Tensile creep performance of unidirectional sisal/epoxy composites under different curing conditions

Abstract

This study focused on the effects of three different sets of curing conditions on the creep properties of epoxy resin composites reinforced with unidirectional sisal fibers: (i) curing at room temperature, followed by postcuring at 120°C in an oven; (ii) curing in an oven at 100°C for 4 h; and (iii) curing with an accelerator at a ratio of 100:38:1 at room temperature without postcuring. The sisal/epoxy [0°] composite cured at room temperature presented the highest tensile strength, whereas the sisal/epoxy composite cured with an accelerator presented superior tensile creep resistance and long-term performance. The scanning electron microscopy images confirmed good fiber/matrix adhesion. Finally, the Findley and Burger models effectively predicted the creep behavior of the sisal/epoxy composites under long-term service use.

Keywords

sisal Findley’s model Burger’s model creep test

Introduction

Composites reinforced with vegetable fibers are more environmentally friendly than synthetic alternatives, as they are derived from renewable fibers that require less energy and resources and are biodegradable, thus reducing the carbon footprint.¹ However, integrating these composites into existing manufacturing processes is challenging due to the need to adapt curing requirements and standardize fiber treatment methods. Nevertheless, the incorporation of these composites presents opportunities to improve material performance and sustainability.

Sisal fiber stands out due to its high cellulose content, which is one of the primary reasons for its use in industry, as cellulose provides the fiber with mechanical strength.^2,3 Sisal fiber consists primarily of cellulose (65%–70%), hemicellulose (10%–22%), and lignin (9.9%–14%).⁴ Extensive research has highlighted the role of sisal as a reinforcement in composite materials.⁵ Moreover, Senthilkumar et al.,² in their review on the mechanical properties of sisal fiber, achieved that this fiber has a density of 1.33–1.5 g/cm³, elongation of 2.0%–14%, tensile strength of 400–700 MPa and Young’s modulus of 9.0–38.0 GPa. However, expanding the industrial applications of sisal requires addressing limitations such as low matrix compatibility and the hydrophilic nature of plant fibers.⁶

Surface treatments can remove amorphous components such as hemicellulose and lignin, promoting more efficient bonding with the matrix.⁷ Sodium hydroxide (NaOH) is the most commonly used chemical reagent for surface treatment, as it effectively removes lignin and hemicellulose. Additionally, NaOH is cost effective and enhances mechanical anchoring between the fiber and matrix.⁸ Sánchez et al.⁹ studied the effect of NaOH surface treatment on guadua fibers used for reinforcement in polymer composites. They concluded that chemical modification generally increased the mechanical properties of the composites.

Due to their high strength-to-weight ratio, composites reinforced with vegetable fibers present significant potential in construction for semistructural applications, especially in floor panels, wall coverings, deck panels, and thermal insulation materials.¹⁰ These materials can handle long-term stress effectively.¹¹ Due to their lightness and mechanical strength, sisal/epoxy composites exhibit potential for use in diverse areas, including automotive,¹² marine,¹³ and construction applications. They can be used in the interior parts of vehicles (trunk liners, interior roofs, and headrests),¹⁴ aircraft, maritime vessels,¹⁵ and wind turbines.¹⁶ Incorporating natural fiber-reinforced composites in such applications represents a promising approach to reducing the material weight, directly contributing to energy efficiency and lowering greenhouse gas emissions by decreasing fuel consumption.¹⁷ The use of sisal/epoxy composites in industry highlights the potential of these materials to meet the technical requirements of high mechanical strength combined with low density, which are essential characteristics for components subjected to continuous stress. However, investing in life cycle analysis studies of these composites is crucial to optimize their applicability, as it can validate their economic and environmental feasibility.¹⁸

The application of polymeric composites reinforced with sisal fiber can significantly impact the renewable energy industry, particularly in the manufacturing of wind turbines, in response to the demand for more sustainable and efficient materials.¹⁹ Currently, components such as blades and nacelles are produced using glass fiber-reinforced composites, and partial or complete replacement with sisal fibers could provide environmental and economic benefits as long as the necessary technical requirements are met.²⁰ Blades, for instance, require materials with high strength, stiffness, and low density to withstand applied loads without excessive deformation.¹⁹ Furthermore, using these composites reduces reliance on synthetic materials derived from fossil fuels that are typically used for the same applications while offering advantages in terms of fabrication simplicity and reusability.²¹ This approach can reduce production costs, facilitate recycling, and drive innovation through improvements and optimization of fiber-matrix interfacial adhesion.²² Optimization strategies include implementing advanced manufacturing techniques, such as compression molding.²³ Thus, incorporating sisal fibers in blades and nacelles reinforces the commitment to sustainability while contributing to technological advancements in the wind energy sector.

Thus, numerous challenges must be overcome to integrate these composites into manufacturing, such as the need to ensure effective fiber‒matrix bonding, consistent fiber quality, and reliable supply chains. Additionally, scaling up production necessitates a steady supply of sisal fibers, which depends on agricultural and climatic factors.^24,25 Notably, the application of sisal fiber for structural purposes needs to be extensively investigated.

Regarding the mechanical properties and creep deformation, previous researchers have reported that composites can be enhanced by adding materials such as vegetable fibers. For example, Anidha et al.²⁶ found that adding treated sisal fibers to polyamide/epoxy composites significantly improved the tensile, flexural, and impact strength compared to composites with untreated fibers. Abera et al.²⁷ reported that the impact strength increased as the proportion of sisal fiber in a sisal/epoxy composite increased. Similarly, Sahu et al.²⁸ observed that adding sisal fibers to epoxy resin composites enhanced the tensile and flexural strength over those of neat resin samples. Giuliani et al.²⁹ studied the creep behavior of unidirectional flax/epoxy composites and concluded that vegetable fiber reinforcement made the material less susceptible to creep deformation.

We can point out more examples related to our work, such as a recent study²⁵ in which the authors investigated the creep response of unidirectional flax fiber-reinforced bio-based epoxy under different tensile stress levels. They concluded that these composites exhibit noticeable creep even at low stress levels and have a relatively short creep rupture life when subjected to high stress. Balaji et al.³⁰ studied the creep behavior of epoxy resin composites reinforced with short prickly pear fibers in related work. They reported that the neat epoxy resin had a higher creep deformation value than the composites, which resulted in lower deformation rates. Another study³¹ examined the impact of temperature on the time-dependent deformation of flax/polyester composites, reporting that temperature variations significantly influenced creep behavior. Further expanding on this topic, Sala et al.³² analyzed how stress levels and hygrothermal conditions affect the creep and recovery behavior of three advanced composites made from GreenPoxy combined with flax and hemp fibers. They concluded that the extent of instantaneous, time-dependent, and residual strains increases with higher applied loads and harsher environmental conditions.

