Article

Testing Procedure for Fatigue Characterization of Steel‐CFRP
Hybrid Laminate Considering Material Dependent
Self‐Heating
Selim Mrzljak 1,*, Stefan Schmidt 2, Andreas Kohl 1, Daniel Hülsbusch 1, Joachim Hausmann 2
and Frank Walther 1
Department of Materials Test Engineering (WPT), TU Dortmund University, Baroper Str. 303,
D‐44227 Dortmund, Germany; andreas.kohl@tu‐dortmund.de (A.K.);
daniel.huelsbusch@tu‐dortmund.de (D.H.); frank.walther@tu‐dortmund.de (F.W.)
2 Leibniz‐Institut für Verbundwerkstoffe GmbH, Erwin‐Schrödinger‐Str. 58, D‐67663 Kaiserslautern,
Germany; stefan.schmidt@ivw.uni‐kl.de (S.S.); joachim.hausmann@ivw.uni‐kl.de (J.H.)* Correspondence: selim.mrzljak@tu‐dortmund.de; Tel.: +49‐231‐755‐8494
1

Citation: Mrzljak, S.; Schmidt, S.;
Kohl, A.; Hülsbusch, D.; Hausmann,
J.; Walther, F. Testing Procedure for
Fatigue Characterization of Steel‐
CFRP Hybrid Laminate Considering
Material Dependent Self‐Heating.
Materials 2021, 14, 3394. https://doi.org/10.3390/ma14123394
Academic Editor: Tomasz Sadowski

Abstract: Combining carbon fiber reinforced polymers (CFRP) with steel offers the potential ofutilizing the desired characteristics of both materials, such as specific strength/stiffness and fatigue
strength of fiber reinforced polymers (FRP) and impact resistance of metals. Since in such hybrid
laminates multiple material layers are combined, a gradual failure is likely that can lead to changes
in mechanical properties. A failure of the metal partner leads to an increase in stress on the FRP,
which under fatigue load results in increased self‐heating of the FRP. Therefore, a suitable testingprocedure is required and developed in this study, to enable a reproducible characterization of themechanical properties under fatigue load. The resulting testing procedure, containing multiple
frequency tests as well as load increase and constant amplitude tests, enabled characterization of
the fatigue performance while never exceeding a testing induced change in temperature of 4 K. In
addition to the development of the testing procedure, an insight into the manufacturing induced
residual stresses occurring in such hybrid laminates, which impacts the load‐bearing capacity, wasestablished using finite element simulation. The gathered data and knowledge represents a basis
for future in‐depth investigations in the area of residual stress influence on the performance ofhybrid laminates and highlights its importance, since not only the used testing procedure
determines the measured fatigue performance.

Received: 27 April 2021

Keywords: fiber metal laminate; thermoplastic; steel; carbon fiber; testing procedure; fatigue
behavior; self‐heating; residual stress

Accepted: 15 June 2021
Published: 18 June 2021
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1. Introduction
Hybrid materials such as fiber metal laminates (FML) made of fiber reinforced
polymers (FRP) and metals offer the potential of utilizing the desired characteristics ofboth materials. Especially when considering automotive and aerospace applications,where high performance materials are essential, the specific strength/stiffness and fatigue
strength of FRP, as well as impact resistance of metals [1–3] is beneficial. Furthermore, theincorporation of a metal constituent can increase the materials poor electrical conductivity
potentially enabling additional functionalities such as electrical shielding, grounding, or
structural health monitoring [4]. Carbon fiber reinforced polymer (CFRP), being one of
the few electrically conductive FRP, represents the superior FRP considering mechanicalperformance and has already been combined successfully with metals like steel [5],aluminum [6–8], or magnesium [9,10] to form hybrid laminates.

