materials
ArticleDigital Image Correlation Comparison of Damaged
and Undamaged Aeronautical CFRPs During
Compression Tests
Claudia Barile 1,2, * , Caterina Casavola 1
1
2*

and Giovanni Pappalettera 1

Department of Mechanics, Mathematics and Management, Politecnico di Bari, viale Japigia, 70126 Bari, Italy;
casavola@poliba.it (C.C.); giovanni.pappalettera@poliba.it (G.P.)
IMAST S.c.a.r.l., Distretto Tecnologico Per L’ingegneria Dei Materiali Polimerici E Compositi E Strutture,P.zza Bovio 22, 80133 Napoli, Italy
Correspondence: claudia.barile@poliba.it; Tel.: +39-080-5962812

Received: 29 November 2018; Accepted: 9 January 2019; Published: 13 January 2019


Abstract: The diffusion of composite materials in aeronautical and aerospace applications is
attributable to the high specific mechanical properties they offer. In particular, the recent use of Carbon
Fiber Reinforced Polymer (CFRP) materials is highly increased. The main disadvantage in using thiskind of material is related to the possibility of including damages or defects not visible on the surface
that compromise their behavior and make their use extremely unsafe if not properly supervised.
The most conventional nondestructive techniques allow the detection of damages when they alreadycompromise the life of these materials. The use of the same techniques makes it harder to monitor
in-situ of the progress of damages, especially if they occur inside the materials. The implementationof the innovative strain analysis method, like those based on full-field measurements, could provide
additional information about the damage mechanisms by supplying the complete strain distributionof the surface of the sample. The present paper examines the mechanical behavior of two different
CFRP specimens, with and without damage, subjected to compressive load in an anti-buckling
fixture by using the Digital Image Correlation (DIC). The purpose is to measure the out-of-plane
displacements, characteristics of the compression tests, in all the points of the ROI (Region of Interest),
using a full-field and noncontact technique. The innovative aspect of this work is therefore to solve
this problem through an experimental approach with DIC 3D technique.
Keywords: Carbon Fibre Reinforced Plastic Materials (CFRP); compression tests; Digital Image
Correlation (DIC); buckling

1. Introduction
Composite materials are carried out by combining two or more materials with dissimilar
mechanical and physical properties. Provided the matching is well done, the final material will havebetter overall properties if compared to the parents. A well-consolidated class of composite materials is
obtained by using a matrix material combined with a reinforcing one. In this way, it is possible to obtaina continuous material with increased capability of loading transmission and higher resistance. From aclassification point of view composites can be considered anisotropic, nonhomogeneous materials,
in which each of the constituting materials is inextricably connected with the others. Carbon Fiber
Reinforced Plastics (CFRP) is one of the most interesting composite class of materials and it will be
specifically analyzed in this paper.
The great interest in CFRPs is driven mainly by their valuable mechanical properties [1–3]. In fact,
they combine high stiffness and high strength to low density, low crack sensitivity, and good properties

Materials 2019, 12, 249; doi:10.3390/ma12020249

www.mdpi.com/journal/materials

Materials 2019, 12, 249

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under fatigue loading. At the same time, they are appealing for applications where vibration dumping
is required [4]. Another point of strength of these materials is connected with their high-dimensional
stability [5]. In fact, due to the low thermal expansion coefficient, they allow the obtainment of stablestructures, even if a large range of exercise temperatures must be taken into account.
Epoxy resins are typical materials for obtaining the matrix [6]. This is a family of thermosetting
polymers quite interesting because of their low coefficient of reticulation retention and the absenceof sub-product consequent to the curing process. Moreover, epoxy resins are characterized by goodmechanical properties, good environmental resistance, good adhesion to many different materials,
and a low coefficient of thermal conductivity [7]. In addition, it is worth noting that this material ischaracterized by a considerable strength-to-density ratio (SWR). This property, in particular, makes thiskind of material of great interest in the aeronautical and automotive fields and, more in general,
wherever lightweight of structure is a crucial point in the designing process.
Due to the fact that CFRP are, in general, nonhomogeneous and anisotropic, the mechanicalresponse of these materials is dependent on the relative orientation of the applied load with respect
to the fibers. Traditional approaches of experimental characterization can fail in capturing thecomplex behavior of such a kind of materials, so that modern approaches based on full-field opticalmethods [8–11] successfully employed in other applications where nonhomogeneous materials areanalyzed [12–15] can be used for CFRP, as will be illustrated in this paper. More recently, approaches
based on Digital Image Correlation (DIC) were applied to these kinds of problems [16–18]. It worth
noting that the possibility to have a high-resolution full description of the behavior of the component
opens the possibility to make numerical modelling more accurate and refined, so to improve andextend capabilities of the whole design process [19,20].
In this work, in particular, the stereo DIC technique was implemented. In this technique, a random
speckle pattern is applied to the surface of the component under testing. During the test, the variationof the speckle pattern is followed by two CCD (Charge-Coupled Device) cameras observing thesample from two different directions. Correlation of the speckle pattern at each stage of the test allows
obtaining the full 3D displacement field (x,y,z) for each of the point in the region of interest [21].
In this way, it is possible to determine to in-plane strain field, as well as the out of plane displacement
component that occurs, for example, in presence of buckling phenomena.
In this work two types of CFRPs are analyzed: One is a laminate made up by multi-plies CFRP,
the second is a foam sandwich included in two sheets of CFRP, the skin. Specimens of the two materials
were subjected to compression load, as described by the Compression After Impact (CAI) standard [22].
According to the aeronautical requirements, and to better describe the response of both materials,
two types of initial conditions of the specimens were considered: As delivered, and with an impacted
damage [23]. Three specimens per type and condition were tested.
2. Materials and Methods
2.1. Material Description and Preparation for Measurements
In this study, two diverse types of CFRP were analyzed. The first is a laminate made by multi-plies
epoxy resin reinforced by carbon fibers. The second is a sandwich constituted by a foam included in
two skins of CFRP. In both cases the specimens were tested under uniaxial compression conditions,according to the CAI Test (ASTM 7137) [22]. According to the standard, the width and the heightof both the materials’ specimens were 150 mm × 100 mm, while the thicknesses were, respectively,
5 mm for the laminates and 3 mm for the foams.
According to the producer’s reference, the layout of the laminate’s plies is reported following:•[45F/0T/0T/45F/0T/0T/0F/0T/0T/0F/0T/0T/45F/0T]s where F = Fabric, T = Tape.
On the other hand, the layout of the foam sandwich is indicated following:•[45F/0F/FOAM 3mm/0F/45F] where F= Fabric.

