1 Introduction

In earlier days of research and development, scientists and researchers were in a race of finding out better sources of energy like coal, steam, crude oil, etc. But in today’s world researchers have been in an endless search of structural materials which can overcome the limitations of existing conventional materials. This is because all the energy sources available are perishing at a lightning speed which now has started another era of saving our fast depleting energy resources. Now, the efforts of researchers have led to the development of polymer composites [1]. Polymer composites are light in weight, inexpensive [2], non-corrosive and have higher specific strength than conventional steels and alloys used in most of the structural applications. Thus, the use of polymer composites in structural components could help in getting rid of major problems like corrosion and fatigue which results in loss of materials and hence the resources. Moreover, the transportation cost of polymer composite structural components will be lower due to their light weight [3]. The use of polymer composites will not only save cost and material but also reduce the consumption of energy and carbon footprint due to their lightweight characteristics [4]. These are the main reasons for which polymer composites are getting attentions of the research community.

At present, a reduction of weight and carbon emission to minimise pollution is the major challenge for the materials research community. This led many passenger car manufacturers to use polymer composite components [5, 6]. Polymer composites are used in wings components, interiors, tail etc., in aerospace industries [7]. This also finds its application in military war ships, vehicles and bulletproof jackets [8]. Electrical and electronic industries are using polymer matrix composite components in laptops, computers, mobile phones, printers, fans, coolers, air conditioners, watches, etc.

It is noteworthy that most of these applications employ hybrid polymer matrix composites due to their superior mechanical properties than conventional polymer composites (polymer composites with single reinforcement that is either synthetic or natural). Natural and synthetic fibres are two commonly used reinforcements in polymer matrices for fabrication of polymer composites. Natural fibres are mostly cellulosic materials extracted from plants. Abaca, coconut, sisal, bamboo and banana are some examples of natural fibres obtained from their respective plants, whereas glass fibre and carbon fibres are two commonly used synthetic fibre used as reinforcements in polymer matrix composites. Natural fibres are biodegradable [9], eco-friendly, cheap and lighter [10] than most of the synthetic fibres. However, the mechanical strength of synthetic fibres is way better than most of the natural fibres. Table 1 represents the mechanical and physical properties of various synthetic and natural fibres.

Table 1 Physical and mechanical properties of natural and artificial fibres
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A study [11] showed that banana-reinforced epoxy composite exhibited higher tensile and impact strength than hybrid jute- and banana-reinforced epoxy composites, whereas hybrid composite showed better flexural strength than banana epoxy composite. From this study, it can be inferred that for infusing desired mechanical properties in polymer composites, hybridisation is necessary. The present review deals with mechanical properties of hybrid polymer composites. It provides an insight pertinent to factors affecting the mechanical behaviour and performance of hybrid polymer composites such as type, orientation, arrangement, weight fraction and treatments.

2 Hybrid polymer composites

Hybrid polymer composites are the composites with a polymer matrix reinforced with two or more different types of reinforcements [20]. Reinforcements can be categorised as natural and synthetic fibres on the basis of their sources. It can also be categorised as continuous fibres, short fibres, particle reinforcement, mat and woven or fabrics on the basis of their different forms as shown in Fig. 1. The arrangement of reinforcements or fibres in polymer matrix also greatly influences the mechanical properties of composites. Polypropylene, polyesters, epoxies, etc., are some commonly used matrices in hybrid polymer composites.

Fig. 1
figure 1

Types of composites based on the various forms of reinforcements. a Particle-reinforced, b continuous long fibre-reinforced, c short fibre-reinforced, d laminated

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It is obvious that hybrid polymer matrix composites are developed to optimally achieve the desired properties. Synthetic fibre-reinforced polymer composites may have higher mechanical strength than natural fibre-reinforced polymer composites. But, higher strength is achieved as a result of trade-off. Synthetic fibres are expensive and hence are lesser cost-effective than natural fibre-reinforced polymer composites [21]. Moreover, synthetic fibre-reinforced composites are not biodegradable unlike natural fibre-reinforced polymer composites and hence are not eco-friendly. On the other hand, natural fibre polymer composites have a very common but major problem of water uptake which is inherent in all natural fibre polymer composites due to hydrophilic nature of natural fibres. This results in deterioration of mechanical properties of natural fibre-reinforced polymer composites. Therefore, to get rid of such trade-off, hybrid polymer composites were developed by researchers.

