Acoustic emission for monitoring the mechanical behaviour of natural fibre composites: A literature...

Composites: Part A 40 (2009) 1456–1469

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Composites: Part A

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Review

Acoustic emission for monitoring the mechanical behaviourof natural fibre composites: A literature review

Igor M. De Rosa a, Carlo Santulli b,*, Fabrizio Sarasini a

a Department of Chemical Engineering Materials and Environment, Università di Roma ‘‘La Sapienza”, Via Eudossiana 18, 00184 Rome, Italyb Department of Electrical Engineering, Università di Roma ‘‘La Sapienza”, Via Eudossiana 18, 00184 Rome, Italy

a r t i c l e i n f o

Article history:Received 19 November 2008Received in revised form 25 April 2009Accepted 30 April 2009

Keywords:Natural fibresD. Acoustic emissionB. Mechanical propertiesA. Polymer–matrix composites (PMCs)

1359-835X/$ - see front matter � 2009 Elsevier Ltd.doi:10.1016/j.compositesa.2009.04.030

* Corresponding author. Tel.: +39 0644585539.E-mail address: [email protected] (C. San

a b s t r a c t

In recent years, natural fibres are increasingly used as reinforcements for the production of low-cost andlightweight polymer composites: other advantages include non-abrasive nature, high specific properties,and biodegradability. However, their limitations, including moisture absorption, poor wettability andlarge scattering in mechanical properties, and the not sufficient understanding of mechanisms controllingtheir mechanical behaviour and failure modes, still confine the use of natural fibre reinforced compositesin non-structural applications. Acoustic emission (AE) proved useful for its capability of real-time mon-itoring over the whole material volume and high sensitivity to any process generating stress waves.

This paper presents a literature review of AE applications in studies on natural fibre composites. Thefollowing fields of application are covered: (1) interface studies in single fibre composite (SFC) tests,(2) damage evolution and failure mechanisms detection and (3) crack propagation, including also currentlimitations of existing literature and future work.

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1. Introduction

The production of polymer composites reinforced with naturalfibres has substantially increased in the last few years, especiallyin sectors such as the automotive, leisure and furniture industry,where their reduced cost and higher bio-degradability may repre-sent important incentives to use [1]. However, their applicationsare mainly as cosmetic i.e., non-load bearing, materials. A signifi-cant difficulty is offered by the structure of reinforcement itself.In particular, plant fibres are not homogeneous, being cellularstructures assembled in nature through a hierarchical procedure,and presenting a hollow, or lumen, of variable dimensions [2]. Inaddition, their introduction in a polymer matrix may generatecompatibility issues, whose consequence may be a large scatteringof properties in the final laminate, only partially addressed throughfibre surface treatment [3]. This is the cause of persistent uncer-tainty on the mechanical properties of the composite obtained,resulting in the difficulty of setting reliable stress limits for mate-rials design, and assessing the effect of damage on the structure[4,5].

Acoustic emission real-time monitoring has been frequentlyused during mechanical tests on traditional polymer compositesi.e., reinforced with carbon [6], glass [7–9] or aramid [10] fibres.These studies involved different and progressively sounder levelsof analysis, from the classical NDT (non-destructive testing) appli-

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tulli).

cation aimed at accept/reject the material under stress to the mea-surement and localization of accumulated damage. In some casesAE signal features were also correlated with specific damage phe-nomena occurring in composites, such as fibre failure, delamina-tion, etc. The possibility of damage characterization usingacoustic emission depends on the complexity of fibre architecture:the presence of multiple damaged zones can lead to difficult inter-pretation of AE data. In general, however, most acoustic emissioncan be ascribed to friction or rubbing between damaged parts inthe composite [11]. The first application of loading usually gener-ates in traditional composites a large number of AE events, whichis increasing with increasing stress. These events are usually dis-tributed, rather than concentrated, throughout the laminate, ifthere is no significant pre-existing damage. This distributed dam-age which is generated during the first stressing of a compositeis known as ‘‘characteristic damage state”. If the material is un-loaded and then reloaded, AE activity is normally reduced becausethe ‘‘characteristic damage state” has already formed and AE fromthose sources is to a great degree over [12]. In particular, AE activ-ity during reloading resumes at a fraction of the previously appliedload: this fraction is defined as ‘‘Felicity ratio” (FR): FR has beenfound to be higher for composites with higher residual strengthi.e., less damaged by first loading [13].

However, catastrophic damage in composites occurs usually ina concentrated area: here, for the application of a stress usuallyconsiderably lower than ultimate stress, local degradation of mate-rials properties occurs, leading to some stress redistribution in thelaminate [14]. As a consequence, when exceeding this stress level,

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increased AE activity is detected in a number of locations in thematerial, although AE at location where eventual failure will takeplace tends to be prevalent above a given load level. This leads tothe possibility of determining a stress limit for composites use dur-ing mechanical loading, at the stress level for which a marked in-crease in acoustic emission activity is observed [15]. Moreover,once the composite is damaged, generation of AE can occur alsowhilst the composite is at constant load [16]. It is also worthy tonote that the accumulation of AE from multiple local regionsmay lead to false interpretation of the AE data: for this reason, datafrom the different regions may need to be segregated, for examplein clusters, and monitored separately, such as proposed in [17]. Inprinciple, these objectives can be retained also for natural fibrecomposites: however, further difficulties can be envisaged, in par-ticular connected with the scattering in properties and the variablediameter of natural technical fibres. This results also in the diffi-culty of obtaining accurate fibre orientation in the laminate andhas limited so far the use of woven fibre structures other than plainweave with natural fibres. In addition, for the lower stressesinvolved and the nature of reinforcement, it is possible that acous-tic emission activity is less pronounced than in traditionalcomposites, which may lead to alternative ways of analyzing datato retain the possibility of use AE as a predicting tool, to obtaininformation e.g., on failure localization and maximum applicableservice stress.

In spite of all the difficulties mentioned above, a number of is-sues arising when using natural fibre composites have been inves-tigated using acoustic emission. Studies so far focused on a fewmain issues, which are particularly relevant for a possible use ofthese materials in large volume applications. These are in particu-lar the strength of the interface between natural fibres (hydro-philic) and polymer matrices (mostly hydrophobic);characterization of damage, whose patterns may differ from thatof traditional composites, because of variable natural fibre geome-try, as discussed above; and resistance of natural fibre compositesto defects propagation.

