3039, with the utilization of aluminum tabs to prevent any kind of slippage or damage on the surface in case of strong gripping. The specimen’s dimensions are illustrated in Figure 4 and the tests are conducted according to the standard procedure, utilizing a gauge section length of approximately 100 mm with a 2 mm/min crosshead speed. Then, the tensile properties of the composite laminates are determined. Due to clamping pressure, grip and tab detachment tensile test specimens vary from pre-test and post-test.

2.5.2. Flexural Test

The bending characteristics of the composite materials are assessed by performing a three-point flexural test on the universal testing machine (UTM) Zwick/Roell Z030, ZwickRoell GmbH & Co, though with a different clamping system consisting of two support nodes and a loading node. The flexural test specimens are prepared as per the ASTM D7264 standard and their respective dimensions are presented in Figure 5. The tests are processed as per standards, with a support span of 16:1, and the flexural strength and modulus are then evaluated.

2.5.3. Low-Velocity Impact Test

The low-velocity impact load tests for the flax and its hybrid variant are conducted with a drop weight testing system (DWTS) Instron, USA, equipped with a force sensor. The impact responses of the composite samples are assessed at ambient temperature and the test specimens are fabricated with the guidelines mentioned in the ASTM standard D7136, with dimensions illustrated in Figure 6. A load impact system features a 20 mm diameter hemispherical tip integrated with a 5 kg punch mass which imparts an impact energy of 50 J. Then, impact tests are performed and the data with respect to time are graphed to evaluate the impact energy, absorbed energy and rebound energy of the test samples.

3. Results and Discussions

3.1. Tensile Properties

The test data for both the flax and its flax/carbon samples are taken from the computer-controlled universal testing machine by using TestXpert II software, version V3.2, ZwickRoell GmbH & Co, Germany. The composite samples before and post-testing are shown in Figure 7, and the cross-sectional area and thickness of the samples are measured.
Then, the tensile strength and its respective Young’s modulus for all six samples are calculated and presented in Table 3. The average and standard deviation are computed to see the data variations in order to ensure the experimental procedure carried out was consistent and repeatable. Then the average deformation plot for tensile property is shown in Figure 8 for both samples is graphed together for comparative study in order to assess the deformation behavior of the composite materials during tensile loads.
From the tested samples, it is evident that the main failure mode in the flax (F) and the flax/carbon (FC) composite is due to the shear stress exerted by tensile load along the ±45 biaxial orientation of fibers in the composite laminates. However, both samples showed brittle failure which can be quite evident from the deformation plot and the post-test samples. The deformation curve usually consists of two parts: the linear and the non-linear part. The linear segment pertains to the elastic deformation of the composite materials, exhibiting some elastic recovery, whereas the non-linear segment signifies the elastic–plastic deformation of the composite samples beyond the yield point [18,19]. The experiment revealed that, in ±45 biaxial orientation, the flax/carbon composite laminate showed a peak tensile strength and Young’s modulus higher than the flax composite’s values of 383.88 MPa and 32.60 GPa, respectively, shown in Figure 9. Despite the brittle nature of both the composites, the flax/carbon composite showed subtle delamination, with the outer layers curved slightly and the flax composite laminate showed a clear failure in the shear direction.
It is evident from the deformation plot that hybridizing carbon with natural fiber flax moderately increased the strain value, thereby demonstrating a favorable hybrid effect, which can be seen on the non-linear part of the loading curve. This increase in strain rate is due to the pseudo-ductility phenomenon, despite carbon fibers being known for their specific toughness [20]. The experimental analysis concluded with flax/carbon composites showing superior properties in tensile strength as well as Young’s modulus of about 3.5 times and 2.7 times higher than the flax composite samples. The findings suggest that combining flax fibers with traditional carbon fibers results in enhanced tensile properties when assessed against the pure flax composite alternative.

