V-notched beam fixture with specimen
Bulk Resin Tests: The test method was originally developed for measuring the shear strength of metals and welds [31]. The original specimens were cylinders, with a 90° circumferential notch located at the specimen centre. The notch was introduced to promote a uniform state of shear at the specimen centre, and to ensure failure occurred within the test-section. Walrath and Adams [32] subsequently modified the specimen geometry and loading configuration in order to determine the shear characteristics (i.e. shear modulus and shear strength) of fibre-reinforced plastic composites. The modified method, now standardised as ASTM D 5379 [33], is used widely for characterising the shear properties of polymeric materials, including adhesives.
The ASTM method employs a double edge-notched, flat, rectangular specimen with two 90° notches cut at the edge mid-length with faces orientated at ±45° to the longitudinal axis. The notches extend to a depth of 20% of the specimen width (i.e. 4 mm). ASTM D 5379 recommends a notch root radius of 1.3 mm in order to minimise the shear stress concentration at the notch roots, and thus promote a more uniform shear stress distribution along the notch-root axis. The specimen has a length of 76 mm and a width of 20 mm with a specimen thickness between 3 and 4 mm. Specimens with a thickness less than 3 mm require adhesively bonded tabs (typically 1.5 mm thick) to prevent out-of-plane bending or twisting which could lead to premature failure. Local crushing, which can occur near the inner loading regions, can also be avoided by the use of tabs. Test specimen dimensions may be reduced if required.
In principle, this procedure induces a state of pure shear stress at the mid-length of an isotropic specimen, by the application of two force couples. A state of constant shear force is induced through the mid-section of the test specimen, with the induced moments cancelling exactly at the mid-length, thereby producing a state of pure shear at this location. The stress distribution in the test-section of both isotropic and anisotropic specimens, however, is not uniform. The use of a large notch root radius (i.e. 1.3 mm), as recommended in ASTM D 5379, is not entirely successful in promoting a uniform shear stress distribution. Tensile stresses near the notch roots can lead to premature tensile failure of brittle isotropic materials.
The average shear strength S and shear modulus G can be determined using:
where Pmax is the maximum applied, w is the distance between the notches and h is the thickness. The variables Δ P, Δ ε 45 and Δ ε -45 are the change in applied load and +45º and–45º normal strains in the initial linear region of the shear stress-strain (t - g ) curve. Shear strain is measured by bonding two biaxial strain gauges (one on each opposite face of the specimen) to the centre of the specimen in the area between notches. 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.
V-notched specimen with strain gauge
Although a special test fixture is required, testing is relatively straightforward. To minimise potential effects of out-of-plane movement or twisting of the specimen, it is recommended that the strain data used for determining shear modulus be the average of the indicated strains from each side of the specimen. The following improvements to the test fixture will reduce differences in shear moduli obtained from the two sides of specimen and possibly eliminate the need for two biaxial rosettes:
The failure process is highly dependent on the microstructure of the material. Tensile failure is characteristic of brittle polymers (e.g. untoughened epoxy and acrylic resins). Failure initiates in the vicinity of the notch roots in a region of high tensile stress with cracks propagating in an unstable manner along the plane of the principal tensile stress. For these materials, ultimate failure stress does not correlate with shear strength. Thermoplastic polymers (e.g. polycarbonate and polypropylene) tested at room-temperature fail as a result of shear yielding along the notch root axis. Ductile materials can redistribute the tensile stress through plastic deformation, and therefore strain data can be obtained well beyond the shear yield stress of the material. Localised high compressive stress gradients may also produce plastic deformation in the vicinity of the loading points.
Adhesive Joint Tests: Work has been conducted using the V-notched beam test for characterising the shear properties of adhesive joints [34–37]. The test specimen consists of two shaped metal adherends, bonded together, with the bondline forming the test section between the notch roots (Figure 13). Shear strain was measured via a strain gauge based shear extensometer. Tests conducted on a flexible adhesive showed reasonable repeatability for the stress-strain response, however, no shear modulus data was supplied. Specimens can be expected to fail at the adhesive-metal interface due to high transverse tensile stresses present at the bondline ends. This mode of failure has been observed for Arcan joint specimens [35], and there is no indication that V-notched beam specimens should respond differently. Strain gauges and shear extensometry are unsuitable for use with adhesive joints specimens with thin bondlines.
V-notched joint specimen
The V-notched beam test can be used for fatigue testing [38], although the loading fixture would probably need to be modified in order to minimise frictional effects and prevent fretting at the loading points. This method is suitable for creep and environmental testing. The formation of ice within the moving parts of the test fixture could cause problems at sub-zero temperatures. Despite this, the test method has been used successfully over the range -40°C and 180°C.
| Advantages | Disadvantages |
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Bulk Resin
Shear modulus attainable for all polymeric materials Shear strength attainable for thermoplastics Adhesive Joints Yields shear modulus and strength Additional Points Low material requirements Data reduction is straightforward Suitable for use under environmental conditions Suitable for creep testing ASTM D 5379—composites Suitable for cyclic/environmental testing |
Bulk Resin
Strain gauges (2 biaxial rosettes) required Brittle polymers—tensile failure occurs 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 |
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