TAST specimen with extensometers for measuring shear deformation
An alternative approach for determining the shear properties of an adhesive is to apply uniaxial tensile or compressive load to a specimen consisting of thick, rigid adherends, with a short overlap length [51]. This test geometry was developed to overcome the inherent weaknesses of the double-lap shear test. Various thick adherend geometries have been suggested, including those specified in ISO 11003–2 [52] and ASTM D 3165 [53].
The specimen can be produced by one of two methods:
ISO 11003–2 specifies a specimen with an overall length of 110 mm, a width of 25 mm and an overlap length of 5 mm. The adherend thickness is 6 mm and the bondline thickness is typically 0. 5 mm. Slots should be 1.5 mm wide. Load is introduced to the specimen via two 12.7 mm diameter pins inserted in holes 80 mm apart. Care is needed to ensure that the holes are accurately drilled in the centre of each adherend, since small misalignments can result in unwanted rotation and uneven loading of the joint, thus compromising the test data. Testing can be conducted using a standard mechanical test machine. It is important to ensure that no coolants are used when machining the two parallel slots, as the coolant may react with the adhesive or penetrate the adherend-adhesive interface. Localised increases in temperature may also result in thermal degradation of the adhesive properties. The slots must be accurately machined through only one adherend.
In principle, shear forces distributed along the bondline between the slots or notches transmit the entire load. The stress distribution along the overlap length is predominantly shear with large peel stresses present at the corners of the overlap. The large peel stresses may cause premature failure. Adding a fillet to the end of the bondline may reduce the high concentration of peel stresses.
The average shear stress t is given by [51, 52]:
where P is the applied force, L is the overlap length and b the specimen width.
Strain is measured using two extensometers in order to eliminate errors from unsymmetrical specimens. The results are averaged. The shear modulus is obtained by calculating the slope of the linear region of the stress-strain diagram. If the strength results are to be used for design of bonded structures, then only cohesive failure data can be used.
Compressive TAST
TAST specimens can also be loaded in compression using a similar fixture to that specified in ASTM D 695 [54]. The specimen is supported along its entire length to minimise out-of-plane deformation. A procedure developed at NPL, based around ASTM D 695, specifies a specimen length of 80 mm and width of 10 mm [55, 56]. The two parallel notches are 6.5 mm apart and 1.5 mm wide. Testing is relatively straightforward to perform, requiring a support fixture of moderate cost. Results are comparable to those obtained from other test methods (Table 6) [55].
Table 6: Shear Strength Measurements for AV119 Epoxy Adhesive [55] (T =Tension/C = Compression)
| Material/Loading Configuration | Shear Strength (MPa) |
|
CR1 Mild Steel
Bonded 2.5 mm thick adherends (T) Bonded 2.5 mm thick adherends (C) |
47.6 ± 2.9 55.1 ± 0.7 |
|
5251 Aluminium Alloy
Bonded 2.5mm thick adherends (T) Bonded 2.5 mm thick adherends (C) Pre-shaped 5 mm thick adherends (T) Pre-shaped 5 mm thick adherends (C) |
35.1 ± 2.3 43.6 ± 3.8 42.0 ± 2.6 42.3 ± 1.4 |
|
Arcan Test Method
AV119 bulk adhesive Aluminium alloy adherends |
46.0 ± 2.0 47.0 |
Care needs to be taken to ensure that the clamping forces preventing lateral movement are minimal. It is possible that during testing, Poisson’s effects can introduce frictional forces, thereby increasing the failure load. Pre-shaped adherend data tends to be higher due to the presence of a small spew fillet of resin at the end of the bondline. The fillets diminish the influence of peel stresses on failure allowing the adhesive to reach its shear limit. Hence, bondline fillets should be used in a controlled manner to ensure test data consistency. Thinner sections than that recommended by ISO 11003–2 could possibly be used provided the intrinsic stiffness of the adherend material is sufficiently high. Steel is preferable to aluminium for measuring the shear properties of unconditioned joints.
Cyclic and Environmental Testing: The TAST should be suitable for characterising the shear properties of adhesive joints subjected to cyclic or creep loading, and/or hostile environments. The simplicity of the test geometry ensures that the TAST provides no particular loading problems under these conditions, although difficulties are envisaged in monitoring shear strains with the contact extensometers for cyclic loads and in corrosive environments. Clip gauges, used for crack opening displacement measurements, may prove satisfactory.
| Advantages | Disadvantages |
|
Yields shear strength and shear modulus Compatible with all metals, plastics and PMCs Stress state relatively uniform over bondline Straightforward/economic Tension standards—ISO 11003–2/ASTM D 695 Compression—NPL draft procedure Suitable for environmental testing |
Complex/expensive extensometers (2 off) required Special bonding fixture required Accurate specimen machining required Stress concentrations present at bondline ends Stress analysis difficult Limited fatigue capability |
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