3.2. Flexural Properties

The flexural data derived from the performance of the three-point bending test are taken from the TestXpert II software, version V3.2, ZwickRoell GmbH & Co, Germany, integrated with the computer-controlled universal testing machine. The samples post-test is shown in Figure 10 and the support span length for both the flax and flax/carbon samples are calculated. Then, from the individual deformation plot, the flexural strength is computed and the flexural modulus is evaluated using the slope at the deformation curve in order to study the bending nature of the composite samples, which are presented in Table 4. Then, the average and standard deviations are calculated to ensure data consistency and process repeatability [21,22,23]. Finally, the average deformation plots for both the composite samples are graphed together to study the mechanical behavior of the composites under the influence of bending loads, as shown in Figure 11.
In the three-point flexural test, the flax (F) and the flax/carbon (FC) composites failed at low strains, which was accompanied by matrix fiber cracking at the tension side of the composite samples. The flax/carbon composites displayed low levels of delamination on the outer layers of the tension and compression side. From the deformation plot, it is evident that the hybrid flax/carbon composites failed at low strain values relative to the flax composite, despite the higher bending load-bearing capability of the flax/carbon composite laminates. The integration of flax fibers and carbon fibers along the biaxial direction caused the material to fail at relatively low strains. However, the flax/carbon composite laminate displayed an improvement in the flexural strength and modulus, with values of 415.57 MPa and 25.02 GPa, respectively. The arrangement of the synthetic layer of carbon on the exterior of the biaxially oriented composite laminates resulted in enhanced flexural resistance when subjected to bending load [24]. This result can be accompanied by the better interfacial bonding characteristics between the matrix and fibers achieved by post-curing after vacuum bagging. From Figure 12, it can be confirmed that the hybrid flax/carbon composites displayed about 4.1 times flexural strength and 3.8 times flexural modulus relative to the flax composite variant. Nevertheless, this experimental study shows that hybridizing conventional fibers like carbon with natural fiber flax exhibits superior flexural performance when compared with flax composites. The main theory backing this is the combined stress-withstanding capacity provided by both the flax and carbon fibers [25,26,27,28,29,30].

3.3. Low-Velocity Impact Properties

The impact data to be processed are recorded by the computer-controlled drop weight testing system, which is then extracted by the TestXpert II software, version V3.2, ZwickRoell GmbH & Co, Germany. The composite samples before and after being subjected to the impact load are shown in Figure 13 and the force, displacements, energy and time data for both the flax (F) and flax/carbon (FC) composite samples are then graphed, as shown in Figure 14, for further evaluation. Using these data, the maximum displacement, maximum force and maximum energy are computed and presented in Table 5. Then, from the energy data and other plotted graphs, absorbed and rebound energies are evaluated and are displayed in Table 6. Then, the average and standard deviations are calculated for both the composite samples to make sure the experimental procedure is consistent and repeatable [31,32,33,34,35].
The deformation plot shows the shock wave experienced by both the flax (F) and flax/carbon (FC) composite samples when struck with an impact force of 50 J energy, which is represented by the curve. This also indicates the onset of penetration exerted by the drop weight during the impact test. The analysis of the impact behavior is achieved by splitting the force-displacement curve into two stages to analyze the damage behavior in terms of energy dissipation mechanisms. The initial load drop is represented by the potential damage caused in the form of delamination or matrix cracking, whereas the second load drop represents the diverse failure mechanisms, such as delamination, debonding, matrix cracking, fiber fracture and fiber pull-out [36]. The key parameters used to evaluate the impact performance are peak force, maximum displacement and absorbed energy. The flax composite laminate showed a lower peak force relative to its hybrid flax/carbon composite. When examining the energy characteristics data shown in Figure 15, it becomes evident that the flax composite laminate showed a superior energy absorption capacity of 44.76 J. Despite its 33.53% higher energy absorption than the flax/carbon composite, it typically fails at lower peak loads.
Conversely, the hybrid flax/carbon composite exhibited a greater peak load force, indicating that a higher impact force is necessary to initially damage its corresponding samples. Both the flax and flax/carbon composites subjected to visual inspection exhibited matrix cracking and fiber fracture under impact loads. For flax composite laminates, the extent of the damage is more apparent due to the higher levels of indentation depth observed in its samples. This shows that, though flax has better energy absorption properties in biaxial orientation, it has weak resistance to impact force. By evaluating the energy characteristics, it is clear that flax/carbon has extremely high rebound energy values, which are approximately 14 times higher than the flax composite laminates. This is also accompanied by subtle delamination, with the little-to-no indentation indicating a superior toughness to impact loads. The hybridization of flax with conventional carbon fibers resulted in an enhanced composite laminate that displayed better impact properties than the flax composite in the biaxial orientation [37,38].

