materials
ArticleInfluence of Aluminum Surface Treatment on Tensile
and Fatigue Behavior of Thermoplastic-Based
Hybrid Laminates
Selim Mrzljak 1, * , Maik Trautmann 2 , Guntram Wagner 2
1
2*

and Frank Walther 1

Department of Materials Test Engineering (WPT), TU Dortmund University, Baroper Str. 303,
D-44227 Dortmund, Germany; frank.walther@tu-dortmund.deInstitute of Material Science and Engineering (IWW), Chemnitz University of Technology,
Erfenschlager Str. 73, D-09125 Chemnitz, Germany; maik.trautmann@mb.tu-chemnitz.de (M.T.);
guntram.wagner@mb.tu-chemnitz.de (G.W.)
Correspondence: selim.mrzljak@tu-dortmund.de; Tel.: +49-231-755-8494

Received: 12 June 2020; Accepted: 6 July 2020; Published: 10 July 2020


Abstract: Hybrid laminates consist of layers of different materials, which determine the mechanicalproperties of the laminate itself. Furthermore, the structure and interfacial properties between thelayers play a key role regarding the performance under load and therefore need to be investigated in
respect to industrial applicability. In this regard, a hybrid laminate comprised of AA6082 aluminumalloy sheets and glass and carbon fiber-reinforced thermoplastic (polyamide 6) is investigated inthis study with a focus on the influence of aluminum surface treatment application on tensile andfatigue behavior. Four different aluminum surface treatments are discussed (adhesion promoter,mechanical blasting, phosphating, and anodizing), which were characterized by Laser Scanning
Microscopy. After the thermal consolidation of the hybrid laminate under defined pressure, double
notch shear tests and tensile tests were performed and correlated to determine the resulting interfacial
strength between the aluminum sheet surface and the fiber-reinforced plastic, and its impact on tensile
performance. To investigate the performance of the laminate under fatigue load in LCF and HCF
regimes, a short-time procedure was applied consisting of resource-efficient instrumented multiple
and constant amplitude tests. Digital image correlation, thermography, and hysteresis measurement
methods were utilized to gain information about the aluminum surface treatment influence on
fatigue damage initiation and development. The results show that fatigue-induced damage initiation,
development, and mechanisms differ significantly depending on the applied aluminum surface
treatment. The used measurement technologies proved to be suitable for this application and enabledcorrelations in between, showing that the hybrid laminates damage state, in particular regarding theinterfacial bonding of the layers, can be monitored not just through visual recordings of local strain
and temperature development, but also through stress-displacement hysteresis analysis.
Keywords: fiber metal laminate; thermoplastic; AA6082; surface treatment; double notch shear test;
fatigue behavior; damage mechanisms; instrumented fatigue testing; digital image correlation

1. Introduction
Hybrid laminates of light metals and fiber-reinforced plastics (FRP), also known as Fiber Metal
Laminates (FML), combine the advantageous properties of both material groups in one material
compound. Besides a better damage tolerance compared to FRP, the FML have a lower density andhigher strength than monolithic metal alloys. These material compounds are of great importance,
especially as structural materials in aircraft construction compared to conventional aluminum alloys,
due to the lower crack growth based on the fiber bridging effect [1–3]. Much of the scientific researchMaterials 2020, 13, 3080; doi:10.3390/ma13143080

www.mdpi.com/journal/materials

Materials 2020, 13, 3080

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especially in the field of mechanical and fatigue properties is based on GLARE, which is a multi-layer
combination of aluminum alloy AA2024 and glass fiber-reinforced epoxy resin layers [3–9]. The
elaborate and complex production of the thermosetting-based multi-layer materials is time- andcost-intensive and unsuitable for large-scale production. Hybrid laminates based on fiber-reinforced
thermoplastics in combination with aluminum alloys of the 5000 and 6000 series are an interestingalternative. Polypropylene [10], polyamide 6 (PA6) [11], and thermoplastic polyurethane [12] are usedas matrix materials in the FRP layer. The layered combination of unidirectional carbon fiber-reinforced
PA6 and aluminum sheets AA6082 is known as CAPAAL. The individual layers are joined by a thermalpressing process. The used aluminum alloy AA6082 is precipitation hardenable, which is excellent toanodize, corrosion resistant, and is often used in lightweight mass products such as automobiles. It has
been shown that hybrid laminates with a thermoplastic matrix offer basic formability [13,14]. By heating
above the melting temperature of the matrix, complex structures can be formed [15,16]. Bending
fatigue tests on CAAPAL with mechanically blasted AA6082 sheets revealed at low loads only crack
initiation in the aluminum. Fiber cracks and delamination were observed at higher deflections. [17].
An essential aspect of the hybrid laminates is the suitable adhesion between the fiber-reinforced
plastic and the aluminum alloy. Surface treatments to the metal sheets such as mechanical treatment,chemical etching, or electrochemical treatment can improve wettability and adhesion between themetal and polymer, impacting on the mechanical properties of metal fiber laminates (e.g., in [18–20]).
For GLARE laminates, different mechanical (grinding, brushing) and chemical methods for the surface
treatment of EN AW-2024-T3 sheets, which were processed subsequently into hybrid laminates GLARE
4/3, were investigated [21]. By grinding with coarse sandpaper (100 grit), the highest interlaminarshear strengths of 72 MPa were achieved for such pretreatments.
The investigation of different surface treatment processes for aluminum AA6082 components
in thermoplastic-based hybrid laminates CAPAAL showed that plasma electrolytic anodic oxidationand mechanical blasting lead to less than 20 MPa in a simple lap shear test [22]. One possibility todetermine the shear strength is the examination of thermally joined single lap samples, which areeasy to produce but do not reflect the real manufacturing process of hybrid laminates. Another test
method to measure the interlaminar properties of hybrid laminates with ductile components is theshort-beam method according to DIN EN ISO 14130, but this often leads to a failure of the specimens,
which does not allow any statement about the interface. Therefore, in this study, a double notchedsample in accordance with the ASTM D3846-08 was utilized to measure the shear strength between asurface-treated aluminum alloy and fiber-reinforced polyamide 6. In [23], the method was successfully
applied to measure the interlaminar shear properties between surface-treated aluminum alloy sheets
and the matrix of glass fiber-reinforced epoxy. Results between 20 and 70 MPa were achieved, andit was found that a nanoscale sculpturing realized by an etching process significantly increases theinterlaminar properties.
With regard to the damage behavior of hybrid laminates, there are qualitative analogies between
the thermoplastic and thermoset-based variants. The fatigue properties and basic mechanisms ofcrack propagation in hybrid laminates with thermoset polymer matrices were investigated in detail inthe course of the qualification of GLARE [24]. Due to different thermal expansion coefficients of theindividual components, internal stresses occur in the laminate during the manufacturing process. These
stresses may be released at cut edges or inclusions by delamination and internal cracks, which adversely
affect the structural integrity of the material. The degree of residual stress is largely determined by
the thermal behavior and mechanical properties of the FRP and the metal as well as by the layered
architecture of the FRP and the quality of the internal interfaces of the hybrid laminate [25]. While muchresearch regarding the influence of metal surface treatments on the internal interfaces and quasistatic
properties of hybrid laminates is conducted, there is a lack of knowledge about the influence on thecyclic properties, i.e., the fatigue behavior. The aim of this study is the identification of the quasistaticand fatigue performance of thermoplastic-based hybrid laminates with respect to different surface
treatments of the metal component. Especially, the metrological-supported investigation of aluminumMaterials 2020, 13, 3080
Materials 2020, 13, x FOR PEER REVIEW

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3 of 19

surface treatment influence regarding damage initiation, development, and mechanisms due to fatigue
investigationof aluminum
surfacetreatment
influence
regarding damage
initiation,load issupported
a key aspectof this study.
A future useof thematerials
in e.g., automotive
application
requiresdevelopment,
andmechanismsduetofatigue
load
is
a
key
aspectofthisstudy.
A
futureuseofa comprehensive picture of the material behavior, since a long duration of application under thefatiguematerials
in e.g.,automotive
conditions
needs
to be
possible. application requires a comprehensive picture of the material behavior,
since a long duration of application under fatigue conditions needs to be possible.

2. Materials and Methods

2. Materials and Methods

2.1. Materials and Manufacturing

2.1. Materials and Manufacturing

The investigated hybrid laminate is based on CAPAAL—a layered material compound that
The investigated hybrid laminate is based on CAPAAL—a layered material compound thatconsists of surface-treated aluminum alloy sheets AA6082-T4 with a thickness of 0.5 mm and layers ofconsists of surface-treated aluminum alloy sheets AA6082-T4 with a thickness of 0.5 mm and layers
unidirectional
fiber-reinforced
plastics(FRP).(FRP).
The FRP
contains
multiplelayersof CePreg(Cetexof unidirectional
fiber-reinforced
plastics
The layer
FRP layer
contains
multiplelayersof CePreg
InstitutgGmbH,
Chemnitz,a continuously
producedunidirectional
glass (GFR-PA
6), and(Cetex
InstitutgGmbH, Germany),
Chemnitz, Germany),a continuously
producedunidirectional
glass (GFRcarbon
polyamide 6 polyamide(CFR-PA 6)
with a calculated
fiber volume
fractionfractionof 0.5. To
PAfiber-reinforced6), and carbon fiber-reinforced
6 (CFR-PA
6) with a calculated
fiber volumeof 0.5. To
for thein the coefficient
thermal expansion
the aluminumcompensate
forcompensate
the mismatch
inmismatchthe coefficientof thermalofexpansion
betweenbetween
the aluminumalloy and
alloy
and
the
CFR-PA
6,
anadditional
foil
of
PA
6
with
a
thickness
of
40
µm
and
a
layer
of
GFR-PA
the CFR-PA 6, an additional foil of PA 6 with a thickness of 40 µm and a layer of GFR-PA 6 was added.
6 wasadded.
The
consolidationof the materials with a size of 260 × 260 mm2 took place in a dip-edged tool
The consolidationof thematerials
withAamaximum
size of 260 temperature× 260 mm2 took
a dip-edged
with a laboratory
platen press
CollinPM300.
of place295 ◦ in
C washeld fortool
24 minwith a laboratory platen press Collin PM300. A maximum temperature of 295 °C was held for 24 minwith a pressure of 1.5 MPa. For the tensile tests, a hybrid laminate with 2/1-configuration was used,with a pressure of 1.5 MPa. For the tensile tests, a hybrid laminate with 2/1-configuration was used,consisting of two layers of aluminum alloy AA6082 and between these unidirectional FRP (Figure 1a).
consisting of two layers of aluminum alloy AA6082 and between these unidirectional FRP (Figure
To investigate
the interlaminarshear strength
betweenbetween
the metal
and theand
FRP,
aluminum1a). To investigate
the interlaminarshear strength
thecomponent
metal component
thetheFRP,
thealloy waspositionedinthemiddle,andthegradedFRP
layersarearranged
on
top
of
both
sides.
aluminum alloy was positioned in the middle, and the graded FRP layers are arranged on top of both Thisbuild-up
isThisreferred
to as
a 1/2-configuration(Figure 1b).
After1b).
pressing,
the samples
werewerecut by asides.
build-upis referred
to as a 1/2-configuration(FigureAfter pressing,
the samples
watercutjet.by a water jet.

