Towards a green & ecological revolution: Review of natural reinforcing bio-composites and bio-hybrid composites

Abstract

Recently, the use of composite materials has significantly increased in various industries, ranging from automotive to construction, due to their more advantageous properties compared to traditional materials. Nevertheless, with the growing environmental and ecological awareness, researchers are focusing on developing more environmentally friendly and healthier bio-composites while ensuring high-performance, affordable, renewable, biodegradable, and lightweight materials with environmental benefits. This literature review intends to provide an overview of recent work that focuses on bio-composites and bio-hybrid composites with natural reinforcements. Moreover, it presents the natural fillers, their classifications, the cultivation of some plants, their mechanical and chemical characteristics, as well as their potential applications. Additionally, this work describes the modeling, its different approaches, as well as the selection process of a model.

Introduction

Economic growth, increasing world population, and industrialization are leading to an exponential increase in the consumption of conventional materials and petroleum resources. This overexploitation is one of the main factors of pollution and depletion of minerals and raw materials. The exhaustion of these resources had led hundreds of scientists and specialists to search for alternatives that are both efficient and eco-friendly. This ecological and environmental awareness, combined with the desire to improve living conditions for the ultimate benefit of all living organisms on earth, has prompted the development of new biomaterials to replace conventional materials. ¹ These green materials, which respect our health, environment, and planet, are bio-composites/bio-hybrids reinforced with natural fibers. ² Among the different natural fibers that have attracted a lot of attention, there are cotton, alfa, coir, sisal, kenaf, flax, hemp, banana, coconut, jute, wood, clay, coconut, jute, cotton husks etc.^3–6 These natural fillers compete with their synthetic counterparts due to their multiple advantages, which are reflected in low cost, biodegradability, lightweight, renewability, ecological benefits, low abrasion, high resistance to fatigue, corrosion and impacts, plus good electrical insulation.^7,8 Natural element-reinforced composites have acquired great importance owing to their properties, which allow broad access to different industry activities such as construction, rail, military vehicles, automotive, furniture, sporting goods, and aeronautics.^9,10 Due to the variety of the reinforcing elements used (organic and inorganic), a wide range of bio-hybrid composites had been created in the last years. These composites exhibit new behaviors that enable their application fields to be expanded as well as intermediate properties between the inorganic and organic. ¹¹

Several studies have investigated the incorporation of natural fibers into various polymer matrices to examine the mechanical performance and ultimate properties of the reinforcements and associated biomaterials. According to these studies, reinforcement efficiency is significantly dependent on the interfacial adhesion between matrix and natural fibers; consistently, the stress transfer between matrix and fibers is higher with the more efficient the fibers are. ²

In general, experimental tests are used to evaluate the mechanical and thermal properties of composite materials, along with identifying the influence of the different fillers by the variation of their filling volume. These tests require chemical surface treatment of the different constituents before their use to eliminate unnecessary substances contained therein, followed by the manufacture of tested material samples, which is time intensive and costly. ¹² In the face of these constraints, modeling has been implemented to minimize the time and cost of experimental tests. On the other hand, modeling aims to develop solutions to define the deformation and stress fields in heterogeneous materials to calculate their properties based on homogenization approaches that allow determining the effective characteristics (macroscopic properties) of material from the characteristics of its different components (microscopic properties).^12–16 Starting with the definition of REV, the elementary volume representative of the material, on which the heterogeneous medium is replaced by an equivalent homogeneous medium MHE.^17–19 Among the micromechanical models designed to predict the characteristics of composites (Young modulus, stress, Poisson’s ratio …), there are Hashin, Self-coherent, Mori-Tanaka Halpin- Tsai, Voigt, Reuss, etc.

This article reviews the work done on the different types of natural fibers and their potential applications in the fabrication of biomaterials (bio-composites/bio-hybrid composites). It is divided into several parts.

The 1st section is devoted to composite materials, their definition and classification, along with hybrid composites.

The 2nd section presents natural reinforcements, their classifications, and the cultivation of some plant fibers. The 3rd section shows the work performed on bio-composites and bio-hybrid composites with naturally occurring organic/inorganic reinforcements. The 4th section presents the mechanical and chemical properties of natural reinforcements and their application areas. The last section is focused on modeling approaches and on the selection of a micromechanical model.

Composite material

In a broad sense, composite material is generally defined as a mixture of two or more distinct immiscible components (Figure 1). These have excellent penetration capabilities and strongly complement each other concerning their properties.^20,21 The resulting material is an inhomogeneous (heterogeneous) material with properties that the components alone do not have. ²² It is generally composed of a matrix and a reinforcing material, which can take the form of fibers, platelets, or particles. The reinforcement phase is characterized by high stiffness and strength compared to the continuous matrix phase, whose role is to hold the reinforcing elements and ensure load transfer in these elements.^23,24 In recent decades, composites have been able to replace metallic materials in several areas thanks to many criteria as their better shock absorption capacity, ease of manufacture, low density, etc. ²⁵

Figure 1.

Composites.

Composite materials classification

Composite materials can be classified according to the following criteria:

The type of the matrix that can be polymer, ceramic or metallic. The polymer matrix is characterized by low density, relatively low mechanical strength, and high strain at break.^21,26 Polymers are classified into three major categories: thermoplastics (TP), thermosets (TD), and elastomers. Thermoplastics account for 80% of the world’s plastics production. ²⁷ They can recycle several times and they include polystyrene, polypropylene, etc. Thermosets contain polyester, which is regularly used with glass fibers. This matrix is limited to less-performing applications because of an adhesion problem with higher-performance fibers such as carbon and aramid fibers. ²⁸ Epoxy matrixes have high mechanical characteristics and their maximum temperature is between 800°C and 1200°C. ²⁹ It blends very well with carbon fibers for the realization of high-performance structural parts (vehicles, aeronautics). Elastomers are considered low-bridged thermosets. In this category, rubber is the best known. The following table (Table 1) represents the main differences between the first two categories. ²⁷

Table 1.

Main differences between (TP) and (TD). ²⁵

	Thermoplastics	Thermosets
Basic state	Solid ready to use	Viscous liquid to be polymerized
Wettability of reinforcements	Difficult	Easy
Molding	Heating+cooling	Continuous heating
Cycle	Short	Long (polymerization)
Impact resistance	Fairly good	Limited
Thermal resistance	Reduced (except new TP)	Better
Scrap and waste	Recyclables	Lost or used in loads
Working conditions	Cleanliness	Solvent fumes

Metal matrix composites are materials based on metal (titanium or aluminum) reinforced with ceramic (non-metallic) reinforcements. ²⁴ However, the ceramic matrix can be made of glass, and silicon carbide, reinforced with ceramics, metals, and carbon. ²⁴

The figure below (Figure 2) shows the shape of the reinforcements (long, short, particles or platelets) and their nature (organic, inorganic, or mineral).^23,25 Concerning the shapes, it is possible to classify the fillers according to their aspect ratio (L/D: Length/Diameter):

• If (L/D) is infinite, the fiber is long.

