Mechanical Test Methods

Numerous test methods exist for characterising adhesives and bonded joints, and may be used to determine fatigue resistance, environmental durability and creep behaviour. Adhesive tests can be divided into those methods that provide mechanical property data for the adhesive, which aids the selection of adhesives, and those methods that can be used to determine the quality of adhesively bonded structures, and thus aid the design process of adhesive joints. Although an extensive range of test methods is available as national and international standards, most of these tests can only be used for qualitative measurements, providing a means of checking the effectiveness of different surface preparations and comparing mechanical properties of different adhesive systems (i.e. ranking adhesive formulations). A limited number of test methods are suitable for generating engineering data, particularly for determining structural integrity of adhesively bonded structures subjected to static, cyclic and environmental effects. A list of standards issued by the American Society for Testing and Materials (ASTM), British Standards Institution (BS series) and International Standards Organisation (ISO) is presented in Appendix I.

Stress analysis of adhesive joints requires a database of basic engineering properties of the adhesive, adherend and the joint geometry. Properties required are listed below:

Adhesive shear modulus
Adhesive elastic tensile or compressive modulus
Effective adhesive transverse modulus
Characteristic adhesive shear strength
Characteristic adhesive tensile or compressive strength
Adhesive and adherend elastic/plastic shear stress and strain
Adherend tensile or compressive modulus
Adherend Poisson’s ratio
Characteristic adherend through-thickness tensile strength
Adhesive layer thickness
Thickness of adherends
Joint efficiency (i.e. ratio of joint strength to adherend strength)

Most of the commonly used test methods are incapable of providing reliable engineering data because the test geometry induces a complex state of stress in the adhesive layer, thus invalidating the results. Two approaches have been adopted in order to overcome this problem. The first and direct approach is to measure the properties of bulk adhesive specimens. Bulk adhesive specimens are cast and machined to the required shape (e.g. dumbbell tensile specimens). Many liquid adhesives can be easily cast into bulk specimens without the need for machining. To minimise the deleterious effect of surface scratches, which may cause premature failure, the edges and faces of the specimen are carefully polished to remove any surface defects.

Although bulk adhesive specimens are relatively straightforward to test, there are a number of problems associated with casting bulk specimens. For example, exothermic reactions can occur when casting thermoset materials, resulting in material degradation through overheating. The problem is exacerbated with increasing thickness. Porosity, in the form of entrapped air and volatiles, is a common cause of premature failure. In many cases it is virtually impossible to produce void free specimens, particularly for materials with a high viscosity. In addition, residual thermal stresses may be generated, as a result of non-uniform (rapid) cooling. Residual stresses, which are typically compressive on the surface and tensile in the interior, are frozen into the material. This is an undesirable situation, as tensional strain at the surface enhances environmental stress cracking.

The second approach for determining engineering properties of adhesives is to use specially designed joint geometries with a thin bond line, often referred to as in-situ testing. In order for these test geometries to produce reliable engineering data, the test geometry should provide a pure state of stress, uniformly distributed across the contact surface and through the bond line, free of stress concentrations. The surface treatment should be sufficient to ensure cohesive failure in the adhesive layer. Ideally, the test method should employ simple and easily prepared specimens. Failure force should in principle remain constant with fluctuations in the failure force being attributed to variations in adhesive strength.

There are a number of problems associated with adhesive joint configurations. These are listed below:

The stress distribution within the bondline tends to be non-uniform in a majority of test joint configurations with stress concentrations existing at the bondline ends. Premature failure will often occur as a result of these stress concentrations.
Generally, the time taken for environmental effects to become apparent increases with joint size, thus test joints with small bonded areas, or with large bondline perimeters compared with the bonded area are preferred. Evidence suggests that small joints exposed to water may initially exhibit a short-term increase in joint strength, due to plasticisation of the bondline perimeter, reducing the stiffness and relaxing internal stresses. In the longer term, joint strength diminishes.
The accuracy and reliability of displacement measurements are often in question, as the magnitude of displacements is often small.

The four main loading modes of bonded joints are:

Peel loads produced by out-of-plane loads acting on thin adherends.
Shear stresses produced by tensile, torsional or pure shear loads imposed on adherends.
Tensile stresses produced by out-of-plane tensile loads.
Cleavage loads produced by out-of-plane tensile loads acting on stiff and thick adherends at the ends of the joints.

In practice, a bonded joint will simultaneously experiences several of these loading components.

Basic loading modes experienced by adhesive joints

Next: Tensile Test Methods

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