Experimental and Numerical Assessment of Bamboo–Coir Hybrid Composite Panels for Formwork Systems

Abstract

This study evaluates bamboo–coir hybrid composite panels developed for formwork applications using an 80:20 fiber–matrix ratio and a 50:50 bamboo-to-coir distribution. The novelty of this study lies in the combined assessment of formwork-relevant mechanical performance, Mode I and Mode II fracture behavior, finite element validation and post-fracture microstructural correlation for a high fiber volume fraction natural fiber hybrid panel. Mechanical, durability, fracture, numerical and microstructural investigations were performed and benchmarked against 10 mm thick construction-grade plywood. The hybrid panels exhibited a density of 805 ± 10.84 kg/m³, which is within 0.7% of plywood, a tensile strength of 50.20 ± 2.85 MPa, representing an increase of 41.8% over plywood, and a flexural strength of 38.60 ± 2.10 MPa, corresponding to an increase of 12.9% as compared to plywood. The impact energy absorption of hybrid panels was 7.85 ± 0.62 J, which is 26.6% greater than plywood. Mode I fracture testing yielded a fracture toughness of 456.65 ± 15.42 J/m², corresponding to an increase of 9.3% over plywood, while Mode II fracture toughness yielded a value of 792.42 ± 30.18 J/m², representing an increase of 13.7% over plywood. Finite element predictions deviated from experimental load–displacement responses by 5–13%. SEM observations identified fiber bridging, fiber pullout and interfacial sliding in the hybrid panels, consistent with the measured fracture energy values. The results indicate that bamboo–coir hybrid panels satisfy the mechanical and fracture performance requirements for reusable formwork systems.

1. Introduction

Formwork systems are temporary yet critical components in reinforced concrete construction, responsible for defining the geometry, surface finish and dimensional accuracy of structural elements such as slabs, beams, columns and walls. Conventional formwork systems are predominantly fabricated from plywood, steel or aluminum. Steel and aluminum formworks offer high stiffness and reuse cycles exceeding 50–100 reuses. However, their high densities (7.8 g/cm³ for steel and 2.7 g/cm³ for aluminum) result in increased handling effort, higher transportation energy demand and elevated initial costs [1,2]. Plywood formwork continues to be the most adopted solution in construction practice due to its relatively low density (0.7–0.85 g/cm³), ease of cutting and ability to accommodate complex geometries. Construction-grade plywood panels typically exhibit flexural strengths in the range of 30–40 MPa and flexural moduli of about 3–4 GPa, which are generally adequate to withstand wet concrete pressures during casting [3,4].

However, despite these advantages, plywood-based formwork systems suffer from several inherent limitations. Repeated exposure to moisture and the highly alkaline environment of fresh concrete results in thickness swelling of approximately 4–7% after 24 h of water immersion, along with surface abrasion and a gradual reduction in load-carrying capacity. As a consequence, the practical reuse of plywood formwork is often limited to about 6–10 cycles [5]. In addition, the dependence on timber-derived products raises concerns related to deforestation, increased embodied carbon and disposal issues at the end of service life. These drawbacks have driven the construction industry to explore alternative formwork materials that are lightweight, reusable, cost-effective and environmentally sustainable.

In this context, biocomposites consisting of natural fibers as reinforcement within polymeric or bio-based matrices have gained increasing attention as potential alternatives to conventional formwork materials. From the formwork perspective, biocomposites’ lower density (0.6–1 g/cm³) facilitates manual handling and rapid assembly, while their layered architecture allows tailoring of stiffness and strength for bending-dominated applications [6].

Among various natural reinforcements, bamboo and coir represent two complementary fiber systems. Bamboo is characterized by high axial tensile strength in the range of 200–500 MPa and elastic moduli of 20–40 GPa, providing stiffness and load-carrying capacity suitable for flexural applications [7]. In contrast, coir fibers exhibit a lower tensile strength of 100–200 MPa but significantly higher elongation at break of 15–30%, imparting enhanced toughness, crack-bridging capability and resistance to moisture-induced degradation due to its high lignin content [8]. Hybridization of these fibers within a single composite system enables the development of panels that balance stiffness, strength and damage tolerance. Bai et al. developed bamboo-strip-reinforced composite panels with densities in the range of 0.75–0.80 g/cm³, reporting flexural strengths of 42–48 MPa, corresponding to an improvement of approximately 20–25% over commercial plywood panels of comparable thickness [9]. Li et al. examined laminated bamboo panels with orthogonally arranged strips and reported elastic moduli ranging from 4.5 to 6.2 GPa with strength variations of up to 35% between principal material directions, highlighting the intrinsic anisotropy of bamboo-based systems [10]. Amede et al. reported reductions in flexural strength ranging from 10 to 18% after repeated wetting–drying cycles, primarily attributed to moisture ingress and degradation at the fiber–matrix interface [11]. To mitigate these limitations, chemical treatments, thermal modification and surface conditioning have been explored; however, such approaches increase processing complexity and associated environmental impacts.

Reported tensile strengths for coir-based polymer composites typically lie in the range of 15–35 MPa, while flexural strengths vary between 25 and 55 MPa, depending on fiber architecture, volume fraction and matrix system [12]. Particle board and mat-based coir panels have demonstrated thickness swelling values of 6–10% after 34 h of water immersion, which are lower than many agro-fiber composites but remain higher than resin-impregnated plywood products. Nevertheless, the comparatively low elastic modulus of coir fibers limits their effectiveness in bending-dominated components when used as the sole reinforcement, particularly in applications requiring repeated load cycles and high stiffness [13].

Hybridization of natural fibers has emerged as an effective materials engineering strategy to overcome the limitations of mono-fiber composite systems by combining fibers with complementary mechanical characteristics. Quantitative studies on flax–jute hybrids have reported tensile strength increases from 32–38 MPa (jute only) and 45–50 MPa (flax only) to 2–58 MPa, while sisal–coir hybrids showed tensile strength improvements from 20–30 MPa to 35–45 MPa accompanied by 25–40% higher impact energy absorption [14,15]. Hybrid systems involving bamboo combined with lower-modulus fibers retained a tensile strength of 48–55 MPa while increasing failure strain by 20–35% and improving resistance to damage accumulation under repeated loading [16]. Moisture-induced strength losses in hybrid systems are typically limited to 4–8% compared to 8–15% reported for mono-fiber composites, demonstrating the effectiveness of hybridization in enhancing mechanical balance and durability [17].

Despite these improvements in nominal mechanical and durability performance, most existing studies on hybrid natural fiber composites rely on strength-based evaluation and provide limited insight into damage tolerance and crack sensitivity. Panel-type construction materials are frequently subjected to bending-dominated loading, localized stress concentrations and accidental impact, where fracture resistance rather than ultimate strength governs serviceability and reuse potential [18,19,20,21]. Reported Mode I fracture toughness (G_Ic) values range from 150 to 350 J/m² for engineered wood products and 200 to 800 J/m² for natural fiber–polymer composites, while Mode II fracture toughness (G_IIc) values are typically 1.5–3 times higher due to enhanced shear driven fiber bridging and interfacial friction [22]. However, systematic Mode I and Mode II fracture characterization of hybrid natural fiber panels at high fiber volume fractions (>70%) remains scarce, particularly for layered composites.

