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MTS Adhesives Project 1

Basic Mechanical Properties for Design

Report No 7 March 1996

TEST METHODS FOR DETERMINING SHEAR PROPERTY DATA FOR ADHESIVES SUITABLE FOR DESIGN

Part 2: The torsion method for bulk and joint test specimens

R Thomas and R D Adams

Composites and Adhesives Group, Engineering Materials Research Centre, Department of Mechanical Engineering,

University of Bristol, Queen’s Building, University Walk,

BRISTOL BS8 lTR



Report No. 7 March 1996

TEST METHODS FOR DETERMINING SHEAR PROPERTY DATA

FOR ADHESIVES SUITABLE FOR DESIGN


Part A : Summary


Composites and Adhesives Group,

Engineering Materials Research Centre,

Department of Mechanical Engineering,

University of Bristol,

Queens Building,

University Walk,

BRISTOL. BS8 lTR

Summary Report : The torsion method for bulk and joint test specimens

Introduction

In Project 1 of the MTS Programme on adhesives, a number of bulk and joint specimen tests

have been developed or improved for the measurement of shear property data for a variety of

adhesive materials. Four types of adhesives were selected that exhibit different characteristics

in their shear behaviour. One objective of this work was to specify how the accuracy and

reliability of data produced by each method can be optimised through features such as the

design of the apparatus and the extensometers, the choice of specimen geometry and the use of

correction procedures for sources of error.

In this summary report, a torsion test method is described that can be used for testing either bulk

or joint specimens of adhesives. The test results for this method can be found in the full report

(l). Two separate reports (2, 3) detail the notched-beam shear or Iosipescu method, together

with the notched-plate or Arcan method and the thick adherend shear test.

Outline of the Test Method

The principle of the torsion test is the application of a torque to a cylindrical rod, loading the

material in pure shear. Measurement of the applied torque is straightforward with the use of a

load cell, but the determination of the resulting strain is more complicated. The strain resulting

from the torsion of bulk adhesive specimens can be large but the strain experienced by a butt

joint in torsion can be difficult to record accurately due to the very low angular displacement,

this being of the order of a few degrees. The bondline thickness for this method should be large

enough to allow precise measurements, but sufficiently small to simulate the type of joint used

in practical applications. The strain should be measured as close to the adhesive layer as

possible so that the deformation of the adherends can be kept to a minimum. Any strain in the

adherends can be calculated to give the actual displacement due to the torque on the adhesive.

Solid, cylindrical bulk specimens and butt joints have been used in this particular programme.

Since the shear stress through a section of the specimen is not constant, when the strain in any

part of the specimen reaches the non-linear region for the material, a correction must be applied

to give the true stress/strain curve.

Tensile testing machines can

are necessary for this type

specially-designed torsional

specimens or butt joints.

be utilised to load

of loading. The

testing machine

the specimens in torsion but special adaptations

method used in this programme employed a

that can be used with either bulk adhesive

2

Summary Report: The torsion method for bulk and joint test specimens

Test Specimens

Four adhesives with different properties and behaviour were considered during the programme.

The details of these adhesives and the cure cycles followed are shown in Table 1.

Name Manufacturer Type of adhesive Cure Cycle Post Cure

AV119 Ciba Polymers l-part epoxy 120°C for 1 hour none

TE251 Evode 2-part epoxy 23°C for 7 days ½ hour at 80°C

F241 Permabond 2-part acrylic 23°C for 7 days 1 hour at 100°C

3532 3M polyurethane 23°C for 7 days none

Table 1 List of adhesives tested, showing the cure cycles used

Test specimens

To enable bulk specimens to be produced for the torsional testing method, the adhesive was cast

in bulk form, 13mm square and with sufficient length to give a specimen 135mm long after

machining. After curing, the bars of adhesive were machined to the required geometry, with a

diameter of 10mm at the centre of the specimen.

The methods used to manufacture these bulk specimens are described in a separate report (4). It

was possible to cast and machine specimens for the two epoxies considered in this programme

(AV119 and TE251) but bulk specimens of the acrylic adhesive (F241) could not be made to

the required thickness due to the exothermic reaction during the cure cycle. Whilst it was

possible to cast the polyurethane adhesive into bars of the required size, the material was too

flexible for machining to be practical.

The butt joints tested were made using

adhesive layer and a bondline of 0.5mm

Experimental procedure

solid steel adherends with a diameter of 15mm at the

for all the adhesives under consideration.

The angular displacement of the clamped ends of the bulk specimen was used to calculate the

shear strain in the bulk adhesive since the use of any contacting extensometry could affect this

measurement. Since the machined specimens had radiussed ends to the gauge length, a

correction was applied to the length of the specimen to give the true shear strain. As the

adhesive begins to behave in a non-linear fashion,

real stress/strain curve for the experimental data.

3

the Nadai correction is used to develop the


.

In the case of the butt joints, the angular displacement was measured using LYDTs located in

the steel on either side of the bondline with special extensometry. Simple materials theory was

applied to calculate the displacement due to the steel adherends and the Nadai correction was

used to give the true stress/strain behaviour for those polymers that are not highly dependent on

the applied strain rate.

The bulk specimens and the butt joints were tested in the same specially-designed torsional

testing machine. For the initial tests, the torsional loading was applied at a rate of constant

angular displacement to the clamped ends of the specimens. However, during the programme,

it became apparent that the maximum shear stresses of the bulk specimens when taken to failure

were lower than the shear stresses measured with the butt joints. Careful investigation of the

strain rates used in the two types of tests indicated that the strain rate in the joints was

increasing by as much as 10 times over the period of the tests whilst the strain rate in the test to

failure of a bulk specimen remains reasonably constant. A feedback control system was

developed to allow butt joint testing to be conducted under a constant strain rate throughout the

tests. Comparisons were made between the two types of tests on the butt joints and, also, with

the bulk specimen tests.

Results

Bulk specimen tests

Shear modulus measurements were conducted on the bulk specimens to assess the reliability and

consistency of the results from this test method. For both the epoxy adhesives used in this

programme, these results were repeatable and consistent between the specimens. The specimens

made from the 2-part epoxy (TE251) generally failed at a strain of less than 10%, failure being

initiated at a void within the material. The bulk specimens ofAV119 failed at a strain of nearly

50%. This demonstrates the importance of good quality bulk specimens for torsional testing as

failure tends to occur through any voids present. A summary of the results for the bulk

specimen testing is shown in Table 2, showing the consistency of the final data.

Adhesive Shear Modulus GPa Max Stress MPa Strain to failure

AV119 (epoxy) 1.139±0.02 46.4 ± 0.8 0.47 ± 0.14

TE251 (epoxy) 0.938 ± 0.04 24.7 ± 0.6 0.09 ± 0.01

Table 2 Average bulk specimen torsion test results


Butt Joint Tests

Initial tests were conducted to demonstrate the consistency of the shear modulus measurements

taken with this method. For all the adhesive systems, the results were consistent and repeatable,

with all the joints failing at a strain of at least 300A. A summary of the average results for all

the adhesives is shown in Table 3, illustrating the consistency of the test data.