For structural composites, stresses such as long-term constant loads must be considered to understand the response of the material over extended periods. In this context, creep tests provide valuable insight into how polymer composites respond to constant stress, offering predictions of dimensional stability and potential failure.³³ Creep must be assessed to construct load-bearing structures.³¹ A previous work of our group³⁴ investigated Findley’s and Burger’s models to describe the creep behavior of sisal/epoxy composites with different orientations [0°, 90°, and cross-ply (0°/90°)]. They determined creep modeling could adequately predict the short-time creep behavior of the sisal/epoxy composites since Burger’s and Findley’s models prolong the service lifetime of the material and reduce maintenance effort and cost for long-term applications.

Findley’s model focuses on viscoelastic responses, which are ideal for sustained loads, whereas Burger’s model captures elastic‒viscous interactions.³⁵ Thus, through these model-driven predictions, we can derive parameters for creep resistance, optimize composite formulations, and enhance maintenance strategies for applications requiring durability.³⁶ By enabling the analysis of time-dependent deformation parameters, these models support the design of high-performance materials and promote long-term reliability for automotive, aerospace, and structural components subject to continuous loads and environmental stresses.³⁷ Epoxy resin composites reinforced with natural fibers, such as sisal fibers, have emerged as a sustainable and low-cost alternative to synthetic fiber-reinforced composites.³⁸ Creep testing of sisal/epoxy composites provides insights into their behavior under constant stress and varying curing conditions, with fillers such as particles and fibers enhancing stress resistance.³⁹

Therefore, understanding the long-term durability and creep behavior of these materials is essential to ensure their viability and performance.⁴⁰ Curing conditions—such as temperature, time, postcuring and accelerators—directly affect composite properties. Anand et al.⁴¹ reported that the creep behavior of a material is directly related to its characteristics, as factors such as the type of polymer matrix, the chemical method used for surface treatment of natural fibers, the fiber length and orientation within the composite, and exposure to environmental conditions (heat, humidity) all affect the lifespan of the material. Thus, time, temperature, and load are closely correlated with the viscoelastic behavior of a material.⁴²

Different curing processes are used for composite fabrication, and every method has advantages or disadvantages; when any parameter of the epoxy resin curing process is altered, the properties of the final product are directly affected. Aleksendri et al.⁴³ studied the influence of autoclave curing on polymers and concluded that the degree of curing was affected by the autoclave curing process. by Badawy et al.⁴⁴ examined the effect of curing composites at higher temperatures and reported that an increase in temperature had a significant effect on the impact resistance of the composites. Huther et al.⁴⁵ studied the effects of different temperatures on the curing cycle of carbon fiber-reinforced epoxy composites. They demonstrated that composites cured at lower temperatures had a longer fatigue life than those cured at higher temperatures. Thus, understanding the effects of curing conditions on the mechanical properties of composites reinforced with natural fibers, such as sisal fibers, still requires further in-depth research.

Accelerators offer a promising approach for reducing curing time, as they speed up the chemical reaction between the resin and hardener, even at low temperatures.⁴⁶ However, the use of accelerators can introduce compatibility challenges, leading to unwanted reactions and lower mechanical performance.⁴⁷ Moreover, accelerators shorten the resin gel time, potentially causing uneven curing and poor fiber/matrix adhesion.^48,49 Despite these risks, Groh et al.⁵⁰ observed that epoxy resin cured for 20 minutes exhibited minimal matrix deformation compared with that of an epoxy resin cured slowly. In another study⁵¹ in which carbon/epoxy composites were cured with different concentrations (2, 3, and 5 phr) of accelerators, it was concluded that adding 5 phr accelerator to the composite curing cycle increased the flexural strength by up to 5% and increased the strain-at-break. Previous studies have shown that the use of accelerators can cause significant changes in the mechanical properties of composites. Hence, the use of accelerators in this study presents a challenge, as their effects on the mechanical properties of unidirectional sisal/epoxy composites are still unknown.

Postcuring is often used to increase the curing degree of epoxy resins and composites. This process helps relieve residual stresses that arise from resin contraction during polymerization, thereby enhancing structural integrity and reducing the risk of delamination or premature failure.⁵² However, this process subjects the material to high temperatures, which negatively affects productivity and the production schedule. Despite the relevance of this topic, research on the creep behavior of sisal fiber-reinforced composites, especially with respect to variations in curing procedures, is limited. The curing conditions (temperature, time, and pressure) influence the reaction rates, polymer network formation, and, thus, the final properties of the composite.⁵³

This work aims to fill the above gaps by investigating the creep behavior of sisal fiber-reinforced polymer composites under various curing conditions. A further aim is to understand how these conditions influence creep resistance, which will ultimately improve composite design for applications demanding durability and long-term stability.

Methodology

Materials

The sisal fibers were supplied by SisalSul and originated from the sisal region of Bahia, Brazil. The sisal fibers were subjected to alkaline chemical treatment. After manual stirring, the fibers were immersed in a 5% (w/v) NaOH solution for 2 h at 70°C at a ratio of 10:1 (fiber solution). Epoxy resin (Araldite LY 5052), a hardener (Aradur 5052), and an accelerator (DY 062) from HUNTSMAN were used as the matrix.

Composite manufacturing

For composite fabrication, the treated fibers were immersed in water and combed with a brush to remove knots. Subsequently, the fibers were dried and combed again. After this process, unidirectional fibers were prepared. The fibers were initially arranged in the same orientation and secured in place with adhesive tape to maintain their position. Next, the fibers were sewn with 100% cotton thread using an automatic sewing machine. Figure 1 shows the fabrication steps for the unidirectional fibers and the finished composite.

Four layers of fibers were used in the fabrication of each composite via compression molding. A manual mixture of the epoxy resin and its hardener at a ratio of 100:38 was used to prepare the epoxy resin. The material was treated in an ultrasonic bath for 5 min to remove bubbles. Then, a steel mold (220 × 200 × 3 mm) was placed on top of a glass plate with a layer of polyethylene film. Subsequently, a fiber layer was overlaid on the mold, and the first layer of the resin/hardener combination was applied onto the fabric using a spatula. The process was repeated until the final fiber layer was placed. At the end of this stage, the mold was closed with a steel plate.

Then, the assembly was pressed using a hydraulic press under a load of 3 t for 15 min to evenly distribute the resin. Sisal/epoxy composites and epoxy resin plates were produced and subjected to three different curing conditions. Under the first condition, the samples were cured at room temperature for 24 h, followed by a 1-h postcuring process at 120°C in an oven. Under the second curing condition, the samples were cured in an oven at 100°C for 4 h. Under the third condition, the composite formula contained an accelerator; specimens were prepared at a ratio of 100:38:1 and cured at room temperature without postcuring. Unidirectional composites with a fiber volume fraction of 50% were fabricated with a 0°, 45°, and cross-ply [0°/90°/90°/0°] orientation; additionally, neat epoxy resin plates were cured under the different tested curing conditions.

Mechanical characterization

Tensile testing was conducted using specimens with dimensions of 200 × 20 × 2.5 mm. The recommendations from ASTM D3039⁵⁴ were followed. A total of 20 specimens were prepared for the tensile tests: five specimens of epoxy resin, five specimens with a 0° unidirectional orientation, five specimens with a 45° unidirectional orientation, and five specimens with a 0°/90°/90°/0° cross-ply orientation. The specimens were tested using a Material Test System (MTS) Model 810 instrument with a load cell calibrated for an axial force of up to 5 kN. The actuators were set to a contact pressure of 1 MPa and a 1 mm/min speed. Additionally, an MTS 634.11F-24 extensometer was employed to measure specimen deformation.

