Influence of irregular fibre surface and debonding on the elastic properties of jute/epoxy composites: Micromechanics and finite element approaches

ABSTRACT This study examines how irregular surfaces and debonding affect jute/epoxy composites. The study used micromechanics and finite element (FE) analysis to investigate properties such as elastic modulus in the longitudinal (E1) and transverse (E2) directions, major (ν12) and minor (ν21) Poisson’s ratios, and interfacial stresses (σ1, σ2, and τ12, τ23, τ13). The FE models were validated using experimental and analytical results, which showed good agreement. Then, the FE model was extended to analyse the influence of different fibre volume fractions (Vf) on jute/epoxy composites with varied irregular surfaces (IRS%) and debonding (DBS%). The interfacial stress was compared across these variables. DBS% caused significant variation in E2 and σ2, while IRS% led to out-of-shear stresses that crossed the threshold. An increase in IRS% and DBS% at a constant fibre volume fraction did not significantly affect E1. However, increasing Vf from 10–70% increased E1 by 168%. E2, on the other hand, decreased with Vf by 63–68%. Both IRS% and DBS% had a significant influence on interfacial stresses. GRAPHICAL ABSTRACT


Introduction
Numerous industries nowadays focus on natural fibre-reinforced composites (NFCs) as a replacement for manufactured fibre-reinforced composites due to their lightweight nature, sustainability, energy efficiency, low cost, and recyclability [1].The performance of NFCs, much like synthetic fibre-reinforced composites, is governed by their microstructure, orientation, distribution, and volume fraction [2,3].However, in addition to these factors, the performance of natural fibres is also influenced by their components, waviness, cross-sectional splits over the fibre, and irregularities [4,5].Unlike synthetic fibres, the characteristics of natural fibres do not allow for a consistent cross-section and a smooth surface [6].
Micromechanics is crucial in designing and optimizing composite structures by considering the relationship between the microstructure and macroscopic mechanical properties.This can be done through either analytical or computational micromechanics [7][8][9][10][11].Using these models makes it possible to predict the composite material properties.Many analytical methods have been implemented for this purpose, including the Rule of Mixtures, Voigt model, Reuss model, Halpin-Tsai equations, Mori-Tanaka method, and generalized self-consistent method [12,13].The Rule of Mixtures is a simplified method that involves averaging the individual constituents' mechanical properties based on each constituent's volume fraction contribution [14,15].The Voigt and Reuss models can be used to address the objectives of micromechanics studies, assuming materials are either elastically isotropic (Voigt model) or incompressible (Reuss model) [12,16].To estimate the effective mechanical properties of composites, Voigt and Reuss models are used based on constituent properties and volume fractions [17].The Halpin-Tsai equations [16][17][18] are an extension of the Rule of Mixtures, which considers the orientation of the fibre and its geometrical aspects.Incorporating the methods mentioned above leads to more accurate predictions than the rule of mixtures.The Halpin-Tsai and Mori-Tanaka methods consider the effects of fibre geometry, orientation, shape, and distribution on the elastic properties of the resulting composites [19][20][21].The Generalized Self-Consistent Method uses the concept of self-consistency to estimate the effective mechanical properties.It assumes that the composite can be divided into small sub-volumes, and the mechanical properties of these sub-volumes are averaged to determine the effective properties of the composite [22].Another approach to predict the elastic properties of composite materials is through numerical studies, which consider the complex microstructures of the constituents that cannot be analyzed using analytical methods.These methods can handle the complex microstructure and their impact on the material behaviour [20,21].
The finite element (FE) method is used to analyse natural composites.It involves dividing a complex geometry into smaller and simpler elements and solving each element's behaviour with mathematical equations [23].However, the limitations of micromechanics can be overcome by multi-scale modelling methods.This method combines numerical techniques to simulate composite materials at different length scales.For instance, FEA can simulate composite material behaviour at the microscale [23][24][25].
Tombolato et al. [26] conducted a study to investigate irregularly shaped tubules' effect on composite materials' elastic modulus.The researchers discovered that the longitudinal and transverse toughness of the material remained uniform.However, the material exhibited notably higher toughness under compression testing in the radial direction.This difference was attributed to delamination of the lamellae, which was identified as the primary failure mode in both the longitudinal and transverse directions.The porosity with irregular shapes significantly affected the resulting composite's elastic modulus.Drach et al. [27] conducted another study that revealed how crack-like, irregularly shaped pores with negligible volume fractions reduce the overall stiffness of carbon/carbon composite material.Meanwhile, Qian et al. [28] found that sisal fibres have rough surfaces, and the length of these irregular surfaces directly impacts the debonding stress.Fuentes et al. [29] studied the interaction between natural fibre and matrix composite and concluded that it plays a crucial role in the failure mechanism.The dominant interfacial and normal radial stresses at the interface can initiate the crack.The delamination factor reduces the overall strength of the natural composite.These defects act as stress concentrators, and failure occurs at this zone due to localized crack propagation [30].
Several studies have shown that the numerical or computational micromechanics approach is ideal for determining composite materials' elastic properties and interfacial stresses.The surface roughness of natural fibres affects the transformation of interfacial stresses, while debonding between the natural fibre and matrix increases stresses at the interface, which can cause failure.However, there needs to be more research on the impact of natural fibre roughness on elastic properties and interfacial stresses.Additionally, the effects of debonding between the constituents of natural fibres on the composite material's overall performance have not been adequately studied.Furthermore, a comparative analysis is needed to determine the effects of natural fibre surface irregularities and debonding on elastic properties and interfacial stresses.This research addresses these gaps by evaluating and comparing the effects of irregular surface features and debonding areas on the elastic properties and interfacial stresses using micromechanics and FE methods.
Jute fibres are prone to irregularities caused by various factors during cultivation, processing, and handling.These inconsistencies can cause differences in texture, appearance, and fibre properties.Several types of irregularities are found in jute fibres, such as knots, bumps, tangles, hairiness, neps, frayed ends, thick-and-thin sections, splices, twists, and turns.Previous research has shown that natural fibres have non-uniform cross-sections that differ significantly from ideal circular shapes [31].The irregular nature of jute fibres is visible in scanning electron microscope (SEM) images.SEM photographs of different jute fibres demonstrate the changes in their surface features [32].
This study proposes an approach to evaluate the influence of irregular surface (IRS) and debonded surface (DBS) on the elastic properties and interfacial stresses of jute/ epoxy composites.Comparative studies were conducted to assess the impact of these two defects on the mechanical properties such as longitudinal elastic modulus (E 1 ) and transverse elastic modulus (E 2 ), major (ν 12 ) and minor (ν 21 ) Poisson's ratios, and interfacial stresses (σ 1 , σ 2 , and τ 12 , τ 23 , τ 13 ).