Materials 2021, 14, 3394. https://doi.org/10.3390/ma14123394

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However, due to different coefficients of thermal expansion (CTE), thermal residual
stresses (TRS) are induced in the manufacturing process while cooling down fromprocessing temperature. Due to the low CTE of FRP in fiber direction compared to metal,
tensile TRS occur in the metal constituent [11,12]. Hence, compressive TRS are present in
the FRP constituent (in fiber direction). Due to the superposition of TRS, the risk of FRP
buckling is increased and the apparent tensile and fatigue strength of the metal is reduced.
The latter is due to accelerated crack initiation and propagation by superimposed tensile
TRS under cyclic loading [13,14].
A further crucial aspect when considering hybrid laminates for applications undercyclic loading is self‐heating, which mainly influences the mechanical properties of thepolymer component. This is due to the viscoelastic material behavior of polymers, caused
by material damping associated with dissipation of energy [15,16]. The intensity of self‐
heating is load‐ as well as frequency‐dependent [17–19], since the dissipated energy
increases with both and therefore the temperature of the material. A simple reduction in
frequency to reduce the self‐heating is not always sufficient considering the mechanicalproperties of the polymer [15], since at too low frequencies ratcheting outweighs thebenefits of decreased self‐heating, reducing ultimately fatigue lifetime. Using the
proposed frequency adjustment method by Hülsbusch et al. [15], which considers anenergy‐based approach and was exemplified on polyamide 6, the testing duration can be
kept short while limiting self‐heating to a certain temperature. Since hybrid laminates
contain multiple materials stacked as layers, partial failure of single layers can occur,
leading to damaging of the laminate integrity and therefore changes in load distribution.
In case of metal layer failure, while the FRP still withstands the load, increased stress andthereby self‐heating results. Therefore, a further factor, the load distribution after partial
failure needs to be considered to limit the extend of self‐heating.
In this study, a testing procedure for hybrid laminates is developed, which takes
material dependent self‐heating into account. Profound frequency adjustment dependingon the applied stress is carried out, leading to a reproducible testing procedure for thecharacterization of hybrid laminate fatigue behavior while eliminating the temperature
influence. For this purpose, a hybrid laminate consisting of austenitic steel and carbon
fiber reinforced polyamide 6 layers was used and successfully characterized using thedeveloped procedure. Accompanying to the fatigue tests electrical resistancemeasurement and digital image correlation were used to conclude on events of material
degradation and damage propagation with regard to evaluation of the fatigue
performance and its usability for, e.g., structural health monitoring. In addition, themanufacturing process induced TRS, which impact the load‐bearing capacity, as shownby quasistatic tensile tests, were determined using finite element simulation. The results
highlight the presence of TRS and therefore the need of considering TRS for future
investigations, since not only the used testing procedure determines the measured fatigue
performance of a hybrid laminate.
2. Materials and Methods
2.1. Materials and Manufacturing
Laminate plates were manufactured using unidirectional carbon fiber reinforced
polyamide 6 (CFRP) tape (Cetex TC910, Tencate, The Netherlands) with a thickness of 0.15
mm. For the metal part, an austenitic steel sheet (X10CrNi18‐8; 1.4310, AISI 301) with athickness of 0.3 mm was used, which inhibits corrosion at the FRP‐metal interface. This
combination of CFRP and steel form a generic model material, without a specificindustrial application but for the special purpose of developing a testing procedure. To
improve the adhesion between the materials through surface treatment of the metal layer[20–23], the steel sheets were grid blasted with alumina. In addition, polymer foil (PA6,thickness 0.05 mm) was placed between composite and metal layers to improve adhesion,as the excess polymer fills the roughness of the blasted metal surface (Figure 1). The

Materials 2021, 14, 3394

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unidirectional hybrid laminates were manufactured in a metal to FRP configuration of 1
to 2 (hereinafter referred to as 1/2, consistent with the nomenclature introduced by
Gonzalez‐Canche et al. [22]) with symmetrical lay‐up: [C0°/C0°/PA/S/PA/C0°/C0°] (C‐CFRP,
S‐steel, with rolling direction 90° to fiber direction, PA‐polyamide 6 foil interlayer)
resulting in a total thickness of about 1 mm. The metal layer was located in the core of the
FML instead of the outer layers—unlike, e.g., GLARE—to address the challenge ofelectrical conductivity measurements of inner layers for future structural healthmonitoring or damage detection in materials testing. The constituents were driedovernight at 90 °C and subsequently consolidated at 260 °C and 25 bar for 20 min using amodified hydraulic upstroke press (SATIM, France, modified by Wickert, DE) with ashear edge tool. Tensile specimens were extracted from the FML sheets by water jet
cutting along the 0° orientation of the fibers.

Figure 1. Micrograph of the polymer steel interface with well infiltrated rough metal surface.

