Comparing Epoxy Adhesives for Stronger Natural Fiber Composite Joints: Insights From Recent Engineering Research

Natural fiber-reinforced polymer composites continue to gain attention as sustainable alternatives to conventional synthetic composite materials in engineering applications. Their low density, renewable origin, and competitive mechanical performance make them attractive for automotive, construction, marine, and lightweight structural components. However, the structural performance of these composites depends not only on the composite material itself but also on the quality of adhesive joints used during assembly. The reviewed study investigates how two commercially available epoxy adhesive systems influence the bond strength of sisal, jute, and hybrid sisal-jute composite joints using three common joint configurations. Through tensile testing and microscopic failure analysis, the research provides practical evidence for selecting suitable adhesive systems to improve the mechanical reliability of natural fiber composite structures.

Bibliographic Information

Item	Information
Article Title	Effect of Two Different Epoxy-Adhesives on the Bond Strength of Single-Lap, Butt and Scarf Joints of Sisal-Jute Fiber Composites
Authors	Kassahun Gashu Melese; Dessalegn Jaweso Eshetu
Journal	Engineering Reports
Volume	7
Issue	12
Publication Year	2025
Article Number	e70469
DOI	https://doi.org/10.1002/eng2.70469
Publisher	John Wiley & Sons Ltd.
License	Creative Commons Attribution (CC BY)
ISSN	2577-8196 (Online)
Keywords	butt joint; FE-SEM; scarf joint; single-lap joint

Research Background

Natural fiber composites have become increasingly important in modern engineering because they offer a combination of lightweight characteristics, environmental sustainability, and competitive mechanical properties. Sisal and jute fibers, in particular, are widely investigated as reinforcements for polymer composites due to their availability, low cost, and favorable specific strength. As industries seek greener materials without significantly compromising structural performance, these renewable fibers have emerged as promising alternatives to synthetic reinforcements such as glass fiber.

Despite the growing use of natural fiber composites, the structural integrity of assembled components depends heavily on the performance of adhesive joints. Adhesive bonding provides several engineering advantages over conventional mechanical fastening, including more uniform stress distribution, lower structural weight, reduced stress concentration, and simplified manufacturing. Consequently, bonded joints are extensively employed in automotive, aerospace, marine, household products, and other composite-based structures.

The effectiveness of adhesive joints is governed primarily by the quality of the interface between the composite substrate and the adhesive layer. Poor interfacial bonding limits efficient load transfer from the polymer matrix to the reinforcing fibers, resulting in premature debonding, crack initiation, and reduced service life. Engineering factors such as adhesive formulation, joint geometry, curing conditions, and composite architecture therefore play essential roles in determining the overall mechanical performance of bonded structures.

Previous research has investigated various joint configurations, including single-lap, butt, and scarf joints, as well as different epoxy adhesive systems. Earlier studies also evaluated parameters such as overlap length, scarf angle, adhesive thickness, and manufacturing techniques. While these investigations contributed substantially to understanding adhesive mechanics, they generally focused on individual composite systems or single adhesive formulations, leaving limited comparative evidence regarding adhesive selection for hybrid natural fiber composites.

The reviewed study identifies this knowledge gap by systematically comparing two epoxy adhesive systems—LY-556/HY951 and XIN-100 IN/XIN-900—across three commonly used joint configurations. Unlike previous investigations that emphasized either joint geometry or adhesive performance independently, this research evaluates both variables simultaneously while considering three reinforcement systems: sisal, jute, and hybrid sisal-jute woven composites. This integrated comparison enables a more comprehensive understanding of how adhesive characteristics influence bond strength in sustainable composite materials.

In addition to mechanical testing, the study incorporates Field Emission Scanning Electron Microscopy (FE-SEM) to investigate fracture morphology and failure mechanisms. By combining quantitative tensile testing with microscopic observations, the researchers establish stronger evidence linking adhesive performance to fiber-matrix interfacial behavior. This dual approach provides valuable engineering insights for designing stronger bonded composite structures based on renewable natural fibers.

