A comprehensive review on material selection for polymer matrix composites subjected to impact load

Vishwas Mahesh.Sharnappa Joladarashi.Satyabodh M.Kulkarni

Department of Mechanical Engineering.National Institute of Technology Karnataka Surathkal.Mangalore.575025.India

Keywords: Polymer matrix composites Natural fibers Green composite Impact characterization Flexible composite MADM Approach

ABSTRACT Polymer matrix composites (PMC) are extensively been used in many engineering applications.Various natural fibers have emerged as potential replacements to synthetic fibers as reinforcing materials composites owing to their fairly better mechanical properties.low cost.environment friendliness and biodegradability.Selection of appropriate constituents of composites for a particular application is a tedious task for a designer/engineer.Impact loading has emerged as the serious threat for the composites used in structural or secondary structural application and demands the usage of appropriate fiber and matrix combination to enhance the energy absorption and mitigate the failure.The objective of the present review is to explore the composite with various fiber and matrix combination used for impact applications.identify the gap in the literature and suggest the potential naturally available fiber and matrix combination of composites for future work in the field of impact loading.The novelty of the present study lies in exploring the combination of naturally available fiber and matrix combination which can help in better energy absorption and mitigate the failure when subjected to impact loading.In addition,the application of multi attributes decision making(MADM)tools is demonstrated for selection of fiber and matrix materials which can serve as a benchmark study for the researchers in future.

1.Introduction

Fiber reinforced composites (FRC) find their application in almost all fields of engineering as a material for structural components.Fibers in various forms like short fiber.long fibers.fiber mats are used as reinforcements in composites and have gained the attention of researchers due to their main advantage of reducing the weight compared to conventional materials.Apart from this,they also possess excellent resistance to corrosion,fatigue with low maintenance cost.Automobiles industry extensively makes use of PMC for various structural components of automobile as it is found that nearly 250 million barrels of crude oil can be saved by reducing the weight of the automobile by 25%[1].The physical and chemical properties exhibited by FRC differ at micro and macroscopic scale which has made it a matter of interest in the field of engineering.However,there lies a drawback in the use of FRC which is its repair.The FRC when subjected to impact loading during its operational or service condition is very difficult to repair and bring back it normalcy [2].

PMC which are used for structural applications make use for either synthetic or natural fiber as the reinforcement.Synthetic fibers used as reinforcements in composites for various engineering application are expensive[3,4].The recent trend in the composites have shifted towards using the natural fibers in place of synthetic fibers keeping in mind the advantages of natural fibers over synthetic fibers such as environmental friendliness[5,6].Natural fibers also possess numerous advantages over the synthetic fibers in terms of acceptable physical.mechanical.corrosive and thermal properties[7-11].Despite the favourable circumstances the natural fibers provide.the available literature also shows that contrasted with synthetic fibers.natural fibers has more water take-up capacity and lower mechanical properties[12,13].In order to convert this unfavourable condition of the natural fiber.many researchers have come up with an approach of modification techniques[14,15].Nowadays it can be extensively found that composites made out of natural fibers are popularly used in the interiors of the automobiles[16,17].Based on their origin.natural fibers can be classified as plant fiber.animal fibers and mineral fibers [18].The sources ofnatural fibers are vast and accordingly natural fibers are found in wide varieties.Natural fibers when compared to synthetic fibers are lighter in weight and also exhibit acceptable mechanical properties [19].The comparison between natural and synthetic fibers are tabulated in Table 1 [20].

Based on the information provided in Table 1.it can be understood that ion terms of density,cost,recyclability,biodegradability,specific strength and modulus.natural fibers are superior compared to synthetic fibers.The hydrophilic nature of the natural fiber can be tackled with chemical treatment of the natural fibers and also though the mechanical properties of natural fibers are lower compared to synthetic fibers.the range of the mechanical properties of natural fibers are within acceptable limit[7-11].Thus,natural fibers are more attractive as reinforcements compared to synthetic fibers.

The evaluation of impact resistance of natural fiber reinforced PMC is a critical issue since majority of the PMC are subjected to impact loading which is dynamic in nature and researchers are involved in the study of impact behaviour of such composites.While developing a material with maximum resistance to damage against impact.it is also essential to understand its energy absorbing mechanism.PMC which are intended to be used in structural applications must possess the capability to maintain the structural integrity.The behaviour of the glass fiber reinforced composites under both low and high velocity impact was carried out by Safri[21-24]which revealed that in order to sustain higher energy of impact,it is essential to have more number of fiber layers.Since.synthetic fibers are expensive; researchers are in search of PMC with natural fiber as reinforcement which can be effectively used for impact applications.

The objective of the present study is to explore various fiber and matrix combination for PMC subjected to impact with emphasis of naturally available fibers and suggest a possible fiber and matrix combination that can effectively enhance impact performance of the composites due to their inherent properties.Also.it is shown that MADM tools can be effectively used in material selection for composites subjected to impact.This particular study provides the readers with extensive literature on the PMC and materials used for composites subjected to impact.Also an approach of application of MADM tools for material selection for composites subjected to impact can be effectively expanded for material selection for various other applications.In order to achieve these objectives,the review is organized with study of natural fibers.materials for matrix.core materials.impact behaviour of composites.MADM approaches,case studies on selection of fiber and matrix using MADM approach followed by concluding remarks.

2.Natural fibers as reinforcements

The natural fibers are available from different sources and varieties.Classification of natural fibers is shown in Fig.1.

Plant based natural fibers are most commonly used in naturalfiber reinforced PMC.Plant based fibers are extracted from different parts of the plant and Fig.2 illustrates the major fibers extracted from different parts of plant.

Table 1 Comparison between natural and synthetic fibers.

The structure and constituents of the natural fiber are well defined by Ref.[25].Plant based natural fibers consists of cellulose,hemicelluloses.lignin.pectin and other materials and based on these contents.the characterization of plant based natural fibers are carried out.Natural fiber is comprised on one thin primary wall and three thick secondary walls as shown in Fig.3.