2.2. Specimen
For the investigations of this study, a dog bone‐shaped specimen geometry was used.
The geometry was self‐developed with a spline as transition from the parallel gauge
length to the clamping zone (Figure 2a), optimized for minimal stress concentration by
finite element analysis. To further reduce stress concentrations at the clamping, glass fiber
reinforced polymer tabs with a thickness of 1.5 mm were adhesively bonded to the clamping
area of the specimen with epoxy glue (Figure 2b). A speckle pattern was spray painted onto
the surface of the CFRP for digital image correlation (DIC) based strain measurement on thefront surface of the specimen. For selected fatigue tests, the specimen were equipped withelectrically conductive connection plates for measurement of direct current potential
drop. These connection plates were placed in‐between the protection tab and specimen
and adhesively bonded to them. To ensure electrical conductivity between the steel and
CFRP layers of the hybrid laminate and the connection plate, silver conductive paint wasapplied to the edges of the laminate.

(a)

(b)

Figure 2. (a) Specimen geometry, (b) prepared specimen with adhesively bonded tabs, electrically conductive connection
plates and speckle pattern for digital image correlation measurement.

Materials 2021, 14, 3394

4 of 172.3. Characterization and Testing Procedure
The following flow chart (Figure 3) gives an impression of the developed testingprocedure for a reproducible characterization of FML under tension loading. The used
testing methods and connections between those are described more in‐depth in thefollowing Sections 2.3.1 and 2.3.2. In general, this procedure is applicable to any material,
but in case of the FML, which use multiple materials having different material properties,
special consideration needs to be taken into account regarding the individual self‐heatingof each laminate partner.
Initially, quasistatic testing is carried out to determine tensile properties of thelaminate and laminate material partners. These values can be used for material
characterization, but also are needed to estimate maximum stress levels for load increase
tests. Using the load increase test, which needs to be executed under a constant testing
frequency, information about the extent of self‐heating under fatigue load at different
applied maximum stresses can be gathered. With that, multiple frequency tests are
executed on the dominant FML material regarding self‐heating. Using these tests, a
relationship between the change in temperature, testing frequency, and maximum stresslevel can be established. The resulting testing frequencies for a selected maximum change
in temperature are then validated using another load increase test to check if a further
adjustment of testing frequencies other than given from the relationship are necessary due
to partial material partner failure. Further adjustments can again be validated using loadincrease tests until a constant specimen temperature is achieved throughout the loadincrease test. This information can then be used to characterize the fatigue properties of
the FML using a constant specimen temperature and therefore, excluding the influence oftemperature change on the fatigue properties.

Figure 3. Flow chart showing the developed testing procedure for a reproducible characterizationof the mechanical properties of fiber metal laminate under fatigue load.

Materials 2021, 14, 3394

5 of 172.3.1. Quasistatic Behavior
Tensile properties were identified in accordance with DIN EN 527‐5 on a servo‐
hydraulic testing system (MTS 858 Mini Bionix II, Fmax = ±25 kN, MTS Systems, Eden
Prairie, USA). The total strain was measured with the use of DIC. A minimum of three
specimens were tested for single CFRP and steel layer material as well as the hybrid
laminate.
2.3.2. Cyclic Behavior
For fatigue characterization, load increase tests (LIT) and constant amplitude tests(CAT) were carried out on the same servo‐hydraulic testing system as for the quasistatic
tests. Fatigue load was applied with the use of a sinusoidal tension‐tension load‐timefunction at a stress ratio R = 0.1. LIT were conducted with a starting maximum stressσmax,start = 50 MPa, followed by stepwise stress increase of Δσmax = 25 MPa per ΔN = 2.5 ×
103 cycles up to specimen failure. For CAT, the maximum stress levels were derived fromthe LIT test results using the measurement information regarding points of characteristic
microstructural changes to get an impression about the overall fatigue lifetime of thehybrid laminate.
To determine suitable fatigue testing frequencies, the laminate structure dependent
self‐heating was investigated with the use of multiple frequency tests (MFT) [15]. Since
the CFRP is mostly influenced by self‐heating, only this material out of the hybrid
laminate was investigated. Therefore, the determined testing frequencies represent a
conservative estimate for the hybrid laminate since the total self‐heating is reduced dueto the steel partner. Additionally, after a partial failure of the hybrid laminate (steel
failure), the testing frequency can be adjusted to the remaining CFRP and effective stress.
The MFT were conducted at maximum stresses of σmax = 200 to 1000 MPa. The startfrequency and change in frequency for each maximum stress level are shown in Table 1.
Each frequency was held for 5 min during the MFT to enable temperature stabilization.
Due to testing machine restrictions and required consistent high control accuracy, testing
frequencies of 30 Hz were not exceeded. For a comparison to results at constant testing
frequency, self‐heating was also investigated in LIT at a constant testing frequency of f =10 Hz. The finally determined frequencies for the CAT were chosen for a resulting,maximum increase in surface temperature of 2 K due to testing frequency during the test,
resulting in a negligible impact of self‐heating in the specimen’s cross‐section on themechanical properties while attaining acceptable testing duration compared to a constanttesting frequency of f = 10 Hz.
Table 1. Parameters for multiple frequency tests for laminate dependent self‐heating investigation.