Research Objective

To compare the adhesive bond strength of sisal, jute, and hybrid sisal-jute woven epoxy composites using two different epoxy adhesive systems.
To evaluate the mechanical performance of three commonly used bonded joint configurations, namely single-lap, butt, and scarf joints.
To determine how adhesive selection influences the tensile strength and failure behavior of natural fiber composite joints.
To identify the composite material and adhesive combination that provides the highest joint strength under tensile loading.
To investigate fracture mechanisms and fiber-matrix interfacial characteristics using Field Emission Scanning Electron Microscopy (FE-SEM).
To provide practical engineering guidance for selecting adhesive systems suitable for structural applications involving sustainable natural fiber composites.

Why This Research Matters

Supports sustainable engineering materials. The research contributes to the development of renewable natural fiber composites that can replace heavier and less environmentally friendly synthetic materials in structural applications.
Improves structural reliability. By comparing adhesive systems and joint configurations simultaneously, the study provides engineers with evidence for selecting bonding methods that maximize structural performance.
Addresses an important knowledge gap. Few previous studies have systematically compared different epoxy adhesives across multiple joint geometries for sisal, jute, and hybrid natural fiber composites within a single experimental framework.
Combines mechanical and microscopic analyses. Integrating tensile testing with FE-SEM observations allows the researchers to explain not only which adhesive performs better but also why different failure mechanisms occur at the fiber-matrix interface.
Provides practical design guidance. The findings help engineers choose suitable adhesive systems for composite structures used in transportation, lightweight manufacturing, construction, and other engineering sectors.
Promotes wider adoption of natural fiber composites. Improving adhesive joint performance increases confidence in using renewable composite materials for load-bearing engineering applications where reliable structural connections are essential.

Research Methodology

The study employed an experimental research design to evaluate the adhesive bond performance of natural fiber-reinforced epoxy composites. The investigation compared two epoxy adhesive systems across three commonly used joint configurations while examining three composite reinforcement materials. Mechanical characterization through tensile testing was complemented by microscopic examination using Field Emission Scanning Electron Microscopy (FE-SEM), allowing both quantitative and qualitative assessment of adhesive performance.

Composite Materials

Three laminated composite systems were fabricated using woven natural fiber reinforcements embedded in an epoxy matrix:

Sisal fiber reinforced epoxy composite.
Jute fiber reinforced epoxy composite.
Hybrid sisal-jute woven epoxy composite with a SJJS (Sisal–Jute–Jute–Sisal) stacking sequence.

Composite laminates were manufactured using the hand lay-up technique. Four woven fiber layers were arranged according to the designated reinforcement configuration, while maintaining approximately 60% fiber weight fraction throughout all specimens. Prior to fabrication, fiber mats were dried to minimize moisture content, and laminates were cured under controlled pressure at room temperature to ensure consistent composite quality.

Adhesive Systems

The research compared two commercially available epoxy adhesive systems possessing different mechanical characteristics:

LY-556/HY951, representing a conventional epoxy adhesive known for its rigidity and widespread engineering use.
XIN-100 IN/XIN-900, a toughened epoxy adhesive designed to provide improved fracture toughness, crack resistance, and enhanced interfacial adhesion.

This comparison enabled the researchers to determine how adhesive formulation influences the mechanical behavior of bonded natural fiber composites under tensile loading.

Surface Preparation

To minimize variability during bonding, all composite adherends underwent identical surface preparation procedures before adhesive application. Bonding surfaces were mechanically abraded using silicon carbide abrasive paper to create a uniform roughened surface suitable for mechanical interlocking. The prepared surfaces were subsequently cleaned using acetone and isopropyl alcohol to remove contaminants before oven drying to eliminate residual moisture. This standardized preparation protocol ensured consistent bonding conditions across every experimental specimen.