In order to allow strain field analysis by DIC, a random speckle pattern was preliminarily
paintedover theto
surfaceof the samples
to be analyzed.
donewasby spraying
Accordingthe standard,
the evaluationof theThis
localwasstrainobtained theby specimen
applying with
four
electricalstrain spraygaugesto(Figure
two for eachsideof the samples
a white matteobtain 1),
a uniform
white
backgroundand,[22].
successively, by spraying black
speckleson thetosurface2). analysis by DIC, a random speckle pattern was preliminarily
In orderallow (Figure
strain field
paintedoverthesurfaceof the samples
to be analyzed.
was doneby spraying
the specimen
The
aim
of
approach
is to compare
the local Thisbehaviorof materials
recordedby the strain
Materials
2019,
12,
249this3withof 14
thesprayoverall
DIC.background and, successively, by spraying black
agauges
whitewithmatte
to behavior
obtain a obtained
uniform bywhite
Compression
test weremadespeckleson the surface(Figure2). by following the standard ASTM 7137 [22]. Tests were performed
Specimensofbothmaterials
inas delivered
andwith
aftera thetests.
at 1.25mm/minatestedSCHENCK
servo-hydraulicmachine250impact
kNthe
load
cell.
The
aim of displacement
this approachrate
is weretooncompare
thecompression
local
behaviorof materials
recordedbystrainAccordingtothestandard,
theevaluation
of
the
local
strainwasobtainedbyapplying
Initially,withpositioningof thespecimen
was donewith great care to avoid the presence of torsionalfour
andgaugesthe overallbehavior
obtainedby DIC.
electricalstrain gauges(Figure1), twofor each sideof the samples
Compression
test weremadeby following
the standard
ASTM[22].
7137 [22]. Tests were performed
bendingloads.
at 1.25 mm/min displacement rate on a SCHENCK servo-hydraulic machine with a 250 kN load cell.
Initially, positioning of the specimen was done with great care to avoid the presence of torsional andbending loads.

Figure 1. Strain gages application on composite materials [dimensions in mm].
Figure 1. Strain gages application on composite materials [dimensions in mm].

In order to allow strain field analysis by DIC, a random speckle pattern was preliminarily painted
over the surface of the samples to be analyzed. This was done by spraying the specimen with a whitematte spray to Figure
obtain1.a Strain
uniform
whitebackgroundand, successively,by spraying
black speckles on
gages
applicationon composite
materials [dimensionsin mm].
the surface (Figure 2).

Figure 2. Real components sprayed for Digital Image Correlation (DIC) analysis.

2.2. Configuration of the Measurement Chain

Figure 2. Real components sprayed for Digital Image Correlation (DIC) analysis.

Figure 2. Realcomponents sprayed
Digital Image
analysis.
The measurementof displacements
and fordeformation
onCorrelation
the entire (DIC)
surfaceof the sample was
The aimof thisof
approachto compare
local behaviorof materials
recorded
the strain
obtainedby means
the DIC issystem
Dantecthe
Dynamics
Q400 (Dantec
Dynamics
A/S,bySkovlunde,
2.2. Configurationof the Measurement
Chain by DIC.
gaugeswiththeoverallbehavior
obtained
Denmark). It is equipped with ISTRA 4D software (Dantec Dynamics A/S, Skovlunde, Denmark ),
were
made
by following
theManta
standard
ASTM
7137
Tests
were
at
and itCompression
includestwotest1628
pixelsGigE
CCD
by AVT(Allied
VisionTechnologies
GmbH,
The
measurementof× 1436
displacements
anddeformationontheentire[22].
surfaceof
theperformed
samplewas
1.25
mm/mindisplacement
rate
on
a
SCHENCK
servo-hydraulicmachine
with
a
250
kN
load
cell.
obtained by means of the DIC system Dantec Dynamics Q400 (Dantec Dynamics A/S, Skovlunde,Initially,
positioningof the withspecimenwaswith (Dantec
great careto avoid theof torsional
andDenmark).
It is equipped
ISTRA
4DdonesoftwareDynamics
A/S,presence
Skovlunde,Denmark),
bendingloads. two 1628 × 1436 pixels GigE CCD Manta by AVT (Allied Vision Technologies GmbH,and it includes