Secondary wall is surrounded by primary wall which is deposited during the cell growth.Three layers are present in the secondary wall and the mechanical property of the fiber is controlled by the middle layer[26].It is found that when the microfibrils are parallel to the fiber axis.the fiber strength will be more [27].The properties of the fiber are mainly dependant on cellulose which is the major constituent of fiber [28].Hemicellulose comprises a group of polysachrides consisting of five to six carbon ring sugars and is hydrophilic in nature [29].Lignin is amorphous in nature which is also cross linked polymer network consisting of irregular array of variously bonded hydroxy- and methoxy-substituted phenylpropane units which acts as a chemical adhesive within and between fibers [30].Toughness of the fiber depends on the lignin and hemicelluloses content and is directly proportional to each other.At the same time,the strength and stiffness of the fiber is found to increase up to a certain limit[31].Pectins are rich in D-galacturonic acid residues.The hemicelluloses.pectin polysaccharides and aromatic polymer lignin.interact with the cellulose fibrils.forming a rigid structure strengthening the plant cell wall [32].

Natural fibers are currently gaining the attention of many researchers as a reinforcement in the composite owing to several advantages they possess such as low density and cost.ease of availability and high specific strength[7-10].It is also known that natural fibers are non-uniform with irregular cross sections,which make their structures quite unique and much different from manmade fibers such as glass fibers.carbon fibers and so on.Out of the widely available natural fibers.the physical and mechanical properties are provided in Table 2 [26,33-36].

Many researchers have studied the use of natural fibers in PMC for impact applications.Selection of natural fibers for any particular application depends on various factors[40]and are represented in Fig.4 which is inspired by Ref.[40].

Concerned to natural fiber reinforced PMC subjected to impact,various researchers have tried with different fibers as reinforcements in composites.The hybrid composites comprising of carbon,basalt and flax fibers are characterized for their mechanical and impact properties by Ref.[41].This study shows that intercalation of basalt with flax layers proved beneficial.The Influence of low-velocity impact on residual tensile properties of nonwoven flax/epoxy composite was studied by Ref.[42]using different shaped impactors.LVI response of stitched flax epoxy PMC was studied by Ref.[43]with an intention of using the proposed composite for high performance applications.The outcomes led to the conclusion that stitching of the fiber leads to propagation of in-plane cracks leading to lower energy absorption per area of damage.The LVI response of flax reinforced PMC with two different types of matrices (epoxy and MAPP) was studied by Ref.[44]whose outcome concluded that flax fiber reinforced with ductile matrix MAPP results in better energy absorption compared to flex fiber reinforced epoxy composite.The comparative study carried out by Ref.[45]showed that the flax reinforced PLA composite is inferior in impact performance compared to cordenka reinforced PLA composite.

LVI behaviour of hybrid basalt/carbon epoxy PMC was studied by Ref.[46]where it was found that due to the use of basalt fiber,the damage resistance of the laminate was increased.Due to the higher ductility of the basalt fiber,laminate was allowed to undergo larger deformation thus having wider area of damage and higher energy absorption.The ballistic impact performance of the natural curaua fiber-reinforced polyester composites was studied by Ref.[47]for personal protection.The curaua natural fiber with 30%volume fraction opened up interesting results for multi hit applications under ballistic impact.Hemp fibers are another type of natural fibers which can be used along with epoxy resin as a PMC.The LVI response of such PMC was studied by Ref.[48]who concluded that hemp fiber have the potential to replace glass fibers in PMC subjected to LVI loading.A study regarding usage of natural fibers for structural engineering applications was attempted by Ref.[49].The outcome of this work showed that selecting an appropriate manufacturing method along for natural fiber reinforced PMC can lead to better performance of composites.Flax reinforced PMC for impact application was studied by Ref.[44]along with two different types of matrices and the major conclusion from this study showed that the ductility of the matrix largely affects the energy absorption capability of the composite.

Fig.1.Classification of fibers.

Fig.2.Popular natural fibers extracted from different parts of plant.

Fig.3.Structure of natural fiber.

3.Matrix material

Matrix materials used in the PMC can be classified as thermoplastic and thermoset based resin system.Selection of appropriate matrix system for the PMC based on the intended application is a critical task since the final property of the composite is directly influenced by the matrix used[50].Though the tensile property of the composite in longitudinal direction depends on the reinforcement used.the other properties such as tensile property in transverse direction,shear strength,compressive strength,resistance toheat and environment are related to the matrix used.

Table 2 Physical and mechanical properties.

Fig.4.Criteria affecting natural fiber selection in composite.

During the initial days of development of PMC,thermoset based matrix systems were extensively used for the development of composites in military aircraft applications.Although.it was possible to obtain superior mechanical properties with the thermoset matrix based composites,many flaws were discovered with the use of epoxy based composites.This led to the invention of thermoplastic based matrices for use in PMC [51].Nowadays,though it is found that thermoplastics are being used as the matrices in PMC,thermoset still find their application in PMC.The differences in the characteristics of thermoset and thermoplastic matrix systems are tabulated in Table 3 [52].

Wide variety of thermoset resins are found to be used in PMC[53].Epoxy.ethylene co-vinyl-acetate (EVA).polyester.vinylacetate.phenolic.unsaturated polyester.Unsaturated and accelerated orthophthalic polyester,Unsaturated isophthalic Polyester and Phenol Formaldehyde are most commonly used thermoset based resins.Similarly.there are many thermoplastic based resins available for use in PMC namely natural rubber.high densitypolyethylene (HDPE).Polystyrene.Acrylonitrile butadiene rubber,poly methyl methacrylate.polyvinylchloride.low density polyethylene (LDPE) and polypropylene (PP).Various engineering applications make use of PMC for structural applications as listed in Table 4.

Table 3 Characteristics of thermoset and thermoplastic.

The physical and mechanical properties of most popularly used thermoset and thermoplastic matrix systems studied by various researchers[61-68]are shown in Fig.5 from which it is clear that thermosets exhibit better physical and mechanical properties compared to thermoplastics.

Selection of appropriate matrix material for PMC subjected to impact loading is as important as selection of appropriate fiber material.Table 5 provides the various matrix materials used for impact applications.