Despite reported improvements in mechanical and durability performance, existing studies on hybrid natural fiber composites remain largely strength-based, providing limited insight into fracture behavior and crack sensitivity under bending-dominated loading. This study integrates formwork-relevant mechanical evaluation with Mode I and Mode II fracture testing, analytical modeling and microstructural analysis, offering a comprehensive framework for assessing damage tolerance in hybrid composite panels.

Unlike previous studies that focus primarily on the strength characterization of natural fiber composites, the present work integrates high-fiber-fraction bamboo–coir hybrid architecture, combined Mode I and Mode II fracture characterization, finite element validation and SEM-based micro-mechanical correlation within a single structural assessment framework for reusable formwork panels. Such an integrated fracture-mechanics-oriented evaluation of hybrid natural fiber panels remains limited in the current literature [23].

2. Materials and Methods

2.1. Constituent Materials

This section describes the constituent materials, panel fabrication procedure and experimental program and analytical framework adopted in this study. Hybrid composite panels were developed using bamboo and coir as natural fiber reinforcements and polypropylene as the thermoplastic matrix with a constant fiber–matrix ratio and controlled variation in fiber hybridization. The experimental program comprises mechanical characterization to identify the optimal fiber distribution followed by Mode I and Mode II fracture testing using standardized single-edge notch bending (SENB) and end-notched flexure (ENF) configurations. In addition to experimental investigations, analytical finite-element-based fracture analyses were carried out to simulate crack initiation and propagation. The properties, physical form and dimensions of all constituent materials used in the fabrication of the composite panels are summarized in

Table 1. Properties and dimensions of constituent materials.

Constituent Material	Form Used	Source	Density (kg/m3)	Typical Mechanical Properties	Dimensions Used in This Study	Remarks
Bamboo (Dendrocalamus strictus)	Strips	MKS Bamboo Suppliers, Bengaluru, Karnataka, India	800	Tensile strength: 200–500 MPa; elastic modulus: 20–40 GPa	Length: 450 mm; width: 8 mm; thickness: 2 mm	Used as primary load-carrying reinforcement; placed in orthogonal orientations (0°/90°)
Coir fiber	Non-woven mat	Sri Maruthi Coir Industries, Peenya Industrial Area, Bengaluru, Karnataka, India	1200	Tensile strength: 100–200 MPa; elongation at break: 15–30%	Mat size: 450 mm × 450 mm; nominal thickness: 3–4 mm	Used to enhance toughness, crack-bridging and energy absorption
Polypropylene (PP)	Non-woven sheets (geofabric)	Indian Oil Corporation Ltd., New Delhi, India	900–920	Tensile strength: 30–35 MPa; melting temperature: 165–170 °C	Sheet size: 450 mm × 450 mm; thickness per layer: 0.4–0.5 mm	Thermoplastic matrix; facilitates fiber wetting and stress transfer
Construction-grade plywood (reference)	Laminated board	Lal Timbers, Bengaluru, Karnataka, India	810	Flexural strength: 30–40 MPa; flexural modulus: 3–4 GPa	Thickness: 10 mm; standard board size	Used only for benchmarking purposes

. The selected dimensions ensured uniform stacking, consistent compaction and compatibility with the mold size during compression molding while maintaining target panel density and thickness comparable to plywood.

2.2. Panel Fabrication and Fiber Distribution Ratios

Hybrid bamboo–coir composites panels were fabricated using a hot compression molding process with a constant fiber–matrix ratio of 80:20 by weight. This ratio was selected to achieve a high fiber volume fraction suitable for panel-based applications while ensuring sufficient matrix availability for fiber wetting, stress transfer and dimensional stability during molding. Preliminary trials and reported studies indicate that fiber contents exceeding 80% often lead to inadequate matrix impregnation and increased void content, whereas lower fiber contents reduce stiffness and load-carrying efficiency in bending-dominated panels [24,25].

To investigate the influence of fiber hybridization while maintaining constant total fiber content, three bamboo-to-coir weight ratios were adopted: 40:60, 50:50 and 60:40 (Bamboo:Coir). For each panel, the total target composite mass was first determined based on the nominal panel dimensions (1.5 feet × 1.5 feet × 10 mm) and a target density comparable to commercial plywood (0.80–0.83 g/cm³). Of this total mass, 80% was assigned to fibers and 20% to PP, after which the fiber mass was subdivided according to the selected hybrid ratios.

Prior to fabrication, bamboo strips and coir mats were oven-dried at 60 °C for 24 h to minimize residual moisture. The required quantities of bamboo strips, coir mats and PP sheets were weighed using precision balance to the accuracy of ±0.01 g accuracy. Figure 1a–c illustrate the schematic stacking sequence adopted in the hybrid panels, their actual stacking during the process of fabrication and the materials’ visibility post-fabrication. The materials were stacked in an alternating layered configuration consisting of polypropylene sheets and fiber layers. Bamboo strips were arranged in orthogonal orientations (0°/90°) to reduce in-plane anisotropy and enhance flexural stiffness. Such orthogonal layered configurations are commonly preferred in structural panel composites because they provide predictable stress distribution and reduce in-plane anisotropy compared with single-direction fiber arrangements. The coir mats were interleaved to improve toughness and crack-bridging capability. Polypropylene sheets were placed at the outer surfaces to promote uniform heat transfer and surface finish. The same architectural concept was maintained for all fiber weight ratios (40:60, 50:50 and 60:40), with only mass contribution of bamboo and coir adjusted accordingly.

Compression molding was carried out using a hydraulic hot press (Om Shakthi Hydraulics Pvt. Ltd., Bengaluru, Karnataka, India) at a pressure of 3000 psi and a temperature of 180 °C for a dwell time of 17–18 min. Steel spacers were employed to maintain a uniform panel thickness of 10 mm. After the heating cycle, the panels were cooled under sustained pressure until platen temperature reduced to approximately 50 °C, after which they were demolded and conditioned at ambient laboratory conditions prior to specimen preparation.

The cross-sectional image of the fabricated panel (Figure 1c) provides direct experimental confirmation of the intended stacking sequence. The alternating arrangement of bamboo strips and coir mats observed in the cross-section matches the systematic layup verifying that the fabricated laminate follows the designed interfacial architecture without layer inversion.

The influence of fiber distribution on mechanical performance was evaluated by comparing the tensile and flexural properties of hybrid bamboo–coir composite panels with bamboo-to-coir ratios of 40:60, 50:50 and 60:40 while maintaining a constant fiber–matrix ratio of 80:20.

Table 2. Mechanical comparison of 40:60, 50:50 and 60:40 bamboo-coir hybrid ratios.

Ratio	Mean Tensile Strength (MPa)	Mean Flexural Strength (MPa)
40:60	44.3 ± 2.6	33.8 ± 1.9
50:50	50.2 ± 2.8	38.6 ± 2.1
60:40	47.5 ± 2.4	39.1 ± 2.0

presents the mechanical comparison of different bamboo–coir hybrid ratios.