2.2. Configuration of the Measurement Chain
The measurement of displacements and deformation on the entire surface of the sample wasobtained by means of the DIC system Dantec Dynamics Q400 (Dantec Dynamics A/S, Skovlunde,Materials 2018, 11, x FOR PEER REVIEW

4 of 14

Stadtroda, Germany) industrial cameras, equipped with Ricoh 16 mm lenses based the extensions ofMaterials
12, 249analysis. The two cameras are mounted on the same support bar in order
4 of to14
the area2019,
undercompensate vibrations. Image acquisition is triggered by a synchronization unit including National
Instruments® NIDAQ 9171 acquisition card (National Instruments Italy S.r.l., Assago, Italy), which
Denmark). It is equipped with ISTRA 4D software (Dantec Dynamics A/S, Skovlunde, Denmark),
converts the acquired signal analog to digital. The synchronization units are connected both with theand it includes two 1628 × 1436 pixels GigE CCD Manta by AVT (Allied Vision Technologies GmbH,loading machine and a PC dedicated to the system management. The starting of the acquisition was
Stadtroda, Germany) industrial cameras, equipped with Ricoh 16 mm lenses based the extensions ofsynchronized with the starting of the test by using an output TTL (Transistor-Transistor Logic) signal
the area under analysis. The two cameras are mounted on the same support bar in order to compensate
from the loading machine. The signal was active as soon as the test started and was used to trigger
vibrations. Image acquisition is triggered by a synchronization unit including National Instruments®
the synchronization unit. A frequency of 1 Hz was set for the tests. A led source is used to stably and
NIDAQ 9171 acquisition card (National Instruments Italy S.r.l., Assago, Italy), which converts theevenly illuminate the area to be analyzed. By using this kind of illumination, higher intensity stability,
acquired signal analog to digital. The synchronization units are connected both with the loadingin time, is achieved. This results in a more stable correlation process. Initial tuning of the illumination
machine and a PC dedicated to the system management. The starting of the acquisition waslevel was performed in order to optimize the acquisition by avoiding both saturation and low
synchronized with the starting of the test by using an output TTL (Transistor-Transistor Logic) signal
illuminated pixels in the whole object area. An optic with a small focal length was chosen due to thefrom the loading machine. The signal was active as soon as the test started and was used to trigger
possibility of capturing suitably large test specimens and to monitor the whole component (Figure
the synchronization unit. A frequency of 1 Hz was set for the tests. A led source is used to stably
3).
and evenly illuminate the area to be analyzed. By using this kind of illumination, higher intensity
Settings for the DIC apparatus are as follows:
stability, in time, is achieved. This results in a more stable correlation process. Initial tuning of the• Spanlengthcameras
40 cm
illuminationlevel
wasbetween
performed
in orderto optimize the acquisition by avoiding both saturation• illuminatedObject’s distancethewhole
cameras
50 cm
and low
pixelsfrom
in theobjectarea. An optic with a small focal length was chosen• theCameras’
Inclination
21.8°
due topossibilityof capturing
suitably large test specimens and to monitor the whole component• 3).Diaphragm opening f/8(FigureFigure 3. Overview of experimental set-up.

Figure 3. Overview of experimental set-up.

Settings for the DIC apparatus are as follows:

The compressive
fixtureused40for•
Span
length between
camerascm the anti-buckling tests is shown in Figure 4. It includes
adaptable
retainingplates
(base
and
top
•
Object’s distance from the cameras 50plates,
cm and two slide plates) for supporting the specimen
and prevent buckling during theload
application.
The fixture allows regulation for considering small•
Cameras’ Inclination 21.8◦
variations in specimen sizes. The side plates are shorter than the specimens to ensure the proper•
Diaphragm opening f/8
sample absorption of the load. The configuration of the features could deeply affect the test results.
In thestandard
test fixture,
the forbase
the top supports
provideno clamping
but only
someThe
compressive
fixture used
theandanti-bucklingtests is shownin Figure
4. It includes
adaptable
restraints plates
to local(baseout-of-planerotation.
retaining
and top plates,
and two slide plates) for supporting the specimen and prevent
buckling during the load application. The fixture allows regulation for considering small variations inMaterials 2019, 12, 249

5 of 14

specimen sizes. The side plates are shorter than the specimens to ensure the proper sample absorptionof the load. The configuration of the features could deeply affect the test results. In the standard
test fixture, the base and the top supports provide no clamping but only some restraints to local
out-of-planerotation.
Materials 2018, 11, x FOR PEER REVIEW
5 of 14