Figure 1. Hybrid laminates composed of aluminum alloy AA6082-T4 and a graded structure of glass
Figure 1. Hybrid laminates composed of aluminum alloy AA6082-T4 and a graded structure of glassand carbon fiber-reinforced polyamide 6 in (a) a 2/1-configuration for tensile and fatigue tests and (b) aand carbon fiber-reinforced polyamide 6 in (a) a 2/1-configuration for tensile and fatigue tests and (b)1/2-configuration to determine the interlaminar shear strength.
a 1/2-configuration to determine the interlaminar shear strength.

2.2. Aluminum Surface Treatments

2.2. Aluminum Surface Treatments

The surface of the aluminum alloy AA6082-T4 was modified using four different treatments:
The surface of the aluminum alloy AA6082-T4 was modified using four different treatments:

Adhesion
promoter:Afterphosphating
the co-polyamide
co-polyamide
VESTAMELT(Evonik
Resource(I) Adhesion
promoter:Afterphosphating(see(seeIII),
III), theVESTAMELT(Evonik
Resource
Efficiency
GmbH,
Marl,
Germany)waswas applied
applied evenly
as asa powderutilizingan an
Efficiency
GmbH,
Marl,
Germany)
evenlytotothe
thesheetsheet
a powderutilizing◦ C.
electrostatic
spraycoatingandthenthenmelted
melted inin an oven
This
forms
a uniform
glassyelectrostatic
spraycoatingandovenatat200
200°C.
Thisforms
a uniform
glassyadhesionwithapproximatethickness
thickness ofof 75
75 µm.
adhesionfilmfilm
with
an anapproximateµm.
(II) Mechanical blasting: The sheets were blasted manually with a pressure of 1 bar on both sides by(II). Mechanical blasting: The sheets were blasted manually with a pressure of 1 bar on both sides byusing high-grade Al2O3 with a grain size F24 (600–850 µm) in a working distance of 100 mm. The
using high-grade Al2 O3 with a grain size F24 (600–850 µm) in a working distance of 100 mm. The
blasting material is applied at an angle of 90°
to the surface. The duration for blasting a sheet of◦blasting
at an
angle
2/s).to the surface. The duration for blasting a sheet of
260 ×material
260 mm2isonappliedboth sides
is 170s (8ofcm90
260 × 260 mm2 on both sides is 170 s (8 cm2 /s).
(I).

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Materials 2020, 13, 3080

4 of 19
4 of 18(III) Phosphating: The phosphating was carried out using the Bonderite M-ZN process from Henkel.
dipping the
sheetsin a zinc wasphosphateconversion
layerof smallcrystalline
zinc(III). By
Phosphating:The
phosphating
carriedsolution,out usinga theBonderite
M-ZN
process
from Henkel.
phosphatic
manganwillbeformed,which
typically
improves
theadhesionofthelacquer.
By dipping the sheets in a zinc phosphate solution, a conversion layer of small crystalline(IV) Anodizing:
After amangan
pretreatment
etchingwhich
the sheetsin 10%
sodiumthehydroxide
subsequentzinc-phosphatic
will beby
formed,typically
improves
adhesionandof thelacquer.
pickling, a 15 µm thick layer of Al2O3 was formed under direct current in sulfuric acid.
(IV). Anodizing: After a pretreatment by etching the sheets in 10% sodium hydroxide and subsequent
pickling, a 15 µm thick layer of Al O was formed under direct current in sulfuric acid.
2.3. Chracterization and Testing Methodes2 3
2.3. Chracterization and Testing Methodes
2.3.1. Aluminum Roughness and Laminate Microstructure
2.3.1. Aluminum Roughness and Laminate Microstructure
The surface roughness parameters arithmetic mean deviation (Sa), root mean squared (Sq), andskewness(Ssk) of roughness
the treated parameters
aluminum sheets
weremeandeterminedby a(SKeyence
VK X200(Keyence
The surface
arithmetic
deviation
squared(Sq ),a ), root mean
DeutschlandGmbH,
Neu-Isenburg,
Germany)laser
scanning
microscope
(LSM)
in
an
area
of
290
and skewness (Ssk ) of the treated aluminum sheets were determined by a Keyence VK X200×
200 µm2 with
10 analyzed
linesNeu-Isenburg,of each sample.
After consolidationof thehybrid laminates,(Keyence
Deutschland
GmbH,
Germany)
laser scanningmicroscope(LSM) in theanthicknesses
by of
light
microscopya scanningelectronareaof 290 were× 200examinedµm2 withon
10cross-sections
analyzed lines
eachsample. andAfterconsolidationof microscope
the hybrid
Zeiss LEO the
1455VP(Carl Zeiss
Oberkochen,
Germany).
laminates,
thicknesses
wereAG,
examinedon cross-sections
by light microscopy and a scanning electron
microscope Zeiss LEO 1455VP (Carl Zeiss AG, Oberkochen, Germany).
2.3.2. Quasistatic Properties
2.3.2. Quasistatic Properties
The interlaminar shear strength was measured on a universal testing system Zwick 20 kN on
shear
strength
was measured
on a universal testing
systemZwick 20 kN
on waterwaterThe
jet interlaminarcut samples of
the 1/2
configuration
with double-notchedsamplesin geometricalaccordancejetcut
samplesofthe
1/2
configuration
withdouble-notchedsamplesingeometricalaccordancewithwith ASTM D3846-08, as shown in Figure 1b. In contrast to the standard, a tensile load was chosen
ASTM
as showncompressionin Figure 1b. test,
In contrast
to the standard,tensile buckle.
load wasTensile
chosenproperties
instead of
insteadD3846-08,of the proposed
as otherwise,
the FRPawould
theproposedcompression
test,otherwise,
the FRP would
buckle. with
Tensileproperties
wereidentified
through tests
onasthe2/1 configurationin accordance
DIN
EN 527-5were
withidentified
water-jetthrough
tests
on
the
2/1
configurationinaccordance
with
DIN
EN
527-5
with
water-jetcutsamples
type
cut samples type A (length: 160 mm, width: 15 mm) on a servo-hydraulic testing system (Shimadzu
A(length: 160
mm,
width:
a servo-hydraulictestingwas
system(Shimadzu
max.
EHF-EV50,
max.
force
Fmax15=mm)± 50onkN).
The gauge length
reducedto 80EHF-EV50,
mm to gather
force
Fmax = ± 50informationkN). The gauge
was reduced
to 80 bymmthe
to measurement
gather high-resolution
information
high-resolution
aboutlength
local material
reactions
instrumentation
of a
about
local
material
reactionsbythemeasurement
instrumentationofasmaller
area.
The
water
jet
smaller area. The water jet cut edges were ground with SiC abrasive paper down to P4000 to reduce
cutedgesTo
weregroundwith
SiC abrasive at
paper
down to P4000
notches. andTo reduce
notches.
reducestressconcentrations
the transitional
areatoofreducethe clamping
testingstressarea,
concentrations
the transitionalof themm
clamping
and testing bonded
area, aluminumtabs with areaa thickness
aluminum tabsatwith
a thicknessareaof 1.75
were adhesively
to the clampingof theof
1.75 mmwith
wereepoxyadhesively
bondedto theclampingtab
areaandof thespecimen
withofepoxy(Figure 2).were
Thespecimen(Figure2). Thealuminum
clampingarea
the specimen
aluminumtab blasted
and clamping
area of thespecimen were
for adhesion
mechanicallyfor adhesion
enhancement.
A mechanicallyminimum ofblasted
three samples
for enhancement.
each surfaceAminimumof tested.
three samples for each surface treatment was tested.
treatmentwasFigure 2.
Prepared specimen
specimen with
with end
end tabs
tabs andand digital
digital image
image correlation
correlation (DIC) pattern for
2. Prepared
deformation analysis.

2.3.3. Cyclic Properties
For fatigue testing, the same specimen geometry and preparation as for the quasistatic tensile
tests were used. A short-time procedure (e.g., in [26]) was used for fatigue investigations, combining
multiple amplitude tests (MAT) and constant amplitude tests (CAT), which were carried out on
a servo-hydraulic testing system (EHF-EV50 from Shimadzu Europe GmbH, Duisburg, Germany,
maximum force Fmax = ± 50 kN, Figure 3) with sinusoidal tension-tension load-time function at a stressratio R = 0.1 and testing frequency f = 10 Hz under ambient temperature with regard to ISO 13003. For
the MAT, the maximum starting stress σmax,start was 50 MPa, followed by a stepwise stress increase of∆σmax = 25 MPa per ∆N = 103 cycles up to specimen failure. The CAT stress levels were derived fromthe material reactions in MAT, between the onset of first microstructural change (high cycle fatigue(HCF)) and specimen failure (low cycle fatigue (LCF)).

force Fmax = ± 50 kN, Figure 3) with sinusoidal tension-tension load-time function at a stress ratio R = 0.1
and testing frequency f = 10 Hz under ambient temperature with regard to ISO 13003. For the MAT, themaximum starting stress σmax,start was 50 MPa, followed by a stepwise stress increase of Δσmax = 25 MPa
per ΔN = 103 cycles up to specimen failure. The CAT stress levels were derived from the material
reactionsin 13,
MAT,
andMaterials 2020,
3080 between the onset of first microstructural change (high cycle fatigue (HCF))5 of
18
specimen failure (low cycle fatigue (LCF)).