Joint Configurations

Three adhesive joint geometries commonly used in structural engineering were investigated:

Single-lap joint.
Butt joint.
Scarf joint with a 45° scarf angle.

All joint specimens were fabricated according to ASTM D5868 requirements. Adhesive thickness was carefully controlled using calibrated spacers and precision clamping fixtures to maintain consistent bond-line thickness throughout the investigation.

Mechanical Testing

After complete curing, bonded specimens were tested using an INSTRON 5982 Universal Testing Machine under uniaxial tensile loading. Tensile experiments followed ASTM D5868 procedures using constant crosshead displacement rates. Multiple specimens were tested for every combination of composite material, adhesive type, and joint geometry to ensure reliable experimental observations.

The principal mechanical parameters recorded during testing included maximum failure load, tensile strength, displacement at failure, and elastic behavior. These measurements enabled direct comparison of bond performance among different adhesive systems and joint configurations.

Morphological Analysis

Following tensile failure, fractured specimens were examined using Field Emission Scanning Electron Microscopy (FE-SEM). Prior to imaging, fracture surfaces were coated to improve electrical conductivity. The microscopic investigation focused on identifying dominant failure mechanisms, including fiber fracture, matrix cracking, fiber pull-out, interfacial debonding, void formation, and adhesive failure characteristics.

The combination of tensile testing and FE-SEM observations allowed the researchers to relate macroscopic mechanical performance to microscopic interfacial behavior between fibers, epoxy matrix, and adhesive layers.

Key Findings

XIN-100 IN/XIN-900 Produced Superior Bond Strength

Across all composite materials and joint configurations, the XIN-100 IN/XIN-900 adhesive consistently demonstrated higher tensile performance than the conventional LY-556/HY951 system. Statistical analysis confirmed that the improvements were significant, indicating that adhesive formulation plays a major role in determining the structural integrity of bonded natural fiber composites.

The superior performance of XIN-100 IN/XIN-900 was attributed to its higher fracture toughness and improved interfacial adhesion, enabling greater resistance to crack initiation and crack propagation during tensile loading.

Hybrid Sisal-Jute Composite Achieved the Highest Mechanical Performance

Among the three reinforcement systems investigated, the hybrid sisal-jute composite consistently exhibited the highest bond strength regardless of joint configuration or adhesive type. The balanced SJJS laminate arrangement promoted more uniform stress distribution while combining the favorable characteristics of both natural fibers.

The researchers observed that the hybrid configuration generated synergistic reinforcement effects rather than simply averaging the mechanical properties of sisal and jute individually.

Butt Joints Benefited Most from the Toughened Adhesive

Although XIN-100 improved the performance of every joint configuration, the greatest improvement occurred in butt joints. Compared with LY-556/HY951, the XIN-100 adhesive increased butt joint bond strength by approximately:

16% for hybrid composites.
13% for jute composites.
9% for sisal composites.

These improvements demonstrate that adhesive toughness becomes increasingly important in joint geometries subjected primarily to tensile loading.

Scarf Joints Demonstrated the Most Efficient Stress Distribution

Among the investigated joint geometries, scarf joints exhibited the most favorable stress distribution because the adhesive layer remained closely aligned with the loading direction. This geometry reduced stress concentrations commonly observed in lap and butt joints, allowing loads to be transferred more uniformly through the bonded interface.

Microscopic Observations Explained Mechanical Performance

FE-SEM examination revealed clear differences in fracture morphology between the two adhesive systems. Specimens bonded using XIN-100 exhibited shorter fiber pull-out lengths, fewer interfacial voids, stronger fiber-matrix adhesion, and more extensive fiber fracture before joint failure. In contrast, LY-556 specimens showed greater interfacial debonding, longer fiber pull-out, and increased void formation, indicating weaker adhesion between adhesive and composite substrate.