Figure 4.
4. Edge-constraint
Edge-constraint structure
structure adopted
adopted inin thethe experiment.
experiment.
FigureAccording to the standard [22], the specimens used for that kind of test were previously
Method D7136
D7136 [23][23] priorprior toto application
application ofof compressive
compressive
subjected to drop-weight impact per Test Method
wasplacedonon
a fixturebase
andandalignedwiththeforce. As
As prescribedprescribedby
bythe
thestandard,
standard,thethespecimen
specimenwasplaceda fixturebasealignedwith
dropping
axisaxis
by by
using
guiding
pins.
Once
the
dropping
using
guidingpins.
Oncefinished,finished,thethealignmentalignmentoperations
operations thethe specimen was
clamped bybyfourfourrubber
rubberterminatedtoggles.
Velocity
ofimpactor
the impactor
was determinedby sensors
opticalclampedterminatedtoggles.
Velocityof thewas determinedby opticalsensors (manufacturer,city,placedcountry)
placedalong thedropping
axis todeterminethe(manufacturer,city, country)
alongthe droppingaxis
to determine
accurately
the accurately
impact energy.
impact energy.
Experiments
were
led at room
two levelsof energy,50respectively,
50 J
Experiments
were
led at room
temperature
at temperature
two levels ofatenergy,
respectively,
J for CFRP and
forJ CFRP
and 10
J for wasfoams.
Impactbywasobtained by aimpactor
hemisphericalimpactorcity,(manufacturer,city,
10
for foams.
Impactobtaineda hemispherical(manufacturer,
country) having
country)
16 mmof diameter,
as prescribedby thestandard.
The systemreboundcatchersystemwas
16
mm ofhaving
diameter,as prescribedby thestandard. The
reboundcatcherwas
activated
duringactivated
during
the
test
to
avoid
reboundofthe
impactoronthetestedsample.
These
drop
tests
the test to avoid rebound of the impactor on the tested sample. These drop tests are most usedareinmostused in the
aeronautic
field forthe
simulating
impact ofwith
a structureWiththe newtheaeronautic
field
for simulating
impact ofthea structurebirds. with
With birds.
the new
composite
composite thismaterials,of greatfor
interest
for industry
to evaluate
the delaminationmaterials,
aspectthis
is ofaspect
greatisinterest
industryin order into orderevaluatethe delamination
effects
effects occurring.
Moreover,they generate
a damageon the specimens
whoseandshape
and dimensions
occurring.
Moreover,they generate
a damageon the specimens
whose shapedimensions
dependdependon thelevel
energyleveltest,of thetest, as on
well
asstructuralon the structural
lay-upof the materials.
Accordingonthe energyof theas wellthe
lay-up of
the materials.
Accordingto theto the producer’s
requirements,
the indicatedabove indicatedenergyset
levelssettwofor materials
the two materials
were
producer’s
requirements,
the aboveenergy levels
for thewere selected,
selected,
after preliminary
tests, toaensure
BarelyImpact
Visible Damage
Impact Damage
1 mm
in
after
preliminary
tests, to ensure
Barely aVisible(BVID) (BVID)
of aboutof1about
mm in
depth
depth (Figure5).extension
The extensionof damaged
the damaged
1 cm
boththetheanalyzed
analyzedclassclass ofof materials.
(Figure5). Theof theareaarea
waswas
1 cm
forforbothmaterials.
The BVID
BVID is the name given to damage in a composite material caused by a low-velocity impact fromThe
objects. Low-velocity
Low-velocity impacts
impacts cause
cause only
only minor
minor damage
damage toto the surface of the composite, which isvisualinspection,
hence
the term
BVID.
However,they cangenerally not
not readilyreadilydetectabledetectablebyby
visualinspection,
hence
the term
BVID.
However,theycausecanstructurally
considerable
damageinternallywithinlaminatecomposites,
cause
structurally
considerable
damageinternallywithinlaminatecomposites,suchsuchasas delamination,
delamination,
pullout. The
matrix cracking, fiber fracture, and fiber pullout.
The aimaim ofof thethe CAI tests was to evaluate thecompression residual
residual strength
strength properties
properties ofof CFRP.
CFRP.
compression

The BVID is the name given to damage in a composite material caused by a low-velocity impact fromobjects. Low-velocity impacts cause only minor damage to the surface of the composite, which is
generally not readily detectable by visual inspection, hence the term BVID. However, they can cause
structurally considerable damage internally within laminate composites, such as delamination,
Materials 2019,12, 249 fiber fracture, and fiber pullout. The aim of the CAI tests was to evaluate
6 of14
matrixcracking,
thecompression residual strength properties of CFRP.

Materials 2018, 11, x FOR PEER REVIEW

6 of 14The 3D system had acquired a series of images representing the displacement off the plane (zdisplacement). It recorded, for the entire duration of the test, all the configurations of the component
Figure5.
Shapeof the damage created on each specimen.
from the first undeformed
one 5.
toShapethe breakFigureof theoccurrence.
damage created on each specimen.

