This section will examine loading configurations that measure adhesive resistance to cleavage (see also Table 2). Mode I (crack opening) fracture toughness test methods, such as the double cantilever beam (DCB) and the tapered double cantilever beam (TDCB), will be discussed later.
Wedge Cleavage Test
Cleavage stresses (essentially concentrated tensile stresses applied perpendicular to the bondline) represent the most severe form of loading that can be experienced by a bonded joint. The Wedge cleavage test [12, 18], often referred to as the Boeing wedge test, was developed in order to simulate, although in qualitative manner, the forces and effects of cleavage stresses in an adhesively bonded joint at metal-adhesive/primer interface. This test method is widely used throughout the aerospace/defence industry and has proved highly reliable in determining and predicting the environmental durability of adherend surface preparations [19]. Although, the test was developed specifically for aluminium alloy joints, the test is applicable to metal-metal, metal-composite and composite-composite joints.
The test involves forcing a wedge into the bondline of a flat-bonded specimen (i.e. DCB configuration), thereby creating cleavage stresses at the crack tip. The wedge imposes a fixed displacement to the adherends and the energy stored in bending the adherends (i.e. arms) provides the driving force for crack growth. The stressed specimen is exposed to an aqueous environment, usually at an elevated temperature, or an appropriate environment representative of service condition. The resulting crack length is monitored with time, and fracture energies, G, are determined as follows [20]:
where E is the Young’s modulus of adherends, w is the displacement caused by the wedge, h is adherend thickness and a is the crack length. Measurement of crack length can be performed reasonably accurately using a travelling microscope.
Specimens are nominally 25 mm wide and 200 mm long, with an adherend thickness of approximately 2 mm. The bonded joints can be made either separately or sectioned from a bonded panel (150 mm square). An aluminium or steel wedge, 25 mm square and with a 14° taper is specified in ASTM D 3762 [18]. Testing and specimen fabrication is relatively straightforward and economic. A variant on the static wedge test is to force the wedge slowly (2–3 mm/min) through the adhesive.
The wedge test is more severe than conventional lap-shear or peels tests, because water is present at the crack tip, a region of high tensile stress, whereas moisture has to diffuse into the bondline from the edges of the lap-shear and peel tests. The test is not suitable for generating quantitative fracture strength data or for short-term testing. If the bonded joint is sensitive to the test environment, then significant crack growth can be expected to occur within one hour of exposure. A crack growth limit will often be reached within 10 days for most systems, enabling rapid comparative durability measurements [12]. The general accuracy of the wedge test is compromised when testing toughened adhesives and flexible adherends. It is therefore important to ensure that plastic deformation of the adherends is either prevented or at least minimised when inserting the wedge. The effectiveness and sensitivity of the technique is dependent on the ageing environment.
| Advantages | Disadvantages |
|
Self-stressed Yields fracture strength Straightforward/economic Accurate and highly reproducible data Sensitive to environmental effects ASTM D 3762 Suitable for environmental testing (QA only) |
Limited to rigid materials Not suitable for generating design data Limited to rigid adherends No allowance for large adherend deformation Unsuitable for cyclic loading |
Compact tension test
In this method, a rigid joint is subjected to cleavage stress by the application of a tensile force at one edge of the joint. The test was developed for determining the cleavage strength of metal-to-metal adhesive bonds, with steel being the preferable material of construction. Specimens are easily fabricated by bonding the flat adherends together. The spew fillet present at the loading end of the specimen should be carefully removed avoiding cracks and scratches, which will considerably reduce cleavage strength of the adhesive bond. Specimens are typically 25 mm wide, with a bondline length of 25 mm. The depth of the adherend is 12 mm along the bondline. Both BS 5350 Part C1 [21] and ASTM D 1062 [22] specify test methods for determining comparative cleavage properties of adhesive bonds for metal adherends. The compact tension test may be used for assessing compliance of an adhesive with a specification and for quality control. A special fixture is required to test the specimens. Although, the test was designed for the determination of comparative cleavage properties under static loading conditions, the use of the procedure could possibly be extended to fatigue testing.
| Advantages | Disadvantages |
|
Yields fracture strength Compatible with metals and PMCs Straightforward/economic BS 5350: Part C1/ASTM D 1062 Suitable for environmental/fatigue testing (QA only) |
Special test fixture required (moderate cost) Special bonding fixture required Limited to rigid adherends Not suitable for generating design data |
Next: Shear Tests