• If 10 < (L/D) < 50, the fiber is short.

• If 1 < (L/D) < 5, the result is a particle.

• If (L/D) approaches 0, it is a disk.

Figure 2.

Composites classification.

Hybrid composites

According to Bottreau and Vicq, ²⁸ hybrid composites are materials that contain at least two reinforcing elements that have diverse mechanical and physical characteristics. Furthermore, Sanchez et al. ²⁹ consider a hybrid composite to be a mixture of both organic and inorganic components with blending in the micrometer range. For Achby et al., ³⁰ this material combines two or more components with predetermined geometries and dimensions to perform a certain function in an optimal way. Thus, hybridization involves combining two or more reinforcing elements to create new and improved functionality. For a long time, glass, aramid, and carbon are used as reinforcements thanks to their good mechanical characteristics and affordable production costs.^31,32 Although different types of reinforcements can be mixed to generate a hybrid material, several studies show that high-performance hybrid composite obtained by combining two types of reinforcements.^33,34

Natural reinforcements

In front of the recent pressure on industries to reduce the nuisances related to industrial processing and encourage the development of sober and clean technologies respecting the environment and ecology, together with the desire to use ecological and eco-friendly materials with little effect on the health of users,^30,33,35,36 scientists and eco-designers are leaning towards using as much as possible renewable resources as reinforcements due to their lower price. In addition to being more environmentally friendly, natural fiber composites are lightweight and have thermal and mechanical performances as composites with synthetic fiber. ³⁷ Accordingly, the necessary raw materials to produce reinforcing agents are readily available.^30,34 Impressively, the percentage of renewable raw materials used in the manufacture of biomaterials has increased from 5% in 2004 to 13% in 2010 and 18% in 2020. According to predictions, it will reach a 25 percent level by 2030. ³⁸

Natural fiber classification

All components of the plant, mineral, or animal origin penetrating a matrix are called natural reinforcements, as shown in Figure 3.

• Organic reinforcements of animal origin come from secretions (silk), sheep (wool), oxen, buffalo, sheep, goats (hooves, horns), alpacas (hair), birds (feathers, beaks), turtles (shells), claws, baleen of whales, as well as spider silk.^39–42 They can be used in many sectors: thermal and electrical insulation, packaging, mural covers, etc.^23,24 Animal fibers are primarily based on proteins, which include collagen, fibroin, and keratin. Collagen is both the prevalent protein in mammals and the most important structural protein in connecting tissue. ⁴³ Keratin is one of the main structural constituents of vertebrate skin, hair or horns, ⁴⁴ and is also a member of the structural protein family. It is rich in sulfur and contains between 12 and 15% nitrogen ³⁹ According to45, this protein gives silk fiber a very good tensile strength between 650 MPa and 750 MPa. Despite their superior performance, silk fiber composites are rarely used in industrial applications than green fibers. ⁴⁶

• The most used mineral fillers are clay, brucite, basalt, talc, chalk, asbestos, etc.^47–52 In contrast with other green and animal fibers, mineral or geological fillers have superior properties which give them wide use in many industrial and military fields such as aeronautics, electronics, medical, marine, automotive, construction, and defense applications.^48,52,53 These fillers are often in the form of nanoparticles, which allow them to have a higher aspect ratio, this criterion offering them a good surface interaction with the matrix.

• The plant reinforcements come from residues of fruits, leaves, stems, grains, wood, etc.^2,25 The Figure 4 illustrates the green fiber’s structure, which consists of a central tube “lumen” (transporter of nutrition and water), and a cell wall. This wall consists of several layers: the thin main wall, the central lamella, and the secondary wall, itself divided into the external secondary wall S1, the central secondary wall S2, and the internal secondary wall S3. These layers are formed by micro-fibrils whose spatial orientation, called the micro-fibril angle, differs depending on the cell wall layer and the plant cell type.^46,54

Figure 3.

Natural fillers.

Figure 4.

Structure of the plant fiber. ⁵⁵

Chemical composition of the plant material

The chemical composition of the plant material is mainly based on, cellulose, lignin, and hemicelluloses, with traces of other components such as proteins, inorganic salts, pectin, and starch.^56,57 It varies according to the type of the plant, moreover, it is possible to change inside one plant.^58–61 The variability of the chemical composition depends on many parameters as species, climatic circumstances, soil conditions, age, and extraction process. ⁶²

• Cellulose is the most dominant component of plant fibers. It is a macromolecule formed by a very long stereo-regular chain of D-glucose subgroups attached by β-1,4-glycosidic bonds,^63,64 as seen in Figure 5.

• Hemicelluloses are heterogeneous polymers with modified linear structures. It is a readily biodegradable hydrophilic substance that serves as a link between cellulose micro-fibrils. ⁶⁶ Hardwoods, softwoods, and various agricultural residues all contain hemicelluloses in an average proportion of about 26%, 22%, and 30%, respectively.^67,68

• Lignin acts as a collage that assures the support to the cell walls to keep their attachment, ⁶⁹ filling the spaces between the pectin, the hemicellulose, and the cellulose to improve their rigidities in the plant.^70,71 Compared to lignin, hemicellulose has 2.6 more times the moisture content. ⁶⁴

• Pectin is a crucial component of natural fibers, helping to bind them together into bundles and affecting their shine and feel. Pectins can be found in variable amounts in plant tissue, but the most common is in gums and peels of fruit. ⁶⁷ The pectin content of flax ranges from 3 to 4%. ⁶⁴

Figure 5.

Cellulose structure. ⁶⁵

Resources of nature fibers

To properly cultivate, natural fibers require several environmental and climatic conditions, for for example jute fiber is limited to the tropics and sub-tropics. ⁷² However, there are other green fibers do not impose any conditions and can be cultivated worldwide as hemp. ⁷³ The following table (Table 2) provides an overview of the requirements for growing natural fibers and the specific areas in which they are grown.

Table 2.

Conditions and areas for the cultivation of natural reinforcements.

