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Fibre hybridisation in polymer composites: A review
Abstract
Fibre-reinforced composites are rapidly gaining market share in structural applications, but further growth is limited by their lack of toughness. Fibre hybridisation is a promising strategy to toughen composite materials. By combining two or more fibre types, these hybrid composites offer a better balance in mechanical properties than non-hybrid composites. Predicting their mechanical properties is challenging due to the synergistic effects between both fibres. This review aims to explain basic mechanisms of these hybrid effects and describes the state-of-the-art models to predict them. An overview of the tensile, flexural, impact and fatigue properties of hybrid composites is presented to aid in optimal design of hybrid composites. Finally, some current trends in fibre hybridisation, such as pseudo-ductility, are described.
... A further development of this concept, where metal sheets are replaced by metal fibers, so-called metal-fiber hybrids (MFHs), can offer additional advantages. The use of metal fibers instead of sheets allows the application of manufacturing techniques commonly used in fiber-reinforced composites, such as compression molding and vacuum infusion, facilitating the production of more complex structures [13,14]. In addition, due to the significantly larger contact area, the interface between the different reinforcing fibers and the matrix is not problematic [15,16]. ...
... The performance of the metal fiber hybrid composite, especially in post-failure behavior, depends primarily on the properties of the constituents and their composition. Although most studies on MFHs focus on a layer-separated hybridization concept, in principle, there are three general levels of hybridization (see Figure 1) [14]. [14]. ...
... Although most studies on MFHs focus on a layer-separated hybridization concept, in principle, there are three general levels of hybridization (see Figure 1) [14]. [14]. ...
The increasing application of fiber-reinforced polymer (FRP) composites necessitates the development of composite structures that exhibit high stiffness, high strength, and favorable failure behavior to endure complex loading scenarios and improve damage tolerance. Achieving these properties can be facilitated by integrating conventional FRPCs with metallic materials, which offer high ductility and superior energy absorption capabilities. However, there is a lack of effective solutions for the micro-level hybridization of high-performance filament yarns, metal filament yarns, and thermoplastic filament yarns. This study aims to investigate the hybridization of multi-material components at the micro-level using the air-texturing process. The focus is on investigating the morphological and the mechanical properties as well as the damage behavior in relation to the process parameters of the air-texturing process. The process-induced property changes were evaluated throughout the entire process, starting from the individual components, through the hybridization process, and up to the tape production. Tensile tests on multifilament yarns and tape revealed that the strength of the hybrid materials is significantly reduced due to the hybridization process inducing fiber damage. Morphological analyses using 3D scans and micrographs demonstrated that the degree of hybridization is enhanced due to the application of air pressure during the hybridization process. However, this phenomenon is also influenced by the flow movement of the PP matrix during the consolidation stage. The hybrid laminates exhibited a damage behavior that differs from the established behavior of layer-separated metal fiber hybrids, thereby supporting other failure and energy absorption mechanisms, such as fiber pull-out.
... In composite materials like fiberglass, epoxy serves as the matrix that binds the fibers, providing structural integrity. Additionally, due to their high strengthto-weight ratio, epoxy polymers are commonly employed in the aerospace and automotive industries for the fabrication of structural components [11,12]. ...
... The physical properties of vinyl ester polymers, such as their impact resistance and durability, make them a preferred choice in applications where long-term exposure to corrosive environments is a concern. These polymers are also used in the production of composite materials, where they enhance the overall mechanical performance and lifespan of the final product [12,13]. ...
... Notably, considerable focus has been placed on the influence of stacking sequences on the strength and variability in epoxy composites [14,15]. Such hybridization utilizes the benefits of constituent materials while addressing their intrinsic weaknesses, resulting in the creation of superior materials [16]. However, the majority of these studies have focused on hybrid structures in laminated forms, predominantly using the hand lay-up technique, with limited investigation of the changes in mechanical properties in cast forms. ...
... Polymers 2024,16, 3559 ...
In this study, epoxy-based composites were fabricated using a layer-by-layer assembly technique, and their mechanical properties were systematically evaluated. The inclusion of cellulose nanocrystals led to variations in the mechanical properties of the composites. These modified properties were assessed through tensile and flexural tests, with each layer cast to enhance strength. Due to the inherent characteristics of epoxy, a single specimen was fabricated through chemical bonding, even post-curing. This approach demonstrated that a three-layer structure, developed using the layer-by-layer method, exhibited improved elastic and flexural moduli compared to a single-layer composite. This improvement aligns with theoretical predictions, which suggest that stiffness increases when stiffer materials are positioned farther from the neutral axis in a layered structure. Furthermore, numerical analysis validated changes in stress distribution across each layer. Consequently, this method enables the production of composites with superior mechanical properties while minimizing the quantity of cellulose nanocrystals required.
... Hybridizing with another natural or synthetic fibers can also improve fiber-matrix compatibility [30,31]. According to Swolfs et al. [32], the purpose of combining two or more fibers into one composite is to preserve the benefits of each fiber type while minimizing some of the drawbacks. This led to a better balance in the mechanical, chemical, and physical properties of the hybrid composite. ...
... Moreover, while recent studies have shown promising improvements in the mechanical properties of hybrid-oil palm fiber-reinforced polymer composites, there is still a need for further investigation into optimizing the composition and processing parameters to achieve superior performance. This includes exploring the effects of different fiber ratios, treatments, and matrix materials on the mechanical and thermal properties of the composites [32,57]. ...
... Wind turbine composite blades, in particular, incorporate uniaxial, biaxial, and triaxial composite materials in their upper and lower skins and shear webs to balance lightness and load-bearing capacity, thereby enhancing structural performance. The performance of these hybrid composites varies, as they are tailored to offset the limitations of each laminated composite's properties [1,2]. The design strategy evaluates the mechanical properties of each material, but the failure characteristics of hybrid composites may differ from single materials [3]. ...
... . (2) where N max represents the normal tensile strength, T max represents the interlaminar shear strength, S max denotes the transverse interlaminar shear strength, σ n denotes the nominal stress, σ s denotes the shear stress, σ t is the transverse shear stress, G IC is the critical fracture toughness of mode I, G I IC is the critical fracture toughness of mode II, G I is the fracture toughness of mode I, G I I is the fracture toughness of mode II, G C is the critical fracture toughness, and η represents the material exponent of the mixed mode. As a third step, the VCCT is employed for the analytical interpretation based on the criteria for crack initiation and growing fracture toughness values. ...
The composite blade is integral to megawatt-class wind turbines and frequently incurs interlaminar damages such as adhesive failures, cracks, and fractures, which may originate from manufacturing flaws or sustained external fatigue loads. Notably, adhesive joint failure in the spar–web and trailing edge (TE) represents a predominant damage mode. This study systematically explores the failure mechanism in these regions, using mode I fracture toughness tests for an in-depth, quantitative analysis of the adhesive joint’s fatigue crack growth characteristics. Additionally, we conducted extensive material and technical evaluations on specimen units, aiming to validate the reliability of techniques employed for wind blade damage modeling. A damage model, inspired by the NREL 5 MW wind generator’s composite blade structure, meticulously considers the interactions between the TE and spar–web. Utilizing the virtual crack closure technique (VCCT), this model effectively simulates crack growth dynamics in wind blade adhesive joints, while the extended finite element method (XFEM) aids in analyzing crack propagation trajectories under repetitive fatigue loading. By applying this integrated methodology, we successfully determined the lifespan of the spar–web adhesive joint under constant load amplitudes, providing crucial insights into the resilience and longevity of critical wind turbine components.
... The use of hybrid reinforcement as a cost-saving measure and a means for tailoring mechanical properties has been applied in the composite industry for a long time when talc, mica, and other mineral fillers [11] were added to glass fibre reinforcement in polypropylene composites to achieve higher stiffness, which resulted also in higher flexural strength [12]. Currently, a widespread hybrid combination of glass and carbon fibre reinforcements is used in the form of textiles and short fibres in thermoset and thermoplastic composites [13,14]. Beyond the fact that high-performance yet costly carbon fibres provide a great increase in strength and stiffness, cheaper glass fibres provide high toughness [15]. ...
Wood–polymer composites and composites reinforced with natural and man-made cellulose fibres are being extensively used in the automotive and building industries. The main shortcoming of the former is their low-impact resistance and brittleness. The relatively high cost of natural and cellulose fibres is the limitation of the latter. This research uses a hybrid combination of wood flour and short man-made cellulose fibres to develop polypropylene composites for injection moulding that excel in mechanical characteristics and have low material cost. Both reinforcements are of wood origin. The synergistic hybrid effect of this combination of reinforcements helps to achieve their mechanical performance superior to that of wood–polymer composites at preserved low cost. The proposed Response Surface Methodology enables the calculation of necessary weight fractions of two reinforcements to achieve desired mechanical properties like strength, tensile, flexural modulus, and impact resistance.
... One of the main advantages of fiber reinforced composites is the design freedom, which can be better exploited using fiber-hybridization (coupling two or more fiber types). Fiber-hybrids reinforced plastics received particular attention due to the synergetic (or 'hybrid') effects [1,2], which can provide, among other features, the pseudo-ductility, namely a pseudo-ductile response by the combination of brittle composites [3]. Pseudo-ductility is often studied in quasistatic tension and the damage mechanisms underneath are reasonably well-understood [4,5]. ...