Adhesive Shear Modulus GPa Max Stress MPa Strain to failure

AV119 1.110 ± 0.02 48.9 ± 1.0 0.46 ± 0.03 , AV119(strain controlled) I 1.081 ± 0.02 44.8 ± 2.2 0.44 ± 0.08

1 , I TE251 0.976 ± 0.03 28.7 ± 0.9 0.38 ± 0.02

TE251 (strain controlled) 1.022 ± 0.05 25.6 ± 0.9 0.31 ± 0.05

F241 0.223 ± 0.007 38.9 ± 3.1 1.40 ± 0.10

3532 I 0.077* 0.01 I 16.4 ± 0.3 1.26 ± 0.10 I

Table 3 Average butt joint torsion test results

For the two epoxy adhesives, tests to failure were made using both a constant angular

displacement rate (where the strain rate increases substantially as the adhesive yields) and, also,

with the feedback system that gave a constant strain rate. The maximum stress achieved with

the controlled strain rate was lower than the stress measured using a constant angular

displacement rate for both the epoxy adhesives. The controlled strain rate tests gave maximum

stresses that were consistent with the bulk specimen tests.

Conclusions

Torsional testing of bulk adhesive specimens has been shown to give consistent, repeatable

results for both shear modulus measurements and stress/strain data to failure for the two epoxies

considered in this programme. This method is suitable for adhesives that can be cast in bars

13mm thick and machined to a circular cross-section. For adhesives that have a significant

exothermic reaction (e.g. acrylic F241 ) or for polymers that are fairly flexible in bulk form (e.g.

polyurethane), bulk specimens may not be possible to produce

testing may not be appropriate.

and, therefore, this method of

generate the true stress/strain For the bulk specimen testing, two corrections are needed to

behaviour from the experimental data. In order to measure the strain to failure, the angular

displacement of the clamped ends of the bulk specimen has been used. As the gauge length of

the specimen is radiussed at both ends, a correction must be applied to the gauge to generate the

effective length of the specimen. Also, as the shearing stresses across the diameter of the

5


specimen are not uniform

Nadai correction must be

failure.

The butt torsion test is a

once the material enters the non-linear region of its behaviour, the

applied to the torque/twist data to derive the stress/strain curve to

reliable test method for all the types of adhesives studied in this

programme, generating reliable shear modulus values and stress/strain curves. As for the bulk

tests, the Nadai correction should be applied to the experimental data to achieve the true

stress/strain behaviour. However, this correction may not be applicable to a material such as the

polyurethane where the behaviour is highly dependent on the strain rate.

It has been demonstrated that the strain rate experienced by the adhesive in a butt joint tested at

a constant angular displacement rate, increases as the polymer yields. Comparison of these

results with testing conducted at a constant strain rate indicates that the latter tests may produce

a lower maximum shear stress. Since the behaviour of polymers is likely to depend on the

strain rate that is applied to the material, the strain rate should be considered when analysing test

results and comparing bulk specimen testing (where the strain rate is constant) with butt joints.

References

1.

2.

3.

4.

5.

R Thomas and R D Adams :Part 2: The torsion method for bulk and joint test specimens,

Test methods for determining shear property data for adhesives suitable for design, MTS

Adhesives Project 1, Report No 7, March 1996

B C Duncan and G D Dean: Test Methods for Determining Shear Property Data for

Adhesives Suitable for Design. Part 1: Notched-beam shear (Iosipescu) and notched-plate

shear (Arcan) methods for bulk and joint test specimens. Report no 6, MTS Adhesives

Project 1, March 1996

L Vaughn and R Adams: Test Methods for Determining Shear Property Data for Adhesives

Suitable for Design. Part 3: The thick-adherend shear test method. Report no 8, MTS

Adhesives Project 1, March 1996.

L Vaughn and R Adams: Test Methods for Determining Shear Property Data for Adhesives

Suitable for Design. Part 3: The thick-adherend shear test method. Report no 8, MTS

Adhesives Project

Dean G D and B

Adhesives Project

1, March 1996.

C Duncan : Tensile Behaviour of Bulk Specimens of Adhesives, MTS

1, Report No 3 May 1995

6


6. Duncan B C, Girardi M A, Read B E, The Preparation of Bulk Adhesive Samples for

Mechanical Testing, MTS Adhesives Project 1, Report No 1, January 1994

Acknowledgements

This work forms part of a programme on adhesives measurement technology funded by the

Department for Trade and Industry as part of its support of the technological

competitiveness of UK industry.



Report No. 7 March 1996

TEST METHODS FOR DETERMININ G SHEAR PROPERTY DATA FOR ADHESIVES SUITABLE FOR DESIGN


Part B : Full Report


Composites and Adhesives Group, Engineering Materials Research Centre, Department of Mechanical Engineering,

University of Bristol, Queens Building, University Walk,

BRISTOL. BS8 lTR

-.

The torsion method for bulk and joint test specimens

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Outline of test method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Calibration of transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.3.1 LVDTS (linear variable differential transducers) . . . . . . . . . . . . 3.3.2 Rotary Potentiometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Load cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4. Test specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Bulk specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Joint specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.2.1 2-part epoxy TE251 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 l-part epoxy AV119 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Acrylic F241 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4 Polyurethane 3532 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5. Derivation of shear stress/strain data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.2.1 Correction for deformation in the steel adherends . . . . . . . . . . 5.2.2 Nadai correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Correction for radiussed end of bulk specimen . . . . . . . . . . . . . .

5.3 Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6. Some illustrative data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Bulk specimen data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6.1..1, Polypropylene and acetal polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.2 l-part epoxy AV119 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3 2-part epoxy TE251 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3 Acrylic F241 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.4 Polyurethane 3532 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6.2 Butt joint data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 l-part epoxy AV119

I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6.2.2 2-part epoxy TE251 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Acrylic F241 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.4 Polyurethane 3532 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8. List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9. List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10. List of reports from Project 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3 3

4

4 4 6 6 6 7 7

7 7 8

10 11 11 12

12 12 13 13 13 15 16

16 16 17 18 19 19 19 20 20 21 22 23

24

26

27

28

29


1. Introduction

In order to employ finite element methods for design with adhesives, data are needed on their

stress/strain behaviour to failure under well-defined states of stress. An important part of this

data requirement is satisfied by a measurement of the stress/strain curve under a shear stress.

For this purpose, a number of test methods exist that employ test specimens in the form of a

bonded joint. For the determination of strain in these tests, it is very difficult to measure the

very small displacements in the adhesive with accuracy and reliability. Where bulk specimens

of the adhesive can be prepared, there is scope for achieving greater accuracy through the use of

a larger gauge length in the specimen for the determination of strain.

In Project 1 of the MTS Programme on adhesives, a number of bulk and joint specimen tests

have been developed or improved for the measurement of shear property data for a variety of

adhesive materials. Four types of adhesives were selected that exhibit different characteristics

in their shear behaviour. One objective of this work was to specify how the accuracy and

reliability of data produced by each method can be optimised through features such as the

design of the apparatus and the extensometers, the choice of specimen geometry and the use of

correction procedures for sources of error.