Tukey’s test was used to identify significantly different groups at a 5% significance level. Statistical analysis was performed using SPSS 20.0. The Tukey test is a widely used statistical tool for determining whether the difference between two data sets is statistically significant, meaning whether a numerical variation observed in one variable can, with high probability, be attributed to changes in another variable.⁵⁵ In practical terms, it is used to validate experimental hypotheses by identifying causal relationships within the data. This method is particularly useful when comparing three or more groups in statistical analyses, often following an ANOVA (Analysis of Variance). The Tukey test examines the average differences between all possible pairs of groups to determine if these differences are statistically significant based on a specific distribution. It accounts for the cumulative error associated with multiple comparisons, ensuring greater accuracy in the results.⁵⁶

In engineering, the Tukey test has been used to analyze and process data as well as validate experiments, such as in the evaluation of material mechanical properties, component performance, or system efficiency. For instance, it can be employed to identify significant differences between various materials or manufacturing methods, supporting decision-making processes based on reliable data.⁵⁷ Lassila et al.,⁵⁸ in their study, evaluated the reinforcing effect of discontinuous glass fiber fillers with varying length scales on the fracture toughness and flexural properties of composites. Tukey’s test concluded that the experimental composites reinforced with fibers of different length scales demonstrated statistically significant improvements in mechanical performance, specifically in fracture toughness and flexural strength, compared to the other composites analyzed. In a recent study, Santos et al.,⁵⁹ who applied the Tukey test to evaluate the statistical variability of their composites reinforced with bamboo fibers and kaolin waste found that the mean flexural strength was significantly greater in composites made with treated bamboo fibers compared to those using untreated bamboo fibers.

The tensile creep test was conducted following ASTM D2990-17. The tests were conducted in a laboratory at approximately 24°C ± 2°C with a humidity between 45% and 55%. The specimens were tested using an MTS Model 810 instrument with a load cell calibrated for an axial force of up to 5 kN (active interlocking for 4.5 kN), a sensitivity of 20 N, a resolution of 0.01 N, and a 2% error margin. The specimen was restrained in flat grips with a contact pressure of 1 MPa. Tensile load and strain were continuously monitored for 24 h using a 634.11F-24 MTS extensometer with a capacity of 15% (active interlocking at 12%), a sensitivity of 0.01% N, a resolution of 0.1 microns, and 0.6% error margin. For the tensile creep tests, two additional specimens (n = (2) of each composite, distinct from those employed in the tensile test, were used for characterization.

The applied loads were 0.31 kN (sisal/epoxy [45°]), 0.92 kN (neat resin), 1.5 kN (sisal/epoxy [cross-ply]), and 2.4 kN (sisal/epoxy [0°]), corresponding to ∼40% of the maximum tensile strength of each group. To visualize and validate the mathematical models, the experimental creep results were calculated using the commercial software programs ORIGIN PRO 2016 and EXCEL 365. Experimental creep curves were fitted using the models proposed by Findley and Burger, given by equations (1) and (2), respectively:

ε (t) = A t^{n} + ε_{0}

(1)

where ε(t) is the creep strain at time t, A is the transient creep strain amplitude, t is the time-dependent creep strain, n is the loading-independent constant (usually less than 1), and ε₀ is the instantaneous elastic strain,

ε (t) = \frac{σ_{0}}{E_{M}} + \frac{σ_{0}}{E_{K} ({1 ‐ e}^{‐ E_{kt} / ƞ_{k}})} + \frac{σ_{0}}{ƞ_{M}}

(2)

where σ₀ is the applied stress, t is time, E_M is the elastic modulus of the Maxwell model, E_k is the elastic modulus of the Kelvin model, and ƞ_M and ƞ_K are the viscosities of the Maxwell and Kelvin models, respectively.

The Larson‒Miller parameter (P_LM) defines the creep rupture time and is expressed by equation (3):

P_{L M} = (T + 273) \times (\log_{t_{R}} + C)

(3)

where T is the temperature in °C, t_R is the rupture time in hours, and C is a material constant known as the Larson‒Miller constant. To predict the creep rupture data, equation (4) can be written as⁶⁰:

P_{L M} = (T_{1} + 273) \times (\log_{t_{R 1}} + C) = (T_{2} + 273) \times (\log_{t_{R 2}} + C)

(4)

The P_LM constant for sisal/epoxy composites is calculated as C = 20.01. The Larson-Miller parameter is widely used to explore creep rupture data for engineering alloys, and a value of C = 20 is typically adopted. However, this equation can also be applied to the extrapolation of polymeric and ceramic data. For instance, in this study⁶¹ was adopted for polycarbonate C = 20.2, for epoxy/carbon laminate C = 20⁶² and finally, C = 20 for ultra-high molecular weight polyethylene.⁶³

Differential scanning calorimetry (DSC)

The composites were analyzed at temperatures ranging from room temperature (25°C) to 600°C at a heating rate of 5°C/min with a simultaneous thermogravimetric analysis–differential scanning calorimetry (TGA-DSC) thermal analyzer (Q600 SDT, TA Instruments, USA) under a nitrogen atmosphere at a flow rate of 50 mL/min. Samples weighing 10 ± 0.5 mg were deposited on an aluminum pan. The degree of curing of the composites was determined using equation (5):

α_{cure} = 1 ‐ \frac{{∆ H}_{res}}{{∆ H}_{dyn}}

(5)

where ΔH_res is the measured residual heat of the reaction for the sample and ΔH_dyn is the measured heat of the reaction for an unreacted sample of the same material. The resulting degrees of curing for sisal/epoxy cured at room temperature with postcuring, sisal/epoxy cured at 100°C/4 h in an oven, and sisal/epoxy cured with an accelerator were 78.57%, 74.44%, and 77.91%, respectively.⁶⁴

Scanning electron microscopy (SEM)

The fractured surfaces of rectangular composite samples measuring 1.5 × 1 × 1 mm were investigated by scanning electron microscopy (SEM; JSM-7001F, Jeol). All samples were cut with the aid of a DREMEL model 3000 micro grinder. The samples were metalized in gold. Images were then captured at a voltage of 15 kV and an emission current of 81 μA at 120× magnification.

