Investigation into the failure of open holes in CFRP laminates under biaxial
loading conditions
C. Williamson and J. Thatcher.
QinetiQ Ltd.
Bldg A7, Rm 2008, Cody Technology Park, Farnborough, Hampshire, GU14 0LX UK
[email protected] & [email protected]
Abstract
This paper describes the results from a study which investigated the failure of biaxially-loaded carbon fibre-reinforced
plastic laminates with open holes.
The use of fibre-reinforced polymer composite materials in the manufacture of structures, from aircraft to racing cars,
has increased considerably in recent years. This is because of their specific strength and stiffness permit significant
improvements in performance compared to conventional metallic structures. However, the full commercial and strategic
benefits of structural composites have not yet been realised because their failure processes are not fully understood, forcing
components to be designed with conservative safety factors
A test method, developed to give valid failure data under multiaxial loading conditions using a planar cruciform
specimen, has been used to experimentally determine the failure envelopes and failure mechanisms for open hole specimens
under the full spectrum of biaxial loading; that is, tension-tension, tension-compression, compression-tension and
compression-compression. Specimens with quasi-isotropic lay-up, manufactured from T300H/914 carbon-fibre/epoxy pre-preg,
and with thicknesses between 2mm and 10mm, were tested. A single hole was drilled through the specimen centres by means
of a CNC machine. The hole diameters were between 6mm and 25mm. The thin specimens were tested in a Biaxial machine
with four 500kN actuators; whereas, the 10mm thick specimens were tested in a larger Biaxial test machine with four 1500kN
actuators. Good repeatability in specimen failure load was demonstrated.
For the majority of specimens, failure was clearly identifiable as either predominately tensile or compressive in nature.
However, at biaxial loading ratios of (+1.0: -1.0) and (-1.0: +0.7), both tensile and compressive failure modes were apparent.
The use of the biaxial cruciform test data to develop failure criteria provides a clear example of how an experimental
technique, allied to suitable analysis tools, can be used to further our understanding of complex composite structures.
Introduction
A large database has been built up for damage growth in composites resulting from features such as fastener holes
(both filled and open) and impact damage; however, the vast majority of these results have come from uniaxial tests.
Unfortunately, despite the reliance of the certification process on results from uniaxial tests, the majority of structures are very
rarely loaded uniaxially in service. Thus, an improved understanding of composite failure under multiaxial loading conditions is
urgently required.
Whilst polymer composites have been tested under multiaxial loading conditions since the mid-1970s, this data has
generally been produced using filament wound tubes; see for example [1 & 2]. Unfortunately, in the majority of aerospace
applications, composites are deployed in flat or gently curved structures, making the use of data from tubular test specimens
unsuitable for design. For this reason, biaxial testing facilities for planar specimens have been developed at QinetiQ, in
collaboration with the aerospace industry, since 1993 [3].
The study presented here was completed to determine how the presence of holes affects the structural strength of
carbon fibre-reinforced plastic (CFRP). The programme was concerned with the testing of cruciform specimens, under biaxial
loading, in order to study the effect of hole size, specimen thicknesses, lay-up and loading ratio, on observed failure
mechanisms. The biaxial strain ratios were selected to be representative of typical in-service loading. The paper also
discusses the design and manufacture of cruciform specimens, as well as the calibration and test methods employed.
1 Experimental
1.1 Biaxial specimen design and manufacture
The experimental programme used a biaxial cruciform specimen designed and validated in a previous programme [4].
The specimens were manufactured and clad by two of the partners; the cladding being bonded to the specimen using a room
temperature curing adhesive equivalent to that used in previous work [5].
The CFRP composite test specimen was designed using quasi-isotropic T300H/914 pre-preg sandwiched between
two layers of 4mm thick glass-fibre composite cladding. A circular cut-out in the centre of the cladding (which was tapered to
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reduce peel stresses) exposed the region of material to be tested under biaxial loading. Machined aluminium end tabs were
bonded onto the arms of the cruciform and holes were produced using a CNC machining station. The holes were required to
allow the specimen to be bolted into the test machine. Before testing, an appropriate sized hole was also drilled in the centre of
the test specimen (except the calibration specimens).