In this report, a torsion test method is described that can be used for testing either bulk or joint

specimens of adhesives. The procedures used test bulk adhesive specimens and butt joints.

Two separate reports (1, 2) detail the notched-beam shear or Iosipescu method, together with

the notched-plate or Arcan method and the thick adherend shear test.

A further objective of this work was to compare results from these test methods obtained on

specimens prepared from the same source of adhesive. A variety of different types of adhesive

have been studied. It should then be possible to make certain recommendations regarding the

most suitable test method for a particular adhesive type. This work should also shed further

light on the issue as to whether the properties of bulk specimens of a particular adhesive are

representative of the material in the thin layer in an adhesive joint. The comparison of results

from the bulk and joint specimen tests is described in a further report (3).

1.1 Acknowledgements

This work forms part of a programme on adhesives measurement technology funded by the

Department for Trade and Industry as part of its support of the technological competitiveness of

UK industry.


2. Outline of the test method

The principle of the torsion testis the application of a torque to a cylindrical rod loading the

material in pure shear. Measurement of the applied torque is straightforward with the use of a

load cell, but the determination of the resulting strain is more complicated. The strain resulting

from the torsion of bulk adhesive specimens can be large but the strain experienced by a butt

joint in torsion can be difficult to record accurately due to the very low angular displacement,

this being of the order of a few degrees. The bondline thickness for this method should be large

enough to allow precise measurements, but sufficiently small to simulate the type of joint used

in practical applications. The strain should be measured as close to the adhesive layer as

possible so that the effect of the deformation in the adherends can be kept to a minimum. Any

strain in the adherends can be calculated to give the actual displacement due to the torque on the

adhesive.

Solid, cylindrical bulk specimens and butt joints have been used in this particular programme.

The shearing stresses across these specimens are uniform when the material is behaving in a

linear fashion. However, once the material reaches non-linear behaviour, this is not a valid

assumption and a correction must be applied to give the true stress/strain curve.

Tensile testing machines can be utilised to load the specimens in torsion although special

adaptations are necessary for this type of loading. The method described in this report employs

a specially-designed torsional testing machine that can be used with either bulk adhesive

specimens or butt joints.

3. Experimental

3.1 Apparatus

All the torsion testing was conducted utilising a variable speed torsional testing machine that

uses a method described by Chodorowski (4). This equipment was designed to test specimens

under torsional loading, minimizing the axial and bending loads on the specimen. The

specimens are located in square jaws at either end of the machine. The variable speed motor

applies an angular displacement to the specimen under test, driving through reduction gears, and

the torque is transmitted to the specimen through hardened steel balls. Specimens with a

maximum length between 100 and 200 mm can be accommodated, with a variable speed motor

to provide the required surface strain rate for each specific specimen diameter. The direction of

the motor is reversible to facilitate unloading the specimen after a test.

4


For the bulk and butt joint testing, the applied torque was measured with a load cell that was

located between the stationary end of the specimen and the frame of the machine. The output

from the load cell was fed through a Sangamo C30 transducer conditioner to the computer. The

gain of the data acquisition processor could be increased to give greater resolution when

undertaking modulus measurements. Voltages were recorded at specified time intervals and this

was used to determine the strain rate applied to the adhesive.

For the bulk specimens, the angular rotation was measured between each end of the specimen

(see Figure 1). Pulleys were attached to the jaws gripping the specimen and pulleys of equal

diameter were connected to a rotary potentiometer. The position of the pulleys connected to the

moving end of the specimen were adjustable to allow for the different lengths of the specimens.

A nylon cord, tensioned with a light spring, was used to connect the pulleys and this was wound

around the pulleys twice to reduce the possibility of slippage.

In the case of the butt joints, the displacement was measured using linear variable differential

transducers (LVDTs). These transducers were held in place using specially designed

extensometer arms as shown in Figure 2. The jig was located on the joint specimen using pins

positioned approximately 4mm on either side of the adhesive layer. The exact position of these

pins was measured accurately following each test. It was important to ensure that the pins were

tightened sufficiently to hold the extensometry in place, but not too tightly so that it was

difficult to measure the position of the pins accurately because of excessive plastic deformation.

Two transducers were used, one on each side of the joint, to check that no bending was

introduced during the test. The average of the two LVDTs was used to calculate the strain.

During the course of the testing programme, it became apparent that the maximum shear

stresses observed in bulk and butt joint tests were different, with the joints achieving higher

loads before failure. The initial strain rate for all the tests to failure for both bulk specimens and

butt joints was set at 4% per minute. Careful examination of the results from the tests

undertaken at a constant angular displacement rate indicated that the strain rate of the adhesive

in the joints increased from 4% per minute to approximately 40% per minute as the rate of

increase of shear stress in the polymer reduced (or even became negative) whilst the adhesive

yielded (see Figure 3). However, in a bulk specimen, the strain rate remains at approximately

4% per minute throughout the test. To investigate this strain rate effect, the experimental

apparatus for the butt’ joint was altered to accommodate testing with strain control as well as

constant angular displacement rate. The computer generated a specified ramp signal at a

frequency calculated to give the required strain rate. The motor was driven by the computer at

speeds that enabled the voltage output from one of the LVDTs to follow this signal. It was

important to remove as much backlash from the system as possible to reduce

the motor driving too fast and then reversing to achieve the necessary voltage

the possibility of

output. Figure 3

5


shows the comparison between the strain rate in a joint using constant angular displacement rate

and the rate achieved with the strain control system. Using strain control, the rate is similar to

that applied during a bulk specimen test to failure and this should reduce the variations between

the two methods due to the strain rate effect, thus allowing for valid comparisons of the

stress/strain data.

3.2 Transducers

The angular displacement generated when testing the bulk specimens was measured using a

Penny and Giles rotary capacitive potentiometer. This type was chosen as the durability is

superior to the alternative resistive transducer since there is be no wear due to contact within the

transducer. The potentiometer was powered by a voltage source of 5 volt, giving a maximum of

5 volt output for an angular displacement of 330°. For angles greater than this, the output falls

rapidly to zero and starts again. This allows for angular measurements of more than 3300

although there is a danger that the springs and nylon cord will become tangled for such large

angles.

Schlumberger LVDTs type AX/5.00 and AX/10.00 with a stroke of ± 5mm and ±10mm

respective y were used for the measurement of the strain in the butt joints. The larger

transducers were chosen to allow measurement of the anticipated high strain to failure of the

polyurethane. The output from the LVDTs was amplified by two Schlumberger OD5

transducer conditioners, and these voltages were recorded digitally on the computer.

3.3 Calibration of transducers

3.3.1 LVDTs (linear variable differential transducers)

The LVDTs used for the butt joint testing were calibrated using a barrel

calibration of the micrometer was carefully checked with calibrated slip

micrometer. The

gauges before the

LVDTs were calibrated. The transducers were located in the calibration jig and the voltages

were recorded at regular displacements, through the computer, for the whole range of the LVDT

movement. The behaviour was found to be linear over the whole range of the displacements

used. Calibration of the transducers was undertaken on a regular basis, although there was little

variation in the behaviour.

