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Measurements of the Bending Stiffness and Failure in 3-Point Bending of Irradiated and Non-irradiated Sandwich Beams
Tim Jones
University Of Liverpool
16th January 2012
Introduction Four 120mm x 50mm x 5mm carbon-fibre/honeycomb sandwich beams were constructed from 3-ply (0/90/0)
K13D2U/RS-3 (80gsm, 29%RC) carbon-fibre face sheets and UCF-126-3/8-2.0 honeycomb. Pairs of beams were
assembled using two different adhesives: Henkel Hysol 9396 (1) and ACG VTA260 (2). Hysol 9396 is a two-part, room
temperature curable, aerospace grade epoxy adhesive which has been extensively used in stave prototyping whilst
ACG VTA260 is a glue film with cure temperatures ranging from 65C to 120C.
During summer 2011 two such beams, one of each glue type, were irradiated to a dose of 1.6.1015 p/cm2 in the CERN
PS whilst the other two were kept aside. All four beams were subject to 3-point bending tests using a universal
materials tester. Measurements of the bending stiffness were made for a series of load-scans with maximum loads
from 50N to 400N. All beams passed the 50N scan and failed at higher loads with none passing the 400N scan.
In addition to comparing the relative performance of the beams before and after irradiation the results can also be
compared to previous work (3) in which measurements were presented of the bending stiffness and maximum load
of a group of similar sandwich beams assembled with Hysol 9396.
Sample Assembly Each pair of beams of a given glue type were manufactured, together with four 5cm square tokens for sandwich flat-
wise tensile testing, as a single beam of length 450mm. The face sheet material for both beams was taken from a
single panel of dimensions 520 x 310mm formed from 3 ply oriented at 0/90/0 with ‘0’ being the direction of the
longer side. For the Hysol 9396 beam the two sides were glued sequentially at 66C for 2hours whilst for the VTA260
beam both sides were cured simultaneously at 80C for 5 hours following the manufacturer’s recommendation. All
glue steps were done under vacuum. Following assembly, the two long beams were cut into sections (2 x 120mm + 4
x 50mm).
Sample Irradiation For each glue-type, 3 test pieces (one 120mm and two 50mm) were mounted onto carbon fibre support frames
designed to interface with the acrylic rails on the lid of the ATLAS irradiation box, such that each group of samples
occupied the equivalent space to a silicon detector module. The test-box was scanned in the CERN PS irradiation
facility using the same scanning pattern as for module irradiations with the samples receiving a total dose of 1.6.1015
p/cm2. Following irradiation the samples were stored at CERN until December 2011 when they were declared safe
for shipment to Liverpool. Prior to 3-point bend testing the two irradiated and two non-irradiated beams were
sealed into loose-fitting plastic bags roughly 18cm x 6cm to contain any debris from broken samples.
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Three-Point Bending A Lloyd Instruments LRXPlus 5kN (4) universal materials tester equipped with a 1kN load cell and a 3-point bending
fixture with 10mm diameter cylindrical contacts was used for these measurements. The separation of the fixed
rollers was set to be 100 0.1 mm. A new batch test was configured using the non-irradiated Hysol 9396 sample as a
template with a maximum load of 50N. During each test the mid-span roller moves in compression at a speed of
2mm/minute and the time, load and displacement are recorded until the pre-defined load is reached. For each batch
test a load span is defined which is used bending stiffness. Batch tests were configured with maximum loads of 50,
100, 150, 200, 300 and 400N. Figure 1 shows a photograph of one of the samples mounted in the materials tester;
note the presence of the protective bag.
Figure 1: Photograph of materials tester with one of the sandwich beam samples
For each batch test the four samples were placed in the fixture and subjected to up to 3 tests and the results were
recorded. Once a test sample had failed any remaining tests of that sample were skipped. Table 1, below shows a
summary of all the tests indicating passed tests and the failure loads.
Sample 50N 100N 150N 200N 300N 400N
Hysol / non-irrad Pass Pass 114.4
Hysol / irrad Pass 79.4
VTA260 / non-irrad Pass Pass Pass Pass 297
VTA260 / irrad Pass Pass Pass Pass Pass 322
Table 1: Test summary indicating the tests passed and the failure loads
Taking the data at face value a comparison of the irradiated and non-irradiated failure loads gives an estimate of the
percentage change, see Table 2 below.