During biaxial testing the specimen was orientated in the machine such that the principal material directions coincided
with the loading axes; that is, the 0º fibre direction was aligned with the X-axis. For example (+1.0: -1.0) implies that the
specimen was subjected to tensile loading in the X-axis (or 0º fibre direction) and a compressive loading in the Y-axis (or 90º
fibre direction). This is illustrated in Figure 1.
X, 0º
Y, 90º
Figure 1: Biaxial test arrangement, including specimen loading axes and fibre orientation (specimen is shown fitted with antibuckling guide)
In the test machine, multi-fingered grips are bolted directly to the actuators. Holes in the ends of the fingers allow the
grips to be tightened onto the specimen to provide uniform load transfer across its width. It should be noted that the bolts are a
clearance fit through the specimen in order to prevent any direct loading of the specimen via the bolt. Such loading would
usually lead to premature failure.
All specimens to be tested in compression required the use of an anti-buckling guide [6]. The glass-fibre cladding is
designed with machined tapers to gradually apply load into the test region (note that the anti-buckle guide does not contact this
region). The outside of these tapered regions were shimmed with an adhesive which was machined level after cure to
surround the glass cladding. In this way, flat surfaces were created that could maintain contact with the anti-buckling guides,
and thereby stabilise the specimen against buckling.
Prior to testing, foil type strain gauges were attached to the specimens in the test region. The gauges used were a
combination of 0/90 rosettes (in the centre of the test area) and uni-axial gauges. The latter type was mounted on the glass
cladding in order to establish the global strain field at failure.
1.2 Specimen Details
Specimens C1-C14 were between 2 and 4mm thick and were tested in a biaxial machine with four 500kN actuators.
Specimens C15-C23 were 10mm thick and were tested in the larger biaxial testing machine with four 1500kN actuators. A
summary of the specimens tested within this study is given in Tables 1 and 2.
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Test
Specimen
No.
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
C13
C14
XA11358
XA11359
XK04856
XK05069
XK05063
XK04983
XK04984
XK05062
XK04982
XK05065
XK05076
XK04981
XK05068
XK05071
Hole
Diameter
(mm)
15
15
15
15
15
15
15
15
20
20
20
25
25
25
Loading Ratio
(+1.0: +1.0)
(+1.0: +1.0)
(+1.0: -1.0)
(+1.0: -1.0)
(-1.0: -1.0)
(-1.0: -1.0)
(+1.0: -1.0)
(+1.0: -1.0)
(+1.0: -1.0)
(-1.0: -1.0)
(+1.0: -1.0)
(+1.0: -1.0)
(-1.0: -1.0)
(+1.0: -1.0)
Laminate
thickness
(mm)
2
2
2
2
3
3
2
4
2
3
4
2
3
4
Cladding
thickness
(mm)
4
4
4
4
6
6
4
8
4
6
8
4
6
8
Stacking Sequence
[(+,-,0,90)2]S
[(0,90,+,-)2]S
[(+,-,0,90)2]S
[(0,90,+,-)2]S
[(+,-,0,90) 3]S
[(0,90,+,-) 3]S
[(+)2,(-)2,(0)2,(90)2]S
[(0)4(90)4 (+)4 (-)4)]S
[(+,-,0,90)2]S
[(+,-,0,90)3]S
[(04)(90)4(+)4(-)4]S
[(+,-,0,90)3]s
[(+,-,0,90)3]S
[(0)4(90)4(+)4(-)4]S
Table 1: Lay-up details of open hole CFC Specimens C1-C14: All plies 0.125mm
Test
Specimen
no.