Increasing bamboo content from 40% to 60% resulted in a progressive increase in stiffness; however, excessive bamboo content led to reduced strain, failure and less stable post-peak behavior under bending. Conversely, higher coir content improved ductility but resulted in comparatively lower tensile and flexural strengths due to a lower modulus of coir fibers. Among the investigated configurations, the 50:50 bamboo–coir distribution exhibited an average tensile strength of approximately 48–52 MPa compared to 32–38 MPa recorded for plywood of similar thickness, indicating an increase of roughly 30–45%. In flexure, the 50:50 hybrid panels achieved strengths in the range of 35–40 MPa, which are comparable to and in certain cases marginally higher than plywood (30–40 MPa), while also exhibiting stable load–deflection behavior. Panels with higher bamboo content showed increased stiffness but reduced deformation capacity, whereas coir-rich panels demonstrated improved ductility at the expense of tensile and flexural strength. The balanced contribution of stiffness from bamboo and toughness from coir in the 50:50 configurations promoted effective stress redistribution and reduced sensitivity to premature failure. Based on this combined tensile and flexural response relative to plywood, the 50:50 hybrid composition was identified as the optimal fiber distribution and was therefore selected for detailed Mode I and Mode II fracture investigations and numerical analysis.

To complement the reported weight fractions, the effective fiber volume fractions were estimated using the densities of bamboo (800 kg/m³), coir (1200 kg/m³) and polypropylene (910 kg/m³). Volume fractions were calculated using Equation (1).

$$V_i={\displaystyle \frac{\raisebox{1ex}{$w_i$}\!\left/ \!\raisebox{-1ex}{${\rho }_i$}\right.}{\sum \frac{w}{\rho }}}$$

(1)

Here, w_i and ρ_i represent the weight fraction and density of each constituent. Based on the selected 80:20 fiber matrix weight ratio and 50:50 bamboo–coir distribution within the fiber phase, the calculated volume fractions were approximately 47.5% for bamboo, 31.6% for coir and 20.9% for polypropylene, corresponding to a total fiber volume fraction of 79%. This high fiber volume fraction confirms that the developed panels fall within the range typically associated with structural natural fiber laminated composites intended for load-bearing panel applications [22].

2.3. Formwork-Related Mechanical Testing

Tensile tests were conducted to determine the tensile strength and tensile modulus of the composite panels in accordance with ASTM D638 [26]. A rectangular specimen was machined from the fabricated panels with nominal dimensions of 250 mm × 25 mm × 10 mm (length × width × thickness). The gauge length was maintained at 150 mm. Tests were carried out using a universal testing machine (Instron 3369, Instron, Norwood, MA, USA) under displacement-controlled loading under displacement-controlled loading at a constant cross-head speed of 4 mm/min. Axial load and elongation were recorded continuously until failure.

Flexural behavior was evaluated using a three-point bending configuration in accordance with ASTM D790 [27]. Rectangular specimens of dimensions 125 mm × 13 mm × 10 mm were tested with a support span of 100 mm corresponding to span to depth ratio of 10. Load was applied at midspan under displacement control, and load–deflection response was recorded through the test.

The elastic modulus of the tested panels was determined from the initial linear portion of the measured stress–strain and load–deflection responses obtained during tensile and flexural testing. The mechanical properties evaluated in this study were determined using the standard continuum mechanics relations applied to the measured load–displacement responses. For tensile loading, the normal stress and axial strain were calculated as in Equations (2) and (3).

$$\sigma ={\displaystyle \frac{P}{A}}$$

(2)

$$\epsilon ={\displaystyle \frac{∆L}{L_0}}$$

(3)

Here, P is the applied load, A is the original cross-sectional area, ΔL is the measured elongation and L₀ is the initial gauge length.

For tensile loading, the Young’s modulus was evaluated from the ratio of normal stress to the corresponding axial strain within the elastic region, expressed as in Equation (4).

$$E={\displaystyle \frac{\sigma }{\epsilon }}$$

(4)

Here, E is the Young’s modulus, with σ being the applied normal stress and the corresponding axial strain.

For flexural loading under three-point bending conditions, the flexural modulus was calculated from the slope of the initial linear portion of the load–deflection curve using the standard beam relation presented in Equation (5).

$$E={\displaystyle \frac{L^{3}m}{4b{d}^3}}$$

(5)

Here, L is the support span with m being the slope of the slope–deflection curve, b the specimen width and d the specimen thickness.

The moisture absorption behavior of the composite panels was evaluated in accordance with ASTM D570 [28]. Rectangular specimens with dimensions 50 mm × 50 mm × 10 mm were prepared from the fabricated panels. Prior to immersion, specimens were oven-dried at 60 °C for 24 h, cooled to room temperature in a desiccator, and their initial dry mass (W₀) was recorded using a precision balance (Sartorius Entris Series, Sartorius AG, Göttingen, Germany) with an accuracy of ±0.01 g. The specimens were then immersed in distilled water maintained at 23 ± 2 °C. At predetermined time intervals (2 h, 6 h, 24 h, 48 h and 72 h), the specimen was removed, surface water was wiped off and the wet mass (W_t) was measured. Moisture absorption (MA) was calculated as the percentage increase in mass relative to dry mass as mentioned in Equation (6).

$$MA={\displaystyle \frac{W_t-W_0}{W_0}}$$

(6)

Resistance to water absorption was assessed in terms of thickness swelling in accordance with ASTM D1037 [29]. Specimens of dimensions 50 mm × 50 mm × 10 mm were used. The initial thickness (t₀) was measured at three locations using a digital micrometer (Mitutoyo Corporation, Kawasaki, Kanagawa, Japan; accuracy ±0.01 mm). The specimens were immersed in distilled water at 23 ± 2 °C for 24 h maintained using a temperature-controlled water bath (Remi Elektrotechnik Ltd., Mumbai, Maharashtra, India). After immersion, surface moisture was removed, and the final thickness (t₁) was measured at the same location. Thickness swelling was calculated as the percentage increase in thickness relative to the initial value. Thickness swelling (TS) was calculated as the percentage increase in specimen thickness after water immersion using Equation (7).

$$TS = {\displaystyle \frac{t_1-t_0}{t_0}}\times 100$$

(7)

Boiling water resistance of the composite panels was evaluated following procedures commonly adopted on wood-based panels, in line with IS 1734 (Part 1) [30] guidelines. Rectangular specimens of dimensions 75 mm × 75 mm × 10 mm were immersed in boiling water (100 °C) for a duration of 2 h. After boiling, the specimens were removed and immediately transferred to water at room temperature for cooling. Subsequently, the specimens were visually examined for surface degradation, delamination, blistering and fiber–matrix separation. Changes in thickness and mass were recorded before and after the test to evaluate resistance to severe hydrothermal exposure representative of extreme site conditions.

Cyclic durability was assessed through repeated wetting–drying cycles in accordance with ASTM D1037. Specimens measuring 100 mm × 20 mm × 10 mm were subjected to cyclic conditioning comprising immersion in water at 23 ± 2 °C for 24 h, followed by oven drying at 60 °C for 24 h. A total of five wetting–drying cycles were applied to each specimen. Upon completion of the prescribed cycles, the specimens were conditioned under laboratory ambient conditions prior to further evaluation. This test was employed to examine resistance to moisture-induced degradation, dimensional instability and surface deterioration under repeated exposure.

Impact resistance was determined using an Izod impact testing machine (Tinius Olsen Model IT504, Tinius Olsen Ltd., Surrey, UK) in accordance with ASTM D256. [31]. Rectangular specimens of dimensions 64 mm × 12.7 mm × 10 mm were prepared with a standard V-notch of 2.54 mm depth. The absorbed impact energy was recorded to assess resistance to accidental handling and site-induced impacts.

