Accepted Manuscript

Damage to flax fibre slivers under monotonic uniaxial tensile loading

Antoine Barbulée, Jean-Paul Jernot, Joël Bréard, Moussa Gomina

PII: S1359-835X(14)00127-4

DOI: http://dx.doi.org/10.1016/j.compositesa.2014.04.024

Reference: JCOMA 3616

To appear in: Composites: Part A

Received Date: 17 October 2013

Revised Date: 23 April 2014

Accepted Date: 30 April 2014

Please cite this article as: Barbulée, A., Jernot, J-P., Bréard, J., Gomina, M., Damage to flax fibre slivers under

monotonic uniaxial tensile loading, Composites: Part A (2014), doi: http://dx.doi.org/10.1016/j.compositesa.

2014.04.024

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1

DAMAGE TO FLAX FIBRE SLIVERS UNDER MONOTONIC

UNIAXIAL TENSILE LOADING

Antoine Barbulée1, Jean-Paul Jernot1, Joël Bréard2, Moussa Gomina1

1 CRISMAT, UMR 6508 ENSICAEN/UCBN/CNRS, 6 Boulevard Maréchal Juin, 14050 CAEN Cedex 4, France.

2 LOMC, UMR 6294 Université du Havre / CNRS, 53 rue de Prony, 76058 LE HAVRE, France.

Email: antoine.barbulee@ensicaen.fr

ABSTRACT

Usual testing methods developed for synthetic fibre bundles or slivers do not suit for

plant fibres which exhibit specific characteristics. This paper reports damage

investigation in flax fibre slivers (collection of disentangled and aligned fibre bundles

more or less bonded by bark residues or other tissues) especially prepared for uniaxial

tension testing using a new methodology and a dedicated device.

The analysis of the acoustic emission data by using clustering methodology allowed

categorizing three populations of damage mechanisms. Identification of the damage

mechanisms was made by correlation of the tensile loading curves with acoustic

emission data and by optical observations. Different damage parameters defined from

the mechanical data (released elastic energy) were compared with those from acoustic

data (energy of the hits). The results showed that the extension of damage versus

deformation can be expressed by using either of these two data sets.

KEYWORDS

A. Fibres; C. Damage mechanics; D. Acoustic emission; D. Mechanical testing

2

INTRODUCTION

In textile plants, bast contiguous cells specialize each in ultimate fibre and the

organization of these fibres into bundles provides support for the plant. Following the

decortication steps, the size of the initial fibre bundles is reduced and that results in

many sub-bundles and relatively few ultimate fibres. The decortication process also

affects the state of the surface and the mechanical properties of the fibres, and therefore

it is necessary to assess separately its impacts. In plant fibre bundles, ultimate fibres are

bonded by pectins while in synthetic fibre bundles each fibre is isolated from the other.

Thus, in the composites with plant fibres the quality of the fibre-matrix interface may

suffer from insufficient impregnation of the bundles and poor wetting of the individual

fibres by the resin. In addition to its role in charge transfer onto the fibres, the fibre-

matrix interface must also prevent the water intake by the fibre. That requires specific

treatments of the fibre surface (or a chemical modification of the matrix) to ensure an

optimum fibre-matrix interface [1–7]. So far, the mechanical properties of flax fibres are

mostly measured on ultimate fibres to estimate the yearly reproducibility of the

mechanical properties [8], the influence of variety [9], retting process [10], decortication

conditions [11,12], thermal treatment [13,14] or to correlate these properties to those of

the composite[15,16]. Many review papers have addressed these issues which determine

the use of plant fibres in structural materials [17–19]. Indeed the tests on ultimate fibres

allow investigating the variability of mechanical properties, but they cannot provide

information on the cohesion of the fibre bundle, which is however an important factor in

understanding the mechanical behavior of dry slivers or within a composite. Otherwise

the selection of ultimate fibres by manually extracting them affects their properties

because the process probably eliminates the weakest and damages the most resistant.

