Plate-twist test method
The plate-twist method, which was initially developed to measure the shear modulus of plywood (ASTM D 3044 [44]), has proved satisfactory for measuring shear moduli ranging from 0.29 GPa to 88.2 GPa. A version of the test method for polymeric materials is now specified in ISO 15310 [45]. The plate-twist method is unsuitable for determining in-plane shear strength [46].
In the plate-twist test, a square plate is supported on the two corners of one diagonal and load is applied at a constant rate to the corners of the opposite diagonal. The stress state induced in the plate is essentially pure shear. The total load is recorded as a function of the resultant displacement. The plate should be square or rectangular in shape with the diagonals being of equal length. The plate length to thickness ratio (a/h and b/h) should be ³ 35 to minimise through-thickness shear effects. This test method is unsuitable for materials that are not transversely isotropic or homogeneous through the section. For these materials, the shear modulus produced under flexural loads is no longer equivalent to the in-plane shear modulus, but equivalent to the effective flexural shear modulus [46].
Plate-twist test specimen
The shear modulus G can be determined from the displacement of the loading points DwP:
and from the displacement at the plate centre DwC:
where a and b are the plate edge dimensions, h is the plate thickness and DP is the change in load (total) for a change in displacement w. A practical difficulty with the test has been the positioning of the loading points, which are normally in-board, rather than at the actual corner; as assumed in the analysis. This misplacement leads to an error of several percent in the shear modulus value. A correction factor K has been introduced into the above equations to account for in-board loading [47]:
where r is the ratio of S the test span diagonal length and D the plate diagonal length. K is relatively insensitive to variations in the Poisson’s ratio over the range 0.25 to 0.40 [46].
ISO 15310 [45] recommends a standard plate specimen 150 mm x 150 mm, and an in-board ratio of 0.95, with other plate dimensions being optional. The maximum thickness requirement for non-standard plate specimens should meet the requirement of 35h £ a £ b. The modulus is determined over the displacement range of 0.1h to 0.3h, with a maximum allowable plate deflection of 0.5 mm.
As the technique is non-destructive (most shear methods result in the destruction of the test material during testing), a plate-twist specimen can also be sectioned into tensile or compression coupon specimens. This guarantees that the shear modulus relates directly to any other property measured from the plate. An additional bonus is that the test results represent the shear response over a relatively large area, which means variations in microstructure across the plate are averaged.
| Advantages | Disadvantages |
|
Yields shear modulus Compatible with all materials No-destructive Stress state relatively uniform Straightforward/economic ISO 15310/ASTM D 3055 Suitable for use under environmental conditions |
No strength data Large specimens required Not suitable for adhesive joints Limited fatigue capability |
Next: Torsion Methods