Nail and screw withdrawal resistance tests were conducted to evaluate the fastening performance of the composite panels, which is a critical requirement for formwork applications involving repeated fixing and stripping operations. The tests were performed in accordance with ASTM D1761 (Standard Test Methods for Mechanical Fasteners in Wood) [32]. Rectangular specimens of dimensions 100 mm × 50 mm × 10 mm were prepared from the fabricated panels. Steel nails and self-tapping steel screws of standard dimensions were driven perpendicular to the panel surface to a controlled embedment depth of 25 mm, ensuring consistent penetration across all specimens. A minimum edge distance of 25 mm was maintained to prevent premature splitting and minimize edge effects.

2.4. Fracture Testing Process

2.4.1. Mode I Fracture Testing (SENB)

Mode I fracture behavior was characterized using SENB configurations in accordance with ASTM D5045 [33]. Rectangular beam specimens with dimensions of 100 mm × 20 mm × 10 mm were prepared. A sharp notch was introduced at midspan using a precision saw followed by blade tapping. The notch depth(a) to width(W) ratio (a/W) was fixed to 0.40 to investigate geometric sensitivity. Specimens were tested under three-point bending with simply supported boundary conditions and a centrally applied load. Load–displacement data were continuously recorded. Figure 2 presents the dimensional characteristics of the SENB test setup. Figure 3 and Figure 4 present the specimen being subjected to SENB testing.

2.4.2. Mode II Fracture Testing (ENF)

Mode II fracture behavior was evaluated using ENF configurations in accordance with ASTM D7905 [34]. Rectangular specimens of dimensions 150 mm × 20 mm × 10 mm were prepared with a predefined interlaminar pre-crack introduced at mid-plane. Initial crack length(a₀) to span(L) ratio (a₀/L) 0.35 was considered. The specimens were subjected to three-point bending under simply supported conditions and load–displacement responses were recorded till crack propagation.

Post-fracture microstructural analysis was carried out to examine the fracture surfaces of the tested specimen and to identify the dominant damage mechanisms governing the initiation and propagation of cracks. Figure 5 presents the ENF test setup. Figure 6 and Figure 7 present the ENF load application setup on the specimen.

2.5. Finite Element Modeling Methodology

Finite element analysis (FEA) was performed to support the experimental fracture investigations by evaluating displacement response and stress distribution in SENB and ENF specimens fabricated from plywood and bamboo–coir composite panels. The SENB and ENF specimen geometries were modeled in accordance with ASTM D5045 and ASTM D7905, respectively. For both SENB and ENF specimens, macro-analysis modeling was preferred using orthotropic material properties to accurately represent the orthotropic behavior of the material. In the present investigation, the finite element method was implemented using ANSYS Mechanical APDL, Version 14.5 (ANSYS Inc., Canonsburg, PA, USA). An eight-node brick element (SOLID 45) was preferred to develop the three-dimensional models of the SENB and ENF specimens. An aspect ratio of less than 3 was maintained to ensure mesh quality, and a refined mesh was generated using an element size growth rate of 1.2. The finite element model comprised 94,388 nodes and 64,613 elements. Since the results in the vicinity of the crack are highly sensitive to mesh density, a finer mesh was adopted in this region to achieve improved accuracy. Simply supported boundary conditions were applied in both configurations using roller supports at the specimen ends, and a concentrated load was applied at midspan to replicate the experimental three-point bending setup. The homogeneous material properties used in the FE simulations were derived from experimentally measured elastic responses. For bamboocoir hybrid panels, the effective Young’s modulus was calculated to be 4.2 GPa with a Poisson’s ratio of 0.32, while plywood was modeled with a modulus of 3.8 GPa and a Poisson’s ratio of 0.30. These equivalent properties represented macroscopic elastic behavior consistent with LEFM-based modeling assumptions. Mesh discretization was performed with local refinement in regions of high stress gradients. A minimum element size of 0.5–1.0 mm was employed in the refined zones, while a coarser mesh was used away from critical regions to reduce the computational cost. Figure 8 presents the SENB specimen with simple support (roller support) at the ends and load applied at the center span of the beam. The notch depth to the width ratio considered for SENB is 0.40. Figure 9 presents the ENF specimen with roller support at the ends and load applied at center of the beam and a notch at the center of the lamina with a notch length to span ratio of 0.35.

2.6. Scanning Electron Microscopic Analysis

To study the micro-mechanisms of failure and validate the macroscale fracture behavior observed during SENB and ENF testing, post-fracture surfaces of selected specimens were examined using SEM. Fracture specimens from each test group were selected for SEM based on clean fracture surfaces, representative crack propagation regions and consistent macroscopic load–displacement behavior. Each fracture surface was sectioned carefully to fit under the SEM sample holder (10 mm × 10 mm). Samples were sputter-coated with a 10 nm thick gold layer using a JEOL JFC-1600 Auto Fine Coater (JEOL Ltd., Tokyo, Japan). SEM imaging was performed using a JEOL JSM-T300 Scanning Electron Microscope (JEOL Ltd., Tokyo, Japan) operated at an accelerating voltage of 15 kV and working distance of 10 mm.The microscale damage patterns observed through SEM were correlated with the fracture toughness values obtained experimentally.

3. Results and Discussion

3.1. Physical and Durability Properties

The physical and durability properties of the 50:50 hybrid bamboo–coir composite panels were evaluated to assess their suitability for formwork applications, where dimensional stability, moisture resistance and fastening performance govern the reuse potential. The results obtained from the ten specimens for each configuration are summarized in

Table 3. Physical and durability properties of bamboo–coir hybrid panels (n = 10).

Property	Unit	50:50 (B:C)	Plywood (10 mm)
Density	kg/m3	805.30 ± 10.84	810.60 ± 9.75
Moisture absorption (24 h)	%	6.18 ± 0.44	5.35 ± 0.41
Thickness swelling (24 h)	%	5.21 ± 0.47	4.92 ± 0.38
Boiling water thickness change	%	7.14 ± 0.69	6.85 ± 0.57
Thickness swelling after 5 cycles	%	6.48 ± 0.59	6.34 ± 0.48

. Plywood of identical thickness was tested under identical conditions to provide a direct benchmark.

As shown in Table 3, the densities of the 50:50 hybrid panels were 805.30 ± 10.84 kg/m³, closely matching that of plywood exhibiting 810.60 ± 9.75 kg/m³. Maintaining a density comparable to that of plywood is important to ensure similar handling characteristics and load transfer behavior in formwork systems. The measured density of the hybrid panels falls within the range reported for laminated bamboo panels and hybrid natural fiber composites developed for construction applications, typically between 750 and 820 kg/m³. Densities within this range are generally considered necessary to achieve plywood-like handling, stiffness and ease of site operation [35].

The moisture absorption of the hybrid panels after 24 h of water immersion is consistent with values reported for hybrid natural fiber composites, where absorption levels between 5 and 8% are commonly observed depending on fiber architecture and matrix content. By comparison, coir-based panels and other agro-based fiber boards reported in the literature often exhibit higher moisture absorption, typically in the range of 7–12%. Resin-impregnated plywood, on the other hand, generally shows lower moisture absorption values of about 4–6%. The slightly higher moisture uptake of the hybrid panels relative to plywood is therefore consistent with the presence of hydrophilic natural fibers while remaining within acceptable limits for short-term formwork exposure [36].