Furthermore, testing ultimate fibres underestimates the presence of defects which are

3

rather concentrated in specific areas in the stem. Figure 1 shows the preferential attack

of fibre bundles by retting micro-organisms around the attachment point of the leaf

where phloem tissues are exposed to the environment. On the other hand, many

challenges remain regarding the mechanical characterization of ultimate fibres. As an

example, big uncertainties remain about the cross-sectional area used for the evaluation

of the Young’s modulus and the tensile strength [20–24] due to the presence of the

lumen, the non-circularity of the fibre section, the cross-section area change along the

fibre. In order to improve the reliability of plant fibre-reinforced composites, Virk et al.

[25] recommended the use of failure strain as the key design criterion. Furthermore, the

standardized tensile test with a gauge length of 10 mm [26] is questionable regarding

the representativeness of the results.

On the opposite, in the sliver the cooperative effect of a great number of fibres allows

reducing the dispersion of the mechanical properties, revealing better the defects within

the fibre. Moreover, testing slivers brings more realistic insight on the mechanical

properties of the basic entity that constitutes the roving used in the composite.

In view of these points, we attempted to develop a reliable methodology for the

mechanical characterization of plant fibres at the scale of the sliver [27]. Indeed, reliable

identification of damage and failure mechanisms is needed to support shaping of

technical parts [28]. It is also needed for modeling the mechanical behavior of the

composite by homogenization approaches [29].

MATERIALS AND METHODS

1) Obtaining flax fibres

Flax stems of Melina variety grown in 2012 in the plain of Caen (in Normandy, France)

were used in this study. The stems were moderately scutched by the Vandecandelaere

4

Company (Depestele Group in Bourguébus, France). The decortication process was

characterized by a feeding rate of 30 m/min., a crushing intensity of 5 bars and a

threshing rate of 62 rounds per minute. Following these mechanical treatments, the

scutched fibres consist mainly of two entities we have represented schematically in

Figure 2: ultimate fibres and fibre bundles with residues of cortical tissues and

epidermis.

2) Sample preparation

Flax fibres possess high axial mechanical properties but significantly lower values in

the radial directions [30]. Thus, fitting a sliver to the jaw for tensile testing is tricky: on

the one hand the sliver must be maintained in the jaw without crushing; on the other one

must ensure the perfect alignment with the loading axis to avoid inadvertent damage in

the vicinity of the clamping area. In addition, (a) the fibre bundles must remain parallel

to each other in the sliver, (b) all fibre bundles must be loaded at the same time from the

beginning of the test, (c) the parasitic friction among the fibres should be minimized, (d)

two independent fibre bundles should break apart. To this end, one must align the fibre

bundles and spread up the sliver without altering the bonds between the bundles. We

have designed and built a device that can meet these technical rules [31]. The

organization of the sliver thus obtained is saved by molding both ends in steel shells

using a resin, which sets the gauge length of the test piece.

3) Sample testing

3.1. Mechanical testing device

The specimens defined above were loaded on a universal mechanical testing machine

(Instron type, 1185 series) equipped with a 2 kN capacity load cell and ensuring

uniform clamping in the jaws and a perfect alignment. The uniaxial tensile tests were

5

conducted at a deformation rate of 1 %/min. The deformation of the sample was

calculated as the ratio of the specimen elongation, ΔL (estimated by the cross-head

displacement) over the gauge length, L0.

Given the sensitivity of plant fibres to hydrothermal conditions, the tests were

conducted in a climatic chamber at 25°C and environment relative humidity RH= 50%.

Prior to the tests, the samples were set in the climatic chamber for 3 hours to ensure the

stabilization of the water content of the fibre [32].

3.2. Acoustic emission monitoring

The discontinuous release of strain energy within a material leads to the creation and

propagation of transient elastic waves. This phenomenon is called acoustic emission.