Manufacturing
The jute fibres, epoxy resin (LY556), and compatible hardener were provided by Vruksha Composites, based in Tenali, Andhra Pradesh, India.To create an epoxy resin matrix reinforced with jute fibres, the weight fraction was maintained at 50%.8.15 g of jute fibres were placed in a mould using the hand lay-up technique, and the resin was poured over the fibres.The mixture was then cured for 24 hours.The density of the jute fibre and epoxy was 1.30 g/cm 3 and 1.40 g/cm 3 , respectively.The fibre volume fraction was approximately 52%, and the thickness of the laminate was 2.50 mm.
Figure 1 illustrates the difference in straightness between natural jute and synthetic glass fibres.As seen in Figure 1a, the jute fibre appears rough, with a high deviation in straightness.In contrast, Figure 1b shows the glass fibre having high straightness, with all fibres appearing straight and parallel.This occurs due to the controlled manufacturing process parameters for glass fibres, such as diameter, length, and other factors.The jute fibres were combed along the length of the fibres in the reverse direction to create irregularities on jute fibres, as shown in Figure 2.

Tensile test
Tensile tests were conducted according to ASTM D3039, using a universal tensile testing machine (load cell 20kN and speed 2 mm/min) available at Prasad V Potluri Siddhartha Institute of Technology, Vijayawada, Andhra Pradesh, India (Figure 3).The specimen ends were clamped with grips.A uniaxial tensile force was applied along the longitudinal axis of the specimen until it failed.Mechanical properties such as tensile modulus, tensile strength, % elongation, and others were evaluated through the load-displacement response.To ensure the accuracy of the results, five specimens were tested, and their average values were presented.

Micromechanics
The micromechanics approach involves dividing a composite material into known unit cells with a representative volume.This ensures a uniform distribution of fibres within the matrix material.In this approach, a one-eighth portion of the unit cell is commonly analysed due to its symmetry in geometry, loading, and boundary conditions.This is referred to as the reduced symmetry approach [33,34].