Thickness swelling after 24 h water immersion compares favorably with reported values in the literature for bamboo laminates and hybrid fiber panels, which typically range between 4 and 7% depending on fiber orientation and bonding efficiency. Agro-fiber boards often exhibit thickness swelling exceeding 8–10%, whereas exterior-grade plywood shows swelling in the range of 4–6%. The comparable swelling behavior of the 50:50 hybrid panels indicates effective restriction of moisture-induced expansion through balanced fiber hybridization and adequate matrix encapsulation [37].

Under severe hydrothermal exposure, the boiling water thickness change remains consistent with reported values for engineered bamboo panels (6–9%) and hybrid natural fiber laminates subjected to boiling or cyclic soaking conditions. The literature on coir-dominant composites reports significantly higher dimensional changes, often exceeding 9–12% due to fiber swelling and interfacial degradation. The observed performance of the hybrid panels therefore reflects the stabilizing contribution of bamboo fibers and the thermoplastic matrix under elevated temperature and moisture conditions [38].

The results of cyclic wetting and drying further support this inference. After five cycles, the hybrid panels exhibited thickness swelling of 6.48 ± 0.59%, comparable to plywood (6.34 ± 0.48%). Previous studies on natural fiber composites subjected to cyclic conditioning frequently report cumulative swelling in the range of 6–9%, with mono-fiber systems showing greater degradation than hybrid systems. The close agreement between the hybrid panels and plywood indicates satisfactory resistance to moisture-induced damage accumulation during repeated exposure, which is critical for formwork reuse [39].

Fastener withdrawal resistance of the hybrid panels also aligns well with previous benchmarks. Nail withdrawal resistance values of 800–900 N and screw withdrawal resistance values of 1100–1300 N are commonly reported for bamboo-based panels and structural plywood of similar thickness. The measured values fall squarely within these ranges, confirming that the hybrid fiber architecture provides adequate bearing resistance and load transfer at the fastener–panel interface [40].

3.2. Mechanical Performance of Hybrid Panels

Following the assessment of physical and durability characteristics, the mechanical performance of the 50:50 bamboo–coir hybrid composite panels was evaluated to determine their load carrying capacity and deformation behavior under tensile, flexural and impact loading conditions relevant to formwork applications.

3.2.1. Tensile Behavior

The tensile performance of the hybrid panels in comparison with plywood is presented in Figure 10, while tensile load–displacement responses are shown in Figure 11. The hybrid panels exhibited a higher average tensile strength of 50.20 ± 2.85 MPa as compared to plywood (35.40 ± 2.40 MPa), with limited scatter as indicated by the standard deviation bars. This improvement reflects the effective load-carrying contribution of bamboo fibers combined with stress redistribution facilitated by coir fibers. The load–displacement curve further highlights the differences in deformation behavior. The hybrid panels demonstrate higher initial stiffness, evidenced by a steeper initial slope, and sustain higher loads over a larger displacement range compared to plywood. Reported tensile strengths for laminated bamboo panels and bamboo-based composites typically range between 48 and 60 MPa depending on fiber orientation and matrix system, while hybrid natural fiber composites incorporating compliant fibers such as coir or jute generally exhibit tensile strengths in the range of 45–55 MPa, closely matching the values obtained in other studies [41,42,43].

The tensile load–displacement curves further highlight differences in deformation behavior between the two materials. The hybrid panels demonstrate higher initial stiffness, evidenced by a steeper initial slope, and sustain higher loads over a large displacement range compared to plywood. In contrast, plywood generally exhibits lower tensile stiffness and a more abrupt post-yield response with reported tensile strengths typically in the range of 30–40 MPa and limited strain capacity prior to failure. Studies on hybrid bamboo-based systems have shown that the inclusion of lower modulus fibers can increase tensile failure strain by 20–35% relative to mono-bamboo laminates, promoting progressive damage accumulation rather than brittle fracture. The observed tensile response of the hybrid panels is therefore consistent with literature trends, confirming the role of fiber hybridization in enhancing both load-carrying capacity and deformation stability [16,44].

Representative tensile stress–strain curves of the bamboo–coir hybrid panels and plywood are presented in Figure 12. The hybrid panels exhibited a steeper initial slope, indicating higher elastic stiffness, followed by a gradual nonlinear region prior to failure, reflecting progressive damage evolution associated with fiber bridging and interfacial bonding.

To further position the measured tensile strength within the broader class of structural bio-based panels, the obtained value of 50.20 ± 2.85 MPa is comparable to reported tensile strengths of engineered bamboo laminated products, which typically range between 48 MPa and 65 MPa depending on strip orientation and adhesive bonding efficiency. Hybrid natural fiber–polymer composites incorporating bamboo with secondary fibers such as jute, flax or coir commonly report tensile strengths between 45 MPa and 55 MPa. The close agreement of the present hybrid panel with these ranges indicates that the bamboo–coir architecture achieves tensile load-carrying capacity consistent with engineered structural biocomposites rather than merely matching conventional plywood performance [18,19].

3.2.2. Flexural Behavior

The flexural performance of hybrid panels in comparison with the plywood is presented in Figure 13, while representative load–displacement responses are shown in Figure 14. The hybrid panels exhibited an average flexural strength of 38.60 ± 2.10 MPa, which is comparable to and marginally higher than that of plywood (34.20 ± 1.95 MPa) with limited scatter. Reported flexural strengths for laminated bamboo panels and engineered bamboo composites typically range between 36 and 45 MPa, depending on strip orientation, adhesive system and panel thickness. Hybrid natural fiber panels incorporating bamboo with compliant fibers such as jute or coir generally exhibit flexural strengths in the range of 35–42 MPa, closely aligning with the values obtained in another study [45].

The flexural load–displacement curve further illustrates differences in bending deformation behavior. The hybrid panels indicate higher load resistance throughout the loading regime and sustain larger mid-span deflections prior to failure compared to plywood, indicating enhanced deformation stability under bending. In contrast, construction-grade plywood typically exhibits flexural strengths in the range of 30–40 MPa with earlier onset of stiffness degradation and more pronounced load drops after peak load. Previous studies on bamboo-based and hybrid composite panels have reported that fiber hybridization can increase flexural deformation capacity by 15–30%, attributed to delayed crack initiation, fiber bridging and progressive interfacial debonding mechanisms [46,47].

From the bending performance perspective, a measured flexural strength of 38.60 ± 2.10 MPa lies within the structural range reported for laminated bamboo boards, bamboo LVL products and strip-reinforced bamboo panels, which typically exhibit flexural strengths between 36 MPa and 50 MPa. Hybrid natural fiber laminates developed for structural applications similarly report flexural strengths in the range of approximately 35–45 MPa. The agreement between these reported values and the present experimental results confirms that the developed bamboo–coir hybrid panels achieve bending resistance comparable to engineered bamboo structural materials while maintaining stable deformation behavior before failure [48].

3.2.3. Impact Behavior

The impact performance of the hybrid panels in comparison with plywood is presented in Figure 15. The hybrid panels absorbed an average impact energy of 7.85 ± 0.62 J compared to 6.20 ± 0.55 J for plywood, indicating higher resistance to sudden loading and accidental impact. The relatively narrow standard deviation bands reflect a consistent impact response across the tested specimens. Reported Izod impact energies for hybrid natural fiber composites generally lie in the range of 7–10 J, depending on fiber architecture, matrix type and notch geometry, while construction-grade plywood typically exhibits impact energies between 5 and 7 J consistent with values obtained in the present study [49].