Fiber	Specific areas	Cultivation requirements	Ref.
Kenaf	China, India, and Pakistan.	Cultivation is found worldwide with a Tsoil ≈ 15°C for sprouting and growing.	74
Hemp	Canada, China, and the European Union (The main producers are France, the Netherlands, and Romania).	It grows best at T = 14°C but can grow between 5.6 and 27.5°C. For usable harvests, 4 to 5 months without freezing temperatures are required. Ideal yields need 500–700 mm of rainfall. The recommended soil pH is 6.0.	73
Ramie	Indonesia, Japan, China, India, Korea, India, Taiwan, Philippines, and Brazil.	In humid environments where the ambient temperature is moderate. 25% of humidity with Toptimum between 20°C and 31°C. Needs regular precipitation of 1500–3000 mm/year.	75
Bamboo	China, Brazil, Asia, Pacific and India.	Thrives on mountain slopes and in jungle areas. Soils with high moisture content and high water holding capacity. Annual precipitation >1000 mm, with the absence of snowfall.	76,77
Jute	In areas that are tropical, sub-tropical, and equatorial.	During rainy summers, the atmosphere is hot or humid. With an optimal temperature is 24°C–27°C.	72
	In Brazil, China, Vietnam, Bangladesh, Thailand, Myanmar, and India.
Abaca	Philippines (60% of commercialization) and Ecuador (35% of commercialization).	Growing conditions are often tropical. A temperature variation between 28°C and 30°C, an excellent relative humidity, and annual precipitations of 2000–2500 mm are ideal for the development of the fibers.	78
Sisal	Brazil, China, Central America Tanzania, and Kenya.	Tropical or sub-tropical climates.	79,80
		It is necessary to have a high temperature and a humid atmosphere.
Alfa	Delimits the desert, North (Moroccan and Algerian highlands) Africa, Portugal, Spain, Egypt, and Balearic Islands.	Dry and semi-arid climate.	51
Flax	Czech Republic, Canada, India, Argentina, Ukraine, Kazakhstan, France, USA, and Egypt.	At Toptimal between 18°C and 20°C with highest relative humidity.	81,82
Curauá (Ananas erectifolius)	Brazilian Amazon.	Tropical and sub-tropical climates.	83

Properties of natural fibers

Natural fiber is a sustainable and recyclable raw material, and its bio-composites can be recycled into ecological compost after completing their life cycle.55 Furthermore, they are characterized by being biodegradable, skin-friendly (non-irritating), low cost, lighter than synthetic fibers, and having low absolute tensile strength, as illustrated in Figure 6.^46,84–86 Besides, they are characterized by a rough and irregular surface that facilitates their bonding with the matrix of a composite. ⁸⁶ Due to their higher specific tensile strength, found by division the absolute tensile strength by the mass density, these fibers are considered an excellent choice for manufacturing composites for aerospace fields.

Figure 6.

Stress/strain diagrams of bio and synthetic composites. ⁴⁶

Generally, the physical characteristics of natural fibers are determined by several essential fiber-related factors, such as structure, physical and chemical composition, section, micro-fibrillar angle, degree of polymerization, cellulose content, and L/D ratio (a key parameter to ensure the loading transmission between the matrix and the fibers). Cellulose fibrils are helicoidally oriented according to an angle called the micro-fibrillar angle, existing between the fibrils and the fiber axis, is considered a parameter that affects fiber rigidity. In simple terms, at a given cellulose content, the microfibrillar angle (MFA) increases with decreasing fiber stiffness and strength, although the elongation at break increases. In contrast, the elasticity linearly increases with increasing cellulose percentage.^86–88

The following Tables 3 and 4 show the mechanical and chemical characteristics of some natural reinforcements, and their prices. Leaf fibers (Sisal, Pineapple, Banana, and Abaca) are especially noted for their higher strength, which is related to the highest percentage of lignin. ⁴⁵ However, bast fibers (kenaf, jute, hemp, and ramie) have higher mechanical properties than other fibers, also a higher proportion of cellulose, with a higher number of cellulose micro-fibrils oriented in the direction of the fiber, ¹⁰ which makes them favored for use in composites’ structural applications. ⁸⁹ Additionally, silk has proven to have greater tensile strength, as well as piezoelectric properties and thermal stability. ⁹⁰

Table 3.

Mechanical characteristics of natural and synthetic fibers.

Fibers	Density (g.cm−3)	Diameter (μm)	Length (mm)	Tensile strength (MPa)	Young’s modulus (GPa)	Strain at break (%)	Specific modulus (GPa)	Specific strength (MPa)	Moisture content (%)	Price (US$.kg−1)	Ref.
Ramie	1.44–1.55	18–80	40–250	400–938	23–128	2–4	15–29	590	8.5–11	2	2,84
Alfa stems	1.4	90–135	1.3–1.5	134–220	13–17.8	—	—	—	—	—	91–93
Bagasse	1.25	66.9–312	—	96–290	8.5–17	4.03	—	—	—	—	37,61,94
Bamboo	0.6–1.52	8–480	1.5–4	140–713	11–55	1.3–8	18	383	—	∼0.5	2,61,95,96–98
Kenaf	1.45	12–106	1.4–11	295–1200	22–60	1.6–6.9	18–50	246–993	6.2–17	0.3–0.5	2,61,84,98,99,100
Flax	1.38–1.5	5–100	10–65	343–1500	27.6–100	1.2–3.2	34–48	345–620	12	2.1–4.2	2,37,100
Abaca	1.5–1.52	10–30	4.6–5.2	400–813	12–41	2.9–10	—	—	14	0.345	2,37,84,100
Hemp	1.1–1.48	10–51	5–55	389–1110	30–70	1.6–4.5	20–41	210–510	12	1– 2.1	2,84,100
Cotton	1.21–1.55	12–35	15–56	287–597	5–13	2–10	4–6.5	194–452	8.5–34	1.5–2.2	2,84,100
Curaua	1.4	—	—	500–1150	11.8	3.7–4.3	—	—	—	—	61
Banana	1.35	12–250	0.4–0.9	500–914	12–32	5–6	20–24	392–677	10–11	—	2,84,100
Oil palm	0.7–1.55	150–250	—	206–248	3.2–25	3.2		—	—	—	37,100
Date palm (raw)	0.9–1.5	100–1000	—	58–203	2 –7.5	—	—	—	—	—	101
Coir	0.96–1.44	7–30	0.3–3	130–1150	4–6	15–40	5.2	92–152	10–13	0.2–0.5	2,37,61,98,100,101–105
Juncus plant hot treated (8% of NaOH, Na2S2O4 and NaOCl)	1.25	40	—	1800	122	—	—	—	—	—	62
BFF	—	—	—	65.2	4.9	47.2	—	—	—	—	106
Jute	1.23–1.49	5–260	0.8–6	187–800	13–55	1.16–3.1	14–39	140–329	12	0.4–1.5	2,37,84,100,106,107
Sisal	1.45–1.5	7–80	0.8–8	350–855	9–22.0	1.9–7	6–15	55–580	11.0	0.6–0.7	2,37,86,106,108,109,110
Cork	0.15	—		—	2.5	—	—	—	—	—	111
Pineapple	1.5	8–41	3–8	170–1627	60–82	1–3	42–57	287–1130	10–13	0.40 –0.55	2,112
Silk	—	33–49	—	200–650	5–16	15–140	—	—	—	2.6–40.0	37,40,86,113
S-glass	2.5	—	—	3–3.5	63–67	2.8	34.4	1.8	—	2	2,114
E-glass	2.5	15–25	—	2000–3500	70–73	2.5	29	800–1400	—	2–3	2,114
Aramid	1.4	—	—	3100	63–67	3.3–3.7	—	—	—	—	114
Carbon	1.7	—	—	4000	235	1.4–1.8	—	—	—	8–11	114

Table 4.