The transfer of the load between the fibers.alignment and stabilization of the fibers depends on the type of matrix selected[86].Thermoplastics are more commonly used in composites reinforced by short fibers.PEEK and PPS have emerged as the potential substitutes for thermosetting polymers for impact applications.Research was concentrated on why the thermoplastics are able to resist damage during impact loading much better than thermosetting in the early part of 90’s [87].Effect of the type of matrix to resist damage during impact loading in PMC has been studied by few researchers[88].The study carried out by Ref.[69]showed that PMC with PEEK as matrix material provides excellent impact energy absorption capability while exhibiting minimal damage under LVI.Since PEEK is thermoplastic in nature.it also exhibits the advantage of easy and fast repair of the damaged composite.Consequently.PEEK has poor response to high velocity impact applications.Epoxy based carbon nano tubes incorporated PMC was investigated by Ref.[76].Polyester based resin was used to fabricate a PMC subjected to LVI [73].It was found that the use of STF matrix enhances the ballistic impact response of the composites compared to neat fabrics [75].Many researchers make use of epoxy resin widely because of its remarkable mechanicalproperties.low shrinkage.strong adhesion.chemical stability.and dimensional stability [76-79].In epoxy based composites subjected to low velocity impact.the transition of impact event to a stress wave dominated mode happens.The introduction of micro particles to HDPE proved to enhance the high velocity impact properties of the PMC [83].The rubber was used as a matrix material in development of flexible composite subjected to ballistic impact loading with kevlar as reinforcement.It was compared with kevlar/epoxy PMC and it was concluded that kevlar/rubber composite exhibits better ballistic impact response compared to kevlar/epoxy composite [84,85].Various researchers have conducted the impact study on the PMC at different impact regimes.The matrixused decided the stiffness of the manufactured PMC.Pertaining to the role of matrix in deciding the impact property of PMC,the study carried out by Ref.[70]put forth an argument that stiff composites absorb higher energy compared to flexible composites.On the contrary [71].showed that flexible composites are better energy absorbers compared to stiff composites.Supporting the argument made by Ref.[71].the study carried out by Ref.[72].on fabric reinforced PMCs using four different matrices showed that flexible composites have higher energy absorbing capability and undergo larger deformation compared to stiff composites.The matrix used in PMCs governs the extent of deformation and thus affects local strain and impact resistance of PMCs.Though extensive literature is available for the impact studies of the composites,it is found from the literature that the application of flexible ‘green’ composites making use of natural rubber based matrix with natural fiber as reinforcement for impact applications is not studied by any researchers.The natural rubber being a complaint material can be a potential matrix system for flexible PMC subjected to impact.Also,natural rubber is environmental friendly.low cost and easily available.

Table 4 Applications of PMC.

Fig.5.Physical and mechanical properties of Polymer matrix [61-67].

Table 5 Matrix materials used in impact applications.

4.Impact of composites

The stages involved in the analysis of impact in composites ranges from simple analytical model to complex FE approaches as presented in Fig.6.

With each increment in the methodology adopted for impact analysis.the capabilities related to prediction of damage is enhanced.The analytical and experimental methods provide sufficient results pertaining to the damage onset and its quantitative dimensions.Whereas,FE based numerical methods are found to be useful in predicting the type and shape of damage based on the FE methodology adopted.In the early days,analytical methods based on spring mass system was adopted to assess the impact behaviour.First such attempt was made by Abrate in 1991[89]which was later followed by Christoforou [90]in 1998.Going further.the beam plate model was used for impact analysis by Abrate[91]which was able to identify the damage initiation in composites.Layered shell model was used by Karger [92]in 2009 with the capability to identify the damage type.shape and extent of damage.Further progress in the effort level led to stacked layered model by Falzon[93]in 2015 with micro damage prediction capability.Finally,Fiber matrix model was developed by Ivancevic [94]in 2016.

The reported research pertaining to various PMC used for impact applications at different impact regimes and the methodologies adopted to assess the impact response of the PMC are presented in Table 6.

In the literature we can find the analytical approaches used for solving the impact problems.Simplified analytical model was proposed by Ref.[121]to analyse the LVI response of reinforced concrete structures using energy balance and single degree of freedom models.LVI response of sandwich composites was analytically predicted by Ref.[122]using a new three degree of freedom spring mass damper model.The study carried out by Ref.[95]demonstrated the use of analytical approach to predict the effect of radius of impactor of damage of glass epoxy composite subjected to LVI.Impact response of a sandwich composite with honeycomb core was assessed by Ref.[123]using revised energy balance model.The sandwich composite beams were analysed for their LVI behaviour using analytical model by Ref.[124]who developed their model based on the model developed by Ref.[125].The numerical methodology was proposed by Ref.[126]to predict the ballistic impact response of composite impacted by steel sphere.The outcome led to conclusion that the ballistic response ofcomposite is affected by radius of the projectile.The analytical model makes use of simple energy balance approach and shows that ballistic limit of composite is inversely proportional to square root of the projectile radius.The analytical approach to study the high velocity impact of composite based of woven fabric was proposed by Ref.[127].The main mechanism for absorbing energy is found to be deformation of secondary yarns and the target was perforated due to failure of primary yarns.

Fig.6.Stages in development of impact analysis.

Table 6 Reported research on PMC subjected to impact.

The drawback of the theoretical approach lies in its inability to deal with materials having flaws such as cracks.voids,manufacturing defects and inhomogenity.Finite element approach using commercially available simulation packages has been proved to be better than analytical approach [128].

Accurate numerical simulation of impact events can provide physical insights that cannot be captured by experiments.Currently,for impact modelling in composite structures at low and high speeds mainly finite element method (FEM).boundary element method (BEM).finite volume method (FVM).mesh less formulations are used.Many researchers have adopted the FE approach to simulate the impact response of the composites,metals and alloys.Some of the approaches adopted and their outcomes leading to justification of using FE approach in the initial part of the study are presented in this section.

The numerical modelling gives the insight into mechanisms that lead to the damage and fracture of composites.The study carried out by Hamdia and Rabczuk [129,130]showed that the key parameters influencing the fracture toughness of polymer composite can be determined by numerical modelling and influence of input parameter uncertainties on fracture toughness of polymer nano composites was also studied.