2. Application of acoustic emission to interface studies in singlefibre composite (SFC) tests

The mechanical properties of fibre reinforced polymeric com-posites strongly influence their final industrial application.Mechanical properties of composites depend on several factors,such as the properties of constituent reinforcement and matrix,their relative volume fraction, the shape, size and architecture ofreinforcement phase, but to a great extent on the reinforcement/matrix interfacial shear strength (IFSS). A relatively strong interfa-cial bond is needed for an effective transfer of the applied load,since a weak interface will probably lead to a premature failureof the composite. The IFSS is a critical factor affecting the tough-ness, transverse mechanical properties and interlaminar shearstrength of composites, hence a detailed knowledge of the charac-teristics of fibre/matrix interface is necessary when tailoring per-formance to applications [18]. In fact, improving IFSS results inincreasing the tensile and flexural strength of the composite whilstlowering the impact strength and toughness [18]. The quality ofthe interface region represents a remarkable concern for tradi-tional man-made fibre reinforced composites and an even moreworrying aspect for natural fibre reinforced composites. In fact,what has prevented a more widespread use of natural fibres isthe lack of good adhesion to most polymeric matrices. The hydro-philic nature of natural fibres adversely influences adhesion to thehydrophobic matrix, resulting in low compatibility and strength[19]. Furthermore, a strong interfacial bond represents a key aspectfor the durability of composites.

In order to optimize the interface for superior mechanical per-formance, the bond’s strength and the microfailure modes shouldbe accurately investigated and evaluated. In this regard, measuringthe IFSS requires special micromechanical techniques such asmicrobond test [20], single fibre composite test (also known asfragmentation test) [21,22], the pull-out test [23,24], the push-out test [25,26] and the microindentation test [27,28]. To investi-gate the interfacial behaviour, a SFC test is often used due to thesimplicity of the test procedure. When an external stress is appliedto a single fibre embedded in a matrix, the tensile stress is trans-ferred to the fibre by means of interfacial shear stress. As the ten-sile load increases, the tensile strain in the fibre will eventuallyexceed the failure strain of the fibre, and the fibre will fracture.The fibre continues to fracture into shorter lengths as the load in-creases, until the fragment length reaches a critical value. This sit-uation is defined as the saturation in the fibre fragmentationprocess. The shortest fragment length is defined as the critical fibrelength. It is therefore essential to know the exact number of frag-ments in order to estimate the average critical length, which inturn is needed to measure the IFSS. In this regard, in the literaturethere are many papers dealing with the use of AE to monitor theSFC test [29–34]. In these papers, AE has been extensively usedfor the detection and localization of fibre breakage during SFCtests. It has been demonstrated that almost all fibre breakageswere detected and associated with a single acoustic emission eventaccording to a one-to-one correspondence. Furthermore, the corre-spondence between the AE events detected and the observed fibrebreakages (by, for instance, optical microscopy [33]) was generallywell established. This allowed to calculate the IFSS by the Kelly–Tyson model [35] using for the average critical length the totalnumber of fibre fracture signals divided by the distance betweentwo sensors [31,34]. In addition, AE is very sensitive to the natureof the interfacial bonding which affects unambiguously the acous-tic emission response thus giving information about the character-istics of the interface, the level of adhesion and the interfacialfailure modes [36–38].

These techniques have been also successfully applied to interfa-cial studies in natural fibre reinforced composites, thus allowing asounder knowledge of failure modes and of the level of adhesionbetween reinforcement and matrix. Park et al. [39] used dog-boneshaped epoxy composites reinforced with dual basalt (two basaltfibres of different diameter embedded at half-depth in epoxy resin)and SiC fibres. These specimens were tested in tension and wereonline monitored by AE in order to characterize the failure modes.The authors monitored the AE event amplitude, energy and dura-tion using cross plots to highlight the differences during the load-ing of dual fibre composites. They noted that the signals were wellseparated in three distinct ranges, two ranges for breakages of eachbasalt fibre of different diameter (15 and 97 lm) and one for ma-trix cracking, respectively. This was true for both AE amplitudeand energy. The analysis allowed also noting that the thicker fibrebroke in advance with respect to the thinner one. It was reportedthat characteristic frequency peaks detected using fast Fouriertransform (FFT) analysis could provide information for discriminat-ing failure sources. Also for mineral fibres the authors found a one-to-one correspondence between AE events and fibre breakages(Fig. 1). Therefore, AE could be used to analyze interfacial modesfor semi or non-transparent composites which cannot be investi-gated by optical microscopy. A similar analysis was carried outby Park et al. [40] on SFC test using two bast fibres embedded inepoxy matrix, ramie and kenaf, respectively. The AE analysis wassupported by optical microscopy. This enabled to better interpretthe results of the other technique. The AE parameters monitoredwere amplitude and energy along with the FFT analysis. The lowerlevel of amplitude and energy signals occurred during the initialloading period and was ascribed to the axial splitting along the

Fig. 1. One-to-one correspondence between the number of various typed-fibresbreakage and the number of AE event (after Park et al. [39]).

Fig. 2. Microfailure modes of untreated natural fibres/PP systems under tension for(a) jute fibre; and (b) hemp fibre (after Park et al. [41]).

1458 I.M. De Rosa et al. / Composites: Part A 40 (2009) 1456–1469

boundaries of elementary fibres and fibrils. The breakages of theelementary fibrils and fibres occurred during the last loading per-iod close to the failure of fibre bundles. These differences were re-flected in the waveforms of the signals, in particular breakageswere characterized by high frequency peaks, whereas axial split-ting was characterized by low frequency peaks. For single fibreepoxy composites, the following conclusions were drawn: for fibrebreakage, AE energy and frequency were higher than those offibrillation and matrix cracks. In particular, for kenaf fibres a great-er number of wave signals with higher AE energy and amplitudeoccurred than those of ramie fibres. In addition, kenaf fibres wereidentified as more subject to fibrillation than ramie ones.

Not only natural fibre reinforced thermoset matrices but alsothermoplastic ones were investigated, e.g., in [41], where the cor-relation of the interfacial and mechanical properties of jute andhemp single fibre reinforced PP–MAPP (polypropylene–maleicanhydride polypropylene) composites was investigated by micro-droplet test and acoustic emission. The AE and the FFT analysis re-vealed that jute fibres were characterized by events of higherenergy and amplitude than hemp ones, due to microfailure pro-cesses which occur through the crystalline material area (for jute)instead of through the amorphous one (for hemp). This is consis-tent with what observed: jute fibres showed a marked tendencyto fibril splitting (inside the crystal areas), unlike hemp fibres, ascan be clearly seen in Fig. 2.