4. Conclusions

The integration of sustainable materials with synthetic fibers leads to the formation of hybrid composites, which yield enhanced mechanical properties while incorporating elements of sustainability. This experimental study was undertaken in order to characterize the mechanical performance of ±45 biaxially oriented flax and its hybrid flax/carbon composite laminates regarding tensile, flexural and low-velocity impact properties. The important conclusions from this research work can be summarized as follows:
  • The tensile test results show that the flax/carbon variant exhibited superior tensile strength and excellent Young’s modulus values of 383.88 MPa and 32.60 GPa, respectively, which are about 3.5 and 2.7 times higher than the flax composites.
  • In the flexural test, the hybrid flax/carbon composites displayed superior bending performance compared with the flax composite, with flexural strength and flexural modulus values of around 415.57 MPa and 25.02 GPa, respectively.
  • The low-velocity impact test showed that flax/carbon composite has excellent impact resistance, with a higher rebound energy of 12.67 J. The flax composite possesses excellent energy absorption capacity despite failing at lower peak loads.
The experimental results show the potential increase in the mechanical properties of hybridizing natural fiber flax with conventional carbon fibers relative to the flax composite variant. Apart from this, the resulting ±45 biaxially orientated woven flax/carbon hybrid composite has partial sustainability due to the presence of natural flax fibers. This makes the flax/carbon composite a suitable material for wind turbine blade applications i.e., for blade diameters greater than 80 m and a power generation capacity of about 5–10 MW. Future study is planned for material optimization, numerical modeling and simulation, scale-up and application and sustainability and recycling.

Author Contributions

S.R.M.R.: Conceptualization, methodology, validation, formal analysis, investigation, resources, data curation, writing- original draft preparation. P.A.B.: writing—review and editing, supervision. B.V.: funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

All data’s are available within the article.

Acknowledgments

The authors gratefully acknowledge Geoff Britton and Will Keeble for technical assistance with performing experiments at the University of Portsmouth.

Conflicts of Interest

The authors declare no conflict of interest among them.