This experimental study has been focused on the tensile-tensile fatigue performance of a carbon-carbon fiber hybrid quasi-isotropic laminate. The laminate contained two unidirectional carbon fiber prepregs with the same epoxy matrix. One prepreg had carbon fibers T800 for the high-elongation layer (HE), the other prepreg had carbon fibers DIALEAD for the low-elongation layer (LE). The sub-laminate stacking sequence was [HE/LE/HE]. Sub-laminates have been piled to get a quasi-isotropic layup [0/45/90/-45]s. The displacement (strain) controlled tension-tension tests were along the 0° fiber direction. Preliminary quasi-static tests provided the pseudo-ductile behavior considered for the fatigue loading levels. The evolution of the fatigue damage was macroscopically analyzed by the stiffness degradation, and microscopically by X-ray micro-CT observations. As main conclusions, the composite retains its load-carrying ability in the pseudo-ductile regime. The evolution of the fatigue damage involved fracture of 0° LE plies, transverse cracks in ±45° plies and delamination of 0°/+45° and ±45°/90° interfaces.
... Immediately adjacent to fibre breaks, this cannot be expected to hold, due to the following scenarios or combinations that can occur: (1) matrix yielding [34], [35], (2) matrix cracking in the fibre break plane [36], [37], [38], (3) fibre-matrix debonding [35], [39] and (4) dynamic effects (e.g. fibre spring-back) [2], [40], [41]. Since the break openings have been excluded from the correlation algorithm, and as the BaTiO3 fiducial markers inscribed within the matrix are the main features used to identify correlation peaks it may be assumed that matrix deformation will dominate the DVC output. ...
This paper presents a novel application of Digital Volume Correlation (DVC) and in situ Synchrotron Radiation Computed Tomography (SRCT) to uniaxial loading in Carbon Fibre Reinforced Polymers (CFRPs). DVC is a relatively novel tool for quantifying full-field volumetric displacements and implicit strain fields [1]. To permit the application of DVC to displacements and/or strain measurements parallel to the fibre direction in well aligned unidirectional (UD) materials, a methodology was developed for the insertion of sparse populations of sub-micrometre particles within the matrix to act as displacement trackers (fiducial markers). For the novel materials systems we have developed, measurement noise is considered, along with the spatial filtering intrinsic to established DVC data processing. The evolution of individually fractured filaments into clusters of breaks is presented, together with the associated elevated strain region. A balance of spatial resolution and signal-to-noise ratio (SNR) is discussed in relation to measuring local micromechanical phenomena, such as ineffective length, within the bulk material. It is shown that novel, mechanistically consistent measurements may be made in relation to fibre failure events.
... This behavior consists of the 'hybrid effect', as was first reported by Hayashi [20]. The hypotheses formulated regarding the causes of the hybrid effect are well summarized in the review paper of Swolf et al. [21]. Causes of the hybrid effect include (a) residual shrinkage stresses, attributed to the difference in the thermal contraction behavior of the different types of fibers, and (b) progressive failure of the individual fiber components, which have different strength and deformation characteristics and are subjected to the same axial and lateral deformation. ...
Application of hybrid jackets consisting of comparatively stiff FRP materials for the seismic retrofit of substandard RC columns, aiming at reducing the risk of buckling and of brittle failure, which are typical to older columns, is a promising challenge. Given the sparsity of similar experimental data, the objective of this paper is to study the hybrid effect in concrete confined with conventional carbon- and glass- reinforced polymer fabrics (CFRP and GFRP, respectively). Twenty-six concrete cylinders, wrapped by one to three layers of CFRP and GFRP with different fiber configurations, were tested in compression. A clear hybrid effect was observed, consisting of a less brittle failure and an improved confinement as compared to the behavior of simple jackets. Furthermore, hybrid specimens, in which a CFRP layer is substituted by a GFRP layer, appear to display similar efficiency in confinement compared to specimens with a stiffer jacket consisting of more CFRP sheets, which are expected to experience 30 to 40% higher lateral pressure owing to the stiffer jacket. A design model to estimate peak concrete compressive strength and axial strain is proposed. The results are promising towards the potential application of similar hybrid jackets for the seismic rehabilitation of older RC columns.
... A multitude of intricate configurations can be created by combining two of these three structures. [32][33][34] Through hybridization, better mechanical properties have been achieved in the past few decades. Maslinda et al. 35 discovered that the intra-layer Kenaf/Hemp and Kenaf/Jute laminates exhibited higher tensile and flexural strength than individual Hemp, Kenaf, and Jute woven composites, the interlocking structure between the fiber yarns increased the stress uptake of the interwoven hybrid composites, requiring a greater force to break the structure. ...
Bidirectional hybrid polymer-based composites are being introduced in many industrial applications. This is because the weaving patterns of woven composites have significant contributions to resulting composite performances. The main goal of this work was to study the effect of chemical treatments and the effect of hybridization on the mechanical properties of intra-layer hybrid Alfa/Jute fabric composites. Jute and Alfa fibers were used as natural fibers reinforcing the polyester matrix-based composites. First of all, the untreated, alkali and permanganate treated Alfa fibers were analyzed by using physical and mechanical tests (ATR-FTIR, XRD, and MEB). Weibull statistical analysis was employed to estimate the variability of untreated and treated Alfa fiber-resin interfacial shear strength. On the other hand, three plain-woven intra-ply hybrid fabrics were used as reinforcements for the polyester matrix. These three produced composite samples were subjected to mechanical, and physical tests such as three-point flexural strength, compressive strength, water absorption, and optical observation. The results were examined, analyzed, and compared to a jute-woven composite reinforced with the same resin. The results show that the chemical treatment can affect positively the fibers’ properties. In others hand, among the studied composites, the developed treated intra-ply woven fabric reinforced polyester resin have good mechanical properties in term of flexural and compression resistance. Weibull statistical analysis was conducted to evaluate and quantify the variability in the tensile strength of various intra-layer hybrid Alfa/Jute fabric composites.
... In the field of construction and building, the application of 3D printing technology has already been realized [1][2][3][4][5][6][7][8]. While most of the emphasis is on 3D-printed concrete and cementitious materials, there is also rising momentum for an understanding of the behavior of 3D-printed composites in construction. ...
With 3D printing technology, fiber-reinforced polymer composites can be printed with radical shapes and properties, resulting in varied mechanical performances. Their high strength, light weight, and corrosion resistance are already advantages that make them viable for physical civil infrastructure. It is important to understand these composites’ behavior when used in concrete, as their association can impact debonding failures and overall structural performance. In this study, the flexural behavior of two designs for 3D-printed glass fiber composites is investigated in both Portland cement concrete and polymer concrete and compared to conventional fiber-reinforced polymer composites manufactured using a wet layup method. Thermogravimetric analysis, volume fraction calculations, and tensile tests were performed to characterize the properties of the fiber-reinforced polymer composites. Flexural testing was conducted by a three-point bending setup, and post-failure analysis was performed using microscopic images. Compared to concretes with no FRP reinforcement, the incorporation of 3D-printed glass-fiber-reinforced polymer composites in cementitious concrete showed a 16.8% increase in load-carrying capacity, and incorporation in polymer concrete showed a 90% increase in flexural capacity. In addition, this study also provides key insights into the capabilities of polymer concrete to penetrate layers of at least 90 microns in 3D-printed composites, providing fiber bridging capabilities and better engagement resulting in improved bond strength that is reflected in mechanical performance. The polymer material has a much lower viscosity of 8 cps compared to the 40 cps viscosity of the cement slurry. This lower viscosity results in improved penetration, increasing contact surface area, with the reinforcement consequently improving bond strength. Overall, this work demonstrates that 3D-printed fiber-reinforced polymer composites are suitable for construction and may lead to the development of advanced concrete-based reinforced composites that can be 3D-printed with tailored mechanical properties and performance.
... To increase the sustainability, affordability, and performance of NFRCs, hybridization of fiber has been tried to make up for the unavailability of a single particular fiber [94,95]. Depending on the requirements of the application, hybridization approaches provide engineering components with fiber selection flexibility. ...
This chapter provides a concise overview of the mechanical and thermal properties of plant/plant fiber hybrid fabric woven polymeric composites. The growing demand for reduced energy consumption and environmental sustainability has fueled the development of natural fiber-reinforced composites (NFRCs). Natural fibers offer advantages such as recyclability, compactness, affordability, improved stability, and excellent mechanical properties. However, challenges associated with plant-based natural fibers, including variations in yield attributes, limited mechanical capabilities, moisture content, low thermal stability, poor integration with hydrophobic matrices, and encapsulation tendencies, hinder their widespread adoption in NFRCs. Recent research efforts have focused on enhancing the features and applications of plant based NFRCs. This chapter provides a summary of the characteristics and reinforcement techniques employed in fabricating NFRCs using woven structures. It highlights advancements and approaches to improve the functionality of NFRCs, such as fiber adjustment, fiber transfection, integration of lignocellulosic fillers, traditional processing methods, additive manufacturing (including 3D printing), and the exploration of new fiber sources. The mechanical and thermal properties of these composites depend on the performance of plant based NFRCs. By presenting key findings and advancements, this chapter aims to deepen understanding of the potential and limitations of plant/plant fiber hybrid fabric woven polymeric composites. This understanding facilitates their development and utilization in various industrial applications. Concluding remarks underscore the significance of this research, identify potential future directions, and emphasize the ongoing need to explore and optimize plant/plant fiber hybrid fabric woven polymeric composites. These materials hold promise for meeting the evolving needs of industries seeking sustainable and high-performance solutions.
... Interply hybrid composites are produced by stacking fabric layers of different materials on top of each other. Intraply layered composites are manufactured by using in-layer hybrid fabrics where the same ply is woven with different material warp and weft threads [9]. In many studies, the mechanical properties of interply or intraply Carbon/Aramid (CA) hybrid composites such as tensile, flexural, fracture, etc. have been characterized experimentally or theoretically. ...
