Torsion rod specimen
Torsion loading of a circular rod is mechanically one of the most accurate methods for characterising the shear properties of bulk resin materials [48, 49]. Specimens may be moulded or machined directly from rods. The ends of the gauge-section are filleted to minimise stress concentrations present at these locations.
The high degree of machining required exposes this specimen geometry to a high risk of machine-induced damage. This is compounded by difficulties in producing castings free of voids and residual stresses place considerable limitations on the materials that can be evaluated using the torsion method. This test may be inappropriate for polymeric materials that have a rapid exothermic cure reaction (e.g. acrylic) or exhibit considerable flexibility (eg polyurethane). Accurate machining of flexible materials can be a problem.
Torsion testing of solid circular rods is relatively straightforward to perform, provided suitable mechanical test equipment (i.e. torsion facility) is available. Many laboratories however do not have access to suitable equipment. The equipment is relatively complex and requires that no axial load be applied to the test specimen during test set-up or during the test. Servo-hydraulic torsion + tension/compression test machines are more costly than servo-hydraulic tension/compression machines. Costs can be reduced by using a torsion gear box attached to a screw-driven test frame.
Surface shear strain is measured using either strain gauges or contact extensometers. Strain gauges are adhesively bonded to the specimen surface at the centre of the gauge-section. The gauges are oriented at ±45° to the longitudinal axis of the specimen. An additional axially aligned strain gauge is recommended to monitor longitudinal strain. Tensile or compressive axial strains must not be present throughout the test duration. The shear modulus is determined from the linear region of the stress-strain curve.
Simple data reduction procedures are available for both linear and non-linear stress-strain response of circular rods in torsion. The analysis procedure developed by Nadai [50] for metals can be used to correct for the non-uniform shear stress distribution through the specimen cross-section, which occurs during non-linear stress-strain behaviour. The accuracy and repeatability of strain measurements, resulting from the large gauge-length, is relatively high compared with other test methods. As with all bulk test specimens, the strength data is dependent on the quality of the specimen. Internal stress concentrations in the form of entrapped air introduced during processing can result in premature failure.
Butt torsion test is a suitable method for all types of adhesives and is capable of generating reliable shear modulus values and stress-strain curves. Specimen geometry is similar to butt tension specimen. Contact extensometers are used to measure the angular rotation across the adhesive. A major disadvantage of this method is the difficulty of obtaining high precision strain measurements, due to the very low angular displacements (1–2 degrees) associated with this test configuration [49].
Shear modulus G can be determined from the following equation:
where T is applied torque, R is the radius, t is the adhesive layer thickness and q is the angular displacement.
The stiffness of the adherends needs to be considerably higher than the adhesive stiffness, in order to minimise the effect of adherend deformation on the stress-strain response. Strain measurements need to be corrected for the effect of adherend deformation and non-linear stress-strain behaviour (i.e. non-uniform shear stress distribution across the bond area). The shear distribution is high on the external surface and lower in the centre.
Cyclic and Environmental Testing: Bulk or joint torsion tests are compatible with cyclic, creep and environmental loading conditions. Environmental equipment can be adapted to meet the requirements of torsion equipment. Concern must be expressed with regard to materials, such as polyurethane, where the behaviour is highly rate dependent. For polyurethanes the glass-to-rubber relaxation transition occurs at ambient temperatures. The corrections for non-linear behaviour may prove inadequate.
| Advantages | Disadvantages |
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Bulk Resin
Shear modulus attainable Shear strength attainable for thermoplastics Adhesive Joints Yields shear strength and shear modulus Compatible with metals and composites Additional Points Suitable for use under environmental conditions Suitable for cyclic/creep testing |
Bulk Resin
Strain gauges/extensometers required Brittle polymers—tensile failure occurs Axial strains need to be monitored Adhesive Joints Special bonding fixture required Non-uniform shear stress state Small strains—difficult to measure accurately Additional Points Accurate specimen machining required Torsion facility required Stress concentrations present at end grips Complex data reduction (non-linear stress/strain) No existing national or international standards |
Next: Thick Adherend Shear Test (TAST)