Results and discussion

Tensile and morphological behavior of the composites

Representative tensile curves of the sisal/epoxy composites and epoxy resin are shown in Figure 2. Initially, the epoxy matrices exhibited brittle and rapid failure. This failure can be attributed to the limited ability of the epoxy resin to absorb energy and thus its brittleness. The results highlight the importance of the reinforcement provided by fibers in composites, which impart greater strength and toughness to the material. Specifically, sisal/epoxy [0°] cured at room temperature and postcured exhibited an average tensile strength of 110 ± 1.13 MPa, representing an 81% increase compared to epoxy resin cured at room temperature and postcured, which recorded a tensile strength of 20 ± 1.76 MPa under the same experimental conditions. These data demonstrate that fibers enhance resistance to abrupt failure and significantly improve energy dissipation during deformation.⁶⁵

In a study conducted by Mangestiyono et al.,⁶⁶ the mechanical strength of a glass/epoxy composite for wind turbine blades showed a tensile strength of 37.9 MPa. When compared to the sisal/epoxy composites analyzed in this study, the following differences were observed: sisal/epoxy [0°] cured at room temperature and postcured (110 ± 1.13 MPa) exhibited a tensile strength 190.3% higher than the glass/epoxy composite. Sisal/epoxy [0°] cured at 100°C for 4 hours in an oven (80 ± 0.95 MPa) showed a 111.1% increase in tensile strength compared to the glass/epoxy composite. Sisal/epoxy [0°] cured with an accelerator (76 ± 1.61 MPa) demonstrated a 100.5% higher tensile strength than the glass/epoxy composite. Sisal/epoxy [cross-ply] cured at room temperature and post-cured (72 ± 2.64 MPa) had an 89.9% higher tensile strength than the glass/epoxy composite. And sisal/epoxy [cross-ply] cured with an accelerator (51 ± 2.01 MPa) exhibited a 34.6% increase in tensile strength compared to the glass/epoxy composite.

These results highlight the superior tensile performance of the sisal/epoxy composites, particularly in the [0°] fiber orientation, which provided significantly improved mechanical resistance compared to the glass/epoxy composite analyzed by Mangestiyono et al.⁶⁶ This suggests that sisal/epoxy composites could be a promising alternative for use in wind turbine blades. However, further studies are needed to fully assess their long-term performance, durability, and environmental resistance before they can become a standard material in turbine blade manufacturing. This study did not consider factors such as blade length and weight, which are critical parameters, as they fall outside its scope.

The sisal/epoxy composites exhibited a greater deformation rate than the epoxy matrices. In other words, incorporating the fibers strengthened the polymer matrix, providing greater resistance to applied forces and increasing the energy absorption capacity of the materials.⁶⁷

The results for the tensile strength, Young’s modulus, and deformation of the sisal/epoxy composites and neat epoxy matrices are presented in Table 1. The composite with the [45°] orientation had a lower tensile strength than the other composites, indicating that the fiber orientation significantly impacts this mechanical property. Sisal/epoxy [45°] cured at room temperature and postcured demonstrated a tensile strength of 25 ± 1.33 MPa, which is 77.3% lower compared to sisal/epoxy [0°] cured at room temperature and postcured (110 ± 1.13 MPa). Sisal/epoxy [45°] cured at 100°C for 4 hours in an oven exhibited a tensile strength of 22 ± 1.48 MPa, representing a 72.5% reduction relative to sisal/epoxy [0°] cured at 100°C for 4 hours in an oven (80 ± 0.95 MPa). Sisal/epoxy [45°] cured with an accelerator achieved a tensile strength of 20 ± 1.48 MPa, indicating a 73.7% decrease in comparison to sisal/epoxy [0°] cured with an accelerator (76 ± 1.61 MPa). These quantitative results emphasize the substantial impact of fiber orientation on tensile strength, where the [45°] orientation results in significantly reduced performance due to less effective load transfer along the fiber direction.

Table 1.

Mechanical tensile properties of epoxy resin and sisal/epoxy composites cured under different conditions.

Sample	Tensile strength (MPa)	Young’s modulus (GPa)	Ultimate strain (mm/mm)
Epoxy resin cured at room temperature and postcured	20a ± 1.76	1.8a ± 0.86	0.55 ± 0.57
Epoxy resin cured at 100°C/4 h in an oven	15a ± 1.35	1.5a ± 1.59	0.41 ± 0.06
Epoxy resin cured with accelerator	13a ± 1.75	1.5a ± 1.39	0.43 ± 0.20
Sisal/epoxy [0°] cured at room temperature and postcured	110d ± 1.13	8.9e ± 1.36	2.33 ± 0.23
Sisal/epoxy [0°] cured at 100°C/4 h in an oven	80c^,d ± 0.95	6.9d ± 1.72	1.32 ± 0.03
Sisal/epoxy [0°] cured with accelerator	76c^,d ± 1.61	5.4d ± 1.72	1.02 ± 0.12
Sisal/epoxy [45°] cured at room temperature and postcured	25a ± 1.33	3.8b ± 1.51	1.97 ± 0.07
Sisal/epoxy [45°] cured at 100°C/4 h in an oven	22a ± 1.48	3.6b ± 1.10	1.65 ± 0.01
Sisal/epoxy [45°] cured with accelerator	20a ± 1.48	3.4b ± 1.86	1.36 ± 0.18
Sisal/epoxy [cross-ply] cured at room temperature and postcured	72c^,d ± 2.64	6.2d ± 1.17	2,51 ± 0.21
Sisal/epoxy [cross-ply] cured at 100°C/4 h in an oven	33a^,b ± 1.85	4.9c ± 1.06	0.72 ± 0.04
Sisal/epoxy [cross-ply] cured with accelerator	51b^,c ± 2.01	5.6d ± 1.87	0.75 ± 0.05

^ab,c,d,eLetters indicate significant differences between samples according to Tukey’s test.

According to a previous study,⁶⁸ this reduction in tensile strength is attributed to fibers oriented transversely to the tensile load. Thus, the insertion of fibers into the matrix did not effectively contribute to the mechanical tensile strength; however, a 45° fiber arrangement can contribute to energy dissipation and crack retardation.

The results showed that the curing conditions significantly affected the failure stress of the composites. The stress of the sisal/epoxy composites [0°] cured at room temperature with postcuring increased by approximately 27% and 30% compared with those of the sisal/epoxy composites [0°] cured at 100°C/4 h in an oven and the sisal/epoxy composites [0°] cured with an accelerator, respectively. The superior tensile mechanical performance of the sisal/epoxy composites [0°] cured at room temperature is associated with the higher degree of crosslinking resulting from the applied postcuring process.⁶⁹

Additionally, Tukey’s test was applied in this study to identify significant differences in the tensile testing results. This statistical tool is used to test experimental hypotheses by assessing whether interactions among three or more variables are statistically significant.⁵⁵ A 5% significance level was used to compare the average results of all the epoxy resins with those of the sisal/epoxy composites, and the results revealed that adding sisal fibers influenced the tensile strength and Young’s modulus. Statistical analysis indicated no significant differences among the sisal/epoxy [0°] composites, as all samples were grouped together (group “d”). In contrast, the epoxy resin samples were significantly different from the sisal/epoxy [0°] composites (group “a”). Accordingly, the letters in Table 1 indicate significant differences between different groups.

Furthermore, as mentioned above, the tensile strength of the sisal/epoxy [0°] composites cured at 100°C for 4 hours in an oven (80 ± 0.95 MPa) was 27.3% lower than that of the sisal/epoxy [0°] samples cured at room temperature with postcuring (110 ± 1.13 MPa). The premature mechanical failure indicated that the degree of crosslinking was insufficient. The group exposed to a shorter curing time (4 h) and higher temperature (100°C) showed no significant improvement compared with the group cured at room temperature (24 h) with postcuring.