... The benefits of synthetic materials used to increase fibrous reinforced polymer composites include their good strength, higher hardness, prolonged fatigue resistance, ability to adjust to the functionality of a structure, resistance to corrosion, and stability (Dinesh et al., 2022). The high cost, high density, low recycling potential, and no biodegradability of this kind of material are other disadvantages (Mishra et al., 2003;Safri et al., 2018;Swolfs et al., 2014). Table 16.1 shows the evaluation of the mechanical and physical characteristics of natural and synthetic fibers. ...
Many developments have made extensive use of hybrid composites. These are lightweight and simple to produce. The hybrid fiber-reinforced composites’ characteristics have improved compared to composites. Hybrid composites have a matrix system that contains more than one material. The different materials are combined with the proper matrix by using a variety of production processes to create the hybrid composites. In many uses, hybrid composites take the role of traditional materials. The mechanical properties (tensile, bending, and impact) and tribological, thermal, and moisture content characteristics of hybrid polymer composites are covered in this chapter. In addition to the latest innovations in hybridized polymeric materials, the choice of materials for a particular function is also discussed.
... Besides, the use of hybridisation has been used in many engineering applications. The hybridisation approach combines the natural plant fibres with other synthetic fibres in a single matrix system to attain desired properties [6]. In addition, using hybrid composites would benefit from the synergistic effects of the different reinforcing phases, resulting in enhanced fibre-matrix adhesion, decreased moisture absorption and superior mechanical properties [7]. ...
This study investigated the effects of different temperatures on the low-velocity impact damage behaviour of flax fibre-reinforced epoxy composites and their glass/flax hybrids. Composites reinforced with flax, glass, and hybrid flax/glass onto epoxy matrix Subjected to low-velocity drop weight impact tests at 5 J of incident impact energy at sub-zero temperatures (−10 °C and −20 °C) and at room temperature (RT) are presented. Under the different temperatures, the experimental findings showed a beneficial hybrid effect where the temperature played a significant role. At RT, the Lam-GFGFGFG exhibit improved impact resistance, with enhanced energy absorption capabilities compared to glass-only laminates (Lam-G). Besides, Lam-GFFFFG laminates exhibit a significant difference in the force–displacement curves at − 20 °C, with a maximum load of 801.95 N in contrast to RT and − 10 °C resulting in a gradual increase in force with increasing displacement. This indicates that Lam-GFFFFG laminates can resist the impact and maintain structural integrity at sub-zero temperatures. The alternation of glass and flax layers in the hybrid structure contributes to the synergistic effects, resulting in improved damage resistance and tolerance. Also, the highest impact tolerance in a laminate is achieved through the hybridisation of flax fibre-reinforced composites with glass-reinforced layers on the outer surfaces (Lam-GFFFFG) at − 10 °C. Subsequently, experimental results were compared with finite element analysis (FEA) results, derived from a model built using a VUMAT subroutine integrated with ABAQUS/Explicit for a more accurate representation of the damage characterisation of the composite laminates under low-velocity impact.
... The increasing importance of rational material usage promotes materials modification. Combining fibers with different properties allows to use the advantages of individual fibers, their mechanical and physical properties, and allows for a more rational use of the material in terms of cost [9][10][11]. To obtained high properties of Hybrid Fiber Reinforced Polymer (HFRP) bars many factors should be control during manufacturing; among others, the fiber placements in cross-sectional, which can affect their properties. ...
The FRP (Fiber Reinforced Polymer) bars are increasingly used as the main reinforcement of concrete structures, replacing traditional steel reinforcement. In this paper results of Basalt Fiber Reinforced Polymer bars (BFRP) modification by partial replacement of basalt fibers with carbon fibers were presented. The analysis of an effect of hybridization on a microstructure and mechanical properties of BFRP bars were performed. This analysis was thought out based on tests performed: tensile strength and shear strength and a microstructure observation with scanning electron microscope. The results obtained indicate that the hybridization effectively increases elasticity modulus compared to unmodified BFRP and the tensile strength and shear strength increase in lower extant. The nonhomogeneous distribution of carbon fiber in the cross-section of HFRP bars has relatively small effect of mechanical properties and their scattering.
... In Microscopic Analysis, Fractured surfaces were examined using Scanning Electron Microscope (SEM). Swolfs et al. [23] In structural applications, fiber-reinforced composites are quickly increasing market share; nevertheless, their lack of toughness is preventing further progress. One approach that shows promise for toughening composite materials is fibre hybridization. ...
This research project delves into the strategic augmentation of composite laminates for aerospace and structural applications, recognizing the pivotal role of weight in material design. Employing intricate handloom weaving techniques and precision hand layer methods, three distinct composite laminates are crafted, each distinguished by varying concentrations of aluminum nanoparticles/nano oxides such as 1%, 3%, and 5%. Rigorous quasi-static loading protocols within a Universal Testing Machine (UTM) reveal a discernible enhancement in impact strength for the 3% nanoparticle composite laminate compared to counterparts with 1% and 5% nanoparticle concentrations. This improvement is attributed to the uniform nanoparticle dispersion and closely spaced fiber architecture within the laminate. Beyond the laboratory, this strategic integration of nanoparticles demonstrates promise in optimizing material properties, with potential applications in advanced technologies such as bulletproofing. This study, characterized by meticulous precision and scientific rigor, contributes nuanced insights to the field of composite material science.
... The interaction effect becomes less advantageous when there is an increase in the deflection. The study found that combining polypropylene fibers, which have a low modulus, with basalt fibers, which have a high modulus, effectively reduced fracture widths and improved the energy absorption capacity of high-performance concrete (Swolfs et al. 2014). ...
Strength is an important characteristic of high performance concrete (HPC). The article explains how the water-to-binder (w/b) ratio, curing procedure, superabsorbent polymer (SAP), expansion-promoting additive (EPA), shrinkage-reducing additive (SRA), and supplemental cementitious materials (SCMs) affect the durability of high-performance concrete (HPC). Fly ash tends to reduce early age strength, however this can be compensated by mixing it with nanosilica or other admixtures, as long as adequate levels of SCMs are present in the mix. The strength is maximised by reducing the aggregate size while increasing the aggregate strength. Fibres frequently have little effect on compressive strength, however they usually improve flexural and splitting tensile values. Using specific SCMs, the negative impacts of high temperature curing on later age strength can be reduced. When the w/b ratio is reduced, both strength and autogenous shrinkage (AS) rise. Although SAP and SRA hybridisation, as well as appropriate management of excess SAP added water, have the potential to increase strength, additional study is needed.
... In this study, the first considered source of uncertainty is the inherent uncertainties in the mechanical properties of FRP constituents (fibers and matrix). The experimental works in the literature [32][33][34][35][36] have concluded that the probability density function (PDF) of Weibull distribution is the fittest PDF to model the uncertainties in the mechanical properties of the fibers, specifically fibers' tensile strength, which is explained by the weakest link theory [37]. Meanwhile, for the polymer matrix stiffness and strength, the PDF of Gaussian distribution is good enough [38,39], especially if the matrix material is isotropic. ...
Enhancing the understanding of the behavior, optimizing the design, and improving the predictability and reliability of manufactured unidirectional (UD) FRP plies, which serve as primary building blocks for structural FRP laminates and components, are crucial to achieving a safe and cost-effective design. This research investigated the influence of fiber volume fraction (vf) on the predictability and reliability of the homogenized elastic properties and damage initiation strengths of two different types of UD FRP plies using validated micromechanical virtual testing for representative volume element (RVE) models. Several sources of uncertainties were included in the RVE models. This study also proposed a modified algorithm for microstructure generation and explored the effect of vf on the optimal sizes of the RVE in terms of fiber number. Virtual tests were systematically conducted using full factorial DOE coupled with Monte Carlo simulation. The modified algorithm demonstrated exceptional performance in terms of convergence speed and jamming limit, significantly reducing the time required to generate microstructures. The developed RVE models accurately predicted failure modes, loci, homogenized elastic properties, and damage initiation strengths with a mean error of less than 5%. Also, it was found that increasing vf led to a concurrent increase in the optimal size of the RVE. While it was found that the vf had a direct influence on homogenized elastic properties and damage initiation strengths, it did not significantly affect the reliability and predictability of these properties, as indicated by low correlation coefficients and fluctuations in the coefficient of variation of normalized properties.
... Natural fiber-based composites have a wide range of applications in automotive and construction industries [2]. In addition to the fiber and matrix properties, fiber treatment, fiber orientation, fiber volume and hybridization are the most common factors that govern the properties of natural fiber composites [3][4][5][6]. ...
Natural fibers are among the most employed reinforcements in the manufacturing process of innovative fiber-based composite materials. As with any composite materials, the properties of composites depend on the type and properties of the fiber, fiber structure, composition (hybridization), and treatment. In this study, the composite was fabricated by using hand lay-up with 100/0, 75/25, 50/50, 25/75, and 0/100 Enset/Sisal (E/S) hybridization ratio. Three cases, i.e., untreated, 5%, and 10% NaOH treatment were considered. The effects of hybridization and treatment on the mechanical and water absorption properties of woven and unidirectional orientation of E/S hybrid composite were evaluated by using a two-factors analysis of variance. The fiber–matrix interfacial fractured surface was characterized by scanning electron microscopy. The treated (5% NaOH) and woven fiber orientation exhibited better mechanical properties than untreated and unidirectional hybrid composites. The flexural and tensile strength of the woven composite was improved by 5% and 9%, respectively, when compared with woven untreated 50/50 volume ratio of composites. In both samples and orientations, the hybridization effects show a higher percentage contribution to the mechanical properties. But, in both orientations of composite samples, the treatment effects show a higher percentage contribution for water absorption properties.