There are some of the review articles based on hybrid polymer composites through which researchers have been making attempts to bring out the insights related to hybrid polymer composites. One of such reviews [22] based on the thermal, electrical and mechanical properties of hybrid nanofiller polymer composites suggested that interaction among the fillers and the type of fillers used in fabrication of hybrid polymer composites plays an important role in deciding the mechanical properties of hybrid composites. It is also pointed out in this study that hybrid nanocomposites are not used at their full potential so as to exploit them for applications. Similar study was carried out on hybrid polymer composites [23] to understand the synergic effect of hybridisation of fibres in polymer composites on their mechanical properties which also dealt with the models related to hybrid polymer composites and failure development of unidirectional hybrid polymer composites. Another recent study [24] on hybridisation of fibre-reinforced thermoset composites based on energy absorption tube application focussed on mechanical properties of metal/synthetic, natural/synthetic and synthetic/synthetic fibres-reinforced thermoset polymer composites. In another research on hybrid polymer composites, their impact properties were analysed for structural applications. This study also stated that hybridisation is an important and effective method to tailor the mechanical properties in polymer composites so as to achieve desired strength for specific application. Thus, the present study is based on mechanical properties like tensile, flexural and impact strengths of hybrid polymer composites irrespective of the types of reinforcement (synthetic fibre, natural fibre, particles) and matrix (thermoset polymer, thermoplastic polymers).

3 Mechanical properties of hybrid polymer composites

3.1 Tensile properties

A study on hybrid polymer composites comprising jute, pineapple leaf and glass fibres as reinforcement in epoxy matrix examines [25] the tensile properties of composites. In this study, jute, pineapple and glass fibres were reinforced in epoxy in ratio 1:1:1, respectively. Volume fraction of hybrid composite was varied from 0.18 to 0.42. Tensile strength and modulus were examined in which it was observed that both tensile strength and tensile modulus of hybrid composite increased with an increase in total volume fraction of composites. This increase in tensile strength is due to the increase in stiffness of composite, as a result of which the tensile modulus of composite also increased. Similar work [26] was carried out on sisal and glass fibre-reinforced polypropylene composites with 42 wt% of total fibre content to investigate the tensile properties of hybrid composites which showed that sisal/glass composite exhibited lesser value of tensile modulus than sisal/carbon composite and on increasing the carbon fibre and glass fibre contents in sisal/carbon and sisal/glass hybrid composite, respectively, tensile modulus values increase for both the composites. An increase in tensile modulus led to an increase in tensile strength of hybrid composites.

Researchers have also found that the length of fibre in hybrid composites affects their tensile properties. A study on coir/glass fibre-reinforced [27] polymer composites showed that as the length of coir fibre increases the tensile strength of hybrid composite decreases, whereas an increase in content of coir fibre increased tensile strength of hybrid composites. This decrease in tensile strength with an increase in length of coir could be due to tangling of long fibres.