Figure 3. Experimental
Experimental setup
setup containing
containing aa servo-hydraulicservo-hydraulic testing
testing system, digital image analysis
cameras perpendicular to the specimen (face and side view),
thermocamera,view), thermocamera, and tactile
tactile extensometer.
extensometer.

thethemicrostructuresand propertiesof specimens
were measured
a combination
The changes
changesin in
microstructuresand propertiesof specimens
were bymeasuredby aof systems toof
enablea combinedof multiple measurands
in [27]). (e.g.,
A Limess
Q400
combination
systems
to enableinvestigation
a combined investigationof multiple(e.g.,measurands
in [27]).
A
digital image(DIC)
system (Limessund Messtechnik
Software GmbH,Germany)
Limess
Q400 correlation
digital image
correlation(DIC) Messtechnik
system (Limess
undKrefeld,
SoftwareGmbH,
wasusedGermany)for surface
deformation
analysisof the front
aluminum
sheet
lens,
28 (precision
mm focalKrefeld,
wasused for surface
deformation
analysisof thefront(precision
aluminumsheet
length)edgeand(macrolens, 70mm(macro
focal length,withtubes)
to observe
interfacelens,
28 andmmlaminate
focal length)
laminateedge
lens, 70
mmextension
focal length,withextension
tubes)
reactions
betweenthelaminate
partners(DIC
specklepatternandarea
analysis
directions
are
shown
in
to observe interface reactions between the laminate partners (DIC speckle pattern and area analysis
Figures 2 andDIC pictures
were
taken
maximumtheathelpof a Limessbox.
directionsare3).
shown
in Figures
2 and
3).at
DIC
picturesstresswerewithtaken
maximum
stressMaxtrigger
with the help
A
TIM 160box.
thermocamera(Micro-Epsilon
Messtechnik GmbH& Co. KG,Messtechnik
Ortenburg,of MicroEpsilona Limess Maxtrigger
A MicroEpsilon
TIM 160 thermocamera(Micro-Epsilon
Germany)
recorded
the front aluminum
surface temperature.
For the investigationof stress-strain
GmbH
& Co.
KG, Ortenburg,
Germany) recorded
the front aluminum
surface temperature.
For thehysteresis development,a Shimadzu
1 LH (l0 =a Shimadzu
25 mm, ∆l TCK= ± 1 mm)theinvestigationof stress-strainhysteresisTCK
development,
LH (l0extensometer,= 25 mm, Δl = ±and
1 mm)engineering stress,
derivedfrom thestress,
load cellat thefrom
initialspecimenused.
Additionally,
theextensometer,
and theengineering
derivedtheload cellarea,at thewere
initialspecimenarea, weremachinepiston displacement(s)piston
was recorded
by a (s)linear(LVDT)
used.
Additionally,
the machine
displacement
wasvariable
recordeddifferential
by a linear transformer
variable differential
for stress-displacement
analysis, ashysteresis
the occurring
developmentof aluminumcracks andtransformer(LVDT) forhysteresis
stress-displacement
analysis,as the occurring
developmentofaluminum delamination
from
the
FRP
led
to
a
slippageoftheextensometer,
resultinginrecordeddata
cracks and aluminum delamination from the FRP led to a slippage of the extensometer,
deviationsinand
less reliability.
resultingrecordeddata deviations and less reliability.
3. Results
Results andand Discussion
Discussion
3.
3.1. Aluminum Roughness and Laminate Microstructure
3.1.
Aluminum Roughness and Laminate Microstructure
The quantification of the surface roughness parameters measured by laser scanning microscopyThequantification of the surface roughness parameters measured by laser scanning microscopy(LSM, Figure 4) shows that the mechanical blasting and anodizing process, in particular, can cause(LSM, Figure 4) shows that the mechanical blasting and anodizing process, in particular, can cause
an increase
compared toto anan untreatedsheet, whereas
anincrease ofof SSaa andand S
Sqq compared
untreated aluminum
aluminum sheet,
whereas thethe otherother investigatedinvestigatedprocesses
do
not
cause
any
pronounced
changes,
as
shown
in
Table
1.
The
increaseinroughnessisprocesses do not cause any pronounced changes, as shown in Table 1. The increase in roughness is
associated with
with anan increase
increase inin thearea andand aa formationof further
further bonding
bonding areas
areas forfor thetheassociatedthe total
total surface
surface area
formation ofthermoplastic melt. The skewness describes the distribution of the height profile to the reference line.
A positive value, as measured by phosphating, describes a profile determined by narrow peaks, whichis caused by the grown phosphate crystals. In contrast, the anodized surface is characterized by aslightly negative skewness.

Materials 2020, 13, x FOR PEER REVIEW

6 of 19

thermoplastic melt. The skewness describes the distribution of the height profile to the reference line.
A positive value, as measured by phosphating, describes a profile determined by narrow peaks,
which is causedby the grown phosphate crystals. In contrast, the anodized surface is characterized
Materials 2020, 13,3080 2020, 13, x FOR PEER REVIEW
Materials
6 of 19 6 of 18
by a slightly negative skewness.
thermoplastic melt. The skewness describes the distribution of the height profile to the reference line.
A positive value, as measured by phosphating, describes a profile determined by narrow peaks,
which is caused by the grown phosphate crystals. In contrast, the anodized surface is characterized
by a slightly negative skewness.

Figure 4. Laser scanning microscopy of (a) untreated and surface-treated aluminum alloy AA6082:
Figure 4. Laser
microscopyof (a)
aluminumAA6082:
Figure scanning
4. Laser scanningmicroscopyof untreated(a) untreatedandand surface-treatedsurface-treated aluminumalloyalloy
AA6082:(b) adhesion promoted, (c) mechanical blasted, (d) phosphated, and (e) anodized.
(b) adhesion
promoted,(c)mechanicalblasted,(d)phosphated,and(e)anodized.
(b) adhesion promoted, (c) mechanical blasted, (d) phosphated, and (e) anodized.
Table 1. Roughness parameters of surface-treated aluminum alloy AA6082-T4 measured by laser
Table 1. Roughness parameters of surface-treated aluminum alloy AA6082-T4 measured by laser
Table microscopy
1. Roughness
parametersof surface-treatedaluminum alloy AA6082-T4 measured by laser
scanning
transvers
to rolling
direction.
scanning microscopytransvers
to rolling
direction.
scanning microscopy transvers to rolling direction.
Mean
Surface Treatment
Arithmetic Mean Root
Root
Mean Squared
Skewness
Arithmetic Mean
Skewness

Surface Treatment

SaSquaredSurface AA6082-T4
Treatment
Root
Mean
Skewness
AA6082-T4Arithmetic
Sa Meanin µm
Sq in Squaredµm
Sa
Sa in µm
Ssk Ssk
Sq in µm
AA6082-T4
Sa in0.55µm± 0.12
S0.66
q in ±µmsk
Untreated
0.01
0.6 ±S
0.1
Ssk > 0
Untreated
0.55 ± 0.49
0.12 ± 0.16
0.66 ± 0.01
0.6 ± 0.5
0.1 ± 0.5
Adhesion promoted0.55
0.59 ± 0.18
Untreated
0.12
0.6 ± 0.1
Adhesion promoted
0.49 ±±0.16
0.59 ±0.66
0.18 ± 0.01
0.5 ± 0.5
Ssk > 0Mechanically
6.88 ± 1.13 −0.2 ±−0.2
Mechanically
blasted blasted0.49
5.74 ±±5.74
1.18
6.88 ±0.591.13
0.4Adhesion
promoted
0.16± 1.18
± 0.180.5± ±0.40.5
Phosphated
0.95 ± 0.24
Phosphated
0.67 ± 0.67
0.15 ± 0.15
0.95 ± 0.24
1.3 ± 1.3
0.8 ± 0.8
Mechanically
blasted5.74
1.18± 0.13
± 1.13−0.2± 0.4 Ssk < 0Anodized
2.13 ±±2.13
0.13
2.39 ±6.88
0.10
0.1
Anodized
2.39 ± 0.10 −0.4 ±−0.4± 0.1
Phosphated
0.67 ± 0.15
0.95 ± 0.24
1.3 ± 0.8
Ssk < 0The cross-sectionsof hybrid
thickness
Anodized
2.13± 0.13 laminates show
2.39a ±different
0.10 material−0.4± 0.1 by using identical
The cross-sections
ofhybridlaminatesshowadifferentmaterial
thicknessbyto using
identical
pressing parameters, as shown in Figure 5. The rough surface of the blasted sheets leads
laminatespressing parameters,
as shown
in Figure5. Thetheroughof withthe blasted
sheetsleadshaveto laminateswith the highest
thickness.
In contrast,
hybridsurface
laminates
the anodized
surfacesa
The cross-sections of hybrid laminates show a different material thickness by using identicalsignificantly
lower thickness,
althoughthehybridarithmetic
mean roughness
isthe
higherthan by phosphating,withthehighestthickness.
In
contrast,
thelaminateswithanodized
surfaceshave apressing parameters,
as shown
in Figure5. The roughsurfaceblastedsheetsleads to laminates
etching, and adhesion
promoter.
In particular,
the thickness
of ofthethe
CFR-PA
6 layeris significantly
lesssignificantly
lower
thickness,
althoughthearithmetic
meanroughnessishigherthanbyphosphating,with the highest
In 6contrast,
theindicateshybridthat
laminateswiththe strongly
anodized
surfacescomparedthickness.
to the GFR-PA
layer, whichthese layers
are more
displacedfromhave aetching,andadhesion
promoter.
In
particular,thethicknessofthe
CFR-PA6
layerissignificantly
lessthelaminate
thanthe
GFR-PA
6
layers.
Therheologicalbehaviorofthethermoplastic
meltduringthesignificantly lower thickness, although the arithmetic mean roughness is higher than by phosphating,comparedtothe
GFR-PA
6
layer,
which
indicates
that
these
layersaremorestrongly
displacedfromconsolidation
processisinfluencedbythesurfacetreatment,
thefibersinsidethe
FRP
layer,
andtheetching, and adhesion promoter. In particular, the thickness of the CFR-PA 6 layer is significantly lessmaterial.
The results
show that rheologicalthe anodization
causes thestrongest
displacement meltof the during
thecomparedlaminate surface
than
GFR-PA
6 layers.
behavior
thermoplastic
to thethe
GFR-PA
6 layer,whichThe
indicates that theselayers ofarethemorestrongly displacedfromthermoplastic material in the process. The material is pressed out through a gap between the dipping
thetheconsolidation
process
is influencedbythesurface treatment,
thefibersinside themelt
FRPduringlayer, theandlaminate
thanthe
GFR-PA
6
layers.
Therheologicalbehaviorofthethermoplasticedge and tool cover plate. This also results in a strong change in the metal volume content, which intheconsolidation
surface material.
The
results
show
that
the
anodization
causesthestrongest
displacementoftheprocessisinfluencedbythesurfacetreatment,
thefibersinsidethe
FRP
layer,
andtheturn influences the strength of the material compounds.
thermoplastic
material
the process.
is pressedout througha gap between
the dipping
surface material.
Theinresults
show The
that material
the anodization
causesthe strongest
displacementof theedge
and tool coverplate.inThis
also resultsa strongchange out
in themetalavolume
content,
thermoplastic
material
the process.
The inmaterial
is pressed
through
gap between
the whichdippinginedge
and toolthe
coverplate. ofThis
resultscompounds.
in a strong change in the metal volume content, which inturninfluences
strengththealsomaterialturn influences the strength of the material compounds.