In the current research work, an attempt has been made to fabricate hybrid cellulose-reinforced composites as a step towards the development of sustainable and ecofriendly materials. Cellulose is extracted from sisal and jute fibers using water pre hydrolysis synthesis. Hybrid cellulose composites are fabricated using different wt% combinations of sisal and jute celluloses and epoxy resin by hand layup technique. These composites are exposed to different environmental conditions that is, saltwater, tap water, and kerosene to explore the impact of moisture absorption on tensile, flexural and impact properties. It was observed that samples immersed in tap water absorbed maximum moisture content when compared with samples in saline water and kerosene. Exposure to various moisture conditions deteriorated mechanical characteristics that is, tensile, flexural, and impact properties in all compositions. Dry hybrid cellulose composite with 10S + 5J composition exhibited the maximum tensile strength (i.e. 25.04 MPa) and flexural strength (i.e. 74.24 MPa). In contrast, 5S + 10J dry composites exhibited better impact strength (1.6733 kJ/m ² ). SEM micrographs of tensile fracture surfaces confirmed that pull out of fibers, presence of micro voids and delamination are the significant causes for the failure of composites. Further, SEM micrographs correlated with moisture absorption effects to probe mechanical behavior and durability of the composites under different environmental effects. The study demonstrates that hybrid cellulose composites are ideal for low-load automotive applications, enhancing sustainability and biodegradability.
The aim of this study was to estimate the effect of weft blending on low-velocity impact properties. Based on whether the jute fibers have undergone alkali treatment the fabrics are systematically classified into two categories, to examine its impact on the jute fiber. The weft mixing ratio is 1:1,1:3 and 1:5. These fabrics were then converted into composite materials utilizing Vacuum Assisted Resin Transfer Molding (VARTM). An instrumented drop hammer impact test setup was employed to investigate the effects of varying impact energies (10J, 20J, and 30J). The findings revealed a notable 27% enhancement in impact resistance following an increase in jute content. This augmentation can be credited to the high flexibility and strain rate of jute fibres, which serve to prevent sudden fractures in the hybrid composite materials. Subsequent to the alkali treatment of jute, the impact strength of the resulting composite material has been observed to enhance by a range of 11% to 15%. For a more profound comprehension of the damage mechanism, the damage behaviour of the composite material following impact was investigated under a three-dimensional optical microscope. The impact surface mainly shows the separation of the yarn from the matrix, and the non-impact surface shows the fracture and resin damage of the yarn.
Hybrid fiber‐reinforced polymer (HFRP) composites are gaining attention due to their impressive strength and stiffness with low density. However, their high‐strength often comes with reduced toughness, and achieving a balance between these properties involves combining fibers using hybrid methods. This study used fabricating HFRP laminates (carbon/Kevlar, carbon/glass, and carbon/glass/Kevlar) with different stacking sequences and hybridization ratios created through molding, followed by tensile testing to evaluate the mechanical behavior. The results showed that hybridization ratios significantly influenced the tensile strength, modulus, and elongation at break. For example, compared to the single Kevlar fiber composites, the tensile strength and tensile modulus of the laminate with the optimal configuration of carbon/Kevlar fiber‐reinforced composites increased by 102.93% and 131.65%, respectively, and the elongation at break decreased by 76.13%. This improvement was attributed to the synergistic effect of combining carbon fibers with Kevlar fibers through effective hybridization. The stacking sequences also had a significant effect on tensile strength and elongation at break, although the effect on the tensile modulus was weaker. Additionally, the different tensile properties were obtained by inter hybridization between the three types of fibers. Microscopic observations provided insights into the fracture behavior of HFRP, highlighting phenomena such as brittle/ductile fracture, delamination, fiber pullout, crack suppression, and potential interactions. These observations underscored the complex mechanics governing the mechanical performance of HFRP.
Highlights
The tensile properties of HFRP laminates with carbon fiber, glass fiber, and Kevlar fiber hybrids are studied.
The effect of hybridization parameters on the tensile properties of HFRP are studied.
Indicators of the tensile properties of HFRP laminates are compared and analyzed.
The hybrid effect and interaction failure mechanism of HFRP are revealed.
The deflection responses of intact and damaged hybrid curved shell structures are investigated numerically and experimentally under mechanical loading conditions. The damaged hybrid composite is numerically modelled in MATLAB using higher‐order mid‐plane polynomials and finite element technique. The consistent characteristics and model accuracy have been established by comparing the consequences to those accessible in the existing article. Additionally, the study examined experimentally and computationally obtained elastic properties from the DIGIMAT tool, substantially boosting the extent and reliability of the analysis. Finally, numerous examples are provided to illustrate the impact of damage, considering various parametric input constraints, on the deflection characteristics of the hybrid composite.
At present, there is limited literature available on the fatigue life estimation of glass fiber fabric‐reinforced thermoplastic polymer composites. This study performed constant amplitude fatigue testing to assess the fatigue life of twill‐woven glass fiber‐reinforced thermoplastic composites made from polypropylene (PP) and polyamide (PA) under four different stress levels (70%, 60%, 50%, and 40% of the ultimate tensile strength) with a stress ratio of 0.1. The fatigue life of the specimens was analyzed statistically using two‐parameter and three‐parameter Weibull distribution functions. Linear regression analysis results demonstrates a strong correlation between the independent and dependent variables in the two‐parameter Weibull analysis (correlation coefficient above 0.80), with an even better correlation observed in the three‐parameter analysis (correlation coefficient above 0.94). Based on the parameters derived from the Weibull distribution, the fatigue life of the laminates at varying survival rates has been evaluated. A P‐S‐N fatigue life prediction model has been developed from these results to account for different failure rates. Comparison with the tensile fatigue performance data of fiber‐reinforced polymer composites from the literature indicated that glass fiber‐reinforced thermoplastic composites exhibit excellent fatigue resistance.
Highlights
The fatigue of twill‐weave glass fiber reinforced PA/PP was investigated.
The fatigue life was analyzed using three‐parameter Weibull distribution functions.
A fatigue life prediction model was established based on the Weibull distribution function.
The P‐S‐N fatigue life prediction model provides a reference for material design.
This work presents the results of investigations into the thermal, dynamic mechanical, and tribological propertiesof epoxy resin reinforced with titanium dioxide nanoparticles and organically modified montmorillonite invarying proportions. Additionally, reinforcement of an aramid honeycomb core was used to compare theproperties of samples with and without the honeycomb structure. A modified T-07 tester was employed toassess the wear resistance of materials and metal coatings during rubbing with loose abrasive. Thermodynamictests were performed using the three-point bending mode on an Instrument DMA SDTA861 thermal analyzerin the temperature range from 20C to 140C. It was found that at an operating temperature of around 57C,the lowest stiffness was observed in the sample without an aramid core and with 2 vol.% particulate filler inthe composite. Tribological tests determined the wear resistance, with composites containing montmorilloniteexhibiting the highest wear rates, while the sample with the highest proportion of TiO2 nanoparticles was themost wear-resistant. Notably, the wear rate decreased further when the friction area was along the walls of thearamid honeycomb core.
Carbon Fiber Reinforced Polymer (CFRP) Hybrid Bonded/Bolted (HBB) joint structures are distinguished for their superior jointing performance among current mechanical joint systems, making them a favored option for mechanical joints. These structures are characterized by many structural parameters, with the relationship between these parameters and jointing performance being notably complex. Additionally, the laminate's brittleness and the uneven distribution of bolt loads in multi‐bolt joint structures impair the overall jointing performance. To investigate the impact of structural parameters on the uneven load distribution within CFRP HBB joint structures and improve their jointing performance, a finite element analysis (FEA) model grounded in 3D Hashin failure criterion is developed. Validation of the model with experimental data confirmed the uneven load distribution among bolts in multi‐bolt joints. The study elucidated the influence of changes in structural parameters (overlap length, bolt‐hole spacing, and clearance fit) on the uneven load distribution and the connection strength of CFRP HBB joint structures. A negative correlation is found between the unevenness of load distribution and connection strength, offering insights for enhancing and researching connection strength in CFRP HBB joint structures.
Highlights
Developed a FEA model based on the 3D Hashin failure criterion for CFRP Bonded‐Bolted joints.
Identified key factors affecting HBB joint load distribution and jointing performance.
Evaluated the impact of overlap length, bolt‐hole spacing, and clearance fit on CFRP joints.
Suggested design optimizations for enhancing the performance of HBB joints.
The performance of polymer composites not only addresses challenges in aircraft components but also contributes to industries, such as automotive, architecture, marine, military, sports, and construction. Current manufacturing techniques and the expertise of engineers are crucial in identifying the most suitable biomimetic materials for specific applications. Based on the current literatures, the study on integrating biomimicry into fiber-reinforced thermoplastic composites to develop aircraft radome is still lacking. Thus, this article reviews various types of composites used in aircraft manufacturing, emphasizing the potential of nature-inspired designs to enhance structural performance, with a particular focus on radomes, which protect radar equipment. Bio-inspired designs, shaped by millions of years of evolution, have proven to be highly effective in creating optimized, complex forms that complement the versatility of polymer composites. Given that many current aircraft components are made from metals with little or no shape optimization, applying biomimicry to aircraft radome design offers significant potential for creating lightweight, high-strength structures. The biomimetic approach using fiber-reinforced thermoplastic composites has emerged as a promising strategy for developing improved structural components, offering enhanced mechanical properties, reduced weight, and greater sustainability, paving the way for more efficient and environmentally friendly radome materials. A general overview of biomimicry in relation to aircraft radomes is provided, highlighting how composite materials have already contributed to successful innovations. The economic and environmental benefits of fiber-reinforced thermoplastic composites and biomimetic approaches are also discussed, with insights into materials that offer superior impact and chemical resistance at a lower cost.
