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b. Three Rail (Symmetric) Shear Test:
The three rail shear test is an improved version of the rail shear test. Using one more rail in two rail shear test fixture it can produce a closer approximation to pure shear. The fixture consists of 3 pairs of rails clamped to the test specimen as shown in Figure 8.14. The outside pairs are attached to a base plate which rests on the test machine. Another pair (third middle) pair of rails is guided through a slot in the top of the base fixture. The middle pair loaded in compression as shown in Figure 8.14. The shear force in laminate is generated via friction between rail and specimen. The strain gages bonded to the specimen at  to the specimen’s longitudinal axis. The specimen geometry is shown in Figure 8.15.
The shear strength is given as
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(8.30) |
And the shear modulus is given as
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(8.31) |
where, all variables in these two equations are given previously.
It should be noted that the holes in the specimen are slightly oversized than the bolts used for clamping. Further, the bolts are tightened in such a manner to ensure that there is no bearing contact between the bolt and specimen in the loading direction. It is recommended that each bolt is tightened with a 100 Nm torque.
Figure 8.14: Three rail shear fixture for shear testing |
Figure 8.15: Specimen dimensions for three-rail shear test
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