Loading ratio
Stacking Sequence
Thickness
(mm)
C15
C16
C17
C18
C19
C20
C21
C22
C23
XK05088
XK07485
XK07486
XK05091
XK05090
XK05089
XK07489
XK07487
XK07488
(+1.0: -1.0)
(+1.0: -1.0)
(+1.0: -1.0)
(+1.0:-0.7)
(+1.0:-0.7)
(+1.0:-0.7)
(-1.0:+0.7)
(-1.0:+0.7)
(-1.0:+0.7)
[(+,-,0,90)10]s
[(+,-,0,90)10]s
[(+,-,0,90)10]s
[(+,-,0,0,+,-,0,90,0,0)4]s
[(+,-,0,0,+,-,0,90,0,0)4]s
[(+,-,0,0,+,-,0,90,0,0)4]s
[(+,-,0,0,+,-,0,90,0,0)4]s
[(+,-,0,0,+,-,0,90,0,0)4]s
[(+,-,0,0,+,-,0,90,0,0)4]s
10
10
10
10
10
10
10
10
10
Hole
Diameter
(mm)
6
6
10
10
6
6
6
6
6
Table 2: Loading ratios, thickness and size of open hole drilled in specimens C15-C23
1.3 Specimen calibration
Prior to drilling holes, one specimen from each batch was calibrated in the appropriate test machine in order to
establish the loading rate ratio on the two axes that would give the desired strain ratio in the test region.
The machine was operated in displacement control; load was applied through the application of a linear displacement
ramp on the two axes.
Initially the required displacement ramp rates on the two axes of the machine were estimated. The specimen was
then loaded to approximately 1000με (1500με for specimens C15-C23) and unloaded. These strain levels were assumed to
be sufficiently low to give elastic behaviour only, with no damage. The actual central strain ratio was then calculated from this
non-destructive test. Slight adjustments were made to the ramp rates to achieve the required strain ratio. The displacement
ramp ratio obtained in this way was then used for all those specimens with the same specimen geometry and loading ratio.
For each test, a PC-based data-logger was used to record load, displacement and strain time histories throughout the
testing, at a frequency of 1Hz.
1.4 ‘Test-to-failure’ procedure
The specimens were mounted in the biaxial test machine in exactly the same manner as for the calibration tests. The
specimens were loaded in displacement control until failure occurred.
For the 10mm thick specimens an anti-buckling guide was used in early tests (e.g. for specimens XK05088 and
XK05091). However, latterly, a Moiré fringe monitoring technique, described at reference [3], indicated no out-of-plane
movement and therefore that no buckling was occurring. The anti-buckling guide was therefore deemed surplus to
requirements and was not used for subsequent tests. This was later verified using LVDTs to monitor any out-of-plane
displacement.
With all the calibration data now available, the ramp rates on the test machine were adjusted to achieve the correct
strain ratios. Following calibration, detailed in the previous section, holes were drilled in the specimens using a CNC machine
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and a tungsten carbide tipped tool. This procedure reduced the possibility of fibre fretting and subsequent premature initiation
of delamination. The open hole specimens were then loaded until failure occurred. Upon failure, the loading ramps were
terminated and the specimens unloaded.
Load, displacement and strains were logged throughout the test, hence allowing the failure loads, Px and Py, and the failure
strains at the gauge positions to be obtained. In the interests of clarity, initial failure was defined as the origin of a deviation
from the load-strain time history in excess of 10%. Final failure is assumed to have occurred when the load carrying capacity of
the test specimen is reduced by at least 10%. A valid failure was defined as a failure that originated from the hole at the centre
of the test region.
2 Results and Discussion
2.1 Experimental findings
A summary of the failure loads and strain gauge values at failure is presented in Tables 3-5.
Test
Specimen
No.
P x (kN)
Py (kN)
ex_gauge
ey_gauge
C1
XA11358
209.5
194.7
3787
3452
C2
C3
C4
C5
XA11359
XK04856
XK05069
XK05063
228.1
151.4
134.0
-358.8
201.7
-182.1
-176.5
-360.5
4047
-4071
3135
-5239
C6
C7
C8
XK04983
XK04984
XK05062
-246.7
160.5
140.6
-339.4
-160.3
-178.9
-2755
3666
-4383
-4436
C9
C10
C11
C12
XK04982
XK05065
XK05076
XK04981
138.9
-306.3
169.7
130.1
-154.4
-338.3
-178.1
-151.1
-4669
8873
-
-5966
-4817
-
C13
C14
XK05068
XK05071
-242.7
153.1
-251.3
-177.5
-2757
-1245
-2781
-3318
Table 3: Summary of failure loads, Px and Py, and measured failure strains, ex_gauge and ey_gauge.