Natural fibers have both environmentally friendly and ecological advantages in the fiber material industry. Improving the mechanical properties and durability of natural fiber composites is an important research approach. As one of the representatives of synthetic fibers, carbon fiber has excellent mechanical properties and chemical stability, but its high cost and low toughness limit further application. Combining environmentally friendly, low-cost natural fibers with carbon fibers can broaden the development of natural fiber-reinforced composites in industrial applications. In this paper, the composites made of jute fibers and polypropylene (PP) as well as carbon fibers coupled with KH550 modification were prepared and their properties were investigated. By introducing a macroscopic model similar to the core-shell structure, an environmentally friendly material with better mechanical properties was achieved, with carbon fiber woven fabric serving as the shell and jute fiber mat as the core. The results show that a low-cost hybrid composite was successfully prepared by using a small amount of carbon fiber woven fabric instead of jute fiber mat, and the tensile, flexural and impact properties of the composites were improved by 488.27%, 70.75% and 463.39%, respectively, compared with those of the pure jute fibers composites. This study provides a rapid and reliable approach to improve the mechanical properties of the natural fiber composites.
Translaminar fracture toughness is pivotal for notch sensitivity and damage tolerance of fibre-reinforced composites. Hybridisation offers a promising pathway for enhancing this parameter in thin-ply composites. Three novel mini-compact tension specimen geometries were investigated for their competence in microscale characterisation of translaminar fracture using in-situ synchrotron radiation computed tomography (SRCT). Only “mini-protruded” design resulted in stable crack propagation with adequate crack increments. Based on this design, five baseline and hybrid cross-ply configurations incorporating low- and high-strain carbon fibres were studied. Crack propagation in low- and high-strain baseline configurations was stable. For interlayer and intrayarn fibre-hybrid configurations, a correlation between load–displacement curves and delamination is observed. The SRCT data confirmed that 90° ply-blocks cushion the interaction between 0° plies, enabling independent fracture. Additionally, crack fronts in 90° plies advance further than those in 0° plies. Moreover, mechanical interlocking and bundle bending within 0° plies serve as supplementary mechanisms for energy dissipation.
This research work aims to investigate the effect of fabrication techniques on the physico‐mechanical properties of pistachio shell (PS) filled glass‐epoxy composites and to compare the experimental findings with numerical simulation results using finite element analysis (FEM). These hybrid composites are fabricated by vacuum‐assisted resin transfer molding (VARTM) and hand layup (HL) techniques with fixed glass fiber fraction (20 wt. %) but different PS powder loadings of 0–30 wt. %. The composites are then characterized for physical, compositional, micro‐structural and mechanical properties. The micro‐structural analysis of the filler and composites is performed using electron microscopy and stereo‐microscopy. To identify the phases present in both the raw filler and the composite, an x‐ray diffraction test is performed. The presence of functional groups is identified with the help of Fourier transform infrared spectroscopy (FTIR). It is observed that there is a reasonable improvement in the mechanical properties of the composites with increase in PS powder content, irrespective of the fabrication process used. The density and void fraction are also greatly affected by the amount of filler particles present in the composites and also on the composite fabrication technique. The FEM analysis is also carried out using ANSYS 22.0 workbench to determine the characteristic properties numerically. The comparison of experimental and numerical results yields that the properties obtained by using VARTM results are close to experimental ones with errors lying in the range of 1%–4%. These composites can possibly find potential applications in light duty structures and wear resistant applications.
Highlights
Development of hybrid composites using HL and VARTM techniques.
Physical and mechanical properties alter with filler loading.
Evaluation of mechanical properties using finite element analysis.
Validating the model by comparing experimental and numerical results.
In this paper, the ductility design methods of fiber‐reinforced polymer (FRP) bars were reviewed. It was observed that the graded fracture theory was typically used as the ductility design method of hybrid fiber‐reinforced polymer (HFRP) bar. However, the ductile HFRP bar designed based on the graded fracture theory had the inherent defects of low modulus of elasticity, high yield strain, and post‐yielded sudden drop in stress, which prevented its large‐scale application in civil engineering. In order to eliminate these deficiencies, the authors proposed a novel design concept for a single‐type FRP bar. This novel single‐type FRP bar consisted of highly aligned discontinuous fiber and continuous fiber. The failure mode of this discontinuous/continuous single‐type FRP bar was different from that of the ductile HFRP bar designed based on the graded fracture theory of composite. The tensile ductility of discontinuous/continuous single‐type FRP bar originated from the debonding and stable pull‐out of the discontinuous fiber layer under increasing load. As a result, the post‐yielded sudden drop in stress can be removed for the ductile HFRP bar designed based on the graded fracture theory of composite. In addition, the yield strain can be controlled by adjusting the length of discontinuous fiber layer. In addition, the design configuration, innovative production process, and corresponding theoretical calculations of this novel single‐type FRP bar will be presented in the future.
Highlights
The ductility design methods of fiber‐reinforced polymer bars were reviewed.
Deficiencies of ductile HFRP bar composed of continuous fibers were reported.
A novel discontinuous/continuous single‐type FRP bar was foreseen.
Conventional carbon fibre reinforced plastics (CFRP) possess high specific strength and stiffness, can provide good chemical resistance and achieve long fatigue lives. However these materials are relatively brittle and have a low strain to failure and so structural failure can be catastrophic with little warning. In order to enhance the ductility of CFRP and change its catastrophic failure mode into a progressive one, carbon fibre tows with different failure strains were carefully selected and combined together into intra-tow hybrid reinforcement by using a gas-flow-assisted commingling process. This hybrid reinforcement was used to manufacture a polyamide-12 matrix composite using a polymer powder suspension impregnation method. By controlling the manufacturing parameters (speed and air flow rate in commingling process), a hybrid composite with significantly improved failure characteristics was obtained. Compared with corresponding single-fibre type composites, this hybrid composite has an improved tensile failure strain and still retains good tensile strength and stiffness properties.
This paper first deals with the stress concentration factors for a general fiber breakage model. The knowledge of stress redistribution at fiber fracture is then used for a Monte Carlo simulation of composite strength. The theoretical analysis has predicted the multiple fracture pattern of the low elongation fibers and the progressive nature of failure of hybrid composites. The enhanced ultimate failure strain of the hybrid as compared with that of the low elongation fiber composites signifies a hybrid effect. The major fin dings of this theoretical analysis coincide well with experimental observa tions.
Carbon fibre reinforced polymers often suffer from limited toughness. To drastically increase their toughness, hybridisation with self-reinforced polymers is proposed. The present study focuses on interlayer hybrids of carbon fiber/ polypropylene and self-reinforced polypropylene (SRPP). These hybrid composites are produced in various layups and their tensile behaviour is investigated. It is found that these hybrids possess a unique combination of stiffness, strength and ultimate failure strain. The most interesting observation is the multiple fracture behaviour of carbon fibre plies, which appears at low fibre volume fractions. This has the potential to increase energy absorption in impact loading.
Hybridization of ductile steel fibres and self-reinforced polypropylene (SRPP) is investigated. The goal is to create a material with a high specific stiffness (stiffness per density) while maintaining a high toughness of constituent materials. Three types of hybrids are produced to assess the effect of the stacking order on the tensile behaviour. The hybrid composites have shown to possess a five times higher stiffness and two times higher specific stiffness compared to the SRPP without any loss in toughness. The classical laminate theory was modified to account for non-linear behaviour of the plies and applied to predict tensile curves of the hybrid laminates.
The focus of this review is primarily on the sequence of structural changes at micro and molecular level during carbonization of cellulosic fibres. The influence of various operational parameters such as the pyrolytic temperature and the stabilization agents also discussed as is the effect of the initial properties of the cellulose fibre on the final properties of the carbon fibre.
Hybrid composites are manufactured by combining two or more fibers in a single matrix. Hybrid composites can be made from artificial fibers, natural fibers and with a combination of both artificial and natural fibers. Hybrid composites can help us to achieve a better combination of properties than fiber reinforced composites. The constituent fibers in a hybrid composite can be altered in a number of ways leading to variation in its properties. The importance of this review can be attributed to the significant aspects of natural fiber based hybrid composites which are found to be predominantly affected by factors which include variation in fiber volume/weight fraction, variation in stacking sequence of fiber layers, fiber treatment and environmental conditions.
In this article, the effects of ply thickness on the impact damage mechanisms in CFRP laminates are discussed based on the experimental observations. Quasi-isotropic CFRP laminates were manufactured using 38 µm thick thin-ply prepregs. Impact damage inside the laminates was evaluated by using ultrasonic scanning and sectional fractography. Compression after impact strength was also evaluated. Thin-ply laminates showed 23% higher strength than standard-ply laminates. Transverse cracks decreased drastically in thin-ply laminates, and localized delamination was largely extended. Based on the discussions in our previous study and the literature, the specific ply thickness without drastic crack propagation appears to be less than or equal to 40 µm. Therefore, the thin laminates showed few and localized transverse cracks and delamination was largely propagated in the midplane.
The variations of impact strength and compressive strength of unsaturated polyester based sisal/glass hybrid composites with fiber loading have been studied. The impact strength of these hybrid composites has been found to be higher than that of the matrix, whereas a marginal decrease was observed in the compressive strength of the hybrid composites over that of the matrix. The effects of NaOH treatment and trimethoxy silane (coupling agent) treatment on the impact and compressive properties of these sisal/glass hybrid composites have also been studied. No significant improvement in impact strength of the sisal/glass hybrid composites has been observed by these treatments, whereas a marginal increase in compressive strength of these hybrid composites has been observed.