In a butt joint shear test, the strain is measured using 2 LVDTs to ensure that the joint is not

subjected to any bending. The strain is

Comparison of the voltage outputs from the

calculated using the average of

two LVDTs revealed no difference

6

the readings.

in the angular


displacements and that no measurable bending occurs during the testing. It is important to

check after each test that the strain measured by each LVDT is the same and that no bending or

slippage of the extensometry has occurred.

3.3.2 Rotary Potentiometer

The rotary potentiometer was connected to the bulk specimen using strings and pulleys. The

angular displacement of the ends of the specimen can be determined using knowledge of the

gearing ratios of the torsional testing machine. The voltage output can be plotted against the

known angular movement over 1800. This calibration assumes that the specimens tested have

clearly defined radii and gauge lengths. The bulk specimens were machined with a radiussed

end to the gauge length and the necessary correction is discussed in section 5.2.3.

3.3.3 Load Ceil

The design of the torsional testing machine incorporated a method of calibrating the load cell

statically by the application of a moment at the stationary end of the apparatus. A bar is located

on two lugs and weights are added at different distances along its length. The voltage output

can be measured and plotted against the moment to find the calibration. As for all the

calibrations, the measured voltages were digitally recorded through the computer.

To confirm the calibration of the rotary potentiometer and the load cell, a specimen was made

from aluminium with a gauge length of approximately 100mm but without a radius at the end.

The geometry is easy to machine in this material and the square end to the gauge length

removed the uncertainty over the length of the specimen under test. The shear modulus of the

aluminium was calculated using two different methods, namely using the torsional testing

machine and dynamically in a torsional pendulum. The torsional testing machine gave a

modulus of 25.57 GPa compared with the dynamic measurement of 25.42 GPa. This result

gives confidence to the testing method under investigation and confirms the validity of the

calibrations.

4 Test specimens

4.1 Bulk specimens

The bulk specimens were machined from blocks of adhesive cast in 13mm square bars. It is

important that the bars of adhesive to be machined are as void free as possible. The specimens

were machined carefully to the shape shown in Figure 4, with square ends of approximately

7


It was shown by Coppendale (6) that a fillet has a stiffening effect on the adhesive in a butt joint

and causes stress concentrations. AS a result of this research, the fillet was machined away from

the all joints before testing. The actual bondline thickness was measured after the fillet had

been removed and before the start of the testing programme.

Four adhesives were tested for this particular programme. The details of the adhesives and the

cure cycles followed are shown in Table 1.

Name Manufacturer Type of adhesive Cure Cycle Post Cure

AV119 Ciba Polymers l-part epoxy 120ºC for 1 hour none

TE251 Evode 2-part epoxy 23ºC for 7 days ½ hour at 80ºC

F241 Permabond 2-part acrylic 23ºC for 7 days 1 hour at 100ºC

3532 3M polyur ethane 23ºC for 7 days none

Table 1 List of adhesives tested, showing the cure cycles used

4.2.1 l-part epoxy AV119

The joints were assembled in the standard manner, ready for the curing cycle. The adhesive was

dispensed using a standard cartridge gun. Since the adhesive cures at an elevated temperature,

time can be taken with this adhesive to

perfect the manufacture.

The thermal history of a cured adhesive

make sure that the joints are of good quality and to

may have an effect on the behaviour of the polymer.

To ensure that all the joint and bulk specimens were manufactured using the same cure cycle,

investigations were conducted to determine the length of time taken to reach the cure

temperature for the one-part epoxy AV119. The large bulk of the aluminium jig used to make

the butt joints was found to take at least 1 hour to heat in an oven. The time taken to heat the

specimens from 90ºC to 120ºC was considered to be the important aspect of this cure cycle.

When manufacturing the bulk specimens, the time taken to heat the adhesive over this

temperature range was approximately 15 minutes. To achieve this time requirement for the butt

joints, a procedure was developed utilising the heated plates of a hot press.

10


The method determined for the cure cycle was:

● heat the press to 120ºC

● place the jig in the press and heat to 60ºC

● assemble the joints

● wrap the ends of the jig with insulating material

● place the jig in the press and apply a low pressure

● monitor the temperature of the joints with a thermocouple until the required

temperature is reached and time the cure for the specified time.

The specimens were allowed to cool in the jig before placing them in a desiccator cabinet. All

the specimens, both bulk and butt joints were stored in this way after cure and prior to testing to

ensure that all the specimens were tested in a dry condition.

4.2.2 2-part epoxy TE251

using either bench mixer or a cartridge gun with a mixer nozzle This adhesive was dispensed

to blend the two parts together. Any air trapped in the nozzle should be expelled and some of

the mixed adhesive discarded before application to the prepared surface. Following the

assembly of the butt joints, the adhesive was left to cure at 23ºC for 7 days. Post-curing was

carried out in a hot press, with the temperature monitored using a thermocouple and the time for

the post-cure (in this case 1/2 hour) taken from the point at which the joints reached the required

temperature of 80ºC. The joints were left to cool to room temperature before placing them in a

desiccator cabinet for storage. All the joints were stored under the same conditions as the bulk

specimens, i.e. in a desiccator cabinet with silica gel to maintain dry conditions.

4.2.3 Acrylic F241

The working life of this adhesive is very short when the two constituent parts are mixed. A

bench mixer was used to dispense the mixed adhesive into a small syringe and, for this

adhesive, only 1 or 2 joints could be made before the cure of the adhesive had advanced too

much for it to be of any use. If the adhesive gels too much prior to assembly, voids can appear

in the adhesive layer. These voids may not become apparent until testing has commenced.

Butt joints were also made using the two parts of the adhesive separately, by putting one part on

each adherend and holding them together until cure had been achieved. The adhesive is very

non-viscous for this system, so it was found necessary to hold the jig almost upright to ensure

that the liquid did not run out of the joint before cure.

11


For both these methods, the joints were left at room temperature for 7 days and then post-cured

at 100ºC for 1 hour. Again, the joints were all stored in a desiccator cabinet following the post-

cure to ensure dry conditions.

4.2.4 Polyurethane 3532

The working life of this adhesive is only a few minutes. Thus, small quantities of adhesive were

dispensed into a small syringe using a bench mixer. The adhesive was applied to the adherends

but in this case only 2 or 3 joints could be made before gelation was initiated. It is important to

ensure that the adhesive has pot started to gel before the joint is placed in the jig. Polyurethane

is so flexible that, if the adhesive has cured in the joint before the specimen has been clamped in

position, the joint may be mis-aligned. The joints were left to cure at room temperature for 7

days in a desiccator cabinet to make sure that the joints did not absorb any water during this

process.

5 Derivation of shear stress/strain data

5.1 Equations

For a solid of circular cross-section, the torsional shear stress is given by

= Tr r—

J

where z is the torsional stress

T is the applied toque

r is the radius

J=+ is the polar moment of inertia

The shear strain for a circular section is given by the equation

ri3 Yl =—

where y is the shear strain

r is the radius

O is the angular displacement in radians

1 is the length of the section

(1)

(2)

(3)

12


For the bulk specimens, equations 2 and 3 can be used to give the measured stress and strain for

the adhesive.