Adhesive Selection Had Greater Influence Than Joint Geometry

One of the most important conclusions of the study is that adhesive formulation exerted a stronger influence on bond strength than changes in joint configuration alone. While joint geometry affected stress distribution, selecting an adhesive with superior fracture toughness and interfacial compatibility produced the largest improvements in structural performance across all composite systems.

Scientific Contribution

Provides one of the first comprehensive comparisons of two epoxy adhesive systems across three structural joint configurations using sisal, jute, and hybrid natural fiber composites within a single experimental framework.
Demonstrates that adhesive selection has a greater impact on bonded joint performance than joint geometry alone for the investigated composite systems.
Shows that hybrid sisal-jute reinforcement generates synergistic improvements in adhesive bond strength compared with single-fiber composites.
Links mechanical performance directly to microscopic failure mechanisms through integrated tensile testing and FE-SEM characterization.
Provides experimentally validated evidence supporting the use of toughened epoxy adhesives to improve the durability and reliability of sustainable natural fiber composite structures.
Offers practical engineering data that can support future adhesive selection and structural design for renewable composite materials.

Industrial Implications

The findings support the development of stronger adhesive joints for lightweight composite structures used in automotive, marine, aerospace, and construction industries.
Manufacturers can improve structural reliability by selecting toughened epoxy adhesives that provide superior interfacial bonding with natural fiber composites.
Hybrid sisal-jute composites offer a sustainable alternative to conventional synthetic composite materials while maintaining competitive mechanical performance.
Improved adhesive bonding may reduce premature joint failure, lower maintenance requirements, and extend component service life.
The experimental methodology provides a useful reference for evaluating future adhesive formulations designed specifically for bio-based composite materials.
The research contributes to broader industrial efforts aimed at replacing synthetic reinforcement materials with environmentally sustainable natural fiber composites without compromising structural integrity.

Research Limitations

Like most experimental investigations, this study has several limitations that should be considered when interpreting its findings. Although the experimental procedures were carefully controlled and standardized, the conclusions are primarily applicable to the specific materials, adhesive systems, and testing conditions investigated. Future studies involving broader material combinations and environmental conditions would further strengthen the generalizability of the findings.

The investigation evaluated only two epoxy adhesive systems. Other commercial structural adhesives, including polyurethane, acrylic, methacrylate, and bio-based adhesives, were outside the scope of the study.
Only three joint configurations—single-lap, butt, and 45° scarf joints—were examined. Other structural joint designs commonly used in engineering applications were not included.
Mechanical characterization focused primarily on static tensile loading. Fatigue performance, impact resistance, creep behavior, cyclic loading, and long-term durability were not investigated.
The research evaluated woven sisal, woven jute, and one hybrid laminate architecture (SJJS). Different fiber orientations, stacking sequences, and reinforcement architectures may produce different bonding characteristics.
Testing was conducted under controlled laboratory conditions at room temperature. Environmental factors such as elevated temperature, humidity, ultraviolet exposure, chemical degradation, and weathering were not considered.
Microscopic observations focused on fracture morphology after tensile failure. Additional analytical techniques could provide complementary information regarding interfacial chemistry and adhesive interactions.
The study concentrated on bond strength evaluation rather than manufacturing cost, production efficiency, lifecycle assessment, or economic feasibility for industrial-scale implementation.

Future Research Opportunities

The findings presented in this study provide several promising directions for future investigations on adhesive bonding of sustainable natural fiber composites. Expanding the experimental framework would improve understanding of long-term structural performance and facilitate broader industrial implementation.