σmax (MPa)fstart (Hz)Δf (Hz)

200
5
5

300
5
5

400
5
5

500
5
5

600
2
2

700
2
2

800
2
2

900
2
2

1000
2
2

Figure 4 shows the experimental setup. Besides monitoring of the hybrid laminates’change in dynamic stiffness Cdyn (change of stress divided through the change of piston
displacement: (σmax − σmin)/(smax − smin)) the underlying microstructural changes weremonitored by multiple sensors for correlation. A Limess Q400 DIC system (Limess
Messtechnik und Software, Krefeld, DE) containing one camera with a precision lens (28mm focal length) was used to measure the 2‐dimensional local deformations on the front
specimen’s surface. Pictures were taken during the quasistatic tests time‐triggered, while
for the fatigue tests a Limess Maxtrigger box was used to acquire pictures at the maximum
stress with an acquisition rate of every 100th cycle during LIT. This rate was adjusted for
the CAT according to the used testing frequency, aiming at around 1000 pictures per test
to achieve a sufficiently high data density while taking a processable amount of data
volume into account. With the evaluation software Istra 4D V4.4.7 (Dantec Dynamics,

Materials 2021, 14, 3394

6 of 17Ulm, DE) the total strain and maximum total strain were extracted from the DIC
measurements using 25 mm virtual gauge line elements in the central section of thespecimen.

Figure 4. Experimental setup containing servo‐hydraulic testing system and metrology
instrumentation.

The change in surface temperature was recorded using a MicroEpsilon TIM 160(Micro‐Epsilon Messtechnik, Ortenburg, DE) thermocamera. Additionally, the change inelectrical resistance of the hybrid laminate was measured during selected fatigue tests toindicate its potential for monitoring the electrical conductivity during load to conclude
onto the damage state of the steel as well as CFRP. For this direct current potential drop
measurement and Ohm’s law were applied. A Sorensen XG 100‐8.5 power supply(AMETEK Programmable Power, San Diego, CA, USA) was used as an electric current
source in combination with a National Instruments cDAQ 9174 and NI‐9238 module(National Instruments, Austin, TX, USA) for voltage drop measurement.
To ensure that the induced electric current does not increase the change intemperature of the specimen due to its electrical resistance, investigations were conducted
regarding the temperature and electric current relationship of the laminate partners. The
steel and CFRP have different electrical resistivity and temperature coefficients andtherefore, lead to different magnitudes of resistance and temperature at applied electric
current. Figure 5 shows the influence of electrical current on the change in temperature ofa single layer of steel (Figure 5a) and CFRP (Figure 5b) of the hybrid laminate. The
electrical resistance value scatter illustrates the measurement deviation for selected
electric currents. While for steel up to an electrical current of 1.5 A the change intemperature stays below 2 K, for CFRP this is the case only up to 200 mA. After this, thetemperature increases significantly with increasing electric current, which is why for thedevelopment of the testing procedure and fatigue performance characterization the
maximum electric current of 200 mA is seen as a suitable value for the whole laminate tolimit the temperature increase to a negligible level.

Materials 2021, 14, 3394

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Figure 5. Influence of electrical current on the change in temperature of (a) 1.4310 and (b) carbon fiber reinforced PA6. Thechange in temperature axis in (b) is also valid for graph (a).

2.4. Thermal Residual Stress Calculation
Thermal residual stresses (TRS) occur due to the mismatch of the thermal expansion
coefficients of the constituent materials. As a first estimation, the following equation (1)can be used to calculate TRS (derived from [24]):

TRS

E ∙ ∆T CTE

CTE

(1)

where index i denotes the constituent (CFRP, steel or PA interlayer), index FML theoverall FML property, the temperature difference ( T) between consolidation andambient temperature, and Young’s modulus (Ei) of constituent i. CTE of FML can becalculated using the following equation (2) [24]:

CTE

E

CTE

V
E

E
V

CTE
E
V

V
E

E
V

CTE

V

(2)

V denotes the volume fraction of the corresponding constituent.
Table 2 summarizes the elastic material properties used as input taken fromcorresponding tests. Calculations were done for FMLs with and without PA interlayers.
Table 2. Material properties for stress calculation.

