The measurement of in-plane shear modulus from shear stress-strain curve is done as follows. The shear stress-strain curve for  specimen is obtained first. A typical shear stress-strain curve for such a specimen is shown in Figure 8.9. The shear modulus is obtained from the initial slope of the this curve in the range of 0.1-0.5% strain as 

(8.20)

The tensile test on  specimen provides an acceptable method for the measurement of in-plane shear modulus. However, one should be careful while interpreting the ultimate shear strength and strain. It should be noted that the laminae are subjected to a biaxial state of stress and not a pure shear. The normal stresses act along the shear planes causing the onset of mixed mode fracture. Other kind of failure like multiple ply cracking, fibre rotation and edge or internal delaminations occur prior to final failure. Therefore, the true failure is very difficult to determine. The shear strength is specified by different standards corresponding either to the ultimate load generated during the test or to a specified strain level. It is recommended in ISO standard that the test be terminated at  . The shear strength is taken as the peak load at or before 5% strain.