AE provided useful information also in durability studies [42].SFC tests were carried out and monitored by AE. The jute fibreswere used both as untreated and treated using alkali or silane.The durability test was performed by soaking the specimens in100 �C boiling water for 1 h, in order to reduce the testing time.The treatments and the boiling water test influenced the acousticemission response thus improving the knowledge of the microfai-lure modes. It was reported that the AE events occurred in a greatnumber after the boiling water test and the AE energy became low-er. This was ascribed to the growing importance of the swelled fi-brils in controlling the microfailure processes, as can be seen inFig. 3. AE energy, amplitude and events were increased by thechemical treatments. After the durability test, the number of AEevents did not increase for the composite with silane-treated fibrescase: however, an increase in the average amplitude and energy ofAE events was recorded. This different behaviour was likely due tothe IFSS increase and improved cohesion between fibril and fibril

caused by silane treatment. The results of this study are interestingsince show the promising capabilities of AE to analyze the effectsof surface treatments on composite performance.

In the previous papers, only AE descriptors such as amplitudeand energy have been used to identify the microfailure mecha-nisms. These studies were performed in the time domain togetherwith the FFT analysis of the acquired waveforms, based on the con-sideration that each failure mechanism is characterized by a differ-ent peak frequency. In particular, three different microfailuremechanisms were investigated and tracked down: fibre breakages,fibrillation and matrix cracking. As a general conclusion, events ofhigher energy and frequency were ascribed to fibre breakages,whilst signals of lower energy and frequency were ascribed tofibrillation and matrix cracking (a clear distinction between thesetwo mechanisms was not always achieved). It is also worth noting,that differences in chemical composition between natural fibresare supposed to be responsible for the extent of fibrillation whichin turn affects the AE response, which is characterized by a largernumber of AE signals coming from fibril splitting.

An examination of the literature dealing with the application ofAE to interface studies in SFC tests shows promising results even ifa number of issues can be raised. First, in most papers the IFSS val-ues were obtained from microdroplet test. This is motivated by thefact that the clarification of microfailure mechanism is a veryimportant step towards the understanding of the fracture behav-iour of composite materials. The application of fragmentation testto estimate the IFSS is not pursued. This can be due to the complex-ity of failure mechanisms in natural fibre composites in compari-son with glass or carbon fibre composites. It is not an easy task,

Fig. 3. AE amplitude and AE energy of jute fibres/PP composites for: (a) the untreated; (b) alkaline treated; and (c) silane treated (after Park et al. [42]).


for instance, to identify the difference between fibril splitting(which has been widely observed and investigated in natural fi-bres) and matrix cracking, since their AE features are often overlap-ping ones. As previously demonstrated, an analysis of AE can beuseful for identifying the modes involved and their relative impor-tance, even though not in a quantitative way. This is particularly

true and effective, if the measured AE features are coupled withsupplementary information such as applied stress field or micro-structural observations of the test specimen, as reported in theexamined papers [39–42]. Second, adhesion studies based on sin-gle fibre model methods can be subject to errors due to the simpli-fied representation of the stress state existing at the fibre/matrix


interface. Moreover, measuring the interface properties using realcomposites is further complicated by the interferences resultingfrom several mechanisms during composite testing. A solutioncould be the use of simple multi-fibre composites in order to limitthe failure mechanisms to those which are known to be associatedwith interface failure, an approach used with traditional compos-ites [36]. In this way, a sufficiently accurate evaluation of adhesionstrength for several composite systems made of different naturalfibres (treated and untreated) and matrices could be obtained. Thisrepresents an important issue in natural fibre composites, consid-ering the variety of physical and chemical treatments developed toenhance the fibre/matrix adhesion. Furthermore, since both timeand frequency domains contain valuable information, a techniquebased on a joint time-frequency analysis could be useful with theaim of failure modes identification; one of the best candidate couldbe the wavelet analysis of AE signals. As a conclusion, these prom-ising results should be also supported by further investigations ona larger number of natural fibres.

3. Acoustic emission in damage evolution and failuremechanisms detection

Several test methods and techniques have been developed toevaluate and monitor the damage mechanisms and failure modes.As a result of these techniques, AE has gained fundamental atten-tion in recent years. Several studies were carried out in the use ofAE method analyzing the damage evolution and failure mechanismdetection in traditional polymer composites reinforced with glass[43,44], carbon [45–47] or aramid fibres [48–50]. However, therehave only been few papers dealing with acoustic emission associ-ated with damage mechanisms in composites reinforced with nat-ural fibres e.g., flax, jute, hemp and kenaf [51–68]. It is worthy tonote that some of these studies have successfully demonstratedthe possibility of determining the failure modes of these green-composites using the acoustic emission technique.

Dogossy and Czigány [53] used AE monitoring to characterizethe failure modes mechanism as a function of the amplitude vs.time in maize hull filled (at different contents) polyethylene com-posites during a tensile test. The examination showed that it waspossible to reveal and distinguish the presence of three main fail-ure modes for these composites: matrix deformation (below25 dB), maize hull pull-out (26–40 dB), and maize hull breakage(over 41 dB). Anuar et al. [54] proved that AE monitoring is a usefultool to identify the different failure modes, and to evaluate the evo-lution of the damage zone in thermoplastic natural rubber rein-forced with untreated and treated kenaf fibres (in the form offlakes). DENT (double edge notched tensile) specimens at differentligament length (4, 6, 8, 10 and 12 mm) were tested. From AE anal-ysis it was noted that higher amplitude signals (55 dB) were emit-ted in the composites reinforced with untreated kenaf fibrescompared with composites reinforced with treated ones (50 dB).Furthermore, they found a higher average number of AE eventcounts for the composites with untreated fibres and for the sam-ples having a higher ligament length. It could be related to astick-slip mechanism caused by the rough surface of the kenaf fi-bre, which opposes the pull-out mechanism. As a consequence,the pull-out showed higher amplitude signals. On the contrary,the treatment improving fibre–matrix adhesion resulted in lesserpull-out. The examination of AE signals amplitude during force-elongation curves showed that it was possible to associate AE sig-nals to three main failure modes. In particular, during the tensiletest they observed only few and low amplitude signals related tomatrix cracking, at the onset of the load. It has been detected thatby increasing the load, the intensification of acoustic activity canbe associated to fibre debonding and pull-out. During the load, in

the proximity of the ultimate fracture, higher amplitude signalsand acoustic activity can be related to fibre breakage. These results,in accordance with the previous study by Dogossy and Czigány[53], indicate that AE monitoring can be a useful tool to followand characterize damage progression during tensile test in kenaf fi-bre reinforced composites.