Materials 2020, 13, 3080
Materials 2020, 13, x FOR PEER REVIEW

7 of 18
7 of 19

Figurecross-sections ofof hybridhybrid laminates
laminateswith
withanan(a)(a)adhesion
adhesion
Figure5.5. Scanning
Scanning electron
electron microscope
microscope cross-sections
promoted,and (d)(d) anodized
anodized aluminum
aluminumalloy
alloyAA6082-T4
AA6082-T4
promoted,(b)(b)mechanicallymechanicallyblasted,blasted, (c)(c) phosphated,
phosphated, andandfiber-reinforcedpolyamidepolyamide66(PA(PA6)6)inina a2/1
2/1configuration,
configuration,anda agradedgradedstructure
structureof
ofglass
glass andand carbon
carbon fiber-reinforced
below:
below:detailed
detailedarea
areaofofthe
theinterface.
interface.

Detailedconnectionof
ofthe
thePA
PA66without
withoutdelamination
delamination
Detailedvisualvisualobservations
observationsof
ofthe
the interface
interface show
show aa connection
forallvariants,
as
shown
in
Figure5a–d.
Especiallywiththemechanically
blastedaluminum
sheet,
for all variants, as shown in Figure 5a–d. Especially with the mechanically blasted aluminum sheet,a agood
can bebe achievedachievedby
byundercut
undercutareas,
areas,
shown
in Figuregoodmechanicalmechanical interlocking
interlocking can
asas
shown
in Figure5b. 5b.
OnlyOnlyin
inthe
theinterface
interfacewith
withthetheadhesion
adhesionpromoter,promoter,a apore
poreseam
seambetween
betweenthe
theGFR-PA6
GFR-PA6andandthetheadhesion
adhesion
promoter
low surface
surface roughness
roughnessSSa a(1(1µmµmsmaller
smallerpromoterisisvisible,
visible,asasshown
shownin
inFigure
Figure5a.
5a. Surfaces
Surfaces with a low
thanadhesion toto thethePA
PA 6.
6. ThisThisimplies
impliesthat
thatnot
notonly
only
thanthetheuntreateduntreatedrolling
rollingsurface)surface) also
also show
show good adhesion
thebetweenmetal
metalandandFRP
FRPisisresponsible
responsibleforforthemicroscopic
microscopicform
formclosure
closurebut
butalso
also weak
weak bonding forces between
theconnection.
connection.
theThecross-sections
cross-sectionsshow
showthetheunsteadyunsteadyposition
positionofofthe
theCFR-PA
CFR-PA
6 layer,e.g.,
Figureusing
The
6 layer,e.g.,
Figure5d.5d.
ByBy
using
the
the unidirectional
orientationof FRP
all FRP
tapes,
small
differencesininthe
thefiberfiber distribution
distribution ofunidirectional
orientationof alltapes,
small
differences
ofthe
theCePreg
CePregmateriallead
leadtotodisplacement
displacementprocesses
processesamong
among each
each other in the viscousmaterial
viscous state
state during
duringthethepressing.
pressing.
3.2.Quasistatic
QuasistaticProperties
Properties
3.2.
Theresults
results
interlaminarshear
are displayedin Figure
6 in comparison
to theTheof ofthetheinterlaminar
shear
teststests
are displayedin Figure
6 in comparison
to the measured
measuredarithmetic
mean
surfaceroughness.
Withamaximum
value
of
28
MPa
for
mechanicalarithmetic mean surface roughness. With a maximum value of 28 MPa for mechanical blasting, ablasting, a significantly
can be thandeterminedthan thoseon overlap
jointsmaterialof thesignificantly
higher valuehigher
can be value
determinedthose measured
onmeasured
overlap jointsof the same
same materialincombinationsin [22].
On blasted
specimens,
FRP
seen onwhich
thecombinations[22]. On blasted
specimens,
residuesof theresidues
FRP canofbethe
seen
oncan
thebe
surface,surface,
which
is
due
to
the
cohesive
failureofthecompositematerial.
The
realinterfacial
strengthofis due to the cohesive failure of the composite material. The real interfacial strength of this surface
thissurfacetreatmentisevenhigher
than
the
value
determined.
It
is
not
possibletodraw
conclusions
treatment is even higher than the value determined. It is not possible to draw conclusions about theabout thestrengthinterfacial
strength
onlyfrom the The
roughness.
samples
withsurfaces
anodized
surfacesinterfacial
onlyfrom theroughness.
samplesThewithanodized
have
similarhave
highsimilarhigh
shear
strengthvaluesdespitetheirlowerroughness.
Itisassumedthat
different
adhesionshear strength values despite their lower roughness. It is assumed that different adhesion mechanisms
mechanisms
are effective.
Throughmechanicalthe PA 6interlocks
mechanicallyinterlocks
theareeffective. Throughmechanicalblasting,
the PAblasting,6 mechanicallywith thesurface,withwhereassurface,whereas binding
with anodization,
forcesresponsible for cohesion.
withanodization,
forces arebinding
responsible
forarecohesion.

Materials 2020, 13, 3080
Materials 2020, 13, x FOR PEER REVIEW

8 of 18
8 of 19

Materials 2020, 13, x FOR PEER REVIEW

(a)
(a)

8 of 19(b) (b)FigureResultsof(a)(a)ananinterlaminarinterlaminarshear
sheartest
test
hybridlaminateswithsurface-treatedaluminum
Figure
6.6.
Results
test
ofof
hybridlaminateswithsurface-treatedaluminum
Figure6.
Resultsofof(a)aninterlaminar
shear
ofhybridlaminateswithsurface-treatedaluminumalloy
AA6082-T4
andagradedstructure
of
glass
and
carbonfiber-reinforced
PA
6
in
a
1/2
configuration;
alloy
AA6082-T4
andagradedstructure
of
glass
and
carbonfiber-reinforced
PA
6
a 1/2
alloy AA6082-T4 and a graded structure of glass and carbon fiber-reinforced PA 6 in ain1/2(b)fractography
images.
configuration;(b) fractography
fractographyimages.
images.
configuration; (b)Stress-displacement
and-strain(measuredvia
aaDIC
virtualgauge
length)curvesareinStress-displacement
and-strain(measuredvia
a DIC
virtualgauge
length)curvesaredisplayed
Stress-displacement
and-strain(measuredvia
DIC
virtualgauge
length)curvesare displayed
displayed
Figure7, wheresimilarsimilar
stress-displacement
development
is foundtheinvestigatedaluminum
Figure7, wherestress-displacement
development
is acrossfound
acrossthe theinvestigatedinin Figure
7,
similarstress-displacement
developmentisfoundacrossinvestigatedsurfacetreatments,
exceptforadhesion
promotion,
which
shows
lower
tensilestrengthandasignificant
aluminumsurfacetreatments,
exceptforadhesion
promotion,
which
shows
lower
tensilestrengthaluminum surface treatments, except for adhesion promotion, which shows lower tensile
strength
stiffnessdecrease
beforefailure.
DIC
imagesprovedthatthelatterisduetointerface
failure.
At theandaasignificant
significant stiffness
stiffness decrease
imagesprovedthatthethelatteris dueto interface
anddecreasebefore
beforefailure.
failure.DIC
DIC
imagesprovedthat
latteris dueto interface
failure.
At thethe start
starttest,of
tensiletest,
which
is representing
assumedto berepresenting
thethestartof eachtensilea kinkis visible,
assumed
to be
the
of
failure.
Atof eacheach
tensiletest,awhich
akink
kinkisisisvisible,
visible,which
is assumed
tobeexceeding
representing
theexceedingofthealuminumyield
stress,resultingsubsequentlyinanelastoplastic
deformationofthealuminumyield
stress,resultingsubsequentlyinanelastoplastic
deformationofthealuminumandexceeding of the aluminum yield stress, resulting subsequently in an elastoplastic deformation of thealuminumdeformationand viscoelastic
viscoelastic
deformation
ofoftheviscoelasticof theFRP.
aluminumanddeformation
theFRP.
FRP.

