J. Acoustic Emission, 23 (2005) 25 © 2005 Acoustic Emission Group

ACOUSTIC EMISSION TECHNIQUE FOR DETECTING DAMAGE AND

MECHANISMS OF FRACTURE IN A KNITTED FABRIC REINFORCED

COMPOSITE

CARLOS R. RIOS1, STEVE L. OGIN

2, CONSTANTINA LEKAKOU

2 and K. H. LEONG

3

1) Unidad de Materiales, Centro de Investigación Científica de Yucatán, Mérida Yucatán, Mex-

ico; 2)

School of Engineering, University of Surrey, Guildford, Surrey GU2 5XH, UK;

3) Cooperative Research Centre for Advanced Composites Structures Ltd., Fishermans Bend,

VIC 3207, Australia. Currently PETRONAS Research, Malaysia.

Abstract

The relationship between fiber architecture and the damage accumulation sequence under

tensile loading has been investigated for a Milano weft-knitted reinforced composite oriented at a

range of angles to the loading direction (0° to 90

°) using acoustic emission (AE) technique among

others. In order to observe progressive damage accumulation, model sandwich laminates were

fabricated using a single layer of the knitted fabric sandwiched between outer plies of unidirec-

tional glass-reinforced epoxy resin. For comparison, commercial composites manufactured with

the same knitted fabric as a reinforcement and Derakane vinyl-ester resin as matrix were ana-

lyzed. These materials have been characterized for monotonic and cyclic tensile loading. Results

have been compared with those found earlier for a single layer and the sandwich model material

with epoxy resin as matrix. AE technique was demonstrated to be an excellent method for mate-

rials analysis especially in the second case where the matrix is not transparent. AE count rate was

used for damage detection and various failure mechanisms were observed in failed specimens,

such as cracking at loop cross-over points, resin matrix cracking, fiber-bundle debonding and

tensile fracture of fiber bundles.

Keywords: Composite materials, Textiles, Mechanical properties.

1. Introduction

Experimental results obtained on a model material and a composite manufactured by RTM

are presented in this paper. First, the structure of the knitted fabric is described, followed by the

results on a single knitted layer/epoxy composite using the 2x68 tex fabric. Second, after outlin-

ing the mechanical behavior of this material, and the difficulties encountered in analyzing dam-

age propagation due to the premature failure of the material, results on a model sandwich lami-

nate made up of the knitted fabric between the outer 0°-unidirectional glass fiber plies are dis-

cussed. These results include tests where the knitted fabric within the sandwich laminate is ori-

ented at different angles to the loading direction. A detailed microstructural relationship between

fabric geometry and cracking behavior is discussed; the acoustic emission technique was used to

support the investigation of crack initiation and accumulation, especially to understand the dam-

age development along the sample in order to relate the damage behavior and failure to the tex-

tile architecture. These results together with tests on the cyclic behavior of the material enable

some conclusions to be drawn about the mechanical behavior of the knitted fabric layer during

damage accumulation. At the same time, an RTM composite manufactured with the same knitted

fabric as a reinforcement but using Derakane vinyl ester resin as matrix was analyzed in order to

26

understand the similitude on damage development with the model material. The purpose of this

last material was the observation of damage behavior on a structural composite of high fiber vol-

ume fraction.

Many authors have commented on the development of damage during the tensile loading of

knitted-fiber composites. There has been general agreement that (a) for loading in the wale di-

rection, the initiation of damage occurs due to fiber bundle/resin matrix interface failure at the

needle or sinker loops, with subsequent linking of this damage to form long cracks; and (b) for

loading in the course direction, that the damage initiates from the sides or legs of the loops [1- 3].

However, in most studies to date, it has only been possible to draw conclusions based on analysis

of the final fracture surfaces of the composites. In this work, a knitted fabric reinforced compos-

ite layer has been fabricated as part of a sandwich laminate, in which the fabric layer is sand-

wiched between outer plies of unidirectionally reinforced epoxy resin. The resultant model

specimens have enabled the sequence of damage to be investigated directly during tensile load-

ing.