Szabó et al. [55] used AE monitoring of SENT (single-edgenotched tensile) samples (notch length ca. 10 mm) in polypropyl-ene/polyamide (PP/PA) blends (with different concentration ofPA: 0, 10, 20, 30, 40 and 50 wt%) reinforced by short basalt fibre(BF) at various contents (0, 10 and 20 wt%). It was showed thatthe number of events increases when the reinforcement contentincreases. Furthermore, they found that the most events were de-tected for composites with pure PP (0% PA) and with 10% and 20%basalt content, whilst the least number of events were detected forcomposites with 30% PA (70% PP) at both basalt content. They con-cluded on the basis of AE data that the PP/PA interfacial debonding,involving a huge plastic deformation, did not produce AE signalsunlike the large number of AE events induced by the friction of fi-bre/matrix failure. Furthermore they associated the low amplitudesignals to the matrix deformation and tearing, whilst the highersignals to the debonding and fibre pull-out.

Czigány et al. [56] correlated the force-displacement curves (di-vided into four sections in regard to the maximum load) to the AEamplitude during tensile test of different composites. SENT speci-mens of treated–untreated basalt fibre mat reinforced vinylester/epoxy (at different concentration) composites were tested. Theyshowed that the AE activity is not related to the type of resin orof the basalt fibre surface treatment. By observing the number ofAE events, it was possible to detect the crack propagation and todistinguish the ductile and brittle failure behaviour of the differentspecimens.

Acha et al. [57] used the AE technique to study the fracture andfailure behaviour of biodegradable jute fabric reinforced thermo-plastic polyester composites. During the tensile test, they observedthat AE amplitude increased in accordance to the following rankingfrom lowest to highest: debonding, fibre pull-out, fibre fracture.Czigány [58] demonstrated that it is possible to correlate the AEparameters, such as number of events, amplitude and energy tothe mechanical properties and to other structural factors (fibre ori-entation and fibre content) during different applied loading. Basalt,hemp, glass, carbon and various hybrids were tested. Specimenswere cut out from the composite plates in the direction of cardingand perpendicular to that. He used AE to characterize damage evo-lution during the different types of loading: matrix deformation atthe beginning (21–35 dB), then delamination (35–45 dB) and fibrepull-out (45–60 dB), and ultimately fibre breakage and the failureof the composites (over 60 dB). In particular, four characteristicranges on the basis of the maximum force value of the force-displacement curves were considered; (0–0.5 Fmax), (0.5–0.8 Fmax),(0.8-Fmax) and (Fmax-0). The first range is characterized by matrixdeformation and small extent fibre/matrix delamination; fibrepull-out starts in the second range whilst fibre breakage occursat the end of the third one; the forth range is characterized bythe presence of all the failure modes. Furthermore, no significantdifferences, concerning the characteristic failure mode and AEamplitude, among different composites were found.

Szabó and Czigány [59] used AE monitoring to study the behav-iour during loading of basalt and ceramic short fibres reinforcedcomposites. Three loading ranges were considered: I (0–0.4 Fmax),II (0.4–0.8 Fmax) and III (0.8–1 Fmax). The AE data showed that thenumber of acoustic events mainly depended on the fibre contentand on the direction of these fibres since different types of damageoccur. In particular, the number of events in specimens having thedirection of fibres perpendicular to the direction of loading, butparallel to the notch (L) was higher than in ones with the fibres


parallel to the direction of the loading, but perpendicular to thenotch (T), as shown in Table 1.

As regards direction L the typical form of damage besides thematrix deformation was dominated by debonding and by tearingof the matrix, whilst in direction T the form of damage was domi-nated by fibre pull-out and debonding, as can be seen from theanalysis of the AE amplitude of Table 1 (higher values are referredto pullout; lower ones are referred to debonding). The analysis ofthe AE distributions showed: in the range I, values typically below50 dB in the direction T (it was ascribed to matrix deformation andsmall extent delamination); in the range II, pull-out occurredreaching AE amplitudes of 70 dB; in the range III, besides the fail-ure modes above mentioned, a certain amount of fibre breakagewas highlighted (below 70 dB in the direction L; 70–80 dB in thedirection T). The presence of a certain amount of fibre breakagewas due to high number of weak points caused by the large quan-tity of fibre-ends.

Romhány et al. [60] used AE monitoring to track the failuremodes in technical flax fibre during a single fibre fracture test.They showed that an AE range of amplitudes can be assigned toidentify three failure mechanisms: longitudinal splitting of thepectin boundary layer among the elementary fibres (AE amplitudeless than 35 dB); transverse cracking of the elementary fibre (35–60 dB); fracture of elementary fibres and their microfibrils (over60 dB), as shown in Figs. 4 and 5. In contrast, no correlation hasbeen found between AE signals characteristics and individual fail-ure events. Romhány et al. [61] also showed that AE monitoringcan be used to identify the failure mode sequence of flax fibre rein-forced composites during a tensile test. Furthermore they foundthat it was possible to identify the AE amplitudes released by theflax fibres during the failure of the composite even if, obviously,the damping effects of the matrix lowered the previous values[60] of about 5 dB. The flax fibres were arranged both unidirection-ally (UD) and crossed-ply (CP) at different content (20%, 40% and

Table 1Relative distribution of amplitude of basalt fibres reinforced composites at different conten0.4 Fmax; II: 0.4–0.8 Fmax and III: 0.8–1 Fmax) (after Szabo and Czigany [59]).