(a)

(b)

(a)
(b) with surfaceFigure7.7. (a)curvesand and(b) stress-strain
curves curvesof hybrid
Figure(a) Stress-displacement
Stress-displacement
curves(b) stress-strain
oflaminateshybrid laminateswithtreatedaluminumalloy
AA6082-T4
andagradedstructure
of
glass
and
carbonfiber-reinforcedPA
6
surface-treatedaluminumalloy
AA6082-T4
andagradedstructure
of
glass
and
carbonfiber-reinforced
Figure 7. (a) Stress-displacement curves and (b) stress-strain curves of hybrid laminates with surfaceina2/1
configuration.
The
curve
color
represents
analuminumsurfacetreatmentvariantofthehybrid
PA
6
in
a
2/1
configuration.
The
curve
color
represents
analuminumsurfacetreatmentvariantoftreated aluminum alloy AA6082-T4 and a graded structure of glass and carbon fiber-reinforced thePA 6laminate;
dotteddottedlines represent
different
specimens.
hybridlaminate;linescurve
represent
specimens.
in
a 2/1
configuration.
The
color different
represents
an aluminum surface treatment variant of the hybrid
laminate; dotted lines represent different specimens.

Measuredultimate
ultimatetensile
tensile strengths
strengths and total
withdifferent
Measured
total strains
strains ofofthe
thehybridhybridlaminates
laminateswithdifferent
aluminumsurfacetreatments
are
shown
in
Figure8.
Thequalitative
results
agree
with
the
aluminumsurfaceultimate
treatments
are shownin Figure8. Thestrains
qualitativeagreewith thewith
distribution
Measuredtensilestrengths
and totalof theresults
hybridlaminates
different
distribution
observedbytheinterlaminar
shear
test.
Mechanicalblasting
leads
with
a
maximum
observed
by theinterlaminarsheararetest.shown
Mechanicalblastingwith a maximum
aluminumsurfacetreatments
in Figure
8. leads
The qualitative
resultsaverage
agree value
with oftheaveragevalue of 625
MPa.anAnodizingshows anultimatetensile strengthofwhilearound610 MPa, while625
MPa.
Anodizingshows
ultimatetensilestrengthofaround610
MPa,phosphating
reaches
distribution observed by the interlaminar shear test. Mechanical blasting leads with a maximum
phosphating
reaches 580
MPa. Nodifferences
in bestraindistribution
could be foundvia580
MPa. Nosignificant
differences
insignificant
straindistribution
could
found
via DIC
theMPa,laminateaveragevalue
of 625 MPa.
Anodizingshows
an ultimate
tensilestrengthofanalysis
around of610
while
DIC analysis of the laminate interface across these three different surface-treated aluminum variants.
interfaceacrossreaches
these threedifferent
surface-treatedaluminumvariants.
While thesethreebealuminum
phosphating
580
MPa.
No
significant
differencesinstraindistribution
could
found
While these three aluminum surface treatments lead to similar tensile performance, the adhesion via
surfacetreatments
lead
to similartensileperformance,
thedifferent
adhesion
promoter achieves
significantly
DIC
analysisof thelaminateinterface
threealuminumvariants.
promoterachieves
significantly
loweracrossvaluestheseof 475MPa,whichsurface-treatedcan be attributed
to extensive
While these three aluminum surface treatments lead to similar tensile performance, the adhesion
promoter achieves significantly lower values of 475 MPa, which can be attributed to extensiveMaterials 2020,
2020, 13,
13, x3080
Materials
FOR PEER REVIEW

of 19
18
99 of

Materials 2020, 13,of
x FOR
PEER REVIEW
of 19
delamination
the aluminumleading to an earlier failure of the hybrid laminate. The partially 9large
lowervalues
of
475
MPa,
which
can
be
attributed
toextensive
delaminationofthealuminumleading
scattering of the results is due to the inhomogeneity of the fiber distribution inside the FRP
delaminationof thealuminumleading
to an The
earlier
failurecausesof thescattering
hybridlaminate.
The
partiallylarge
to an earlierThefailureof the hybrid
laminate.
partiallylarge
ofwhichthe results
isinduetothecomponent.
displacement
processduringproduction
fiber
shifts,resultdifferent
scatteringofoftheresults
duefibers
to theinhomogeneityof the fiber
theFRP
inhomogeneityof thefiber
distribution
the FRP component.
The distribution
displacementinsideprocessduringproportions
carbonand isglass
ininsidetheindividual
specimen.
component.
The displacement
duringcauses fiberofshifts,
which
resultfibers
in different
production causes
fiber shifts, process
which result
inproduction
different proportions
carbonand glassin theproportionsof carbon and glass fibers in the individual specimen.
individual specimen.

Figure 8. Arithmetic tensile strength and strain values for hybrid laminates with surface-treatedaluminum alloy AA6082-T4 and a graded structure of glass and carbon fiber-reinforced PA 6 in a 2/1
Figure 8. Arithmetic
Arithmetic tensile
tensile strength
strength andand strainstrain values
values forfor hybrid
hybrid laminates
laminates with surface-treatedconfiguration.
aluminum alloy
alloyAA6082-T4
AA6082-T4andanda agradedgradedstructure
structureglass
andcarbonfiber-reinforced
aluminum
of of
glass
and
carbonfiber-reinforced
PA PA
6 in6ain2/1a2/1
configuration.
configuration.
3.3. Cyclic Properties

3.3. Cyclic
Properties
For a Properties
first estimation of the aluminum surface treatment influence on fatigue behavior, MAT
3.3. Cyclic
were For
carriedwhich ofrevealed
discernible
differences
regardfatiguelife anddamagea firstout,
estimation
the aluminum
surfacetreatmentwith
influenceonto
fatiguebehavior,
MAT
were
For a firstAnodizing
estimationleadsof thealuminumsurfacetreatment
influenceon fatigue
behavior,
MAT
development.
tothehighestMAT
fatiguelife,
while
adhesion
promotion
treatment
carried out, which revealed discernible differences with regard to fatigue life and damage development.
were carriedout, which 9a).
revealed
discernible
differences withregardto fatigue
life anddamageshows
the lowest
TheMATchangein temperature
reveals
differences
betweenthesurface
Anodizingleads (Figure
to the highest
fatiguelife, while ΔT
adhesion
promotion
treatmentshowsthedevelopment.
Anodizing
leads
to
the
highestMAT
fatiguelife,
while
adhesion
promotion
treatment
treatments,
hintingatdifferinglaminate
properties
anddamageprogressions(Figure9b).
Mechanicallowest (Figure 9a). The change in temperature ∆T reveals differences between the surface treatments,
shows thelowest(Figureshow
9a). Thechangein temperature
ΔT reveals
differences
betweenthe surface
blasting
anodizinga highertemperature
increaseearlier
thanpromotion
hinting atand
differinglaminate properties
anddamage progressions(Figure9b).adhesion
Mechanicalblasting andandtreatments, hinting at differing laminate properties and damage progressions (Figure 9b). Mechanicalphosphating.
anodizing show a higher temperature increase earlier than adhesion promotion and phosphating.
blasting and anodizing show a higher temperature increase earlier than adhesion promotion andphosphating.

(a)

(b)

Figure 9. Influence
Influence ofof(a)aluminum surface
surface treatment
treatment onon (a)(a) fatigue
fatigue strength
strength andandlife andand (b)(b) changechange inin
Figurealuminum(b)lifetemperature during multiple amplitude tests.
temperature
Figure 9. Influence of aluminum surface treatment on (a) fatigue strength and life and (b) change in
The changes
changesin dynamic
dynamic
stiffnessC(change ofof stress
stress divided
divided throughthrough thethe changechange ofof pistonpistontemperatureduringmultiplestiffness
amplitude
tests.
The
in
Cdyn
dyn
(changedisplacement: (σ
(σmax
)/(s
smin
), seeFigure10a)
illustrate
thatthetheearlier
earliertemperaturetemperatureincreases
increases
max––σσminmaxmindisplacement:)/(smax– s–min), seeFigure10a)
illustratethat
Thechanges
indynamic
stiffnessCdyn (change of stress divided through the change of piston
can be attributed to a load-increased cyclic softening and the hardening processes of the aluminum,
displacement: (σmax – σmin)/(smax – smin), see Figure 10a) illustrate that the earlier temperature increases
can be attributed to a load-increased cyclic softening and the hardening processes of the aluminum,Materials 2020, 13, 3080

10 of 18

2020, 13,
PEER REVIEW cyclic softening and the hardening processes of the aluminum,
10 of 19can beMaterials
attributed
tox aFORload-increased
whichMaterials
are particularlypresentinanodized
surfacetreatment.
Adhesion
promotionandphosphating
13, x FOR PEER
REVIEWin anodized surface treatment. Adhesion promotion and phosphating
10 of 19
which 2020,are particularlypresent
exhibit the same but delayed cyclic hardening processes, which could be due to the phosphating of both.
exhibit the same but delayed cyclic hardening processes, which could be due to the phosphating of
which strain
are particularlypresent
in anodized
treatment.
promotion
phosphating
The plastic
amplitude
εa,p (Figure10b) surface
correlates
directlyAdhesion
to the stiffness
andandtemperaturechange,both. The plastic strain amplitude
εa,p (Figure 10b) correlates directly to the stiffness and temperature
exhibitthe
theplasticallysame but delayed
cyclic aluminum
hardening processes,
which
could
be dueto thefor
phosphatingof
revealing
deforming
component
as
the
main
cause
the
temperaturechange, revealing the plastically deforming aluminum component as the main cause for the
both.
The
plastic
strainamplitudeεa,p (Figure 10b) correlates directly to the stiffness and temperature
increase.
After theincrease.
temperature
maxima,stiffnessdecreasedue todamagepropagationin the interface
temperature
After thetemperature
maxima,stiffnessdecreasedue
to damage propagation
change,revealing theplastically
deforming aluminum component as the main cause for theand thefiber-reinforced
polymers
isvisible.
in the interface and the fiber-reinforced polymers is visible.

temperature increase. After the temperature maxima, stiffness decrease due to damage propagationin the interface and the fiber-reinforced polymers is visible.

(a)

(b)

10. Influenceof aluminum
surfacetreatment
onnormalized(a) normalized
dynamic
stiffness
FigureFigure
10. Influenceof aluminum
surfacetreatmenton (a)
dynamic
andand(b) (b)plastic(a)(b)stiffness
strainduring
amplitude
duringamplitude
multiple amplitude
strain plastic
amplitude
multipletests. tests.