Many physical processes can give rise to acoustic emission: plastic deformation,

dislocation motion, sliding at the grain boundaries, phase transformation, corrosion,

hydrogen embrittlement, mechanical impact, leakage, cavitation, partial discharges,

micro and macro-ruptures... Acoustic emission can be used as a nondestructive testing

method to detect the presence of growing defects, to follow their evolution in real time

and to subsequently study the underlying damage and failure mechanisms [33,34].

The method includes detecting and converting into electrical signals the transient elastic

waves by using one or more piezoelectric sensors coupled to the test piece (using silicon

grease as coupling agent) subjected to a thermomechanical loading (Fig. 3a). These

signals can then be analyzed by using a suitable electronic device (acoustic emission

set).

On an oscilloscope, the elastic waves appear as transient signals consisting of thousands

of acoustic hits. Considering the alternations above a preset threshold (chosen to avoid

background noise), a hit can be defined using a set of parameters (Fig. 3b) [34,35]

- The number of counts (N) is the number of alternations above the preset threshold,

6

- The peak amplitude (A) is the maximum amplitude of the hit,

- The duration (D) is the elapsed time between the detection of the first alternation and

the last alternation,

- The rise time (R) is the elapsed time between the first alternation and the peak

amplitude,

- The energy (U) is proportional to the square of the peak amplitude,

- ...

The shape of the monitored signal depends on a number of factors such as:

- The physical nature of the phenomenon or damage mechanism responsible for the

acoustic emission,

- The material and the wave propagation characteristics (attenuation and reflections...),

- The spectral response of the sensor,

- …

In most cases, acoustic emission is used correlatively with the loading curve for

specifying the damage threshold or determining the domains within which the different

damage and fracture mechanisms occur [36].

In order to study the damage to the sliver under load, an acoustic emission set (Mistras

type) was used. It includes a PCI-2 data acquisition system, two Pico piezoelectric

sensors for the precise location of damage sources in the gauge length. The operating

frequency of these broadband sensors is in the range 200-750 kHz. The parameters set

for monitoring acoustic emission were a preset threshold of 40 dB and a gain of 40 dB.

To isolate the hits by eliminating echoes, acoustic emission was recorded by using a set

of timing parameters:

- The hit definition time HDT = 800 μs. This parameter determines the end of the hit; its

duration must be chosen short enough to avoid taking into account the multiple echoes.

7

- The hit lockout time HLT = 1000 μs. It is the duration between the end of the hit and

the end of the related echoes. It insures no triggering occurs while echoes are

manifested.

- The peak definition time PDT = 200 μs. It allows separating the peaks of the volume

and surface waves.

RESULTS AND DISCUSSION

1. Validation of the testing device

1.1. Precautions for making the samples

The nature of the resin for making the samples is important because we must ensure that

it does not induce premature aging, deterioration or breakage of the flax fibres. The

choice was a chemically stable acrylic resin Versocit 2 from Struers, i.e. non-reactive

with the fibre after setting. The chosen fabrication technique avoids the infiltration of

the resin in the calibrated zone of the sliver, which would stick the fibres together and

reduce the gauge length.

1.2. Alignment and iso-tension of the fibre bundles within the

sliver

To assess the fair alignment of the loading device and the iso-tension of the fibre

bundles in the sliver, the deformation at the onset of linear behavior of the sliver, (εl), is

compared to the deformation associated with the first bundle rupture (εbf). To this end,

the domain of linear elastic behavior was identified on the tensile loading curve (Fig. 4)

and the line of highest slope was drawn (O’z); it intersects the deformation axis at point

O’. This point is identified as the origin of the loading curve that would be obtained if

the loading assembly had infinite stiffness and all fibre bundles were equally strained.