Test
C15
C16
C17
C18
C19
C20
C21
C22
C23
Specimen No.
XK05088
XK07485
XK07486
XK05091
XK05090
XK05089
XK07489
XK07487
XK07488
Hole Diameter (mm)
6
6
10
10
6
6
6
6
6
Px (kN)
586.4
586.5
469.5
914.0
835.5
1004.2
-1080.0
-1124.0
P y (kN)
-544.4
-557.2
-462.2
-55.4
-106.4
-47.6
40.1
38.1
ex_3.5
2957
-4737
-4965
ex_7
6224
9373
-
ex_20
6741
6230
4200
3893
4348
4928
-5709
-5498
ex_40
6617
6593
5178
5017
5383
5724
-7139
-7535
ex_100
3240
-4849
-5151
Table 4: Summary of loads and strains at the point of failure in direction X. (Measured failure strains are shown at 3.5mm,
7mm, 20mm, 40mm and 100mm from the centre of the specimen).
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Test
C15
C16
C17
C18
C19
C20
C21
C22
C23
Specimen No.
XK05088
XK07485
XK07486
XK05091
XK05090
XK05089
XK07489
XK07487
XK07488
Hole Diameter (mm)
6
6
10
10
6
6
6
6
6
Px (kN)
586.4
586.5
469.5
914.0
835.5
1004.2
-1080.0
-1124.0
Py (kN)
-544.4
-557.2
-462.2
-55.4
-106.4
-47.5
40.1
38.1
ey_3.5
-18260
7202
4458
ey_7
-5662
-3962
-
ey_20
-6346
-6702
-1547
-2994
-4479
-3904
4764
4443
ey_40
-6460
-6884
-5306
-3471
-4935
-3952
4622
4936
ey_100
-1585
1343
1410
Table 5: Summary of loads and strains at the point of failure in direction Y. (Measured failure strains are shown at 3.5mm,
7mm, 20mm, 40mm and 100mm from the centre of the specimen).
2.2 Fractography
After testing to failure, under a range of biaxial loading ratios, the specimens were examined fractographically to
determine the mode and extent of fracture. The assessment of the specimens initially consisted of visual examination and
photography, followed by a more detailed assessment using dye penetrant X-radiography. Photographs illustrating typical
failure modes are presented in Figures 2-3. A typical X-ray assessment can be seen in Figure 4.
Figure 2: Specimen XK04856 (15mm hole, 2mm thick, +1.0:-1.0), taken post-failure and depicting the discoloration in the
cladding due to the damage beneath.
XK07486 (10mm hole, 10mm thick, +1.0:-1.0)
Figure 3A: Example of failure mode: specimen XK07486 (C17).
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XK05091 (10mm hole, 10mm thick, +1.0:-0.7)
Figure 3B: Example of failure mode: specimen XK05091 (C18).
Figure 4: X-ray of specimen XK05071 (25mm hole, 4mm thick, +1.0:-1.0)
2.2.1 Fractographic observations
Some of the specimens were further dissected using a dry diamond saw to examine the fracture surfaces in more
detail using a stereo microscope or scanning electron microscope (SEM). The SEM was used primarily to confirm the mode of
fracture in cases where this could not be established with any certainty during the initial examination.
Table 6 presents a summary of damage observed in the failed specimens. The table also shows the corresponding
loading conditions and specimen geometry.
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Test
Panel
Thickness
(mm)
2
Loading
ratio
Damage observed.
