3. Thin Walled Tube in Torsion:
In this method a direct shear stress is applied to a thin walled tube by pure torsion applied along the longitudinal axis of the tube. The thin tube is an ideal specimen for testing composite laminae in shear as it can provide a uniform state of pure shear stress. Due to small thickness of the tube the shear gradient in the thickness direction can be neglected. The specimen should have a gauge length to diameter (L/D) ratio >1, and a wall thickness to diameter ratio (t/D) of 0.02, or less.
Figure 8.10: Shear test by torsion of tube |
The in-plane average shear stress  in a thin walled unidirectional tube  under pure torsional loading, is related to torque  by
 |
(8.24) |
where,  is the outer diameter and  is the inner diameter of the tube.
The shear strain is measured by means of two bonded triaxial strain gauges (0°/45°/90°). The strain gauges are bonded diametrically opposite each other, at the centre of the specimen. The strain gauges have a gauge length of 6 mm. The longitudinal and transverse strain gauges are monitored to ensure there are no significant bending forces applied to the specimen during the test set-up and no bending loads present during the test. The shear strain is determined from the average of shear strains measured using the  strain gauges. The shear modulus is then given as
 |
(8.25) |
where  and  denote the strain measured by  and  and strain gages. It should be noted that above equation indicates that the shear modulus is obtained from the initial slope of the shear stress strain curve in some range.
More details on this test can be seen in ASTM D5448/D5448M-93. |
