Bonding Fixture
Alignment Fixture
Extensometry
Butt Joint Specimen
Transfer of load to an adhesively bonded structure by tension, either directly or indirectly (peel), represents the most severe form of loading, since the strength of the joint relies on the tensile strength of the adhesive, which is low. It is good design practice, therefore, to ensure that the load is transferred by shear or compression and that direct or induced tensile stresses are minimised. Tensile stresses are virtually impossible to avoid, however there is a need for reliable test methods to measure the tensile properties of the adhesive.
This section examines the tensile butt joint, which is used to evaluate the tensile properties of adhesive joints (see also Table 1). Test methods that can be used to measure the static, fatigue and creep behaviour of engineering adhesives, in bulk form, will not be covered in this section. The reader should refer to the ISO standards relating to the testing of polymeric materials [1, 2].
|
|
|
|
|
|
Bonding Fixture |
Alignment Fixture |
Extensometry |
Butt Joint Specimen |
The cylindrical butt joint can be used to test thin bondlines in tension, torsion and compression. The test provides data on the moduli of rigidity and elasticity, and Poisson’s ratio. The average strength is taken as the applied load at failure divided by the bond area. The test is difficult to perform. Significant bending can be induced due to misalignment of the adherends, resulting in premature failure. Care needs to be taken to ensure good alignment during specimen preparation (i.e. bonding of adherends) and testing. A small misalignment can severely reduce strength data. A further consideration, relates to the removal of the spew fillet (i.e. excessive adhesive) around the outer edges of the joint. The spew is able to transmit some of the applied load, resulting in a significant reduction in the level of axial stresses within the adhesive joint, and thus contributing to the uncertainty in moduli and strength data.
Variations of this test configuration have been included in ASTM D 897 [3] and ASTM D 2095 [4], and in BS EN 26922 (ISO 6922) [5]. Specimens may be designed with either a circular or square cross-section. Circular cross-section specimens are usually used to facilitate manufacture and maintain symmetry. Both circular and square cross-sections, 10 to 50 mm wide, are included within BS EN 26922.
The adhesive in the bondline is restrained in the radial and circumferential directions by the adherends. In the absence of this restraint, the adhesive is free to undergo radial contraction with respect to the stiffer adherend. The presence of the adherends has the effect of inducing radial and circumferential stresses, thereby increasing the effective stiffness of the joint. The apparent Young’s modulus E* is given by [6]:
where E is the Young’s modulus and v is the Poisson’s ratio of the adhesive. This analysis assumes that the adhesive has a much lower stiffness than the adherends (i.e. zero contraction of adherends).
The tensile stresses in the central region of the adhesive layer for butt tension specimens are uniformly distributed and there are no shear stresses present [6–7]. In marked contrast, the stress state in the outer edges is highly complex, varying through the thickness of the adhesive layer. The stress state at the overlap edges (i.e. adhesive-adherend interface) is complicated by through-thickness shear stresses and the presence of stress singularities. Large, indefinable, stress concentrations at the periphery of the adhesive layer can be expected to render the strength data meaningless. Rounding the adherend butt edges to a diameter equal to the adhesive thickness can facilitate their removal. This effectively increases the bondline thickness at the outer edges of the bondline, and as a result non-linear stress effects may result, especially at elevated temperatures and exposure to high humidity environments [6–7].
It is difficult to envisage using the butt joint for assessing long-term performance of adhesive joints under combined cyclic and hostile environments, considering the difficulties associated with this configuration (inc. monitoring of strain, which is relatively small).
| Advantages | Disadvantages |
|
Yields tension, torsion and compression Straightforward/economic Data reduction BS EN 26922/ASTM D 897/ASTM D 2095 Suitable for cyclic/environmental testing (QA only) |
Qualitative adhesive property data only Strain measurements difficult High shear and peel stresses at bondline edges Strength sensitive to spew fillet Sensitive to specimen misalignment Limited to thick and rigid adherends Failure attributed to peel stresses Special bonding fixture required |
Next: Peel Tests