Linear and nonlinear finite element analyses are used to examine the effects of friction, test geometry, and fixture compliance on the perceived toughness as obtained from threeand four-point bend end-notched flexure tests. To this end, a newly developed ‘direct energy balance approach’ is used to obtain the ‘true’ energy release rate for any given specimen, test geometry, and coefficient of friction. Finite element analyses are also used in a simulated compliance calibration technique, which is combined with experimental results from fixture compliance tests to obtain a perceived toughness, i.e., the value that would be obtained by experiment. By varying the different parameters, the individual and combined effects of friction, test geometry, and fixture compliance on the ratio of the perceived to true toughness is obtained. The approach is applied to two graphite/epoxy materials for which toughnesses by the three(3ENF) and four-point bend end-notched flexure (4ENF) tests are obtained experimentally for a range of geometries. These experiments produced larger perceived mode II toughnesses, GIIc, by the 4ENF than the 3ENF test, and GIIc values from the 4ENF test were observed to decrease with increasing outer span length. The finite element simulations were shown to accurately recreate the perceived values of GIIc obtained from these tests. Moreover, the finite element simulations indicate that the true toughness values are essentially constant for a given material. These findings are used to make some general recommendations for choosing 3ENF and 4ENF specimen and test geometries, as well as to discuss the relative advantages and disadvantages of the 3ENF and 4ENF test methods.
A new approach and material architecture is presented in order to overcome the inherent brittleness and unstable failure characteristic of conventional high performance composites. The concept is the use of thin-ply hybrid laminates. Fracture mechanics calculations were carried out to determine the critical carbon layer thickness for stable pull-out in a three layer unidirectional hybrid laminate, which can provide a pseudo-ductile failure. Unidirectional hybrid composites were fabricated by sandwiching various numbers of thin carbon prepreg plies between standard thickness glass prepreg plies and tested in tension. Specimens with one and two plies of thin carbon prepreg produced pseudo-ductile failure, whereas ones with three and four plies failed with unstable delamination. An explanation of the different failure modes is given in terms of the different energy release rates for delamination in various specimens. The observed damage characteristics agreed well with the expectations according to the estimated critical carbon layer thickness.
Initiation of fatigue damage for a hybrid polymer matrix composite material was studied via 3-Dimensional viscoelastic representative volume element modeling in order to gain further understanding. It was found that carbon fiber reinforced composites perform better in fatigue loading, in comparison to glass fiber reinforced composites, due to the fact that the state of stress within the matrix material was considerably lower for carbon fiber reinforced composites eliminating (or at least prolonging) fatigue damage initiation. The effect of polymer aging was also evaluated through thermal aging of neat resin specimens. Short-term viscoelastic material properties of unaged and aged neat resin specimens were measured using Dynamic Mechanical Analysis. With increasing aging time a corresponding increase in storage modulus was found. Increases in the storage modulus of the epoxy matrix subsequently resulted in a higher state of predicted stress within the matrix material from representative volume element analyses. Various parameters common to unidirectional composites were numerically investigated and found to have varying levels of impact on the prediction of the initiation of fatigue damage.
Solutions are presented for two stress distribution problems which result from breaking of the filaments in a composite material composed of high modulus elements embedded in a low modulus matrix. Both problems represent extensions of the two-dimen sional filamentary structure stress concentration problem: the first concerns the determination of static stress concentration factors in the unbroken elements of a three-dimensional square or hexagonal array where specified filaments are broken; the second involves the stress concentration factor in the element adjacent to a broken filament in a two-dimensional array where the shear stress in the matrix adjacent to the broken filament is restricted by a limit stress in an ideally plastic sense.
Jute—carbon and glass—carbon hybrid composites of mixed matrix material [epoxy resin of bisphenol-C (EBC) and bisphenol-C-formaldehyde (BCF) of 50 wt% of the fibers] have been prepared by hand lay-up technique at 150°C under 7.6MPa pressure for 2h. Alkali-treated jute fibers have been acrylated to improve their physico-chemical properties. Tensile strength, flexural strength, electric strength, and volume resistivity of untreated (JCEBCF-50), treated (TJCEBCF-50) jute—carbon and glass—carbon (GCEBCF-50) composites are 10 MPa, 17 MPa, 1.60 kV/mm, and 5.9 × 10¹² Ω-cm; 14.65 MPa, 19.33 MPa, 2.09 kV/mm, and 6.79 × 10¹² Ω-cm; and 21.4 MPa, 24.53 MPa, 1.62 kV/mm, and 5.71 × 10¹² Ω-cm, respectively. Alkali treatment and acrylation of jute fibers resulted in 46.50, 13.71, 24.40, and 15.15% improvement in tensile, flexural, electric strengths and volume resistivity, respectively. Water uptake tendency of jute—carbon composite is considerably reduced upon alkali treatment and acrylation of jute fiber. Observed equilibrium water content in all the three composites is HCl > H2O > NaCl. Observed reduction in water uptake in TJCEBCF-50 is due to esterification of hydrophilic OH groups. In boiling water saturation time is reduced 20 times for JCEBCF-50 and TJCEBCF-50, and 16 times for GCEBCF-50 without any damage. Hybrid composites may be useful for low load bearing application and also in marine field.
Composites have been introduced to obtain the desirable properties from two different materials which have different properties and they are the prime subject of interest in the research of metallurgy and material sciences field. The advancement in the composites has uplifted itself from primitive to its next stage. The hybrid composites navigated for purpose of lightweight, wear resistance and for combining advantages of the reinforcements based upon the necessity and need of applications. In this work the hybrid composites consisting of Silicon carbide and Molybdenum disulphide reinforced Aluminum 7075 has been developed using stir casting technique The SEM analysis and mechanical characterization of hybrid composites has been studied. The composition of silicon carbide is varied in steps of 5% wt. and Molybdenum disulphide is kept constant with 3% wt. Three samples with 5, 10 and 15% wt of silicon carbide added with 3% wt of Molybdenum disulphide are considered for analysis.
Comparison of the tension and shear fatigue of hybrid systems with that of all-glass and all-graphite composites indicates that hybrids made with HTS graphite and S-glass have excellent fatigue resistance even when as much as 50% of the fiber volume is composed of glass. The fatigue life of HMS graphite is not significantly improved with the replacement by S-glass. Static residual ultimate strength tests on unidirectional hybrids indicate some reduction in strength and strain due to fatigue cycling but practically no change in modulus. Extensive experimental results are presented in curves and photomicrographs.
This work concerns the production by vacuum infusion and the comparison of the properties of different hybrid composite laminates, based on glass, flax and hemp fibers in different combinations, keeping constant basalt laminates as the inner core of the layered material and a 21±1% fibers volume throughout. The laminates have been subjected to tensile, flexural and falling weight impact tests. Mechanical tests show quite limited differences between the three hybrid configurations, a fact which is also suggested by the not large variation in material density obtained. The main differences have been observed dealing with falling weight properties, carried out at energies between 6 and 24 Joules using a half-inch impactor with a mass of 1.25 kg. Here, the hybrid configuration containing together flax and hemp fibers shows some energy dissipation properties with the onset of complex damage modes in the laminate.
Current applications of carbon fibre reinforced plastics (CFRP) can be found mostly in sectors where their use is not principally cost-driven and which have limited production volumes, such as aerospace and sports cars. In order to achieve a step-change in the application of high-performance composites in larger-volume applications, new materials systems are needed that combine very short production cycle times with performance that meets automotive requirements. The EU-FP 7-project HIVOCOMP is developing two material systems that show unique promise for cost effective, higher-volume production of high performance carbon fibre reinforced parts. These materials systems are: Advanced polyurethane (PU) thermoset matrix materials offering a combination of improved mechanical performance and reduced cycle times in comparison with conventional matrix systems, Thermoplastic PP- and PA6-based self-reinforced polymer composites incorporating continuous carbon fibre reinforcements offering increased toughness and reduced cycle times in comparison to current thermoplastic and thermoset solutions. In this introduction, the global concept of the HIVOCOMP-project is presented, and an overview is given of the achievements during the first three project years. Emphasis is then put on the demonstrators (see figures) and how they show the advantages of both innovative material concepts, both in processing (reduced cycle times) and in properties (increased toughness). Finally, the environmental impact of these new material concepts is evaluated by reporting on the results of the LCA (life cycle assessment) and LCC (life cycle costing) studies.
A study has been made of the fatigue behaviour of carbon/Kevlar-49/epoxy hybrid composites. Stress-life data have been obtained for both unidirectional and [(±45, 0, 0)2]s laminates in repeated tension and compression-tension cycling tests at various values of the stress (or R) ratio. Goodman (constant life) diagrams are presented for the unidirectional composites which indicate that the fatigue resistance of hybrid mixtures varies linearly with composition. The presence of the Aramid fibre, whose natural resistance to compression loads is suspect, does not appear to exert any unexpected damaging influence on the response of the hybrids either in tensile or tension-compression loading. This is also true of the behaviour of hybrid laminates containing plies at an angle to the main loading direction.
