investigation into the failure of open holes in cfrp...

© Copyright QinetiQ ltd 2007

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]

AbstractThis 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.

IntroductionA 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 manufactureThe 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


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.

Figure 1: Biaxial test arrangement, including specimen loading axes and fibre orientation (specimen is shown fitted with anti-buckling 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 DetailsSpecimens 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.

X, 0º Y, 90º


Test Specimen No.

Hole Diameter

(mm)

Loading Ratio Laminate thickness

(mm)

Cladding thickness

(mm)

Stacking Sequence

C1 XA11358 15 (+1.0: +1.0) 2 4 [(+,-,0,90)2]SC2 XA11359 15 (+1.0: +1.0) 2 4 [(0,90,+,-)2]SC3 XK04856 15 (+1.0: -1.0) 2 4 [(+,-,0,90)2]SC4 XK05069 15 (+1.0: -1.0) 2 4 [(0,90,+,-)2]SC5 XK05063 15 (-1.0: -1.0) 3 6 [(+,-,0,90) 3]SC6 XK04983 15 (-1.0: -1.0) 3 6 [(0,90,+,-) 3]SC7 XK04984 15 (+1.0: -1.0) 2 4 [(+)2,(-)2,(0)2,(90)2]SC8 XK05062 15 (+1.0: -1.0) 4 8 [(0)4(90)4 (+)4 (-)4)]SC9 XK04982 20 (+1.0: -1.0) 2 4 [(+,-,0,90)2]S

C10 XK05065 20 (-1.0: -1.0) 3 6 [(+,-,0,90)3]SC11 XK05076 20 (+1.0: -1.0) 4 8 [(04)(90)4(+)4(-)4]SC12 XK04981 25 (+1.0: -1.0) 2 4 [(+,-,0,90)3]sC13 XK05068 25 (-1.0: -1.0) 3 6 [(+,-,0,90)3]SC14 XK05071 25 (+1.0: -1.0) 4 8 [(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)

Hole Diameter

(mm)C15 XK05088 (+1.0: -1.0) [(+,-,0,90)10]s 10 6C16 XK07485 (+1.0: -1.0) [(+,-,0,90)10]s 10 6C17 XK07486 (+1.0: -1.0) [(+,-,0,90)10]s 10 10C18 XK05091 (+1.0:-0.7) [(+,-,0,0,+,-,0,90,0,0)4]s 10 10C19 XK05090 (+1.0:-0.7) [(+,-,0,0,+,-,0,90,0,0)4]s 10 6C20 XK05089 (+1.0:-0.7) [(+,-,0,0,+,-,0,90,0,0)4]s 10 6C21 XK07489 (-1.0:+0.7) [(+,-,0,0,+,-,0,90,0,0)4]s 10 6C22 XK07487 (-1.0:+0.7) [(+,-,0,0,+,-,0,90,0,0)4]s 10 6C23 XK07488 (-1.0:+0.7) [(+,-,0,0,+,-,0,90,0,0)4]s 10 6

Table 2: Loading ratios, thickness and size of open hole drilled in specimens C15-C23

1.3 Specimen calibrationPrior 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’ procedureThe 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


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 findingsA summary of the failure loads and strain gauge values at failure is presented in Tables 3-5.

Test Specimen No.

Px (kN) Py (kN) εx_gauge εy_gauge

C1 XA11358 209.5 194.7 3787 3452C2 XA11359 228.1 201.7 4047 3135C3 XK04856 151.4 -182.1 - -C4 XK05069 134.0 -176.5 - -C5 XK05063 -358.8 -360.5 -4071 -5239C6 XK04983 -246.7 -339.4 -2755 -4383C7 XK04984 160.5 -160.3 - -C8 XK05062 140.6 -178.9 3666 -4436C9 XK04982 138.9 -154.4 - -

C10 XK05065 -306.3 -338.3 -4669 -5966C11 XK05076 169.7 -178.1 8873 -4817C12 XK04981 130.1 -151.1 - -C13 XK05068 -242.7 -251.3 -2757 -2781C14 XK05071 153.1 -177.5 -1245 -3318

Table 3: Summary of failure loads, Px and Py, and measured failure strains, εx_gauge and εy_gauge.

