Annex 2 Impact Testing of Adhesive Joints

MTS Adhesives Project 2 Failure Modes and Criteria

Report No. 5: Annex 2

Impact Testing of Adhesive Joints

A Taylor*

July 1996

The work described in this report was funded by the Department of Trade and Industry under the Measurement Technology and Standards Programme. AEA Technology plc accepts no liability for the use by third parties of any information contained in the report.

Copyright AEA Technology plc 1996. Enquiries about copyright and reproduction should be addressed to John McCarthy at the address below.

AEA Technology * Imperial College of Science, Technology& Harwell Medicine Didcot Department of Mechanical Engineering Oxfordshire. OX11 0RA Exhibition Rd United Kingdom London. SW72BX Telephone 01235432476 United Kingdom Facsimile 01235432481 Impact Testing of Adhesive Joints

Final Report

By

Ambrose Taylor

Imperial College of Science, Technology & Medicine, Department of Mechanical Engineering, Exhibition Road, London SW7 2BX.

-1- Title MTS Adhesives Project 2: Report No. 5: Annex 2: Impact Testing of Adhesive Joints

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Date Summary The performance of adhesives under high loading rates was investigated using four common test methods.

The impact wedge peel test is an ISO Standard test method. The average force required to drive a wedge through a bonded ‘tuning fork’ specimen is measured. Most failures were stable at room temperature at test rates of 10-5 and 2 m/s, At -40°C only the toughest adhesive tested exhibited stable failure. Higher cleavage forces were measured for steel than for aluminium alloy substrates. There was a significant amount of plastic deformation of the substrate, and thus this method gives an indication of the performance of the adhesive/substrate combination rather than a measure of the ‘material’ properties of the adhesive. However a correlation between the cleavage force and the adhesive fracture energy, Gc, has been identified. This has been achieved by developing a finite element model for the failure of the impact wedge peel specimens. This allowed the cleavage force, and the fracture energy of the adhesive, to be predicted accurately from crack length versus time data, obtained from high speed photography. It has also been found that friction between the wedge and the specimen must be taken into account for accurate predictions.

The block shear test is both an ASTM and British Standard test method. This is a comparative method, and was found to have poor discrimination between the adhesives compared to the other methods. No correlation between the measured force and the adhesive fracture energy was found.

The three point bend test is commonly used to determine the fracture energy of polymers and metals. Tests were performed using bulk adhesive and joint specimens. Problems were encountered inserting a sharp crack into the specimens. A blunt crack leads to a value of the fracture energy higher than the true ‘lower bound’ value being recorded. The stiffness of the specimens, especially of the joints, lead to problems with dynamic effects even at relatively low test rates, under 0.3 m/s. These were resolved by damping the contact between the striker and the specimen. The minimum amount of damping must be used to measure a ‘lower bound value of the fracture energy, and thus a large degree of trial and error is required, The measured fracture energy values were similar to those from the tapered double cantilever beam tests,

Tapered double-cantilever beam specimens were used to investigate how the fracture energy, Gc, of the adhesives changed with loading rate. Test rates of between 10-5 and 5 m/s were used. At the high test rates, dynamic effects are superimposed on the load signal. Hence the true failure load is uncertain, and a load-independent analysis method must be used. The fracture energies were found to decrease with increasing rate. -2- Reductions of between 9 and 36 % for various adhesives were recorded over a test rate range of 10-5 and 1 m/s.

The tapered double-cantilever beam test is the easiest and most reliable method for measuring the adhesive fracture energy. The three point bend test is time consuming and care must be taken that the values measured are meaningful. The block shear test is poor at discriminating between adhesives, even over a large range of fracture energies. The impact wedge peel test gives an indication of the performance of the adhesive/substrate combination, data which cannot be obtained from the tapered double cantilever beam tests. However the values obtained from the impact wedge peel test do depend upon the choice and dimensions of the substrate material.

-3- I Contents

I Summary 2

Table of Contents ...... 4

Nomenclature ...... 7 English Alphabet ...... 7 Greek Alphabet ...... 7 Abbreviations ...... 8

1 Introduction ...... 9

2 Materials and Methods ...... 10 2.1 Introduction ...... 10 2.2 Adhesives ...... 10 2.3 Substrates ...... 10 2.4 Solace Treatments ...... 10 2.4.1 Introduction ...... 10 2.4.2 Gritblast/Degrease ...... 11 2.4.3 Chromic-Acid Etch ...... * ***.** 11 2.5 Adhesive Application...... 11 2.5.1 Single-Part Epoxy *** * ...... *...... 11 2.5.2 Two-Part Epoxy ...... *...**...... 11 2.5.3 Two-Part Acrylic ...... 12

3 Impact Wedge Peel Tests ...... 13 3.1 Introduction ...... 13 3.2 Analysis ...... 13 3.3 Specimen Manufacture ...... 14 3.4 Experimental Procedure ...... 16 3.5 High Speed Photography ...... 17 3.6 Results ...... 17 3.7 Finite Element Modelling ...... 18 3.8 Conclusions ...... 18

4 Block Shear Tests 20 4.1 Introduction ...... *...... 20 4.2 Analysis ...... * ...... 20 4.3 Specimen Manufacture ...... 20 -4- I

I 4.4 Experimental Procedure ...... 21 4.5 Results ...... 21 1 4.6 Conclusions ...... 22

5 Three Point Bend Tests . 24 5.1 Introduction ...... 24 5.2 Analysis ...... 24 25 5.3 Dynamic Effects ...... ● ...... 5.4 Specimen Manufacture ...... 25 5.4.1 Bulk Specimens ...... 25 5.4.2 Joint Specimens ...... 26 5.5 Experimental Procedure ...... 27 5.5.1 Pendulum Mach ine...... 27 5.5.2 High Speed Instron ...... 28 5.6 Results ...... 28 5.7 Conclusions ...... 29

6 Tapered Double-Cantilever Beam Tests ...... *** *** ...... *.*.. ***** ***** **** 31 6.1 Introduction ...... 31 6.2 Analysis ...... 31 6.2.1 General Analysis ...... 31 6.2.2 Quasi-Static Analysis ...... 32 6.2.3 Dynamic Effects 32 6.2.4 High Rate Analysis ...... 32 6.2.5 Constant Crack Speed Analysis ...... 33

6.3 Specimen Manufacture ● ...... 33 6.4 Experimental Procedure ...... 4 6.4.1 Quasi-Static ...... 34 6.4.2 High Rate ...... 34 6.5 Results ...... 34 6.6 Conclusions ...... 34

7 Conclusions ...... 35

References ...... 36

Acknowledgements ......