When carbon fibre is combined with less-stiff higher-elongation glass fibre in a hybrid composite an enhancement of the failure strain of the carbon fibre reinforced phase is observed. This "hybrid effect" is only partially accounted for by internal compressive strains induced by differential thermal contraction during fabrication. The predominant factor is shown to be a relationship between the strength and effective bundle size of the carbon fibre ligaments which is a consequence of the statistical distribution of strengthreducing flaws in the carbon fibres. A lamina or ligament (bundle) of carbon fibres fails when there is a local critical accumulation of fibre fractures. A model based on this concept is used to relate the two-parameter Weibull strength distribution of the carbon fibre reinforced composite phase to that of single carbon fibres. The model suggests that the critical number of fibre fractures is of the order of 3, and experimental observations of the failure process support this hypothesis.
This paper studied the mechanical behaviors of unidirectional flax and glass fiber reinforced hybrid composites with the aim of investigation on the hybrid effects of the composites made by natural and synthetic fibers. The tensile properties of the hybrid composites were improved with the increasing of glass fiber content. A modified model for calculating the tensile strength was given based on the hybrid effect of tensile failure strain. The stacking sequence was shown to obviously influence the tensile strength and tensile failure strain, but not the tensile modulus. The fracture toughness and interlaminar shear strength of the hybrid composites were even higher than those of glass fiber reinforced composites due to the excellent hybrid performance of the hybrid interface. These macro-scale results have been correlated with the twist flax yarn structure, rough surface of flax fiber and fiber bridging between flax fiber layers and glass fiber layers.
Self-reinforced polypropylene is a very tough material. It is even thought that its impact resistance increases with decreasing temperature. This was investigated by examining the constituent tapes and matrix. Tensile tests on both drawn polypropylene tapes and self-reinforced polypropylene were similar: the stiffness increased and the failure strain slightly decreased at low temperatures. The matrix, however, embrittled below room temperature due to the glass transition. In contrast with literature data on Izod impact resistance, the penetration impact resistance did not increase at low temperatures. At lower temperatures, the damaged area after non-penetration impact was significantly reduced. This was caused by a change in the damage mode from tape–matrix debonding to matrix cracking, as the matrix went through its glass transition. These conclusions provide the first understanding of the failure behaviour of self-reinforced polypropylene below room temperature, and can be exploited to further optimise the excellent impact resistance of self-reinforced polymers.
Hot compacted self-reinforced polypropylene composites have good tensile properties and excellent impact resistance, but they have a limited processing window. Therefore, the influence of compaction temperature, dwell time and the application of interleaved films on the tensile and impact properties was assessed. Increased compaction temperature allows more molecular relaxation, thereby melting more matrix and creating a stronger interlayer bonding. This results in reduced 0° tensile properties and penetration impact resistance, while the 45° tensile properties and non-penetration impact resistance are maintained or improved. The dwell time only has minor influences on tensile and impact properties, while interleaved films have a similar influence as increased compaction temperature. These films increase the interlayer bonding, which increases the tensile strength and non-penetration impact resistance, but reduces penetration impact resistance. This paper demonstrates a wide property range depending on the processing parameters, helping in future tailoring of self-reinforced composites to specific applications.
Aramid fiber/glass fiber hybrid composites were fabricated to investigate the impact behavior of four-layer composites through the analysis of delamination area. The effect of position and content of aramid layer on the impact properties of hybrid composites was examined by using driven dart impact tester. The surface-treated composites were prepared by treating the surface of aramid fiber with oxygen plasma and silane coupling agent. The trend of total impact energy was correlated to that of delamination area in both untreated and treated composites. The impact energy and delamination area of hybrid composites depended on the position of aramid layer. When aramid layer was at back surface, the composite exhibited the higher impact energy and delamination area. In surface-treated composites, however, the position of aramid layer had a minor effect on the impact energy of hybrid composites. This was due to the restriction in deformation of aramid fiber. The impact behavior of four-layer hybrid composites was affected by the delamination area at each interface. The deformation at neighbored-aramid layers increased the deformation at adjacent interfaces.
This paper describes the production and properties of hybrid single polymer composites made from co-mingled tows of carbon and oriented nylon 12 fibres using the Leeds hot compaction process. For 22% volume fraction of carbon fibres, a well consolidated UD sample was made at a temperature of 176 degrees C, 2 degrees C below the temperature at which major melting of the oriented PA12 fibres, and loss of molecular orientation, occurs. For braided cloth a higher temperature of 178 degrees C was required to give a good sample, which is too close to the melting point of the PA12 multifilaments. Reducing the carbon fraction to 13% allowed a well consolidated sample (braided cloth) to be made at a lower temperature of 175 degrees C, giving a wider temperature processing window. In tension the hybrid samples were found to fail in a brittle manner while in bending the behavior was ductile as long the molecular orientation was retained.
Due to the intrinsic brittleness of high performance fibres, traditional structural fibre-reinforced composites have limited ductility and toughness. In the present work a new class of fibres is explored for the reinforcement of polymers: continuous stainless steel fibres that simultaneously possess a high stiffness and a high strain-to-failure. The fibres are combined with brittle and ductile matrix systems (epoxy and PA-6) to produce unidirectional and cross-ply composites. The composites are investigated in quasi static tensile tests accompanied with acoustic emission registration. The steel fibre composites are found to exhibit a 3 to 4 times higher strain-to-failure than typical carbon or glass fibre composites.
The aim of this paper was to evaluate the effect of hybridizing glass and curaua fibers on the mechanical properties of their composites. These composites were produced by hot compression molding, with dis-tinct overall fiber volume fraction, being either pure curaua fiber, pure glass fiber or hybrid. The mechan-ical characterization was performed by tensile, flexural, short beam, Iosipescu and also nondestructive testing. From the obtained results, it was observed that the tensile strength and modulus increased with glass fiber incorporation and for higher overall fiber volume fraction (%V f). The short beam strength increased up to %V f of 30 vol.%, evidencing a maximum in terms of overall fiber/matrix interface and com-posite quality. Hybridization has been successfully applied to vegetable/synthetic fiber reinforced poly-ester composites in a way that the various properties responded satisfactorily to the incorporation of a third component.
A study on the flexural properties of hybrid composites reinforced by S-2 glass and TR30S carbon fibers is presented in this article. Test specimens were made by the hand lay-up process in an intraply configuration with varying numbers of glass/epoxy laminas substituted for carbon/epoxy laminas. These specimens were then tested in the three point bend configuration in accordance with ASTM D790-07 at a span to depth ratio of 32. The failed specimens were examined under an optical microscope, and the results show that the dominant failure mode is at the compressive side. The flexural behavior was also simulated by finite element analysis (FEA). Based on the FEA results, the flexural modulus and flexural strength were calculated. Good agreement is found between the experiments and FEA. It is shown that flexural modulus decreases with increasing percentage of S-2 glass fibers, positive hybrid effects exist by substituting carbon fibers for glass fibers, and applying a thin layer of S-2 glass fiber-reinforced polymer on the compressive surface yields the highest flexural strength. The modeling approach presented will pave a way to the effective design of hybrid composites. POLYM. COMPOS., © 2012 Society of Plastics Engineers
This paper proposes a new FE-based approach for modelling all of the possible damage modes in glass/carbon UD hybrid laminates in tensile loading. The damage development is modelled by two sets of cohesive elements, (i) periodically embedded in the carbon layer for modelling carbon fibre failure and (ii) at the glass/carbon interface to capture delamination. The analysis is stopped when the glass layer failure is predicted by integrating the stress distribution over the glass layer to calculate an equivalent stress for unit volume of the glass. The proposed method is validated against the experimental results and then used to simulate the progressive damage process of other hybrid configurations and finally produce a damage-mode map for this material set. The method can easily be applied to other hybrids to assess their performance by producing damage-mode maps.
To improve the toughness of carbon fibre composites, unidirectional carbon fibre prepregs were hybridised with highly oriented polypropylene (PP) tapes. The latter tapes are used in self-reinforced PP composites (SRPPs) with high toughness, but relatively low stiffness. The tensile behaviour of intralayer hybrids of oriented PP tapes and CFPP prepreg tapes was investigated by changing the layup and weave pattern, and by adding interleaved PP films. While stiffness and strength of these hybrids was decreased compared to CFPP, their ductility significantly increased by adding oriented PP tapes. The parallel behaviour of the two constituent materials was caused by delaminations, which developed after the CFPP failure and spreads over the entire sample. This behaviour was modelled and proven to be independent of layup and weave pattern. Interleaved films, which are often necessary for thermoformability, limited the delamination development by increasing the interlayer bonding and yielded a lower ultimate failure strain.
A computational study of the effect of microstructure of hybrid carbon/glass fiber composites on their strength is presented. Unit cells with hundreds of randomly located and misaligned fibers of various properties and arrangements are subject to tensile and compression loading, and the evolution of fiber damages is analyzed in numerical experiments. The effects of fiber clustering, matrix properties, nanoreinforcement, load sharing rules on the strength and damage resistance of composites are studied. It was observed that hybrid composites under uniform displacement loading might have lower strength than pure composites, while the strength of hybrid composites under inform force loading increases steadily with increasing the volume content of carbon fibers.
The relationship between the structure and the compressive strength of carbon fibres has been studied in detail. In order to determine the compressive strength, a combination of single-fibre composite tests and Raman spectroscopy was employed. It was found that the compressive stress-strain curves showed nonlinear behaviour, with modulus softening in compression. The compressive strengths for the fibres with a modulus a parts per thousand yen400 GPa were measured as a parts per thousand currency sign2 GPa and those with a modulus < 400 GPa were > 2 GPa. We have introduced a model to explain this behaviour that assumes that the fibres behave as composites consisting of both crystallites and amorphous carbon. It is suggested that the compressive strength is controlled by the critical stress for kinking the crystallites in the fibres. Hence, the compressive strength of carbon fibres is found to depend upon the shear modulus of the fibres and the orientation of the crystallites within them.