Material
CFRP (0°)
Steel
PA interlayer

E (GPa)
108
175
1.844

CTE (ppm/K)
2.85
16.5
95

For a detailed estimation of TRS, a finite element (FE) simulation was carried out
using Abaqus 2020 (Dassault Systèms, Vélizy‐Villacoublay, France). For the CFRP, anorthotropic material model was implemented. Engineering constants were taken fromcorresponding
tests(E1 = 108 GPa,
E2 = 6.95 GPa,
ν12 = 0.3,
G12 = 3.59 GPa,
CTE2 = 101 ppm/K). For steel and PA plasticity was implemented using measurement datataken from tensile tests. The FML was modeled with shell elements and a composite
layup. A homogeneous cooling of 240 K was carried out using a predefined field.
Additionally, for one calculation the PA interlayers were replaced by E‐glass twill weaves(E = 21.9 GPa, CTE = 23 ppm/K, thickness = 0.23 mm) reinforced PA to reduce the tensile
TRS of the metal constituent.

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3. Results and Discussion
3.1. Quasistatic Behavior
The obtained results gained through quasistatic tensile tests are visualized in Figure
6 for the single materials (Figure 6a) steel 1.4310 and CFRP (Figure 6b) as well as for the
hybrid laminate (Figure 6c). Up to the nominal stress level of around 1200 MPa, before theappearance of a significant increase in plastic strain of the steel, the stress–strain
development of steel and the CFRP show similar trends. While the strain of the steel
increases constantly after this point due to plasticity until failure, CFRP is able towithstand further load with a linear increase in strain as well as partial fiber breakage.
The combination of both materials, therefore, should represent a good match regarding
quasistatic properties, which is visible in Figure 6c. The resulting stress–strain curves canbe seen as a representative overlay of the single materials stress–strain curves, besides themissing visibility of plasticity of the steel due to the dominant CFRP stiffness. In its hybrid
laminate state, this combination retains the high mechanical performance of steel and
CFRP, while improving the electrical conductivity of CFRP.

(a)

(b)

(c)

Figure 6. Stress–strain curves of (a) 1.4310, (b) carbon fiber reinforced PA 6, and (c) both combined as hybrid laminate ina 1/2‐configuration. Each color represents one specimen tested. For the hybrid material the stress–strain curves of theconstituents are given for comparison in the diagram.

3.2. Testing Frequency Induced Self‐Heating of the Hybrid Laminate
In a first instance, a LIT with a constant testing frequency of 10 Hz was conducted on
the hybrid laminate to get an impression regarding the self‐heating induced change intemperature of the specimen (Figure 7). With increasing maximum stress and therefore
strain, the surface temperature increases up to around 4 K until failure of the steel layer(at about 66,000 cycles). The initiation of upcoming steel layer failure is visible in the
dynamic stiffness and electrical resistance values, showing that the measurement of thelatter can be used to replace conventional measurement techniques for damage state
assessment. In the following, where the load is applied just to the remaining CFRP‐layers,
the electrical resistance increases with failing fibers and the temperature increases up to 8
K before failure. The damage induced change in temperature can be a relevant factor thatneeds to be taken into account for, e.g., the upcoming steel failure, but is considerednegligible at times where the mechanical properties show constant development. Looking
at the results a significant temperature increase is visible when using a constant testing
frequency of 10 Hz.

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Figure 7. Load increase test for hybrid laminate in 1/2‐configuration of 1.4310 and carbon fiber
reinforced PA 6; development of dynamic stiffness, change in temperature and electrical resistancewhen testing with constant frequency.

Figure 8a shows exemplarily the frequency dependent change in temperature
measured during MFT at σmax = 500 MPa. The testing method enables the measurement of
stable temperature plateaus during fatigue load for a characterization of the temperature‐
testing frequency relationship of CFRP. For this test, the change in temperature appears
as constant increases with establishment of temperature plateaus up until the maximum
testing frequency of f = 30 Hz. If in such a test continuous increases in temperature occur
within one frequency step, which hints at are more complex causes such as too excessive
strain rates and material damage, these steps are not considered for the determination of
the temperature and testing frequency relationship. Figure 8b visualizes the derived
testing frequency‐maximum stress relationship for ΔT from 1 to 5 K. Due to different
temperature development before and after σmax = 600 MPa, two different fitting functions
are needed and implemented, in this case, an exponential and parabolic fit.

(a)

(b)

Figure 8. (a) Exemplary multiple frequency test on carbon fiber reinforced PA6 and (b) derived testing frequency‐
maximum stress relationship visualized for ΔT from 1 to 5 K.

