ACOUSTIC EMISSION PERFORMANCE FOR DAMAGE
MONITORING OF IMPACTED FRP COMPOSITE LAMINATES
M.P. Amoroso, C. Caneva, F. Nanni, M. Valente
Dept. of Materials and Chemical Engineering, University of Rome "La Sapienza"
Via Eudossiana 18, Rome, Italy
ABSTRACT. The purpose of paper is to emphasize the AE capabilities on detection and
characterization of the impact damage of FRP composite laminates. These materials, in particular
FRP, have anisotropic and non homogeneous property that confer it a hardy predictable mechanism
of damage such as initiation, growth and propagation of failure, delamination, breaking of matrix and
fibers, debonding, pull-out and more else, as a consequence of impact. Impact in composite materials,
even if it is performed with low velocity can cause considerable and also invisible damage, so,
knowing the behavior of this material after impact is useful especially to the aeronautical industry
that uses a great deal of composite material. AE is a suitable NDT method to detect in real time the
progressive damage that occurs, giving information about nature and location of the dam. By means
of AE we have characterized the damage of composite laminates and evaluated the cumulative
damage of FRP. In the spirit of the impact field we have also analyzed many aspect of the damage
like the magnitude of the damage, the residual life of the composite.
INTRODUCTION
Excellent resistance to low velocity impact is always more and more requested in
most of polymer composite materials applications. Low velocity impacts usually are
characterized by small amplitudes and cause damages contained in very well defined
areas of the structure. Low velocity impacts are therefore pretty much different from
high velocity impacts (or ballistic impacts), which are limited to specific applications
specially in the military field. Low velocity impacts, on the contrary, happen very
frequently in many common industrial applications such as in the transportation
industry (aeronautics, railway systems, automobile and naval industry), industrial plants
and civil engineering.
Low velocity impacts on traditional monolithic materials do not cause severe
problems, since they usually happen on the material surface where the phenomenon
cause local plastic deformation, indentations or superficial microcracks. In fiber
reinforced materials, instead, the situation is completely different since usually there can
be no external (i.e. superficial) evidence of the occurred impact, but, on the contrary,
there can be severe internal damages such as delaminations, matrix and fibres fracture,
debonding and pull outs.
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
1447
Damage onset and increase can cause mechanical performance decrease in
composite structures, which can often remarkably reduce their residual life. In fact, even
if the effect of low velocity impact is initially localized and does not bring immediately
to structure failure, nevertheless such impacts are extremely dangerous since the small
damaged areas usually act as point of onset for large and fatal structural failures. This
phenomenon is, therefore, particularly dangerous so that it can bring to a drastic
limitation in the use of composite materials for many structural uses (as in the field of
aeronautics and spatial structures) where availability is the major goal and has always to
be guaranteed.
It is therefore of great importance to be able to detect and evaluate quickly the
effect of impacts on composite structures (usually made of laminates), by using non
destructive evaluation methods.
To detect impact damage (usually small and localized), Acoustic Emission AE is
particularly suitable since it is very sensitive, easy to use, can be use on large structures
and allow to perform in-field evaluations. Moreover this technique can insure a dynamic
evaluation of damage, which can point out the real effect of the damage and evaluate
the structural resistance modification of the material subjected to mechanical loads,
independently from the damage extension, localization and depth. AE can foresee
damage evolution and therefore help to evaluate critical stress that can cause damage
propagation.
AE can select and distinguish impact damage characteristics. As previously
reported, in fact, it is well known that impact damage can cause matrix and fibres
fracture, matrix-fiber debonding and fiber pullout. Such effects are easily recognized
and identified by using AE. This aspect is particularly important in structural composite
materials since the effect caused by impact damage depends on the laminate type and
composition, which include the type of matrix (polyesther or epoxy), the type of
reinforce (glass, kevlar or carbon) as well as on the stacking sequence, the type of cloth.
Purpose of the present research was to evaluate AE ability to characterize the
effects of low velocity impacts so that it was possible to evaluate damage (in terms of
decrease of mechanical resistance) and to obtain quantitative information on residual
life of a specific material after impact.
The research was carried out on laminates. At first impacts were performed then
the effect of such impact on the laminate mechanical behavior was evaluated by
performing AE measurements during tensile tests. At the end, the analysis of AE data
together to those recorded during the mechanical tests allowed to point out the presence
of a damage threshold above which the material mechanical properties start to decrease.
