in-situ damage comparison between fabric-fabric and fabric … · different load steps with the...
TRANSCRIPT
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IN-SITU DAMAGE COMPARISON BETWEEN FABRIC-FABRIC AND
FABRIC-UD BONDED CFRP
Christian Hannesschläger1, Stefanie Rauchenzauner2, Christian Gusenbauer1, Dietmar Salaberger1 and Johann Kastner1
1University of Applied Sciences Upper Austria, Stelzhamerstrasse 23, 4600 Wels, Austria, e-mail:
[email protected], [email protected], [email protected], johann.kastner@fh-
wels.at
2FACC, Fischerstraße 9, 4910 Ried im Innkreis, Austria, e-mail: [email protected]
Abstract
An interrupted in-situ X-Ray computed tomography (XCT) was used to investigate the influence of different Carbon fibre
reinforced polymers (CFRP) material systems on the crack progression and propagation of adhesive jointed CFRP plates. The
damage behaviour of adhesive jointed CFRP is decisive for the mechanical strength. For this paper the material systems (I)
CFRP fabric plate with another CFRP fabric plate and (II) a CFRP fabric plate with a unidirectional (UD) CFRP plate were in-
situ investigated with a high resolution X-Ray computed tomography (XCT) with a voxel size of (6 µm)³ at different load
steps. The damage volumes of both material systems were segmented and compared with each other. The results confirmed the
stress concentration at the end of the specimens [14]. In general the CFRP-cracks in the plates can be identified as major
damage mechanism. The different strength of the two material systems is caused by the missing damage in the UD-plate. This
lack of damage is caused by the high crack resistance of the UD-plies in respect to the force direction and results in a higher
strength of the system fabric-UD.
Keywords: x-ray tomography, in-situ, lap shear testing, adhesive bonded CFRP
1 Introduction
Carbon fibre reinforced polymers (CFRP) are widely used in aeronautic industries for their special properties such as light
weight, high specific stiffness, high specific strength and the high corrosion resistance [1]. An important manufacturing issue is
the bonding of CFRP plates. Therefore adhesives are widely used. A main advantage of the adhesives is the homogenous stress
distribution in comparison to other joining techniques [2].
The microstructure of the jointed area is decisive for the mechanical strength of adhesive jointed CFRP plates. In the past
several studies investigated the failure behaviour of bonded composites joints [3-6]. Therefore, light optical microscopy or a
visual inspection [7] was used to characterize the failures. With these methods a non-destructive 3D tracking of the crack
propagation is not possible. High resolution X-Ray computed tomography (XCT) as common non-destructive technique (NDT)
is used to characterise defects in CFRP specimens [8-9]. In the past in-situ XCT has already been used to investigate damage
mechanism in CFRP [10-11]. H. Kunz et al. showed a technique for particle tracking in polyurethane adhesive bonded CFRP
and steel with in-situ XCT while performing a lap shear test [12].
In this paper, the method in-situ XCT is used to determine the damage progression in adhesive during an adapted slotted single
lap shear test [13]. With interrupted in-situ XCT measurements under different load conditions it is possible to mechanically
test the adhesive jointed CFRP plates and to visualise and study the influence of different CFRP material combinations on the
crack progression and propagation.
2 Experimental Method
2.1 Material and Specimen
A supported epoxy adhesive was used to bond two different material systems: (I) CFRP fabric plate with another CFRP fabric
plate and (II) a CFRP fabric plate with a unidirectional (UD) CFRP plate. In both cases the adhesive layer is 0.2 mm thick. An
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epoxy resin based adhesive, with a polyester fabric, was used for bonding. The adapted slotted single lap shear test specimens
were cut out of the bonded plates by jet cutting (red area in Figure 1).
Figure 1: (a) sketch of adapted slotted single lap shear test specimen (dimensions are given in mm, measurement area is indicated in red); (b)
cut section in detail, in which regions “top” and “bottom” are discussed in more detail by in-situ XCT slice images.