Natural fiber chemical characteristics.

Fibers	Cellulose %	Hemicellulose %	Lignin %	Micro-fibrillair angle (deg)	Ref.
Alfa stems	45	24	24	—	91–93
Bagasse	55	17	25	—	61,94
Date palm (raw)	46	18	20	—	100
Juncus plant hot treated	40	21	29	—	62
Bamboo	26–65	30	21–31	2–10	61,95,98,102,115
Kenaf	72	20.3	9	9–15	61,98,99,102,115,116
Coir	26.8–43	17.2–37	22.2–45	30–49	61,198,102,103,117
Jute	58–71.5	13.6–20.4	12–14	8	6,30,106,107
EFB	49.6	—	21.2	—	107
Sisal	67–78	10.0–14.2	8.0–11.0	10–22	106,108,109,110
Banana	63–64	10	5	11	108,109,118
Cork	—	—	—	—	108,109
Flax	71–78	18.6–20.6	2.2	5–10	6,30,118
Eucalyptus	—	—	—	—	108,110
BFF	45.67	32.76	21.53	—	106
Hemp	70.2–74.4	17.9–22.4	3.7–5.7	2–6.2	2
Ramie	68.6–76.2	13.1–16.7	0.6–0.7	7.5	6,30,118
Olive	25	35	35	—	119
Pistache	53.98	20.1	25.25	—	120
Abaca	56–63	21.7	12–13	—	61,118
Pineapple	80–83	15–20	8–12	—	28,121,122
Abricot	29.57	17	47.97	—	123
Noisette	25.9	28.7	44.4	—	124
Untreated (raw) coconut sheath fiber	37–43	24–28	26–28	—	125
Treated coconut sheath fiber	53–63	14–20	21–25	—	125

Chemical modification

The bio-fiber’s use faces several limitations, ⁸⁰ such as high moisture absorption, low compatibility between fiber and matrix, hydrophilic structure, etc. Therefore, to increase the surface properties of natural fiber and reduce its moisture absorption, it has been subjected to chemical modifications such as alkalization, Bleaching, silanization, acetylation, dewaxing, Seawater, benzoylation, cyanoethylation, graft copolymerization, etc.

Alkaline treatment

This method is one of the most widely used and economical techniques. It acts on the molecular structure of the fibers by removing pectins, hemicellulose, lignin, and wax which clothe the fiber surface to be cleaner, by adding aqueous sodium hydroxide (NaOH) to the natural fiber to facilitate the ionization of the hydroxyl group to the alkoxide natural fibres (fibre-Cell-O-Na).^127,128

Bleaching treatment

The fibers are subjected to a bleaching test with calcium hypochlorite Ca (ClO)₂ for about 45 min, after which they are washed with deionized water and dried in an oven at 80°C for 24 h. ¹²⁷

Silane treatment

Silane is a versatile chemical element that is utilized to change fiber surfaces as a coupling agent. Among various coupling agents tested for natural fiber surface treatment, silane proved to be the most successful, forming a chemical bond between the fiber surface and the matrix via siloxane molecules. The absorption of silane is highly reliant on a number of parameters, including hydrolysis time, temperature, pH, etc. After the hydrolysis phase, the silanol created combines with the OH groups of the cellulose, while the second end reacts with the functional groups of the matrix. This process promotes the formation of hydrocarbon chains that reduce fiber swelling.^2,127

Benzoyl treatment

This treatment is done in two steps: removal of waxes and impurities, then immersion of the fibers in a benzoyl chloride solution where the benzoyl group (C₆H₅CO) replaces the OH group of the natural fiber. It reduces the hydrophilic properties of the fiber and improves the interfacial adhesion between the fiber and the matrix.^2,127

Acetylation treatment

During this treatment (esterification method), the acetyl group reacts with the hydrophilic hydroxyl groups of the fiber. To expedite the reaction, the fibers are immersed in acetic acid and acetic anhydride, respectively. As a result, the fiber’s hydrophilic nature is reduced due to the substitution of OH groups with acetyl groups, and its surface becomes smoother, thus improving the transfer of mechanical stresses to cell interfaces.^2,127

Seawater treatment

This is the easiest and least expensive way to treat natural fibers. After testing the pH and salinity of the seawater, the fibers are placed in the water for 30 days before being cleaned and dried at ambient temperature. ¹²⁷

Nature fiber reinforcement of biocomposite and biohybrid composite

Composites with natural reinforcements

Several kinds of natural fibers have been used in the development of biocomposites such as pehuen cellulosic husk, ¹²⁹ Juncus plant, corn stalks, alfa, resinous wood (SW), ¹³⁰ coconut sheath, ramie, ¹³¹ silk protein, ¹³² date palm, bamboo, ¹³³ coir, basalt, banana stem ¹³⁴ flax, and caraua. ¹³⁵ Table 5 lists the work that has been done that evaluates and adds value to the manufacture of biocomposites. Chen et al. ¹³⁶ have compared corn stalk fiber, basalt, and lignin fiber. They showed that corn stalk fibers and lignin fibers reduced the penetration capacity of the asphalt binder. The addition of more than 6% corn stalk fiber has been shown to decrease the penetration of the asphalt binder very rapidly. Corn stalk fiber was excellent at dispersing asphalt binder because of its short length. The asphalt modification effect improved as the amount of inclusion increased. Regarding the penetration of basalt fiber into the asphalt binder, it shows a slight decrease with the addition of basalt fiber due to the smooth surface of basalt as well as its low adsorption capacity of asphalt. Lignin fiber in asphalt binder is characterized by its similar performance to that of corn stalk fiber.

Table 5.

Recent work on natural fibre reinforced biocomposites.