Relative distribution of amplitude (%) Amplitude interval (dB)

11–20 21–30 31–40

Panel a0 I 53.8 38.5 7.7

II 53.3 13.3 26.7III 50.8 28.6 14.3

5 I 73.2 21.6 5.2II 65.0 17.5 13.3III 58.0 25.4 12.2

15 I 67.3 21.8 9.1II 63.0 18.3 11.6III 55.6 22.9 15.4

25 I 68.2 15.9 13.3II 62.9 19.5 11.5III 50.4 23.3 16.5

Panel b0 I 78.3 21.7

II 42.7 10.3 23.5III 30.5 15.0 20.1

5 I 82.9 8.6 2.9II 64.0 18.4 10.8III 56.0 22.6 14.4

15 I 73.4 15.2 9.6II 59.8 23.1 11.3III 46.4 23.4 18.5

25 I 79.6 13.0 7.4II 64.2 20.0 12.4III 61.7 21.8 12.2

60%). On the basis of correlations between AE amplitude/cumula-tive number of AE events/duration vs. elongation and relative AEamplitude distribution, it was possible to characterize the failuremodes. At low load levels (in range I), fibre/matrix debondingand axial splitting of the elementary fibres occurred (20–35 dB).When increasing the load (in range II), also fibre pull-out andtransverse microcracks within the elementary flax fibres becomesignificant (35–55 dB). When reaching the maximum load (inrange III), the previous failure mechanisms were accompanied bymultiple fibre breakage (above 55 dB). In Fig. 6 it was clearlyshown the possibility of classifying the AE amplitudes in range IIIat 40 wt% flax content into three groups (where all the failuremodes are present): A (20–35 dB), B (35–55 dB) and C (above 55dB). Furthermore it can be noted that in composite reinforced withCP oriented fibres the breakage of the technical flax (AE amplitudeabove 55 dB) does not occur. This is due to the fibres laid perpen-dicular to the loading direction which do not contribute to thereinforcement since they split easily along their longitudinal axis(fibre/matrix debonding and axial splitting at fibre/fibre interface).

Shin et al. [62] used the AE cumulative counts under the tensileand flexural loading test to predict and evaluate the onset of frac-ture and material failure of unidirectional bamboo fibres epoxycomposites. They showed that the onset of fracturing can be pre-dicted with the occurrence of small AE events, whilst the onsetof critical damage with the occurrence of higher AE activity (explo-sive AE), thus defining the maximum load value safety region foreffective service of these composites. Dányádi et al. [51] demon-strated that a correlation of AE amplitude vs. elongation and cumu-lative hits vs. elongation on wood flour filled polypropylenecomposites can be obtained, resulting in three main failure mech-anisms: debonding at matrix/filler interface, fracture of large woodparticles and filler pull-out. The decreased acoustic activity forcomposites reinforced with 70 wt% was correlated with the de-creased strength due to considerable aggregation of the particles.

t (0, 5, 15 and 25 wt%) in: (a) transverse direction and (b) longitudinal direction (I: 0–

Number of events

41–50 51–60 61–70 71–80 81–90

576.76.3

4313.3 0.84.4

1.8 12145.8 1.0 0.34.1 1.1 0.7 0.2

2.6 71864.6 1.3 0.27.2 1.8 0.5 0.3

6823.518.8

5.7 74465.1 1.0 0.75.4 1.4 0.2

1.1 0.6 118125.0 0.5 0.38.8 1.9 0.5

195243.0 0.3 0.23.4 0.6 0.2

Fig. 4. Failure sequence in a technical flax fibre: (a) axial debonding and fibrillation along elementary fibres and (b) radial cracking in the elementary fibres (after Romhanyet al. [60]).

Fig. 5. Schematic lay-up of the technical flax fibre (a) and its failure sequence along with the related AE amplitude ranges; (b) axial splitting of elementary flax fibre (AEamplitude <35 dB); (c) transverse microcracking of elementary fibres (AE amplitude 35–60 dB) and (d) multiple fracture of technical flax fibre (AE amplitude >60 dB) (afterRomhany et al. [61]).


Fig. 6. Relative amplitude distribution of AE events in the loading ranges (I: 0–0.9 Fmax; II: 0.9-Fmax and III: Fmax – maximum slope of the cumulated events vs. time) for thecomposites with: (a) UD reinforcements and, (b) CP reinforcements as a function of flax content. (the horizontal axis of the diagram is amplitude (dB), the vertical one isrelative distribution (%) (after Romhany et al. [61]).


Kocsis and Czigany [52] used AE to detect debonding betweenbeech wood short fibres and the matrix material. They tested awood/PP system in tensile mode and different weight contentswere analyzed. They reported that the number of AE events de-creased with increasing wood content because of the formationof aggregates. They noted also a decreasing tendency of the ampli-tudes with increasing wood content. In addition, at smaller tensilestrength values the number of AE counts tended to decrease due topoor adhesion between matrix and wood fibres. The AE count dis-tribution showed two local maxima for higher wood content whichare likely to be due to slipping of wood fibre aggregates and fibresdebonding, whilst at lower wood content the sole debonding wasresponsible for the large number of AE counts.

Santulli [63] used AE during three-point bend test on impactedspecimens of jute fabric polyester laminates to investigate theresidual properties, and to predict the occurrence of failure. Thecorrelation between AE data and mechanical results suggested thatimpact damage and residual properties of the laminates are de-fects-driven characteristics. Therefore these results confirmed thatthere is no real dependence of the damage produced on the energyapplied. Santulli [64] also proved elsewhere the AE capacity ofmeasuring the level of damage on natural fibre reinforced compos-ites. In particular, AE has been applied during tensile, flexural andstaircase and continuous indentation test, on jute fabric/polyesterlaminate. He showed a correspondence between the stress–straincurve and the onset of AE activity. Therefore, investigating the var-iation of slope of the AE curve against the applied stress, ‘‘AE knee”,he evaluated the elastic limit of the laminate. The author alsoshowed that it was possible to measure the AE limit only for lowimpact energies since at high impact energies the AE activity wascontinuous during loading. It was also shown during three-pointbend test that it is possible to clearly evaluate the damage develop-ment and the mode of failure of the composite.

De Rosa et al. [65] used AE monitoring to evaluate the effect ofdamage dissipation offered by the jute fibre core in glass/polyesterlaminates. Correlating the AE activity to the applied cyclic bendstress on impacted specimens at various energies, the level of im-pact loading and the residual strength of the laminates was evalu-ated. Furthermore, it was clarified that the failure in the hybridlaminates started on the compressive side close to the interface be-tween the skins and the jute core.

Finkenstadt et al. [66] correlated AE hit rate to the time andcumulative AE hits to the stress–strain curve in poly(lactic acid)

(PLA) oil seed composites, discerning between three differentstages of deformation. At first, debonding of the filler from PLA ma-trix occurs; during the second stage the yielding of matrix occurs;in the last stages the ductile fracture of the matrix occurs. They alsonoted that as the oil seed content increased the ductile fracture didnot occur; it indicated that the defects in material integrity pre-vailed over any advantageous interaction between oil seed andmatrix.

Whilst investigating on the correlation between cumulative AEevent counts and stress–strain curves in unidirectional flax rein-forced composites, Hughes et al. [67] found evidence of yieldingat low values of stress and strain. This evidence indicated that mi-cro-structural damage occurred in the proximity of yield point.