The 3D system had acquired a series of images representing the displacement off the plane
3. Results and Discussion(z-displacement). It recorded, for the entire duration of the test, all the configurations of the component
6–9, the out-of-plane
field obtained at different instant times during the
fromIntheFigures
first undeformed
one to thedisplacement
break occurrence.
uniaxial compression tests on CFRPs and foams, damaged and undamaged, are shown. Main analysis
3.
Results
settings
forandthisDiscussion
evaluation are listed as follows: 3D residuum lower that 0.4 pixels, facets size 19
pixels,accuracy
0.1out-of-planepixels [16]. Adisplacement
total of 1180 field
grid points
areatcomputed
on average
the ROI.
In andFigures
6–9, theobtaineddifferent instanttimesinduring
theReferringtothelaminate
specimens
and
from
the
analysisoftheout-of-plane
displacements,
uniaxial compression tests on CFRPs and foams, damaged and undamaged, are shown. Main analysis
several for
considerations
canarebelisteddone.
it canbe observed
that,
the fixture
and 19thelowsettings
this evaluation
as First,
follows:
3D residuum
lower
thatboth
0.4 pixels,
facets size
pixels,displacement
ratepixels(1.25 mm/min)contributeto keep
lowarethecomputed
amount ofon
theaverageout-of-planeandaccuracy 0.1[16]. A totalof 1180 grid
pointsin the displacement
ROI.
in the initial part of the test, for both the CFRP and foam components.

CROSS SECTIONANALYZED

Figure 6. Out-of-plane displacement of the undamaged Carbon Fiber Reinforced Plastics (CFRP)
Figure 6. Out-of-plane displacement of the undamaged Carbon Fiber Reinforced Plastics (CFRP)
recorded by DIC at t = 0 s (A), t = 80 s (B) and t = 116 s (C).
recorded by DIC at t = 0 s (A), t = 80 s (B) and t = 116 s (C).

CROSS SECTION

ANALYZED

Figuredisplacement of the undamaged Carbon Fiber Reinforced Plastics (CFRP)7 of 14
Materials
2019, 6.
12,Out-of-plane249
recorded by DIC at t = 0 s (A), t = 80 s (B) and t = 116 s (C).

CROSS SECTIONANALYZED

Figure 7. Out-of-plane displacement of the damaged CFRP recorded by DIC at t = 0 s (A), t = 60 s
(B)
t = 101s (C).
andandt = 101s (C).

Figuredisplacement of the damaged CFRP recorded by DIC at t = 0 s (A), t = 60 s (B)Materials
2018,7.11,Out-of-plane
x FOR PEER REVIEW
7 of 14CROSSSECTIONANALYZED

Figure 8. Out-of-plane displacement of the undamaged foam recorded by DIC at t = 0 s (A), t = 45 s
Figure 8. Out-of-plane displacement of the undamaged foam recorded by DIC at t = 0 s (A), t = 45 s(B), t = 60 s (C).
(B), t = 60 s (C).

Referring to the laminate specimens and from the analysis of the out-of-plane displacements,
several considerations can be done. First, it can be observed that, both the fixture and the low
displacement rate (1.25 mm/min) contribute to keep low the amount of the out-of-plane displacementin the initial part of the test, for both the CFRP and foam components.

Figuredisplacement of the undamaged foam recorded by DIC at t = 0 s (A), t = 45 s8 of 14
Materials
2019,8.
12,Out-of-plane
249(B), t = 60 s (C).

CROSS SECTION
Materials 2018, 11, x FOR PEER REVIEWANALYZED

8 of 14

9. Out-of-planeOut-of-plane
displacement
thedamaged
damagedFoam
recordedt 0=s0(A),s (A),= 55s
therecorded
byby
DIC
at at
t =sandwich.
t = t55s(B) that,valueFigure
corresponds
to thedisplacement
maximumofof
resistanceof Foamthefacesheets
ofDIC
the
After
(B)
t =s70s (C).
andandt = from70(C).
differently
theprevious reported case load is still recorded as a consequence of the presence offoam which is, in this stage, still capable to transmit load. Load starts to increase again at time t = 122
Initial settlement
settlement ofofthe
theloading
loading plateis observablein thefirst
20 thes oftest,
theastest,
as showninis observablein thefirstof
20the
s of
shown
Figures dueInitial
to the reached contact
between plate
the upper
and the lower
part
fixture.
It should
be in
observed
Figure
10A.
From
that
point,
onload
startstobeeffectivelytransmittedtothesample.
The
CFRP
10A.
point, appearance
on load starts
be effectively
to the bysample.
The CFRP
laminates
that, Fromin thisthat
situation,of toinstability
is not transmitted
directly inferable
the temporal
load
law.
laminates
begin to collapse
after
80 s (inflexion
point)
from
the
start
of
the
test,when
whenanan out-of-plane
beginHigh-definition
to collapse
after
80
s
(inflexion
point)
from
the
start
of
the
test,
out-of-planemaps of quantitative performance of out-of-plane displacements collected bydisplacementof 800µm
recordedby
DIC system.
system. After
After 116
116 s, out-of-plane
out-of-plane displacements
displacements
displacement
800techniqueμm isis clearlyclearly
recordedby thethe DIC
means of the of
DIC
allow
an immediate
understandingof thes,global
and local behavior of
becomemore
evident
and
they
reach
a
maximum
value
of
3200
µm,
while
the
specimenbrokesoon
becomemore evident
and
they
reach
maximum
value of 3200
whileresponses
the specimen
broke
soon
the components,
beforeandafter
the adamage,highlighting
the μm,differentof thematerials
after,
namely
at
120
s.
The
abrupt
cut
of
curve
is
causedbythefailureofthecomponent.
Thishappened
after,
namelyat120
seconds.
The
abrupt
cut
of
curve
is
causedbythefailureofthecomponent.
This
investigated.
for
a
load
equal
to
−
200
kN.
happened for a load equal to −200 kN.
Similar analysis, but different considerations, can be done for the foam specimens, as shown in[s][s]stage of adjustment of the loading fixtureTime
Figure 10B. In this case, anTime
initialis observed.
Starting from
0
50
100
0
50
100
150
t = 45 s, load is effectively transmitted to the specimen and a linear trend is followed until a rapid0
collapse of 0the specimens occurred at t = 63 s, when the applied load reached a value of −6.6 kN. This