C3
Hole
size
(mm)
15
(+1.0: -1.0)
C4
C5
15
15
2
3
(+1.0: -1.0)
(-1.0: -1.0)
C6
15
3
(-1.0: -1.0)
C7
15
2
(+1.0: -1.0)
C8
15
4
(+1.0: -1.0)
C9
20
2
(+1.0: -1.0)
C10
20
3
(-1.0: -1.0)
C11
20
4
(+1.0: -1.0)
C12
25
2
(+1.0: -1.0)
C13
25
3
(-1.0: -1.0)
C14
25
4
(+1.0: -1.0)
Cracking was visible either side of the hole along both the X and Y axes.
Cracking along X-axis showed local buckling typical of compression failure.
Fine crack along Y-axis indicative of tensile fracture. X-radiography also
revealed fine cracks along the Y-axis which was consistent of tensile fracture.
Delamination was found along the X-axis, consistent with compressive failure.
As C3.
Cracking was visible either side of the hole along the X-axis only. Local
buckling, indicative of compression failure, was found. X-radiography showed
fracture in the specimen which was surrounded by extensive delamination.
This was again consistent with compressive failure.
Cracking was visible along the X-axis only, either side of the hole. Local
buckling was also apparent. X-radiography confirmed visual examinations
including localised delamination around the fracture site, consistent with
compressive failure.
Cracking was only visible along the X-axis, either side of the hole. Xradiography revealed fracture in the specimen along both axes; the fractures
along the X and Y axis appeared to be consistent with failure in compression
and tension respectively.
Cracking was only visible along the X-axis. Localised buckling indicated
compressive failure. X-radiography showed fracture in both the X and Y axes,
but it was not possible to determine the modes of fracture due to the extensive
delamination present in the specimen.
Severe cracking was not visible, but splitting of the surface plies was visible on
both sides of the specimen. Splits were oriented at 45° to the main axis of
loading. X-radiography revealed cracking along both the X and Y axes, either
side of the hole. Fracture along the X-axis was consistent with compressive
failure, whilst failure in the Y-axis was tensile in nature.
Cracking was only visible along the X-axis at an angle of approximately 20° to
the vertical. Delamination and local buckling was observed. X-radiography also
revealed localised delamination consistent with compressive failure.
Splitting of the surface plies was observed perpendicular to the X-axis. Tensile
fracture along the Y-axis was seen but sectioning showed this was confined to
the two surface plies only. Delamination not visible from the surface was found
in the X-axis, indicating compressive collapse. X-radiography revealed
extensive delamination around hole.
Splitting was visible in surface plies at 45° to principal loading axis. Surface
cracking was seen on either side of the hole, in the X-axis. Sectioning revealed
further evidence of delamination and buckling. Cracking along the Y-axis was
observed which was typical of tensile failure. Fractography confirmed that the
cracking along the X-axis was consistent of compressive failure, whilst the
delamination in the Y-axis was consistent with tensile failure.
Cracking either side of the hole was visible only the X-axis and oriented at an
angle of 25° to the X-axis. Localised buckling around the fracture indicated
compressive failure. X-radiography revealed delamination around the crack
suggesting compression fracture.
Splitting of the surface plies was visible at 90° to the principal loading axis,
whilst fracture of the surface plies around the hole was confined to the Y-axis.
Local buckling along the Y-axis was consistent with compressive failure.
Sectioning showed extensive delamination around the hole. X-radiography
revealed delamination around the crack along the Y-axis and around most of
the hole. There was no evidence of tensile failure along the X-axis.
Table 6: Summary of the fractographic results for specimens C3-C14
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2.3 Discussion
The failures observed in the specimens could be divided into two broad groups; some containing both tensile and
compressive fractures and some containing compression fractures only. All specimens tested under (+1.0: -1.0) conditions,
with the exception of specimen C14, contained both compressive and tensile failures. As expected, the tensile and
compressive fractures visible in the composite were oriented perpendicular to the tensile and compressive loading axes,
respectively. The fractographic assessment was not able to determine which of the two fractures occurred first. However, since
the strength of carbon laminates in compression is generally lower than in tension, it seems likely that the former would have
initiated first. Specimens tested under (-1.0: -1.0) conditions contained only compressive failures and this was usually
restricted to one axis.