Longitudinal modulus: (E 1 model)
All the constituents present share the load acting on the RVE: Where F C ; F f F m are the forces acting on the composite; fibre; matrix; respectively: Where σ C ; σ f ; σ m are the stresses and A C ; A f ; A m are the cross-sectional areas of composite, fibre, and matrix, respectively.Using Hooke's law, the stress is directly proportional to the strain: Substituting the Eq. ( 3) in Eq. ( 2): E 1c ; E 1f ; E m are the longitudinal moduli and ε 1c ; ε 1f ; ε m are the strains of the composite, fibre, and matrix, respectively, under longitudinal directional loading.
Under the condition of a perfect bond between the constituents of the fibre and the matrix, the strain generated in the RVE is equal to the strain in the fibre and the strain developed in the matrix: The equation ( 4) becomes: Where V f and V m are the volume fractions of fibre and matrix, respectively.

Transverse modulus: (E 2 model)
The transverse modulus was obtained from the RVE subjected to transverse loading.The transverse elongation under the applied load equals the transverse extension generated in all the constituents, such as fibre and matrix.Again, jute fibre is considered with lumen, lignin, hemicellulose, and cellulose, thus: Δ C ; Δ f ; Δ m are transverse deformation of RVE, fibre and matrix, respectively.
Using the strain in the above is modified as: w c ; w f and w m are the width fraction of the composite, fibre and matrix, respectively.The ε 2C is obtained by following equations: Using the relation between the strain and stress in terms of modulus in the respective directions, the Eq. ( 11) becomes: σ 2C ; σ 2f ; σ m are the stresses in the composite, fibre and matrix under transverse loading, respectively.
After applying the assumption of Eq. ( 13), the Eq. ( 12) becomes: Substituting the corresponding values of fibre constituents, matrix elastic modulus and their percentage, in Eq. ( 7) and Eq. ( 14), the longitudinal modulus, and transverse modulus of the will be estimated, respectively.E 2c ;E 2f ; E m are the Young's moduli of the composite, fibre, and matrix in transverse direction, respectively.

Finite element analysis
Numerical analysis was conducted using the FE-based commercial software ANSYS.The geometrical details required for the study were determined based on the volume fraction of the unit cell.Once the unit cell dimensions were fixed, the radius of the fibre was calculated according to its volume fraction.Symmetry boundary conditions were applied to three sides (x = 0, y = 0, z = 0) of the model, while the RVE model comprised 100 units [35][36][37].The fibre with an irregular surface was modelled by varying the percentage from 20% to 80% at different fibre volume fractions (ranging from 10% to 70%) considered for the analysis.The representation of the IRS% and DBS% are in Figure 4. Figure 5 shows the FE mesh of different IRS percentages at varying fibre volume fractions, DBS between the fibre and matrix.The model was discretised into small elements using the SOLID186 element.Target 170 and Contact 174 were used to create debonding elements to simulate the separation of the fibres from the matrix.After creating the FE models, conducted a mesh convergence test to ensure that the results were not sensitive to the size of the FEs used.Mesh size was refined until the results converged on a consistent value.For this investigation, a fixed element size of 2 mm was used, resulting in 106,215 elements for 20% V f and 40% DBS in the FE Model.There was no discernible effect on the outcome by changing the element size beyond this threshold, except for increased computing time.As a result, fixed the element size to 2 mm since it produced satisfactory results while maintaining computational efficiency.The final output included various mechanical properties of the composite material studied.To run the simulation, jute/epoxy properties were adopted from Ref [38].The principal material directions in the jute/epoxy composite were aligned to the global coordinate system by aligning 1-direction (longitudinal or fibre direction) with the Z-axis, 2-direction (Transverse direction) with the X-axis, and 3-direction (Transverse direction) with the Y-axis.Figure 5 shows the irregularity of natural fibres and debonding surfaces expressed as a percentage value ranging from 20% to 80%.For instance, if the irregularity of a natural fibre is 40%, the fibre deviates from a straight curve by an average of 40% of its diameter.Similarly, if the debonding surface is 60%, 60% of the fibre-matrix interface is not bonded.
The composite material was analysed using FE simulation to determine the deformation and stress distribution under longitudinal or transverse loading.The elastic properties obtained from the FE models were validated against micromechanical model results and then used to evaluate interfacial stresses.These stresses offer insights into the mechanism of load transfer between fibres and the surrounding matrix, making it possible to optimize the fibre-matrix interface for improved mechanical properties.Further, the elastic modulus was predicted for perfect and defective fibre-reinforced composite.The elastic modulus was predicted by implementing irregular fibres into FE models and changing the fibre volume fraction from 10 to 70%.The micromechanical models consider the effect of fibre irregularities and debonding on the overall composite properties.