When testing the butt joints, the angular displacement is measured using LVDTs located at a

distance of a mm from the centre of the joint. This angular displacement (assuming a small

angular rotation) is calculated according to the relationship

~= 6 — (4) a

where 8 is the displacement measured by the LVDTs

a is the length of the extensometer arm

5.2 Corrections

5.2.1 Correction for deformation in the steel adherends

The extensometry was located on the butt joint at approximately 4mm on other side of the

bondline. The measured angular displacement includes the movement due to the steel

adherends and the adhesive. The joint is loaded in pure shear and the loads applied to the

specimen ensure that the steel adherend is always operating in the linear elastic region.

Therefore, a simple strength of materials equation can be used to calculate the strain in the steel.

i.e. .G=~ Y

where G is the shear modulus of the material

The strain can be calculated simply from this relationship, using the measured stress. The shear

modulus of the steel adherends, determined from a resonance test, was found to be 82.14 GPa.

5.2.2 Nadai correction

The shearing stresses across the diameter of a cylindrical bar subjected to a torque are uniform

when the loads applied keep the behaviour of the material in the linear region. However, once

the material starts to behave in a non-linear fashion, this is not a valid assumption. Nadai (7)

developed a correction to the measured

curve for a butt joint or a bulk specimen.

torque-twist curve that derived the true stress-strain

13


If the stress for the material is given by the relationship

then the torque is

Since

Substituting y for r, this equation becomes

where

Differentiating with respect to (3 gives

Thus,

‘= *(’T+ ‘a In terms of the torque-twist curve obtained from a torsion test shown in Figure 6, this equation

becomes

‘r — — L(3 AC

2m3 + AB)

Application of this correction to a typical butt joint torque/angle experimental curve is shown in

Figure 7. The adjustment has a similar effect on the torque/angle data for both the epoxies. The

effect of the correction on a material such as the acrylic is shown in Figure 8.

Originally, Nadai applied this correction when testing metals. The method assumes non-linear

behaviour and one stress-strain curve for the material. Polymers are very often strain rate

dependent and, for these adhesives, Nadai’s assumptions may not strictly apply. For the

14


particular adhesives under test in this programme, the behaviour of the two epoxiesTE251 and

AV119 does not vary significantly with strain rate and, therefore, this correction can be applied

to the torque/twist curve. The behaviour of the acrylic F241 does not depend too heavily on the

strain rate and the correction can be used in this case, but the polyurethane is both temperature

and strain rate dependent. For the polyurethane, the data shown in this report excludes any

effect due to Nadai’s correction.

5.2.3 Correction for radiussed end of bulk specimen

All the bulk specimens were machined with a radius at each end of the gauge length (see Figure

4). The formulae relevant to the application of a torque to a cylindrical rod require the

knowledge of the length and the radius of the specimen for the calculation of the strain

(equation 3). With the radiussed end, neither of these measurements are known accurately. To

assess the effect of this geometry, the extensometry used for the butt joint tests was fixed to a

bulk specimen of the geometry employed in these tests. Modulus measurements were made on

the specimen, using the rotary potentiometer and the LVDTs, and comparisons were made

between the results. It was assumed that the radius of the specimen was the measurement taken

at the centre of the gauge length. The gauge length itself was assumed to be the dimension of

the straight part of the specimen, between the radiussed ends.

A comparison between the two measurements of strain is shown in Figure 9. The calibration

using the LVDT extensometry gives the correct strain measurement for the specimen since the

length over which the angular displacement is measured is at the centre of the specimen and

does not include the radiussed ends. It was found that the effective length for a specimen with

this geometry when” measuring the strain using the rotary potentiometer is 8% greater than the

measured gauge length between the radiussed ends. It is important to note that this correction

only applies to specimens with the geometry shown in Figure 4. Any significant variation in

these dimensions would require are-calibration using revised measurements.

The extensometry used for the butt joints utilises pins located in the specimen. For the bulk

adhesive, this may introduce a stress concentration in the polymer that may affect the results,

particularly near to failure. For a strain of 40%, bulk specimens of the dimensions used in this

method will experience an angular displacement of the order of 360º and, therefore, the LVDT

extensometry will be unable to measure such large angles. For these two reasons, a non-

contacting method of measuring the strain is to be preferred. In this case, a rotary potentiometer

was used, with pulleys and strings to attach the specimen to the extensometry.

15


5.3 Uncertainties

To enable comparisons to be made between tests and materials, the experiments were conducted

under the same conditions so as to reduce the number of possible variables. A strain rate of 1%

per minute was chosen for the modulus measurements for TE251, AV119 and F241. For all the

measurements taken on the polyurethane and all the tests to failure, a strain rate of 4% per

minute was selected.

For a solid cylindrical specimen, the strain rate at the surface can be determined, but this rate

will not be the same across the radius of the specimen. It is important to note that, as a result of

this variation of the strain rate through the specimen , the strain rate set for all the tests is the

maximum strain rate i.e. the strain rate apparent on the surface. For a material such as the

polyurethane that is very strain rate dependent, the actual effect of the strain rate variation is not

known. The Nadai correction does not apply rigorously to these types of materials. For this

reason, all the results for the polyurethane adhesive discussed in this report exclude the Nadai

correction. Further investigations would be required to determine the effect of the strain rate

across a solid butt joint for such a strain rate dependent adhesive.

6. Some illustrative data

6.1 Bulk specimen data

For comparison reasons, all the modulus measurements for the joint and bulk specimens were

taken at a maximum surface strain rate of 1% per minute. A strain rate of 4% per minute was

selected for the tests to failure to allow the testing to be completed within a reasonable time

scale and to reduce the effect of creep on the behaviour of the polymers. All the tests were

undertaken at a temperature of 23ºC ± 0.5ºC. For the bulk specimen testing, the strain rate on

the surface of the specimen is constant as the adhesive is taken to failure and the motor is run at

a constant, pre-determined speed, although the strain rate does vary through the radius.

For each material considered, the shear modulus was calculated from the linear part of the

stress/strain curve. The gradient of the stress/strain plot between 0.3°/0 and 0.7°/0 strain was used

since the relationship appears linear at these strains and ignores any inaccuracies there may be

around the zero points of the data when taking up slack or backlash in the equipment.

16


6.1.1 Polypropylene and acetal polymer

Bulk specimens were tested using the two epoxy adhesives, TE251 and AV119. However, the

quality of the bulk material was found to be variable so, for comparison purposes and

confirmation of the test method specimens were machined from polypropylene and acetal

polymer provided by the NPL. These materials are considered to be uniform in their bulk form

and to have little material variability.

Machining the polypropylene was difficult as the material is rather ductile. This resulted in

specimens that were not perfectly round. For the four specimens made in this polymer, using

the torsion method the modulus measurements were found as shown in Table 2.

