Off-Axis Laminae

One can measure the tensile properties by conducting experiments on off-axis laminae. However, there are certain issues associated with this kind of experiments. For example, the presence of axial-shear coupling is associated with the nonzero  . Alternately, one can say that this term is associated with coefficient of mutual influence . Hence, these tests are not straight forward as in case of symmetric laminates with and  laminae. Therefore, sometimes these tests are called as specialized tests.

When the experiments are conducted to measure the properties like ,  and   one can get the other properties along with these tests. For example, the coefficient of mutual influence  , the nonlinear response and strength of an off-axis lamina for given fibre orientation can also be obtained.

There is an important issue associated with these tests is that what boundary conditions one should impose on the specimen? If a pure, uniform state of axial stress   can be applied to the ends and sides of a specimen and the specimen is free to assume any desired deformation pattern, the state of stress will be uniform and constant through-out the specimen. The deformation pattern is shown in Figure 8.4(a).

For uniform, far-field axial stress loading, that is , the stresses in principal material directions can be given as

(8.11)

Further, the global elastic constants associated with axial stress loading are measured as

(8.12)

Combining above two equations, we get

(8.13)

From the above equation all three strain components can be obtained for non zero value of axial stress. Thus, from the third of the above equation we can find the shear modulus.

It is not easy to apply pure, uniform tensile stress to an off-axis coupon. The specimens are gripped in such a manner that the ends of the specimen are constrained and boundary condition is actually a specification of the axial end displacement. Further, there are more issues with these tests like the constrained displacement induces a doubly curved displacement field in the specimen. The deformed shape of the coupon with restrictions on the ends is depicted in Figure 8.4(b). We will not deal the complete analysis for the measurements of the properties with tests on off-axis laminae.

The bone shaped specimens for chopper-fiber, metal matrix composite tensile tests. More details can be seen in ASTM D3552-77(1989). Further, for the tensile testing for transverse properties of hoop-wound polymer matrix composite cylinders are used. The details of this testing can be seen in ASTM D5450/D5450M-93.