Test Specimen No. Hole Diameter (mm) Px (kN) Py (kN) εx_3.5 εx_7 εx_20 εx_40 εx_100C15 XK05088 6 586.4 -544.4 - 6224 6741 6617 -C16 XK07485 6 586.5 -557.2 - - 6230 6593 -C17 XK07486 10 469.5 -462.2 - - 4200 5178 -C18 XK05091 10 914.0 -55.4 - 9373 3893 5017 -C19 XK05090 6 835.5 -106.4 - - 4348 5383 -C20 XK05089 6 1004.2 -47.6 2957 - 4928 5724 3240C21 XK07489 6 -1080.0 40.1 -4737 - -5709 -7139 -4849C22 XK07487 6 - - - - - - -C23 XK07488 6 -1124.0 38.1 -4965 - -5498 -7535 -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).


Test Specimen No. Hole Diameter (mm) Px (kN) Py (kN) εy_3.5 εy_7 εy_20 εy_40 εy_100C15 XK05088 6 586.4 -544.4 - -5662 -6346 -6460 -C16 XK07485 6 586.5 -557.2 - - -6702 -6884 -C17 XK07486 10 469.5 -462.2 - - -1547 -5306 -C18 XK05091 10 914.0 -55.4 - -3962 -2994 -3471 -C19 XK05090 6 835.5 -106.4 - - -4479 -4935 -C20 XK05089 6 1004.2 -47.5 -18260 - -3904 -3952 -1585C21 XK07489 6 -1080.0 40.1 7202 - 4764 4622 1343C22 XK07487 6 - - - - - - -C23 XK07488 6 -1124.0 38.1 4458 - 4443 4936 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 FractographyAfter 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).


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 observationsSome 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.


Test Hole size(mm)

PanelThickness

(mm)

Loading ratio

Damage observed.

C3 15 2 (+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.

C4 15 2 (+1.0: -1.0) As C3.C5 15 3 (-1.0: -1.0) 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.C6 15 3 (-1.0: -1.0) 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.C7 15 2 (+1.0: -1.0) Cracking was only visible along the X-axis, either side of the hole. X-

radiography 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.C8 15 4 (+1.0: -1.0) 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.C9 20 2 (+1.0: -1.0) 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.C10 20 3 (-1.0: -1.0) 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.

C11 20 4 (+1.0: -1.0) 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.

C12 25 2 (+1.0: -1.0) 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.

C13 25 3 (-1.0: -1.0) 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.

C14 25 4 (+1.0: -1.0) 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


2.3 DiscussionThe 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 (C1-C14), 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 ConclusionsThere 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 WorkQinetiQ 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).

AcknowledgementsThe 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.


References1. Hinton, M.J., Soden, P.D. and Kaddour, A.S., “Biaxial failure of composite laminates under combined loads”, Applied

Composites Materials, V.3, pp 151-162, 1996.2. Kaddour, A.S., Soden P.D. and Hinton, M.J., “Failure of ±55° filament wound tubes under biaxial compression”, J.

Composites Materials, V.32, no.18, pp1618-1645, 1998.3. Wiliamson, C., Cook, J., Clarke, A., “Investigation into the failure of open and filled holes in CFRP laminates under biaxial

loading conditions”, ECCM 11, Rhodes 2004.4. Hopgood, P., Cook, J., Clarke, A., “Multi-axial testing of planar composite specimens”, ICCM12, Paris 1999.5. P.J. Hopgood, P.J. Mobbs, B.J. Millson, R.C. Radley and S.M. Bishop, ‘Design and validation of a specimen of a biaxial

test machine’, DRA/SMC/CR941019/1.0, June 1994.6. B.J. Millson, P.J. Hopgood, R.C. Dadey and S.M. Bishop, ‘Failure at open holes in composite under full spectrum of

biaxial loading’ DRA/SMC/CR951076/1.0, June 19957. http://www.npl.co.uk/materials/cog/multiaxial

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