Figure 10. Influence of aluminum surface treatment on (a) normalized dynamic stiffness and (b)plastic
strainfront
amplitude
during multiple
amplitude
tests.
Throughobservationand in-plane
observationof the laminar damage propagation via

Through front observation and in-plane observation of the laminar damage propagation via
DIC, differences depending on the surface treatment and related cyclic hardening progression were
DIC, differenceson theandsurfacetreatment
and related
cyclic hardening
progression
Throughdepending
front observation
in-planeobservationof the laminardamage propagation
viawererecognized. Figure 11 visualizes the specimen damage through strain analysis, representing therecognized.
Figure depending
11 visualizes
thesurface
specimendamagethroughstrainanalysis,
representing
DIC, differenceson thetreatment
and related
cyclichardening
progression
were thecondition immediately before MAT failure. Visible aluminum cracks formed primarily from the outer
recognized.
Figure before
11 visualizes
the specimen
damagethroughstrainanalysis,
representing
theoutercondition
immediately
MAT
failure.
Visiblealuminumcracksformedprimarily
fromthespecimen edges to the inside, developing into interface cracks and delamination. For the variants ofcondition
immediately
beforeMAT
failure.
Visiblealuminumcracksformedprimarily
from
the
outer
specimento the (adhesion
inside, developing
crackstreatment),
and delamination.
Foramounts
the variants
later edges
hardening
promotioninto
andinterface
phosphating
substantialof of
specimen edges
to the promotion
inside, developing
into interface
cracks andsubstantial
delamination.
For theof
variants
oflater hardening(adhesionandphosphating
treatment),
amounts
delamination
delamination between the aluminum and FRP are present, while the variants that are inclined to anlater hardening (adhesion promotion and phosphating treatment), substantial amounts of
betweenthe andaluminumand FRPare present,
while
the variants
thatandare anodizing
inclined totreatment)
an earlierenable
and high
earlierhigh quantityof crack
initiation(mechanicalblastingdelamination between the aluminum and FRP are present, while the variants that are inclined to an
morestress concentrations
and anodizingshow therefore
locally restricted
delamination.
quantityof localcrackreductions
initiation of(mechanicalblasting andtreatment)
enable more
local reductions
earlier and high quantity of crack initiation (mechanical blasting and anodizing treatment) enableof stress
concentrations
andshow
therefore
locally
restricted
delamination.
more local reductions of stress concentrations and show therefore locally restricted delamination.

Figure 11. Digital image correlation-based strain analysis images of front aluminum perspective andin-plane laminate perspective immediately before multiple amplitude tests (MAT) failure for hybrid
Figure11. Digital
image
correlation-basedstrainstrain analysis
analysis images
front
aluminum
perspective
and andFigure11. Digital
image
correlation-based
imagesofof
front
aluminum
perspective
laminateswithdifferent
aluminum surface treatments.
in-plane laminate perspective immediately before multiple amplitude tests (MAT) failure for hybrid
in-plane laminate perspective immediately before multiple amplitude tests (MAT) failure for hybrid
laminates with different aluminum surface treatments.

laminates with different aluminum surface treatments.

Materials 2020, 13, 3080
Materials 2020, 13, x FOR PEER REVIEW

11 of 18
11 of 19

The S-NS-N relationshiprelationship forfor eacheach hybrid
hybrid laminate
laminate with
with thethe individual
individual treatedtreated aluminum
aluminum surface
surfaceThe
confirms thetheMAT
MATfindings
findingsregarding
regardingfatigue
fatiguelifetime,
lifetime,showing
that
it anodized
is anodized
andphosphated
confirms
showing
that
it is
and
phosphatedin
Materials 2020, 13, x FOR PEER REVIEW
11 of 19ingeneralwiththehighestfatiguelife(Figure12).
Theextensive
delamination
visibleforanadhesion
general with the highest fatigue life (Figure 12). The extensive delamination visible for an adhesion
promoterduring
during
MAT(Figuretakes
place
in representingCAT,
representing
low interfacial
load
promoter(Figure11)11)
alsoalso
takes
place
in CAT,
thetreatedlowtheinterfacialloadcapacityThe
S-NMAT
relationshipforeachhybridlaminatewiththe individualaluminumsurfacecapacity
betweenmetal
andreducing
theFRP,thereducing
the fatigue
analysisduringMAT
andconfirms
theMAT
regardingfatigueshowingthat it DIC
isduring
anodizedandphosphated
betweenmetal
andthe findings
FRP,
fatiguelifetime,
lifetime.
DIClifetime.
analysis
MAT
and
CAT
showedingeneralwiththehighestfatiguelife(Figure12).
Theextensive
delamination
visibleforanadhesion
CATtheshowedthat thevariant
phosphated
variant delaminates
to fewer
cracksextent,
to a larger
but morethatphosphateddelaminates
due to fewerdue
cracksto a larger
but extent,
more graduallypromoter
MAT(Figure 11)also takes
place indamage-induced
CAT, representing
theinterfacialload
gradually
thanduring
adhesion
promotion,
to similar
reliefand distribution
thanadhesion
promotion,
leading
to leading
similardamage-induced
stress reliefstressandlow
distributionasfor thecapacity
between
metal
and
the
FRP,
reducingthefatiguelifetime.
DIC
analysisduring
MAT
and
as
for
the
anodizedandmechanically
blastedlaminates,
as
well
as
fatiguelife.
anodized and mechanically blasted laminates, as well as fatigue life.
CAT showed that the phosphated variant delaminates due to fewer cracks to a larger extent, but more
gradually than adhesion promotion, leading to similar damage-induced stress relief and distribution
as for the anodized and mechanically blasted laminates, as well as fatigue life.

Figure 12. S-N relationship for hybrid laminates with different
different aluminum
aluminum surface
surface treatments.
treatments.
Figure 12. S-N relationship for hybrid laminates with different aluminum surface treatments.

This
This more
more gradual
gradual damage
damage propagation
propagation ofof thethe phosphated
phosphated variant
variant is
is shown
shown with
with regard
regard toto the
the
This
more
gradualdamagepropagationofthephosphated
variant
is
shown
with
regardtothestiffness
stiffness development
development inin CAT
CAT exemplary
exemplary forfor aa maximum
maximum stressstress level
level ofof 300300 MPa
MPa upup to
to 14E4
14E4 cycles
cyclesstiffness development
in CAT exemplary
for a maximum
stress level of crack
300 MPa up to 14E4
cycles(which(which represents
representsdamage
damagedevelopment
developmentup
upuntil
until nearly
nearly full
full aluminum
aluminum crack extension)
extension) inin Figure
Figure 13a13a(whichrepresents
damagedevelopmentup
until
nearlyfull
aluminumcrack
extension)inFigure13a
directlycompared
to
the
other
aluminumsurfacetreatments.
Themaximum
stress
level
of
the
shown
directlycompared
to
the
other
aluminumsurfacetreatments.
Themaximum
stress
level
of
the
shown
directly compared to the other aluminum surface treatments. The maximum stress level of the shown
phosphated
variant
specimenisMPalower duetoachieving
more
evenlydistributed
impression
phosphated
variant
specimen
is 10
1010
MPa
to achieving
achievinga aamore
more
evenlydistributed
impression
phosphated
variant
specimenis
MPalower
lowerduedue toevenlydistributed
impression
of
the
S-N
relationship,
and
in
view
of
the
results
obtainedwithregardtofatiguelifetimeandbehavior,of theof S-N
relationship,
andininview
obtainedwith
withregard
regardto fatigue
lifetime
the S-N
relationship,andviewofof thethe results
results obtained
to fatigue
lifetime
and anditbehavior,is considerednegligible
andtherefore
comparable
to 300 MPa.
As
was
theAs
case
for
MAT,
the
it is itconsiderednegligible
andtherefore
300
MPa.
was
the
case
for cyclic
MAT,behavior,
is considerednegligible
andtherefore comparable
comparable
toto300
MPa.
As
was
the
case
for MAT,softeningandhardeningappearformechanically
blastedandanodizedlaminates,while
adhesionthecyclicsofteningandhardeningappearformechanically
blastedandanodizedlaminates,while
the cyclic softening and hardening appear for mechanically blasted and anodized laminates, while
promoted
andpromoted
phosphated
exhibitcyclichardeningfrom the beginningof the test.
adhesion
phosphated
exhibitcyclic hardening
from
beginningof theadhesion
promoted
andandphosphated
exhibitcyclichardening
fromthethebeginningof test.
the test.

(a)

(b)

Figure13. Influenceof aluminum
surfacetreatment
onnormalized(a) normalized
dynamic
stiffness
Figure13. Influenceof aluminum
surfacetreatmenton (a)
dynamic
stiffness
andand(b) (b)change(a) (plotted linearly) during constant amplitude tests.
(b)changeintemperaturein temperature (plotted linearly) during constant amplitude tests.
Figure 13. Influence of aluminum surface treatment on (a) normalized dynamic stiffness and (b)change in temperature (plotted linearly) during constant amplitude tests.