It was noted that neither the diameter of the central hole, nor the thickness of the laminate, had any significant effect
on the mode of fracture observed in the specimens. Samples C11 and C14 showed more delamination around the hole,
compared with other specimens, but it was unclear whether this was caused by the increased thickness of the laminates or the
larger hole diameters.
A comparison of failure loads between specimens of the same geometry, and subjected to the same loading ratio,
suggested that good repeatability was achieved. However, while the failure loads were repeatable, there was evidence of
differences between the failure mechanisms (even for the same loading ratio). For example, specimen XK07489 failed
following an audible signal that some damage had occurred, whilst specimen XK07485 failed catastrophically, i.e. with no prior
audible warning.
It is also worthy of note that, whilst audible warnings of imminent failure were apparent for the thinner specimens (C1C14), for the 10mm thickness specimens this was not the case; the specimens generally failed in a catastrophic manner.
For all of the 10mm thick specimens the actuator loads did not fall immediately to zero upon failure; typically 80% of
the load remained. At this stage, a crack was consistently observed running from the edge of the hole into the cladding. Upon
unloading from that point, considerable tearing was noticed; corresponding to delamination around the edge of the glass
cladding (it was possible to observe these events because no anti-buckling guide was needed for the thick specimens).
The delamination around the edge of the test area was accompanied by higher values being recorded in the strain gauges at a
distance of 40mm from the specimen centre, compared with those at 20mm.
Fractography suggested that splitting of surface plies in the direction perpendicular to the primary loading direction
was common. A dominant compressive or tensile mode was also apparent. However, there are examples of both tensile and
compressive fracture in a single specimen tested in a (+1.0: -1.0) loading ratio, thus confirming the presence of a biaxial stress
state at failure.
Whilst fractography was not performed on specimens C15-C23, the photographs taken post-failure suggest that the
failure modes were similar to those of the thinner specimens. The (+1.0: -1.0) loading ratio used for specimens C15-C17
resulted in both tensile (dominant) and compressive (recessive) failure modes. In addition, extensive delamination of the
woven fibre was evident. For the other specimens in this series, either tensile or compressive failure clearly dominated.
The strain fields viewed at failure for specimens C15-C23 suggested that the tensile cracks which extended across
the full width of the test area, appeared to originate from the drilled hole.
2.4 Conclusions
There was no evidence of buckling during testing of the 10mm thick specimens (C15-C23), even when the specimen
was subjected to a (-1.0: -0.7) biaxial loading ratio.
The experimental results described herein suggest good repeatability for all of the loading ratios investigated.
Both tensile (dominant) and compressive (recessive) failure modes have been observed simultaneously in those
specimens subjected to a (+1.0: -1.0) biaxial loading ratio, irrespective of the specimen thickness. For other loading ratios the
failure is usually clearly either tensile or compressive.
While the limited number of specimens tested prevented detailed trends from being established, with complete
certainty, the investigation described herein has provided a valuable overview into the multi-axial failure behaviour of
composite cruciform specimens containing open holes.
Future Work
QinetiQ is a member of the Measurement of Multiaxial Properties Standards Working Group that has recently been
formed under the Versailles Project on Advanced Materials and Standards (VAMAS) agreement [7]. The objective of this
international working group is to propose acceptable methodologies that will lead to internationally accepted test methods for
the evaluation of the multiaxial performance of fibre-reinforced composites. Other current members include NPL (UK), Vrije
Universiteit Brussel (Belgium) and Universiteit Gent (Belgium).
Acknowledgements
The authors would like to acknowledge the UK Department of Trade and Industry (DTI) and the partners of the
multiaxial testing programmes for their considerable (and continued) support in the field of multiaxial testing and its
applications.
© Copyright QinetiQ ltd 2007
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loading conditions”, ECCM 11, Rhodes 2004.
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test machine’, DRA/SMC/CR941019/1.0, June 1994.
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biaxial loading’ DRA/SMC/CR951076/1.0, June 1995
http://www.npl.co.uk/materials/cog/multiaxial
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