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When applying the testing frequency relationship from Figure 8b, considering amaximum change in surface temperature of 2 K, to a LIT on the hybrid laminate, the results
below (Figure 9) are achieved. The change in temperature develops until σmax = 400 MPa
similarly compared to the LIT with a constant testing frequency of 10 Hz, since only after
this step the constant testing frequency of 10 Hz is higher than the adjusted ones. After
that, the change in temperature does not reach 2 K until failure of the steel layer, since thedetermined frequency values are valid for the CFRP layer, rather than the whole hybrid
laminate. After the failure of the steel layer the change in temperature increases above 2 K.
This is related to the significantly higher effective stress on the CFRP layers, leading tohighly increased strain and therefore self‐heating.

Figure 9. Load increase test for hybrid laminate in 1/2‐configuration of 1.4310 and carbon fiber
reinforced PA 6, showing the development of dynamic stiffness and change in temperature when
testing with adjusted frequency for a change in temperature of 2 K.

To compensate for this temperature increase, a laminate integrity dependent
adjustment of the testing frequencies needs to be taken into account. When considering
the remaining cross‐sectional area of the specimen after the failure of the steel, presuming
a mostly intact CFRP laminate, the testing frequencies can be adapted to the effective
stress on the CFRP layers. Using this approach, the maximum change in surface
temperature does not exceed 2 K during LIT until specimen failure (Figure 10). The
presumption of mostly intact CFRP seems valid, since the change in temperature, using
the determined frequencies for CFRP (Figure 8b), is kept as low as it is supposed by thedetermined relationship. The DIC‐based measured maximum total strain on thespecimen’s surface supports this. Immediately after the failure of steel the related stiffness
drop correlates to a change in maximum total strain, leading to different slopes of thelinear increase in strain from start until failure of the hybrid laminate. Only at the end of
the test, significant changes in stiffness, strain, and temperature are visible, leading to thenon‐linear change in surface temperature, which is material damage related and therefore
not covered by the used method of testing frequency adjustment.

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Figure 10. Load increase test for hybrid laminate in 1/2‐configuration of 1.4310 and carbon fiber
reinforced PA 6, showing the development of dynamic stiffness, change in temperature andmaximum total strain for testing with adjusted frequency for a change in temperature of 2 K in
combination with further adjustment of the frequency after failure of 1.4310 related to the effective
stress.

3.3. Fatigue Behavior
As shown in Section 3.4 in the LIT results, the hybrid laminate exhibits a
stepwise damage sequence and thus undergoes graduated damage development.
The failure of the steel layer is the most prominent, leading to a significant change
in the stiffness of the laminate. The DIC measured maximum total strain t,max(Figure 11) shows that with increasing maximum stress an earlier failure of thesteel layer is evident, represented by the high sudden increase in strain. This
failure mode highlights that the steel layer is the weakest part of the laminate.
Considering the residual stress calculations, which showed to be in tensile
direction and therefore, reduce the lifetime for the investigated stress ratio of 0.1,
the significance of the influence of thermal residual stresses superimposed toexternal stresses is an important factor.

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Figure 11. Maximum total strain measured via DIC during CAT on the surface of the hybrid
laminate in 1/2‐configuration of 1.4310 and carbon fiber reinforced PA 6.

After the failure of the steel layer, the CFRP endures higher stress proportional to theloss in cross sectional area of the steel layer. This leads at maximum stresses higher than
600 MPa to initial partial fiber detachment and failure at the edges and specimen radii,
visible in the sudden and afterward steady increase in strain after steel failure. This
propagates at stresses higher than 900 MPa and ultimately reduces lifetime significantly.
Figure 12 shows the local strain occurring on the CFRP surface during CAT with σmaxof 600 and 900 MPa, which highlights the failure of the steel layer and strain development.
The observable steel failure, which initiation is visible by the local increases in strain at
23.4 × 103 cycles for 600 MPa and at 2.3 × 103 cycles for 900 MPa, shows to be comparable
despite the different applied maximum stress. The reason for the angled strain patternduring steel failure lies in the one‐sided crack initiation, leading to strain accumulation
development in the adjacent region. When steel failure starts, in only a few hundred cycles
the whole CFRP layers delaminate, leading to nearly a doubling in strain. The damage
mechanisms and resulting change in CFRP load are the same, leaving only the initiation
time as a major difference. After the steel failure, the increase in strain for σmax = 900 MPa
leads to the continuous development of fiber detachment and failure visible at the edgesof the laminate (n = 2.5 × 103 to 105), which still does not lead to failure, demonstrating thehigh load bearing capabilities at even higher effective stresses considering the missing
cross‐sectional area of the steel layer.