Some studies indicate that factors such as temperature and time directly interfere with curing and, consequently, with the mechanical properties of composites.⁷⁰ Therefore, polymer composites cured between 60°C and 100°C may not complete the curing process, contributing to a lower degree of crosslinking.⁷¹ It was also observed that the sisal/epoxy composites [0°] cured with an accelerator (76 ± 1.61 MPa) exhibited mechanical behavior similar to that of the sisal/epoxy [0°] samples cured at 100°C for 4 h in an oven (80 ± 0.95 MPa).

Rapid crosslinking, which may result in a lower degree of crosslinking and render the material more brittle, is a significant issue when an accelerator is used in curing. Nevertheless, the tensile strength of the sisal/epoxy composites [0°] cured with an accelerator in this study was satisfactory, as indicated by the DSC results. Statistical analysis confirmed that the tensile strength of this group was similar to that of the sisal/epoxy [0°] group cured at room temperature with postcuring. A similar result was reported in a previous paper,⁷² where the use of an accelerator simultaneously enhanced the crosslinking density and made the resin more ductile without compromising its mechanical properties.

The fracture morphology of all composites after the tensile test was analyzed by SEM to evaluate resin/fiber adhesion. Figure 3(a)–(c) correspond to composites reinforced with fibers oriented at 0°. Notably, the micrographs indicate that the fibers were pulled out of the matrix. Furthermore, the removal of amorphous components (lignin and hemicellulose) via chemical treatment was similar for the three curing conditions, as the sisal fibers exhibited flat cross sections. The observed flat cross-sections of the sisal fibers may indicate that the fibers are inherently susceptible to fracture without large prior deformation.⁷³

Figure 3(d)–(f) correspond to composites with a 45° orientation. Mechanical anchoring occurred between the fibers and the resin, resulting in uniform and consistent mechanical properties for this group of composites. According to a previous study,⁷⁴ the adhesion between sisal fibers and epoxy is directly attributed to the chemical treatment, which improves fiber/matrix bonding and overall composite performance. This adhesion can be confirmed by this study,⁷⁵ which observed an increase in tensile strength for the treated fiber composites, reaching 41 MPa, compared to the untreated fiber composites, which achieved 30 MPa. This represents a 36.7% improvement in tensile strength. The results are further supported by the micrographs presented in this study. Finally, Figure 3(g)–(i) show the sisal/epoxy [cross-ply] composites. The fibers oriented at 0° have a flat cross-section, and fibers are oriented in the longitudinal direction. The nature of fibrous materials can explain the flat cross-sectional cut of the fibers oriented at 0°: fibers parallel to the cutting surface exhibit flatter cross-sections due to the plane orientation of the cutting blade.⁷⁶

Effective fiber/matrix adhesion can be attributed to several factors. During composite manufacturing, the sisal fibers are impregnated with the epoxy resin, allowing interaction between the two phases. The resin infiltrates the fiber structure, encapsulating it and forming a durable and cohesive interfacial bond, which enhances mechanical performance and load transfer within the composite. For instance, studies have shown that the tensile strength of natural fiber-reinforced polymer composites can increase by up to 40%–50% when the resin effectively infiltrates the fiber structure, as demonstrated by^5,77 their research. This improvement is attributed to the enhanced interfacial adhesion and stress distribution between the fibers and the matrix.

Applying surface treatments to sisal fibers before composite manufacturing can improve adhesion between the fibers and the matrix.⁷⁸ Thus, combining an optimized epoxy resin impregnation process and a surface treatment enhanced fiber/matrix adhesion in the longitudinally oriented fibers.¹⁴ This interfacial bonding was crucial for ensuring the high mechanical performance and structural integrity of the sisal/epoxy composites, as previously mentioned in the stress behavior topic. Silva et al.⁷⁹ have demonstrated that effective resin infiltration and NaOH surface treatment can improve the tensile strength of natural fiber-reinforced composites by up to 28%, highlighting the role of resin infiltration and interfacial adhesion in enhancing the overall mechanical properties. SEM analysis did not reveal a clear correlation between the different curing conditions and the fracture morphology of the sisal/epoxy composites. Regardless of the curing conditions applied, all samples exhibited a pull-out phenomenon, indicating consistent fiber detachment behavior across the curing variables tested.

Tensile creep properties

Figure 4 shows the experimental creep curve plotted alongside the fitted curves of the Findley and Burger models for the composite groups and epoxy resins cured under different curing conditions. Table 2 provides the values of the parameters of the Findley model obtained from equation (1).

Table 2.

Parameters of the Findley model for epoxy resin and sisal/epoxy composites.

Curing conditions	Parameter	Epoxy resin	Sisal/epoxy [0°]	Sisal/epoxy [45°]	Sisal/epoxy [Cross-ply]
Cured at room temperature and postcured	-	σ = 15 MPa	σ = 40 MPa	σ = 5 MPa	σ = 25 MPa
	A	(2.95 ± 0.3) × 10⁻³	(4.11 ± 0.3) × 10⁻³	(1.23 ± 0.5) × 10⁻³	(2.56 ± 0.5) × 10⁻³
	ε₀	(12.98 ± 0.5) × 10⁻⁵	(33.01 ± 0.02) × 10⁻⁵	(4.73 ± 0.5) × 10⁻⁵	(20.85 ± 0.5) × 10⁻⁵
	n	0.1151 ± 0.4	0.2566 ± 0.4	0.0860 ± 0.2	0.1542 ± 0.3
	R²	0.992 ± 0.2	0.993 ± 0.22	0.990 ± 0.2	0.992 ± 0.2
Cured at 100°C/4 h in an oven	A	(2.67 ± 0.5) × 10⁻³	(5.51 ± 0.2) × 10⁻³	(2.31 ± 0.1) × 10⁻³	(3∙78 ± 0.2) × 10⁻³
	ε₀	(12.09 ± 0.5) × 10⁻⁵	(37.47 ± 0.1) × 10⁻⁵	(5.79 ± 0.2) × 10⁻⁵	(21.21 ± 0.4) × 10⁻⁵
	n	0.1196 ± 0.2	0.2585 ± 0.5	0.0864 ± 0.2	0.1539 ± 0.3
	R²	0.990 ± 0.2	0.990 ± 0.4	0.991 ± 0.2	0.991 ± 0.2
Cured with an accelerator	A	(2.13 ± 0.4) × 10⁻³	(3.01 ± 0.02) × 10⁻³	(1.04 ± 0.4) × 10⁻³	(1.05 ± 0.2) × 10⁻³
	ε₀	(11.69 ± 0.3) × 10⁻⁵	(31.86 ± 0.3) × 10⁻⁵	(3.71 ± 0.5) × 10⁻⁵	(20.73 ± 0.4) × 10⁻⁵
	n	0.1199 ± 0.2	0.2587 ± 0.2	0.0863 ± 0.3	0.1540 ± 0.2
	R²	0.993 ± 0.3	0.998 ± 0.2	0.992 ± 0.3	0.995 ± 0.2

According to Table 2, A follows a trend similar to ε₀, which represents instantaneous elastic deformation. The sisal/epoxy [0°] cured at 100°C for 4 hours in an oven exhibited the highest values for these parameters (5.51 ± 0.2 × 10⁻³), which were 45% higher than those of the sisal/epoxy [0°] cured with the accelerator (3.01 ± 0.02 × 10⁻³). Additionally, the postcuring curves of the sisal/epoxy [0°] cured at room temperature (4.11 ± 0.3 × 10⁻³) showed intermediate values, representing a 25% decrease compared to the oven-cured samples and a 37% increase compared to the accelerator-cured samples.