Sreekala et al. [68] used AE monitoring to investigate the failuremodes of oil palm and pineapple reinforced phenol formaldehydecomposites. They highlighted that a correlation between thecumulative AE events and average amplitude vs. elapsed time ex-ists during the application of loading. At the beginning of the load-ing, the distribution of events showed small amplitude signals dueto matrix deformation and debonding. In contrast, in proximity ofthe maximum load value, AE events distribution was characterizedby the presence of higher amplitude signals due to pull-out and fi-bre fracture.

All the above results emphasize that AE monitoring can be asuitable tool for detecting, evaluating and better understandingthe damage mechanisms evolution and the failure behaviour innatural fibres reinforced composite during mechanical loading. Inaddition, these studies provide remarkable and unique experimen-tal data for development and correlation with theoretical work,even if a number of issues can arise. For instance, it is very difficultto identify damage mechanisms evolution and the failure modes incomposite material reinforced with natural fibres in a quantitativeway. This is easily possible in the case of the single fibre fragmen-tation test, whilst in contrast difficulties arise in the case of tests oncomposite laminates. Here, a number of different damage mecha-nisms take place and the corresponding AE signals overlap andare attenuated in an unpredictable way from the matrix. As a con-sequence, a single damage mechanism such as matrix deformationor fibre pull-out can produce a wide range of AE signal features.

In the previously mentioned works, well defined amplituderanges of AE events were assigned to failure modes such as matrixdeformation, fibre pull-out, fibre breakage. Conflicting conclusionsregarding the AE parameters attributed to each failure mechanism

Fig. 7. AE sensors position on a SENT specimen and likely crack growth location(after Acha et al. [57]).


could be found. For instance, various amplitude range of AE eventswere assigned to polypropylene matrix deformation and cracking:11–40 dB [58,59] and 40–55 dB [7]. This disagreement is not due toany mistakes in the interpretation of AE data but the authorsdeveloped their conclusions on AE data which were generated ina specific test without quantifying the effect played by attenuation.Therefore, all the conclusions were accurate within the context ofthe given specimen geometry and experimental test and couldnot be necessarily absolute or representative of a specific failuremechanism for a different experimental set up. Many factors, suchas equipment setting, material damping (depending on fibre andmatrix materials, fibre content and orientation), specimen geome-try and propagation distance should be accounted for includingphenomena of attenuation and dispersion, edge reflection andpropagation of different wave modes. Furthermore a particularattention should be paid to how all the following effects might af-fect the results. This is particularly important, when trying to applythese results to more complex structures.

An alternative approach should be developed to improve theidentification of damage modes of natural fibre reinforced compos-ites not only based on traditional parameters such as amplitude,duration and event counts. Concerning this, a suitable methodol-ogy could be an AE waveform-based analysis associated to ad-vanced pattern recognition techniques [69].

In conclusion, the above-mentioned studies clearly demon-strate the fundamental role that AE monitoring plays as a tool formaterials scientists working with composite materials reinforcedwith natural fibres.

4. Acoustic emission for crack propagation studies

Acoustic emission can assist crack propagation studies in a two-fold sense, either enabling crack tip localization or allowing thecorrelation of AE data with toughness measurements.

As regards the former aspect, localising the crack tip encountersthe typical difficulties of damage localization by acoustic emissionin composite samples. These are especially due to the short dis-tances involved, which require a high accuracy of stress wavevelocity measurements, not always easy to obtain on non-uniformand anisotropic materials. Specific algorithms for this purpose havebeen developed in a number of studies, including [70,71].

In experimental practice, it is also significant noting that soundvelocity in a composite laminate can be modified also by polymermatrix modification e.g., hybridization, as suggested in [56], wherea decrease of over 20% in sound velocity is measured passing fromvinylester to vinylester/epoxy resin. In addition, the localizationaccuracy of acoustic emission systems is in the order of one milli-meter, and problems do exist for optimal data representation. Ontraditional composites, damage localization can be obtained usingacoustic emission with accuracy in the order of the centimeter orlower. In particular, during monotonic tests it is possible to predictthe area of the laminate where failure will finally occur [72]. In thisregard, three-dimensional plots have sometimes been used, whichallowed including another AE parameter other than events loca-tion, e.g., amplitude, duration or energy [73]. Another more empir-ical possibility exists, which is to divide the laminate into regionsand quantify the percent distribution of AE events in the differentregions: this method was for example used by Aicher et al., in astudy of transverse damage progression in wood [74]. A thirdand more mathematically rigorous possibility is the use of weight-ing algorithms based on a bell-shaped function, which account forthe neighbourhood of the detected events. By achieving a smooth-ing of the events curve, weighting allows to discern betweenevents representing a substantial amount of concentrated AE andisolated events. In addition, this enables removing the artifacts

coming from the locating algorithms used by AE systems, espe-cially connected with the detection of falsely superposed eventsin the same location [75].

In principle, tests on notched specimens to assess the elasto-plastic fracture mechanics behaviour of composites would appeareasier to be characterized using acoustic emission, since the pro-gression of damage should approximately follow an oriented direc-tion. Using AE sensors position scheme, such as the one depicted inFig. 7, it is possible to follow in real time the evolution of cracklength during toughness testing. The weighing centre of AE ampli-tudes is supposed to correspond to the notch root, whilst the dam-aged area is supposed to be equivalent to the area in which 90% ofAE events are detected, to account for occasional events unrelatedwith crack growth [57].

Passing to the second aspect of acoustic emission analysis i.e.,correlation with toughness data, tests procedures such as SENB(single-edge notched bend) or SENT provide already some wayfor reconnecting indications supplied by force-elongation curvesto the fracture mechanics behaviour. For example, load drops inSENB were associated to the onset of crack bifurcation in the mate-rial [76].