Then, enhanced impact energy absorption of the hybrid panels can be attributed to the synergistic contribution of bamboo–coir fibers. Bamboo fibers provide stiffness and constrain cracks while coir fibers are characterized by high elongation at break, thereby promoting energy dissipation through fiber pull-out, crack deflection and interfacial friction. The literature on bamboo hybrids reports 20–35% increases in absorbed impact energy compared to wood-based and mono-fiber composites, attributed to progressive damage evolution and delayed catastrophic fracture. The absence of abrupt failure during impact loading further indicates a stable damage accumulation, an advantageous aspect with respect to on-site applications [50].

The absorbed impact energy of 7.85 ± 0.62 J also aligns closely with the values reported for hybrid natural fiber composites intended for structural purposes, where Izod energy impact energies typically fall between 6 J and 10 J depending on the fiber architecture and matrix system. Engineered bamboo laminates frequently exhibit lower impact tolerance due to their comparatively brittle strip bonding behavior, whereas hybrid systems incorporating compliant fibers demonstrate improved energy dissipation through fiber pull-out and interfacial sliding. The present hybrid panel behavior therefore reflects the expected toughening contribution of coir fibers and confirms its suitability for applications involving accidental handling loads and site-induced impacts [51].

3.3. Fracture Studies

Having established the mechanical competitiveness of the hybrid panels relative to both plywood and engineered bio-based panel systems, the fracture behavior governing crack initiation and propagation are examined.

3.3.1. Mode I Fracture Behavior (SENB)

• Experimental Results of Mode I Test

The Mode I fracture behavior of the 50:50 bamboo–coir hybrid panels was investigated using SENB configuration in accordance with ASTM D5045 to evaluate crack initiation and propagation under bending-dominated loading conditions. The experimental load–displacement response obtained from SENB testing indicated a stable pre-cracking regime followed by gradual crack propagation, indicating progressive damage evolution. In comparison, the plywood specimen exhibited a comparatively steeper post-peak load drop, reflecting limited crack-bridging capability and reduced damage tolerance.

The experimentally obtained SENB fracture parameters are summarized in

Table 4. Experimental SENB fracture parameters for bamboo:coir (50:50) hybrid panels and plywood (ASTM D5045, n = 10).

Specimen No.	Bamboo–Coir			Plywood
	Pc (N)	δc (mm)	GIc (J/m2)	Pc (N)	δc (mm)	GIc (J/m2)
1	372	0.69	432.15	355	0.63	402.80
2	385	0.71	445.80	362	0.65	410.25
3	398	0.74	458.30	370	0.67	418.90
4	410	0.76	471.60	378	0.69	425.35
5	380	0.70	439.25	350	0.62	398.60
6	402	0.75	462.40	382	0.70	430.15
7	388	0.72	447.85	365	0.66	415.70
8	420	0.78	478.10	392	0.72	438.40
9	395	0.73	455.70	358	0.64	407.90
10	408	0.77	469.35	375	0.68	422.60
Mean	395.80	0.74	456.65	368.70	0.67	417.67
Standard Deviation	14.95	0.03	15.42	13.84	0.03	12.86

. The hybrid bamboo–coir panels exhibited a higher average critical load (P_c = 385.80 ± 14.95 N) and critical displacement (δ_c = 0.67 ± 0.03 mm) compared to plywood (P_c = 368.70 ± 13.84 N, δ_c = 0.67 ± 0.03 mm). Correspondingly, the average Mode I fracture toughness of the hybrid panels (G_Ic = 456.65 ± 15.42 J/m²) was marginally higher than that of plywood (417.67 ± 12.86 J/m²). The overlapping scatter bands indicated comparable crack resistance, with the hybrid panels benefiting from additional energy dissipation through fiber bridging and interfacial sliding mechanisms.

The critical displacement at crack initiation provides insight into deformation tolerance prior to fracture. The hybrid panels exhibited a critical displacement of 0.74 ± 0.03 mm, indicating a higher value than hybrid, which can be attributed to enhanced deformation capacity before the onset of cracking. The literature on laminated bamboo and hybrid fiber composites reports critical displacements in the range of 0.65–0.85 mm, confirming that the values obtained in the present study indicate the contribution of fiber hybridization to improving crack tolerance. The Mode I fracture toughness of the hybrid panels falls within the upper range reported for natural fiber–polymer composites, where G_Ic values typically vary between 200 and 500 J/m² depending on fiber architecture and matrix system. Laminated bamboo and bamboo-based composite panels reported in the literature exhibit G_Ic values in the range of approximately 380–520 J/m², while engineering wood products generally show fracture toughness values between 300 and 450 J/m². The fracture toughness of plywood obtained in the present study is therefore consistent with established reference values [52,53].

The marginal increase observed for the hybrid panels can be attributed to additional energy dissipation mechanisms, including bamboo fiber bridging, coir fiber pull-out and interfacial sliding at the fiber–matrix interface. Overall, the SENB-derived fracture parameters of the bamboo–coir hybrid panels demonstrate a systematic but restrained improvement over conventional plywood. The relatively small increase in G_Ic, coupled with a higher critical displacement at crack initiation, indicates that the primary benefit of hybridization lies in improved deformation stability and damage tolerance rather than a pronounced enhancement in intrinsic crack resistance. Such behavior is particularly advantageous in applications where resistance to crack initiation and stable crack propagation under repeated bending loads govern serviceability and reuse performance [54].

• Finite Element Analysis of Mode I Test

The numerical displacement response of the SENB specimens was evaluated for an a/W ratio of 0.40 under incremental loading. For plywood, the numerically predicted mid-span displacement increased from 0.115 mm at an applied load of 100 N to 0.585 mm at 400 N, exhibiting an approximately linear load–deformation relationship characteristic of bending-dominated elastic behavior. The numerical response indicates comparatively stiffer behavior, which is expected due to the idealized linear elastic material representation adopted in the finite element model [55].

To further examine the fracture mechanisms governing the SENB response, finite element displacement and stress contour plots were analyzed for both plywood and bamboo–coir hybrid panels at applied loads of 100 N, 150 N, 250 N and 400 N. Figure 16 presents the displacement and stress contours for the SENB bamboo–coir hybrid panels at different load levels, while Figure 17 illustrates the corresponding results for the SENB plywood specimens.

The displacement contours shown in Figure 16 indicate a progressive increase in mid-span deformation with increasing load, with the maximum displacement consistently occurring at the central loading point, confirming bending-dominated behavior. At lower load levels (100 N and 150 N), the displacement field remains smooth and symmetric, indicating an elastic response prior to crack initiation. As the applied load increases to 250 N and 400 N, the magnitude of displacement increases substantially while maintaining a continuous deformation profile without abrupt localization, reflecting stable deformation behavior prior to fracture. The corresponding stress contours reveal pronounced stress concentrations at the notch root at all load levels, validating the SENB configuration and confirming that crack initiation is governed primarily by opening-mode stresses. With increasing load, the peak tensile stress increases, and the stress field extends progressively into the ligament ahead of the notch tip. This distributed stress transfer indicates the development of a larger fracture process zone in the hybrid panels, which can be attributed to fiber bridging by bamboo strips and interfacial energy dissipation associated with coir fibers. Unlike the highly localized stress intensification typically observed in brittle laminated systems, the hybrid panels exhibit gradual stress redistribution, particularly evident at applied loads of 250 N and 400 N [56,57].