Researchers have been also exploring carbon nanotubes (CNTs) as reinforcements [28] with bamboo fibre in polymer matrix. In this study, the effect of variation of CNT was analysed on the tensile strength of hybrid composites keeping the bamboo fibre content constant. It showed an improvement in tensile strength of hybrid composites as the CNT content was increases. Another work was carried out to investigate [29] mechanical properties of jute/glass and bamboo/glass-reinforced epoxy composites. Inter-ply hybrid composites were fabricated such that each composite has 4 laminas. As glass fibre lamina increases from 1 lamina to 3 laminas in bamboo/glass composites, tensile strength of hybrid composite increases from 72.02 MPa to 190.94 MPa, whereas in case of jute/glass composites the tensile strengths were 41.04 MPa, 75.64 MPa and 153.39 MPa for single, double and triple layer of glass fibre composites, respectively. Results showed that tensile strength of hybrid composites increases drastically with the addition of glass fibre laminas in both cases of jute/glass and bamboo/glass composites. However, it is noteworthy that bamboo/glass hybrid composite completely outperforms jute/glass composite. But it also remarkable that an increase in tensile strength of bamboo/glass composite is 1.65 times when glass fibre lamina increases from one to three, whereas in case of jute/glass composite, the increase in lamina of glass fibre from one to three resulted in 2.74 times increase in tensile strength of composite. Hence, tensile strength of jute/glass hybrid composite increases more than bamboo/glass composites with the addition of glass fibre laminas. Likewise, in another similar work [30] based on investigation tensile strengths of jute/glass (sample-1), banana/glass (sample-2) and jute/banana/glass (sample-3) fibre-reinforced epoxy composites were evaluated as 51.12 MPa, 68.42 MPa and 85.91 MPa, respectively. Result shows that jute/glass hybrid composite performed the worst compared to the other two hybrid composites. However, it is also seen that jute when used in combination with banana and glass fibre that is jute/banana/glass hybrid composite performed best among all the three hybrid composites.

Orientation of fibres in hybrid composites is also an important factor which is considered while fabricating hybrid composite for any applications. A research [31] on abaca/GFRP (GFRP—glass fibre-reinforced polymer), neem/GFRP and abaca/neem/GFRP hybrid composites showed how the tensile properties are influenced when the orientation of fibres changes. In this study, three different types of composites type-1, type-2 and type-3 were fabricated with parallel, perpendicular and 45° inclined orientation of natural fibres in three successive layers for all the three hybrid composites, respectively. Results showed that when fibres are in parallel orientation (that is type-1), abaca/neem/glass composites exhibited highest tensile strength among the three composites which is evident from Table 2. Table 2 illustrates the tensile strengths of type-1, type-2 and type-3 composites respectively, for all the three composites. In type-2, abaca/glass composite performed better than neem/glass, whereas in type-3, neem/glass composite performed better than abaca/glass composites. Moreover, abaca/neem/glass composite exhibited the highest tensile strength among the three composites for all three types. Therefore, it could be concluded that selection of fibre for hybridisation is more critical and dominant factor than orientation of fibres in hybrid composites.

Table 2 Tensile strengths of polymer composites with parallel, perpendicular and 45° orientation of natural fibre in successive layers
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In another research on kenaf/coconut epoxy composite revealed that increasing the kenaf content in composites results in enhancing the tensile strength of composite [32]. A study on the effect of layering sequence of treated kenaf mat in hybrid kenaf–kevlar epoxy composites on tensile strength showed tensile strength is maximum when kevlar lamina is placed at the outer surface of laminate composite covering the kenaf lamina [33]. The effect of red mud particle on tensile strength of hybrid banana/polyester composite was examined in a study and found that an increase in wt% of red mud increases tensile strength of hybrid composites. Also, the composite with smaller particle size of red mud performed well under tensile loading conditions as smaller particles of red mud help in its uniform distribution in polymer matrix [34].

3.2 Flexural properties

Flexural strength of polymer composites determines the maximum bending stress sustained by composite when subjected to bending load conditions. Flexural property helps in analysing the bending performance of hybrid polymer composites which is of great importance for structural applications. Therefore, one of the researchers has studied [35] the effect of red mud on the flexural strength of E-glass hybrid polymer composites. Three types of hybrid composite were fabricated having 0, 10 wt% and 20 wt% of red mud which exhibited 390 MPa, 280 MPa and 400 MPa flexural strength, respectively. This shows that incorporation of 10 wt% of red mud in composite resulted in the deterioration of flexural strength. This decrease and then increase in flexural strength may be attributed to the presence of voids due to the addition of red mud particles (10 wt%) which disappears on further addition of sufficient amount of red mud (20 wt%) in hybrid composites. In a similar study on hybrid polymer composites with red mud as filler [36], it was again observed that at 10 wt% of red mud, flexural strength of composite deteriorates when compared to without red mud. Further, on increasing the red mud content to 20 wt%, flexural strength of composite again shoots up to 401.81 MPa which is more than the flexural strength of composites with 10 wt% and without red mud. However, similar trends were not observed in case of bamboo-reinforced epoxy matrix with red mud as filler. In this case, flexural strength of hybrid composite increased continuously with an increment in red mud composition from 0 to 20 wt% in hybrid composites as shown in Fig. 2a.