Assisting the analysis of force-elongation curves using acousticemission, a typical procedure for AE data analysis is dividing thesecurves in different phases with growing AE activity [77]. This hasbeen done in the understanding that the number and amplitudeof AE events reflect the fibre-related failure processes of notchedsamples. In this way, the application of acoustic emission can beuseful to detect the onset of fibre-related breakage events, assum-ing that events detected in the matrix have lower amplitude thanthose due to fibre pull-out, which have in turn lower amplitudethan those due to fibre failure [78]. On natural fibre composites,AE amplitude distributions plots from SENB tests allowed forexample establishing that in hemp-basalt hybrid laminates crackpropagation starts before the maximum load is reached [58]. Inother cases, the aforementioned weighing functions were able tomeasure damaged area and to follow its evolution with loading

Fig. 8. Number of acoustic events (E) as a function of fracture toughness (KIC) (afterCzigany [80]).


in these hybrids, where no clear effect of basalt fibre treatment onthe results was observed [58]. A similar measurement was per-formed in flax-starch (MaterBi) composites, where data were com-pared, finding good agreement, with damage area measurementusing IR thermography suggesting that this material failed in aductile way with pronounced crack growth and enabled the deter-mination of the J–R integral [79]. The quasi-unidirectional flax fibrelayers were stacked in a cross ply manner. In this way, the real-time determination of the central point of the damaged area, cor-responding to the actual crack length, was determined usingacoustic emission localization, allowing the reliable tracing of J–Rcurves [79]. The same method, employed on jute cloth reinforcedcomposites, clarified that the value of J-integral is consistentlyhigher in SENT specimens with the introduction of a larger volumeof reinforcement (40 wt% against 20 wt%) [57]. The presence of aquasi-linear relation between material toughness and AE activitywas also clarified in plotting the values of KIC against the numberof detected AE events in flax-polypropylene composites, as it isshown in Fig. 8 [80]. Here again, the determination of damagedarea using AE highlighted a reliable correlation of the crack tip po-sition measured with the moisture content in the samples: sub-stantially higher AE amplitudes were also revealed for the ‘‘dry”samples than for the ‘‘wet” ones in both directions, parallel or per-pendicular to the fibres [80].

It is significant noting that most AE analysis in crack propaga-tion studies is based on amplitude distribution plots, includingsometimes also 3D plots, such as in [56], where AE amplitude dis-tribution is also correlated with the fibre length effect on staticproperties. This is based on the assumption that a larger numberof fractured fibres leads to a larger number of AE events and hence,due to damage accumulation, also to higher AE amplitudes. In gen-eral, this is true, especially in plant fibre composites, where most ofAE activity is connected with fibre breakage phenomena, althoughit does not appear by any means general the fact, mentioned in[54], that AE events related with fibre pull-out have lower ampli-tudes than the ones related to fibre breakage. In any case, the pres-ence of shorter fibres, hence to a larger number of fibres in thecomposite, would lead to a higher percent of AE high amplitudeevents. In this regard, any data evaluation based on the numberof AE events, for example the relation suggested in [80] with frac-ture toughness, may encounter the difficulty of being dependenton the fibre length.

To summarize, the analysis of toughness data with acousticemission appear quite reliable and repeatable, when concentratingon localization data and on use of bell-shaped amplitude distribu-tion to follow crack growth progress. In contrast, any relationshipestablished with toughness data using AE counts or number of

events is probably of less general use in natural fibre composites,although may be useful in specific cases, as it has been widelyproved in basalt fibre reinforced laminates [56,58].

5. Summary

In Table 2, a summary of the most significant works includingacoustic emission as a tool for studying the behaviour of natural fi-bre composites is reported. Trying to isolate some characteristicfeatures of acoustic emission application on natural fibre compos-ites, a first consideration would involve the lower mechanicalresistance of these materials, which reduces the margins for pre-dictive use of real-time monitoring techniques. It does not comeas a surprise therefore that some studies in this field concern basaltfibre composites [39,55,56,58], whose resistance is in the samerange than that of glass fibre composites, or hybrids includingplant fibres and glass fibres, to evaluate the grade of success ofhybridisation for the specific material [65,67]. The significance ofhybrids in this field goes well beyond the possibility to lead to apositive hybridisation effect, in that hybrids may represent an ac-tive way to cover the gaps in properties between the materials[81]. However, it is suggested that the rationale of using acousticemission on these materials could be principally the measurementof load limits for use of natural fibre composites, both before andduring service. In this respect, a number of studies offer some indi-cations, although the inherent variability of the properties of thesematerials needs also to be accounted for [63–65]. It is also remark-able, as a significant issue, that little is known so far on the effect ofbiological factors (e.g., geographical location, different cultivars ofsame species, mode of extraction) on the properties of plant fibreswhen used as a reinforcement in a composite. It is perhaps moreunexpected the limited attention dedicated so far to the study ofenvironmental degradation of plant fibres when included in com-posites, even if acoustic emission, being essentially a comparativetechnique, rather that providing absolute results, would appearideal for this purpose. Similar studies are available already on glassfibre composites, such as [82], dealing with environmentally-en-hanced fatigue monitoring by AE. Variables used so far for acousticemission analysis in these studies tend to be more traditional ones,such as amplitude and energy distributions and cumulative countcurves: a few involved the study of frequency content in wave-forms [39–42], or the use of basic clustering analysis [57,58]: inboth cases this was mainly employed to assist in events localiza-tion. From a mechanical point of view, it is possible to note thatsome studies move towards the direction of recognising the cellu-lar structure of biological ‘‘products”, such as plant fibres. This sug-gests the need for identification of particular micro-mechanicaldamage features in natural fibre composites. Also in this field,acoustic emission analysis can have some say: in particular fibril-lation of plant fibres in composites was identified by AE and thenconfirmed by microscopy observations, using once again AE ampli-tude distributions [40]. This can suggest that further possibilities ofAE studies in these materials are mainly linked to understandinghow the hierarchical restructuration, involving a number of levelsfrom the cell up to the fibre itself, and the calibrated presence ofdefects affects damage modes and failure behaviour [83]. Thiswould possibly lead to mechanical modelling of plant fibre com-posites, in a similar way to what is largely available on traditionalman-made fibre reinforced composites.

6. Conclusions

The starting point in using acoustic emission on natural fibrecomposites reflects obviously work which has been performed onother polymer composites, such as glass, carbon and Kevlar fibre

Table 2Summary of the most significant works of acoustic emission as a tool for natural fibre composites studies.