Materials 2020, 13, 3080

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Materials 2020, 13, x FOR PEER REVIEW

12 of 19

The change in temperature in Figure 13b also shows the same characteristics as in the MAT, wherechange within temperature
Figureamplitude.
13b also shows
thepoint,
same characteristics
the MAT, thatit directlyThecorrelates
the plasticinstrain
At thisit can be seenasasinestablishedwhereitdirectly
correlates
withtheplastic
strainamplitude.
At
thispoint,
itcanbeasthrough MAT, many characteristics of the hybrid laminates behavior appearing in CAT canseen
be derived.
established that through MAT, many characteristics of the hybrid laminates behavior appearing in
Adhesion promoted and phosphated show just small increases in temperature and plastic strain
CAT can be derived. Adhesion promoted and phosphated show just small increases in temperatureamplitudewith an afterward approximately constant leveling, which can be a reason for the postponed
and plastic strain amplitude with an afterward approximately constant leveling, which can be aand slower
to the laminates
withcomparedmechanically
blastedand anodized
reason damage
for the development
postponed andcomparedslower damage
development
to thelaminateswithaluminum.
This
could
be
attributedtoanoverallhigherstiffnessofthephosphated
aluminum,leadingmechanically blasted and anodized aluminum. This could be attributed to an overall higher stiffness
to a more
stiffnessdecreasethan a plastic strain-induced
stiffness.
of thecrack-dominatedphosphated aluminum,leading
to a rather
more crack-dominatedstiffness decrease
rather than aplastic
strain-induced
stiffness.
Thelogarithmicplot ofthe normalized stiffness (Figure 14a) and change in temperature (Figure 14b)
The logarithmicview
plot ofstiffness (Figure14a) andin temperature(Figureenable a supplementaryof thethenormalized
damage progression
by focusing
onchange
the major
events at the
beginning
5
14b)enableasupplementary
view
of
the
damageprogression
byfocusing
on
the
major
eventsat
theof fatigue load appliance and showing the later on development, which here is up to 4 × 10 cycles.
It
beginning of fatigue load appliance and showing the later on development, which here is up to 4 × 105
becomes apparent that the mechanically blasted and anodized laminates undergo a stiffness reduction,
cycles. It becomes apparent that the mechanically blasted and anodized laminates undergo a stiffness
inflicting an increase in temperature up to approximately 600 cycles followed by a stiffness increase
reduction, inflicting an increase in temperature up to approximately 600
cycles followed by a stiffness
and starting
damage initiation at the maximum stiffness at around 104 cycles. The adhesion promoted
increase and starting damage initiation at the maximum stiffness at around 104 cycles. The adhesion
and phosphated
laminateincreasedin stiffness
damageinitiation
around at
9×
103 andpromoted andphosphated
laminateincreasedup
in until
stiffnessup untildamageatinitiation
around
9 ×2.7 ×
4
10 respectively
showing while
a significantly
increasein temperature.
103 and 2.7 ×while
104 respectively
showing alower
significantly
lower
increase in temperature.

(a)

(b)

Figure14. Influenceof aluminum
surfacetreatment
on normalized(a) normalized
dynamic
stiffness
Figure14. Influenceof aluminum
surfacetreatmenton (a)
dynamic
stiffness
andand(b)(b)
changechange in temperature(plotted logarithmically)
during constant
amplitude
tests; dottedindicator
in temperature(plotted logarithmically)
during constant
amplitude
tests; dottedindicatorlines forfor cycles ofhysteresis investigation.
cycleslinesof hysteresis
investigation.

Comparingthe initiationof large
the large
decreasesin stiffness
with
large
decreasesinComparingthe initiation
timetimeof thedecreasesin stiffnesswiththe thelarge
decreasesin changechange in temperature, a delay in between those is noticeable. Especially for the mechanically blasted
in temperature, a delay in between those is noticeable. Especially for the mechanically blasted andand anodized laminates, an earlier onset of stiffness decrease is visible compared to temperature
anodized laminates, an earlier onset of stiffness decrease is visible compared to temperature decrease.
decrease. Furthermore, the stiffness development of the phosphated variant shows discernable
Furthermore,thea prolonged
stiffness developmentof the phosphated
variantcrackshowsdiscernabledifferences: a
differences:
decrease in stiffness,
and after aluminum
extension,
the normalized
prolonged
decreasein stiffness,andafterthanaluminum
crack
the normalized
dynamic
stiffness
dynamic
stiffnessis reduced
morecompared
to extension,
the mechanically
blasted andanodized
is reduced
more than compared to the mechanically blasted and anodized laminates.
laminates.
To getbetterunderstanding
ofofthis
temperaturedevelopment,
development,
corresponding
Toageta betterunderstanding
thisstiffness
stiffness andand temperature
the thecorresponding
stress-displacement
hystereses
investigated(Figure15).
15).Single
Singlehysteresis
hysteresis curves
curves arestress-displacement
hystereses
areareinvestigated(Figureareplottedplottedfor
for thestate
right
the fatigue
test(Nstart(N and= 10)selected
and selected
numbersof cycles
accordingto thestate the
right
after
theafter
fatiguetest start= 10)
numbersof cycles
accordingto thedottedlines14infor
Figure14 fora direct correlation
to the measured
and temperature(andlines dottedin Figurea directcorrelation
to the measured
stiffnessstiffness
and temperature(and therefore
therefore plastic strain amplitude) development. In order to avoid overlapping hystereses and thus
plastic strain amplitude) development. In order to avoid overlapping hystereses and thus reduced
reduced visibility, the hystereses are provided with offset x-axes related to an increasing number ofvisibility, the hystereses are provided with offset x-axes related to an increasing number of cycles, which
cycles, which is indicated by the different saturation of hystereses curves and x-axes print. Looking
is indicatedby the differentof hysteresesand x-axes
print.
Looking
at the anodized
at the anodizedmaterials,saturation
immediately,
the decreasecurves
in stiffnessand high
increasein temperatureatmaterials, immediately, the decrease in stiffness and high increase in temperature at the beginning of
the CAT (Figure 14, N = 10 and 600) can be correlated with a high opening and area of the hysteresis
curve, which represents the amount of dissipated energy. Since the other aluminum surface treatment

Materials 2020,
13,2020,308013, x FOR PEER REVIEW
Materials

13 of 19 13 of 18

the beginning of the CAT (Figure 14, N = 10 and 600) can be correlated with a high opening and area
the hysteresis
which
represents
the amountof different
dissipatedsurface
energy. Since
the other aluminum
variantsofshow
this to acurve,
smaller
extent,it is assumed
that
treatment-induced
aluminumsurface treatment
variants
show this toa smaller
extent,it is assumed
surfaceintreatmentstates regarding
mechanicalproperties
arepresent,
which
needs tothat
be different
investigatedfurther studies
aluminum states regarding mechanical properties are present, which needs to bethroughinduced
hardnesstesting, etc. Even if anodized shows the shortest time until stiffness decreases andinvestigated in further studies through hardness testing, etc. Even if anodized shows the shortest
the highest extent of experienced loss energy, this seems to have no significant impact on the overall
time until stiffness decreases and the highest extent of experienced loss energy, this seems to have nolifetime significantof the hybrid
laminate(Figure12, compare
lifetime
at a maximumof lifetimearoundat300
impacton the overall
lifetimeof the hybrid
laminate(Figure 12, stresscomparea MPa).
However,
it immensely
stiffnessdevelopment
andaffects
therefore
durationof maintainingthemaximum
stress of affects
around the
300 MPa).
However,
it immensely
the stiffness
development
andtherefore
durationof laminate
maintaining
the structural
integrityof the laminate (compare, e.g., phosphated).
structural
integrityof the(compare,e.g.,
phosphated).

Figure15. Stress-displacement-hysteresisdevelopment
development in
amplitude
tests tests
with awith
maximum
Figure 15.
Stress-displacement-hysteresis
inconstantconstant
amplitudea maximum
stress of 300 MPa with regard to aluminum surface treatment; dotted indicator lines visualize
stress of 300 MPa with regard to aluminum surface treatment; dotted indicator lines visualize maximum
maximum stress at which hysteresis starts exhibiting a kink due to aluminum failure.
stress at which hysteresis starts exhibiting a kink due to aluminum failure.

The most prominent conspicuousness between the different aluminum surface treatments lies

Theinmostprominent
conspicuousness
betweendifferent
aluminum
treatments lies
the differing
establishment
and developmentof athekinkin the hysteresis.
Duesurface
to the hysteresis
in the differing
establishment
anddevelopment
of
a
kink
in
the
hysteresis.
Duetodevelopment before and after the significant stiffness reduction, it is assumed that the lowerthe
parthysteresisofthehysteresis
below
the
kink
represents
thefullhybridlaminate
stiffness,whereastheupperpart part ofdevelopment before and after the significant stiffness reduction, it is assumed that the lower
represents
the FRPwith aluminum
opened.laminate
For all variants,
the initialstiffness
at loadthe hysteresis
below
thestiffness
kink represents
the cracks
full hybrid
stiffness,whereastheupper part
application (full hybrid laminate) stays more or less the same until failure, while the stiffness at load
represents the FRP stiffness with aluminum cracks opened. For all variants, the initial stiffness at load
removal (FRP with aluminum cracks open) and the position of the kink point (determined via
application(full hybrid laminate) stays more or less the same until failure, while the stiffness at loadtangents between loading and unloading curve of the hysteresis; kink point height indicated byremovaldotted(FRP lines
withassociatedaluminumcracks open)
andthe positionof thekink point(determined
viathetangentsto maximum
stress)changessignificantly.
Moreover,approaching
failure,betweenkink
loading
andunloading
curve
of
the
hysteresis;
kink
point
heightindicatedbydottedpoint is lowered in terms of its appearance at a maximum stress level, which is especially visible lines
associated to maximum stress) changes significantly. Moreover, approaching failure, the kink point
is lowered in terms of its appearance at a maximum stress level, which is especially visible for thephosphated variant. The stiffness reduction of the phosphated variant, which overall is slower, buthigher than for the other treatments, can be traced back to a drastic reduction in the stiffness of the FRP
as well as lowering of the kink point. Even though the phosphated variant shows the highest stiffness
decrease overall, compared to the anodized and mechanically blasted variant, a comparable fatigue

Materials 2020, 13, 3080

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life and kink point height at failure is present. This indicates that the kink point height could be usedin further studies as a parameter representative for interface condition and failure prediction.
The occurrence of the kink point seems to be related to the failure of the weaker laminate partner
aluminum and the height of the kink point to the grade of damage development in the interface tothe FRP. Figure 16 shows DIC analysis images for selected cycles representing characteristic statesof damage for each aluminum surface treatment variant at applied maximum stress, which support
this relationship. While adhesion promoted undergoes extensive interface damage already from thestart of the test and fails early due to it, the other three aluminum surface treatments show a gradualdamage development. This includes (1) aluminum crack initiation, (2) interface damage initiation, and(3) interface delamination, which can be reflected on the overall stiffness development and also kink
point establishment and development.
Materials 2020, 13, x FOR PEER REVIEW

15 of 19

Figure 16. Digital image correlation-based strain analysis images of the front aluminum perspective