Both experimental and FE approach to analyse the LVI impact performance of the carbon fiber reinforced composite was adopted by Ref.[97]in their study.Commercially available ABAQUS/Explicit was made use of to simulate the LVI behaviour of the composite.Damage mechanisms are modelled based on continuum damage mechanics (CDM).The results showed that there is a good agreement between experimental and FE results.Different modelling techniques for analysing the LVI response of composite plates was studied by Ref.[131]with an intention of comparing results obtained from different techniques and provide a guideline for those researchers who opt for FE simulation to study the LVI response of composites.Impact simulation was carried out using LS-DYNA by Ref.[132]to study the different modes of failure in composites using a 3D solid element.It was found that results vary with mesh size.Parametric study was also carried out by varying the boundary conditions and impact velocity.The validation of the obtained FE results with experimental results showed that FE method can be effectively used to simulate the LVI response of the composite to near real life situation.The cross ply composite laminates subjected to LVI was studied by Ref.[133]using a FE approach using a CDM based method with the help of ABAQUS/Explicit software.The results obtained led to respectable comparison of FE and experimental results.The delamination type of damage induced in composite due to LVI loading was studied by Ref.[134]using LSDYNA with the help of transient non-linear finite element code.The material model proposed in this study proved to be the good approach to assess the LVI behaviour of composite using FE approach for non penetrating response of composite laminates.The progressive damage analysis of the graphite epoxy composite laminates using CDM was proposed by Ref.[98]using ABAQUS/Explicit FE software.Weibull distribution of the composite strength was employed in the proposed model along with consideration of non linear shear behaviour of composite.In addition to these considerations.the model was also incorporated with irreversible strain due to the damage.The FE results well matched with the experimental results supporting the proposed model.Impactresponse of the composite ranging from low to hyper velocity was studied by Ref.[99]using AUTODYN hydro code.The results concluded that the penetration of the proposed composite by bullet is more difficult when the composite is fully restrained.In case of high velocity impact.damage of the composite plate is localized.The obtained FE results agreed well with the available literatures.The LVI response of carbon/epoxy laminates was carried out by Ref.[96]and the results matched with the experimental results.Numerical analysis with the aid of FE software was carried out for impact analysis of steel plates and graphite/epoxy composite where the flexible nature of the composite was investigated.The results proved that flexible nature of the composite has a positive effect on the energy absorption due to impact.High velocity impact of the sandwich composite was studied by Ref.[100]using ABAQUS/Explicit.The accuracy of the current FE approach used was assessed by comparing the obtained results with the experimental results available in literature in terms of residual velocity,ballistic limit and damage area.The conclusion of this study reveals that FE approach can be successfully adopted in the initial part of study to save time and effort.High velocity response of composite plates was studied by Ref.[101]using FE modelling and the results reveal that FE approach efficiently simulates the minute details pertaining to impact response of the composite which are difficult to obtain through experiment.Shear thickening fluid impregnated with kevlar fabrics was assessed for their high velocity impact behaviour using numerical simulation method with the help of LSDYNA by Ref.[102].At the time of carrying out the simulation,an assumption was made that friction between the yarns and fabric layers was the major energy absorption mechanism.The obtained results were compared with analytical results and proved that they are in agreement with each other.Similarly high velocity impact response of the kevlar-polypropylene composite was assessed by Ref.[82]using ANSYS.The FE model was validated with experimental results.The extension of FE simulations was used to study the influence of projectile mass of high velocity impact response of the composite.Three dimensional FE model was adopted to assess the normal and oblique performance of the ceramic composite armours by Ref.[103].The experimentation is done with the same conditions in which the simulation was carried out and it was found that the deviation in experimental and FE results are within acceptable range.The LVI impact response of the napier grass fibre/polyester composites was studied by Ref.[104]and it was found that there exists a relationship between the amount of fiber used and the impact properties of the composite.The incorporation of napier fiber led to increased damage resistance of the composite compared to pure polyester.The energy absorption of crash box made up of glass fiber reinforced composite was studied by Ref.[105].It was found that the geometry of the crash box played a vital role in deciding the energy absorption of the composite.Energy absorption of polyurethane foam,silicon carbide inserts/plate bonded to glass fiber composite was assessed by Ref.[106].The study revealed that composite with silicon carbide plate absorbs higher energy compared to composite having silicon carbide inserts.LVI impact response of 3D woven composites intended for aerospace applications are carried out by Ref.[107]and results are compared with numerical results.The outcome revealed that the model used gives exact damage mechanism and also it was possible to predict the residual depth accurately.The pandanus composites were assesses by Ref.[108]for their impact response which showed that pandanus fibers offer excellent impact properties.Increase in volume fraction of these fibers resulted in enhanced toughness of composite.Impact response of the pultruded glass fiber reinforced composites was studied by Ref.[109]at different impact energy levels.It was found that as the impact energy increases,the extent of damage increases in pultruded glass fiber reinforced composites.Also,when the composites are subjected to higher impact energy,a multiple shear damage mode was found to occur.The effect of orientation of plies in the carbon-silicon carbide composite subjected to LVI was studied by Ref.[110].It was noted that the damage mechanisms were different on the impacted side and the rear side of the laminate.The laminate with 0/90 orientation is found to experience extreme fiber fracture.whereas the ones with 45/45 and 0/45/90/45 showed less fracture of fiber.The process for analysing the damage of the composite subjected to LVI was proposed by Ref.[135]where C scan methodology was adopted to assess the damage.Based on the damage model.the numerical models were built with cohesive contact method as reference.The LVI response of three different polypropylene based composites with fibers kevlar,basalt and hybrid combination was assessed by Ref.[81].The outcome led to a conclusion that basalt composite has higher resistance to damage and hybrid composite exhibited better energy absorption characteristics.Inspired by the woodpecker head.a sandwich beam was developed and analysed for LVI by Ref.[111].The sandwich consisted of facesheets made of carbon fiber reinforced composite and rubber and aluminium were used as core materials.The results indicate that inclusion of rubber enhances the damage resistance with matrix cracking as the only damage mechanism observed.The sandwich composite proposed for use in trains structures were evaluated by Ref.