8

The fact that all the fibre bundles are not equally strained explains why the experimental

loading curve intersects the (O’z) line at the R point of abscissa εl. The ratio (εl/εfb) is an

indicator of the iso-tension of the fibre bundles at the start of test. The iso-tension is the

more effective this ratio is the lower. An average ratio of 18 % was noted when testing

10 flax slivers with a gauge length of 100 mm [27].

1.3. Reliability of strain measurements

As ultimate flax fibres are not continuous along the gauge length, we could not use a

mechanical axial extensometer for strain measurements as is customary with synthetic

fibres [37]. For gauge length in the range 5 - 70 mm we estimated the specimen

deformation from the crosshead displacement (refer to paragraph 3.1.) and compared it

to the results obtained using a contact-free deformation measurement device (Imetrium

from Mescan). It appears that the compliance of the loading device is of about 3.2x10-5

m/N. Hence, for higher gauge lengths, the contribution from the testing device to the

total elongation of the specimen is negligible (Fig. 5).

1.4. Identification of damage mechanisms

The loading curve of a sliver consisting of several fibre bundles (Fig. 4) shows a first

domain corresponding to the linear elastic behavior, followed by a non-linear zone in

which acoustic emission appears. The end of this second domain is marked by the first

breakdown of a fibre bundle and is manifested by a more or less pronounced load drop.

Further increase in strain results in a recovery of the load until the failure of another

fibre bundle. The occurrence of these events leads to the typical saw-tooth shape of the

load-deformation curve.

9

Acoustic signals monitored during loading have been used to identify several damage

mechanisms in carbon fibre composites [38], refractory material [39] or metals [40].

As regards the slivers, successive reduction of the rigidity and the associated variations

of elastic energy can be correlated with the acoustic emission parameters (duration,

energy and amplitude of the hits, rise time, number of counts…). Conventionally, in AE

the identification of damage and failure mechanisms is performed using a single

acoustic parameter, the peak amplitude is most usual. Thus, in the study of fibre

composites, fibre/matrix debonding is associated with low AE amplitudes, matrix

cracking with average amplitudes and fibre ruptures with the highest amplitudes [41–

50]. In the case of fine microstructure materials, the Pollock plot methodology [49] was

used to characterize inter- and transgranular ruptures [50]. But in composite materials,

the intuitive approach mentioned above is often ineffective because of overlaps in the

distribution of AE amplitudes. It is then necessary to use a more sophisticated analysis

of the acoustic data. Anastassopoulos and Philippidis [38] conducted a review of

various recent techniques implemented to select the most relevant acoustic parameters

for partitioning data into populations characterizing the damage mechanisms in a

composite.

To investigate damage in flax fibre slivers, the evolution of the stiffness ΔK (defined by

successive load drops ΔP (ε)) during mechanical loading is analyzed. In deciding whether

a given acoustic parameter, S, can be part of the panel to use for partitioning the

acoustic emission data and defining distinct populations of damage mechanisms, we

proceeded as follows. First, the n load drops observed on the loading curve and used to

define the stiffness ΔK are ordered by descending values ΔK1 > ΔK2 > ΔK3>... ΔKi > ... >

ΔKn . So, for the acoustic parameter S under study, the values xi and yi are defined by:

10

i

ix

n= (1)

1

1

i

k

i n

k

Sy

S= ∑

∑ (2)

The representation of (xi, yi) data is shown in Figure 6 for different acoustic parameters.

For flax fibre slivers, two types of curves are identified essentially: (a) some parameters

(such as amplitude) vary almost linearly with rigidity up to failure, i.e. rigidity is little

discriminating with respect to these acoustic parameters; (b) other acoustic parameters

(such as absolute energy or duration of the hit) significantly change in the range of low

xi values and then the growth rate slows down. Thus, the absolute energy and duration

of the hits are more relevant than the amplitude for data partitioning to determine

characteristic populations of damage mechanisms. The separation of the acoustic events

in different populations is shown in Figures 7 and 8.