Results and discussion
This section presented the first experimental results, followed by FE validation.In addition, the study presented the changes in mechanical properties based on variations in V f , IRS%, and DBS%.

Tensile properties
The stress-strain response of jute/epoxy composites with regular and irregular fibres is shown in Figure 6.The results show that the composite with regular fibres takes load gradually without significant fluctuations, unlike the composite with irregular fibres that exhibited a non-gradual load-carrying capacity.Up to a displacement of 2 mm, the slope of the composite with irregular fibres was gradual and then changed until it reached a maximum value.After reaching the maximum value, the composite with regular fibres failed abruptly, whereas the composite with irregular fibres showed some fluctuations, which could be due to the influence of irregularities.The change in slope indicates the failure initiation in the composite, and the fluctuations indicate different failure modes evolved during the load application.The failure in the composite with regular fibres (Figure 6b) was perfectly straight within the gauge length, while it was at an angle for the composite with irregular fibres.From the stress-strain response, the tensile strength and tensile modulus were calculated and presented in Table 1.The elastic modulus of jute/epoxy composites was validated using analytical solutions such as the rule of mixtures and the inverse rule of mixtures in both longitudinal and transverse directions.The validated models were then extended to evaluate interfacial stresses, and the agreement between the Finite Element (FE) and analytical results is presented in Table 2.The validated FE models were used to assess the impact of IRS% and DBS% on the composites' elastic properties and interfacial stresses.Poisson's ratio characterizes the tendency of composite material to undergo transverse contraction when subjected to longitudinal expansion along the direction of an applied load [33].When subjected to a load, representative Volume Elements (RVEs) undergo deformations in all three directions: ux, uy, and uz.However, due to the heterogeneous nature of composite materials, these deformations differ from those observed in isotropic materials.The strain in each respective direction was obtained by dividing deformations with the original dimensions of the RVE.The longitudinal direction resulted in longitudinal strains, while the other direction resulted in transverse strains.Poisson's ratio was calculated using the obtained strains.
The combination of micromechanics and the FE method is a powerful approach to studying the influence of fibre irregularities and debonding on the mechanical behaviour of natural fibre composites [25,39].The agreement between the experimental and FE models is compared in Fig. 7. Figure 8 displays SEM micrographs of the jute/epoxy composites that failed under tension, containing regular and irregular fibres.The composites' failure mechanism was analysed to investigate the effect of fibre surfaces on their behaviour.In the case of jute/epoxy with regular fibres (Figure 8a), the primary failure mode was due to fibre pull-out, with plastic deformation, fibre failure, and matrix cracks also observed.Conversely, in the case of jute/epoxy with irregular fibres (Figure 8b), fibre pull-out and fibre-slitting were observed, indicating the influence of the irregular fibre arrangement.Furthermore, fibre deformation, plastic deformation, and fibre distortion were observed due to the complex failure mechanism exhibited by irregular fibres, which was not observed in the case of regular fibres.