Year	Material	Methods	Results of the study	Ref.
2019	Asphalt binder/corn stalk fiber	Tests: penetration/softening point, BBR (bending beam rheometer), rotational viscosity, and dynamic shear.	At high temperatures, the penetration of the asphalt binder was reduced by the added corn stalk fiber, but the softening point and complex modulus of the asphalt increased. The viscosity of the asphalt binder also increased.	136
2018	PP/talc	Mathematical models: Halpin-Tsai, Kerner-Nielsen model and Guth modified.	The experimental and mathematical models results indicate the effect of these particles, where young’s modulus obtained progress, even with higher particles contents.	137
		Experimental tests: tensile and morphological tests.
2018	Banana-stem fibers	Experimental tests: mechanical-physical, morphological, chemical, durability, thermal (TGA,DSC), degradability, and antibacterial properties.	This chapter outlines the production, extraction method, and possible applications of banana stem fiber. According to tests, banana pseudo-stem fiber can be kept until 3 months before losing its durability. After this period, the fiber’s strength is diminished.	134
2017	Juncus fibers	Experimental tests: SEM,FTIR, X-ray diffraction, physical-mechanical, and chemical tests.	At hot temperatures, the processing with 8% NaOH, (Na2S2O4), and (NaOCl) gave better results in terms of both strength and tensile modulus due to the successful delignification of the fibers. Unlike the cold alkaline treatment.	62
2017	Sand, cement/date palm	Hygric and thermal characteristics (sorption isotherm, water vapor permeability, thermal conductivity, and moisture diffusivity).	The results show good suitability and efficiency of the composite strengthened by date palm fibers in the construction domain.	138
2016	Flax and hemp fiber composite		This paper was discussed several technical and non-technical reasons to choose the flax and hemp fiber composites.	153
2015	PP/alfa	Mechanical characterization, chemical treatment, acoustic emissions, and SEM.	Young’s modulus for virgin PP significantly increased after alkaline treatment, going from 28.67 to 132.22%, and tensile strength for untreated fibers increased from 11.34% to 30.14%.	139
2014	Epoxy/raw coconut sheath/treated coconut sheath	Dynamic mechanical analysis FTIR, SEM, and TGA.	Treated coir fiber composites perform better than non-treated fiber composites in mechanical strength and thermal stabilities, and according to SEM, they exhibit better fiber-matrix adhesion than other composites.	125
2014	Cement/coir	Mechanical, thermic properties, and XPS.	The combination of fiber affected the water absorption capacity of mortars, augmented their thermal and mechanical properties when decreases their amount. These properties were increased when using the alkali-treated coir fibers.	154
2013	PP/borassus fruit fine (BFF)/sisal/jute/coir	FTIR, tensile and flexural test.	In the presence of the maleated PP compatibilizer, the maximum tensile resistance of the fiber was at 15 wt.%, and the maximum flexural strength of the BFF was at 10 wt.%, according to FTIR analysis. In the same study, composites composed of 15 wt.% jute, coir, and sisal fibers were created. The mechanical properties of the BFF/PP composites were superior to those of the coir/PP composites by 15 wt% but equal to those of the sisal/PP and jute/PP composites. However, in comparison to the coir/PP and BFF/PP composites, the jute/PP and sisal/PP composites had higher water absorption capabilities.	106
2012	PLA + EAGMA/basalt fibers	Tensile, flexural, and impact properties.	At 20% basalt and 20% EAGMA, the impact strength has risen by 19 kJ/m2 to 34 kJ/m2 without notching the PLA. The phases (fiber/matrix) are well bonded; no void separates them, while the matrix barely wets the fiber surface.	144

Racca et al. ¹³⁷ studied the influence of talc particles in a PP matrix with experimental tests and mathematical models to determine the elastic modulus. The results indicate that the obtained modulus of elasticity grows with the proportion of particles. Nailia et al. ⁶² have valorized the green biomaterial, where this study aimed to characterize the fibers extracted from Juncus and to examine the influence of the chemical treatments performed on these fibers. They revealed that its Young’s modulus and tensile resistance are higher when treated with 8% NaOH, sodium dithionite (Na₂S₂O₄), and sodium hypochlorite (NaOCl) at elevated temperature. Haba et al. ¹³⁸ focus on measuring the hygienic and thermal properties of a composite material used in construction reinforced with 15 wt% date palm fibers by comparing the results obtained with other types of bio-composite materials used in the same field. Mechakra et al. ¹³⁹ were interested in the mechanical characterization of composite with short fibers of alfa plants. The results obtained showed the influence of alkaline chemical treatment by the significant progression of the elastic modulus from 28.67% to 132.22% compared to virgin PP and also tensile resistance from 11.34% to 30.14% relative to the untreated fibers at 30% content.

In the same context, Rokbia et al. ¹⁴⁰ investigated the impact of these alkali-treated fibers on the bending characteristics to define the ideal parameters for chemical treatment (NaOH). During the treatment of Alfa fibers with 10% NaOH in 24 h, the strength and Modulus of bending increased from 23 MPa to 57 MPa and 1.16 to 3.04 GPa. In contrast, these results decreased during treatment with 5% NaOH for 48 h, which was explained by the reduction of lignin. The two previous articles^62,140 show the importance of chemical pretreatment of the fiber to eliminate unwanted substances and render it suited to use as a reinforcement in a composite, where the alkaline treatment affects the mechanical behavior of the fibers, especially on the strength and stiffness of the fibers. ¹⁴¹ Kumar et al. ¹²⁵ performed a comparative study between untreated and treated raw coconut sheath fiber. They showed the impact of fiber chemical treatment on the composite properties; better fiber-matrix coupling. Song et al. ¹⁴² evaluated the thermal and viscoelastic characteristics of Poly Lactic Acid matrix composites with twilled and plain hemp tissues. With an increase in fiber volume fraction from 6 to 20%, they demonstrated that the thermal expansion coefficient of composites constructed with hemp fabrics dropped from 70 µm/°C to 10 µm/°C; however, their tensile and impact properties were 10% and 15% higher than those of the plain tissue. To reinforce a soybean resin, Behera et al. ¹⁴³ studied woven and unwoven jute fibers with different contents ranging from 40 to 80 wt%. The maximum mechanical properties were obtained at 60 wt% jute for both materials, woven and non-woven jute composites. For the non-woven jute composites, the tensile properties are: 37.1 MPa for tensile strength and 1.04 GPa for Young’s modulus (E), yet the modulus and flexural strength are 1.12 GPa and 38.4 MPa, respectively. Regarding the woven jute composites, the tensile resistance and elastic modulus are 35.6 MPa, and 0.972 GPa Regarding the modulus and flexural resistance are 1.02 GPa and 33.5 MPa. According to Liu and al., ¹⁴⁴ a basalt fiber with the right dimension can improve the traction, bending and impact characteristics of PLA. The non-notch impact strength of matrix improved to 34 * 10³ J/m² by combining 20% basalt fiber and 20% ethylene acrylate glycidyl methacrylate copolymer. In addition, the phases are tightly bound, as there is no void between the matrix and the fiber, while the matrix barely wets the fiber surface.