Load [N]Load [N]-40000-80000-120000

-2000
-4000
-6000

-160000-200000

-8000
(A)

(B)

Figure 10.
10. Load
Load law
law evolution
evolution ofof thethe machinemachine forfor thethe CFRP
CFRP specimen
specimen (A)(A) and
andfoamfoam composite
composite(B).
(B).
FigureNext, a analysis,series of plots
reportingthe normal strain
y, thestrainand the displacement
Similarbut different
considerations,can alongbe donefor shear
the foamspecimens,
as shown inalong z10B.
for CFRP
foam,
respectively,
will be presented.
They arefixture
representativeof whatobtained
FigureIn thisandcase,
an initialstage of adjustmentof the loading
is observed.
Startingfrom
thes,three
oftransmitted
each combinationmaterial/damage
condition.
ton= 45
loadreplications
is effectively
to the ofspecimen
and a linear
trend is followed until a rapid
Theoflongitudinal
deformationcross
sectionof theCFRP aspecimens
is6.6
shown
collapsethe specimens
occurredyy
atalongt = 63thes, medianwhen theappliedload
reachedvalue of −kN.
in Figureunder conditions
maximum
deformation.
Cross-section
waschosenin orderto avoidThisvalue11,
corresponds
to the ofmaximum
resistanceof theface sheets ofthesandwich.
After
that,
regions where no correlation was observed as shown in Figure 7. The * indicates areas with noinformation due to the impossibility of recognizing speckle pattern by the DIC system. The lack of
information is caused by the shadow zones created by the fixture.
A different behavior is clearly observable for the damaged and the undamaged components: themean values of maximum load are −102714 ± 8286 N and −192674 ± 10081 N, respectively, while theMaterials 2019, 12, 249

9 of 14differently from the previous reported case load is still recorded as a consequence of the presence offoam which is, in this stage, still capable to transmit load. Load starts to increase again at time t = 122 s
due to the reached contact between the upper and the lower part of the fixture. It should be observed
that, in this situation, appearance of instability is not directly inferable by the temporal load law.
High-definition maps of quantitative performance of out-of-plane displacements collected bymeans of the DIC technique allow an immediate understanding of the global and local behaviorof the components, before and after the damage, highlighting the different responses of thematerials investigated.
Next, a series of plots reporting the normal strain along y, the shear strain and the displacement
along z for CFRP and foam, respectively, will be presented. They are representative of what obtainedon the three replications of each combination of material/damage condition.
The longitudinal deformation εyy along the median cross section of the CFRP specimens is shownin Figure 11, under conditions of maximum deformation. Cross-section was chosen in order to avoid
regions where no correlation was observed as shown in Figure 7. The * indicates areas with noinformation due to the impossibility of recognizing speckle pattern by the DIC system. The lack ofMaterials 2018, 11, x FOR PEER REVIEW
9 of 14
information is caused by the shadow zones created by the fixture.

15000
CFRP undamaged

Normal strain [m]*

CFRP damaged

*

10000

5000

0
0

15

30

45

60

75

90

105

120

135

150

-5000

-10000-15000Specimen Length [mm]Figure 11. Maximum normal strain εyy for damaged and undamaged CFRP.
Figure 11. Maximum normal strain yy for damaged and undamaged CFRP.

A different behavior is clearly observable for the damaged and the undamaged components:
In Figuremaximumshear
strainfor ±damagedis alsoreported.
the meanvalues12,
ofthemaximumload
are −102,7148286 N andand undamaged−192,674 ± CFRPs
10,081 N,
respectively,No
pointsofmaximumshear
strainconcentration,
along
the
analyzedcross-sections,areobservable
while the mean values of maximum CAI stress are −202 ± 15 MPa and −385 ± 20 MPa, respectively.
for theundamaged
CFRPs. displays
For the damaged
very
is observedThe
undamaged
specimena normalCFRPs,
strain instead,negativea inthedifferent
outlyingbehavior
areas, showingawithaclearlyevident
maximumatthecenterofthecomponent
withapositivestrainof
5000
μm.
symmetrical shape with respect to the middle part of the component. This is in accordance with
Rupturethe component
occurs component
in correspondenceof the areacolumnof maximum
shear
strain. A
what
was ofexpectedfor undamaged
under fixed-fixed
constraints
conditions.
symmetric
behavior
in
the
lower
part
of
the
samplewithrespect
to
the
upper
part
is
observed,
The damaged composite undergoes a more evident buckling deformation in the center of its structure.
indicating
the12,
presenceof a relativerotation
two parts
duringthe test.CFRPs is also reported.
In Figurethe maximum
shear
strain offorthedamagedandundamaged
No points of maximum shear strain concentration, along the analyzed cross-sections, are observable
5000
for the undamaged CFRPs. ForCFRP
the damaged
CFRPs, instead, a very different behavior is observed
undamaged* maximumwith a clearly evident
at the center of the component with a positive strain*of 5000 µm.
CFRP damaged
Rupture of 4000
the component occurs in correspondence of the area of maximum shear strain. A symmetricn [m]

3000
2000

Figure 11. Maximum normal strain yy for damaged and undamaged CFRP.