Tables .. 39

-5- Figures ...... 43

-6- a Crack Length b Width c Compliance d Depth E Young’s Modulus Ea Young’s Modulus of Adhesive Es Young’s Modulus of Substrate F Load per Unit Width G Strain Energy Release Rate Gc Fracture Energy h Substrate Thickness m Mass mb TDCB Geometry Factor P Load Pc Critical Load T Temperature ta Adhesive Thickness tf Time to Failure u Energy Uc Energy Consumed in Test u Displacement v Velocity Vo Impact Velocity z Height

Gree k Alphabet 4 Geometry Factor for Three Point Bend Test o Stress 1) Poisson’s Ratio

-7- Abbre viations ASTM American Society for Testing Materials BS British Standard CAE Chromic-Acid Etch ESIS European Structural Integrity Society FE Finite Element FEA Finite Element Analysis GB/DG Gritblasted and Degreased HSI High Speed Instron IS0 International Standards Organisation Impact Wedge Peel LMD Lost Motion Device LVDT Linear Variable Displacement Transducer MTS Measurements, Technology and Standards Adhesives Progmmme TDCB Tapered Double Cantilever Beam TPB Three Point Bend

-8- 1 Introduction In recent years the use of adhesives has increased as manufacturers become aware of the advantages that they can offer over conventional joining techniques. There is however a lack of confidence in their perfomance under ‘extreme’ conditions, especially impact loading. This work investigates how epoxy and acrylic adhesives perform under these conditions when used to bond steel or aluminium substrates. Various methods will be used to evaluate the impact performance, including those laid down by National or International Standards. This work will use the two test methods set out in BS/ISO/ASTM Standards - the impact wedge peel and the block shear tests.

Two fracture mechanics approaches will also be used. Firstly, three point bend tests on bulk adhesive and joint specimens will be used to determine the mode I fracture energies, Gc, of the adhesives under impact conditions. Secondly, recent work has shown that the tapered double cantilever beam (TDCB) test, used with adhesives at quasi-static loading rates, can also be used successfully at high rates of test. Thus TDCB tests will also be performed to investigate the variation of fracture energy with loading rate. This will also allow comparison with the other test methods.

The dynamic response expected at high loading rates will be modelled. This work will concentrate on the impact wedge peel test, as no finite element modelling has previously been published on this geometry. The above data will then be compared to identify the preferred methods of testing adhesives at high rate.

-9- 2. Materalsi and M ethods 2.1 Introduction This section provides some details of the materials and surface treatments that are common for many of the test methods used in this work.

2.2 Adhesives The MTS programme specified the main adhesives used. These were a rubber-toughened epoxy, ‘AV119’ (Ciba Polymers), and an acrylic, ‘F241‘ (Permabond). A range of rubber- toughened epoxy adhesives were used additionally for some tests. All the adhesives used are listed in Table 1.

2.3 Substrates The mild steel and aluminium adherends used throughout this work were specified under the MTS programme.

Mild steel grade EN3A was used for the impact wedge peel test, and EN32B for the block shear, three point bend and TDCB geometries. The EN32B is a hardened mild steel which is not available in the thin sheet required for the impact wedge peel specimens.

The aluminium used was grade BS5251, except for the TDCB tests where grades with higher yield strengths were required, as plastic deformation of the adherends occurred. Grade BS5083 was used for the majority of these tests, though grade 2014A was required for the ‘LMD 1142’ adhesive as the yield strength of the BS5083 was insufficient.

2.4 Surface Treatments 2.4.1 Introduction The basic surface treatment used was a gritblast and degrease (GB/DG). However the CAE treatment was used for some aluminium adherends to provide a surface With a higher surface energy and encourage cohesive-in-adhesive failure. Bonding was completed as soon as possible after surface preparation to reduce the risk of surface degradation or contamination, Further details of these surface treatments can also be found in report 3 of MTS project 42.

-10- 2.4.2Gritblast/Degrease Larger adherents were wiped with acetone until the tissue used remained clean after each stroke, showing that no further contamination was being removed. They were then gritblasted with alumina grit at 60 or 80 psi for aluminium and steel respectively. The grit was sprayed at an angle of 45° to the surface so that the particles bounced off the surface rather than becoming embedded in it. The adherends were then degreased again by wiping with acetone as above.

Smaller adherends were degreased in a bath of Genklene (this was later changed to Triklone for environmental reasons). They were immersed in the hot liquid solvent for 5 minutes, before being removed and allowed to dry and cool for 5 minutes. After this time they were placed in the vapour for another 5 minutes. They were then gritblasted as above and degreased again in the Genklene as before.

2.4.3 Chr omic-Acid Etch The adherends were prepared as above, then placed in a bath of chromic-acid etch2 at 68°C for 10 minutes. They were then rinsed with tap water and placed in a bath of tap water for 15 minutes before being rinsed with distilled water and dried in warm air.

Ad hesive Application 2.5.1 Single-Part Epoxy After surface preparation, a bead of adhesive was applied to both sides of the joint and spread with a spatula until the whole surface was covered. Care was taken not to trap air bubbles. The joint was closed, and the adhesive cured, see Table 1.

The time required to cure the adhesive was calculated by adding the manufacturer’s recommended cure time to the time required for the joint to reach the cure temperature, as measured b y a thermocouple placed in the bondline of a sample joint.

2.5.2 T we-Part Epoxy The ‘EA9309’ adhesive is a two-part system consisting of a resin and a hardener, which were mixed in the ratio of 100:22 by weight. The adhesive was mixed thoroughly, but attempting to minimise air entrapment. A bead of adhesive was applied to both surfaces and spread with a spatula. The joint was closed and allowed to cure at room temperature. Full strength is obtained after 3 to 5 days.

-11- I 2.5.2 Two-Part Acr ylic The 'F241’ adhesive consists of a resin and a liquid hardener (aniline). A thin layer of I hardener was painted onto both surfaces, ensuring that the whole area was wetted. The resin was applied as a bead to both surfaces, but not spread (as per manufacturer’s instructions). The joint was closed and allowed to cure for at least 24 hours.

-12- 3 Impact Wedge Peel 3,1Introduction The impact wedge peel (IWP) test was introduced as an International Standard in 19933. It specifies 20 mm wide, sheet metal adherends, between 0.6 and 1.7 mm thick. These are bonded over a length of 30 mm, the unbended arms being formed to give a ‘tuning fork’ profile. No starter crack is used with these specimens.

The free arms of the specimen are clamped, and a wedge is driven through the bonded portion, see Figure 1. The wedge velocities specified by the Standard are 2 m/s for steel adherends, and 3 m/s for aluminium. However for consistency and ease of comparison, this work will use 2 m/s throughout. A range of epoxies and the acrylic adhesive will be tested.

Previous work by Ford Research4 has shown that more consistent results may be achieved by forming the adherends prior to bonding. This prevents damage to the specimen caused by forming the profile after bonding. However Ciba Polymerss point out that, as with T-peel specimens, pre-forming the adherends results in a larger bead in the fillet and so they bend the adherends by hand after bonding. There is some concern that this may damage the adhesive, and that the variability of the adherend profile produced may influence the results. It may also be impossible to bend the thicker adherends by hand. Thus this work used pre-formed adherends. The bead size is minimised by removing as much excess adhesive from the fillet as possible before curing.

Most of the work carried out on this test, for example by Ford Research and Ciba Polymers, has been for the evaluation of adhesives, and so has not been published except in general terms6’7.