The representation of the hit duration vs. the rigidity of the sliver (Fig. 7) allows

identifying three populations of damage mechanisms shown in the last column in Table

1. Population #1 is associated with low duration hits without significant reduction of the

rigidity. Hits in population #2 also show short duration (< 3 ms) and are accompanied

by a low reduction of the rigidity (< 0.8 N/% deformation). Unlike the first two

families, the duration of the hits in population #3 is longer and the rigidity variation is

more consistent. These results are similar to those reported by Romhány et al. [51],

except that herein the determination of the different populations of damage and failure

mechanisms is based on a multi-criteria analysis (clustering methodology) of the

acoustic emission, which is a less subjective methodology. The main mechanisms

identified by correlation of the acoustic and mechanical data to direct optical

observations are shown in Table 1. Examples of delamination within fibre bundles and

ruptures of ultimate fibres or fibre bundles are illustrated in Fig. 9. In Figure 10 the

11

three populations are shown on the loading curve. We note that population # 1 is present

almost continuously during mechanical loading while population # 2 is manifested

between the end of the elastic behavior and the maximum load, i.e. the domain where

the accumulative rate of broken bundles is highest (see Fig. 11). Population # 3 is

observed since the first bundle failure up to the ruin of the sliver.

2. Analyses of damage accumulation in the sliver

The results presented above indicate that various mechanical and acoustic parameters

can be used to describe the evolution of the damage to the sliver. In the following we

analyze damage growth in the sliver through the variations of three mechanical

parameters and an acoustic parameter.

3.1. Density of broken fibre bundles

When the deformation rises, the number of broken fibre bundles increases. The density

of broken fibre bundles, D1 (ε), can then be considered for the evaluation of damage to

the sliver. In this analysis, an equal importance is given to each fibre bundle, regardless

the number of ultimate fibres or sub-bundles included:

( )1

Number of broken fibrebundlesD

Total number of fibrebundles inthe sliverε = (3)

This method allows a simple estimation of the proportion of broken fibre bundles. But,

to reproduce effectively the damage occurring to the sliver, it is essential to separate two

phenomena occurring in closely spaced intervals of time (breaks of two bundles almost

concomitantly, for example). The evolution of D1 (ε) vs. deformation is shown in Figure

11 in correlation with the loading curve. We notice a significant load drop for a

deformation of 0.6% and a little beyond, correlated to a sharp rise of the damage

12

parameter. The resolution of the acoustic set identifies the ruptures of four fibre bundles

but the possibility remains that several bundles break simultaneously.

3.2. Reduction of the sliver rigidity - Accumulative released elastic

energy

As the deformation is increased, damage associated with the mechanisms identified in populations #1 and #2 does not induce significant load drops, except rare breaks of ultimate fibres. Indeed, delamination within and among fibre bundles involves little loss of rigidity of the sliver. Only partial or total ruptures of fibre bundles (included in population #3) have a significant influence on the loss of rigidity. Considering a load drop PΔ , the associated loss of rigidity is defined by:

( ) ( )0 *

P PK

L Lε

ε= =� �

��

(4)

where Lo is the gauge length. We define the damage parameter, D2 (ε), associated with

the load drops by the relative variation of the sliver rigidity:

( )( )

02

0

( ) K d

DK d

εε ε

εε ε

+∞= ∫∫

�

�

(5)

Consequently, the released elastic energy associated with the load drop at deformation ε

is expressed by:

( )*

2released

PE =

�� (6)

With (4) and (6) we obtain:

( ) ( )( )

02

0

2* *

*releasedL E

KL

εε

ε=

�� (7)

Equation 5 can then be used to define the parameter D3 (ε) whose evaluation is based on

incremental mechanical energy measurements:

( )

( )( )

( )( )

20

3

20

ΔEreleased ε

*

ΔEreleased ε

*

dLo

D

dLo

εε

εε

εε

∞=

∫

∫ (8)

D2 (ε) and D3 (ε) are plotted in Figure 12. It is obvious that both measurement methods

give the same results.