[136].The results revealed that cavitations in core could lead to critical damage in the sandwich structure.A methodology for predicting the incipient point of impact force and related damage in the composites was proposed by Ref.[137].Initially potential failure modes were assessed.Later,the damage was simulated along with assessing the effect of damage mode on the stiffness of the composite.High compressive stress suppresses the delamination adjacent to the point of impact thereby resulting in negligible reduction of laminate stiffness.Damage investigation of the jute based PMC through ultrasonic method was studied by Ref.[112].Special attention is given to jute fiber due to limited or nor literature available on the impact behaviour of jute based composite.Impact response of innovative sandwich composites made of core having epoxy resin filled with alumina tri hydrate particles was assessed by Ref.[113].The results revealed that this type of sandwich exhibited better performance in terms of damage resistance which is due to the nature of the core used which can undergo good amount of plastic deformation prior to failure.Thus this type of core material is suitable for sandwich composites subjected to impact loading.The fiber metal laminates subjected to LVI was studied by Ref.[138].It was found that in fiber metal laminates.with increase in impact energy.the damage area increases with delamination and matrix cracks as the major source of failures.The study carried out by Ref.[139]concluded that PMC are more susceptible to damage due to impact and this damage is generally more difficult to detect and repair and can be serious threat to the integrity of the component.Glass fiber reinforced composites subjected to dynamic loading was assessed by Ref.[114]under different shaped impactors.It was found that the shape of the impactor also plays a vital role in deciding the impact response of the laminate.Delamination is found to be the most influenced damage condition compared to others.It was also found that when glass fiber reinforced composite is sandwiched in between kevlar fiber reinforced composite.better penetration resistance is obtained.Ballistic impact performance of glass epoxy composite laminate was studied by Ref.[115]in terms of ballistic limit and was concluded that S2-glass/epoxy composite is ranked high in ballistic capability.New type of sandwich structure comprising aluminium foam as the core and fiber metal laminate as skin was studied by Ref.[140].It was found that the energy absorption can be increased by increasing the thickness of the skin.Also it was found that blunter projectile showed greatest decrease in the velocity,followed by the hemispherical.semi-ellipsoid and conical projectiles.A comparative study on energy absorption of glass epoxy and Dyneema composites was carried out by Ref.[116].Dyneema composite emerged as better energy absorbers compared to glass epoxy composites when subjected to high velocity impact.Failure analysis shows that the nature of damage in both the composites are different with Dyneema composite undergoing plastic deformation and fiber stretching due to tension.Whereas.glass epoxy composite undergoes elastic deformation,delamination and brittle failure of fibers.Parametric studies was carried out on single,multi layered steel plates and kevlar composites subjected to high velocity impact loading [117].The results revealed that impact performance of composites are dependent on thickness and layer configuration.High velocity impact studies on carbon kevlar and glass fiber reinforced composites were carried out by Ref.[118]out of which kevlar is found to absorb maximum energy compared to carbon and glass.Ballistic impact performance of composites were studied by Ref.[141]in terms of absorbed energy and damage mechanism.It was found that energy absorbing mechanisms are different with different target thickness.Ballistic response of glass phenolic composites are studied by Ref.[79]with varied thickness and impact velocity.A non linear relationship between thickness and energy absorption was found.Also the projectile deforms more based on the thickness of target compared to impact velocity.The post-mortem study of the laminates shows that failure mechanism is different for different thickness of laminate with more damage area for laminate with larger thickness.Tensile failure and delamination are found to be the predominant mechanism of failure in case of laminate with larger thickness.Whereas,for laminate with smaller thickness.failure is dominated by shear cutting of fibers.The influence of impact angle on the response of the composite laminates was studied by Ref.[142,143].The delamination type of damage occurs due to failure of matrix in tension in case of normal impact and in case of oblique impact.the interlaminar stresses leads to delamination.The fiber at the front portion of the composite laminate breaks due to shearing and no signs of crater were found in case of normal impact.Whereas in case of oblique impact,crater were formed with breakage of fiber was visible.The oblique impact response of composite was studied by Ref.[144]and was found that 15°is the critical angle of impact below which no significant effects are seen in impact response of composite.The oblique impact response of sandwich structure was carried out by Ref.[145].It was found that the extent of damage is highly dependent on the firing axis or in other words on the impact angle.The effect of impactor shape on the ballistic impact response of the single.two layered steel sheet and sandwich composite was studied by Ref.[146].The results revealed that layered plate lost more energy compared to single layered plate when impacted by flat nosed projectile.But no significant changes were observed for hemispherical projectile.A study on normal and oblique impact response of aluminium plates was taken up by Ref.[147].The results revealed that critical oblique angle was less than 60°.As the oblique angle increased beyond 60°.bullets went unpenetrated through the targets.Experimental approach for assessing the high velocity impact response of the composites were carried out by Ref.[148].Below the ballistic limit of the composites.the damage caused due to oblique impact is smaller compared to normal impact.Whereas the behaviour if found to get reversed above ballistic limit.Chopped strand mat reinforced composites were analysed for their impact performance at oblique angles by Ref.[149].It can be concluded that the impact of obliquely inclined plates is less onerous than the impact of flat plates.Dynamic response of delaminated composite shells at oblique impact was studied by Ref.[150].It was found that with increase in angle of impact.resistance to damage is found to reduce.Preliminary studies related to assessing the LVI of fiber metal laminates was assessed by Ref.[151].The impact response is found to be dependent on the size of impactor.Damage resistance and damage tolerance post impact of the hemp fabric reinforced composite subjected to LVI was addressed by Ref.[119].From the results it is evident that compared to standard hemp epoxy composite laminate.hemp and bio based resin resulted in superior performance.Impact test was carried out by Ref.[120]on the glass fiber reinforced nitrile rubber modified epoxy resin.The results showed higher indentation for modified composite with lower delamination extension.It also showed that addition of nitrile rubber resulted in more difficult damage propagation.Impact response of rubber reinforced concrete(RRC)was carried out by Ref.[152].Due to addition of rubber the energy absorption of RRC was found to be higher compared to normal concrete.