Influence of IRS% and DBS% on properties
Figure 9 shows how the longitudinal modulus (E 1 ) changes with varying IRS% and DBS% in jute/epoxy composite with different fibre volume fractions.Interestingly, neither the IRS% nor the DBS% significantly impacted E 1 .The stiffness of the fibres and the matrix primarily determine the E 1 of a composite material, but the stiffness of the fibres dominates.Since the fibres are usually much stiffer than the surrounding matrix and are oriented longitudinally in this case, they constantly contribute to the longitudinal stiffness of the composite, regardless of any irregularities or debonding effects (IRS% or DBS%).Furthermore, despite such effects, the E 1 value increased by 157-170% when the fibre volume fraction increased from 10% to 70%.In Figure 10, the transverse modulus (E 2 ) of jute/epoxy is shown to vary with IRS% and DBS% at different fibre volume fractions.When the volume fraction was constant, E 2 decreased significantly due to DBS%, which caused delamination.However, E 2 did not change with IRS% for the same fixed fibre volume fraction.This is because, under a transverse load, the matrix primarily bore the load perpendicular to the fibres.Debonding compromised the load transfer between fibre and matrix, resulting in a decrease in the effective stiffness of the composite in the transverse direction.As the fibre volume fraction increased from 10% to 70%, E 2 decreased by 62-70%.Although the uneven surface of natural fibres could affect E 2 , it was not as significant as DBS%.Due to the irregularities of the natural fibre at the cross-sectional surface, the load applied in the transverse direction could be transferred better than the debonding defect.
Figure 11 illustrates how the major Poisson's ratio (ʋ 12 ) of jute/epoxy composite varies with the changes in IRS% and DBS% at different fibre volume fractions.Poisson's ratio shows how much deformation occurs perpendicular to the direction of the applied load.The effect of IRS% and DBS% on the ʋ 12 is minimal as the matrix properties play a more significant role in determining Poisson's ratio.Any changes in the load transfer due to debonding or irregular surface area would significantly affect the transverse modulus of the material.The microstructure of the composite material has little impact on ʋ 12 .This means that changes in the properties of the fibres or their arrangement may not significantly affect ʋ 12 .However, if the IRS% and DBS% are increased, the ʋ 12 decreases by 8-9% and 8-11%, respectively, at a constant fibre volume fraction.
In Figure 12, the minor Poisson's ratio (ʋ 21 ) of jute/epoxy is demonstrated at different fibre volume fractions while varying the IRS% and DBS%.As the fibre volume fraction increases, the composite becomes stiffer and less likely to deform in the axial direction.This results in a decrease in the Poisson's ratio of the composite material.Irregular fibres in the composite resist transverse direction deformation more than the composite with debonding.Consequently, the ʋ 21 for the composite with an irregular surface is lower.In a composite with debonding, weak adhesive between the fibres and the matrix material causes fibres to separate from the matrix.The debonded fibres in the composite deform more easily in the transverse direction than the composite with irregularly surfaced fibres, leading to a lower ʋ 21 .With an increase in IRS% and DBS%, ʋ 21 decreased by 53-54% and 87-92%, respectively, as the fibre volume fraction increased.For a constant fibre volume fraction, ʋ 21 did not significantly change with IRS%.However, in the case of DBS%, ʋ 21 did not significantly change up to a fibre volume fraction of 40%.Beyond this point, ʋ 21 showed significant variation with an increase in DBS%.