3.2 A nalysis The IS0 Standard specifies that the average cleavage force is calculated from the force versus time data, disregarding the first 25 % and the last 1090 of the curve, see Figure 2. The energy is also calculated, by integration over the same part of the curve. The trace shown displays an initial peak and a plateau region, where stable crack growth occurs. Some test results do not show this stable region, see Figure 3, and will give a high force value indicative of a ‘tough’ adhesive when the joint is exhibiting very brittle behaviour.

Other methods of analysing the test data have been suggested, as shown in Figure 4, and these have been discussed in more detail previously. The author has tried various methods, but has concluded that the average force calculated by the Standard method is preferable, Providing that this section is contained within the plateau region. Where no

-13- plateau region exists, a zero value will be quoted. This provides greater differentiation than, say, the total energy absorbed in the test, which is effected by the adherend profile and the adhesive fillet, see section 4.7(FEA).

Figure 2 shows an example result, with the average force and energy values that have been calculated from the force versus time trace. The energy is calculated by integrating the force versus time data, finding the area under the curve using Simpson’s rule, then multiplying by the test velocity. These values are calculated from a ten point moving average of the original data. This smoothes out some of the noise and makes it easier to locate the start and end of the test. This approach may be used due to the large number of data points taken for each test, typically about 8000 between initial contact and final failure. The difference between values from this trace and the original trace has been shown to be less than 1 %, often less than 0.1 %.

Prior to bonding the surfaces of the adherends were gritblasted and degreased, as detailed in section 3.3. Adhesive was applied to both adherends, with a loop of copper wire to ensure a constant bondline thickness of 0.36 mm. Work by Ciba Polymers5 and Ford4 has shown that the presence of this wire has no effect on the results, but they have used copper wire because its low stiffness should minimise any possible effect, Further tests at Ford to investigate whether the use of a stiffer wire, e,g. stainless steel, has any effect have proved inconclusive 4. The adherends were brought together and clamped in individual bonding jigs, shown in Figure 6, the excess adhesive being removed prior to curing. Care was taken to remove all the adhesive from the specimen fillet before curing. Any excess adhesive present after curing was removed with a knife or file. The bead of cured adhesive in the fillet, if present, was not removed to prevent any possible notching effect.

3.4 Experimental Proced ure The specimens were tested on a high-speed servo-hydraulic machine, using a lost-motion device (LMD). This allows the ram to accelerate for a short distance, and reach the required velocity before it picks up the specimen, see Figure 7. The specimen grip and

-14- LMD are made from titanium to reduce the inertia of the system. The specimen grip is attached to the moving part of the testing machine, rather than the wedge shackle as recommended by the Standard, for the same reason.

A piezoelectric load cell mounted below the wedge shackle was initially used to measure the force on the specimen, see Figure 7, This was found to resonate at just over 2 kHz, and the traces produced were unsuitable for the calculation of results, see Figure 8. Therefore strain gauges were bonded onto the arms of the wedge. Initially these were wired up individually. The signals were recorded on two oscilloscopes, and then combined once the data had been transferred to a computer analysis package. This process was time-consuming because the oscilloscopes triggered at different times, with a variable interval between the triggers. However the strain gauges produced a much clearer trace than the load cell, and were connected into a bridge circuit producing a single signal. This was calibrated up to 3 kN, and showed perfect linearity. The signals were not filtered.

Analysis of the signals from individual strain gauges shows that the wedge exhibits bending in some tests. This puts pairs of gauges into tension and compression, so the effect cancels out when the signals are combined in the bridge circuit.

The signal from the strain gauge bridge and the displacement, taken from an LVDT on the machine ram, were recorded. The test velocity was calculated from the gradient of the displacement versus time trace. Room temperature tests were conducted at 2.5 x 10-5 and 2 m/s. Further 2 m/s tests were conducted at -40 and 60°C.

The strain gauge bridge was calibrated at all the test temperatures used. This showed a linear relationship between the calibration factor and the test temperature, with a gradient of 320 mV/N°C.

The -40 and 60°C tests were performed using an environmental chamber mounted on the testing machine. The time taken for the specimens to attain the test temperature was measured using a thermocouple embedded in the bondline of a sample specimen. After the specimen was placed in the testing fixture, it was allowed to acclimatise for 15 minutes before being tested. The broken specimen was removed and the chamber left for a further ten minutes before the next specimen was fitted.

A few 2.5 x 10-5 m/s tests were conducted at -40C to investigate how the specimens failed, and compare with the 2 m/s tests. It had been noted from observation of the room temperature failure of specimens at 2.5 x 10-5 m/s that these showed a similar failure to that of the 2 m/s tests, as seen from the high speed films.

-15- . 3.5 High Speed Photography A 16 mm Photec IV rotating prism high speed camera (Hadland Photonics) was used to film some of the IWP tests to investigate how the specimens failed at the high test rate, The specimens were illuminated with floodlights and a focused tungsten spotlight. These were activated immediately before testing to prevent any heating of the specimen. Tungsten-balanced Eastman 7250 colour film was used.

A typical wedge peel test at 2 m/s lasts about 15 ms. To enable sufficient data to be taken from the film to feed into the finite element model, the camera was run at its maximum speed of 5000 frames per second. The camera was fitted with a half-frame converter, enabling two pictures to be taken on each frame of film, giving a total of 10,000 pictures per second (pps). This would be expected to give about 150 pictures of the test in progress. The film used was 100 feet long, and so when the specimen was tested, it would still be accelerating, thus the final framing rate was less than that specified. Analysis of the films showed that a framing rate of approximately 6000 pps was obtained.

The disadvantage with using such a high framing rate is that as the film speed is increased, the picture quality deteriorates, producing a grainy picture. The picture also becomes darker as less light is absorbed by the film when it is traveling faster.

Due to the geometry of the wedge peel shackle, the camera cannot be placed parallel to the plane of the crack, i.e. side-on to the specimen. Instead it was placed at an angle of approximately 45°, making the crack less clear. These effects combine to make the crack tip difficult to locate on the final image, and may lead to errors in the measured crack length.

The camera printed a timing mark on the film every millisecond. This enabled the duration of the test to be calculated and compared to the force/time output.

To analyse the high speed films, they were projected onto a screen and the distance from the wedge tip to the crack tip was measured from the image, The magnification factor was calculated from the projected size of the wedge shackle,

3.6 Results A typical IWP force versus time trace consists of two peaks and a plateau region, see Figure 2. The initial rise occurs when the wedge makes contact with the specimen. Rapid crack growth occurs and the measured force drops. The crack runs for about 5 mm and arrests. This is repeated for the second peak, after which the crack growth is

-16- stable through the rest of the specimen. This ‘plateau region’ is typically 10 ms long for a test at 2 m/s. As the crack velocity is observed to be equal to the test velocity, this is equivalent to 20 mm of crack growth.

The crack velocity was found to be constant and equal to the test velocity over the plateau region for those specimens that exhibit one. Observations at 2.5 x 10-5 m/s indicate that the crack tip is a constant distance ahead of the wedge over the plateau region of the force/time trace. The smaller this distance the higher the recorded force. Analysis of the high speed films confirms that this is also the case at 2 m/s.

