Arcan test fixture and specimen
Bulk Resin Tests: The Arcan test method employs a compact butterfly specimen with two symmetrical notches cut at ±45° to the loading axis [34, 35, 39–43]. This specimen is gripped in a loading frame with the load applied via the faces to avoid the potential problem of instability associated with edge loading [35]. This method has been used to measure the shear properties of fibre-reinforced plastic composites and adhesives. The original Arcan specimen consisted of a diametrically loaded circular disc containing asymmetric cut outs with the mid-section designed to establish a state of uniform stress. Depending on the loading angle, various biaxial stress states can be produced. In principle, a uniform state of pure shear should exist when the loading axis and the notch axis are aligned.
Specimens are typically 40 mm wide, 50 mm long with a 12 mm distance between notches. Modulus measurements obtained from bulk resin specimens have been shown to be consistent for both 2 mm and 4 mm thick specimens. A notch radius of 1.5 mm is used to ensure a relatively uniform shear stress state exists within the specimen gauge-section [35]. Specimens are clamped in a specially designed fixture, with four bolts serving to supply the necessary pressure to the jaws. The specimen is loaded along the faces. Alignment is critical with small misalignments resulting in pre-stressing of the specimen, which could lead to premature failure.
Determination of shear properties is the same as that for the V-notched beam test. Shear strain is measured by either bonding two biaxial strain gauges, one on each opposite face of the specimen, to the centre of the specimen, in the area between notches, or by using a shear extensometer [35]. The strain gauges have a gauge-length of 1 mm or 2 mm, to keep within the region of uniform stress, and are aligned at ±45° to the longitudinal axis of the specimen. The extensometer is suitable for measuring shear modulus, but not satisfactory for determining failure strains. Measurements of shear modulus have been found to correlate well with other techniques, whereas the shear strength value tends to be low. This is because failure occurs as a result of high transverse tensile stresses on the cut out boundary, away from the gauge-section.
High compressive stresses also exist near the notch roots, and as a result localised buckling may occur. Compliant materials, such as polyurethane, are prone to deform at high stresses. Measurements are invalidated unless constraints are applied the specimen clamps to maintain alignment.
Adhesive Joint Tests: Arcan joint specimens consist of two symmetrical metal adherends, each forming half of the test specimen. Each adherend is 19.75 mm wide to allow for a 0.5 mm bondline thickness. The adherends are typically 6 mm thick. Bondline length may range from 5 to 10 mm, without producing significant differences in test data. The two halves are bonded together in a clamping fixture. This fixture ensures good specimen alignment and maintains suitable pressure on the specimen during cure.
Arcan joint specimen
Specimen alignment is particularly critical as small misalignments can result in premature failure of the specimen. Rigid adhesives (e.g. epoxy) are particularly sensitive. An approach that has proved successful has been to use a clamping fixture for specimen manufacture that replicates the actual loading fixture [36].
The shear stress distribution at the specimen centre is uniform, however there are shear stress concentrations at the notch roots. Tensile and compressive peel stress concentrations occur near the ends of the bonded region, at the interface between adhesive and adherend. The use of concave fillets at the bondline ends lowers the shear and peel stresses in these regions, preventing premature failure of the specimen. The shear displacement is measured using an extensometer with the average shear stress as with the V-notched beam test. Strain gauges are unsuitable for this application. The method is particularly suitable for measuring the shear moduli of compliant materials (i.e. G < 1 GPa), such as polyurethane and acrylic adhesives.
The application of shear extensometry to adhesively bonded Arcan specimens proved more difficult than for bulk adhesive specimens. Due to the hardness of the metal adherends, it is difficult to achieve an acceptable contact between the extensometer needles and the adherend surface. This approach is unsuitable for fatigue testing, as the extensometer needs would be unable to maintain satisfactory contact with the specimen.
Cyclic and Environmental Testing: Provided the necessary precautions are taken, the Arcan test is probably suitable for long term testing under cyclic loading conditions. There will probably be a tendency under cyclic loading for slippage within the grips of the test fixture, resulting in the loading bolts directly bearing onto the specimen. This would cause bulk adhesive specimens to fail within the gripping regions. Although this may not be a serious problem for adhesive joint specimens, concern must be expressed as to the validity of the strength data, since these specimens invariably undergo interfacial failure as result of the presence of high transverse tensile stresses at the bondline ends. This method is expected to be suitable for creep and environmental testing, provided the measurements are restricted to the measurement of shear modulus.
| Advantages | Disadvantages |
|
Bulk Resin
Shear modulus attainable for all polymeric materials Shear strength attainable for thermoplastics Adhesive Joints Yields shear modulus and strength Suitable for compliant adhesives Additional Points Low material requirements Data reduction is straightforward Suitable for use under environmental conditions Suitable for creep testing |
Bulk Resin
Strain gauges/extensometer required Brittle polymers—tensile failure Adhesive Joints Special bonding fixture required Interfacial tensile failure occurs Non-uniform shear stress state Additional Points Accurate specimen machining required Special test fixture required Unsuitable for use under cyclic loading conditions No existing national or international standards |
Next: Plate-Twist Test