MATERIALS AND METHODS
Many tests have been carried out by the authors on different polymer composite
materials in order to evaluate the AE effectiveness in detecting damage on both
impacted and virgin specimens. In this paper, though, only the results achieved on
GFRP impacted laminates will be presented. The results obtained from tests carried out
on other class of polymer composite materials are described in other papers reported in
the references.
The materials involved in this research were GFRP laminates containing glass
woven roving as reinforce. The laminates, manufactured by RTM process, had these
characteristics:
1448
Matrix:
polyesther resin ( Lonza 1629NT)
Catalizator: MetiletilChetone Peroxide
Reinforce:
10 layers of glass woven roving (type K1555 510 g / m 2 )
Volume
percentagepolyesther
of fibres: resin
37,7%
Matrix:
(Lonza 1629NT)
The
samples,
120
mm
long, Peroxide
20mm wide and 3.5 mm thick, were finished with
Catalizator: MetiletilChetone
tabs to
allow the 10correct
during
Reinforce:
layers ofgripping
glass woven
rovingthe
(typemechanical
K1555 5 lOgtest.
/ m2) Moreover, two
semicircular notches were performed in the central zone of the specimens, in order to
Volume percentage of fibres: 37,7%
localize the AE signal and allow a better analysis of the acquired data. Each notch was 4
The4 samples,
120leaving,
mm long,
20mm wide
and specimen
3.5 mm thick,
wereoffinished
mm high and
mm wide,
therefore,
a new
width
16mm.with
The
tabs to part
allow
theresearch
correct was
gripping
duringinto
thedifferent
mechanical
test. Moreover, two
experimental
of the
articulated
phases.
semicircular
notches
wereimpacts
performed
in carried
the central
of the
specimens,
in order
During
the first
phase,
were
out zone
at three
different
energies:
5, to
10
localize
the
AE
signal
and
allow
a
better
analysis
of
the
acquired
data.
Each
notch
was
and 15 J by using an impact tower. The steel impactor has a hemispheric head with4 a
mm high and 4 mm wide, leaving, therefore, a new specimen width of 16mm. The
radius experimental
of 5mm. The
to be was
impacted
were
a steel plate having a
partsamples
of the research
articulated
intoplaced
differentonphases.
thickness of During
20mm.the first phase, impacts were carried out at three different energies: 5, 10
In
phase
of thetower.
experimentation,
mechanical
tests were head
carried
andthe
15 second
J by using
an impact
The steel impactor
has a hemispheric
without
a
both onradius
virginofand
impacted
samples
an Instron
8033 universal
machine
5mm.
The samples
to by
be using
impacted
were placed
on a steel tensile
plate having
a
thickness
at a strain
rate of
of20mm.
2 mm/min. At first simple tensile tests were performed in order to
the secondproperties
phase of the
experimentation,
teststests
werewere
carried
out
evaluate the Inmechanical
of the
material, thenmechanical
cyclic tensile
carried
both
on
virgin
and
impacted
samples
by
using
an
Instron
8033
universal
tensile
machine
out in order to evaluate the effect of increasing damage on both damaged and
at a strain
rate of 2Inmm/min.
At first
simple tensile
were performed
in orderthat
to
undamaged
materials.
particular,
a specific
cyclictests
procedure
was adopted
evaluate the mechanical properties of the material, then cyclic tensile tests were carried
consisted
steps ofthe
50 effect
cyclesofeach,
performed
between
0 and
σmax. This
out ofinconsecutive
order to evaluate
increasing
damage
on both
damaged
and
maximum
load
value
was
increased
each
step
following
this
procedure
(1):
undamaged materials. In particular, a specific cyclic procedure was adopted that
consisted of consecutive steps of 50 cycles each, performed between 0 and amax This
σmax-i each
= σmax-(i-1)
+ 10% σ
(1)
maximum load value was increased
step following
this
rott.procedure (1):
In all cases, strain gauges were used in the notched area to evaluate the occurring
strain.
In allacquisition
cases, strain(Vallen
gauges were
used in was
the notched
evaluate the occurring
AE
AMSY-4)
carried area
out tocontemporarily
to the
strain.
mechanical tests and to this aim four piezoelectric probes were applied on each
AE acquisition (Vallen AMSY-4) was carried out contemporarily to the
specimen.
Two probes
wereto positioned
nearpiezoelectric
the grips and
werewere
usedapplied
as guards,
mechanical
tests and
this aim four
probes
on while
each
the other
two (active
sensors)
were
placed just
andand
below
area.
The
specimen.