Visual observation at 2.5x 10-5 m/s showed that the distance from the crack tip to the wedge increased towards the end of the test, causing a step down in the recorded load. This was confirmed by the high speed films, and can be seen on Figure 2 after 14.5 ms.

Specimens that exhibit a plateau region in their force/time trace exhibit plastic deformation of the adherends, see Figure 9a. Unstable failure, as shown in Figure 3, does not result in plastic deformation, see Figure 9b.

The results show a correlation between the measured cleavage force and the adhesive fracture energy from the TDCB tests, see Figure 10. This data is summarised in Table 2. The Gc values shown are taken from tapered double cantilever beam tests at crack velocities approximately equal to those seen in the IWP tests. The tougher adhesives perform better when used to bond steel, while the weaker adhesives have marginally higher force values on aluminium substrates.

At the lower test velocity the wedge cleavage force values are higher than those at 2 m/s for both steel and aluminium adherends, see Table 2. However it should be noted that as the adhesive fracture energy and the IWP force change with rate, the correlation between IWP force and Gc also changes. This is shown in Figure 11.

At higher temperatures the wedge cleavage force was generally higher, see Table 3. The exception was the ‘LMD 1142’ adhesive, which performed better at -40°C than at room temperature, At -40°C it was the only adhesive to exhibit stable failure with the steel adherends.

3.7Finite Element Modelling A finite element (FE) model was setup by Dr. Yu Wang9 using the ABAQUS commercial FE package. This modelled the unbended region of the specimen and 20 mm of the bond, as crack propagation is stable for most of this region. The mesh used is shown in Figure 12. Eight node quadratic elements were used. The model was two-dimensional. Half the specimen was modelled due to symmetry. The adherends were modelled as elastic/plastic. Tensile tests were performed on the steel and aluminium used, to provide modulus and yield data. The measured modulii agree well with values quoted in the literature. The adhesive was modelled as purely elastic, using data from Project 1 of the MTSA Programme10velocity of 2 m/s was applied to the wedge, which is assumed to be rigid.

Data from the force versus time traces and the high speed films was used. The former gave an approximate crack initiation load, and the latter gave the distance from the crack tip to the wedge. This enabled the nodes in the centre of the bondline to be released in turn when the crack tip reached them, see Figure 12.

The contact between the wedge and the adhesive was initially assumed to be frictionless. However this was found to give lower peel force and Gc values than those seen experimentally, Table 4. Values of 0.4 and 0.5 were used for the coefficient of friction as these are quoted for the unlubricated contact between steel and PMMA1l. The value of 0.5 gives the closest fit to the experimental data. This model has also confirmed that the force required to fracture the specimen, in the plateau region, is dependant on the distance of the crack tip from the wedge.

The predicted force versus time response is shown in Figure 13. The agreement between the FEA and the experimental results is extremely good.

The FE model has also shown that the adherend profile will alter the peak force. A gradual transition between the gripped end of the specimen and the bonded region will result in a lower peak force. However, the force in the plateau region of the trace is not affected, and so the adherend profile has no effect on the quoted value. A fillet of adhesive in the ‘V’ formed by the shape of the adherends will also increase the peak force.

A virtual crack closure technique was used to calculate the adhesive fracture energy from the IWP data. The agreement between the predicted value and that from the TDCB tests is excellent. For the ‘XB5315’/steel specimens, the predicted value was 1.5 kJ/m2, while the experimental value, from the TDCB tests, was also 1.5 kJ/m2.

3.8 Conclusions

-18- The impact wedge peel test has been shown to provide good discrimination between adhesives. The measured force depends on both the adhesive and the substrate used. Thus it evaluates the energy-absorbing capabilities of the system, not the adhesive alone. Almost all the room temperature and 60°C tests gave stable failure. Failure at -40C was mostly stable for aluminium adherends, but mostly unstable for steel. In general at higher temperatures, and the lower test rate, the recorded force values were higher.

A correlation between the wedge cleavage force and the adhesive fracture energy has been shown. The form of this correlation is dependent on the test conditions. A finite element model was used to predict the cleavage force and adhesive fracture energy measured from the TDCB tests. The agreement between the FE theoretical model and the experimental results is extremely good.

-19- 4 Block Shear 4.1 Introduction This test method is specified by ASTM D95012 and BS 535013. Until the impact wedge peel test was introduced these were the only Standards governing the impact testing of adhesive joints. The block shear test uses a comparative method, so it does not give any material parameters, The specimen consists of a 25 mm square, by 10 mm thick, block bonded to a larger base block, see Figure 14. This is tested on an Izod machine as shown in Figure 15. The energy required to remove the upper block, i.e. the energy lost by the pendulum, is taken to be the impact strength of the adhesive. No starter crack is used with this test.

4.2 Analysis The Izod machine available did not allow the height from which the pendulum was dropped to be varied, and so only one test velocity could be used, This can be calculated by equating the loss of potential energy with the gain in kinetic. At the moment of impact:

(1) where the height from which the pendulum is dropped, z, is 1.3 metres. Hence the velocity at impact, vO= 5 m/s.

The energy lost by the pendulum, as indicated by a scale on the front of the machine, was recorded.

4*3 Spe cimen M anufacture The Standard specifies a lower adherend height of 20 nun. However, this was increased to 65 mm to allow the specimen to be held in the machine vice. Though it is not supported all the way to the top, its size should prevent any significant bending, see Figure 16.

The steel and aluminium alloy adherends were sawn to the required length and de-burred. Aluminium adherends were prepared with a gritblast and degrease or CAE surface treatments, steel specimens were gritblasted and degreased, as described in section 3.4. The adhesive was applied and a ‘z’-shaped loop of copper wire, as used in the impact wedge peel specimens, was placed in the joint to give a constant bondline thickness of 0.36 mm. Four replicates were used for each test.

-20- Care was taken to ensure that the impact face of the top adherend was parallel to the face of the lower, so that the specimen would be struck squarely. After curing any excess adhesive was removed using a file.

4.44 Experimental Procedure The specimen was placed in the vice of the testing machine with the impact face of the specimen vertically below the centre of rotation of the pendulum, so that it would be struck when the pendulum was at its lowest point. Hence all the pendulum energy would be kinetic.

The height of the lower adherend was chosen such that the striker would make contact as close to the bondline as possible without the leading edge of the pendulum fouling on the lower adherend. The lower adherend height was slightly less for the aluminium specimens so shims were placed under the specimens to raise the bondline to the same height as the steel specimens. Contact was made at 1 mm from the bondline. This lies outside the ‘preferred’ distance specified by the Standards, but was constrained by the geometry of the testing machine.

Before testing the pendulum was allowed to swing with no specimen in place, and the scale set to read zero. The zero error was also checked frequently during testing, and was never found to exceed ±0.5 Joules.

4.5 Results Initially the ‘AV119’ and ‘F241’ adhesives were used, but as the discrimination was found to be poor, see Table 5, the range of adhesives tested was increased to provide comparison over a wider range of adhesive fracture energies. However, the discrimination was still found to be poor, see Table 6. The means and standard deviations of the measured energies are quoted. The apparent locus of failure is also given.