2.2 High-resolution XCT
The specimens were scanned with a Nanotom 180NF XCT device (GE phoenix | X-ray) with a voxel size of (6 µm)³ at
different load steps. This system consists of a sub-µ-focus X-ray tube and a 2304x2304 pixels flat panel pixel detector
(Hamamatsu). The scanning time was 80 min per load step. A beam hardening correction was applied to the data. The 3D
volume was analysed by using the software VGStudio MAX 3.0.
2.3 In-situ tensile stage
The experiment was performed with the CT5000 5 kN in-situ tensile stage (Deben) at room temperature (see Figure 2). Main
specifications of CT5000 stage can be found in Table 1.
A displacement was applied with 1 mm/min. An interrupted slotted single lap shear test, with adapted dimension, was used to
identify damage at several load stages. Reference specimens of both material systems were tested to determine the quasi static
load steps for the in-situ experiment. The reference investigations showed a first detectable damage at ~2/3 of Fmax. Larger
material failures can be segmented at around 90 % of the maximum load.Table 2 shows the applied load steps in relation to the
maximum force of the reference specimen.
relative percentage of reference Fmax [%] stepi. breakage
Fabric-Fabric:
Force [N]
Fabric-UD:
Force [N]
0 1 - 0 0
66.6 2 - 1000 1550
90 3 - 1400 2100
95 4 - 1500 2200
~100 5 yes 1606 2205
Table 2: planned load steps for measurement
To ensure a stable scan without any motion artefacts, a relaxation time is needed (see Figure 3).
Specifications Tensile stage CT5000
Max. load 5000 N
Max. motor speed 1 mm/min
Modes tensile/compression
Max. extension 10 mm
Accuracy 1% of full scale range
Weight ~5.5 kg
Table 1: Specification of tensile stage CT5000
Z
Y
Ten
sile
sta
ge
X-r
ay s
ou
rce
mo
un
ted s
pec
imen
(a) (b)
(a) (b)
top
bott
om
adhesive
CFRP
CFRP
Figure 2: (a) Position of tensile stage within XCT-device; (b)
specimen position with respective coordinate system
Y
Z
X
Z
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(a) (b)
Figure 3: (a) Examplaric diagram of all force cycles during an in-situ experiment over time (fabric-UD); (b) relaxation of the specimen: a
relaxation time of 6 minutes is used to ensure almost stable conditions during XCT scanning
2.4 Segmentation
The 32bit data was mapped to 16bit, to ensure that the average grey level of the air (10000) and adhesive (50000) peak is
always at the same grey value position. For noise reduction a median 3x3x3 filter was applied. The advanced surface
determination and the porosity analysis tool of VGStudio MAX 3.0 were used to segment cracks, debondings and pores. To get
rid of the surrounding air, the adaptive rectangular tool (ISO-90 Threshold) was applied. The resulting region of interest was
used for defect detection (region 1). To distinguish between the different failure modes, first of all the defects of the unloaded
specimens were segmented. For the pore segmentation the VGStudio pore analysis tool (only threshold: threshold ISO-50) was
used. Big pores are defined by size (larger than 1100 voxels) and sphericity (threshold of 0.36). The value 0.36 was determined
empirically .The sphericity threshold separates pores from cracks. In the system fabric-UD the biggest pore was calculated
separately without sphericity threshold, due to the low sphericity of this defect. To determine the cracks of the initial state an
inverted advanced surface determination (ISO-25) was used for segmentation (see Figure 4).