Fig. 2
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Reproduced with permission from Elsevier

Effect of wt% of red mud on a flexural strength and b impact strength of various hybrid composites [36].

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Flexural strength of hybrid polymer composite also depends on the particle size and wt% of filler which is evident from a study [34] which deals with the effect of red mud on flexural strength of banana polyester hybrid composite with red mud as filler. Different hybrid composites were fabricated with varying particle size and wt% of red mud. Flexural strength of composites with least particle size of red mud (4 µm) exhibited the best flexural strength than other composites. Analysis of results also indicates that flexural strength of hybrid composites with different particle sizes of red mud shows deterioration in flexural strength at 4 wt% of red mud which is very evident from Fig. 3a. Lower flexural strength values at 4 wt% of red mud were reported due to the cross-linking of fibres which was confirmed by scanning electron microscopy (SEM) as shown in Fig. 3b. This study concludes that flexural strength of hybrid composites majorly depends on particle size of red mud (filler).

Fig. 3
figure 3

a Influence of red mud on flexural strength of banana/polyester hybrid composites, b SEM image of fractured banana/polyester hybrid composite under flexural loading and c impact strength for various banana/polyester hybrid composites [34]

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Surface modifications in natural fibre-reinforced polymer composites are very common which are done to improve the compatibility of natural fibres with polymer matrices. Moreover, surface modification also improves the surface roughness and reduces the hydrophilicity of natural fibre. Thus, it is obvious that when natural fibres are introduced as reinforcement in hybrid polymer composites, surface treatments become necessary. Coconut and kenaf fibres were treated with 5 wt% NaOH solution [32] for 4 h for better surface morphology and improved mechanical performance of hybrid composites. Figure 4 shows the dependence of flexural strength of hybrid composites with varying proportions of kenaf and coconut fibres. Results of the study show that flexural strength of hybrid composite was maximum for the 7.5 K/2.5C specimen; that is, in other words flexural strength is highest for the composite specimen which constitutes higher proportion of kenaf fibre. However, on increasing the coconut fibre composition up to 10 wt%, there is an increase in flexural strength of composites. The addition of higher amount of coconut fibres may have increased the tangling between fibres which reduces the flexural strength of hybrid composites.

Fig. 4
figure 4

Reproduced with permission from Elsevier

Effect of kenaf and coconut fibre content on flexural strength of hybrid composite [32].

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A study on abaca/neem hybrid epoxy composite reported [31] the effect of orientation of reinforcements on the flexural strength of composites. Hybrid composites with reinforcements (abaca and neem laminates) having 45° inclination were observed with highest flexural strength than the composites with their reinforcements laid horizontally and vertically. A similar work portrayed the influence of lamina sequencing in woven kenaf/aramid hybrid composites [33] on their mechanical properties. Figure 5 shows the flexural properties of various composites. It clearly shows that composite with kevlar lamina at the outer surface (A/k/A/k) has better flexural strength than the other composites having kenaf layer at outer surface (A/k/A/k*). In other words, when load is subjected to kevlar layer surface of the composites higher flexural strength is realised. Thus, kevlar is better alternative than kenaf as reinforcement in hybrid composites under flexural loading conditions.

Fig. 5
figure 5

Reproduced with permission from Elsevier

Flexural modulus and strength of kenaf/kevlar/epoxy, kenaf/epoxy, pure epoxy and kevlar/epoxy composites [33].