Hybrid composites

Hybrid composites with synthetic/natural fibers

In this regard, several investigations have been realized to evaluate the effectiveness of the composite materials, see Table 6. Saidane et al. ¹⁴⁵ studied the effects of the hybridization of glass/flax fibers on diffusion kinetics and mechanical properties. They showed that the glass fiber additive decreased the water absorption and diffusion coefficient while improving the tensile characteristics at ∼20°C. However, at 55°C, this combination adversely affected both the specific tensile resistance and the tensile resistance. Yahaya et al. ¹⁴⁶ investigated how the orientation of kenaf fibers (woven, unidirectional UD, and matted) affected the mechanical properties of the kenaf/aramid hybrid composite. The mechanical and morphological results indicated that the tensile strength of the woven kenaf hybrid composite sample was better than that of hybrid composites with unidirectional and matted fibers, which showed that the use of kenaf in the form of a weaved structure is capable of generating a hybrid composite material with high tensile strength and impact resistance properties. Benmansour et al. ¹⁰⁸ studied the possibility of applying the composite containing sand, cement, and date palm fibers (2 different dimensions of this fiber) to use as an insulating building material. The results showed that the incorporation of fibers reduces the compressive strength and thermal conductivity of this composite. In the same year, El Makssoudi et al. ¹⁴⁷ studied the tensile behavior of a hybrid composite based on an epoxy matrix reinforced with oil palm and glass fibers. They showed that hybridization improved the tensile resistance and elastic modulus of the hybrid composite. In 2013, Ramesha et al. ¹⁴⁸ made a comparative study between sisal/glass fiber reinforced epoxy matrix - jute/glass fiber based hybrid composites. They showed that the hybrid composite based on an epoxy matrix and sisal/glass fibers had the highest properties than the other hybrid composite reinforced with jute/glass fibers in fiber-reinforced hybrid composite concerning tensile properties, but the jute/glass fiber-reinforced hybrid composite with jute/glass fibers is better in bending. Ramnath et al. ¹⁴⁹ compared the mechanical the mechanical behavior of three hybrid composites reinforced with abaca/glass, jute/glass, and abaca/glass/jute. The tensile and shear tests indicate that the hybrid abaca/jute/glass composite has better properties than the abaca/glass composite. Nevertheless, the abaca/glass hybrid composite is superior to the abaca/jute/glass composite in terms of impact and bending strength. Ornaghi et al. ¹⁵⁰ studied hybrid composites comprising a polyester matrix strengthened with glass/sisal fibers prepared by the transfer resin molding process. They showed that this hybridization increased the storage modulus and loss modulus.

Table 6.

Hybrid composites with natural-synthetic reinforcements.

Year	Material	Measurements	Results of the study	Ref.
2016	Epoxy/flax-glass fibers	Tensile tests, and water absorption testing.	In contrast to 55 C, a low temperature of 20 C has a beneficial impact on the mechanical properties of the flax/glass hybridization. For hybrid laminates containing 6% and 11% glass fibers, respectively, the elastic modulus was reduced by 50% and 41%. For the flax laminate was reduced by 67%. Glass fibers reduce the thickness swelling and water absorption of the flax fibers.	145
2015	Kenaf–aramid fibers	Mechanical, SEM, and water absorption tests.	In comparison to unidirectional and matted kenaf fibers, the hybrid composite containing woven kenaf has tensile strengths that are almost 20.78% and 43.55% greater, respectively. According to SEM, the woven kenaf hybrid material has a high tensile and impact strength.	146
			The high void content and hydrophilic properties of kenaf affect fiber swelling and dimensional stability negatively and change the material’s capacity to absorb water.
2014	Cement, sand,/date palm fibers (two different sizes)	Thermal conductivity, and water absorption compressive strength testing.	The inclusion of date palm fibers decreased the bio-compressive composite’s strength and thermal conductivity.	108
2014	Epoxy/oil palm-glass fibers	Tensile tests.	The elastic modulus and tensile resistance were enhanced by the hybridization.	147
2013	Epoxy/glass – sisal fibers; glass – jute fibers	Mechanical characteristics, SEM.	The bending properties of the hybrid composite reinforced with glass and jute fibers were superior, with a peak bending load rating of 1.03 KN; however, the material with glass/sisal fibers had better tensile properties. These natural fiber composites perform worse than glass fiber when utilized in applications that call for medium strength.	148
2013	Epoxy/abaca–jute–glass fibers	Tensile, shear, flexural, and impact tests.	The abaca/jute/glass hybrid composite exhibits superior qualities over the abaca/glass hybrid composite, according to the findings of shear and tensile tests. The glass/abaca hybrid composite has a superior impact and flexural strength over the abaca/jute/glass hybrid composite.	138
2013	UD flax/glass fiber	Tensile properties.	The fracture strength and inter-laminar shear resistance of the material were superior to those of composites with glass fibers due to the better interface of hybrid, which also led to a rise in tensile property results from the increasing proportion of glass fibers.	155
2010	Polyester/glass–sisal fibers	Mechanical and dynamic properties.	The hybridization with sisal/glass fibers causes an increase in storage modulus and loss.	150
2007	Short hemp-glass fiber	Mechanical, water absorption, and thermal properties.	The hybridization of hemp/glass fibers revealed improved mechanical and thermal properties. At 15 wt% hemp fibers and 25 wt% glass fibers being combined, the flexural modulus was 5.5 GPa. The thermal properties and resistance to water absorption of the hybrid composites were also enhanced by the addition of glass fiber.	156

Hybrid composites with natural fibers

Faridul et al. ¹⁰⁵ studied the effect of melamine urea formaldehyde (MUF) on trilayered bio-composites reinforced with coir fibers and fibrous chips. They demonstrated the successful formation of bio-composite panels. Mbeche et al. ¹⁵¹ focused on the fabrication of hybrid bio-composites with a polyester matrix reinforced with sisal and cattail fibers. At a fiber content of 20 wt % (fiber mixture ratio varied between 0 and 75%), tensile, compression, and bending strength increased with optimum contents of 75/25 sisal/cattail ratio. Boujmal et al. ⁵¹ studied hybrid PP matrix bio-composites (70% wt) reinforced with Alfa fibers and clay particles with their total fillers constant at 30 wt %. They showed that the dispersion and interface interaction between the two fillers were good, that the elastic modulus, yield strength, tensile resistance changed with the filler content, and that the inclusion of clay enhanced the thermal stability of the matrix.

In 2016, Rosni et al. ⁹⁸ compared the mechanical characteristics of three hybrid composites based on a polylactic acid (PLA) matrix strengthened with coir-kenaf, coir-bamboo-kenaf, and coir-bamboo fibers. Abdellaoui et al. ¹⁵² were interested in the detection of the effect of natural hybridization on the mechanical characteristics of hybrid resin (clay/epoxy) and bi-hybrid composites (hybrid matrix with natural hybrid reinforcement (coir/jute and doum/jute)), see Table 7.

Table 7.

Hybrid composites with natural fibers.