unloaded
1) pore segmentation on region 1 (step1)
2) subtract pores of region 1 = region 2 (step1)
3) region 2: invert surface (ISO-25)
4) invert surface = roi cracks of initial state 0N (region
cois(step1))
loaded
1) pore segmentation on region 1 (stepi)
2) subtract pores of region 1 = region 2 (stepi)
3) region 2: invert surface (ISO-25) = region 3 (stepi)
4) subtract dilated region cois (step1) of region 3 =
region 4 (stepi)
5) region 4: defect detection (threshold: 65 535)
6) region 4: sorting of defects by their x-coordinate
7) subtract defects inside the adhesive (x-coordinate) of
region 4 defects = CFRP cracks (region 5(stepi))
8) subtract region 5 of region 4 = debonding of
polyester fabric (region 6 (stepi))
9) subtract regions 5 and 6 of region 3 = cois (stepi)
region
Figure 4: Segmentation flow chart of unloaded (step1) and loaded (step3-4) specimen
measurement time
of an XCT scan
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At the following load steps a separation of CFRP-cracks, debonding of polyester-fabric and cracks of initial state is necessary
because all three failure types are segmented by the advanced surface determination.The cracks of initial state can be easily
eliminated by dilating and subtracting the defect volume of 0 N initial state of the inverted advanced surface determination,
due to the fact that the volume of the initial state crack is not overlapping with the volume of any other defects. Afterwards
defect detection was applied on the remaining region. To separate debonding from CFRP-cracks, the defects were sorted by
their X coordinate position. Every defect inside the adhesive along the X-axis is detected as debonding of polyester-fabric.
After this step the CFRP-cracks and debonding were subtracted of the actual inverted advanced surface determination volume
to calculate the load step specific cracks.
3 Results and Discussion
The progression of damage could be characterised by registering the individual load steps on each other. Different damage
types such as pores, debonding of polyester fabric and CFRP cracks were identified and segmented. A comparison of the
different load steps with the unloaded specimen allows a determination of the influence of different damage types on the final
breakage. Figure 5 shows the obtained results of fabric-fabric.
Figure 5: Fabric-fabric: comparison of unloaded (a) and broken status (b) shows a final fracture mainly in the CFRP. The volumes top and
bottom regions are visualized in detail in Figures 6-8.
Figure 6 shows a slice images (Z-X) of the fabric-fabric system at top and bottom region. From the beginning of damage
initiation to 95% Fmax,reference the defects can be observed only within these regions. Defect propagation can be seen at certain
load steps. The area of defect origin is highlighted by a red cross. It can be observed that the defect starts by a debonding of the
polyester fabric within the adhesive (see Figure 7). After this initiation the observed cracks are growing mainly through the
CFRP (see figure 6: 1400 N and 1500 N). It can be noticed that the CFRP cracks are expanding in load direction (z-axis) after
they have reached the first CFRP-layer. The defects can be detected only in one CFRP plate at the positions top and bottom.
These stress concentrations at the end of the overlap is a well-known problem of single lap-joints [14]. The final breakage goes
primarily through the CFRP. It is noticeable, that the debonding failure in the bottom region is closed after the final breakage
(see Figure 6: bottom 1606 N).
carbon fibre bundle CFRP-matrix
polyester fabric adhesive to
p
bo
tto
m
CFRP-cracks
top-region bottom-region
top-region bottom-region
1 mm 1 mm
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0 N 1000 N 1400 N 1500 N 1606 N
Figure 6: Fabric-Fabric slice images comparison (top/bottom region Z-X) of the defect propagation at certain load steps; failure mode:
debonding of polyester fabric (green) and CFRP-cracks (red). Region (a) is shown in detail in figure 7.
Figure 7: Debonding of the polyester fabric within the adhesive in detail at load step 1000 N
Figure 8 shows the comparison of slice images (Z-X) of the fabric-UD system at top and bottom position. The pictures of
column 0 N show CFRP cracks in the initial state. These initial cracks are not changing throughout all load steps. At 1550 N
the first force induced defects are visible (debonding, CFRP-crack). It is obvious that no damage can be detected in the UD-
plate at any load step.
