I have measured anisotropy of a sheet metal directly in an uniaxial tensile test. I have also determined it indirectly using inverse identification method in a biaxial bulge test. However, they are different (R11 which is the ratio of the yield stress in thickness to the yield stress, in uniaxial and biaxial tests are different). How can I explain them.
In addition, in the uniaxial test, lankford coeficient varies from 0.75 in low strain to 0.5 in high strain.
Any suggestion?
Anisotropy means; the response of the material depends on the direction of the applied load with respect to the principal axis. In very special cases such as where the sheet plane {111} belongs to the four fold symmetry axis then the direction of the uniaxal tensile test , which lies in plane will be immaterial. In the case of bi-axial test the results related to the elastic properties mearuments should be changed not more then few percent compare to the uniaxial tensile test.
On the other hand the effect of the applied biaxial tension stress on the surface morphological evolution is factor of two greater than the uni-axial tensile counter part as it was proven by Ogurtani below givenin 3th reference paper.
yes, anisotropy can differ in uniaxial and biaxial due to the texture effects arising during testing. i.e deformation induced texture can effect the mode of testing.
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