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Adhesive Load at Failure (N)
% Change Non-irrad Irrad
Hysol 9396 114.4 79.4 -31%
VTA260 297 322 8%
Table 2: Comparison of non-irradiated and irradiated failure loads
Table 3 below shows the bending stiffness for each batch test and the averages over all passed tests (up to 3) for
each of the beam samples. Please note that the calculation of the bending stiffness does not take into account the
compliance of the materials tester and therefore the absolute accuracy of these numbers is not understood.
Nevertheless, the comparison of relative bending stiffness at similar loads is likely to be accurate.
Limit (N) Range
(N) Hysol 9396 (N/m) VTA260 (N/m)
Non-irrad Irrad Non-irrad Irrad
50 20 - 40
244520 232580 315710 344020
263550 247590 315020 356140
266810 250220 333030 344100
Ave 50 258293 243463 321253 348087
100 40 - 60
289940 271970 297000 303270
281530 296720 308620
282400 306560 308140
Ave 100 284623 271970 300093 306677
150 80 - 100
321550 367920 375520
369680 364380
392530 366900
Ave 150 321550 376710 368933
200 120 - 140
449220 424230
456790 434790
481310 418660
Ave 200 462440 425893
300 180 - 200
557280 507670
549250
543080
Ave 300 557280 533333
400 260 - 280
566280
Ave 300 566280
Table 3: Summary of average bending stiffness for all passed tests
Comparing the non-irradiated and irradiated bending stiffness at each load point and normalizing to the non-
irradiated sample value results in the data shown in Table 4. There is little evidence for any irradiation induced
reduction in bending stiffness.
Adhesive Maximum Load (N)
50 100 150 200 300 400
Hysol 9396 94% 96%
VTA260 108% 102% 98% 92% 96%
Table 4: Irradiated sample bending stiffness expressed as a percentage of the non-irradiated measurement
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Failure Characteristics As noted above there is a clear distinction between the failure loads of the Hysol 9396 and VTA260 samples which is,
in fact, a result of two different failure mechanisms. Figures 2 and 3 below show the loading curves for batch-tests
which induce breakages for the Hysol and VTA260 samples respectively. In addition to the clear difference in the
average failure loads (93N vs. 300N) the data exhibit different behaviour. For the Hysol samples failure results in a
sudden drop in load and then as the deflection increases further the stiffness increases again until another failure
point is reached. It appears as if failure proceeds as a series of successive failure points. Conversely, for the VTA260
samples there is one single clear load at which failure occurs which is followed by a smooth decrease in the load as
further deformation is imparted. Note that the failure characteristics relate to the adhesive used and not whether
the sample has been irradiated or not.
Figure 2: Loading curve for the Hysol 9396 samples
Figure 3: Loading curve for the VTA260 samples
0
20
40
60
80
100
120
140
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Load
(N
)
Extension (mm)
Hysol / Irrad
Hysol / Non-irrad
0
80
160
240
320
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
Load
(N
)
Extension (mm)
VTA260 Irrad
VTA260 / Non irrad
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The strong similarity in the behaviour of the non-irradiated and irradiated samples means that a partial exploration
of the failure mechanisms can be obtained by visual inspection of the non-irradiated samples. Figure 4, below, shows
a photograph of the non irradiated samples post testing – the Hysol sample is at the top. Visually there appears to be
little wrong with the Hysol beam but in handling it is clear that the bond between the honeycomb and the face sheet
has failed on one side. Figure 5 shows a close-up of the edge of the Hysol sample indicating that almost 1/3 of the
width of the sample has become de-bonded. In contrast, figure 4 shows that the VTA260 sample has a clear
compression line corresponding to the upper loading roller which has cracked the face sheet; see also Figure 6.
Elsewhere the face sheets are still fully bonded to the honeycomb core.
Figure 4: Photograph showing non-irradiated beams post testing (Hysol – top, VTA260 - bottom)
Figure 5: Close-up of Hysol sample showing extent of honeycomb/face sheet bond failure
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Figure 6: Close-up of edge region of VTA260 sample showing clear compression line
The conclusion from the load-curves and visual inspection is that the Hysol 9396 sample has suffered progressive
failure of adhesion between the honeycomb and the face sheet. By contrast the VTA260 sample has failed due to
core crushing immediately below the central load point on the upper side. Had a load spreader plate been used to
distribute the force the failure load would have been above that achieved in these tests. In that sense, the measured
failure load represents a lower limit.
The relatively poor performance of Hysol 9396 does need further study as this adhesive is a strong candidate for
stave assembly. Whilst the ease of handling of the preformed VTA260 glue film is an advantage, the mass of glue is
considerably larger than for the Hysol beams (Hysol 9396 mass = 5.7g, VTA260 mass = 7.1g) and the requirement to
cure at elevated temperature are drawbacks. In other work we have demonstrated the use of a pattern cutter to
remove glue from the central regions of honeycomb cells to try and reduce the excess mass (5).