The influence of hybridization on the crashworthiness and energy-absorption characteristics of pultruded glass-graphite/epoxy composite beams was investigated. Lowvelocity drop weight instrumented impact tests were conducted on these hybrid composites to determine the load-deformation behavior for evaluating the impact performance in terms of the ductility index, damage initiation, propagation, and total absorbed energies. Three-point static flexural tests were also conducted to compare the static load-deformation characteristics with those of the dynamic low-velocity impact tests. The behavior under both static and dynamic loading conditions was simulated using finite element modeling procedures to identify the failure mechanisms for optimizing the performance of pultruded hybrid composites.
Experimental results show that the load and strain to failure of all-graphite/epoxy, all-glass/epoxy and other hybrid composites obtained from impact tests are significantly higher as compared to the static test data. The load-bearing capability of composites after damage initiation (which is dictated by the ductility and failure index) has shown marked improvement for the graphite-outside hybrids when compared with the all-graphite, all-glass, and glass-outside hybrids. The high strain to failure glass fibers absorb considerably higher energy before ultimate failure compared to the brittle graphite fibers; as a result, the fiber content and geometric placement of each type of fiber significantly influenced the energy-absorption characteristics of hybrids. Results indicate that the energy-absorption behavior of pultruded hybrids predicted using finite element modeling is in close agreement with the behavior characterized from experimental impact tests. The effectiveness ofhybridization to improve the impact performance of composites was demonstrated.
Polymeric matrix composites are susceptible to degradation and material properties changes if subjected to low-temperature environmental conditions. This paper attempts to present a study on effective coefficient of thermal expansion for various hybrid carbon fibers/glass fibers polymeric composite structures previously subjected to low-temperature environmental conditioning. The hybrid composite architectures were made from various layers of glass mat and/or glass woven embedded along with layers of unidirectional carbon fibers into a polymeric matrix. The samples were preconditioned to a low-temperature environment at a constant temperature of −35 °C for 1-week long, 24 h/day. The instantaneous CTE and thermal strain fields were recorded with a DIL 402 PC/1 dilatometer from Netzsch GmbH (Germany) by setting a monotonically linear rise of temperature from 20 to 250 °C, at a rate of 1 °C min−1. The experimentally retrieved data were compared with the values obtained by running a micromechanical-based approach simulation on a representative volume element.
The aim of the present study is to investigate and compare the mechanical and thermal properties of raw jute and banana fiber reinforced epoxy hybrid composites. To improve the mechanical properties, jute fiber was hybridized with banana fiber. The jute and banana fibers were prepared with various weight ratios (100/0, 75/25, 50/50, 25/75 and 0/100) and then incorporated into the epoxy matrix by moulding technique to form composites. The tensile, flexural, impact, thermal and water absorption tests were carried out using hybrid composite samples. This study shows that addition of banana fiber in jute/epoxy composites of up to 50% by weight results in increasing the mechanical and thermal properties and decreasing the moisture absorption property. Morphological analysis was carried out to observe fracture behavior and fiber pull-out of the samples using scanning electron microscope.
The weave architecture is vital for hot compaction and the mechanical properties of self-reinforced polypropylene. Low compaction quality resulted in early damage initiation and reduced tensile strength. Interleaved films and decreased crimp in the weave architecture increased the compaction quality. The best compaction quality and tensile properties were obtained by standard fed weaves with interleaved films. The penetration impact resistance and peel strength was independent of the weave architecture. Interleaved films increased the peel strength drastically, but the impact resistance only slightly decreased. These conclusions help to select the correct weave architecture and facilitate the hot compaction process.
Strength models for fibre-reinforced composites often rely on the calculation of the stress concentrations around a single broken fibre. This paper presents the first results for stress concentrations in unidirectional hybrid composites, more specifically around a broken carbon fibre. The centres of the carbon and hybridisation fibres are randomly placed in a two-dimensional packing. The common assumption that both fibre types have the same fibre radii, is proven to lead to significant errors. The relative ratio of the volume fraction of the two fibre types only has a minor influence on the stress redistribution. A small increase of the stress concentration factors on both fibre types is noted with decreasing carbon fibre content. The ineffective length, which is a measure of the length of the influenced zone, remains unaffected. A stiffer hybridisation fibre reduces the SCFs on the hybridisation fibre, while this influence on the SCFs on carbon fibres is much smaller. The influence of the hybridisation fibre on the ineffective length is again small. The differences with existing literature are explained based on the more realistic packings in this paper. These results should now be implemented in a model to predict the influence on the strength of hybrid composites.
The effect of thermal exposure in an atmospheric environment for up to 1 year on the flexural performance, under both static and fatigue loading, of a glass fiber/carbon fiber hybrid polymer matrix composite material was evaluated. It was found that exposure to a temperature near, but below, the glass transition temperature resulted in diminished flexure strength as well as reduced fatigue performance. The magnitude of property reduction was, in general, proportional to the amount of aging time, and was found to be dictated by the dominant aging mechanism. Scanning electron microscopy revealed that the modest reduction in mechanical properties at intermediate aging times was predominantly attributed to thermal oxidation, while for longer aging times thermal aging (dimensional relaxation) was the primary cause for the substantial reduction. Dimensional relaxation of the composite was measured at several isothermal aging temperatures, from which, the activation energy of the aging process was determined. This work provides insight into the evolution of mechanical properties as a function of aging time in an atmospheric environment for a hybrid polymer matrix composite.
The stress redistribution after a single fibre break is a fundamental issue in longitudinal strength models for unidirectional composites. Current models assume hexagonal or square fibre packings. In the present work, random fibre packings were modelled using 3D finite element analysis and compared to ordered fibre packings. Significant differences in the stress redistribution are found. Compared to square and hexagonal packings, random fibre packings result in smaller stress concentration factors for fibres at the same distance from the broken fibre. These random packings, however, also show higher maximal stress concentration factors. The influence of the fibre breakage is more localised, which results in lower ineffective and overload lengths. The presence of fibres at smaller distances from the broken fibre explains these phenomena. For an accurate representation of the stress redistribution after a fibre breakage, random fibre packings should be used.
A study on the flexural behaviour of hybrid composites reinforced by S-2 glass and T700S carbon fibres in an intra-ply configuration is presented in this paper. The three point bend test in accordance with ASTM D790-07 at various span-to-depth ratios was simulated using finite element analysis (FEA). For the purpose of validation, specimens of selected stacking configurations were manufactured following the hand lay-up process and tested in a three point bend configuration. The validated FEA model was used to study the effects of fibre volume fractions, hybrid ratio and span-to-depth ratio. It is shown that flexural modulus increases when the span-to-depth ratio increases from 16 to 32 but is approximately constant as the span-to-depth ratio further increases. A simple mathematical formula was developed for calculating the flexural modulus of hybrid composites, given the moduli of full carbon and full glass composites, and the hybrid ratio. Flexural strength increases with span-to-depth ratio. Utilisation of hybridisation can improve the flexural strength. A general rule is in order to improve flexural strength, the fibre volume fraction of glass/epoxy plies needs to be higher than that of carbon/epoxy plies. The overall maximum hybrid effect is achieved when the hybrid ratio is 0.125 ([0G/07C]) when both Vfc and Vfg are 50%. The strength increases are 43.46% and 85.57% when compared with those of the full carbon and glass configurations respectively. The optimisation shows that the maximum hybrid effect is 56.1% when Vfc = 47.48% and Vfg = 63.29%.
This paper presents an analytical model for size effects on the longitudinal tensile strength of composite fibre bundles. The strength of individual fibres is modelled by a Weibull distribution, while the matrix (or fibre–matrix interface) is represented through a perfectly plastic shear-lag model. A probabilistic analysis of the failure process in hierarchical bundles (bundles of bundles) is performed, so that a scaling law relating the strength distributions and characteristic lengths of consecutive bundle levels is derived. An efficient numerical scheme (based on asymptotic limits) is proposed, hence coupon-sized bundle strength distributions are obtained almost instantaneously. Parametric studies show that both fibre and matrix properties are critical for bundle strength; model predictions at different scales are validated against experimental results available in the literature.
This paper deals with the ultimate failure strength of four-layered hybrid laminated composites. A shear-lag model is first applied to obtain the stress redistributions after the breakage of layers. On the basis of the knowledge of these stress redistributions, the probabilistic ultimate failure strength is evaluated by applying the approach of Harlow and Phoenix. The effects on the ultimate strength of hybrid laminates, due to the scatter of lamina strengths, relative fiber volume fractions, composite size and laminate stacking sequence, are identified.
An adaptation of the chain-of-bundles probability model for unidirectional intraply hybrid composites consisting of two types of fibres is given. Local load sharing, which is sensitive to the different elastic moduli of the fibres, is assumed for the non-failed fibre segments in each bundle. A sequence of tight upper bounds is developed for the probability distribution of strength for the hybrid. The upper bounds are based upon the occurrence of k or more adjacent broken fibre segments in a bundle; this event is necessary but not sufficient for bundle failure. This development allows for a description of a critical crack size k*, dependent upon the load on the hybrid, which is a characterization of the length of a crack that catastrophically propagates causing bundle failure with virtual certainty. The upper bound developed with k*, based upon the hybrid median strength, is essentially identical to the true probability distribution of hybrid strength. It is also shown that the strength distribution for the hybrid composite has a weakest link structure in terms of a characteristic distribution function that is highly dependent upon the local load sharing rule, the fibre properties, and the geometrical structure of the hybrid. Numerical results from the model show that typically there is a negative 'hybrid effect' for hybrid breaking strain, but there is a positive 'hybrid effect' for hybrid tensile strength.