Figure 16. Digital image correlation-based strain analysis images of the front aluminum perspective andand in-plane laminate perspective at defined constant amplitude test cycles (σmax = 300 MPa,
in-plane laminate
at definedconstant
amplitude
test cycles
max = 300 MPa, phosphated
phosphatedperspective
290 MPa) for hybrid
laminateswith different
aluminumsurface(σtreatments.
290 MPa) for hybrid laminates with different aluminum surface treatments.
In addition to a comprehensive analysis of all the investigated hybrid laminates with different
aluminum surface treatments at an exemplary maximum stress level, the phosphated andmechanically blasted variants were compared at different maximum stress levels (Figure 17) toachieve a more profound understanding about the fatigue performance differences. For each of theshown maximum stress levels, a major temperature decrease only occurs after the stiffness has
already decreased (see gray dotted lines), not simultaneously with it. This decrease in temperature

Materials 2020, 13, 3080

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By combining the results from Figures 14–16, for, e.g., mechanical blasting, it can be concluded
that the decrease in stiffness visible starts right after 104 with (1) aluminum crack initiation, which isfollowed by (2) damage initiation in the interface in form of continuing cracks and (3) delamination
along the aluminum (see Figure 16, exemplary 1.6 × 104 ). Taking the aforementioned decrease intemperature into account (Figure 14), it becomes evident that the change in temperature, respectively
in relation to plastic strain amplitude, is reduced greatly after the first crack initiation when cracks
propagate, not simultaneously. This crack propagation results in the kink of the hysteresis, as shown in
Figure 15 exemplary for 2.7 × 104 cycles. With ongoing cycles, the delamination develops further (seeFigure 16, exemplary 1.5 × 105 ), correlating primarily to FRP stiffness decrease. After 1.5 × 105 cycles
along with the FRP stiffness decrease, a lowering of the kink point occurs (compare cycle 2 × 105
from Figure 15) while approaching failure. When evaluating the phosphated variant, a different
damage development is present. Cracks develop to a smaller extent while having a similar stiffness
decrease (compare 6 × 104 from Figures 14 and 16), which is assumed to be due to the higher surface
treatment-induced aluminum stiffness, causing more stress concentration relief through interface
damage rather than aluminum crack initiation. This seems plausible looking at the smaller number ofcracks at 1.5 × 105 (see Figure 16) cycles. Comparing the kink point height in the hystereses at cycle 2 ×
105 and Nf (Figure 15) with the delamination extension visible in Figure 16 between the exemplary
images shown for cycles 1.5 × 105 and 4 × 105 , a connection between the reduction of kink point heightand interface damage can be seen.
Anodized undergoes a similar fatigue damage development as mechanical blasting, showing
differences just in a slightly earlier onset of cracks and more delamination growth, which is attributed
to inferior polymer adhesion due to lower surface roughness. Still, as the phosphated variant did, theanodized variant as well shows an overall longer fatigue life than the mechanically blasted, pointing
out that the stress concentration relief through delamination expansion does not necessarily appear tohave a negative effect on fatigue life, rather a prolonging one.
In addition to a comprehensive analysis of all the investigated hybrid laminates with different
aluminum surface treatments at an exemplary maximum stress level, the phosphated and mechanically
blasted variants were compared at different maximum stress levels (Figure 17) to achieve a more
profound understanding about the fatigue performance differences. For each of the shown maximum
stress levels, a major temperature decrease only occurs after the stiffness has already decreased (see graydotted lines), not simultaneously with it. This decrease in temperature always seems to occur at similar
stiffness losses after the initiation of the strong stiffness decrease, indicating the visible appearance of ahysteresis kink. It can be assumed that the decrease in temperature for each of the maximum stresses
shown correlates with the initiation of crack propagation, resulting in the fact that the aluminum canonly bear a load as long as the interfaces can transfer it from the FRP to the aluminum. Looking at thephosphated variant, it becomes apparent again that crack initiation is delayed and occurs on a smallerscale, which is why crack propagation only occurs at a significantly higher decrease in stiffness. On
the other hand, the phosphated variant has generally a higher overall decrease in stiffness than thecompared mechanically blasted variant, which is why, depending on the application, the phosphated
variant can be more suitable (longer retention of stiffness with low crack formation) or less suitable(greater delamination propagation with strong stiffness reduction).

transfer it from the FRP to the aluminum. Looking at the phosphated variant, it becomes apparent
again that crack initiation is delayed and occurs on a smaller scale, which is why crack propagation
only occurs at a significantly higher decrease in stiffness. On the other hand, the phosphated variant
has generally a higher overall decrease in stiffness than the compared mechanically blasted variant,which is why, depending on the application, the phosphated variant can be more suitable (longer
retentionwith low crack formation) or less suitable (greater delamination propagation 16 of 18
Materials
2020, of13, stiffness3080
with strong stiffness reduction).

FigureInfluence ofofphosphating
andandmechanicalblasting
aluminumsurface treatmenton (a)
Figure17.17. Influencephosphating
mechanicalblasting
aluminumsurface treatmenton
stiffness
andand(b) (b)changein temperature(plottedlogarithmically)
during during(a) normalized
normalizeddynamic
dynamic
stiffnesschangein temperature(plottedlogarithmically)
constant
amplitudetests
tests at
at aa variety
variety of
constant
amplitudeofσσmax.
max.

4. Conclusions
4. Conclusions
The
following
conclusionscan
canbebedrawn
drawnfromfromthetheinvestigations
investigations onon thetheinfluence
influence ofof surface-treatedsurfaceThefollowing
conclusions
treatedaluminum
sheetsAA6082-T4
inhybridlaminates,
based
on
glass
and
carbonfiber-reinforced
aluminum sheets AA6082-T4 in hybrid laminates, based on glass and carbon fiber-reinforced polyamidepolyamide 6, regarding mechanical properties.
6, regarding mechanical properties.
The surface of the aluminum alloy affects the rheological behavior of the melt during the
The surface of the aluminum alloy affects the rheological behavior of the melt during theconsolidation process. The thickness of the laminates varies with different surface treatments in aconsolidation
process.process.
The thicknessof thelaminates
varieswithdifferent
surfacetreatments
in apressure-controlled
Mechanically
blasting
resultsin thehighestsurfaceroughnessandpressure-controlled
process.
Mechanically
blasting
results
inthehighestsurfaceroughnessandachieves
achieves the highest interlaminar shear strength of 30 MPa of all the investigated aluminum surface
thetreatments.
highest interlaminar
shear
strengthofstrength30 MPaofofthe
allhybrid
the investigatedaluminumsurfacetreatments.
However, theultimatetensilelaminate does
not seem tobe heavily

However, the ultimate tensile strength of the hybrid laminate does not seem to be heavily dependent
on the aluminum surface treatment as long as no delamination occurs as with the applied adhesion
promoter, since the mechanically blasted, phosphated, and anodized variants show similar results.
By using multiple amplitude tests, digital image correlation analysis, and hysteresis-stress-strain
measurement, an overview about the fatigue performance was established and validated successfully
through constant amplitude tests. The fatigue damage development and fatigue life of hot-pressure
consolidated thermoplastic-based hybrid laminates are highly influenced by the aluminum surface
treatment. Anodizing and phosphating achieve the highest fatigue life, while in comparison, theadhesion promoter led to a significantly reduced fatigue life due to a low interfacial load capacity.
Furthermore, results showed that the aluminum surface treatment affects the stiffness development
and therefore the duration of maintaining the structural integrity of the hybrid laminate more than
the fatigue lifetime itself. It appears that of all the investigated treatments, phosphating enables thelongest retention of stiffness with low crack formation but causes greater delamination propagation
with the highest overall stiffness reduction, which needs to be considered regarding applications. In

Materials 2020, 13, 3080

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conclusion, a graded crack initiation and propagation—which is influenced by the aluminum state,aluminum surface, metal-FRP interface, and the onset of aluminum cyclic hardening—contributes
to higher fatigue life, but it needs further investigation regarding the relevant factors for remaining
fatigue lifetime after aluminum crack initiation (when mainly the FRP is bearing the load).
Through stress-displacement hysteresis analysis, an aluminum surface treatment-dependent
appearance of a hysteresis kink was detected, which seems to be related to aluminum and aluminum-FRP
interface crack initiation and propagation. The visible appearance of the hysteresis kink was foundto establish at a significant aluminum surface temperature (or correlating plastic strain amplitude)
reduction, independent of the applied maximum stress (as long as aluminum cracks initiate). In
addition, the position of the kink point showed dependency of the aluminum surface treatment withregard to load-bearing capabilities at applied stress and the grade of damage in the aluminum-FRP
interface (mainly delamination). A connection between the kink point height regarding its occurrence
at maximum stress and the amount of interface damage can be seen, which needs to be validated infurther studies through, e.g., digital image correlation analysis of whole cycles rather than strain states
at applied maximum stress.
The interfacial, microstructural differences between the metal and FRP of the hybrid laminate need
to be characterized further regarding occurring damage mechanisms for a better understanding of thesurface treatments regarding improvement possibilities and should be analyzed during fatigue tests
through, e.g., accompanied microscopy. Since the investigated aluminum surface treatments lead todifferent laminate thicknesses, the related ratio of fiber-reinforced polymer and fiber density needs to beconsidered for each laminate variant with regard to a relative comparison of the individual and integral
laminate partners fatigue performance for a more accurate description of the fatigue performance.
Author Contributions: Mechanical testing for the characterization of tensile and fatigue properties,stress-displacement hysteresis analysis, and digital image correlation-based strain analysis were performed by
S.M.; specimen manufacturing, microstructural investigations (surface roughness measurements and laminate
structure SEM imaging) and mechanical testing for the characterization of interlaminar shear strength were carriedout by M.T.; the original manuscript draft was conceived by S.M. written by S.M. and M.T.; F.W. and G.W. have raised
the funding, supervised the project, and reviewed the manuscript. All authors have read and agreed to the publishedversion of the manuscript.
Funding: The authors thank the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) for its
financial support within the research project “Mechanism-correlated characterization of the deformation anddamage behavior of thermoplastic-based fiber metal laminates for property-oriented process development” (WA
1672/56-1, WA 1665/9-1).
Acknowledgments: The authors would like to acknowledge Martin Risthaus from Evonic for providing surface
treatment and Marco Fischer for supporting the experimental tests in Chemnitz.
Conflicts of Interest: The authors declare no conflict of interest.

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