The fracture surfaces of the specimens were examined using optical microscopy. The locus of failure was close to the interface for all the epoxy adhesives, most specimens having a thick layer of adhesive on the top block and a thin layer on the base block, as shown in Figure 17.

The ‘AV119’/steel joints exhibited apparent interracial failure, but there were patches of cohesive failure, accounting for about 5 % of the bonded area.

-21- The epoxies used on the aluminium showed more cohesive-in-adhesive failure on the gritblast/degreased substrate than on the chromic-acid etched. This is a counter-intuitive result, as the latter is considered to be a ‘better’ treatment. However larger energies were measured for the chromic acid etched specimens. Therefore, it is possible that a thin layer of adhesive was left on the surface, but was not visible via the methods used. This could be confirmed by more detailed surface analysis.

The ‘F241’ adhesive failed cohesively within the adhesive on aluminium with either surface treatment. The mean energies measured are approximately equal, lying within one standard deviation of each other, see Table 5,

The ‘LMD 1142’ adhesive was observed to peel off both substrates when used to bond steel, see Figure 18. This effect was also seen with the ‘AV119’/steel specimens. Figure 19 shows this effect, the light areas on the top block indicating where the adhesive has become detached from the underlying substrate.

If the top block was not bonded squarely on the base block, i.e. the striker and the struck face of the specimen were not parallel on impact, the measured energy was significantly reduced. For example, for the %SP110’/steel specimens the mean value obtained was 41.7 J, but a value of only 28 J was obtained from a specimen where the top block was not struck squarely.

In some cases the locus of failure changed from closer to one adherend to closer to the other when the crack reached the copper wire, Figure 18.

Some plastic deformation of the aluminium adherends occurred where the pendulum struck the specimen. However the extent of the plastic deformation was small, and its effect can be considered to be minimal.

The energy values obtained were plotted against the fracture energies obtained from the TDCB tests, see section 7. However no relationship is apparent, see Figure 20.

4,6 C onclusions Without the aid of surface chemical analysis it is difficult to identify the locus of joint failure for the block shear specimens. Apparent interracial failure was observed for some specimens, but the presence of a thin coating of adhesive cannot be ruled out. Those joints that exhibited apparently interracial failure showed more scatter in the energy values obtained. It is important that the striker makes contact squarely with the specimen, otherwise there can be a significant reduction in the energy value obtained. -22- I I The data obtained were plotted against the Gc value obtained for the adhesives from the I TDCB tests, but no relationship was apparent. No mode II fracture energy data was available within the MTS programme. I

-23- 5 Three Point Bend 5.1 Introduction The three point bend (TPB) or Charpy test is a Standard method for measuring the fracture energy of metals and polymers14. It has also been used with bulk adhesive and joint specimensl5’ 16.

The ‘AV119’ epoxy and 'F241’ acrylic adhesives will be used in joint specimens with steel adherends. Bulk ‘AV119’ specimens will also be tested, The ‘F241’ adhesive was not used for bulk specimens due to problems with manufacturing the relatively thick (10 mm) specimens’.

The specimens will be tested on two machines at various rates. A pendulum (CEAST) machine will be used, while for higher rates a servo-hydraulic (Instron) machine will be used. Tests will be conducted at 0.3 and 2 m/s. Where machine velocities overlap, tests will be conducted to ensure consistency between the results from the two machines. When the time-to-failure, tf, is in the order of 1 ms or less, dynamic effects start to become significant.

. 5.2 Analysis The current work used both bulk and joint specimens, 80 mm long with a 10x 12 mm cross-section as shown in Figure 21. These were notched, and may have a natural crack inserted. The crack tip should be sharp enough that a true ‘lower bound’ value of the fracture energy is measured. A blunt crack will give a fracture energy that is erroneously high. The depth of the notch was varied in order to give a range of crack lengths, and enable a value of Gc to be calculated using:

(2) where UC is the energy consumed in the test, b is the width and d is the depth of the specimen. Values of the geometry factor, ~, are available in tabular forml8. Hence the fracture energy is the gradient of a plot of the energy consumed in the test against bd$, where @is dependant on the initial crack length.

The measured energy can be partitioned as shown in Figure 22. The inertia and kinetic energy is assumed to be constant across the range of crack lengths, and thus Gc is unaffected, The energy terms caused by the damping (see section 6.3), indentation and machine compliance will affect the calculated value of fracture energy.

-24- .! ,

5.3 Dynami ‘c Effects At low rates of test, the force versus time response of the three point bend specimen is linear. Failure occurs at the maximum load, Pc. When the test velocity is increased, dynamic effects become significant. Oscillations become superimposed on the linear response, see Figure 23. These are caused by the specimen flexing and losing contact with the supports and the striker, and hence with the load cell, as shown in Figure 24, The measured load may even become negative. There are large differences between the measured values and those acting in the specimen 19,20. Thus the area under the curve, and hence the calculated energy, becomes meaningless.

An Appendix to the ESIS protocol on the TPB testing of polymers18 discusses the experimental aspects of this problem. The recommended method is to place some damping material on the specimen where the striker makes contact. Initially ‘Blutak’ or similar material was used, but the latest work favours grease21, because it will be displaced while damping the initial contact. Thus, though it reduces the dynamic effects, it will only affect the contact stiffness at the beginning of the test, see Figure 25. The aim is to reduce the oscillations between PQ/2 and PQ to within a band of width ±5 % of PQ, see Figure 26. The protocol18 explains how to determine the value of PQ, but PQ = Pmax. Including the damping increases the time to failure, but only increases the energy consumed, Uc, slightly. It is possible to correct the calculated value of Gc by performing rebound tests on unnotched specimens with damping in place, see Figure 27. The parasitic energy terms Ud, Ui and Um can be calculated from the area under this graph. Tests must be performed for each specimen, damping and velocity combination. These tests were not performed for the current work, due to the risk of damaging the testing machines and load cells, especially with the joint specimens. This method is more applicable for polymers as these will “give’when struck.

Castrol ‘LM’ and ‘heavy’ grease were used as damping material. This was applied as a bead and spread to the required thickness with a razor blade. The thickness was controlled by pieces of shim on either side, as shown in Figure 29. The bulk specimens required very little damping to reduce the dynamic effects to within the acceptable limits, even 0.05 mm of grease could cause the response to be overdamped, as can be seen in Figure 25.

5.4 Spe cimen Manufacture 5.4.1 Bulk Spe cimens Bars of bulk ‘AV119' adhesive were prepared by AEA Technology. The adhesive was injected into an evacuated mould and cured for 8 hours at 80°C, followed by a post-cure of 2 hours at 120°C, to prevent the exothermic reaction produced during curing

-25- damaging the adhesive 17. This produced a bar approximately 15 x 25 x 400 mm long, The use of the vacuum ensured that air entrapment was kept to a minimum.

Specimens of the required size were milled from the bars and notched with a single-point cutter, giving a V-shaped notch. Kinloch and various co-workers16,22 inserted a natural crack into their specimens by tapping a razor blade into the notch, causing a crack to jump ahead of the notch tip. This method was tried, but a natural crack could not be generated. Care must be taken when using this technique as the crack must arrest after a short distance, and not fracture the specimen.