2. Experimental Procedures

Milano weft-knitted fabric produced from E-glass yarns (2x68 tex) was used for manufac-

turing the sandwich panel using a wet impregnation technique. The knitted cloth was cut to size

and fixed inside a steel frame using tape around its edges and the frame was placed in a filament

winder to wind unidirectional glass fibers around the cloth. The knitted cloth was fixed in the

frame in which the wale direction of the cloth was at different angles to outer 0° plies (0

°, 30

°,

45°, 60

° or 90

°). The matrix used for the sandwich panel was an epoxy resin (Astor Stag Epoxide

Resin, Astor Stag NMA curing agent and Ancamine K61B accelerator). The panel dimensions

were 250 mm x 250 mm and the unidirectional plies on each face of the sandwich panels had a

nominal thickness of 1 mm, with an overall panel thickness of about 3.1 mm. The overall fiber

volume fraction of the laminates was 0.29 although the fiber volume fraction within the central

(knitted fabric) layer was much lower (about 0.13). In addition to the sandwich specimens, pan-

els of knitted-fabric reinforced epoxy resin, without the reinforcing outer 0° plies were also

manufactured. RTM materials were produced by placing 5 layers of the Milano weft-knitted fab-

ric in a mold, which was injected with Derakane vinyl ester resin resulting in a high fiber volume

fraction of 0.47.

Samples of 230-mm length and 20-mm width were cut from the panels for mechanical test-

ing. Aluminum end tabs (50 mm long by 20 mm wide) were used for the tests and strain gauges

were bonded transversally and longitudinally near the center of the coupons. An Instron 1196

testing machine was used for the monotonic tests with a constant crosshead speed of 0.5

mm/min. Load/strain data were collected using a data-logger and acoustic emission (AE) events

were recorded using an AECL 2100M system, which employs two guard transducers to reduce

external noise. The threshold for the AE equipment was established by performing preliminary

analysis in order to enable recording of the AE signals from the tests without spurious noise. The

processing unit was set to count the number of acoustic events in 0.1 s intervals. Photographs

were taken for two purposes: first to examine partially tested samples at different percentage of

strain in order to identify the source of the cracks and to obtain the crack density, and second, to

examine the fracture surfaces to obtain an overall idea of the crack propagation and fracture sur-

face contours. Optical microscopy was used to analyze and identify the cracking initiation sites

and to related them to the architecture of the knitted textile.

27

3. Results and Discussion

Figure 1 shows the architecture of the Milano 1x1 knitted fabric, which consists of the inter-

locking of loops of fiber bundles where two singles threads knitted in two separate sets of nee-

dles are held together by a row of 1x1 rib course. Note that this fabric consists of two main di-

rections: course, the row of the loops in the width direction; and wale, the row of knit loops in

the longitudinal direction.

Fig. 1 Architecture of the Milano 1x1 knitted fabric.

Initially laminates containing only one knitted cloth embedded in epoxy resin were manu-

factured and tested at different angles in order to observe damage progression under tensile

loading. However no damage was observed along the samples since samples failed catastrophi-

cally when the first signal of damage appeared. No pre-damage was produced. Wale direction

presented higher mechanical properties than the others due to a higher portion of fibers oriented

at 0°. Fiber volume fraction for single layer laminates was calculated to be 0.13±0.005.

Due to the sudden fracture of the single layer composite materials, sandwich laminates were

made by winding unidirectional glass fibers over the 2x68 tex knitted fabrics as described before.

Observation of damage accumulation was facilitated by the transparent nature of the composite,

which allowed the plan view damage development to be observed during testing by in situ pho-

tography. For all sandwich laminates, the average fiber volume fraction was measured obtaining

a value of 0.29±0.002. Figures 2 and 5 display the strain-stress curve for the samples tested in the

0° (wale), and 90

° (course) direction, respectively. For wale direction, the stress-strain curve dis-

plays a region of pre-damage, which was registered by AE signal where visual damage is not

detected below 1% strain. After this strain value, a first “knee” is observed and a second discon-

tinuity appears at about 1.5%, which is intimately related to the knitted architecture as explains

on Fig. 4. AE signals give additional information with respect to the damage development. A

small number of AE events are observed between 0.80% and 0.95% strain. This is in advance of

the development of the cracks and is related to crack initiation at crossover points in the knitted

fabric architecture. Next, in a range of 1.1% to 1.2% strain, the first significant cracks appear

producing many more AE events until total fracture of the sample.

1

2

3

Wale

Course

28

Model material in wale direction (0 o)

0

50

100

150

200

250

300

350

400

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4

Strain (%)

Str

ess (

MP

a)

0

20

40

60

80

100

120

AE

ev

en

t c

ou

nt

rate

Unloaded 1.10% 1.15% 1.30% 1.60% 2.30%

Fig. 3 Crack development for the model sandwich composite tested in wale direction (0°). Ap-

plied strains are indicated

The cracking pattern shown in Fig. 3 is intimately connected with the fabric architecture.