13

3.3. Accumulative acoustic energy

The damage parameters D2 (ε) and D3 (ε) have been defined taking into account only the

mechanisms associated with load drop ΔP. On the contrary the acoustic energy

collected during the test includes the contributions of all the damage phenomena listed

in Table 1.

Assuming that the monitored acoustic energy is proportional to the released elastic

energy during damage growth, the accumulative acoustic energy, D4 (ε), is expressed as:

( ) 0

4

0

ΔEacoustic( )( * )²

ΔEacoustic( )( * )²

dLo

Dd

Lo

ε ε εεε ε εε

∞=

∫

∫ (6)

The definition of this parameter includes other contributions than the elastic energy: (a)

delamination between the fibres within the bundles; (b) delamination between adjacent

bundles linked together, accompanied by load transfer onto the remaining bundles. The

accumulated acoustic energy D4 is shown in Figure 13 as a function of the deformation

of the sliver, together with D1, D2 and D3. The similarity among the different trends

suggests that damage accumulation with deformation can be evaluated equally by each

of the exposed methods.

CONCLUSIONS

The mechanical behaviour of flax fibre slivers was assessed by using a tensile testing

methodology specially developed for plant fibres. Three populations of damage

mechanisms were found by analyzing the acoustic emission data using a clustering

methodology. The particular saw-tooth shape of the loading curves was correlated to

acoustic emission data and optical microscopy observations to identify the damage

mechanisms which characterize these slivers of plant fibres, especially: (a) delamination

in the bundles by breakage of the pectin links between ultimate fibers, (b) delamination

14

among adjacent bundles, (c) successive ruptures of the bundles. Different damage

parameters were identified and evaluated using mechanical or acoustic data. Very good

agreement was noted between the evolutions of these damage parameters as a function

of strain, this indicates that the damage to the plant fiber slivers can be analyzed with

either one or the other of these methods. The obtained results open the way for the

possibility of evaluating the effects of physical, mechanical, hydrothermal or chemical

treatments applied to the fibres during the various preparation stages of a fibre preform

(from decortication to preforming) and the composite manufacturing process.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the Region of Lower Normandy and the European

Regional Development Fund (FEDER) for their financial support to the LINT project

that includes this study. They also thank the Depestele Group, member of the LINT

consortium, for providing the scutched flax fibres.

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Figures caption

Fig. 1: Example of defect zones in a fibre bundle made of 5/6 sub-bundles linked with cortical tissues.

Fig 2a: Main entities in sliver.

Fig. 2b: Different arrangements of the flax fibres in the sliver.

Fig 3a: Acoustic signal monitoring under load

Fig 3b: Hit characterization by different acoustic emission parameters

Fig 3c: Definition of the timing parameters allowing hits recognition

Fig. 4: Evaluation of the iso-tension of the fibre bundles

Fig. 5: Relative error on the deformation of the sliver vs. gauge length.

Fig. 6: Evolution of acoustic parameters vs. hits associated with the rigidity of the sliver.

Fig. 7: Hit duration as a function of the reduction of sliver rigidity.

Fig. 8: Reduction of the sliver rigidity as a function of hit absolute energy.

Fig 9: Close to a leaf attachment point (L), five sub-bundles covered with cortical

tissues (CT) are loaded up to rupture. A1: delamination among bundles; A2:

delamination within a bundle; A3: delaminated and broken ultimate fiber; B:

delaminated and broken fibre bundle.

Fig. 10: Representation of the three populations of damage mechanisms identified along the loading curve.

Fig. 11: Damage parameter estimated by the ratio of broken fibre bundles D1 ( )together with the loading curve ( ).

Fig. 12: Damage parameters estimated vs. deformation by the accumulative reduction of the sliver rigidity D2 ( ) or the accumulative released elastic energy D3 ( ) together with the loading curve ( ).

