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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
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
Part A : Summary
R Thomas and R D Adams
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
Summary Report: The torsion method for bulk and joint test specimens
.
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
Summary Report: The torsion method for bulk and joint test specimens
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
Summary Report: The torsion method for bulk and joint test specimens
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
Summary Report: The torsion method for bulk and joint test specimens
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.
MTS Adhesives Project 1
Basic Mechanical Properties for Design
Report No. 7 March 1996
TEST METHODS FOR DETERMININ G SHEAR PROPERTY DATA FOR ADHESIVES SUITABLE FOR DESIGN
Part 2: The torsion method for bulk and joint test specimens
Part B : Full Report
R Thomas and R D Adams
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
The torsion method for bulk and joint test specimens
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.
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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 torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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.
6.1.3 2-part epoxy TE251
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
6.1.4 Polyurethane 3532
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
The torsion method for bulk and joint test specimens
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 torsion method for bulk and joint test specimens
6.2.2 2-part epoxy TE251
.
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.
Specimen Shear modulus GPa Max Stress MPa Strain to failure
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
constant angular displacement rate
Specimen Shear modulus GPa Max Stress MPa Strain to failure
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
The torsion method for bulk and joint test specimens
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
Specimen Shear Modulus MPa Max Stress MPa Strain to failure
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
The torsion method for bulk and joint test specimens
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.
6.2.4 Polyurethane 3532
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%.
Specimen Shear Modulus MPa Max Stress MPa Strain to failure
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 torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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
The torsion method for bulk and joint test specimens
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-
29
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0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Shear strain
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Figure 13 Shear stress/strain data for TE251 - bulk specimens
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Figure 14 Shear stress/strain data for AV119 - butt joints
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