Two probes
were
positioned
near above
the grips
were the
usednotched
as guards,
while
total gain
was settled
at 60
dB and
theplaced
threshold
at 45 and
dB.below
The frequency
the other
two (active
sensors)
were
just above
the notchedacquisition
area. The
window
wasgain
between
100 and
300
total
was settled
at 60
dBkHz.
and the threshold at 45 dB. The frequency acquisition
window was between 100 and 300 kHz.
Impact side
Impact side
AeAe
sensors
sensorsspacing
spacing70
70 mm
mm
Toapparatus
AE apparatus
To AE
FIGURE 1. Specimen sketch.
FIGURE 1. Specimen sketch.
1449
Cumulative Counts
Cumulative Counts
Stress (MPa)
(MPa)
Stress
o
U
I
Strain
Strain
Strain
FIGURE 2 Stress (MPa) and AE cumulative counts vs strain for woven roving laminates impacted at 5 J.
FIGURE
Stress (MPa)
(MPa)and
andAE
AEcumulative
cumulativecounts
counts
strain
woven
roving
laminates
impacted
FIGURE 22 Stress
vsvs
strain
forfor
woven
roving
laminates
impacted
at 5 J.at 5 J.
RESULTS AND DISCUSSION
RESULTS
ANDDISCUSSION
DISCUSSION
RESULTS AND
Figure 2 reports the stress (MPa) and AE cumulative counts vs strain for woven
Figure 22 reports
the
stress
(MPa)
and
cumulative
counts
vs strain
for woven
theat
stress
(MPa)
andAE
AE
counts
vs strain
for woven
rovingFigure
laminatesreports
impacted
5 J. As
it is possible
tocumulative
see from this
diagram
the counts
vs
roving laminates
impacted
atat55J.J.As
it itisispossible
to to
seesee
from
thisthis
diagram
the the
counts
vs vs
roving
laminates
impacted
As
possible
from
diagram
counts
strain
curve
confirm
the
results
obtain
from
the
mechanical
test
since
a
continuous
strain curve
confirm the
results obtain
from
testtest
since
a continuous
strain
curve
obtain
fromthe
themechanical
mechanical
since
a continuous
increase
was confirm
recorded,the
butresults
seems to
to be
be more
more
sensitive:
acoustic activity
activity
starts
before
increase
was
recorded,
but
seems
sensitive:
acoustic
starts
before
increase
was
recorded,
but
seems
to
be
more
sensitive:
acoustic
activity
starts
before
any
appreciable
change
in
the
stress-strain
curve.
Therefore,
AE
activity
is
far
more
any appreciable
appreciable change
ininthe
stress-strain
curve.
Therefore,
AEAE
activity
is far
more
any
change
the
stress-strain
curve.
Therefore,
activity
is
far
more
representative of
of the
the status
status of
of the
the material
material respect
respect the
the simple
simple mechanical
mechanical test.
test.
representative
representative
the status
of thethe
material
respect
the simple
mechanical
test. of the
Figure 33ofreports,
reports,
instead,
AE mean
mean
amplitude
distribution
analysis
Figure
instead,
the AE
amplitude
distribution
analysis
of the
Figure
3
reports,
instead,
the
AE
mean
amplitude
distribution
analysis
of the
events.
The
mean
is
done
taking
the
values
at
five
different
load
level
(as
UTS
%). The
The
events. The mean is done taking the values at five different load level (as UTS
%).
events.
The
mean
is
done
taking
the
values
at
five
different
load
level
(as
UTS
%).
behavior
of
the
impacted
specimens
is
different
since
the
first
step
of
loading.
The
behavior of the impacted specimens is different since the first step of loading. The The
behavior
of the
impacted
specimens
is different
since the
firstfracture.
step ofAmplitude
loading. The
phenomenon
involved
is of
of
high energy
energy
and involves
involves
fibre
fracture.
Amplitude
phenomenon
involved
is
high
and
fibre
increases from
frominvolved
to 15
impacted
specimens,
that
there
is still
large
phenomenon
high energy
andsignificant
involves that
fibre
fracture.
increases
55 JJ to
15isJJ of
impacted
specimens,
significant
there
is
still Amplitude
aa large
contribution
of 5fibre
fibre
to15the
theJ strength
strength
ofspecimens,
materials. At
At 80%
80% of
of that
the failure
failureisthere
there
a
increases
from
J toto
impactedof
significant
there
still isis
a alarge
contribution
of
materials.
the
decrease of
of the
the
amplitude
Value,
meaning
that the
the main
main80%
phenomenon
that is
isthere
nowis a
contribution
of fibre
to theValue,
strength
of materials.