Comparison with Previous Work A few years ago Oxford manufactured a series of 10cm x 5cm sandwiches using 3-ply K13D2U/RS3 face sheets, UCF-
126-3/8-2.0 honeycomb and Hysol 9396 adhesive (3). The depth of the glue bath into which the honeycomb was
dipped was varied to alter the amount of adhesive used. Samples were tested using 3 point bending with the
equivalent homogeneous beam modulus and the failure load being evaluated. Over the ensemble of 7 samples the
failure loads varied from 67N to 141N and the flexural modulus varied over a rather large range from 3,000 to 10,000
MPa.
The flexural modulus of a homogeneous beam is related to the beam geometry and bending stiffness, D, as follows;
where, L is the span (=100mm), b is the width (=50mm) and d is the thickness (=5.36mm) giving;
Taking the data for the 100N scans the equivalent flexural moduli are 8.1GPa for the Hysol 9396 beams and 11.3GPa
for the VTA260 beams. Assuming the Oxford beams were built to the nominal stavelet specification with a 4.6mm
core then a proper comparison between the two data require the Liverpool flexural moduli to be scaled down by the
ratio;
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which gives moduli of 6.5GPa and 8.6GPa.
The Liverpool samples lie in the middle to upper end of the range encompassed by the Oxford samples. It is also
worth noting that the loading curves for the Oxford beams indicates a similar failure mode to that observed in the
Liverpool samples manufactured using Hysol 9396.
Conclusions Four sandwich test beams have been assembled using carbon fibre face sheet material and honeycomb used in stave
assembly and two adhesives; Hysol 9396 and ACG VTA260. During summer 2011 one pair of beams were irradiated
at the CERN PS to a dose of 1.6.1015 p/cm2. Results of a series of 3-point bending tests on the beams have been
presented. These tests show that:-
The failure load for VTA260 is three times that for Hysol 9396
The bending stiffness for VTA260 is 9% higher than that for Hysol 9396
For both adhesives the bending stiffness of the irradiated beams is essentially the same as the non-irradiated
beams
The failure load for an irradiated a Hysol 9396 beam is 30% lower than an un-irradiated beam whilst the
irradiated VTA260 beam has a failure load 8% larger than the non-irradiated beam
An analysis of the loading curves indicates that failure for the Hysol 9396 beams proceeds as a series of failures in a
similar fashion to the Oxford-made samples from 2009. In addition, visual inspection of the broken samples shows
large areas of face sheet / honeycomb bond failure. By contrast, the loading curve for the VTA260 beams indicates a
single point failure. Visual inspection confirms this with the only damage being a full-width crack directly beneath the
mid-span roller which is likely to originate in an inability of the core to support the applied load.
The failure load for the VTA260 glue film is high ( 300N) and insensitive to irradiation however, there is some
concern at both the low level ( 114N) and large difference (30% fall) in the failure load for the Hysol 9396 beams.
Whilst it is true that there is considerable more glue mass in the VTA260 beams, it would be prudent to understand if
the relative performance of the two glue types is simply related to the amount of glue or whether improvements to
the failure loads for Hysol 9396 beams can be achieved through improved assembly techniques.
References 1. Henkel. Hysol EA9396. [Online]
https://tds.us.henkel.com//NA/UT/HNAUTTDS.nsf/web/44D0528FE441CD318525715C001BD530/$File/Hysol_EA_93
96-EN.pdf.
2. Advanced Composites Group, UK. VTA260. [Online] http://www.advanced-
composites.co.uk/data_catalogue/catalogue%20files/pds/PDS1174_VTA260_Issue3.pdf.
3. Viehhauser, Georg. WP4 Twiki. [Online] 30 11 2009.
https://twiki.cern.ch/twiki/pub/Sandbox/TrackerExchange/Glueing_Honeycomb.doc.
4. Lloyd Instruments, UK. LRXPlus product information. [Online] http://www.lloyd-
instruments.co.uk/products/sku.cfm?ProductCAtegory_Id=3755&Product_Id=158&SKU_Id=286.
5. Jones, Tim. UK Stave & Stavelet Development. INDICO - Oxford AUW. [Online] 30 3 2011.
https://indico.cern.ch/getFile.py/access?contribId=129&sessionId=17&resId=1&materialId=slides&confId=116547.