Specimen I Shear Modulus GPa ! Yield Stress MPa I P01 0.582 ± 0.04 19.7

P02 0.569 ± 0.02 19.2

P03 0.588 ± 0.02 ! 19.9

P04 0.560 ± 0.02 not tested to failure

Average 0.575 ± 0.02 19.6 * 0.04

Table 2 Shear moduli and yield stress for polypropylene

These modulus measurements were taken at a maximum strain rate of 1% per minute. The

polypropylene specimens were tested to failure at a strain rate of 4% per minute. The results of

these tests are shown in Figure 10. The stress/strain curves are shown to approximately 15%

strain, giving a good indication of the similarity and consistency of the tests to failure.

Although a maximum shear stress had been reached for each test to failure and the adhesive had

yielded, none of the specimens broke. The strain achieved for each test was in excess of 60%

and, when unloaded, the material exhibited a significant amount of relaxation. The specimens

were not taken to failure because the angular displacement required was so large that, although

the specimen was turned through two complete turns (720°), there was no failure. The rotary

potentiometer can measure large strains but, for this level of strain, the strings attached to the

pulleys can become twisted and the test has to be halted.

The acetal polymer was easier to machine and the resulting specimens were more uniform in

shape. However, although four specimens were machined, only three were tested as the fourth

specimen had a diameter of 8.75mm, significant y less than the 10mm for the other specimens.

As discussed in section 5.2.3, the correction for the radiussed end only applies to specimens

with the geometry shown in Figure 4. A summary of the measurements taken on these

specimens is shown in Table 3.

17


Specimen Shear Modulus GPa Yield Stress MPa Strain to failure

N02 1.096 ± 0.02 51.8 0.48

N03 1 .099 ± 0.02 52.9 0.47

N04 1.091 ±0.01 N/A N/A

Table 3 Shear moduli and failure data for acetal polymer

The tests to failure are shown in Figure 11, showing good consistency between the tests. The

final specimen (N04) was used to determine the effect of the radiussed end on the strain

measurements. As a result of the damage caused by the pins used with the LVDT extensometry,

this specimen was not tested to failure.

For both the polypropylene and the acetal polymer, the stress/strain curves to failure are similar

for the different specimens and the modulus measurements are consistent. Variations will occur

in the curves since the materials are not easy to machine and, as a result, the tolerances achieved

will not be as high as those expected with metals. The resulting cylindrical shape can become

elliptical and this will produce tests to failure with some variability. However, the materials do

not contain the voids that are present in the bulk adhesives and, therefore, the specimens do not

fail prematurely. The acetal polymer specimens broke in the centre of the gauge length,

indicating that the specimen was aligned correctly in the torsional testing machine for a shearing

load to be applied.

6.1.2 l-part epoxy AV119

The results of the shear moduli measurements and the tests to failure are shown in Table 4 and graphically in Figure 12.

Specimen Shear modulus GPa Max Stress MPa Strain to Failure

M01-281 1.124±0.01 45.0 0.50

M01-282 1.145±0.01 46.8 0.64

M01-283 1.136±0.02 46.5 0.53

MO 1-284 1.137 ±0.01 46.9 0.25

M01-285 1.153±0.02 46.8 0.43

Average 1.139±0.02 46.4 ± 0.8 0.47±0.14

Table 4 Shear moduli and failure data for bulk l-part epoxy AV119

The stress/strain data from all the specimens showed good agreement and the modulus

measurements were very consistent, with an average shear modulus of 1.139GPa. This bulk

adhesive breaks, on average, at a strain close to 50% and at a stress of approximately 46MPa.

18


Failure usually occurs at or through a void and the presence of these voids can result in

premature failure at a lower strain than the average, e.g. specimen M01-284.


Shear modulus measurements and results of tests to failure for this 2-part epoxy are shown in

Table 5, with the stress/stain curves illustrated in Figure 13.

Specimen Shear modulus GPa Max Stress MPa Strain to failure

HTElll 0.921±0.01 23.9 0.09

HTE112 0.977 ± 0.02 25.3 0.08

HTE113 0.948 ± 0.02 24.4 0.07

HTE114 0.929 ± 0.01 24.9 0.08

HTE115 0.988 ± 0.01 25.2 0.09

HTE116 0.864 ± 0.01 24.3 0.11

Average 0.938 ± 0.04 24.7 ± 0.6 0.09 ± 0.01

Table 5 Shear moduli and failure data for bulk 2-part epoxy TE251

The different specimens gave consistent results although, for this particular adhesive, the

specimens broke at strains of 10% or less and the stress at failure was found to be approximately

25MPa only about half the stress measured with AV119. Examination of the pieces indicated

that failure, in general, occurred at or through a void. The bulk adhesive bars cast in this

adhesive appear to have a significant number of voids and this can cause premature failure of

the test specimens. The possibility of voids can be reduced by vacuum stirring which was the

technique used for the, l-part epoxy AV119.

6.1.3 Acrylic F241

It was not possible to make 13mm thick bars from this adhesive for

the high exothermic reaction of this system during the curing cycle.

bulk torsion testing due to


Although it was possible to cast bars of this adhesive in the required thickness, machining the

material to shape could not be achieved. Testing in this form could only be carried out if the

polymer could be cast in the correct shape, with no requirement for machining. This procedure

would require a special mould and experimentation to achieve a good specimen for testing.

19


6.2 Butt joint data

6.2.1 l-part epoxy - butt joints

Since the strain rate in the epoxy joints increases as the adhesive yields (see 3.1), tests were

carried out with both constant angular displacement rate and constant strain rate. Figure 14

shows some typical shear stress/strain curves for this adhesive. Typical modulus measurements

and the results of the tests to failure for this particular adhesive are shown in Tables 6 and 7,

illustrating the variation seen when testing under constant strain rate conditions. The modulus

measurements were taken at a strain rate of 1% per minute whilst the tests to failure were

conducted at an initial surface rate of 4% per minute.

Specimen Shear Modulus GPa Max Stress MPa Strain to failure

AV/ST/lE 1.l10±0.02 49.2 0.42

AV/ST/1F 1.100±0.02 48.0 0.48

AV/ST/3B 1.096 ± 0.02 50.1 0.48

AV/ST/3C 1.133±0.02 48.1 0.45

Average 1.110 ± 0.02 48.9 ± 1.0 0.46 ± 0.03

Table 6 Shear moduli and failure data for l-part epoxy AV119 - butt joints tested with

constant angular displacement rate

Specimen Shear Modulus GPa Max Stress MPa Strain to failure

AV/ST/3D 1.099 ± 0.01 45.8 0.35

AV/ST/3E 1.095 ± 0.01 44.6 0.50

AV/ST/3F 1.047 ± 0.02 41.7 0.50

AV/ST/4C 1.084 ± 0.01 46.9 0.40

Average 1.081 ± 0.02 44.8 ± 2.2 0.44 ± 0.08

Table 7 Shear moduli and failure data for l-part epoxy AV119 - butt joints tested at a

controlled 4% strain per minute

The behaviour of this epoxy is reasonably linear until yield at approximately 45-50 MPa and a

shear strain of nearly 7%. The adhesive finally fails at a strain of nearly 50%. The average

yield stress when the joints are tested under strain control (44.8MPa) appears to lower than for

those tests conducted using a constant angular displacement rate (48.9MPa). Figure 15

compares, graphically, the maximum shear stresses measured for joints tested with and without

strain control, including also the results of the bulk specimen tests. The butt joints tested under

strain control show a larger variation in maximum stress but the trend demonstrates that, with

strain controlled tests, the maximum stresses measured in bulk specimens and butt joints are

similar.