At
of the failure
decrease
amplitude
meaning
that
phenomenon
that
now
occurring of
is less
less
energetic.
AtValue,
this step
stepmeaning
delamination
debonding
and
pull-out occur.
occur.
This
occurring
is
this
delamination
pull-out
decrease
the energetic.
amplitudeAt
that debonding
the main and
phenomenon
that This
is now
behavior is
isis the
the
same
for all
allAtthe
the
impacted
compositesdebonding
but itit isis possible
possible
to select
select
theThis
behavior
for
composites
but
to
the
occurring
lesssame
energetic.
thisimpacted
step delamination
and pull-out
occur.
several
levels
of
damage
by
low
velocity
impact,
or
residual
mechanical
strength,
in the
several
levels
of
damage
by
low
velocity
impact,
or
residual
mechanical
strength,
in
behavior is the same for all the impacted composites but it is possible to select
terms
of
AE
amplitude.
several levels of damage by low velocity impact, or residual mechanical strength, in
terms of AE amplitude.
WOVEN ROVING
^ 40
Sa
!*
1 25 -
|2Q
f-15
< to
3 5tO%
40%
60%
80%
9l%
% Strength to Failure
FIGURE
t for
FIGURE 3.
3. Mean
Mean Amplitude
Amplitude over
over threshold
threshold vs
vs %
% aσrarupt
forall
alllaminates.
laminates.
FIGURE 3. Mean Amplitude over threshold vs % σrupt for all laminates.
1450
One of the aim of the present research was to find a quantitative characterization
of damage caused by low-velocity impacts in PMCs, by using AE technique coupled to
a test methodology
the material
can foreseecharacterization
its residual life
One of thethat
aimsimulate
of the present
researchservice
was tolife
findand
a quantitative
[1].of damage caused by low-velocity impacts in PMCs, by using AE technique coupled to
This
methodology,
already
investigated
thecan
authors
reported
a test
methodology
that has
simulate
the been
material
service lifebyand
foreseeand
its was
residual
life
in previous
papers
[1-3].
It
is
based
on
the
performance
of
cyclic
tests
and
correlates
the
[1].
onset or This
increase
of
damage
to
the
material
Young
modulus
decay,
as
depicted
methodology, has already been investigated by the authors and was reportedby
Equation
(2).
in previous papers [1-3]. It is based on the performance of cyclic tests and correlates the
onset or increase of damage to the material Young modulus decay, as depicted by
Equation (2).
D = 1-(E/E0)
(2)
1-(E/E
)
the 0Young
modulus of the virgin material (2)
and
In which D is a damage parameter,DE=0 is
E is the actual modulus [2]. In this research, instead, a new damage parameter is
In whichwhich
D is a is
damage
is the Young
of thebyvirgin
material
introduced
based parameter,
on acousticE0emission
data modulus
and is given
Equation
(3). and E
is the actual modulus [2]. In this research, instead, a new damage parameter is
introduced which is based on acoustic emission data and is given by Equation (3).
D = counts / total counts
(3)
D = counts / total counts
(3)
Where the term counts indicates the number of counts until a partial state and total
counts
is the
the term
number
of total
counts
fracture.
number
is also
Where
counts
indicates
the registered
number of atcounts
until Count
a partial
state and
total a
significant
related
to impact
damage phenomenon:
an high
countis number
counts isparameter
the number
of total
counts registered
at fracture. Count
number
also a
means
an extent
impact damage.
significant
parameter
related to impact damage phenomenon: an high count number
means
an extent
Diagram
of impact
figuresdamage.
4 to 7 report the trend of D parameter evaluated by
Diagram
of figures
4 tomesurements
7 report the
of Dof parameter
mechanical
test monitored
by AE
as atrend
function
σrupt. %. evaluated by
mechanical test monitored by AE mesurements as a function of am t %.
Damage vs %σrupt. forr virgin specimens
Damage vs %(?rupt. f° virgin specimens
IIIIIIIII
iiill!
ili
D
Illi;
iilll
till!
iiii
llllililliiilill
%σrupt
FIGURE 4. Parameter D as a function of % σrupt. for virgin specimen evaluated mechanically (pink line)
FIGURE 4. Parameter D as a function of % anqrt for virgin specimen evaluated mechanically (pink line)
and by AE measurements (blue line).
and by AE measurements (blue line).
1451
Damagevs
vs %a
%σrrupt.
forvirgin
virginspecimens
specimensimpacted
impactedaa55JJ
Damage
upt. for
DD D
Damage vs %σrupt. for virgin specimens impacted a 5 J
Damage vs %σrupt. for virgin specimens impacted a 5 J
%σ
rupt
rupt
%σ
rupt
%σrupt
FIGURE
5.