Figure 12. DIC strain measurement during CAT with σmax of 600 and 900 MPa on the surface of the hybrid laminate in 1/2‐
configuration of 1.4310 and carbon fiber reinforced PA 6. Selected cycles are shown, visualizing the strain development
before and after 1.4310 failure as well as fiber failure at the edges of the laminate (red arrows).

As a result, the hybrid laminate can withstand significant structural damage withoutcatastrophic failure while still delivering high performance during tension‐tension
loading. Looking at Figure 13, the S–N relationship shows that only at comparably low
maximum stresses of 300 MPa or lower the steel layer remains intact, maintaining the
integrity of the hybrid laminate for the full lifetime of 2 × 106 cycles investigated in thisstudy. This aspect has to be kept in mind when considering such a hybrid laminate forMaterials 2021, 14, 3394

13 of 17applications, where the electric conductivity of the structure is a necessary property, e.g.,
for electrical data transfer to substitute, e.g., electrical cables. The relationship between
applied maximum stress and number of cycles to failure of the steel layer (Figure 13, bluedots) follows almost a straight line in the half‐logarithmic graph and can therefore bedescribed as a power function, which is well suited for lifetime prediction using only a
few tests. Minimizing the residual stresses occurring in the steel layer should lead tolonger retention of the steel layers load bearing capabilities and thus, longer lifetime. Thispossibly is simply describable with help of the S–N graph through a shift of the steel layer
lifetime to the right, depending on the intensity of residual stress reduction.

Figure 13. S–N relationship for hybrid laminate in 1/2‐configuration of 1.4310 and carbon fiber
reinforced PA 6. Failure for 1.4310 and postponed failure of carbon fiber reinforced PA 6 are marked.
For tests with σmax below 900 MPa the tests were stopped after steel failure, since carbon fiber
reinforced PA 6 proved to withstand 2 × 106 cycles at 900 MPa after failure of 1.4310.

3.4. Residual Stresses
Stresses calculated analytically and numerically using the properties and calculation
methods presented in Chapter 2 are summarized in Table 3. Figure 14 visualizes an
exemplary TRS constellation.
Table 3. Calculated thermal residual stresses (S11  fiber direction) using different methods andcomposite layups.

Interlayer
none
none
50 μm PA50 μm PA
Glass weave/PA

Calculation
Material Model
Methodanalytical‐numerical
orthotropic
analytical‐numerical
orthotropic
numerical
orthotropic

CFRP
−158
−119
−162
−122
−148

Steel
(MPa)
317
239
311
231
191

Interlayer
‐
‐
38
38
69

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Figure 14. Exemplary visualization of the calculated thermal residual stress (numerical simulation
with orthotropic material model) in the investigated hybrid laminate.

Generally, it has to be noted that all the calculated stresses have to be considered as
an idealized upper limit. For all calculating methods temperature dependent properties
and viscous material behavior were neglected. Especially at elevated temperatures, it canbe expected that residual stresses are partly reduced due to relaxation. Furthermore, arigid connection of both components at the interface will not occur while the polymer isin a liquid state, so the effective temperature difference can be expected to be less than
240 K.
The different calculating methods (analytical and numerical) yield very different
results. Stresses calculated analytically are about 25% higher than stresses calculated
numerically with an orthotropic material model. In comparison, the lower TRS from thenumerical method can be considered the most accurate since anisotropy and transversal
strains are neglected by the other simplified method. However, due to the describedcalculating difficulties of all methods, TRS need to be accurately measured for a more
precise stress analysis.
The PA interlayer only shows a minor influence on TRS. The compressive stresses in
CFRP are intensified by about 2% and the tensile stresses in steel are reduced by about 1–
3%. This minor influence is due to the low modulus and thickness fraction of PA despite
its high CTE.
The calculation using glass weave reinforced PA as interlayer shows the lowest
tensile TRS in the metal constituent and higher compressive TRS in the CFRP. This is dueto the higher CTE of the FML, resulting from the relatively high CTE and moderate
stiffness of the glass weave reinforced PA interlayer. The higher CTE of the FML causes alarger shrinkage during cooling, yielding in the different TRS distribution.
Given the tensile nature of the residual stresses in the metal constituent, a detrimental
influence on fatigue strength is evident [13]. The results of the quasistatic investigations
shown in Figure 6 show how the performance of the overall laminate can be affected by
TRS since the combination leads to lower ultimate tensile strength compared to the single
materials. Therefore, investigating and reducing TRS are critical parts of fatigue/lifetime
analysis.