(a)
to
p
CFRP-cracks
500 µm
bo
tto
m
Z
X
100 µm
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0 N 1550 N 2100 N 2200 N 2205 N
Figure 8: Fabric-UD slice images (top/bottom region Z-X) of the defect propagation at certain load steps; failure mode: debonding of
polyester fabric (green), CFRP-cracks (red) and initial cracks (blue)
The CFRP-cracks are expanding in load direction after they have reached the first ply. No further crack deflection can be
observed. The same has been seen before at the system fabric-fabric. The CFRP-cracks are the dominating failure leading to
the final fracture.
A comparison of minimum intensity projections at top and bottom position shows that the cracks are growing in both material
systems only in the CFRP fabric layers next to the adhesive. (see Figure 9). The observed damage initiation occurs mainly in
the adhesive layer (see Figure 9: column ~2/3 Fmax). Afterwards the cracks are continuously growing through the adhesive
layer and the CFRP with increasing load. It is noticeable, that there are no defects in the UD-plies. A growth of initial cracks
and pores cannot be observed.
to
p
bo
tto
m
Z
X
Debonding
of polyester
fabric
CFRP-cracks
init
ial
crac
ks
500 µm
init
ial
crac
ks
deb
on
din
g o
f p
oly
este
r fa
bri
c
CFRP-cracks
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Top: ~2/3 Fmax,ref Top:95% Fmax,ref Bottom: ~2/3 Fmax,ref Bottom: 95% Fmax,ref
Top: ~2/3 Fmax,ref Top:95% Fmax,ref Bottom: ~2/3 Fmax,ref Bottom: 95% Fmax,ref
Figure 9: Comparison of different load steps of top and bottom regions: damage propagation is visualised minimum intensity projections (thickness: 300 µm) of (a) fabric-fabric and (b) fabric-UD; failure mode: debonding of polyester fabric (green), CFRP-cracks (red), initial
cracks (blue) and pores (violet).
In particular four different defect types can be classified that are colour coded. To understand the different breakage behaviour
of the two different material systems, a segmentation and quantification of each load step is essential.
First visual cracks can be seen at a load of ~2/3 Fmax. These first CFRP-cracks and debonding failures are too small for
segmentation. Nevertheless a quantitative comparison of the load steps 0, 90 and 95% of Fmax,reference was performed. The
volume of each defect type at every position represents the damage evolution. Figure 10 shows colour coded defect volume
progression of the system fabric-fabric (pores = violet, debonding of polyester fabric = green, CFRP-cracks = red). It is
obvious that the initial pore volume is nearly constant over all load steps. The shape of the debonding is defined by the
polyester fabric (see Figure 11: row (a)). CFRP-cracks can be identified as the main defect type. It can be observed that they
are growing from top and bottom, in an angle of 45° to the y-axis, to the specimen centre. This behaviour is expected, since the
orientation correlates to the ply orientation.
0 N 1400 N 1500 N
Figure 10: Fabric-Fabric system colour coded defect type 3D comparison at different load steps (pores = violet, debonding of polyester fabric
= green, CFRP-cracks = red). The regions (a) and (b) are visualized in detail in Figure 11.
bo
tto
m
to
p
Z
Y
(b
) fa
bri
c-U
D
(a)
fab
ric-
fabri
c
Y
X 1500 µm
(b)
(a)
CF
RP
-cra
cks
deb
on
din
g o
f p
oly
este
r fa
bri
c
init
ial
crac
ks
po
res
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Figure 11 shows the visualized failure types, debonding and CFRP cracks in detail.
unsegmented slice segmented slice segmented: 3D
Figure 12 shows the defect volume progression of the fabric-UD. In the unloaded stage, larger pores and smaller cracks in the
fabric-plies are visible. A propagation of these initial cracks cannot be detected. It also can be observed that these initial
defects are not connected with the CFRP-cracks and debonding volume. Due to this fact, an influence of these defects as a
crack origin can be neglected. Furthermore, a growth of debonding volume from load step 2100 N to 2200 N can be observed.
It is noticeable that CFRP-cracks can only be detected in the fabric in the bottom region. The orientation of these cracks
correlates with the orientation of the fabric plies.