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Some of the studies which dealt [37] with abaca/jute/glass, abaca/glass, jute/glass hybrid composites also revealed some interesting aspects pertinent to hybrid polymer composites. All the three types of composites were fabricated such that each composites specimen had three layers of natural fibres sandwiched between glass fibres. All the composites had the mid layer reinforcement arranged horizontally and the other two layers of natural fibres in vertical orientation. In case of abaca/jute/glass composite midlayer consisted of jute fibres. Flexural properties of various composites were determined in which it was reported that abaca/glass hybrid composite showed the highest value for flexural strength that is 12.5 MPa at break load of 1.55 kN as shown in Fig. 6. Higher stiffness of abaca and better adhesive bonding with epoxy than other two fibres might be the two reasons for high flexural strength of hybrid abaca composites. But, abaca/jute/glass hybrid composite also contained abaca despite it failed to perform as good as abaca/glass composites. Hence, it was concluded from this study that abaca possesses better compatibility and adhesion with epoxy matrix. Another work [12] on abaca/jute/glass hybrid composites also reported that composites with higher abaca content exhibited better strength than the other composites with lower content of abaca.

Fig. 6
figure 6

Reproduced with permission from Elsevier

Load–displacement curves for abaca/glass/epoxy, abaca/jute/glass/epoxy and jute/glass/epoxy composites under flexural loading [37].

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3.3 Impact properties

Impact strength determines the resistance offered by hybrid composite against impact load without failure. Therefore, it is obvious for a design engineer or scientist to know the impact properties of component before its deployment in any structural applications. Especially in automobile industries, these properties are of great significance for components like fenders, under floor panels, doors, etc. A study reported that impact strength of hybrid composites depends on fibre length and fibre loading [27]. Carbon nanotube (CNT) and bamboo-reinforced epoxy composite was fabricated [28] with varying wt% of CNT and was subjected to impact loading conditions. Results showed that the introduction of CNT in hybrid composite adversely affects the impact strength. Likewise, jute/banana/glass-reinforced epoxy composites showed a better impact resistance than jute/glass and banana/glass hybrid composites which was attributed to the presence of different laminas of jute and banana which absorbs more energy during impact test [30]. Another similar work [38] was reported on evaluation of mechanical performance of banana, jute glass fibre-reinforced polyester. Jute/glass composite exhibited highest impact strength (752 J/mm) than glass/banana (326 J/mm) and jute/banana/glass composites (500 J/mm). The effect of red mud [34] on impact strength of banana/polyester composites was evaluated. Particle sizes and wt% of red mud were varied, and their effect on impact strength was analysed as results of which it was inferred that with a decrease in particle size of red mud impact strength of hybrid composite increased which may be attributed to uniform distribution of red mud particles. Also, from Fig. 3c it can be clearly seen that impact strength is maximum for the composites with 13 µm particle size of red mud at 10 wt% of red mud, whereas for composites with lower particle sizes of red mud like 6 µm and 4 µm exhibited highest impact strength at 8 wt% of red mud. This study also showed that untreated banana hybrid composite with 8 wt% and 4 µm particle size of red mud was observed with the highest value of impact strength when compared to silane and NaOH treated banana.

Hybrid composites with neem, abaca and glass fibres [31] as reinforcement in epoxy polymer matrices were also investigated for their impact strength. Various composite specimens were fabricated with different orientations of fibres like horizontal, vertical and 45° inclined in laminates. And it was reported that hybrid composite with neem as its one of the reinforcements possessed better impact strength than the others. However, abaca/glass hybrid epoxy composite has better impact strength [37] than jute/glass and abaca/jute/glass composites. This better performance of abaca composite was again credited to its better adhesive strength with epoxy. However, it was noticed that impact strength of jute/glass composites was also close to that of abaca/glass composite. In a similar work [12] on abaca/jute/woven glass hybrid composite with epoxy matrix, it was observed from the results of impact tests that with an increase in content of abaca fibre, impact strength of composite increases. This study also dealt with the determination of the effect of orientation of reinforcements on impact strength of hybrid composites. Again, the results showed that hybrid composites with their fibres in the second layer aligned at 45° to the first and second layers exhibited highest impact strength. Figure 7a illustrates the SEM image of hybrid composite after failure under impact loading conditions. It was also stated that failure was not due to the lower adhesive strength between fibres and epoxy polymer matrix as shown in Fig. 7b.