Figure 4 gives an indication of the sequence of events. The first sign of damage develops at the

loop crossover sites, represented by A, which is detected as micro-debonding at low AE signal.

Significant cracks (matrix cracking) appeared firstly in the plane where the float stitch of course

2 and 3 are together (represented by the dashed lines) possibly due to the strain magnification

caused by adjacent tows. These cracks have a spacing of 4 mm, which is the dimension of the

repetitive unit in the knitted fabric. When these sites have been used, then at a higher strain

cracks develop where the rib holds threads 2 and 3 (represented by dotted lines), giving a final

crack spacing of 2 mm, which is the wale direction dimension of a single loop. This cracking

pattern is registered by AE signals in accord to the discontinuities in the stress-strain curve.

For sample tested in 90o (course) direction linearity can be appreciated up to 0.7% strain. In

this case visual and audible flaws are observed around 0.9% strain, and the first damage ob-

served by AE is again in the form of micro-cracks at the loop crossover points at lower values.

Matrix cracking appeared at a strain of about 0.8% producing a small knee in the stress-strain

4 mm20 mm

Fig. 2 Stress vs. strain curve for sandwich composite tested in wale (0°) direction.

29

curve. During loading in this direction, branched-like cracks were observed to be produced per-

pendicular to the load direction.

Fig. 4 Schematic diagram showing crack development for the model sandwich laminate with the

fabric layer in the 0o direction (Wale).

Therefore, in both cases the AE information suggests that micro-debonding in the initiation

sites (cross-over points) occurs due to stress magnification at lower levels below the knee point,

registered by AE signal. Later above the knee point, when visual damage is observed in the sam-

ple, interfacial debonding and resin matrix cracking take place as demonstrated by some re-

searchers [4, 5]. Finally, fiber fracture arises toward the end of the tensile test.

Model material in course direction (90 o)

0

50

100

150

200

250

300

350

400

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4

Strain (%)

Str

ess (

MP

a)

0

10

20

30

40

50

60

70

80

AE

ev

en

t c

ou

nt

rate

Fig. 5 Stress vs. strain curve for sandwich composite tested in course (90°) direction.

The cracking development sequence can be seen in Fig. 6. At low strains, cracking at the

crossover points is visible, again as black spots in the photographs. In this orientation, it is easy

for the developing matrix cracks to link up as they grow. Hence, the crack density increases

fairly uniformly. Figure 7 shows a schematic of crack formation. The crack initiation points are

4 mm

Course 1 (rib)

Course 2

Course 3

A

30

Unloaded 0.9% 1.00% 1.20% 1.60% 2.00%

Fig. 6 Crack development for the model sandwich composite tested in course direction (90°).

Applied strains are indicated

represented by A, where the yarns are joined together (crossover). All these sites coalesce to

propagate the cracks along the sample perpendicularly to load direction by running on the sides

or legs of the loops.

Fig. 7 Schematic diagram showing crack development for the model sandwich laminate with the

fabric layer in the 0° direction (Course).

In Fig. 8 the crack density can be seen and compared. In wale direction crack initiation oc-

curs at a strain of 0.85% (pre-damage) although a significant increase in crack density occurs

after a strain of 1.1%. The crack initiation strain of 0.9 % is in reasonable agreement with the

strain to failure of a single layer of knitted fabric reinforced resin tested alone, which was found

to be 0.87%. In Fig. 8 it is noticed that the crack density increases in a course direction sample

more rapidly than in samples tested at wale direction. This is due to the transversal branched-like

matrix cracks that grow almost randomly along the sample showing no steady section in the

curve.

In order to visualize the damage initiation detected by AE signals, data from crack density

was plotted against the AE event count rate. Thus, it is possible to observe in Fig. 9, for the sam-

ple tested on the wale direction, that the first AE signal appearing before the main cracking dam-

age (just before 1% strain) suggesting that a lower values of strain a kind of micro-damage is

2 mm

A

31

detected (i.e. interfacial debonding). On the other hand, Fig. 10 confirm this statement since AE

signal is located before the initiation of significant cracks at 0.85% strain when the sample is

loaded to the course direction.

Crack density for samples at course and wale directions

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Strain (%)

1/2

s m

m-1

Course Wale

Fig. 8 Crack density vs. strain for sandwich composites.

Model material in wale direction (0 o)

Crack density vs. AE event count

0

10

20

30

40

50

60

70

80

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4

Strain (%)

AE

ev

en

t c

ou

nt

rate

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1/2

s m

m-1

Fig. 9 AE event count rate vs. crack density loaded in wale direction.