0 N 2100 N 2200 N
Figure 13 shows a defect volume comparison between the investigated material systems over the segmented load steps. It can
be clearly seen that there is no noticeable increase of initial cracks and pores at the final breakage. In general the CFRP-crack
volume is always larger than the debonding volume. At the fabric-UD system a nearly parallel progression of debonding and
CFRP-cracks can be observed. In opposite to this, a much higher increase of CFRP-crack volume than debonding volume can
be observed at the fabric-fabric system, from load step 1400 to1500 N. Also the volume proportion of debonding to CFRP
cracks is at the system fabric-fabric higher than at the system fabric-UD. The difference can be explained by the missing cracks
in the UD-plate.
Z
Y
Z
Y
Figure 11: Visualization (slice- and 3D-images) of the segmented defects: (a) debonding and (b) CFRP-crack
Figure 12: Fabric-UD system colour coded defect type 3D comparison at different load steps (initial cracks = blue, pores =
violet, debonding of polyester fabric = green, CFRP-cracks = red)
Z
X
UD Fabric
300 µm
300 µm
(a)
(b)
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Figure 13: Defectvolume comparison of the systems fabric-fabric and fabric-UD over the segmented load steps
In both material systems the crack initiation occurs mainly in the adhesive layer (see Figure 9). But only in the fabric-fabric
system CFRP cracks are in both plates visible. This lack of CFRP-cracks in the UD-plate explains the lower CFRP-crack
gradient (step from 1400 to1500 N) at the fabric-UD system in comparison to fabric-fabric system. It can be concluded that the
stress is not high enough for a crack initiation in the UD-plies. The main advantage of UD in this case is that the force and the
plies are orientated in the same direction.
5 Conclusion and Outlook
In general the interrupted in-situ XCT measurements allowed a detailed look on the defect propagation of adhesively bonded
CFRP during an adapted slotted single lap shear test. It has been seen shown that the influence of the substrate on the damage
mechanism can be determined in a qualitative and quantitative manner. The results of the interrupted in-situ scanned fabric-
fabric specimen were compared with the fabric-UD specimen. The stress concentration at the end of the specimen [14], in the
overlapping region, can be seen in both materials. A voxel size of (6 µm)3 is sufficient to detect visually the first defects at ~2/3
of Fmax,reference. At a load step of 90% of Fmax,reference the defects could be segmented and quantified. Four different failure types
could be observed. An influence of initial defects (pores, cracks) from the unloaded step, on the final breakage cannot be seen.
The crack initiation event can be determined as the debonding of the polyester fabric. After this first damage step, the cracks
are growing mainly through the adhesive nearest fabric ply until the final fracture occurs. This major influence of the CFRP
cracks is determined by the high defect volume ratio of CFRP cracks to debonding of polyester. It can be concluded that the
higher strength of the fabric-UD system is caused by the higher crack resistance of the UD in comparison to the 45° plies of the
fabric. All plates show a potential crack initiation behavior by debonding of the polyester fabric, but only the strength of the
UD-plate is high enough to prevent a growth of those early cracks. The lack of damage in the UD plates leads to a significant
higher strength of the system fabric-UD. In a further step, an improved system of fabric-fabric with one UD-layer next to the
adhesive and the system UD-UD should be investigated. This single UD-layer may increase the crack resistance of the plate.
To sum up in case of single lap shear strain the system fabric-UD shows a better resistance against crack growth. With the
usage of fabric-UD instead of fabric-fabric the maximum force can be increased by ~37 percentages.
Acknowledgements
The authors like to acknowledge FACC Operations GmbH and CoLT Prüf und Test GmbH for the manufacturing of the
specimen. This work was supported by the K-Project for “non-destructive testing and tomography plus” (ZPT+) and by the
COMET program of FFG and the federal government of Upper Austria and Styria and supported by the project “multimodal
and in-situ characterization of inhomogeneous materials” (MiCi) and by the European Regional Development Fund (EFRE) in
the framework of the EU-program IWB2020.
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