20



.

The significant change in strain rate that was experienced in the butt joints made with AV119

was, also, seen in the tests to failure undertaken with TE251. Examples of the modulus

measurements and the stress and strain to failure for the TE251 butt joint specimens are shown

in Tables 8 and 9. All the modulus measurements were taken at a strain rate of 1 % per minute

and the tests to failure were conducted at an initial strain rate of 4% per minute.


TE/ST/2A 0.958 ± 0.04 27.9 0.35

TE/ST/3A 0.983 ± 0.05 28.0 0.40

TE/ST/3B 1.011±0.06 29.6 0.37

TE/ST/3C 0.953 ± 0.06 29.3 0.38

Average 0.976 ± 0.03 28.7 ± 0.9 0.38 ± 0.02

Table 8 Shear moduli and failure data for 2-part epoxy 9X251 - butt joints tested with



TE/ST/2C 0.941 ± 0.02 26.6 0.40

TE/ST/2D 1.008 ± 0.02 24.5 0.35

TE/ST/2E 1.008 ± 0.02 26.8 0.40

TE/ST/3E 1 .043 ± 0.02 25.8 0.30

TE/ST/3F 1.110*0.02 25.7 0.30

Average 1.022 ± 0.05 25.6 ± 0.9 0.31 ± 0.05

Table 9 Shear moduli and failure data for 2-part epoxy TE251 - butt joints tested at a

controlled 4% strain per minute

The shear stress/strain behaviour of some typical joints are shown in Figure 16, with a curve

from a bulk specimen test for comparison. This epoxy exhibited similar behaviour to AV119,

with a failure strain of 30-40% but a yield stress of approximately 25-30 MPa almost half that

of AV119. However, the strain to failure for the butt joints made with TE251 is significantly

higher than that achieved with the bulk specimens (less than 10%). The bulk specimens tested

with this adhesive had a large number of voids present that initiated premature failure.

Examination of the butt joints after failure indicated that there were no visible voids and a strain

of at least 30°/0 was achieved.

Figure 15 shows, graphically, a summary of the maximum stress measurement for all the joints

tested with TE251, illustrating the yield stress change observed using strain control. The trend

21


appears to be a reduction in the stress level achieved when the joints are tested under strain

control, as was also seen with the AV119.

6.23 Acrylic F241

Some difficulties were experienced with the manufacture of the joints using this system. The

adhesive appears to shrink significantly during cure and voids seemed to be introduced as a

result of this reaction. The quality of some of the joints made with the one-part method was

reasonable, but that of the specimens made with the two-part application was very poor. A

good indication of the quality of the adhesive joint can be obtained from the modulus

measurements, even if the voids cannot be easily seen. Joints with voids on the surface of the

bondline were obviously poor joints but many of the voids were only visible when the joint was

broken. Only after testing the joints to failure was it possible to assess the success of the

manufacturing process.

Examples of the shear moduli and tests to failure for this system, when applied using the one-

part method are shown in Table 10 and some typical measurements taken on the joints made

with two-part application are listed in Table 11.

Specimen Shear Modulus MPa Max Stress MPa Strain to failure

FA/ST/lA 223.0 ± 6.2 42.1 1.50

FA/ST/lC 224.0 ± 2.2 38.6 1.40

FA/ST/lF 226.8 ± 5.1 40.0 1.40

FA/ST/2F 218.9 ± 13.0 34.8 1.30

Average 223.2 ± 7.3 38.9 ±3.1 1.4 ± 0.1

Table 10 Shear moduli and failure data for acrylic F241 (l-part application) - butt joints


FB/ST/2A 176.3 ± 4.1 25.5 1.17

FB/ST/2C 153.4 ± 4.2 15.6 0.90

FB/ST/2D 195.6 ± 8.8 15.7 0.85

FB/ST/2E 242.7 ±6.1 20.0 1.00

Average 192.0 ± 28.1 19.2 ± 4.7 0.98 ± 0.14

Table 11 Shear moduli and failure data for acrylic F241 (2-part application) - butt joints

The joints made with the pre-mixed adhesive gave the most consistent results when tested to

failure (Figure 17). Using the two separate constituent parts of the system to manufacture joints

of 0.5mm thickness was not successful. Most of these joints had large voids present and the test

22


results showed a large variability (Figure 18). The maximum stress measured in the joints made

with two-part application was less than half the stress seen in the joints manufactured with the

one-part method.

As many of the joints had a large number of voids present it may be that the bondline thickness

used for all the testing (0.5mm) was too large for this adhesive system. A bondline thickness of

this size was particularly difficult to achieve with the two-part application so it may be

necessary to consider the bondline thickness very carefully when testing this type of adhesive.


Butt joint shear stress/strain data for this material are shown in Figure 19 and the data is

summarised in Table 12.

All these joints were tested using a constant angular displacement rate as the strain rate remains

nearly constant over the time of the test. Figure 20 illustrates the strain variation with time over

the period of a typical torsional test. The strain rate for this adhesive is reasonably constant as

the polymer continues to bear an increasing load until failure at a strain of more than 100%.


PU/ST/lE 86.2 ± 15.0 16.8 1.17

PU/ST/2A 80.6 ± 8.2 16.0 1.20

PU/ST/2B 71.3 ± 11.4 16.4 1.25

PU/ST/2C 68.5 ± 5.1 16.5 1.40

Average 76.7 * 10.1 16.4 * 0.3 1.26 ± 0.10

Table 12 Shear moduli and failure data for polyurethane 3532- butt joints

As the stress/strain relationship does not appear to be linear at any point, the calculation of the

shear modulus from the experimental data is not easy to define. For comparison purposes, the

secant modulus at 1% strain has been used. Since the behaviour of this polymer is very

dependent on the rate of displacement and the temperature, it is important to ensure that these

factors are constant for each of the tests. The temperature was controlled at 23°C and the

surface strain rate was set at 4% per minute for all the tests, Since the strain rate through the

joint varies from zero at the centre to the maximum at the surface and the material is rate-

dependent, the Nadai correction is not strictly applicable to this adhesive.

Butt joints made with the polyurethane adhesive and tested

with increasing strain until the joint fails at approximate y

23

in torsion, show an increasing stress

16MPa and 120% strain. Although


the load drops at this level of strain, the joints do not separate into two pieces and remain joined

by the adhesive.

When testing this adhesive, it is important to ensure that the polymer has sufficient time to relax

before applying the load. As the stress/strain relationship does not appear to be linear at any

point, it is important to allow the adhesive to relax to a state of zero load and displacement

before tests are commenced. If it is possible, any backlash in the machinery should be removed

before starting any tests. With the torsional testing machine, this can be achieved. Since the

stresses applied, particularly for the modulus measurements are so low (less than lMPa), any

backlash present in the testing equipment can affect the experiment results. As the machine

takes up any backlash, the loading is halted and for materials such as the polyurethane, the

polymer relaxes before the loading is recommenced. The stress/strain curve, in these

circumstances, may not reflect the true behaviour of the adhesive and the secant modulus will

not provide an accurate shear modulus.