Fig. 7
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Reproduced with permission from Elsevier

a SEM image of fractured hybrid abaca/jute/woven glass composite under impact loading; b fibre–matrix bond [12].

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There are some of the interesting studies made on hybrid composites which analysed the effect of red mud on the impact strength of composites. E-glass fibre/red mud-reinforced epoxy composites [35] were fabricated with varying wt% of red mud. Impact strength for e-glass/10 wt% red mud composite was increased almost about 32% when compared to composites with 20 wt% red mud and without red mud. Therefore, an optimum quantity of filler dispersion is desirable for hybrid composites. A comparative study [36] on red mud filler bamboo/epoxy and glass/epoxy composites presented that impact strength of hybrid composites majorly relies on geometry and orientation of reinforcements in composites, adhesive strength at interface, type of reinforcements and test conditions. Figure 2b represents how the impact strength (J) of composites varies with wt% of red mud in glass/epoxy and bamboo/epoxy composites. It is clear from the results that glass/epoxy possesses superior impact strength than bamboo/epoxy composites irrespective of red mud content. Moreover, with an increase in wt% of red mud impact strength of bamboo/epoxy composite increases, whereas in case of glass/epoxy composites, impact strength increases drastically with an increase in red mud content from 0 to 10 wt%. But, on further increasing the content of red mud up to 20 wt%, abrupt downfall is seen in impact strength of glass/epoxy composite which is also seen in many past literatures based on red mud hybrid composites. The results also stated that failure under impact loading condition may take place due to fibre pull outs, matrix failure, insufficient adhesive strength between matrix and reinforcements. Table 3 illustrates the tensile, flexural and impact strength of hybrid polymer composites.

Table 3 Mechanical properties of hybrid polymer composites
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4 Conclusions and future scope

In today’s world where researchers are in a quest to develop new alternatives for present traditional materials, this study provides a comprehensive review on the mechanical properties of hybrid polymer-based composites. The literature shows that jute and banana fibres are the two natural fibres which are mostly used reinforcements in hybrid polymer composites. However, it is also observed that pineapple, bamboo, ramie, coir and neem fibres are not utilised at their best as reinforcements in hybrid polymer composites. It is remarkable that abaca-reinforced hybrid polymer composites performance was excellent under tensile, flexural and impact loading conditions. Likewise, kevlar hybrid composites also have remarkable mechanical properties especially under impact loading conditions which makes them suitable for military applications. Epoxies are the most exploited polymer matrices in hybrid composites, whereas polyesters, polypropylene and polylactide acids (PLAs) are the least. Mechanical properties like tensile, flexural and impact strengths of hybrid composites are mainly affected by the types of reinforcements and matrices, surface treatments, wt% of filler and fibres, particle size of filler, adhesion between matrix and fibres, orientation of fibres and geometry of hybrid composites. However, impact strength of hybrid composites was least affected by the particle size of fillers. Thus, to tailor the desired mechanical properties in hybrid polymer composites these factors must be properly managed and optimised.

On the basis of past literature, it is very evident that there are very less reports made on the use of fillers like high entropy alloys (HEAs), red mud, basalt powder, fly ash on hybrid polymer composites. Hybridisation of natural fibres with existing structural composites such as concrete can be explored for further improvement in their mechanical performance. Developed novel hybrid composites may be characterised for their mechanical, thermal, electrical, fire retardant, biomedical, acoustic properties for a wide range of applications. More reporting on modelling and optimisation of mechanical properties such as tensile strength, flexural strength, impact strength and toughness of hybrid polymer composites by using finite element method and statistical methods (like response surface method and Taguchi method) would be beneficial for the upcoming researchers.