Figure 17 presents the displacement and stress contours of the SENB plywood specimens at the same load levels. In contrast to the hybrid panels, the stress field in plywood remains confined to a narrow region ahead of the notch, with limited spread into the surrounding ligament as the load increases. This pronounced stress localization is characteristic of laminated wood products and is indicative of a relatively small fracture process zone. This behavior is further reflected in the displacement contours, where deformation remains more localized in plywood compared to the higher mid-span deformation and smoother strain gradients observed in the hybrid panels. The stress localization observed in plywood is consistent with its lower deformation tolerance and the sharper post-peak load drop recorded during SENB testing.

• Comparison between Experimental and Analytical Results (SENB)

The load–displacement comparison of SENB hybrid panels is presented in Figure 18. A close agreement between experimental and numerical responses across the investigated load range is observed. At an applied load of 100 N, the experimental displacement is approximately 0.12 mm, while the numerical prediction is about 0.13 mm, indicating a deviation of 8.3%. Additional deviations of 7.1% at 200 N, 12.5% at 300 N and 6.7% at 400 N are observed. Across all load levels, the numerical model consistently predicts slightly higher displacements than those measured experimentally, indicating a marginally more compliant response. This discrepancy can be attributed to the idealized linear elastic assumptions adopted in the numerical framework, which do not explicitly capture micro-damage mechanisms such as fiber bridging, interfacial debonding and localized crushing beneath the loading rollers [50]. The observed deviation range of approximately 6–13% lies within generally accepted limits for numerical validation of fracture-related responses.

For the plywood specimens shown in Figure 19, the experimental and numerical load–displacement curves exhibit similar trends, with numerical predictions again marginally exceeding the experimental measurements. At an applied load of 100 N, the experimental mid-span displacement is approximately 0.11 mm compared to a numerically predicted value of 0.12 mm, corresponding to a deviation of about 9.1%. Comparable deviations of 9.1% at 200 N, 10.5% at 300 N and 5.5% at 400 N are observed. In comparison with the hybrid panels, plywood specimens exhibit lower absolute displacement values at all load levels, reflecting a stiffer bending response [16].

The experimentally obtained Mode I fracture toughness of 456.65 ± 15.42 J/m² falls within the upper range reported for laminated bamboo composites and natural fiber–polymer systems, where G_Ic values commonly vary between approximately 350 J/m² and 520 J/m² depending on fiber orientation and matrix bonding. Engineered wood-based panels typically exhibit lower fracture energy values due to their limited fiber bridging and smaller fracture zones. The present hybrid panels therefore demonstrate fracture resistance consistent with advanced hybrid biocomposites, supporting the observed stable crack propagation behavior during SENB testing [55].

3.3.2. Mode II Fracture Behavior (ENF)

• Experimental Results of Mode II test

The Mode II fracture behavior of the 50:50 bamboo–coir hybrid panels was investigated using the ENF configuration in accordance with ASTM D7905 to evaluate crack initiation and propagation under shear dominated loading conditions. The experimental load–displacement response obtained from ENF testing indicated an initial linear elastic regime followed by stable crack initiation and gradual crack propagation along the mid-plane interface, suggesting controlled shear driven damage evolution. The experimentally obtained Mode II ENF fracture parameters are summarized in

Table 5. Experimental ENF fracture parameters for bamboo:coir (50:50) hybrid panels and plywood (ASTM D7905) (n = 10).

Specimen No.	Bamboo–Coir			Plywood
	Pc (N)	δc (mm)	GIIc (J/m2)	Pc (N)	δc (mm)	GIIc (J/m2)
1	485	0.82	742.30	455	0.74	668.40
2	498	0.85	765.20	468	0.76	682.10
3	512	0.88	792.45	472	0.77	695.80
4	525	0.91	815.60	480	0.79	712.35
5	495	0.84	758.90	452	0.73	661.70
6	518	0.89	805.30	486	0.80	728.15
7	502	0.86	778.65	469	0.76	689.40
8	535	0.93	842.10	492	0.82	745.90
9	508	0.87	798.25	460	0.75	675.60
10	522	0.90	825.40	475	0.78	705.80
Mean	510.00	0.88	792.42	470.90	0.77	696.92
Standard Deviation	15.62	0.03	30.18	13.45	0.03	27.85

. The bamboo–coir hybridized panels exhibited a higher average critical load of 510.00 ± 15.62 N and critical displacement (δ_c = 0.88 ± 0.03 mm) compared to plywood. Correspondingly, the average Mode II fracture toughness of the hybrid panels (G_IIc = 792.42 ± 30.18 J/m²) was higher than that of plywood (696.92 ± 27.85 J/m²), indicating enhanced resistance to shear driven crack propagation.

The average critical load obtained for the hybrid panels lies within the range reported for laminated bamboo and hybrid natural fiber composites where ENF peak loads typically vary between 480 and 550 N for specimens of comparable thickness and span. The literature on natural fiber–polymer composites reports critical ENF displacements in the range of 0.80–1 mm, while plywood and engineered wood products typically exhibit lower values between 0.65 and 0.8 mm. The values obtained in the present study therefore align closely with reported trends and confirm the contribution of fiber hybridization to improved interfacial deformation capacity. The Mode II fracture toughness values further substantiate these observations. The obtained fracture toughness of the hybrid panels lies within the upper range reported for natural fiber–polymer composites where G_IIc values typically span 500–900 J/m² depending on fiber architecture interface quality and matrix ductility [53]. Overall ENF results indicate a systematic but restrained improvement in Mode II fracture resistance for the bamboo–coir hybrid panels relative to plywood. The relatively small increase in G_IIc combined with higher critical displacement suggests that fiber hybridization primarily enhances interfacial damage tolerance and crack stability rather than inducing a dramatic increase in shear fracture toughness.

• Finite Element Analysis of Mode II Test

The numerical response of ENF specimens was analyzed to examine displacement evolution and shear stress redistribution along the crack interface under Mode II loading conditions. Finite element simulations which were performed for an initial crack length ratio of 0.35, consistent with the experimental configurations presented in Figure 20 and Figure 21.

For the bamboo–coir hybrid panels, the displacement contours indicate a progressive and smooth increase in deformation with increasing load. At 100 N, the displacement field is uniformly distributed, indicating elastic shear bending behavior prior to crack initiation. As the applied load increases to 200 N, the displacement magnitude increases while remaining distributed over a larger portion of the specimen, indicating stable shear deformation without abrupt localization. In contrast, the displacement contours of the ENF plywood specimens exhibit lower overall deformation and a more localized displacement field concentrated near the crack region. At higher load levels, deformation becomes increasingly confined, suggesting limited shear deformation capacity prior to crack propagation. The corresponding shear stress contours reveal pronounced stress concentrations along the crack front, with minimal stress redistribution into the surrounding ligament. This highly localized stress field is characteristic of laminated wood products and indicates the presence of a relatively small fracture process zone [54].