7. Conclusions

Torsional testing of bulk adhesive specimens has been shown to give consistent, repeatable

results for both shear modulus measurements and stress/strain data to failure for the two epoxies

considered in this programme. This method is suitable for adhesives that can be cast in bars

13mm thick and machined to a circular cross-section. For adhesives that have a significant

exothermic reaction (e.g. acrylicF241) or for polymers that are fairly flexible in bulk form (e.g.

polyurethane), bulk specimens may not be possible to produce and, therefore, this method of

testing may not be appropriate.

For the bulk specimen testing, two corrections are needed to generate the true stress/strain

behaviour from the experimental data. In order to measure the strain to failure, the angular

displacement of the clamped ends of the bulk specimen has been used. As the gauge length of

the specimen is radiussed at both ends, a correction must be applied to the gauge to generate the

effective length of the specimen. Also, as the shearing stresses across the diameter of the

specimen are not uniform

Nadai correction must be

failure.

The butt torsion test, is a

once the material enters the non-linear region of its behaviour, the

applied to the torque/twist data to derive the stress/strain curve to

reliable test method for all the types of adhesives studied in this

programme, generating reliable shear modulus values and stress/strain curves. As for the bulk

tests, the Nadai correction should be applied to the experimental data to achieve the true

24


stress/strain behaviour. However, this correction may not be applicable to a material such as the

polyurethane where the behaviour is highly dependent on the strain rate.

It has been demonstrated that the strain rate experienced by the adhesive in a butt joint tested at

a constant angular displacement rate, increases as the polymer yields. Comparison of these

results with testing conducted at a constant strain rate indicates that the latter tests may produce

a lower maximum shear stress. Since the behaviour of polymers is likely to depend on the

strain rate that is applied to the material, the strain rate should be considered when analysing test

results and comparing bulk specimen testing (where the strain rate is constant) with butt joints.

25


8 List of Tables

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 8

Table 9

List of adhesives tested, showing the cure cycles used

Shear moduli and yield stress for polypropylene

Shear moduli and failure data for acetal polymer

Shear moduli and failure data for bulk l-partepo~AV119

Shear moduli and failure data for bulk 2-part epoxyTE251

Shear moduli and failure data for 1-part epoxyAV119 - butt joints tested with constant angular displacement rate

Shear moduli and failure data for l-part epoxyAV119 - butt joints tested at a

controlled 4% per minute

Shear moduli and failure data for 2-part epoxyTE251 - butt joints tested with constant

angular displacement rate

Shear moduli and failure data for 2-partepo~TE251 - butt joints tested at a controlled

4% per minute

Table 10 Shear moduli and failure data for acrylic F241 (l-part application) - butt joints

Table 11 Shear moduli and failure data for acrylic F241 (2-part application) - butt joints

Table 12 Shear moduli and failure data for polyurethane 3532- butt joints

26


9 List of Failures

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Extensometry for bulk specimen testing

Extensometry for butt joint testing

Shear stress and strain versus time for atypical TE251 butt torsion joint

Bulk specimen geometry

Butt joint geometry

Calculation of the true stress from the torque/angle data using the Nadai correction

Shear stress/strain data for a typical bulkTE251 specimen, illustrating the effect of the Nadai correction

Shear stress/strain data for AcrylicF241, illustrating the effect of the Nadai correction

Comparison of strain measurements for bulk specimens using LVDTs and rotary potentiometer

Figure 10 Bulk torsion tests for polypropylene

Figure 11 Bulk torsion tests for acetal polymer

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Figure 20

Shear stress/strain data for bulkAV119

Shear stress/strain curves for bulkTE251

Shear stress/strain data forAV119 - butt joints

Comparison of maximum shear stress measurements forTE251 andAV119

Shear stress/strain data forTE251 - butt joints

Shear stress/strain data for acrylic F241(one-part) - butt joints

Shear stress/strain data for acrylic F241 ( two-part) - butt joints

Shear stress/strain data for polyurethane 3532- butt joints

Strain variation with time for a typical polyurethane butt joint

27


10 List of reports from Project 1

Report no 1

Report no 2

Report no 3

Report no 4

Report no 5

Report no 6

Report no 7

Report no 8

Report no 9

Report no 10

Preparation of Bulk Adhesive Samples for Mechanical Testing. January 1994 B C Duncan, M A Girardi and B E Read. Measurement of Strain in Bulk Adhesive Test pieces. October 1994 B C Duncan and P E Tomlins. Tensile Behaviour of Bulk Specimens of Adhesives. May 1995 G D Dean and B C Duncan. Working Draft for an International Standard. Adhesives - Methods for preparing bulk specimens. Part 1: Low-temperature, two-part systems. September 1995 B C Duncan, G D Dean and H Simon. Correlation of Modulus Measurements on Adhesives Using Dynamic Mechanical and Constant Strain Rate Tests. March 1996 G D Dean and B C Duncan. Test Methods for Determining Shear Property Data for Adhesives Suitable for Design. Part 1: Notched-beam shear (Iosipescu) and notched-plate shear (Arcan) methods for bulk and joint test specimens. March 1996 B C Duncan and G D Dean. Test Methods for Determining Shear Property Data for Adhesives Suitable for Design. Part 2: The torsion method for bulk and joint test specimens. March 1996 R Thomas and R Adams. Test Methods for Determining Shear Property Data for Adhesives Suitable for Design, Part 3: The thick-adherend shear test method. March 1996 L Vaughn and R Adams. Comparison of Bulk and Joint Specimen Test for Determining the Shear Properties of Adhesives. March 1996 G D Dean, R Adams, B C Duncan, R Thomas and L Vaughn. Final Report. March 1996 G D Dean.

28


12 References

1 B C Duncan and G D Dean: Test Methods for Determining Shear Property Data for Adhesives Suitable for Design. Part 1: Notched-beam shear (Iosipescu) and notched-plate shear (Arcan) methods for bulk and joint test specimens. Report no 6, MTS Adhesives Project 1, March 1996

2 L Vaughn and R Adams: Test Methods for Determining Shear Property Data for Adhesives Suitable for Design. Part 3: The thick-adherend shear test method. Report no 8, MTS Adhesives Project 1, March 1996.

3 Dean G D and B C Duncan: Tensile Behaviour of Bulk Specimens of Adhesives, MTS Adhesives Project 1, Report No 3 May 1995

4 Chodorowski W.T. Fatigue strength in shear of an alloy steel, Int. Conference on Fatigue in Metals Proceedings, Sept 1956

5 Duncan B C, Girardi M A, Read B E, The Preparation of Bulk Adhesive Samples for Mechanical Testing, MTS Adhesives Project 1, Report No 1, January 1994

6 Coppendale J. The stress and failure analysis of structural adhesives PhD pp13-16, 1977

1-

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