A study conducted by Reddy et al.⁸⁰ reported a 15% increase in the stiffness of composites made from E-glass fibers compared to the polymer matrix, with these fibers being the most widely used in the wind turbine blade industry. Similarly, the sisal/epoxy composites cured with an accelerator (3.01 ± 0.02 × 10⁻³) exhibited the lowest deformation and greater stiffness among all the sisal/epoxy composites developed in this study. This suggests that the curing cycle with an accelerator could be a potential candidate for partially replacing synthetic fibers for the wind turbine blade. However, further specific studies are required to evaluate its feasibility, particularly regarding interlaminar failure between the matrix and fibers, which can occur due to layer overlaps and potentially lead to mechanical failure of the composite. Therefore, in-depth research on optimizing delamination resistance is essential cause these investigations can ensure the material’s structural integrity before it can become a standard in wind turbine blade manufacturing.

Parameter A of the Findley equation is related to the transient deformation amplitude, which represents the initial deformation characteristics of the composite material.⁸¹ Sisal/epoxy [0°] presents A= (5.51 ± 0.2) × 10⁻³, sisal/epoxy [45°], A= (2.31 ± 0.1) × 10⁻³ and sisal/epoxy [Cross-ply] A= (3.78 ± 0.2) × 10⁻³ composites cured at 100°C for 4 hours in an oven exhibited a more pronounced initial deformation, as evidenced by the increased A values compared to the other cure conditions. This outcome was directly influenced by the elevated curing temperature, indicating a greater transient deformation amplitude within the composite structure. This effect can be attributed to the enhanced segmental motion within the polymer network, facilitating greater short-term deformation under stress. Elevated curing temperatures likely reduce structural constraints within the composite matrix, making it more susceptible to deformation in the initial stages of creep.⁸²

Parameter A in the Findley equation indicates creep behavior within the primary region, representing the initial phase of the creep process, characterized by high deformation rates and low creep resistance. Therefore, elevated A values suggest reduced creep resistance during this initial phase, with the material being more prone to rapid deformation.⁸³ Variations in curing conditions influence this behavior by modifying molecular mobility and network density within the composite matrix, ultimately impacting the ability of the material to withstand deformation under sustained stress. Elevated curing temperatures can lead to an increase in the A values, indicating a higher susceptibility to transient deformation. In a study conducted by Youssef et al.,⁴² E-glass/vinyl-ester composites cured at elevated temperatures exhibited A parameter values similar to those found in Table 2. The recorded values were 1.92 × 10⁻³ (E-glass/vinyl-ester 166), 2.00 × 10⁻³ (E-glass/vinyl-ester 169), 2.08 × 10⁻³ (E-glass/vinyl-ester 173), 2.27 × 10⁻³ (E-glass/vinyl-ester 179), and 2.38 × 10⁻³ (E-glass/vinyl-ester 183).

As demonstrated in the preceding analysis, sisal/epoxy [0°] composite cured with the accelerator displayed significantly improved creep resistance compared to the other sample groups. This conclusion is supported by the lower A = [(3.01 ± 0.02) × 10⁻³] values presented in Table 2, which quantitatively reflect the reduced susceptibility to transient deformation in these samples. This suggests that curing with the accelerator promoted a more resilient structure in the primary creep region, reducing initial deformation under continuous stress. The accelerator likely enhances the cross-linking density or matrix cohesion, thereby restricting molecular mobility and increasing resistance to early-stage creep deformation. This structural modification reinforces the composite’s stability, slowing the rate of initial deformation and enhancing the overall creep resistance.⁸⁴ Reis et al.⁸⁵ reported higher A values for an epoxy/cork powder composite ranging between 1.3 × 10⁻¹ and 8.4 × 10⁻¹, which are higher than the values obtained for the sisal/epoxy [0°] samples in this study. Specifically, the sisal/epoxy [0°] samples cured at 100°C for 4 hours exhibited 5.51 ± 0.2 × 10⁻³, which is 95.7% lower than the lowest value reported by Reis et al. The sisal/epoxy [0°] samples cured with the accelerator showed 3.01 ± 0.02 × 10⁻³, which is 96.5% lower, and the samples cured at room temperature exhibited 4.11 ± 0.3 × 10⁻³, which is 95.2% lower than the values reported for the epoxy/cork powder composite.

The data in Table 2 indicate that n remained largely unaffected by variations in curing conditions. This stability can be attributed to the strong temperature dependence of n, which plays a key role in regulating creep rate and is responsive to applied stress levels. The minimal change in n values across curing conditions implies that other factors, such as the thermal conditions during mechanical testing, are the primary drivers for this parameter rather than the specific curing process itself.⁸⁶ However, this study conducted creep analysis at a constant room temperature of 27°C, which may explain the lack of significant variation in n values.

According to the literature,⁸⁷ ε₀ is related to the instantaneous deformation capacity of a material, indicating the extent of initial deformation during the onset of creep. Higher ε₀ values denote increased susceptibility to creep deformation. The results demonstrate that the sisal/epoxy [0°] samples cured at 100°C for 4 hours in the oven exhibited reduced creep resistance (37.47 ± 0.1 × 10⁻⁵), with greater initial deformation compared to those cured at room temperature postcuring (33.01 ± 0.02 × 10⁻⁵), which showed a 12.1% lower creep resistance. In contrast, the sisal/epoxy [0°] samples cured with the accelerator (31.86 ± 0.3 × 10⁻⁵) exhibited increased creep resistance, suggesting a 15.0% higher resistance and reduced initial deformation compared to the oven-cured samples. This behavior was also observed in all samples. This improved resistance likely results from the structural effects of the accelerator, which potentially increase network stiffness and reduce early deformation under a sustained load. Postcuring can enhance the stiffness of the polymer matrix due to increased cross-linking, resulting in a more rigid molecular structure. This additional cross-linking creates a tighter network within the composite, which can slightly increase the stiffness of the material. Consequently, postcuring enhances the composite’s resistance to deformation under prolonged stress by reducing molecular mobility within the polymer network. This process improves stability during the initial and primary creep phases, as it restricts the rearrangement of polymer chains, thereby minimizing strain accumulation over time.⁸⁸ This effect was observed in a polymeric composite reinforced with vegetal fibers, where postcuring led to a 40% increase in resistance to deformation.⁸⁹

In summary, the Findley equation was suitable for describing the creep behavior of the sisal/epoxy composites under the different curing conditions applied in this study. The Burger model was used to describe the creep behavior of the sisal/epoxy composites. The values of the parameters E_M, η_M, E_K, and η_K were determined from the experimental data and are presented in Table 3.