• Comparison between Experimental and Analytical Results (ENF)

The ENF load–displacement comparison for hybrid panels presented in Figure 22 demonstrates good agreement between experimental and numerical responses under shear-dominated loading. At 100 N, the experimental displacement is approximately 0.78 mm while the numerical displacement is 0.82 mm, yielding a deviation of 5.1%. Further, at 200 N, the deviation increases to 5.6%, followed by 6.7% at 300 N. The numerical model slightly overpredicts the displacement, which is expected due to the absence of frictional sliding resistance, fiber pullout and progressive interfacial damage in linear elastic formulation. The deviation range of 5–7% indicates excellent agreement and confirms that the numerical model captures the global Mode II deformation behavior of the hybrid panels with high fidelity.

For the ENF plywood specimens presented in Figure 23, the experimental displacement at 100 N is approximately 0.65 mm while the numerical value is 0.70 mm, corresponding to a deviation of 7.7%. Further, a deviation of 7.5% is observed at 200 N and 5.3% at 300 N. The plywood specimen shows lower deformation capacity as compared to hybrid panels, consistent with their lower Mode II fracture toughness. The numerical–experimental deviation remains within 5–8%, which is within the acceptable limits for ENF numerical validation and supports the reliability of the numerical approach.

Similarly, the measured Mode II fracture toughness values are consistent with reported ranges for hybrid natural fiber laminated systems, where G_IIc values are typically 1.5–3 times higher than Mode I toughness due to enhanced shear driven fiber bridging and interfacial frictional dissipation. The observed behavior confirms that the bamboo–coir-layered architecture promotes effective shear transfer and progressive crack propagation resistance comparable to engineered laminated biocomposite systems [54].

Beyond the mechanical validation, the developed laminated biocomposite panels demonstrate strong potential as sustainable construction materials. Their improved Mode II fracture resistance, combined with progressive crack-bridging behavior, indicates suitability for structural as well as semi-structural applications [58].

3.3.3. SEM Analysis Results

SEM was employed to examine the fracture surfaces of bamboo–coir hybrid panels and plywood specimens after SENB and ENF testing, as shown in Figure 24, Figure 25, Figure 26 and Figure 27. SEM examination of the hybrid panels after SENB failure reveals a rough and heterogeneous fracture surface characterized by exposed bamboo fibers, coir fiber pull-out, matrix remnants adhering to fiber surfaces and clear evidence of interfacial debonding. The presence of both fractured and pulled-out fibers indicates effective stress transfer and energy dissipation through fiber–matrix interactions, which is consistent with the fracture mechanisms commonly reported for hybrid natural fiber composites.

In contrast, SEM images of plywood specimens tested under SENB conditions show comparatively smoother fracture surfaces with distinct interlaminar separation and limited fiber bridging. This fracture morphology suggests a more localized crack propagation mechanism, which is consistent with the steeper post-peak load drop and lower deformation tolerance observed experimentally, as well as the numerically predicted stress localization near the notch tip.

Following ENF testing, the hybrid panels exhibit pronounced fiber pull-out, matrix smearing and shear-induced fiber misalignment along the crack interface, indicating enhanced resistance to interlaminar sliding under Mode II loading. Conversely, plywood specimens display predominantly interlaminar shear failure, characterized by fiber splitting along the grain direction and minimal plastic deformation, reflecting reduced shear energy dissipation.

The extensive fiber bridging, coir fiber pull-out and interfacial sliding observed in the SEM images of the bamboo–coir hybrid panels directly correspond to the higher fracture energy absorption measured experimentally, with Mode I fracture toughness values of approximately 450–480 J/m² and Mode II fracture toughness values in the range of 780–820 J/m². This can be compared to the relatively smoother interlaminar fracture surfaces of plywood, which exhibited lower G_Ic (400–430 J/m²) and G_IIc (670–710 J/m²) due to limited fiber bridging and localized crack propagation [59].

Thus, the adoption of bamboo–coir hybrid laminated panels contributes towards net-zero construction targets as bamboo and coir are rapidly renewable, bio-based resources that sequester atmospheric carbon. Consequently, substitution of such natural fiber laminates in structural applications supports carbon-efficient material selection [60].

4. Conclusions

The experimental, numerical and microstructural investigations conducted in this study provide a quantitative basis to assess the suitability of bamboo–coir hybrid composite panels for formwork applications.

• Bamboo–coir hybrid panels fabricated with a 50:50 fiber distribution exhibited densities in the range of 805–816 kg/m³, which are closely comparable to those of plywood (801–820 kg/m³). This similarity ensures equivalent handling characteristics for formwork applications. The achieved density reflects the combined influence of bamboo strip packing and the inherent porosity of the coir mat within the polymer matrix, resulting in mass characteristics comparable to laminated wood products.
• The hybrid panels demonstrated higher tensile and flexural strengths, measuring 50.20 ± 2.85 MPa and 38.60 ± 2.10 MPa, respectively, compared to 35.40 ± 2.40 MPa and 34.20 ± 1.95 MPa for plywood. This improvement is attributed to efficient axial load transfer through bamboo fibers and enhanced transverse stress redistribution provided by the coir fibers within the hybrid architecture.
• Impact resistance of the hybrid panels was significantly higher, with absorbed energy values ranging from 7.23 to 8.47 J, compared to 5.65 to 6.75 J for plywood. The increased energy absorption is associated with fiber pull-out, matrix deformation and interfacial friction mechanisms activated during impact loading.
• Mode I fracture behavior obtained from SENB testing showed critical loads of 395.80 ± 14.95 N, critical displacements of 0.74 ± 0.03 mm and G_Ic values in the range of 432–478 J/m² for the hybrid panels. In comparison, plywood exhibited lower critical loads (355–392 N), critical displacements of 0.62–0.72 mm and G_Ic values between 398 and 438 J/m².
• Mode II fracture testing using the ENF configuration yielded G_IIc values of 742–842 J/m² for the hybrid panels and 661–746 J/m² for plywood, with corresponding critical displacements of 0.82–0.93 mm and 0.73–0.82 mm, respectively, indicating enhanced resistance to interlaminar shear fracture in the hybrid system.
• Finite element simulations successfully reproduced the experimental load–displacement responses for both SENB and ENF configurations, with numerical deviations limited to approximately 5–13%, which is considered acceptable for LEFM-based modeling approaches.
• SEM observations revealed extensive fiber bridging, coir fiber pull-out and interfacial sliding in the hybrid panels, consistent with the higher fracture energy absorption measured experimentally, with Mode I fracture energies of approximately 450–480 J/m² and Mode II fracture energies of 780–820 J/m². In contrast, plywood exhibited predominantly interlaminar fracture surfaces, corresponding to lower fracture energy values of about 400–430 J/m² in Mode I and 670–710 J/m² in Mode II.

While the measured mechanical and fracture properties indicate strong potential for reusable formwork applications, long-term reuse performance will depend on cyclic loading, repeated moisture exposure and site handling damage. Future scope includes focus on the evaluation of formwork reuse cycles beyond 10to 15 repetitions, correlating stiffness degradation with fracture energy reduction, long-term alkaline exposure studies simulating concrete contact durations and life-cycle assessment to compare embodied energy and CO₂ emissions relative to plywood formworks.

Based on the measured mechanical and fracture performance, the bamboo–coir hybrid panels are suitable for applications including reusable formwork panels, which are subjected to bending pressures corresponding to flexural strengths in the range of 35–40 MPa, temporary shuttering systems requiring impact resistance, modular formwork components requiring screw and nail withdrawal resistance in the range of 800–1300 N and temporary construction platforms and protective panels subjected to repeated wetting and drying exposure.