In Figure 12, the maximum shear strain for damaged and undamaged CFRPs is also reported.
No points of maximum shear strain concentration, along the analyzed cross-sections, are observablefor the2019,
undamaged
CFRPs. For the damaged CFRPs, instead, a very different behavior is observed
Materials
12, 249
10 of 14with a clearly evident maximum at the center of the component with a positive strain of 5000 μm.
Rupture of the component occurs in correspondence of the area of maximum shear strain. A
behavior in the lower part of the sample with respect to the upper part is observed, indicating thesymmetric behavior in the lower part of the sample with respect to the upper part is observed,
presence of a relative rotation of the two parts during the test.
indicating the presence of a relative rotation of the two parts during the test.

5000
CFRP undamaged

*

4000

*

CFRP damaged

Shear strain [m]

3000
2000
1000
0
0

15

30

45

60

75

90

105

120

135

150

-1000
-2000
Specimen Length [mm]-3000

Figure 12. Maximum shear strain εxy for damaged and undamaged CFRP.

FigureMaximum shear strain xy for damaged and undamaged CFRP.
Materials 2018, 11, x FOR
PEER12.
REVIEW

10 of 14

The off-plane displacement behavior allows to localize the buckling (Figure 13). For the damaged
The off-plane displacement behavior allows to localize the buckling (Figure 13). For the
CFRPs, it results located exactly at the center, reaching a value w equal to 750 µm at 75 mm along thedamaged CFRPs, it results located exactly at the center, reaching a value w equal to 750 µ m at 75 mm
y axis, and a value of w equal to 3200 µm for the undamaged CFRP.
along the y axis, and a value of w equal to 3200 µ m for the undamaged CFRP.

3500

CFRP undamaged

*

3000

*

CFRP damaged

Displacement [mm]

2500
2000
1500
1000
500
0
0

15

30

45

60

75

90

105

120

135

150

-500
Specimen length [mm]Figure 13. Out-of-plane displacement w for damaged and undamaged CFRP.
Figure 13. Out-of-plane displacement w for damaged and undamaged CFRP.

In Figure 14, the trend of longitudinal deformation along the cross-section for the foam
specimens is reported under conditions of maximum longitudinal deformation. The maximum
values of yy along the median cross section are, respectively, −8000 με for the undamaged foam and−9700 με for the impacted foam. The correspondent mean values of maximum load are −6618 ± 206
N and −4178 ± 413 N, respectively, while the mean values of maximum CAI stress are −84 ± 3 MPa

Materials 2019, 12, 249

11 of 14In Figure 14, the trend of longitudinal deformation along the cross-section for the foam specimens
is reported under conditions of maximum longitudinal deformation. The maximum values of εyyalong the median cross section are, respectively, −8000 µε for the undamaged foam and −9700 µε
for the impacted foam. The correspondent mean values of maximum load are −6618 ± 206 N and−4178 ± 413 N, respectively, while the mean values of maximum CAI stress are −84 ± 3 MPa andMaterials 2018, 11, x FOR PEER REVIEW
11 of 14−53 ± 5 MPa, respectively.

15000
FOAM undamaged*

FOAM damaged

*

10000

Normal strain [m]

5000
0
0

15

30

45

60

75

90

105

120

135

150

-5000

-10000-15000-20000Specimen Length [mm]Figure 14. Maximum normal strain εyy for damaged and undamaged foam.
Figure 14. Maximum normal strain yy for damaged and undamaged foam.

Shear strain [m]

The maximum deformations of the component are not recorded at the center. It is noticed that the
not impacted specimens buckle in the lower part while the impacted foams reached the buckling in
1,000
the upper part.
FOAM undamaged
In Figure 15, the*maximum shear strain recorded under conditions of maximum deformation
is*
FOAM damaged
shown. It can be observed that the shear strain, for the undamaged foam, is quite uniform along the500
plotted cross section. In the case of damaged foam, we can observe points of shear strain concentration
along the analyzed cross section; in particular high negative values of shear strain are recorded incorrespondence of the upper part of the component reaching a minimum value of γxy of −1800 µm,
0 to the region where skin instability occurs.
that corresponds
0
15foam,30the graph
45 of6075
90 displacement105
120 (Figure
135 16)150
For the undamaged
the out-of-planeshows amaximum value of w equal to 1400 µm in correspondence of a length of 120 mm, just before therupture at t-500= 62 s.

-1,000-1,500-2,000Specimen Length [mm]Figure 15. Maximum shear strain xy for damaged and undamaged foam.

For the undamaged foam, the graph of the out-of-plane displacement (Figure 16) shows amaximum value of w equal to 1400 μm in correspondence of a length of 120 mm, just before therupture at t = 62 s.
In the damaged foam, a globally higher out-of-plane displacement field is observed, reaching a

-20000

Specimen Length [mm]Materials 2019, 12, 249Figure 14. Maximum normal strain yy for damaged and undamaged foam.