Typical stress-strain curves for samples with the wale and course direction of the fabric are

shown in Figs. 11 and 12 for a commercial material, made of 5 knitted layers by RTM process,

which was tested in order to compare the behavior of a model material with a commercial com-

posite. In this case, Derekane vinyl ester was used as matrix. In general, the stress-strain curves

appear non-linear from even very small strains. Overall, the curves show a remarkable similarity

to the stress-strain curves of ductile metals. Indeed, they even show a zero “work-hardening rate”

region above a strain of 1.7%, with failure at 2%. In these opaque RTM materials, it was impos-

sible to determine the onset of cracking visually and the AE technique was used to monitor dam-

age. AE activity is shown together with the stress-strain curves. AE activity begins at quite low

32

Model material in course direction (90o)Crack density vs. AE event count rate

0

10

20

30

40

50

60

70

80

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4

Strain (%)

AE

ev

en

t c

ou

nt

rate

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1/2

s m

m-1

Fig. 10 AE event count rate vs. crack density loaded in course direction.

strains (approximately 0.2% to 0.4%). From strains of about 0.2% to 1%, the AE event count

rises slowly, but a dramatic increase is observed from 1.0% strain. For this material, it was diffi-

cult to identify the different stages of damage as was demonstrated before. However, further mi-

croscopic analysis revealed that micro-debonding were as well presented in the crossover points

detected as pre-damage before significant matrix cracking and fiber fracture. Although matrix

cracking was difficult to observe, there were indications of such damage on the surface of the

sample in the form of light-colored lines crossing the width of the coupon.

RTM material in wale direction (0 o)

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Strain (%)

Str

ess (

MP

a)

0

20

40

60

80

100

120

AE

ev

en

t c

ou

nt

rate

Fig. 11 Stress-strain curve for 5 layers RTM material tested at 0o (Wale) direction.

33

RTM material in course direction (90 o)

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Strain (%)

Str

ess (

MP

a)

0

20

40

60

80

100

120

AE

even

t co

un

t ra

te

Fig. 12 Stress-strain curve for 5 layers RTM material tested at 90o (Course) direction.

For these materials, identifying the limit of no significant damage was extremely difficult

even with the AE signal, since it increases gradually while the sample is loaded. Samples cut at

different angles from commercial RTM composite were tested in tensile loading by taking them

to a certain strain and, after reaching this point, they were unloaded and reloaded again to a

higher strain, and so on until fracture. The samples were tested at the same crosshead speed as in

the earlier work (0.5 mm/s) and stress, strain and AE data were recorded. Samples at 0° direction

were taken to 0.1% strain, unloaded, reloaded to 0.6%, unloaded, reloaded to 1.0%, unloaded,

reloaded to 1.5% strain, unloaded and then reloaded to failure. Figure 13 shows the stress-strain

behavior for these cyclic tests, and Fig. 14 shows the stress-cumulative strain results. There are a

number of features to be noted, which were common to all the specimens cyclically loaded in

this way. Firstly, there is elastic behavior when the material is loaded to 0.1% and then un-

loaded. It is difficult to see this in Fig. 13 since the two lines superimpose. Although there is

elasticity, the curves do not appear to be entirely linear even at this low strain. Secondly, loading

to 0.6% strain and unloading produces not only a small hysteresis loop, but also a residual strain

(of 0.03%). It should be noted, for comparison, that a strain of 0.6% is above the strain, at which

the change in the Poisson’s ratio during the monotonic tests suggested as the strain for the onset

of matrix cracking. Thirdly, when reloading the sample to 1% strain, the hysteresis loop is

‘closed’, in the sense that the stress passes through the previous peak stress value at a strain of

0.6% while the specimen is being loaded to a higher strain. Unloading from a strain of 1% pro-

duces a much larger residual strain (about 0.15%). Fourthly, when reloading the sample, now to

a strain of 1.5%, the hysteresis loop is this time not ‘closed’. During this reloading, the stress at

1% strain is lower (by about 3 MPa) than the value on the previous cycle. Fifthly, when unload-

ing and reloading to failure, the hysteresis loop is again not closed, and the stress reduction at

1.5% strain is now larger (about 4.5 MPa). Finally, for each successive reloading to higher

strains (above the initial strain of 0.1%) the hysteresis loops have a lower overall slope.

Figure 14 shows two acoustic phenomena, which are important in understanding the mecha-

nism of internal failure. The first one is the Kaiser effect (Ke) in which it is necessary to reach

the same former stress value to go on whit the AE signal and it is observed at low strain values.