Table 3.

Parameters of the Burger model for the sisal/epoxy composites.

Curing conditions	Parameter	Epoxy resin	Sisal/epoxy [0°]	Sisal/epoxy [45°]	Sisal/epoxy [Cross-ply]
Cured at room temperature and postcuring	-	σ = 15 MPa	σ = 40 MPa	σ = 5 MPa	σ = 25 MPa
	E _M	2496 ± 0.33	4547 ± 0.5	1103 ± 0.2	3458 ± 0.3
	E _K	4159 ± 0.4	10526 ± 0.5	2569 ± 0.3	7003 ± 0.2
	η _K	(7.25± 0.4) × 10⁵	(6.42 ± 0.2) × 10⁵	(2.29 ± 0.3) × 10⁵	(4.77 ± 0.6) × 10⁵
	η _M	(2.74± 0.2) × 10⁶	(18.16 ± 0.3) × 10⁶	(4.20 ± 0.5) × 10⁶	(9.29 ± 0.5) × 10⁶
	R²	0.992 ± 0.2	0.993 ± 0.3	0.989 ± 0.2	0.994 ± 0.2
Cured at 100°C/4 h in an oven	E _M	2226 ± 0.3	4860 ± 0.2	1197 ± 0.3	3669 ±0.5
	E _K	4325 ± 0.3	10368 ± 0.4	2151 ± 0.5	6855 ±0.4
	η _K	(5.30± 0.2) × 10⁵	(11.00 ± 0.4) × 10⁵	(6.36 ± 0.2) × 10⁵	(9.82 ± 0.4) × 10⁵
	η _M	(9.91± 0.3) × 10⁶	(25.90 ± 0.03) × 10⁶	(5.56 ± 0.5) × 10⁶	(12.87 ± 0.3) × 10⁶
	R²	0.990 ± 0.2	0.997 ± 0.2	0.989 ± 0.1	0.993 ± 0.2
Cured with an accelerator	E _M	2117 ± 0.3	4270 ± 0.5	1084 ± 0.3	3303 ± 0.2
	E _K	4499 ± 0.3	10753 ± 0.3	2693 ± 0.4	7212 ± 0.3
	η _K	(3.48± 0.4) × 10⁵	(3.95 ± 0.2) × 10⁵	(1.47 ± 0.2) × 10⁵	(2.21 ± 0.3) × 10⁵
	η _M	(2.09± 0.3) × 10⁶	(11.70 ± 0.2) × 10⁶	(2.16 ± 0.5) × 10⁶	(5.01 ± 0.5) × 10⁶
	R²	0.993 ± 0.1	0.994 ± 0.2	0.988 ± 0.3	0.993 ± 0.2

E_M is closely associated with the capacity of a material for immediate deformation upon loading. Higher E_M values suggest a greater tendency for instantaneous deformation, indicating a less rigid structure that deforms more readily. Conversely, lower E_M values indicate reduced susceptibility to initial deformation, reflecting a more robust structural network. The variation in E_M across different curing conditions reveals the impact of these processes on the composite’s response to stress and, thus, its immediate deformation behavior.⁹⁰ The sisal/epoxy [0°] composites cured at 100°C for 4 hours in the oven exhibited the highest E_M values (4860 ± 0.2), which were 13.8% higher compared to the sisal/epoxy [0°] composites cured with an accelerator (4270 ± 0.5).

The above results suggest that the resin–fiber interface influences creep deformation. A stronger interaction between the polymer matrix and fibers constrains the mobility of the polymer network, effectively increasing the resistance to deformation under prolonged stress. Improved bonding at this interface results in a more rigid and integrated system, where reduced movement within the composite matrix contributes to heightened creep resistance. Different curing conditions that promote adhesion at this interface may thus enhance the overall stability of the composite and minimize its deformation over time.⁹¹ Furthermore, curing with an accelerator increased the stiffness of the material, resulting in reduced deformation, as evidenced by the tensile test results for the corresponding group.

Generally, lower E_K values correspond to lower rigidity in amorphous polymeric chains.⁹² Thus, the sisal/epoxy [0°] composites cured in an oven at 100°C for 4 hours (10368 ± 0.4) exhibited a lower deformation rate (3.7%) compared to sisal/epoxy [0°] composites cured with an accelerator (10753 ± 0.3).

Compared with the other composites, the epoxy resin composites reinforced with sisal fiber and cured with an accelerator showed the lowest η_K values, suggesting delayed onset of material flow due to restricted polymer chain relaxation. This restriction likely arises from the formation of a more effective fiber/matrix interface, which constrains molecular movement and limits the flow capacity of the material. Consequently, the composites cured with an accelerator demonstrated a lower apparent viscosity, reflecting increased structural rigidity and enhanced resistance to deformation. These findings reveal that curing with an accelerator impacts the creep behavior of the resulting composite by promoting greater dimensional stability under prolonged stress and reinforcing interfacial adhesion and network density.⁹³

Higher η_M values indicate that the viscosity of a material predominates over its elasticity.⁹⁴ The data in Table 3 indicate that the sisal/epoxy [0°] composites cured at 100°C for 4 hours in an oven (25.90 ± 0.03 × 10⁶) exhibited a 121.4% higher flow capacity compared to the sisal/epoxy [0°] composites cured with an accelerator (11.70 ± 0.2 × 10⁶). This suggests that the oven-cured composites displayed higher viscosity and reduced deformation resistance, facilitating material flow under applied stress. The increased flow capacity in these composites implies a less rigid structural network, likely resulting from the curing conditions facilitating molecular movement within the polymer matrix.⁹⁵

Even though this study primarily examined the effects of curing conditions on the creep behavior of sisal/epoxy composites, the interaction between the applied stress and fiber orientation also influences the parameters of the Burger model. Fiber orientation plays a significant role in stress distribution and load transfer within composites, impacting deformation behavior and modifying the values of analytical model parameters under varying stress levels. This interplay underscores the critical need to account for applied stress and fiber alignment when evaluating creep behavior, as these factors jointly shape the composite’s response to long-term loading through stress dispersion and interfacial bonding.

For E_M, an increase in value was observed with increasing stress across all samples, reflecting the material’s stiffness response under loading. This trend suggests a reduction in stiffness as stress levels increase.³⁵ Notably, among the tested materials the sisal/epoxy [0°] composites exhibited the highest E_M values, indicating a greater tendency toward immediate elastic deformation. The alignment of fibers in the [0°] orientation likely enhances this initial elastic response, as it supports efficient load transfer along the fiber axis, thus promoting deformation under stress. These findings reveal the role of fiber orientation in shaping the elasticity and overall creep response of the composite. The η_K value also rose with increasing applied stress, reflecting a reduction in material stiffness. This behavior arises from the enhanced mobility of the polymer chains under sustained stress, revealing the inherent viscous characteristics of the material. The impact of fiber orientation on η_K is significant, as specific alignments can either resist or facilitate chain movement, influencing how the composite accommodates stress over time. Consequently, fiber orientation not only affects the creep behavior but also plays a pivotal role in balancing the elastic and viscous responses within the composite and is therefore important in tailoring material performance under loading. Similar behavior was reported by Ornaghi et al.⁹⁶ in their study on epoxy resin composites reinforced with carbon fibers.