Investigate additional structural adhesive systems, including polyurethane, acrylic, methacrylate, nanomodified epoxy, and bio-based adhesives.
Evaluate the long-term durability of bonded joints under fatigue loading, cyclic loading, creep, vibration, and impact conditions.
Study the influence of environmental aging, moisture absorption, ultraviolet radiation, elevated temperatures, and corrosive environments on adhesive performance.
Investigate different fiber architectures, woven patterns, laminate stacking sequences, fiber treatments, and hybrid reinforcement combinations.
Examine additional joint geometries commonly used in composite engineering, including double-lap, stepped-lap, T-joints, and tubular bonded structures.
Integrate numerical simulations using finite element analysis (FEA) with experimental testing to predict stress distribution and failure mechanisms more accurately.
Explore the incorporation of nanofillers or surface modification techniques to enhance fiber-matrix adhesion and improve fracture toughness.
Conduct lifecycle assessment and economic analysis to evaluate the commercial feasibility of natural fiber composite adhesive joints in large-scale manufacturing.
Investigate recyclable and environmentally friendly adhesive systems that complement the sustainability advantages of natural fiber composites.
Develop predictive design guidelines for engineers by combining experimental data, computational modeling, and artificial intelligence-based optimization techniques.

Potential for Public Policy Citation (Overton)

This study demonstrates meaningful potential for citation in public policy documents because it contributes to the development of environmentally sustainable engineering materials and manufacturing technologies. Governments and industrial agencies worldwide are actively promoting renewable materials that reduce dependence on synthetic fiber composites while maintaining structural reliability and product safety.

The research may provide useful technical evidence for policies related to sustainable manufacturing, circular economy initiatives, lightweight transportation systems, renewable material utilization, and green industrial innovation. The findings also support broader engineering strategies aimed at reducing environmental impacts through increased adoption of bio-based composite materials.

Although the study is fundamentally experimental and does not directly evaluate regulatory frameworks, its results provide scientifically validated information that may assist standards organizations, engineering regulators, material certification bodies, and government agencies responsible for promoting sustainable material technologies. Consequently, the article possesses moderate potential for future citation within technical guidelines, engineering standards, and policy documents concerning sustainable composite materials and advanced manufacturing.

Who Should Read This Paper?

Mechanical engineers working with composite materials.
Materials scientists investigating natural fiber composites.
Researchers specializing in adhesive bonding technologies.
Composite manufacturing engineers.
Automotive engineers developing lightweight vehicle components.
Aerospace engineers interested in sustainable structural materials.
Civil and construction engineers utilizing fiber-reinforced composites.
Graduate students studying composite mechanics and material engineering.
Researchers working in renewable materials and green manufacturing.
Industrial product designers seeking lightweight and sustainable structural solutions.
Quality assurance engineers responsible for bonded composite assemblies.
Academics teaching composite materials, polymer engineering, and adhesive technologies.

Final Thoughts

This research provides a comprehensive experimental evaluation of adhesive bonding performance in natural fiber-reinforced epoxy composites by simultaneously examining two epoxy adhesive systems, three composite materials, and three widely used structural joint configurations. Rather than focusing exclusively on composite reinforcement or joint geometry, the study demonstrates that adhesive selection is a critical factor governing the overall mechanical performance of bonded composite structures.

One of the most significant contributions of the research is its clear demonstration that the toughened XIN-100 IN/XIN-900 adhesive consistently outperformed the conventional LY-556/HY951 system across all investigated composite materials. The hybrid sisal-jute laminate further enhanced structural performance through synergistic reinforcement mechanisms, while FE-SEM observations provided convincing microscopic evidence explaining the superior interfacial bonding achieved by the tougher adhesive system.

The integration of standardized tensile testing with detailed fracture morphology analysis strengthens the scientific value of the investigation. Rather than relying solely on numerical strength measurements, the researchers successfully connected macroscopic mechanical behavior with microscopic failure mechanisms, providing a more complete understanding of adhesive performance in sustainable composite materials.

Although additional research involving long-term durability, environmental exposure, and broader material systems remains necessary, this study establishes a solid experimental foundation for future developments in renewable composite structures. Its findings provide valuable guidance for engineers, researchers, and manufacturers seeking stronger, more reliable, and environmentally sustainable bonded composite components across numerous engineering applications.