Year	Material	Measurements	Results of the study	Ref.
2021	Coir fiber/fibrous chips	Thermo-mechanical, and morphological properties.	The results demonstrate the successful formation of bio-composite panels from coir fibers and fiber chips.	105
2020	Polyester/sisal-cattail	Mechanical properties.	For a constant fiber ratio of 50/50, the mechanical characteristics increase when the fiber content is raised from 5 to 20%. For 20 wt% fiber, tensile, compression, and bending strength enhanced when the fiber mixture ratio ranged from 0 to 75%, between 0 and 75%, with optimal obtained at the sisal to cattail ratio of 75/25.	151
2017	PP/alfa fibers-clay	Morphological, mechanical, and thermal properties.	With 15 wt % of clay and 15 wt % of alfa, Young’s modulus optimized being of 2520 MPa. Morphological tests showed good dispersion and interfacial interaction between alfa fibers and clay particles.	51
			The addition of clay and graphite improved the thermal stability of the PP, raising the maximum temperatures of decomposition from 21°C to 34°C to 30 wt% clay, respectively.
2016	Polylactic acid/bamboo, kenaf, and coir fibers	Tensile, flexural, and SEM characteristics.	The bamboo-kenaf-coir hybrid composite has a significant tensile strength of 187 MPa, representing a 78% and 20% improvement over kenaf-coir and bamboo-coir, respectively. The flexural strength for coir-bamboo hybrid composite and -kenaf-bamboo hybrid composite was 206 MPa and 199 MPa, respectively, which is 20% and 16% better than PLA/coir-kenaf. The flexural modulus of the PLA/coir-kenaf hybrid composite was the highest (70% better than for both materials).	98
2016	Thermoset/clay, doum, jute, and coir	Mechanical characteristics.	The tensile strength enhanced with clay content; good wettability of the fibers by the hybrid matrix (epoxy/clay). The yield strain decreased with decreasing clay content, which refers to the brittle nature of the reinforcements and the stiff nature of the clay. At 15% clay and 85% epoxy, the hybrid matrix had the highest elastic modulus (E).	152
			E of the two-hybrid is best in the case of the laminate with doum/jute fibers.
2013	Epoxy/oil palm - jute	Mechanical properties.	The tensile characteristics improved with the increase fiber content compared to oil palm, thus it showed that the addition of jute fiber to matrix/palm composite increased the storage modulus and the damping factor shifted to a hotter temperature region.	156
2010	Epoxy/empty fruit bunch (EFB)-jute	Mechanical characteristics.	The flexural and tensile findings increased with the employment of jute woven in the pure EFB composite.	107
2005	Polyester/banana-sisal	Mechanical properties.	The results indicated a positive influence of a combination of both fibers on flexural strength.	157
			The highest tensile resistance was achieved with the banana/sisal ratio of 1:4.

Application sectors of bio-composites

With climate change and resource exhaustion, the public’s desire for sustainable products made from renewable resources has grown, instead of products made from seemingly exhaustible and very expensive petroleum-based raw materials. This desire aims to alleviate environmental and economic constraints. Naturally reinforced bio-composite materials are commonly employed in many industries as railway, marine, ship bodies, automotive, military vehicles, weaving, construction, medical, engineering and technology, sporting goods, aerospace, and radial fan blades,^{126,142,158–160} see Table 8. In the automobile industry, bio-composites are made not only to be ecologically benign but also to lighten the weight of car parts by substituting high-density glass fibers by low-density natural fibers. Using light vehicles, which are cheaper to buy and fuel, and whose efficiency can be increased, CO₂ emissions can be decreased. ¹⁶¹ According to one analysis, a 25% weight reduction in vehicles is estimated to reduce crude oil consumption by 250 million barrels and 220 billion pounds of CO₂ emissions per year. ¹⁶² According to another, the utilization of sugarcane bagasse-based composites can reduce the weight of automotive components. In addition, growing sugarcane absorbs carbon, helping to minimize planetary warming. ¹⁶³

Table 8.

Application fields of natural reinforcements.

Application areas	Natural reinforcements used	Ref.
Textile weaved, clinically as the thread of sutures, fishing nets and lures.	Silk	40
Paper-making.	Talk	49
Insulating materials in building.	Date palm	102
SMA pavement.	Corn stalk	136
Aerospace.	Hemp	142
Military vehicle’s spall.	Kenaf	146
Areas of engineering and technology.	Sisal/jute	148
Sporting goods, musical instruments interior design, and lighting.	Flax/hemp	153
Automobiles and the building industry.	Oil palm/jute	156
Trunk shelf (Toyota).	Kenaf	163
Automotive components.	Sugarcane bagasse	163
Engine and transmission coverage.	Sisal/flax/coir	163
Panel trim.	Flax	163
Defense, military research, and aeronautical applications.	Basalt	164,165,166
Automotive industry and construction, manufacture of ropes, baskets, and fishing lines.	Curauá (Ananas erectifolius)	83,167

The use of bio-composites is expected to become more diffused in industries foreseeable future. In this regard, it is important to broaden their applications by developing improved and innovative treatment techniques to address the shortcomings of natural fillers. Successful marketability of these biomaterials also requires surmounting the impediment of not recognizing the quality of R&D in developing countries (DCs), wherein natural reinforcements are abundant.

Modelling approaches

Modeling approaches have been employed to approximate the mechanical performance of composite based on the characteristics of their structural components (matrix/fiber), that is, they predict the responses of structures based on these materials to external stresses before their manufacturing. ¹³ Thus, modeling attempts to minimize the need for experimental tests and thus reduce their costs.^168,169 Among the models used are Voigt “Rule Of Mixtures ROM,”^170,171 Hirsch model, Reuss “Inverse Rule of Mixture IROM,” ¹⁷² rule of hybrid mixtures,^164,169,173 Halpin-Tsai model,^169,174,175 Hashin and Shtrikman model, ³⁰ Mori-Tanaka, ¹⁵⁰ Neerfeld-Hill Model, ¹⁷⁶ self-consistent, ¹⁷⁷ method of finite elements “MEF,” ¹⁷⁸ etc.

Generally, the geographical shape of the fibers conditions the micromechanical model used. If the fibers are continuous and unidirectional, the mixing rule (ROM-IROM), the Hashin-Shtrikman model, and the classical laminate theory can be used. However, for shorter (random) fibers, there is the mixing rule, the Halpin-Tsai equations, the Tsai-Pagano equation, ¹⁷⁹ Nielsen-Harris, Mori-Tanaka, Bowyer–Bader,^180–182 self-consistent, Manera-Cox-Krenchel, Hirsch, Kelly-Tyson,^183,184 generalized shear-lag analysis, ¹⁸⁰ the stress transfer model, ¹⁸⁵ etc. For particles, it is appropriate to use the mixing rule, the Halpin-Tsai equations, the Kerner-Nielsen model, ¹⁸⁶ the Tsai-Pagano equation, etc.

• Voigt (1910) - Reuss (1920) are the two simplest models to apply, represent respectively the maximum and minimum bounds of elastic properties (framing model).^187,188 They are calculated from the volume fractions of the components (fiber/matrix), and their individual properties. The longitudinal^178,189 and transverse^175,190,191 moduli of the composite are predicted with Voigt and Reuss models, respectively.