The notches were sharpened by drawing a razor blade across the notch tip. This method is also recommended for use with polymers by the ESIS protocol. However it should be noted that this produces a blunter crack tip radius than that of a natural crack, and hence an erroneously high fracture energy value can be obtained.

5.4.2 Jo int Spec imens In previous work three point bend joint specimens have been made by moulding silicone rubber around a blank the same size as the finished specimen16’22.This blank is removed, and the adherends, cut to length to give the required adhesive thickness, placed in the mould. The adhesive is poured into the gap between the adherends, and cured.

This method was not practicable for the adhesives used in the current work due to their high viscosity. The adhesive would not flow into the gap between the adherends, but bridged across it. Specimens were made using longer silicone rubber moulds, allowing the adhesive to be injected into the gap between the adherends before they were pushed together. It was found that due to the high coefficient of friction and the low stiffness of the rubber, the adherends tended to be misaligned in the finished specimen. A metal mould was designed, see Figure 29, which proved to be successful in eliminating these problems, though the mould needed to be pre-heated and the cure time for the ‘AV1 19’ adhesive extended due to the high thermal inertia of the mould.

The adherends were sawn to length before bonding to give a finished specimen length of approximately 80 mm, including the adhesive. The saw marks were removed by grinding the surfaces to be bonded. A jig was used to ensure that the finished surface was perpendicular to the sides of the specimen. This kept a constant adhesive thickness across the joint. Prior to bonding the surfaces of the adherends were gritblasted and degreased, as section 3.4.2. They were placed in the mould, about 10 mm apart, and adhesive was injected into the gap between them. The adherends were then pushed

-26. together using the bondline adjustment bolts until the required adhesive thickness was achieved, see Figure 29. The excess adhesive was removed, and the joints were cured.

The narrow bondline of the joint specimens prevented the V-shaped single point cutter being used, and so the notch was initially inserted by using a razor blade mounted as a cutter. Attempts were made to insert a natural crack into the specimens by tapping a razor blade into the notch. However the narrowness of this notch made this impossible as the blade tended to stick part of the way down the notch without propagating a crack.

To check whether the production of a natural crack was going to be necessary, some ‘AV1 19’ joints were tested on the pendulum machine with the razor blade notch alone. However the force required to fracture these specimens was greater than the capacity of the load cell, and considerably higher than for the bulk specimens. From this it must be concluded that this method does not give a sufficiently sharp notch tip.

To produce a wider notch a jeweller’s saw was used. This allowed the razor blade to reach the bottom of the notch. However there were still problems with trying to propagate a natural crack, and the notch was sharpened by drawing a razor blade across the tip.

The 'F241’ specimens were notched by sawing down to the required depth and drawing a razor blade across the bottom of the notch to sharpen it, as it is not possible to propagate a natural crack in such a rubbery adhesive.

5.5 E xperimental Procedu re 5.5.1 Pendulum Mach i ne Tests The pendulum testing machine used is shown in Figure 30. The test velocity could be altered by dropping the pendulum from various angles, e.g. 15° corresponds to 0.5 rids. It was not possible to alter the mass of the pendulum, and hence the energy available at a given velocity. The energy available was insufficient to break some specimens and limited the range of crack lengths that could be used. Tests were performed at 0.5 m/s rather than 0.3 m/s so that more energy would be available. Dynamic effects were found to be larger than on the servo hydraulic machine. As the striker is falling freely rather than being driven, there is likely to be more bounce on contact.

-27- 5.5.2. High SpeedInstron (HSI) Tests The load cell and steel striker were attached to the ram of the high speed servo-hydraulic machine, as used with the IWP specimens. The striker is made as small as possible, whilst retaining the same tip radius and included angle as the pendulum striker, This is to minimise the mass, and hence the inertia.

Tests were performed at 0.3 and 2 m/s using a total travel of 40 mm. The 20 mm pre-travel used allows the ram to accelerate to the required velocity before striking the specimen. The extra travel means that the ram only starts to decelerate after the specimen has broken. Thus the specimen experiences a constant velocity during the test.

5.6 Results Bulk specimens of ‘AV1 19’ were tested at 0.5 m/s on the pendulum machine. Though dynamic effects were present these were tested undamped because even a 0.05 mm thickness of grease caused the response to be overdamped. A Gc value of 0.64 kJ/rn2 was obtained.

Bulk ‘AV1 19’ specimens were tested on the HSI at 0.3 m/s. These showed dynamic effects, and the oscillations were greater than the ±5 % band specified by the ESIS protocol18Further tests were carried out using damping. These gave a Gc value of 0.89 kJ/m2, see Figure31. If the tests with dynamic effects outside the ±5% band are included a value of 0.96 kJ/m2 is obtained. Thus the calculated fracture energy is affected by the dynamic effects and it is important to keep them within the specified limits. Tests at 2 m/s gave a value of 0.64 kJ/m2, as shown in Figure 32. For comparison, the TDCB tests gave a static value of 0.7 kJ/m2, and 0.45 kJ/m2 at 1 rids. The TPB values are about 2570 higher, but show a similar reduction in the fracture energy with increasing rate.

The ‘AV119’/steel joint specimens gave a Gc value of 0.53 kJ/m2 when tested at 0.3 m/s, see Figure 33. This compares well with the TDCB data mentioned above of 0.7 kJ/m2 statically and 0.45 kJ/m2 at 1 rids. The value obtained from the joint tests is lower than that from the bulk specimens, possibly because the specimen is stiffer and hence the adhesive experiences a relatively higher loading rate. For example a bulk specimen with a 2 mm crack had a time-to-failure, tf, of 1.35 ms, while for a joint specimen with the same crack length it was 0,45 ms,

The ‘F241’ joint specimens tested on the HSI at 0.3 m/s gave a GC value of 0.70 kJ/m2. This compares to *** kJ/m2 from the TDCB tests at a test velocity of *** m/s

-28- 5.7 Co nclusions The results of the three point bend testing show that Gc decreases with increasing test rate. The values obtained are similar to those from the TDCB tests, see section 7, but the test method is more time-consuming, The TPB test is a multi-specimen approach, the ESIS protocol recommends at least 15 valid tests to obtain a reliable value of Gc. It may be necessary to test three times this many specimens to find the correct amount of damping and obtain valid data. Even though the data are valid with relation to dynamic effects the value of fracture energy obtained may not be a ‘lower bound’ value unless the notch is sufficiently sharp. Obtaining data on the pendulum machine was more difficult than on the servo-hydraulic due to larger dynamic effects, possibly because the striker is falling freely rather than being driven. The available energy is dependent on the test velocity so a light duty machine may not have sufficient energy to break specimens at a low test velocity.

-29- 6 Tapered Double-Ca ntilever Beams 6.1 Introductoni This test geometry, as shown in Figure 34, is normally used under quasi-static test conditions, using crosshead speeds in the order of 1 mm/min23. However recent work has shown that it may be used successfully at high rate 24. Tapered double-cantilever beam tests were performed to determine fracture energy, Gc, values for a range of test velocities to investigate the effect from increasing the test rate. The data allowed comparisons to be made with the fracture energy obtained from the three point bend tests, section 5, and to investigate the correlation between the adhesive Gc and the impact wedge peel force or energy, see section 3.