Ref. Material Mechanicalcharacterization

AE analysis (variables) Damage modes detected and main results

Matrix Reinforcement

[39] Epoxy Basalt (single fibres) Fragmentationtest

Amplitude distributionAmplitude vs. timeEnergy vs. timeDuration vs. amplitudeEnergy distributionAE waveforms and their FFT

Discrimination of different failure modesone-to-one correspondence between AE events and fibrebreakage

[40] Epoxy Ramie, kenaf (single fibres) Single fibrecomposite test

Amplitude vs. strainEnergy vs. strainAmplitude vs. timeEnergy vs. timeAE waveforms and their FFT

Matrix crackingFibrillationFibre breakage

[41] PolypropyleneMaleic anhydridepolypropylenecopolymer

Jute, hemp (single fibres) Single fibrecomposite test

Amplitude vs. strainEnergy vs. strainAE waveforms and their FFT

Evaluation of microfailure mechanisms of natural fibres inthe single fibre composite specimens

[42] Polypropylene Jute (single fibres) Single fibrecomposite test

Amplitude vs. strainEnergy vs. strainAE waveforms and their FFT

Identification of microfailure modes of natural fibres in thesingle fibre composite specimens prior and after boilingwater test

[51] PolypropyleneMaleinated-Polypropylene

Wood flour Tensile test Cumulative hits vs. elongationAmplitude vs. elongation

Three main failure mechanisms were identified:debonding at matrix/filler interfacefracture of large wood particlesparticle pullout

[52] Polypropylene Beech wood (short fibres) Tensile test Number of AE eventsAmplitude distributionCounts distribution

Fibre–matrix debonding

[53] Polyethylene Maize hull Tensile test Amplitude vs. time Matrix deformationMaize hull pull-outMaize hull breakage

[54] Thermoplastic nat-ural rubber

Kenaf (flakes) Double edgenotched tensile(DENT)

Amplitude vs. elongationAmplitude distributionEvent counts vs. ligament length

Matrix deformationFibre–matrix debondingFibre pulloutFibre breakage

[55] Polyethylene/polyamide

Basalt (short fibres) Single edgenotched tensile(SENT)

Number of AE eventsAmplitude distribution

Matrix deformationMatrix tearingFibre–matrix debondingFibre pullout

[56] VinylesterVinylester/Epoxy

Basalt (mat) Tensile testThree-pointbend testSingle edgenotched tensile(SENT)

Number of AE eventsAmplitude distribution3D location maps

Matrix fractureCrack propagationShape and size of the damage zone

[57] Thermoplastic Poly-ester (Ecoflex�)

Jute fabric (plain weave) Tensile testSingle edgenotched tensile(SENT)

Cumulative number of AE events vs. strainAmplitude distribution3D location maps

Crack propagation and damage zone estimation for SENTspecimensFibre–matrix debondingFibre pulloutFibre fracture

[58] Polypropylene Basalt, hemp, glass, carbon and hybrid(basalt/hemp basalt/glass basalt/carbon)(short fibres)

Tensile testThree-pointbend testSingle edgenotched tensile(SENT)

Number of AE eventsAmplitude distributionCorrelation matrices of the mechanical proper-ties and AE parameters (amplitude, energy andevents)

Damage development and growth during loading of notchedspecimensPosition of the propagating crackDetermination of J–R curves

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[59] Polypropylene Basalt (short fibres)Ceramic (short fibres)

Single edge notched ten-sile (SENT)

Number of AE eventsAmplitude distribution

Failure mechanism characterizationExistence of a significant correlation between AE parameters and mechanical propertiesfor each type of loading

[60] – Flax (technical fibres) Single fibre fracture test Amplitude vs.displacementAmplitude distribution

Identification of two different damage forms:pullout in transverse directiondebonding in longitudinal direction

[61] Thermoplasticstarch (MaterBi�)

Flax (unidirectional andcrossed-ply fibres)

Tensile test Amplitude vs.elongationCumulative number ofAEevents vs. elongationDuration vs. elongationAmplitude distribution

Fibre–matrix debonding

[62] Epoxy Bamboo (unidirectional fibres) Tensile testThree-point bend test

Cumulative counts Prediction of the onset of fracture and material failure

[63] Polyester Jute fabric (plain weave) Post impact cyclic three-point bend test

Planar localization plotsCounts vs. durationAmplitude distribution

Flexural damage is more significant than impact damageImpact damage and residual properties are defects-driven characteristics

[64] Polyester Jute fabric (plain weave) Tensile testCyclic three-point bend testStaircase indentationContinuous indentation

Count rate vs. stressLinear localization plotsPlanar localization plots

Determination of the elastic limit of the material by using the shape of the AE count ratevs. stress curve (only for low impact energies)Damage evolution during cyclic three-point bend testsMeasurement of impact-damaged areas by means of AE localization plots

[65] Polyester Jute/glass fabrics (plain weave) Post-impact cyclic flex-ural tests

Number of AE eventsLocalization plotsAmplitude distributionDuration distribution

Identification of failure modesAssessment of the strength-limiting role played by the core–skin interface

[66] Poly(lactic acid) Oilseed coproducts Tensile test Hit rate vs. timeCumulative Hits vs.strain

AE shows ductile behaviour of the compositesThree different stages of deformation: debonding, yielding of matrix, ductile failure

[67] Polyester Flax (unidirectional) Tensile test Cumulative counts vs.strain

Evidence of yielding at low values of stress and strain

[68] Phenolformaldehyde

Oil palm empty fruit bunchfibresPineapple leaf fibres (shortfibres)

Single edgenotched tensile

Number of AE eventsAmplitude and cumula-tiveevents vs. elapsed timeAmplitude distribution

Identification of failure modesCorrelation between distribution of amplitudes and fracture toughness

[79] Thermoplasticstarch (MaterBi�)

Flax (quasi-unidirectional andcrossed-ply fibres)

Single edgenotched tensile(SENT)

2D–3D location mapsCumulative amplitudevs. displacement

Damage development and growth during loading of notched specimensPosition of the propagating crackDetermination of J–R curves

[80] Polypropylene Flax (short fibres) Single edgenotched tensile(SENT)

Fracture toughness vs.number of AEevents3D location maps

Linear correlation between KIC and number of AE eventsDetermination of the size of the damage zone

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reinforced ones. However, this review proves that some consider-ation of the particular structure and behaviour of natural fibrecomposites has been also attempted in more recent studies involv-ing acoustic emission monitoring. In particular, some critical as-pects of the structural properties of natural fibre compositeshave been also dealt with, such as detection of damage initiationand measurement and characterization of damage area. These areparticularly difficult in natural fibre composites, where in additionto anisotropic behaviour, typical of composite laminates, a largevariability in fibre geometry and properties is also present. More-over, an optimal interaction between natural fibres and the poly-mer matrix has hardly been achieved so far.

Respect to these issues, acoustic emission may constitute a use-ful tool for mechanical behaviour monitoring of natural fibre com-posites. This would imply contributing to the future transitiontowards fully bio-degradable materials, of which natural fibrecomposites constitute one possible path, which is increasingly fol-lowed in recent years.

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Acoustic emission for monitoring the mechanical behaviour of natural fibre composites: A literature...

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