34

Cyclic test of RTM material at 0 o

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Strain (%)

Str

ess (

MP

a)

0

20

40

60

80

100

120

AE

ev

en

t c

ou

nt

rate

Fig. 13 Cyclic stress-strain curve for 5 layers RTM material at 0° (wale) direction.

Fig. 14 Cyclic stress-cumulative strain curve for 5 layers RTM material at 0° (wale) direction.

On the other hand, Felicity effect (Fe) is observed at higher strain when reloaded, and AE

signal is detected even when the last stress value has not been reached. At the same time, when

load is released AE signal is still detected. These effects suggest that at low strain values the

knitted fabric reinforcement is receiving the load trough the matrix trying to behave elastically;

however at higher strains, the fibers are pulled out from the matrix sockets and when the load is

released there is friction between the fiber and the matrix when trying to accommodate again in

the sockets. Therefore, it is possible to establish that at low strains where Kaiser effect is de-

tected no damage is produced in the materials, and when micro-damage initiates (fiber/matrix

debonding), it originates low AE signal, which increases uniformly when matrix cracks are

35

detected, at higher strain values then Felicity effect is sensed. Finally, load takes the maternal to

fracture and fiber breakage is perceived by AE signal. For these materials, however, it is very

difficult to assure when the significant cracks initiate.

Of particular interest in this work is the relationship of the complex fiber architecture of the

knitted fabric composite to the damage accumulation under load. For this reason AE technique

was used to find out the strain value where micro-damage initiates in the composites materials.

Obtaining the data, investigation was done at a macroscopic level by cutting and polishing sev-

eral pieces in different planes in order to relate the damage initiation and propagation with the

geometry of the textile. The two principal material directions, wale and course, were investigated

since they represent the two extremes in the orientation of the knitted fabric. Finally, analysis of

a course sample loaded to a strain of 0.7%, which is just below the onset of matrix cracking in

these specimens (which occurs at 0.8%) provided further evidence of early damage initiation in

the form of cracking at the loop crossover points (Fig. 15). This represent fiber/matrix debonding

interface detected by AE signal at low strain values (Fig. 5), which some researchers [4, 5] have

identified as the first stage in the composite fracture mechanisms: interface debonding, matrix

cracking and finally fiber breakage.

Fig. 15 Evidence of crack initiation sites on crossover points.

4. Conclusions

The advantage of acoustic emission (AE) technique is shown as applied to composite materi-

als in support of the analysis of damage development at lower strain to understand the cracking

process by identifying initiation sites, cracking progression and final fracture. By AE, various

failure mechanisms such as resin matrix cracking, fiber bundle debonding and tensile fracture of

fiber bundles were distinguished. Initial AE signal (event count rate) was attribute to the fiber

bundle debonding at the crossover initiation sites, followed by visual matrix cracking on the

sample. Micro-cracks originating at the debonded fiber bundles propagated into the resin-rich

regions of the laminate and cracks coalesced until catastrophic fracture. Similar mechanism of

fracture were observed in both the model and the RTM materials that suggest the intimate effect

of the knitted fabric architecture in the final mechanical behavior of the composite. The model

sandwich laminates enabled the damage behavior and failure mechanisms to be related to the

orientation of the fabric. The observed pre-damage in the knitted fabric layer was identified for

36

the first time to be damage occurring at the loop cross-over points. The commercial RTM mate-

rial was difficult to analyze due to the high fiber volume fraction, the tendency for the fabric

layers to be crushed out-of-plane during manufacture and the opaque nature of the Derekane

vinyl ester used as matrix. However, some features of the knitted fabric architecture were identi-

fied and related with the cracking damage. Finally, the influence of the knitted fabric architecture

in determining the damage development in the composites was also shown in the results.

Acknowledgements

The author wishes to thank to the Mexican Council for Science and Technology (CONA-

CyT) that financed this research. In the same way, gratitude is expressed to Cooperative Re-

search Centre for Advanced Composite Structures Ltd from Australia for providing the textiles

used in this work.

References

1. S. Ramakrishna, N.K. Cuong, H. Hamada. (1997), J. of Reinforced Plastics and Composites,

16 (10), 994-965

2. K.H. Leong, S. Ramakrishna, Z.M. Huang, G.A. Bilbo, (2000) Composites: Part A, 31, 197-

220

3. X. Ruan, T.W. Chou, (1998) J. of Composite Materials.32 (3), 199-221.

4. S. Ramakrishna, D. Hull (1994), Composites Science and Technology, 50, 237-247.

5. J. Bohse (2004), European Working Group on Acoustic Emission (EWGAE 2004), pp. 339-

348.