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Figure 1. Graphical representation of the stacking sequence.
Figure 1. Graphical representation of the stacking sequence.
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Figure 2. Vacuum bag method.
Figure 2. Vacuum bag method.
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Figure 3. Manufactured samples. (a) Flax and (b) flax/carbon composites.
Figure 3. Manufactured samples. (a) Flax and (b) flax/carbon composites.
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Figure 4. ASTM D3039 dimension with Al tabs.
Figure 4. ASTM D3039 dimension with Al tabs.
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Figure 5. ASTM D7264 dimension.
Figure 5. ASTM D7264 dimension.
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Figure 6. ASTM D7136 dimension.
Figure 6. ASTM D7136 dimension.
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Figure 7. Tensile test samples. (a) Pre-test and (b) post-test.
Figure 7. Tensile test samples. (a) Pre-test and (b) post-test.
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Figure 8. Cumulative average deformation plot for tensile property.
Figure 8. Cumulative average deformation plot for tensile property.
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Figure 9. Tensile characteristics. (a) Tensile strength and (b) Young’s modulus.
Figure 9. Tensile characteristics. (a) Tensile strength and (b) Young’s modulus.
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Figure 10. Flexural test samples. (a) Pre-test and (b) post-test.
Figure 10. Flexural test samples. (a) Pre-test and (b) post-test.
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Figure 11. Cumulative average deformation plot for flexural property
Figure 11. Cumulative average deformation plot for flexural property
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Figure 12. Flexural characteristics. (a) Flexural strength and (b) flexural modulus.
Figure 12. Flexural characteristics. (a) Flexural strength and (b) flexural modulus.
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Figure 13. LVI samples. (a) Pre-test and (b) post-test.
Figure 13. LVI samples. (a) Pre-test and (b) post-test.
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Figure 14. Impact Performance. (a) Deformation plot and (b) energy characteristics curve.
Figure 14. Impact Performance. (a) Deformation plot and (b) energy characteristics curve.
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Figure 15. Impact characteristics. (a) Absorbed energy and (b) rebound energy.
Figure 15. Impact characteristics. (a) Absorbed energy and (b) rebound energy.
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Table 1. Comparison of mechanical properties of flax fiber and carbon fiber.
Table 1. Comparison of mechanical properties of flax fiber and carbon fiber.
S.NOPropertyFlax FiberCarbon Fiber
1Density (g/cm3)1.4–1.51.75–1.95
2Tensile strength (MPa)500–11003500–5500
3Young’s modulus (GPa)30–60230–600
4Moisture absorption (%)High Very low
5Thermal stability (°C)Degrades above ~200 °CStable above 400 °C
6SustainabilityRenewable, biodegradableNon-renewable
Table 2. Material specifications and the manufacturer.
Table 2. Material specifications and the manufacturer.
MaterialManufacturerForm/WeaveTensile Strength
(MPa)		Young’s Modulus (GPa)
Flax fabricNet composites Ltd. (Chesterfield, UK)Plain weave500–90050–70
Carbon fiberAeron composites pvt Ltd. (Ahmedabad, India)Unidirectional (UD)~4900~230
Vinyl ester resinINEOS Composites (London, UK)Liquid resin~75
(resin only)		~3.2
Table 3. Tensile characteristics of the composite specimens.
Table 3. Tensile characteristics of the composite specimens.
MaterialSample NoMaximum Force
(N)		Strain
(mm)		Tensile Strength (MPa)Young’s Modulus
(GPa)
Flax (F)17737.770.0885.828.51
28163.820.0991.039.23
37299.090.0779.948.17
Average 7733.560.0885.568.63
Std. deviation 0.014.570.44
Flax/carbon (FC)19183.720.12382.2132.42
29543.410.14390.5633.53
38761.750.10378.8931.87
Average 9162.960.12383.8832.60
Table 4. Flexural properties of the composite specimens.
Table 4. Flexural properties of the composite specimens.
MaterialSample No.Max Force
(N)		Max Displacement (mm)Flexural Strength
(MPa)		Flexural Modulus
(GPa)
Flax (F)1303.7112.26105.216.51
2295.3312.0194.185.92
3310.0712.67109.536.88
Average 303.0312.31102.976.43
Std. deviation 0.336.460.39
Flax/carbon (FC)1359.029.09423.1125.83
2320.306.69408.5724.28
3341.406.97415.0524.97
Average 340.247.58415.5725.02
Std. deviation 1.317.280.77
Table 5. Peak force, energy and displacement data.
Table 5. Peak force, energy and displacement data.
MaterialSpecimen No.Max. Force (N)Max. Energy (J)Max. Displacement (mm)
Flax (F)15808.9745.4720.47
24955.2245.6022.07
35555.1645.4820.53
45220.5846.1722.49
Average 5384.9845.6821.39
Std. deviation 0.331.04
Flax/carbon (FC)113,942.7046.3217.67
213,735.0346.3317.79
313,619.6646.0517.63
413,833.1046.0517.55
Average 13,782.6246.1917.66
Std. deviation 0.160.10
Table 6. Energy characteristics data for the composite specimens.
Table 6. Energy characteristics data for the composite specimens.
MaterialSpecimen No.Absorbed Energy (J)Rebound Energy (J)
Flax (F)144.251.22
244.860.74
344.331.15
445.620.54
Average 44.760.91
Std. deviation 0.630.32
Flax/carbon (FC)133.0513.28
234.3411.99
333.2712.78
433.4212.62
Average 33.5212.67
Std. deviation 0.560.53
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MDPI and ACS Style

Rajendran, S.R.M.; Balakrishnan, P.A.; Visvalingam, B. Determination of the Mechanical Properties of Flax and Its Hybrid Flax/Carbon Composite Laminates with Vinyl Ester Resin for Wind Turbine Rotor Blades. J. Compos. Sci. 2025, 9, 229. https://doi.org/10.3390/jcs9050229

AMA Style

Rajendran SRM, Balakrishnan PA, Visvalingam B. Determination of the Mechanical Properties of Flax and Its Hybrid Flax/Carbon Composite Laminates with Vinyl Ester Resin for Wind Turbine Rotor Blades. Journal of Composites Science. 2025; 9(5):229. https://doi.org/10.3390/jcs9050229

Chicago/Turabian Style

Rajendran, Sriman Ram Marimuthu, Prem Anand Balakrishnan, and Balasubramanian Visvalingam. 2025. "Determination of the Mechanical Properties of Flax and Its Hybrid Flax/Carbon Composite Laminates with Vinyl Ester Resin for Wind Turbine Rotor Blades" Journal of Composites Science 9, no. 5: 229. https://doi.org/10.3390/jcs9050229

APA Style

Rajendran, S. R. M., Balakrishnan, P. A., & Visvalingam, B. (2025). Determination of the Mechanical Properties of Flax and Its Hybrid Flax/Carbon Composite Laminates with Vinyl Ester Resin for Wind Turbine Rotor Blades. Journal of Composites Science, 9(5), 229. https://doi.org/10.3390/jcs9050229

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