The study carried out by Ref.[70]put forth an argument that stiff composites absorb higher energy compared to flexible composites.On the contrary [71].showed that flexible composites are better energy absorbers compared to stiff composites.Supporting the argument made by Ref.[71],the study carried out by Ref.[72],on fabric reinforced PMCs using four different matrices showed that flexible composites have higher energy absorbing capability and undergo larger deformation compared to stiff composites.The matrix used in PMCs governs the extent of deformation and thus affects local strain and impact resistance of PMCs.The damage mechanisms is flexible composites also differ from that of stiff composites as shown in Fig.7.

The flexible composites undergo larger deflection during an impact event and due to the flexural deformation; the damage is introduced at the bottom ply; whereas in case of stiff composites,the limited deformation of the composites leads to localized stress leading to damage at the point of contact [91].Though extensive literature is available for the impact studies of the composites,it is found from the literature that the application of flexible green composites for impact applications are untouched.

The damage mechanism in case of randomly distributed fiber reinforced composites are different from that of layered composites.The random fiber composites when subjected to impact exhibits dent on the impacted surface and cracks on back surface.This leads to reduction in strength and necessity for developing toughening mechanisms [153].Addition of randomly oriented fibers results in randomly oriented heterogeneous layer.This creates potential for a random and disturbed crack path while the interleaving film creates additional two continuous interfacial zones.Also.inclusion of randomly oriented fibers results in induction of semi elastic failure mechanism such as fiber pull out and fiber bridging effect that acts a cause for energy consumption.In case of interleaving laminates,energy absorption is due to the mechanism of plastic deformation of the interlayer.The fracture toughness in case of randomly fibers increases with increase in fiber content.Fibre-matrix interface and interlaminar interfaces in composite laminates constitute the weakest regions for debonding and delamination to initiate.Interface regions being more in randomly oriented fibers lead to more risk of crack propagation.

However,orientation of short fibres is a very important factor in the short fibre reinforcement method.which affects the mechanism of fracture as well as the mechanical properties.Difficulty in achieving random orientation of short fibres poses a major obstacle in this method.Also,in case of randomly oriented fibers,the issues of agglomeration are of major concern due to inadequate uniformity of distribution of fiber in the matrix.This can lead to potential site of crack initiation and stress concentration further leading to catastrophic failure.However.there is limited literature available on damage study of random fiber oriented laminates [154]and needs further investigation.

Fig.7.Damage propagation in (a) flexible composite and (b) stiff composite.

Table 7 provides the details of fibers used for manufacturing PMC for impact applications.It can be seen from Table 9 that natural fibers are used mostly with PP matrix for preparation of composites.The literature reveals that the jute which is the most popular natural fiber reinforced with natural rubber leading to flexible PMC is not studied by any researchers.

5.Multi scale modelling of polymer matrix composites

The behaviour of the PMCs under various loading is complex and it is essential to understand the variation of material properties over large range of temperature and strain rate.Thus it is essential to understand the behaviour of composites,understand the proper constitutive equations.defining fracture mechanisms and structure-property relationship of the materials.The study carried out by Vu-Bac et al.[171]summarized the various physics based models from nano to macro scales.The models are divided into models at nano scale,models at micro scale,models at meso scale,models at macro scale.In case of equivalent continuum model,the discrete of the atomic and the interfacial interactions are simulated naturally.The properties of the effective fibers are obtained and employed in micromechanical models by making use of energy balance between total potential energy and continuum model via intermediate truss model.The total potential energy of the molecular model is given by Eq.(1)

Table 7 Natural fibers in PMC for impact applications.

Where.Paand ρarepresents equilibrium and deformed length of bond“a”.?arepresents equilibrium of bond angle number“a”.Kθa and Kρarefer to the force constants associated with the bond stretching and bond-angle bending number “a” respectively.The total strain energy by truss model is given by Eq.(2).

Where.V represents the volume.

Micro scale models are employed in order to reduce the simulation expenses.Meso scale models are often based on representative volume elements (RVE’s).By using the RVE approach.the heterogeneous material is substituted by an equivalent homogeneous material and the original properties is averaged by different methods.In this way.it is possible to avoid simulating an infinite medium.but still obtain the average properties of the heterogeneous material.At the macro scale.materials are assumed as a continuous homogenized medium.

6.Material selection based on MADM approach

Decision making is one of the critical tasks of a designer.In general.there are eight steps involved in decision making process as shown in Fig.8 [172,173].The most important thing in decision making is selection of an appropriate decision making tools to realise the required goals and objectives [174].which is the first stage of decision making.Further.technical ground or expert judgement should be established to justify the obtained result.

Once the problem is defined.the requirements to realise the result should be established.Further,it is essential to establish the goals positively [175].Once the goal is established the designer/engineer should list out the various alternatives available for selection.Usually.in the initial stage it will be impossible for thedesigner/engineer to decide which alternative exactly fits the goals.Thus.multiple evaluation criteria are selected by the designer which forms the basis for comparison among the alternatives[176].Now the designer should carefully select the appropriate decision making tool and apply it to get the result,which is nothing but the selection of appropriate option among the alternative that meets the goals exactly or nearly exact.

Fig.8.Steps involved in decision making.

Pertaining to material selection.one of the most challenging task for any engineer is to select an appropriate constituent,volume fraction of the selected constituent.stacking sequences of the composite [177]which finally affects the performance of the end product[178-180].It is to be born in mind that there is no hard and fast rule that the engineers are bound to follow as a reference.This makes their task even more complicated.The selection of the various parameters of the composite is totally dependent on the final task the product is intended to perform.There can be various criteria that has to be considered for selection of the material for a particular application and these criteria may be conflicting.Thus,the role of engineers in material selection is very crucial.There are various ranking methods available for material selection as shown in Fig.9.

In order to make the life of engineers easy,a tool called multiple criteria decision making(MCDM)is available for problems such as material selection [181].MCDM can be further sub classified into multiple attribute decision making (MADM) approaches and multiple objective decision making (MODM) [178].Whenever an engineer is facing a task to selecting a material with multiple selection criteria involved.MADM can be efficiently employed to arrive at conclusion[182].There are various approaches that the researchers have adopted for material selection and damage assessment problems namely Elimination Et Choix Traduisant la REalite meaning Elimination and Choice Expressing REality (ELECTRE)[181],technique for order preference by similarity to ideal solution(TOPSIS) [183].weighted product method (WPM) [183].simple additive weight (SAW) method [184].preference selection index(PSI) method [185].analytic hierarchy process (AHP) [178].graph theory and matrix representation approach (GTMA) [184].VIse Kriterijumska Optimizacija Kompromisno Resenje(VIKOR)method[178].TOPSIS-PSI approach [186].combined DEMATEL-VIKOR[187].Fuzzy analytical hierarchical process (FAHP) [188]and so on.Along with minimizing the engineer’s effort in material selection,MADM also provides realistic results which have made it so popular.Out of all the possible approaches.TOPSIS is preferred by most of the engineers in the area of PMC as it considers both positive ideal solution and negative ideal solution while deciding the best material [9,189-191].Along with TOPSIS.VIKOR method also proves to be promising method in material selection when there is a conflict among the criteria considered for material selection.In case of VIKOR,ranking to the materials is given based on the criteria that how close is the alternative to the ideal solution [192].PSI is another simpler and easiest method that aids in material selection that was developed by Ref.[185].It is said to be easiest method since this method does not warrant the assignment of weight to each of the criteria considered.In the field of material selection,many researchers have attempted the use of MCDM methodology for appropriate material selection.The details pertaining to the same are tabulated in Table 8.

