A simple engineering approximation for evaluating the interaction term in the tensor polynomial strength theory is given. The approximation treats the interaction term under plane stress condition as a parameter which depends on the failure properties parallel with the reference axes. Theoretically, the correct way to obtain the value of the normal stress interaction parameter is by way of tests on 45 ° off-axis specimens. However this method has been found difficult to apply to real anisotropic materials. Various approximations have been proposed of which the most noteworthy is the one of "Tsai and Hahn." A detailed discussion of this and the currently proposed approx imation is included.
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