29

Fig. 13: Damage parameter estimated vs. deformation by the accumulative reduction of the sliver rigidity D2 ( ) or the accumulative acoustic energy D4 ( ) together with the loading curve ( ).

Fig. 14: Damage parameter estimated vs. deformation by the accumulative duration of the hits D5 ( ) together with the loading curve ( ) and other.

30

Fig. 1

31

Ultimate fibre

b) Ultimate fibre as constrained within the bundle. Note the angular morphology of the fibre cross-section.

b) Ultimate fibre extracted from the bundle, with less angular morphology.

Clean isolated fibre bundle or sub-bundle

Ultimate fibres bonded by pectic cements within the bundle

Bundlesassociated with bast tissues

After decortication some fibre bundles are still associated with residues of bast tissues (cortical parenchyma, epidermis and cuticle)

Fig 2a

Fig. 2b

32

Fig 3a

Fig 3b

Fig 3c

33

0,0 0,2 0,4 0,6 0,8 1,0

0

20

40

ZLo

ad (

N)

Deformation (%)

Loading Curve

εl εfb

O'

R

Fig. 4

0 10 20 30 40 50 60 70 800

5

10

15

20

25

30

35

40

45

50

Rel

ativ

e er

ror

(%)

Gauge length (mm)

Fig. 5

34

0,0 0,2 0,4 0,6 0,8 1,0

0,0

0,2

0,4

0,6

0,8

1,0

Si/S

n

Hits rate

Rigidity reduction Absolute energy Duration Number of counts Rise time AmplitudeDeformation

Fig. 6

0 5 10 15 20

0

1000

2000

3000

4000

5000

6000

Population 1Population 2Population3

Hit

dura

tion

(ms)

Reduction of sliver rigidity (N/%)

Fig. 7

35

0,0 5,0x109 1,0x1010 1,5x1010 2,0x1010 2,5x1010

0

10

20

Population 1Population 2Population 3R

educ

tion

of s

liver

rig

idity

(N

/%)

Hit absolute energy (ato J)

Fig. 8

Fig 9

36

0,0 0,2 0,4 0,6 0,8

0

10

20

30

40 Loading curve

Population 1

Popolation 2

Population 3

Load

(N

)

Deformation (%)

Fig. 10

37

0,0 0,2 0,4 0,6 0,8

0

20

40

60

80

100

Deformation (%)

Dam

age

(%)

0

20

40

Load

(N

)

Fig. 11

�

0,0 0,2 0,4 0,6 0,8

0

50

100

Deformation (%)

Dam

age

(%)

0

10

20

30

40

Load

(N

)

Fig. 12

38

0,0 0,2 0,4 0,6 0,8

0

50

100

Deformation (%)

Dam

age

(%)

0

10

20

30

40

Load

(N

)

Fig. 13

0,0 0,2 0,4 0,6 0,8

0

50

100

Deformation (%)

Dam

age

(%)

0

10

20

30

40

Load

(N

)

Fig. 14

39

Table 1

Table 1: Proposed damage mechanisms from the correlation of the loading curve and the acoustic emission. (N /% def. means newton per percent deformation).

Analysis of the loading curve Analysis of the acoustic signalsDifferentfamilies of damagemechanisms

Damagemechanisms

Reduction of the sliver rigidity (N / % def.)

Elastic energy variation

Hitduration

(μs)

Absolute hit energy

(109 ato J)

Amplitude(dB)

Friction among

ultimate fibres or fibre bundles

Non detectable Non

detectable Low energy Low

#1

Rupture of ultimate fibres

Very low Non

detectable Low Medium

Delaminationamong or

within fibre bundles

< 0.8 Low > 1500 < 5 < 80

#2Partial

ruptures offibre bundles

< 0.8 Low/Medium > 1500 > 5 > 80

Catastrophicrupture of the fibre bundles

> 0.8 High > 1500 > 5 80-100 #3

Table 1