From the available literature.it is well established that MADM method can be effectively used for material selection.However,from the review carried out by Mardani et al.[212]it is found that merely 5.34%of literature are available on material selection which indicates the necessity of more exposure for the designers regarding the use of MADM approaches for material selection.

7.Discussion

Research carried out in the field of PMC has resulted in development of PMC with wide range of constituents.Thus developed PMC by researchers till date fulfils the need of various engineering applications as discussed in earlier sections.Following section concentrates on the outcomes of the review carried out on various topics in the earlier sections which can be taken forward for further research.

7.1.Natural fiber

The percentage of various natural fibers used as reinforcements in PMC for impact applications either in hybrid form or non hybrid form are presented in Fig.10 and it is evident from Fig.10 that basalt is the most preferred natural fiber for impact applications.The percentage of natural fibers used as reinforcements in hybrid and non hybrid PMC are presented in Fig.11 and Fig.12 respectively.It can be seen that the natural fibers are used more in non hybrid PMC compared to hybrid PMC.

Fig.9.Classification of ranking methods for material selection.

Table 8 MCDM methods used for material selection and damage assessment.

It can be concluded that though jute is the most popular natural fiber with better mechanical properties[29]that is used in PMC,its application in PMC for impact application is not much exposed regularly.Since 2010,there are only two literatures available which has made use of jute in PMC for impact application.These two literatures concentrate on development of the complete bio composite for impact application.However.all the available literatures concentrate on the development of stiff composites for impact applications and the authors have noted that there exists a gap in development of flexible ‘green’ composite for impact applications.Thus.jute has emerged as the potential reinforcing material for developing completely biodegradable PMC intendedfor impact application.

Fig.10.Percentage of natural fibers used in PMC for impact applications from 2000 to 2020.

7.1.1.Case study: application of multi attribute decision making(MADM) method for selection of fiber

MADM based method called PSI is applied for identifying the potential natural fibers for impact application.The steps involved in PSI method are as follows:

Step 1.Problem definition: In this step.the objectives are determined; attributes and alternatives involved in decision making are identified.

Step 2.Decision matrix formulation: The decision matrix is formulated based on the attributes and alternatives.Each row of the decision matrix represents the attributes of each alternative and each column is dedicated to one attribute.Hence.in an element xijof the decision matrix represents x value of jth attribute in real values which in non normalized for ith alternative.Hence.if there are m alternatives with n attributes for decision making.the matrix will be of the order mxn and represented as in Eq.(4)

Step 3.Normalization: It is essential to make the values of the attributes dimensionless in MADM approaches.This is carried out by converting the attribute values to a value between 0 and 1.For the beneficial kind of attributes.the larger values are desired.Hence,normalization is carried out using Eq.(5)and for nonbeneficial kind of attributes.smaller values are desired and thus normalization is carried out using Eq.(6).

Where,xijis the measure of attribute(i=1,2,3,……m and j=1,2,3,….n).

Step 4.Finding the mean of normalized value: for every attribute,the mean value of the normalized data is calculated using Eq.(7).

Step 5.Finding preference variation value: For every attribute,the preference variation value is calculated using Eq.(5).

Step 6.Finding the deviation in preference value:Here,for each of the attribute.the deviation in the preference value is calculated using Eq.(9).

Step 7.Calculate overall preference value: For each of the attribute.the overall preference value is calculated using Eq.(10).The total value ofΔjshould be equal to 1.

Step 8.Calculate preference selection index: For each of the alternative.preference selection index (PSI) is calculated using Eq.(11).

Step 9.Ranking and selection of suitable alternative: The alternative with the highest PSI will be ranked 1 and so on.

The attributes considered for selection of natural fibers are presented in Table 9.

The decision matrix developed based on the fiber properties are presented in Table 10 and is normalized for the purpose of comparison which is presented in Table 11.The PSI values for each of the alternative is calculated and tabulated in Table 12 based on which the fibers are ranked.

Based on the outcome.it can be said that the flax is the best suited fiber for impact applications followed by pineapple.sisal,jute and hemp.However.considering the availability of these fibers,authors put forth their view that jute can be effectively used as reinforcement in PMC and this case study also shows that the MADM method such as PSI can be effectively used for selection of fibers for PMC subjected to impact application.

7.2.Matrix material

Various matrices are used for reinforcing the natural fibers in order to get the PMC.It can be seen from Fig.13 that epoxy is the most popularly used polymer used in PMC followed by PE and PP where natural fibers are reinforced.Coming to the PMC for impact applications.most of the PMC used for impact application makes use of synthetic fibers and the percentage of polymers used with synthetic fibers for impact application are presented in Fig.14 from which it is evident that epoxy,PE and PP are most commonly used in combination with synthetic fibers for impact applications.Shear thickening fluids and natural rubber which are the potential matrix materials for flexible composites are also used along with synthetic fibers.When it comes to the natural fiber reinforced PMC.Fig.15 shows the commonly used matrix materials with natural fibers and it is observed that very few matrix materials have been used such as epoxy.PE.PP.PU.