6.2 Analysis 6.2.1 Ge neral Analvsis Williams25 has shown that for a general linear elastic double cantilever beam, the energy release rate, G, is given by:

(3)

where b is the width of the specimen, C is the compliance, a is the crack length, and u is the displacement.

The tapered double cantilever beam (TDCB) geometry is designed such that the value of dC/da is constant as the crack grows through the specimen26:

(4)

where Es is the modulus of the substrate and

(5)

where z is the height of the substrate, see Figure 34, and mb = 2 mm-l.

.3(3- 6.2.2 Quasi-Stat ic Analysis TDCB specimens are commonly used at quasi-static loading rates, where the value of the adhesive fracture toughness can be calculated from the crack propagation load, Pc. Substituting Equation 4 into Equation 3 gives:

(6)

Hence,

(7)

This will be referred to as the load method,

6.2.3 Dynamic Effects As the test rate is increased, dynamic effects become significant. The measured load oscillates, and an accurate value of Pc cannot be determined, see Figure 35. These can be caused by high contact stiffness between the testing machine and the specimen, or by the loss and regaining of contact between the two, as discussed in section 6.3. These effects may be reduced by damping the contact, here between the machine ram and the lost motion device, see Figure 36. Oscillations can also be caused by inertial effects, and the repeated reflection of stress waves in the specimen.

6.2.4 High Rate Analysis At high rates, the load value becomes unreliable due to dynamic effects, and so a load-independent analysis method is required. Integrating Equation 4 with respect to the crack length, a, gives:

(8)

-31- Substituting into the displacement form of Equation 3:

(9)

This will be referred to as the displacement method.

67,5 CO nstant Crack Speed Ana lysis Analysis of the high speed films of the TDCB tests has shown that the test velocity and the crack velocity are constant. Thus the fracture energy can be found from:

(10)

Tests conducted at 10-5 m/s have shown a good correlation between all three analysis methods, see Figure 36. For the TDCB specimens tested the crack velocity is approximately twenty to fifty times the test velocity, a larger ratio giving a smaller value of fracture energy.

6,3 Spec imen Ma nufacture The tapered profile of the beams was milled on a computer numerically controlled milling machine. Aluminium substrates were used, 10 or 12.5 mm thick. The surfaces to be bonded were chromic-acid etched, and the adhesive was applied as described in section 2.5. Stainless steel wire, 0.4 mm in diameter, was placed at either end of the joint to give a constant bondline thickness. A double layer of aluminium foil coated with release agent was placed in the middle of the bondline as a starter crack to encourage the adhesive to fail cohesively. The two halves of the beam were brought together in a jig, pre-heated where applicable. The top and bottom platens of the jig are shaped such that pressure is applied to three points along the length of the beam. The bolts securing the two platens were torqued to about 5 Nm.

After curing most of the excess adhesive was removed using a chisel, the remainder being sanded off. It was also possible to remove the excess from the softer adhesives with a razor blade when they were removed from the oven after curing. A 20 mm strip,

-32- spanning the bondline, was painted white and marked every 10 mm from the loading point. This enabled the length of the crack to be measured.

6.4 Expe rimental Procedu re 6.4.1 Quas i-Static Test were conducted on a screw-driven Instron tensile testing machine, using a displacement rate of 0.5 to 1 mm/min (=10-5 m/s). The force versus time response was recorded, and the crack length measured optically at intervals with a traveling microscope. The loading pins allowed the halves of the beam to rotate freely as they deformed elastically.

6.4.2 High Rate A 20 mm strip, spanning the bondline, was painted white and marked every 10 mm from the loading point. This enabled the length of the crack to be measured. The beams were tested on the high speed servo-hydraulic machine and filmed with a high speed camera. Analysis of the high speed films enabled the displacement and crack length data to be found. The force data was recorded from a piezo-electric load cell, with a natural frequency of 70 kHz and a rise time of 10 µs. The test rig used, Figure 37, was similar to the IWP arrangement, with titanium shackles in place of the IWP specimen grip and wedge shackle.

6.5 Results The results obtained are shown in Figure 38 and Table 7. The fracture energies decreased with increasing test rate. Reductions in fracture energy of 9 to 36 % were found between test rates of 10-5 and 1 m/s. Increasing the test rate to 5 m/s, showed a further reduction, as shown in Figure 38.

6.6 co nclusions At high test rates the fracture energy calculated from the measured load is not reliable due to dynamic effects, as the force on the load cell is not the same as that experienced by the specimen. However load-independent methods, using displacement and crack length, or test rate and crack velocity, data from the high speed films can be used.

All the adhesives tested showed a reduction in fracture energy with increasing test rate. The magnitude of the reduction varied from less than 10 to over 35 % as the test rate increased from 10-5 to 1 m/s. Increasing the test rate to 5 m/s showed a further reduction. -33- 7 co nclusions The results from the ISO Standard impact wedge peel test show a correlation between the wedge peel force and the adhesive fracture energy (the adhesive fracture energy, Gc, being a ‘material property’ of the adhesive, assuming failure is cohesive-in-adhesive). The form of this correlation is dependent on the test conditions. As the test velocity is increased from 10-5 to 2 m/s the cleavage force is reduced, though most specimens are still exhibiting stable crack propagation. All failures are cohesive-in-adhesive. A finite element model has been used to predict the wedge cleavage force and adhesive fracture energy. The agreement between the predicted values and the experimental results is excellent. An improved method for analysing the experimental data has also been identified.

Standard block shear specimens were tested at 5 m/s. The discrimination between adhesives was poor, even over a wide range of adhesive fracture energies. No correlation was found between the energy values from the block shear test and the adhesive fracture energies.

The three point bend specimens used for the impact tests showed large oscillations in the force versus time response due to dynamic effects. These were more significant with the pendulum testing machine than with the servo-hydraulic machine. These oscillations can be reduced by damping the contact between the striker and the specimen. The data showed a reduction in fracture energy with increasing test rate. The measured values were similar to those obtained from the TDCB testing, however the TPB test method is more time-consuming, requiring a large number of specimens to be tested. Also, considerable care must be taken that the values of fracture energy obtained are valid. For example an insufficiently sharp crack tip will produce a value that is erroneously high.

In the tapered double cantilever beam tests, at high test rates the fracture energy calculated from the measured load is not reliable due to dynamic effects. However, load-independent methods, using displacement and crack length (or test rate and crack velocity) data from the high speed films can be used, All the adhesives tested showed a reduction in fracture energy with increasing test rate, The magnitude of the reduction measured by the TDCB tests varied from less than 10 to over 35 % as the test rate increased from 10-5 to 1 m/s. Increasing the test rate to 5 m/s showed a further reduction.