$C_{\text{Voigt}\hspace{0.17em}}=C_m*V_m+C_{f\hspace{0.17em}}*V_f$

(1)

$C_{\text{Reuss}}^{-1}=V_f*C_f^{-1}+V_m*C_m^{-1}$

(2)

• • Hybrid mixture rule (ROHM) is a derivative of the previous method, except that it is intended for hybrid composite with two types of fibers.^169,179,192

$C_{\text{ROHM}\hspace{0.17em}}=C_m*V_m+{\displaystyle \sum }_{i=1}^{n=2}{C}_{fi\hspace{0.17em}}*V_{fi}$

(3)

• • Neerfeld-Hill Model takes the arithmetic median of the values obtained by the two models Reuss and Voigt.^176,193

$C_{\text{Neerfeld}-\text{Hill}}=\frac{\left(C_{Voigt}+C_{Reuss\hspace{0.17em}}\right)\hspace{0.17em}}{2}$

(4)

With C, $C_f,{\hspace{0.17em}and\hspace{0.17em}C}_m$ respectively denote the stiffness of the composite, fiber, and matrix. ${V_m,V}_f$ represent the volume fraction of matrix and fiber.

• • Hirsch model, a semi-empirical model, combined the two above-mentioned models: Voigt “the parallel law” and Reuss “the series law,” inserting an adjustable parameter “x” that defines the stress transfer between the fiber and matrix.^175,190 Mainly, it is determined by the stress concentration at the extremities of the fibers, their orientations, and their lengths. ¹⁹⁴

$C_{Hircsh}={x(C}_m*V_m+C_f*V_f)+\left(1-x\right)\left(V_f*C_f^{-1}+V_m*C_m^{-1}\hspace{0.17em}\right)$

(5)

• • Halpin-Tsai equations, ¹⁹⁵ a semi-empirical model, is an approximate representation derived from self-consistent micromechanics.”^196,197 They are applied in aligned short-fiber composites. In addition to the individual moduli of the matrix and fibers and their volume fractions, this approach accounts for fiber morphology via theL/D ratio.

$\raisebox{1ex}{$E_{HT}$}\!\left/ \!\raisebox{-1ex}{$E_m$}\right.=\frac{1+\zeta \eta V_f}{1-\eta Vf};\eta =\frac{\frac{E_f}{E_m}-1}{\frac{E_f}{E_m}+\zeta }$

(6)

With ζ the reinforcement factor, determined experimentally, which depends on the loading conditions, arrangement of the fibers, their geometries. ¹⁹⁸ E_f, E_m represent respectively the Young moduli of fiber and matrix.

• • Mori and Tanaka (1973) describes the material containing a high rate of inclusions with properties identical to those of the matrix, ¹⁹⁹ assuming that these inclusions are distributed isotropically and embedded in an infinite matrix submitted to an average deformation.^200–202 This model is applicable for a fiber volume rate not exceeding 30% to 50%. It considers the interaction between the inclusions, ²⁰³ the mechanical characteristics and, the volume fraction of the phases, their shapes, and their orientations. ²⁰⁴

$C={C_m\left[\hspace{0.17em}I+\left({\displaystyle \sum }_{i=1}^n{f}_{i\hspace{0.17em}}{L}_i\right){(I+{\displaystyle \sum }_{i=1}^n{f}_i\hspace{0.17em}\left(S_i^{Esh}-I\right)L_{i\hspace{0.17em}})}^{-1}\right]}^{-1}$

(7)

${\hspace{0.17em}With:L}_i=-[\left(C_f-C_m\right)S_i^{Esh\hspace{0.17em}}+{C_{m\hspace{0.17em}})}^{-1}\left(C_f-C_m\right),S_i^{Esh}:Tenseur\hspace{0.17em}{d}^{\prime }Eshelby$

• • Finite element analysis is a numerical method oriented towards simulation applications used to model physical phenomena, and solve problems of complex bounded structures by partial derivative equations.^198,205 This model is based on the division of the analyzed surface/volume into units of smaller size, this operation is called “mesh,” and it calculates the load at the level of each finite element.

Selection of a micromechanical model

Regardless of their microstructure, the models stated above are used to calculate the effective characteristics of heterogeneous materials. The architecture of the composite has a great influence on the choice of the relevant model. Therefore, it is crucial to take into account the volume fractions of the reinforcements, their spatial distribution, orientation, shapes, aspect ratio, and symmetries of the material. For example, Voigt-Reuss are the two simplest methods (less descriptive). They only consider a small number of microstructural parameters as volume fractions and tensors of all phases. ¹⁸⁷ However, the shape, orientation, and spatial distribution of the reinforcements are not considered in these methods. Hashin and Shtrikman’s approach is constrained to a specific shape of reinforcement (small disks, long fibers, spherical reinforcements). Additionally, the boundary models (Voigt-Reuss and Hashin-Shtrikman) provide a framework for the effective properties. The Halpin-Tsai equations are more efficient than the laws of mixtures. ¹⁷⁹ They take the volume fractions and properties of the phases, and the morphology of the fibers into account. Mori Tanaka and self-consistent have broader applications because of their potential to predict the performance of composites with a higher ratio of reinforcement. Indeed, Mori Tanaka has the advantage of being able to visualize separately the matrix in which the reinforcements are incorporated directly, as opposed to the self-consistent technique, where no distinction is made between the phases. Moreover, compared to the self-consistent model, Mori Tanaka offers the advantage of an analytical resolution. ²⁵ As an application, Chichane et al. ¹²⁸ provided a comparative and applicative study of analytical and numerical models using Voigt-Reuss models, Halpin-Tsai equations, the Hirsch model, and numerical simulation to predict the effective elastic moduli of composites and hybrid composites reinforced with natural reinforcements. The results obtained are compared with experimental data and show the advantages and disadvantages of each model.

Conclusion and perspectives

This article proposes a detailed review of the literature on naturally reinforced bio-composites. It presents the latest work on natural reinforced composites and hybrid composites, the different natural resources of reinforcements, their cultivation, their mechanical characteristics, and their chemical compositions, along with their uses in different industrial fields. It also presents the modeling technique, their approaches, and the procedure for choosing a model. The main conclusions reveal that the choice of green fibers as reinforcement requires consideration of their growing conditions (optimal temperatures, precipitations, etc.) and their geographical location, mechanical properties, and cost. Thus, natural reinforcements may be employed in their native state or modified after undergoing pretreatment procedures to remove unwanted substances and prepare them for use in a composite. They also point out that the chemical composition of the reinforcement has an influence on its mechanical properties. These characteristics can facilitate the determination of the suitable reinforcement type for a given application. Additionally, they highlight that the model providing the most accurate prediction of effective properties requires detailed access to microstructural information.