Parameter D
as aa function
function of
% aσnqrt
For specimens
FIGURE
5. 5.Parameter
ofof%
specimensimpacted
impactedatat5J5Jevaluated
evaluatedmechanically
mechanically
rupt. For
FIGURE
ParameterDDDas
functionof
rupt. For specimens impacted at 5J evaluated mechanically
FIGURE
5. by
Parameter
asasaafunction
%%σσ
(pink
line)
and
AE
measurements
(blue
line).
rupt. For specimens impacted at 5J evaluated mechanically
(pink
line)
and
by
AE
measurements
(blue
line).
(pink line) and by AE measurements (blue line).
(pink line) and by AE measurements (blue line).
Damage
vs
riinf. for
Damage
vsvs%cy
%σ
for
virgin
specimens
impacted
at10
10JJJ
Damage
%σrupt.
forvirgin
virginspecimens
specimensimpacted
impactedat
at
10
D
D
D
rupt.for virgin specimens impacted at 10 J
Damage vs %σrupt.
%σ
%σ
rupt
rupt
%σ
rupt
D D
D
FIGURE
6.6.6.Parameter
DDDas
%%σa^
specimens
impacted
10J
evaluated
mechanically
FIGURE
Parameter
afunction
functionof
σrupt.For
Forspecimens
specimensimpacted
impacted
10J
evaluated
mechanically
FIGURE
Parameter
asasaafunction
ofof%
For
atatat
10J
evaluated
mechanically
FIGURE
6.
Parameter
D as a function
ofline).
% σrupt.
rupt. For specimens impacted at 10J evaluated mechanically
(pink
line)
and
by
AE
mesurements
(blue
(pink
(pinkline)
line)and
andby
byAE
AEmesurements
mesurements(blue
(blueline).
line).
(pink line) and by AE mesurements (blue line). r
Damage
vs
impacted
Damage
vsvs%(?rupt.
%σ
for
specimens
atat
1515
J JJ
Damage
%σrupt.f°
forspecimens
specimensimpacted
impacted
at
15
Damage vs %σrupt.
rupt. for specimens impacted at 15 J
%σ
rupt
%σ
rupt
%σrupt
FIGURE
Parameter
asasaaafunction
function
specimens
mechanically
FIGURE
7.7.7.Parameter
DDDas
%%σa^
specimensimpacted
impactedatat15J
15 evaluated
J evaluated
mechanically
rupt. For
FIGURE
Parameter
functionofof%
σ
rupt. For specimens impacted at 15J evaluated mechanically
(pink
line)
and
AE
measurements
(blue
line).
(pink
line)
and
byby
AE
measurements
(blue
(pink
line)
and
by
AE
measurements
(blue
FIGURE
7.
Parameter
D
as a function
of line).
%line).
σ
For specimens impacted at 15J evaluated mechanically
rupt.
(pink line) and by AE measurements (blue line).
1452
Such diagrams show that a damage threshold is easily recognizable at the point at
which D start to increase from zero. Moreover, it is possible to correlate the stress (as %
arott) at which D >0 with the impact energies. In particular, damage initiation was
recorded at almost 70%arott in the case of virgin specimens, at 60%arott in the case of
specimens impacted at 5J, at 50%arott in the case of specimens impacted at 10J and at
20%arott in the case of specimens impacted at 15J. The amazing thing is that damaged
evaluated by means of AE perfectly match the trend of damage evaluated by
mechanical data.
CONCLUSIONS
The results obtained by AE analysis on impacted composite laminates to assess
the damage effects and explain the microstructural modifications due to impact confirm
and improve the conventional methods based on mechanical tests. AE even improves
the evidence of the damage kinds (delamination, matrix microfractures, fibre failure),
and it is able to evaluate the damage extension. Furthermore it is possible by AE to
define an impact energy threshold below of that impact damage is negligible. In
conclusion, AE is a very important tool to evaluate the health state of structural
composite materials for large volume applications.
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Emission Testing. Senlis-France 24th-26th May 2000.
3. F. Billi, C. Caneva, M. Valente. Damage Tolerance Assessment on Polymeric
Matrix Impacted Composites by means of Acoustic Emission. Proc. of 6th
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2001.
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CA, US A-July 1-6 2002.
1453
7. C. E. Lemaitre, J. L. Chaboche, Mecanique des materiaux solides - CNRS/ONERA
- 1988.
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