The tapered double cantilever beam test is the easiest and most reliable method of measuring the adhesive fracture energy. The three point bend test method is time-consuming and considerable care must be taken that the values measured are meaningful. The block shear test is poor at discriminating between adhesives, even over -34- -

1 a large range of brittle to very tough adhesives. The impact wedge peel test gives an indication of the performance of the adhesive/substrate combination, data which cannot be obtained from the tapered double cantilever beam tests. Although, it appears that the impact wedge peel performance may be quantitatively modelled from a knowledge of the fracture energy of the adhesive, assuming that failure is cohesive in the adhesive , layer.

-35- 1. Kinloch, A,J,, Adhesion And Adhesives: Science And Technology (Chapman Hall, London, 1987). 2. Bull, S.J., Bellamy, B.A., Bishop, H.E., Watts, J.F. & Brewis, D. The Role of Surface Analysis in Adhesive Bonding (1995). 3. ISO, Adhesives - Determination of Peel Resistance of High-Strength Adhesive Bonds Under Impact Conditions - Impact Wedge Method, ISO 11343 (1993). 4. Davis, R.E. & Fay, P. A., Personal Communication, Ford Motor Company Ltd 5. Holmes, B., Personal Communication, Ciba Polymers Ltd. 6. Fay, P.A., Davis, R.E. & Suthurst, G.D. Impact Testing and Performance of Bonded Automotive Structures in Conference Proceedings of Impact and Fatigue Testing of Adhesives (Plastics and Rubber Institute, London, 1990). 7. Powell, J.H. Impact Testing of Adhesives : Peel Joints in Conference Proceedings of Impact and Fatigue Testing of Adhesives (Plastics and Rubber Institute, London, 1990). 8. Taylor, A.C. Impact Loading of Adhesive Joints, MTS Programme, Project 2 (1995). 9. Wang, Y., Personal Communication, Imperial College of Science, Technology and Medicine 10. Dean, G.D. & Duncan, B.C. Tensile Behaviour of Bulk Specimens of Adhesives, MTS Programme, Reject 1 (1995). 11. Greer, A, & Hancock, D.J., Tables, Data and Formulae for Engineers (Stanley Thornes, Cheltenham, 1984). 12. ASTM, Standard Test Method for Impact Strength of Adhesive Bonds, ASTM D950, (1982). 13. BS, Determination of Impact Resistance of Adhesive Bonds, BS 5350, (1986). 14. ASTM, Standard Methods for Notched Bar Impact Testing of Metallic Materials, E23, (1986). 15. Kinloch, A.J. & Kodokian, G. A., Impact Behaviour of Structural Adhesive Joints, Journal Of Materials Science Letters 6, 653-655 (1987). 16. Jamarani, F., Jakusik, R., Kinloch, A.J. & Kodokian, G.K.A. Experimental and Analytical Studies of the Fracture Behaviour of Engineering Adhesives and Adhesive Joints Under Impact Loading, 161-188 (Elsevier Appl. Sci., 1990). 17. Sampson, T,, Personal Communication, AEA Technology 18. ESIS, A Linear Fracture Mechanics (LEFM) Standard for Determining Kc and Gc for Plastics, (1990). 19. Kinloch, A.J., Kodokian, G.A. & Jamarani, M.B., Impact Properties Of Epoxy Polymers, Journal of Materials Science 22,4111-4120 (1987).

-36- I I 20. Williams, J. G., The Analysis Of Dynamic Fracture Using Lumped Mass-Spring Models, International Journal of Fracture 33,47-59 (1987). 21. MacGillivray, H.J. & Chong, M.J.C., Personal Communication, 22. Kinloch, A.J. & Kodokian, G. A., The Impact Resistance Of Structural Adhesive Joints, Journal of Adhesion 24, 109-126 (1987). 23. Jethwa, J.K. & Kinloch, A.J. The Fatigue Performance of Adhesively Bonded Metal Joints in Conference Proceedings of Adhesion ’93, 197-202 (Institute of Materials, London, 1993). 24. Blackman, B .R.K., Personal Communication, Imperial College of Science, Technology and Medicine 25. Williams, J.G., Fracture Mechanics of Polymers (Ellis Horwood, Chichester, 1984). [ 26. ASTM, Fracture Strength in Cleavage of Adhesives in Bonded Joints, ASTM D3433, (1975).

-37- Acknowledgements This work was funded by the DTI Measurements, Technology & Standards programme on the performance of adhesive joints. The author wishes to thank AEA Technology, Professor A,J. Kinloch, and his colleagues at Imperial College, especially Dr. B. Blackman and Dr. Y. Wang.

-38- Tables

Table 1: Adhesives used in the current work.

No.

Table 2: Room temperature impact wedge peel results, 2.5x 10-5 and 2 m/s.

277 I 0.8 I 0.85

512 I 0.9 I 1.06 (88)

553 I 334 I 604 346 I 1.5 I 1.58 (26) (47) (44) (70) 1223 640 1585 1161 I 1.97 I 4.59 (29) (48) (85) (75) I

-39- Table 3: Variation of IWP force with temperature.

Note: * denotes combination not tested.

Table 4: The effect of friction on the cleavage force and fracture energy predicted by finite element modelling. Steel adherends, ‘XB53 15’ adhesive, 2 m/s test velocity.

Coefficientof Wedge to Crack Tip IWP Cleavage Predicted Gc, Friction, µ Distance, mm Force, N kJ/m2 o 4.5 500 1.5 0 6 300 1.3 0.4 6 500 1.4 0.5 6 600 1.5 Experimental 6 600 1.5

-40- Failure loci: C: Cohesive. I: Apparently interracial. C+I: Mixed.

Table 6: Energy values from block shear tests, steel adherends. The Gc values are taken from the TDCB data. Table7: Variation of adhesive fracture energy with test rate, TDCB tests. Also see Figure 38. Figure 1: The impact wedge peel test.

.43-

Ram motion

Specimen grip ,

Figure 7: High rate rig for IWP tests.

-47- 3000

2000

1000

0

-1000 0 24 i Time, ms

Figure8: Force versus time trace from piezo-electric load cell, stable failure, showing oscillatory response.

-48-

Notch

Notch / Adherend

\ Adhesive

Figure 21: Three point bend specimens, (a) bulk adhesive, (b) joint.

uDamping

Figure 22: Partitioning of energy consumed in TPB test, showing effect of damping and other parasitic energy terms on the measured value of Gc.

-58- 20

0

o 0.25 0.5 0.75 1 1.25 1.5 1.75 Time, ms

Figure 23: Force versus time response of three point bend specimen at high rate, showing dynamic effects.

-59- n

I I

Figure 24: Loss of contact (LoC) in three point bend test, which will cause oscillations in the measured force versus time response.

-60-

Gc = 0.65 kJ/m2

Figure 32: Energy versus bw@ graph for bulk 'AVl19' TPB test. Tested on high speed Instron at 2

0.1

0.05 P

specimen.

1500

500

0 . 12

Figure 35: Force versus time trace of TDCB specimen bonded with ‘ESP110’ adhesive tested at 0.5 m/s, showing oscillations due to dynamic effects.

-65-

Figure 37: High rate TDCB testing rig, showing lost motion device and damping unit.

-67- Figure 38: Variation in fracture energy with test rate, from TDCB tests. Also see Table 7.

-68-