1,000

FOAM undamaged*

12 of 14*

FOAM damaged

Shear strain [m]

500

0
0

15

30

45

60

75

90

105

120

135

150

-500

-1,000-1,500Specimen Length [mm]-2,000Materials 2018, 11, x FOR PEER REVIEW

12 of 14

FigureFigure15.
15.Maximum
Maximumshear
shearstrainstrainγxyxy forfor damaged
damaged andand undamaged
undamaged foam.
foam.

2500

Displacement [mm]For the undamaged foam,FOAM
the graphof the out-of-plane displacement (Figure 16) shows aundamaged
maximum value of w
1400 μm
incorrespondence of a length of 120 mm, just* before the* equal to FOAM
damagedrupture at t2000= 62 s.
In the damaged foam, a globally higher out-of-plane displacement field is observed, reaching amaximum 1500
of w = 2000 μm in the upper area of the specimen.

1000
500
0
0

15

30

45

60

75

90

105

120

135

150

-500
-1000
-1500
Specimen length [mm]Figure 16. Maximum out-of-plane displacement w for damaged and undamaged foam.
Figure 16. Maximum out-of-plane displacement w for damaged and undamaged foam.

In the damaged foam, a globally higher out-of-plane displacement field is observed, reaching a
It is worth
notingthat,infor
reported
phenomenon of elastic instability occurs even if
maximumof w =
2000 µm
theallupper
areadata,of thethe
specimen.
the specimens
were constrainedby fixturedata,
properly
designed toofavoid
It is worth noting
that, for all reported
the phenomenon
elasticinstability.
instability However,occurs eventhe
ifcriticalload
ishigherthanthat
referencedbythebibliographyforasimplyanisotropic
the specimens were constrained by fixture properly designed to avoid instability. However, supported
the criticalplateissubject
to uniaxial
compression
Moreover,worthanisotropic
adding some
additionalload
higher thanthat referenced
by the [24].
bibliography
for aitsimply
supported
plategeneralsubject
considerations:First,
presented
analysisindicatesthat
analysisofout-of-plane
displacement
is anto uniaxial compression [24]. Moreover, it worth adding some additional general considerations: First,
additional
indicator
toevaluatethepresenceor
appearanceofdamage
and
this
can
be
flankedtopresented analysis indicates that analysis of out-of-plane displacement is an additional indicator tonormal and shear strain that were used in experiments on CFRP by Reference [25]. This is obtainedby also taking into consideration that a buckling minimizing clamping system was implemented for
performing the test. Strikingly, capability of information provided by DIC to put in evidence thepresence of damage is here confirmed also by the presence of a real impact damage while similar
evidence was previously found for the case of a simulated damage by a drilled and repaired holeMaterials 2019, 12, 249

13 of 14

evaluate the presence or appearance of damage and this can be flanked to normal and shear strain
that were used in experiments on CFRP by Reference [25]. This is obtained by also taking into
consideration that a buckling minimizing clamping system was implemented for performing the test.
Strikingly, capability of information provided by DIC to put in evidence the presence of damage is here
confirmed also by the presence of a real impact damage while similar evidence was previously found
for the case of a simulated damage by a drilled and repaired hole [25]. Another important aspect to
put into evidence is the very different behavior under compression shown by the CFRP and foam,
respectively. While in the first case, by analyzing a cross-section, strain concentration points appear in
the proximity of the damage, in the case of the foams a more homogenous behavior appears with astrain concentration points that are shifted away from the center of the damage. This is a behavior
different from what observed by Reference [26] where, however, a higher impact energy was used.
4. Conclusions
In this paper, the mechanical behavior of composites subjected to compression loading wasanalyzed by DIC. By the 3D DIC, high-resolution maps were obtained, reporting quantitative trends ofthe out-of-plane displacements of the investigated surfaces at different loads. These maps allowed
an immediate understanding of the global and local behavior of the components, highlighting thedifferent responses of the investigated materials. When those maps are used to compare mechanicalbehavior of damaged and undamaged components, they show how the presence of the impact damage
drastically change the response of the component.
Interesting consideration was found concerning with the measurement of the shear strain γxy
before the break of the specimens. In fact, it was found that, for impacted specimens, both CFRPs
and foams, the fracture point can be identified in correspondence of the absolute maximum valuesof γxy . Similar consideration cannot be extended to undamaged specimen. This aspect of the DIC issignificant as it allows to predict the area of component breakage. Potentialities of DIC technique are
evident also in relationship with the capability of observing occurrence of instability during loading.
It was found that buckling can be hardly be inferred by observing the temporal load law for the CFRPs
while no evidence appears when the law is analyzed for foams. DIC, instead, clearly allows to detectbuckling as well as to identify its localization over the component.
Author Contributions: Methodology, C.B., C.C., G.P.; Formal analysis, C.B., G.P.; Investigation, C.B., C.C., G.P.;
Data curation, C.B., G.P.; Writing—original draft preparation, C.B., G.P.; Supervision, C.C.
Funding: This research received no external funding.
Acknowledgments: The authors thank the Regional Apulia Call “Support for Regional Technological Cluster for
Innovation” for financial support, specifically COGEA–Composite Certification in General Aviation (OES4AM4).
Conflicts of Interest: The authors declare no conflict of interest.

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