Suggested Citations

UNP–Teknomekanik Style

Melese KG, Eshetu DJ. Effect of Two Different Epoxy-Adhesives on the Bond Strength of Single-Lap, Butt and Scarf Joints of Sisal-Jute Fiber Composites. Engineering Reports. 2025;7(12):e70469. https://doi.org/10.1002/eng2.70469

APA (7th Edition)

Melese, K. G., & Eshetu, D. J. (2025). Effect of Two Different Epoxy-Adhesives on the Bond Strength of Single-Lap, Butt and Scarf Joints of Sisal-Jute Fiber Composites. Engineering Reports, 7(12), e70469. https://doi.org/10.1002/eng2.70469

IEEE Style

K. G. Melese and D. J. Eshetu, "Effect of Two Different Epoxy-Adhesives on the Bond Strength of Single-Lap, Butt and Scarf Joints of Sisal-Jute Fiber Composites," Engineering Reports, vol. 7, no. 12, Art. no. e70469, 2025, doi:10.1002/eng2.70469.

Harvard Style

Melese, K.G. & Eshetu, D.J., 2025. Effect of Two Different Epoxy-Adhesives on the Bond Strength of Single-Lap, Butt and Scarf Joints of Sisal-Jute Fiber Composites. Engineering Reports, 7(12), e70469. Available at: https://doi.org/10.1002/eng2.70469.

Vancouver Style

Melese KG, Eshetu DJ. Effect of Two Different Epoxy-Adhesives on the Bond Strength of Single-Lap, Butt and Scarf Joints of Sisal-Jute Fiber Composites. Engineering Reports. 2025;7(12):e70469. doi:10.1002/eng2.70469.

Chicago (Author–Date)

Melese, Kassahun Gashu, and Dessalegn Jaweso Eshetu. 2025. "Effect of Two Different Epoxy-Adhesives on the Bond Strength of Single-Lap, Butt and Scarf Joints of Sisal-Jute Fiber Composites." Engineering Reports 7 (12): e70469. https://doi.org/10.1002/eng2.70469.

MLA (9th Edition)

Melese, Kassahun Gashu, and Dessalegn Jaweso Eshetu. "Effect of Two Different Epoxy-Adhesives on the Bond Strength of Single-Lap, Butt and Scarf Joints of Sisal-Jute Fiber Composites." Engineering Reports, vol. 7, no. 12, 2025, article e70469. Wiley, https://doi.org/10.1002/eng2.70469.

Editorial Note

Engineering Research Insights publishes independent scholarly reviews of recently published engineering research to improve knowledge dissemination across academic and industrial communities. This review is based exclusively on the scientific content presented in the original research article together with bibliographic metadata verified from the official journal webpage. The review summarizes the research objectives, methodology, findings, scientific contributions, industrial relevance, limitations, and future research opportunities without altering the authors' original conclusions. Readers are encouraged to consult the original publication for complete experimental details, supplementary data, and additional technical discussions.

SEO Meta Description

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Conclusion

The reviewed study demonstrates that adhesive selection is a decisive factor in achieving high-performance bonded natural fiber composite structures. By systematically comparing two epoxy adhesive systems across three structural joint configurations and three composite reinforcement materials, the research provides convincing experimental evidence that the toughened XIN-100 IN/XIN-900 adhesive consistently delivers superior bonding performance compared with the conventional LY-556/HY951 system. The hybrid sisal-jute laminate further enhances mechanical performance through synergistic reinforcement effects, while FE-SEM observations reveal the microscopic mechanisms responsible for improved interfacial adhesion.

Beyond its immediate findings, the study contributes valuable engineering knowledge for the design of lightweight, sustainable, and structurally reliable composite assemblies. As industries continue to adopt renewable materials in pursuit of environmentally responsible manufacturing, understanding the interaction between adhesive systems, composite architectures, and joint geometries becomes increasingly important. Consequently, this research represents a meaningful contribution to composite engineering, adhesive technology, and sustainable materials development.

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