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4. Conclusions and Outlook
The following conclusions can be drawn from the investigations on the hybrid
laminate in 1/2‐configuration of 1.4310 and carbon fiber reinforced PA 6:
For a comparative characterization of mechanical properties, a testing procedure wasdesigned and evaluated, which considers prevention of the mostly polymer related self‐
heating effect, as well as the laminate integrity dependent changes in this regard due toincreases in effective stress after the failure of the metal component. Using multiple
frequency tests, a relationship between the self‐heating and testing frequency was foundand used to enable fatigue tests with changes in temperature up to a defined value while
retaining as short as possible testing durations. Using the load increase tests, it was shownthat an additional frequency reduction after the failure of the steel layer is necessary tomaintain the self‐heating of the remaining hybrid laminate, excluding temperature as apossible influence regarding the fatigue capability.
The hybrid laminate proved to be capable of high performance, showing a fatigue
lifetime of a minimum of 2 million cycles at a maximum stress of 900 MPa for a stress‐
ratio of 0.1. The steel layer, which is the weaker material in the hybrid laminate, is failingfirst, which needs to be taken into account when using this laminate also because of its
good electric conductivity. A measurement of the laminates’ electrical resistance revealed
that the state of electrical conductivity can be monitored reliably throughout damage
processes, but when considering long‐term measurement in industrial applications otherfactors like electric current induced acceleration of corrosion processes need to be kept inmind and evaluated profoundly.
Due to the differences in coefficients of thermal expansion of 1.4310 and CFRP, thehybrid laminate leads to thermal residual stresses, which impact the mechanicalproperties. The used calculation method gives an impression about the quantitative extent
of the occurring thermal residual stresses for each layer contained in the hybrid laminate.
The CFRP component underlies compressive residual stresses around 120 MPa, while thesteel exhibits tensile residual stresses around 230 MPa. For a closer consideration ofthermal residual stresses, precise measurements need to be included in a stress analysis.
Many parameters such as temperature dependent viscoelasticity or effective temperature
difference are very difficult to incorporate into a bare calculation without validation.
Possible measurement methods include X‐ray diffraction, modified hole drilling, or theobservation of yield strength shifting.
Considering tension‐tension loading, the occurring thermal residual stresses
contribute negatively. To enhance lifetime performance, a reduction of the tensile thermal
residual stresses of the steel layer needs to be approached in future work. Exploiting themetals’ plasticity, post‐stretching of the hybrid laminate decreases thermal residual
stresses or even induces compressive stresses in the steel layer. In addition, adjusting the
FML layup using different interlayer materials can have a beneficial effect due to different
stiffness distributions and the resulting coefficients of thermal expansion of the hybrid
laminate, as discussed in the calculation using a glass weave PA interlayer.
Further, the aspect of corrosion needs to be investigated in detail for such laminate
configurations, since galvanic corrosion is known to occur between steel and carbon fiberin a large extent and the assessment of remaining service life is important. Therefore,
continuous monitoring of electrical conductivity during salt‐spray‐tests as well as within
combined in situ corrosion fatigue tests could lead to necessary data for determining
advanced lifetime prediction methods.
Author Contributions: Conceptualization, S.M. and S.S.; methodology, S.M., S.S., D.H., J.H. andF.W.; validation, S.M., S.S. and A.K.; mechanical testing for developing a testing procedure forcharacterization of fatigue behavior and quasistatic tests were performed by S.M., A.K. and D.H.;
evaluation of the mechanical testing results was performed by S.M., A.K., D.H. and F.W.; specimen
manufacturing and residual stress analysis were carried out by S.S. and J.H.; visualization, S.M., S.S.
and A.K.; the original manuscript draft was conceived by S.M., and written by S.M. and S.S.; D.H.,

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J.H. and F.W. have had discussions on the topic and reviewed the manuscript. All authors have readand agreed to the published version of the manuscript.
Funding: The authors gratefully acknowledge the funding by the German Research Foundation(Deutsche Forschungsgemeinschaft, DFG) of project “Mechanism‐correlated characterization of thedeformation and damage behavior of thermoplastic‐based fiber metal laminates for property‐
oriented process development” (WA 1672/56‐1, WA 1665/9‐1), since approaches of the developed
testing procedure were transferred to the investigations within this paper.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.

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