Various researchers have conducted the impact behaviour assessment of the PMC making use of wide range of polymer based matrices.However.most of the research is concentrated towardsthe assessment LVI of stiff composites used in structural application.Though many literatures are available regarding the study of ballistic impact behaviour of flexible composites used in body armours,the assessment of flexible ‘green’ composite making use of completely biodegradable material subjected to LVI and possibly used as sacrificial structures such as claddings are not available.Shear thickening fluids are extensively used for development of body armours which are flexible composites.The use of naturally available material such as natural rubber which has the potential to be used in PMC subjected to impact loading [213,214]in underexposed.Also,the PMC in which the reinforcements such as kevlar,Dyneema,carbon which are impregnated in shear thickening fluids are explored by the researchers.The matrix used in PMCs governs the extent of deformation and thus affects local strain and impact resistance of PMCs.Though extensive literature is available for the impact studies of the composites,it is found from the literature that the application of flexible ‘green’ composites making use of naturally available fiber and matrix is not studied till date.In this context.the composite consisting of jute and natural rubber emerges as the interesting combination to be evaluated.The natural rubber being a complaint material can be a potential matrix system for flexible PMC subjected to impact.Also,natural rubber is environmental friendly.low cost and easily available.

Table 9 Attributes considered in fiber selection.

7.2.1.Case study: application of multi attribute decision making(MADM) method for selection of matrix material

PSI method which is applied for selection of fibers is applied for selecting the matrix material for PMC especially flexible composite for impact application.The attributes considered for selection of matrix materials are presented in Table 13.

The methodology adopted for PSI method is same as that used for selection of fibers.The decision matrix as shown in Table 14 is developed based on the properties of the polymers from the literature [61-67,215].Normalization of the values are carried out for the purpose of comparison and shown in Table 15.Finally.the PSI values are calculated and tabulated in Table 16.The polymer with highest PSI value is given rank 1 and lowest PSI value is given rank 12.

Based on the outcome,it can be said that the rubber is the best suited matrix material for flexible composite subjected to impact applications followed by PE.HDPE.PU and PP.Rubber being a naturally available material which is also biodegradable aids in development of flexible ‘green’ composite.This case study also shows that the MADM method such as PSI can be effectively used for selection of matrix materials for PMC subjected to impact application.

7.3.Impact of composites

The impact assessment of the PMC can be carried out in different approaches namely: FE.Analytical and experimentation.Currently researchers are able to analyse the impact response of the composite in the micro fiber matrix model using FE approach.Incorporation of FE approach reduces the time.effort and cost involved in other two approaches.The analytical approach is other means of analysing the impact response of the composites where the energy balance and single degree of freedom models form the basis for such an approach.However.the drawback of the theoretical approach lies in its inability to deal with materials having flaws such as cracks.voids.manufacturing defects and inhomogenity.Finite element approach using commercially available simulation packages has been proved to be better than analytical approach.Experimental approach can provide the real time results considering the various environmental factors.All three approaches have been adopted by the researchers in analysing theimpact response of the composites.

Table 10 Decision matrix.

Table 11 Normalized matrix.

Table 12 PSI values and ranking.

Fig.13.Percentage of polymers used as matrix in combination with natural fibers.

Though synthetic fibers are extensively used as reinforcements in PMC subjected to impact,the recent trend witnesses the usage of naturally available fibers such as Kenaf.flax.hemp.jute.bamboo and so on in PMC for impact applications.It is found from the literature that the plat based fibers are most popular choices for PMC subjected to impact.Though,natural fibers do not provide the mechanical properties as good as synthetic fibers,the hybridization of natural fibers with synthetic fibers proves useful.Also.natural fibers alone are found effective in PMC where extraordinary strength is not a requirement in secondary structures such as claddings which act as a sacrificial structure.Among all the available natural fibers.jute proves to be the most popular and useful fiber.The impact response of the composite also depends on the matrix selected.The impact study carried out by various researchers show that the flexible composites are better energy absorbers compared to conventional stiff composites.Also.the nature of damage in flexible composites differs from that of stiff composites.The major damage mechanism in stiff composites is matrix cracking and delamination.However.these types of damages are eliminated in flexible composites and the damage in flexible composite is dominated by matrix tearing.fiber breakage and pullout.Though extensive literature is available for the impact studies of the composites.it is found from the literature that theapplication of flexible ‘green’composites for impact applications is untouched.

Fig.14.Percentage of polymers used with synthetic fibers for impact application.

Fig.15.Percentage of polymers used with natural fibers for impact application.

Based on the present review carried out.the findings can be consolidated and presented as shown in Fig.16 which clearly indicates that the area of flexible ‘green’ composite for low velocity impact application such as secondary structural applications are the areas untouched till date by researchers and jute along with natural rubber can be the potential constituents for the flexible‘green’ composites.

8.Concluding remarks

In recent times,PMC are gaining popularity and are widely used in various engineering applications as substitutes for conventional metals.alloys and also metal matrix composites.This review concentrates on identifying the potential natural fiber,matrix and core material for preparing a PMC for impact application.Following conclusions are drawn from the present study:

· Natural fibers have been used in fabricating the stiff PMC and less attention is given in using the natural fibers for flexible PMC for impact application.

· It is also found that though jute fiber has excellent mechanical properties,it is not utilized to the fullest extent for impact based applications.Exploring the possible use of jute for impact based applications emerges as an area of research.

Table 13 Attributes considered in fiber selection.

Table 15 Normalized matrix.

Table 16 PSI values and ranking.

Fig.16.Outcome of the present review.

· Concerned to the matrix,it is found that natural rubber which is an excellent compliant material can be an ultimate choice for matrix to prepare a PMC for impact application.

· The history of development of impact analysis has been discussed and various PMC used by various researchers for different regimes of impact assessment is studied.Also,extensive study is carried out on the natural fibers used in PMC for impact applications.From the impact study.it is found that there exists a gap in development of completely biodegradable flexible green composite intended for impact application.

· Though enormous amount of effort has been put by the researchers to assess the impact behaviour of the composites experimentally.it is found that FE simulations have gained much importance during the initial feasible study in order to minimize the time and cost involved in experimentation.Also,FE simulation is found to be effectively used for optimizing the composites.The introduction of simulation tools have resulted in major impact on the theoretical study of the composites.

· Vast experimental research carried out for impact studies reveals that though wide variety of composites are extensively used for impact application.the flexible green composites comprising of all naturally available materials with inherent impact resistant properties are found to be untouched.

· It is found that all the research carried out concentrating the impact behaviour of the composites revolve around the conventional stiff composites and no attention is given to sacrificial structures subjected to impact.Since such sacrificial structures does not need higher strength and stiffness compared to primary structures.the naturally available materials such as jute and rubber emerges as the suitable constituents for PMC intended for sacrificial structural applications.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.