Interfacial stresses
Figure 13 shows the maximum fibre directional stresses (σ 1 ) at the interface with IRS% and DBS%.When a fibre surface has irregularities, it creates stress concentrations at the fibre/matrix interface, leading to higher local stresses.However, the stress distribution along the fibre's length will be evenly distributed because the irregularities are along the surface only.In the case of debonding, there is no direct contact between the fibre and matrix.Despite these differences, the fibre directional stress at the interface will be the same for both surface irregularities and debonded surfaces under longitudinal loading.This resistance is insensitive to the fibre surface or debonding defect.Figure 13(b-c) presents the FE contours of σ 1 .The non-separation of fibre and matrix was visible in the same stress for both IRS% and DBS%.Also, the FE contours show a uniform stress distribution in both cases.
The graph in Figure 14 displays the changes in transverse stresses concerning IRS% and DBS% under transverse loading.This graph is derived from the E 2 model, Section 2.3.2.When the fibre/matrix interface undergoes shear stresses due to load transfer between the fibre and matrix, a non-uniform stress distribution occurs along the fibre's length, as shown in the case of composite with DBS%.In contrast, the composite with IRS% didn't separate the fibres from the matrix, which allowed the load transfer between the fibre and matrix to occur, leading to a more uniform stress distribution and a lower interfacial shear stress.In the case of DBS%, the fibre was entirely separated from the matrix, resulting in no load transfer between the fibre and the matrix.The debonding created a space between the fibre and the matrix, leading to stress build-up at the edges of the debonded area.When composite materials undergo debonding, it creates an uneven distribution of stress, accumulating stress at the edges of the separated area.This localized stress concentration had damaging effects and even led to the failure of the composite in certain regions [40].The debonding process usually initiates at a specific point or percentage along the fibre, forming a small debonded zone.The stress exerted on this zone caused it to expand outward and circumferentially encircle the fibre.The overall integrity of the material weakened due to the propagation of this debonded zone along the fibre's circumference, which reduced the bonding between the fibre and the matrix over a broader area [41].The reduced bonding between the fibre and matrix regions resulted in higher stresses, leading to local failure in composites with IRS% under transverse loading.The FE contours for σ 2 are displayed in Figure 14b-c.Stress concentrations were observed in the defected composite with IRS % (Figure 14a), and fibre-matrix separation was shown in FE contours with DBS defect (Figure 14b).The surface irregularities caused the separation of the fibre and matrix.In contrast, in the debonded composite, the already separated zone was further separated by the applied load.Figure 15 demonstrates the variation of transverse directional stress (σ 2 ) for composites with IBS% and DBS%.Transverse stress (σ 2 ) was found to be significantly high as compared to other types of stresses, especially for debonded composite.Transverse stress is transferred efficiently between the matrix and fibres in a perfectly bonded composite.However, when the bond between the two is broken, the transverse stress builds up at the point where they meet, resulting in high-stress concentrations.This, in turn, leads to more than just the IRS effect.The FE contours of σ 2 for IRS% and DBS% under transverse load can be seen in Figure 15b-c.Figure 16 displays the variation of in-plane shear stress (τ 12 ) at the interface of composites with IRS% and DBS%.The magnitude of in-plane shear stress was higher in the composite with IRS% than in the composite with DBS%.In-plane shear stress is a crucial factor at the interface of composite materials as it affects their strength and durability.When defects such as IRS% or DBS% are present, they cause a change in the distribution and magnitude of in-plane shear stress, ultimately reducing the strength and performance of the composite material.Interestingly, the magnitude of in-plane shear stress was higher in the composite with IRS% than in the composite with DBS%.This is because the IRS defects caused a reduction in the effective cross-sectional area of the composite material, leading to higher stress concentrations and increased shear stress at the interface (Figure 16b-c).The composite with IRS% also exhibited debonding indicating higher shear stress at the fibre/matrix interface.Figure 17 shows that out-of-plane shear stress (τ 23 ) acts perpendicular to the plane of the composite.The results indicate that the composite with DBS% had a higher τ 23 magnitude than the composite with IRS%.This was due to debonding between the constituents, which created a gap, leading to stress concentration and higher shear stress at the interface (Figure 17b-c).Moreover, the study revealed that the composite with DBS % exhibited a particularly high τ 23 magnitude at higher fibre volume fractions.The higher fibre volume fraction resulted in more fibre reinforcement, which increased the load-bearing capacity of the composite material, leading to more interfacial shear stress.Additionally, increasing the fibre volume fraction improved the interfacial shear stress in the jute/epoxy composite.
Figure 18 shows the out-of-plane shear stress (τ 31 ) of jute/epoxy composite with IRS% and DBS%.The results indicate that the composite with IRS% exhibited higher τ 31 values.This is because the irregular surface of the fibre in IRS% led to poor bonding between the fibre and matrix, which resulted in stress concentration and • At a constant fibre volume fraction, the increase in IRS% and DBS% did not influence the longitudinal modulus (E 1 ), as fibres undergo primary loading in the longitudinal direction.In addition, the longitudinal modulus increased by 168% with fibre volume fraction.Similarly, the IRS% and DBS% did not influence major Poisson's ratio (v 12 ).• The transverse modulus (E 2 ) was decreased with increase in IRS% and DBS% at a constant fibre volume fraction.For example, at 50% fibre volume fraction, an increase in IRS% and DBS% decreased E 2 by 7% and 2%, respectively.The DBS% exhibited more influence than IRS% because the presence of debonding led to delamination, reducing E 2 .
• In composites with IRS%, fibre surface irregularities could act as stress concentrators, leading to localized stress concentrations, but they did not result in fibre/matrix separation.
• The IRS%-affected composite exhibited significantly higher magnitudes of interfacial shear stresses τ 12 and τ 31 than the DBS%-affected composite.
Hence, while designing natural fibre-reinforced composites for desired elastic properties, surface roughness should be considered as it significantly affects interfacial stresses.Our future studies include surface roughness and other surface properties.

Figure 1 .
Figure 1.Microscopic images of natural and manmade fibres.

Figure 3 .
Figure 3. Tensile testing machine loaded with jute fibre reinforced composites.

Figure 4 .
Figure 4. FE modelling of IRS% and DBS% at a fibre volume fraction of 20%.

Figure 5 .
Figure 5. Discretisation of irregularities and debonding at a fibre volume fraction of 20%.

FailureFigure 6 .
Figure 6.Tensile response of jute/epoxy composites with regular and irregular fibres.

Table 2 .
Validation of E 1 , E 2 and ν 12 with analytical results.
Figure 7.Comparison of longitudinal modulus: experiment vs FE with different IRS%.