The parameter η_M, which reflects irreversible deformation in the amorphous regions of polymers, showed notable variation with increased stress across all samples. This change suggests that higher stress levels amplify permanent deformation in these regions, especially as polymer chain mobility becomes more pronounced.⁹⁴

Importantly, for all the analyzed samples, fitting with Burger’s equation yielded a high average coefficient of determination (R²) of approximately 0.99. These fitting results are consistent with the literature, providing additional evidence for the validity and robustness of the results obtained in this study.

Long-term performance and lifecycle considerations

Importantly, no signs of material failure were observed during the 24-h analysis period, as the applied stresses did not reach the critical levels required to cause failure or rupture. Notably, in this study, the third stage of creep, which typically requires higher stresses or a longer testing time, was not observed. This suggests that the analysis conditions used were adequate for studying the initial stages of creep in the composites but not sufficient to observe the third creep stage.

A technical approach was employed to estimate the rupture of the sisal/epoxy [0°] and sisal/epoxy [cross-ply] composites under different curing conditions through creep analysis. This approach involves extrapolating creep rupture data to long-term scenarios, considering a combination of time, temperature, and stress parameters. This method allows for the estimation of a material’s strength over time under the relevant service conditions.

The service life of the materials was estimated on the basis of the Larson‒Miller parameter, which is widely used for predicting the life expectancy of metallic materials. Although this method is less commonly used for polymeric materials, some studies have employed it to estimate the service life of polymers.^97,98 The Larson‒Miller parameter determines service life on the basis of a combination of time and temperature and represents an established approach for predicting service life under different service conditions.⁹⁹ Figure 5 shows the Larson‒Miller stress‒time curves of the sisal/epoxy [0°] and sisal/epoxy [cross-ply] composites under different curing conditions.

As shown in Figure 5, the rupture times of the sisal/epoxy [0°] and [cross-ply] composites were determined. As the applied stress increased, the rupture time decreased. The sisal/epoxy [0°] composites subjected to a higher stress of 40 MPa have a projected service life of approximately 190–198 years under various curing conditions, with those cured with an accelerator showing the longest rupture time of roughly 198 years. In contrast, the sisal/epoxy [cross-ply] composites subjected to a lower stress of 25 MPa have a longer estimated service life of approximately 230–238 years, with the accelerator-cured composites again standing out with an extended rupture time of approximately 238 years. The difference in service life is due to the higher stress levels of the [0°] composites, while the [cross-ply] composites benefit from a lower applied stress, resulting in greater durability.

A previous study¹⁰⁰ that predicted the lifespan of a polyethylene tube concluded that the Larson‒Miller equation can be used to predict the rupture of polymer materials. However, applying the Larson‒Miller method to fiber-reinforced polymeric composites would require further research to establish reliable correlations between the mechanical properties, thermomechanical conditions, and lifespan of these materials. Therefore, clearer relationships between the mechanical properties of composites, such as tensile strength and Young’s modulus, and temperature must be established for fiber-reinforced polymer composites. Elucidating these relationships would require experimental tests with different temperatures and durations to better understand how composites respond under specific thermomechanical conditions.

Furthermore, moisture absorption can impact the long-term performance of sisal/epoxy composites, particularly in humid environments, as sisal fibers are prone to water uptake, which can weaken the fiber‒matrix interface and cause dimensional changes.¹⁰¹ This moisture can also degrade the epoxy matrix, reducing the composite’s mechanical properties. Applying water-resistant treatments or coatings and optimizing formulations can help mitigate these effects, enhancing the durability of sisal/epoxy composites in humid conditions.¹⁰²

Batista et al.,¹⁰³ who developed sisal/epoxy composites, conducted water absorption testing followed by tensile and fatigue characterization. The authors concluded the treated fiber composites exhibited superior tensile strength (112 MPa) and higher residual strength after cyclic loads compared to neat epoxy (43 MPa) and cross-ply epoxy composites. Additionally, the treated fiber composites demonstrated approximately 160% greater tensile strength than neat epoxy and 50% lower water absorption than untreated fiber composites. These findings suggest potential benefits in using treated sisal fibers to enhance durability in environments where water resistance and mechanical reliability are critical.

In addition, vegetable fiber composites can have lower environmental impacts than synthetic fiber composites if their production impacts are sufficiently minimized or if they enhance the final product’s overall environmental performance. Several studies suggest that replacing synthetic fibers with natural fibers can positively impact energy savings and greenhouse gas reduction, particularly when these materials are used to lower the weight of automobiles and aircraft.¹⁸ Therefore, sisal fiber could be a strong candidate for use in composites to reduce the weight of cars and airplanes, contributing positively to environmental goals. Its natural properties offer the potential for energy savings and lower greenhouse gas emissions, supporting sustainable advancements in the automotive and aerospace industries.

Conclusions

As discussed above, the different curing conditions tested in this study critically influenced the tensile and tensile creep mechanical properties of the prepared samples. The sisal/epoxy composite [0°] cured at room temperature exhibited superior tensile strength. Moreover, the tensile strength was optimized by applying room-temperature curing and postcuring to the [0°] composite. According to the tensile creep results, the epoxy resin and all groups of sisal/epoxy composites cured with an accelerator exhibited the highest creep resistance. For creep, the parameter values obtained through the Findley and Burger models were correlated with the properties of the analyzed materials.

Both models were suitable and adequately fitted the experimental data. The different curing conditions significantly affected the creep properties of the composites. Furthermore, the accelerator curing method differs from that proposed by the manufacturer, which recommends curing at room temperature with postcuring. The accelerator proved to be advantageous, resulting in composites with higher creep resistance. In summary, the analysis of creep under tension, coupled with suitable fitting models, successfully elucidated the properties of sisal/epoxy composites under different curing conditions.

The long-term performance of the tested materials demonstrates that the rupture time of sisal/epoxy composites is influenced by both the applied stress and curing conditions, with accelerated curing significantly enhancing durability. Under higher stress, the [0°] composites have a shorter service life than do the more durable [cross-ply] composites subjected to lower stress.

The adoption of sisal/epoxy composites in sectors traditionally reliant on synthetic materials could yield significant economic benefits. First, the use of renewable natural fibers such as sisal can reduce reliance on petroleum-based products, potentially lowering material costs in the long term. This transition may also improve sustainability, appeal to environmentally conscious consumers, and meet regulatory pressure for greener products. Furthermore, by shifting to these composites, companies might benefit from cost reductions and a strengthened market position due to enhanced corporate social responsibility.

Footnotes

Acknowledgments

The authors would like to thank Decanato de Pós Graduação University of Brasília (DPG-UnB), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for their financial support of this project.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors would like to thank Decanato de Pós Graduação University of Brasília (DPG-UnB), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for their financial support of this project.

ORCID iDs

Linconl Araujo Teixeira

Sandra Maria da Luz

Data availability statement

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.*

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